Local generation of multineuronal spike sequences in the hippocampal CA1 region

Sequential activity of multineuronal spiking can be observed during theta and high-frequency ripple oscillations in the hippocampal CA1 region and is linked to experience, but the mechanisms underlying such sequences are unknown. We compared multineuronal spiking during theta oscillations, spontaneous ripples, and focal optically induced high-frequency oscillations (“synthetic” ripples) in freely moving mice. Firing rates and rate modulations of individual neurons, and multineuronal sequences of pyramidal cell and interneuron spiking, were correlated during theta oscillations, spontaneous ripples, and synthetic ripples. Interneuron spiking was crucial for sequence consistency. These results suggest that participation of single neurons and their sequential order in population events are not strictly determined by extrinsic inputs but also influenced by local-circuit properties, including synapses between local neurons and single-neuron biophysics.A hypothesized hallmark of cognition is self-organized sequential activation of neuronal assemblies (1). Self-organized neuronal sequences have been observed in several cortical structures (2–5) and neuronal models (6–7). In the hippocampus, sequential activity of place cells (8) may be induced by external landmarks perceived by the animal during spatial navigation (9) and conveyed to CA1 by the upstream CA3 region or layer 3 of the entorhinal cortex (10). Internally generated sequences have been also described in CA1 during theta oscillations in memory tasks (4, 11), raising the possibility that a given neuronal substrate is responsible for generating sequences at multiple time scales. The extensive recurrent excitatory collateral system of the CA3 region has been postulated to be critical in this process (4, 7, 12, 13).The sequential activity of place cells is “replayed” during sharp waves (SPW) in a temporally compressed form compared with rate modulation of place cells (14–20) and may arise from the CA3 recurrent excitatory networks during immobility and slow wave sleep. The SPW-related convergent depolarization of CA1 neurons gives rise to a local, fast oscillatory event in the CA1 region (“ripple,” 140–180 Hz; refs. 8 and 21). Selective elimination of ripples during or after learning impairs memory performance (22–24), suggesting that SPW ripple-related replay assists memory consolidation (12, 13). Although the local origin of the ripple oscillations is well demonstrated (25, 26), it has been tacitly assumed that the ripple-associated, sequentially ordered firing of CA1 neurons is synaptically driven by the upstream CA3 cell assemblies (12, 15), largely because excitatory recurrent collaterals in the CA1 region are sparse (27). However, sequential activity may also emerge by local mechanisms, patterned by the different biophysical properties of CA1 pyramidal cells and their interactions with local interneurons, which discharge at different times during a ripple (28–30). A putative function of the rich variety of interneurons is temporal organization of principal cell spiking (29–32). We tested the “local-circuit” hypothesis by comparing the probability of participation and sequential firing of CA1 neurons during theta oscillations, natural spontaneous ripple events, and “synthetic” ripples induced by local optogenetic activation of pyramidal neurons.     

Epileptic baboons have lower numbers of neurons in specific areas of cortex

Epilepsy is characterized by recurrent seizure activity that can induce pathological reorganization and alter normal function in neocortical networks. In the present study, we determined the numbers of cells and neurons across the complete extent of the cortex for two epileptic baboons with naturally occurring seizures and two baboons without epilepsy. Overall, the two epileptic baboons had a 37% average reduction in the number of cortical neurons compared with the two nonepileptic baboons. The loss of neurons was variable across cortical areas, with the most pronounced loss in the primary motor cortex, especially in lateral primary motor cortex, representing the hand and face. Less-pronounced reductions of neurons were found in other parts of the frontal cortex and in somatosensory cortex, but no reduction was apparent in the primary visual cortex and little in other visual areas. The results provide clear evidence that epilepsy in the baboon is associated with considerable reduction in the numbers of cortical neurons, especially in frontal areas of the cortex related to motor functions. Whether or not the reduction of neurons is a cause or an effect of seizures needs further investigation.Epilepsy is associated with structural changes in the cerebral cortex (e.g., refs. 1–6), and partial epilepsies (i.e., seizures originating from a brain region) may lead to loss of neurons (7) and altered connectivity (8). The cerebral cortex is a heterogeneous structure comprised of multiple sensory and motor information-processing systems (e.g., refs. 9 and 10) that vary according to their processing demands, connectivity (e.g., refs. 11 and 12), and intrinsic numbers of cells and neurons (13–16). Chronic seizures have been associated with progressive changes in the region of the epileptic focus and in remote but functionally connected cortical or subcortical structures (3, 17). Because areas of the cortex are functionally and structurally different, they may also differ in susceptibility to pathological changes resulting from epilepsy.The relationship between seizure activity and neuron damage can be difficult to study in humans. Seizure-induced neuronal damage can be convincingly demonstrated in animals using electrically or chemically induced status epilepticus (one continuous seizure episode longer than 5 min) to reveal morphometric (e.g., refs. 18 and 19) or histological changes (e.g., refs. 20 and 21). Subcortical brain regions are often studied for vulnerability to seizure-induced injury (21–27); however, a recent study by Karbowski et al. (28) observed reduction of neurons in cortical layers 5 and 6 in the frontal lobes of rats with seizures. Seizure-induced neuronal damage in the cortex has also been previously demonstrated in baboons with convulsive status epilepticus (29).The goal of the present study was to determine if there is a specific pattern of cell or neuron reduction across the functionally divided areas of the neocortex in baboons with epilepsy. Selected strains of baboons have been studied as a natural primate model of generalized epilepsy (30–36) that is analogous to juvenile myoclonic epilepsy in humans. The baboons demonstrate generalized myoclonic and tonic-clonic seizures, and they have generalized interictal and ictal epileptic discharges on scalp EEG. Because of their phylogenetic proximity to humans, baboons and other Old World monkeys share many cortical areas and other features of cortical organization with humans (e.g., refs. 9 and 10). Cortical cell and neuron numbers were determined using the flow fractionator method (37, 38) in epileptic baboon tissue obtained from the Texas Biomedical Research Institute, where a number of individuals develop generalized epilepsy within a pedigreed baboon colony (31–36). Our results reveal a regionally specific neuron reduction in the cortex of baboons with naturally occurring, generalized seizures.     

Balanced feedforward inhibition and dominant recurrent inhibition in olfactory cortex

Throughout the brain, the recruitment of feedforward and recurrent inhibition shapes neural responses. However, disentangling the relative contributions of these often-overlapping cortical circuits is challenging. The piriform cortex provides an ideal system to address this issue because the interneurons responsible for feedforward and recurrent inhibition are anatomically segregated in layer (L) 1 and L2/3 respectively. Here we use a combination of optical and electrical activation of interneurons to profile the inhibitory input received by three classes of principal excitatory neuron in the anterior piriform cortex. In all classes, we find that L1 interneurons provide weaker inhibition than L2/3 interneurons. Nonetheless, feedforward inhibitory strength covaries with the amount of afferent excitation received by each class of principal neuron. In contrast, intracortical stimulation of L2/3 evokes strong inhibition that dominates recurrent excitation in all classes. Finally, we find that the relative contributions of feedforward and recurrent pathways differ between principal neuron classes. Specifically, L2 neurons receive more reliable afferent drive and less overall inhibition than L3 neurons. Alternatively, L3 neurons receive substantially more intracortical inhibition. These three features—balanced afferent drive, dominant recurrent inhibition, and differential recruitment by afferent vs. intracortical circuits, dependent on cell class—suggest mechanisms for olfactory processing that may extend to other sensory cortices.The recruitment of inhibition is an essential feature of cortical processing. Feedforward and recurrent inhibitory circuits have been implicated in controlling the timing, strength, and tuning of cortical responses (for review, see ref. 1). In sensory cortices, including the olfactory cortex, neural responses to sensory stimuli depend on the relative balance of inhibition with respect to excitation in both feedforward and recurrent pathways (2–7). Moreover, numerous theoretical studies have suggested that balanced cortical networks underlie the selectivity, sparseness, and correlations of cortical activity (8–15). These studies highlight the importance of quantifying the relationship between excitation and inhibition in cortical networks. However, isolating the contributions of feedforward vs. recurrently evoked inhibition to cortical responses is difficult because these circuits are often coactive and frequently share interneurons (16–18). The piriform cortex is an ideal system to address this issue because the interneurons responsible for feedforward and recurrent inhibition differ by class and laminar location and, thus, are differentially recruited by afferent and intracortical excitation (19–22).The piriform cortex is a trilaminar cortex responsible for processing olfactory stimuli. Principal excitatory neurons are found in layer (L) 2/3 and send dendrites to L1, where they receive odor-related excitation directly from the olfactory bulb via the lateral olfactory tract (LOT) (23). LOT afferents also drive horizontal and neurogliaform inhibitory interneurons within L1, yielding feedforward inhibition of principal neurons (24). Within the cortex, principal neurons send axon collaterals throughout L2/3 and to an intracortical fiber tract in L1b (25, 26). Intracortical excitation recruits a number of interneuron classes within L2/3 that, in turn, provide recurrent inhibition to principal neurons (20, 24, 27, 28). Stimulation of the LOT evokes short- latency feedforward inhibition that targets principal neuron dendrites, as well as long-latency, recurrent inhibition that is somatic (24, 28, 29). These findings are consistent with the different laminar locations of inhibitory interneurons mediating feedforward and recurrent inhibition respectively.Previous studies have shown that electrical stimulation of the LOT as well as odors recruit mixed feedforward and recurrent inhibition in vivo (3, 4, 30, 31). However, in vivo and in vitro studies focusing on different principal neuron classes have led to conflicting reports of the relative contributions of feedforward and feedback inhibition during afferent odor processing (3, 24, 27, 28, 32). Furthermore, because of the disynaptic nature of inhibition, estimates of feedforward or recurrent inhibitory strength depend on the quality of afferent and intracortical excitatory recruitment, which varies with the different stimulation protocols used in each study. Here, we resolve these discrepancies by comparing feedforward and recurrent inhibition evoked by direct optical activation of interneurons that express channelrhodopsin (ChR2) (33), as well as electrical stimulation of excitatory pathways in all three classes of principal neuron in the anterior piriform cortex (APC).In the APC, principal excitatory neuron classes differ in laminar location and in the proportion of afferent vs. intracortical excitatory input received (34–37). Within L2, semilunar cells (SLCs) receive predominantly afferent excitation, whereas superficial pyramidal cells (sPCs) receive weaker afferent and stronger intracortical excitatory drive. In L3, deep pyramidal cells (dPCs) receive minimal afferent, but substantial intracortical excitation. Given these differences in excitatory drive, we hypothesized that inhibition mediated by feedforward and recurrent inhibitory circuits also differs between principal neuron classes. In this study, we find that principal neuron classes are weakly inhibited by L1 interneurons that mediate feedforward inhibition, compared with L2/3 interneurons that provide strong recurrent inhibition. As predicted, feedforward inhibitory strength varies in a manner consistent with the amount of afferent excitation received by each class of principal neuron. In contrast, intracortical stimulation of L3 evokes strong recurrent inhibition that dominates excitation in all classes. Moreover, excitatory and inhibitory profiles differ between SLCs, sPCs, and dPCs. Taken together, our results demonstrate that inhibitory circuits in the piriform cortex provide both balanced feedforward inhibition and dominant recurrent inhibition, as well as segregate principal excitatory neuron classes during cortical processing.     

In vivo imaging of immediate early gene expression reveals layer-specific memory traces in the mammalian brain

The dynamic processes of formatting long-term memory traces in the cortex are poorly understood. The investigation of these processes requires measurements of task-evoked neuronal activities from large numbers of neurons over many days. Here, we present a two-photon imaging-based system to track event–related neuronal activity in thousands of neurons through the quantitative measurement of EGFP proteins expressed under the control of the EGR1 gene promoter. A recognition algorithm was developed to detect GFP-positive neurons in multiple cortical volumes and thereby to allow the reproducible tracking of 4,000 neurons in each volume for 2 mo. The analysis revealed a context-specific response in sparse layer II neurons. The context-evoked response gradually increased during several days of training and was maintained 1 mo later. The formed traces were specifically activated by the training context and were linearly correlated with the behavioral response. Neuronal assemblies that responded to specific contexts were largely separated, indicating the sparse coding of memory-related traces in the layer II cortical circuit.In the mammalian brain, memory traces in cortical areas are poorly understood. In contrast to the medial temporal lobe, particularly the hippocampus, which is involved in the temporary storage of declarative memories (1, 2), the neocortex is believed to store remote memories (3–6). However, remarkably little knowledge regarding the sites and dynamics of remote memory storage has been revealed at the cellular level owing to the complexity of the connections and the large number of neurons within the cortical circuit.In vivo electrophysiological recording of neuronal firing revolutionized neurobiology by linking neuronal activity with animal behavior. The small number of neurons recorded by the electrodes, however, was a limitation, as information coding and decoding may use an army of neurons forming neuronal assemblies (7, 8). Efforts to record the activity of larger populations of neurons in cortical volumes have been actively pursued by either increasing the number of electrode probes (7, 9–11) or using calcium indicator–based imaging (12–15) and immediate early gene (IEG)-based reporters (16–18). The expression of IEGs is correlated with the averaged neuronal activation on external stimuli (19, 20), implying that the marked neurons are involved in behavior (1, 21–25). Studies using in vivo imaging of IEGs have revealed cortical coding in the visual cortex and in other cortical areas, reflecting electrical activation in individual neurons (16, 17). Among IEGs, the expression of early growth response protein 1 (EGR1, also known as zif268) is associated with high-frequency stimulation and the induction of long-term plasticity during learning (26, 27). To measure neuronal activation in cortical circuits during a behavioral task, we used an EGR1 expression reporter mouse line in which the expression of the EGFP protein is under the control of the Egr1 gene promoter. We designed offline recording strategies to monitor task-associated neuronal activity by quantifying changes in cellular EGFP signals in the mouse cortex. Patterns of activated neuronal assemblies during different tasks were visualized in the entire cortical volume. Furthermore, through computer recognition-based reconstruction, we were able to track the activity-related cellular EGFP signals from multiple cortical areas for 2 mo to reveal memory-related changes in the cortical circuit.     

Dissecting neural pathways for forgetting in Drosophila olfactory aversive memory

Recent studies have identified molecular pathways driving forgetting and supported the notion that forgetting is a biologically active process. The circuit mechanisms of forgetting, however, remain largely unknown. Here we report two sets of Drosophila neurons that account for the rapid forgetting of early olfactory aversive memory. We show that inactivating these neurons inhibits memory decay without altering learning, whereas activating them promotes forgetting. These neurons, including a cluster of dopaminergic neurons (PAM-β′1) and a pair of glutamatergic neurons (MBON-γ4>γ1γ2), terminate in distinct subdomains in the mushroom body and represent parallel neural pathways for regulating forgetting. Interestingly, although activity of these neurons is required for memory decay over time, they are not required for acute forgetting during reversal learning. Our results thus not only establish the presence of multiple neural pathways for forgetting in Drosophila but also suggest the existence of diverse circuit mechanisms of forgetting in different contexts.Although forgetting commonly has a negative connotation, it is a functional process that shapes memory and cognition (1–4). Recent studies, including work in relatively simple invertebrate models, have started to reveal basic biological mechanisms underlying forgetting (5–15). In Drosophila, single-session Pavlovian conditioning by pairing an odor (conditioned stimulus, CS) with electric shock (unconditioned stimulus, US) induces aversive memories that are short-lasting (16). The memory performance of fruit flies is observed to drop to a negligible level within 24 h, decaying rapidly early after training and slowing down thereafter (17). Memory decay or forgetting requires the activation of the small G protein Rac, a signaling protein involved in actin remodeling, in the mushroom body (MB) intrinsic neurons (6). These so-called Kenyon cells (KCs) are the neurons that integrate CS–US information (18, 19) and support aversive memory formation and retrieval (20–22). In addition to Rac, forgetting also requires the DAMB dopamine receptor (7), which has highly enriched expression in the MB (23). Evidence suggests that the dopamine-mediated forgetting signal is conveyed to the MB by dopamine neurons (DANs) in the protocerebral posterior lateral 1 (PPL1) cluster (7, 24). Therefore, forgetting of olfactory aversive memory in Drosophila depends on a particular set of intracellular molecular pathways within KCs, involving Rac, DAMB, and possibly others (25), and also receives modulation from extrinsic neurons. Although important cellular evidence supporting the hypothesis that memory traces are erased under these circumstances is still lacking, these findings lend support to the notion that forgetting is an active, biologically regulated process (17, 26).Although existing studies point to the MB circuit as essential for forgetting, several questions remain to be answered. First, whereas the molecular pathways for learning and forgetting of olfactory aversive memory are distinct and separable (6, 7), the neural circuits seem to overlap. Rac-mediated forgetting has been localized to a large population of KCs (6), including the γ-subset, which is also critical for initial memory formation (21, 27). The site of action of DAMB for forgetting has yet to be established; however, the subgroups of PPL1-DANs implicated in forgetting are the same as those that signal aversive reinforcement and are required for learning (28–30). It leaves open the question of whether the brain circuitry underlying forgetting and learning is dissociable, or whether forgetting and learning share the same circuit but are driven by distinct activity patterns and molecular machinery (26). Second, shock reinforcement elicits multiple memory traces through at least three dopamine pathways to different subdomains in the MB lobes (28, 29). Functional imaging studies have also revealed Ca²⁺-based memory traces in different KC populations (31). It is poorly understood how forgetting of these memory traces differs, and it remains unknown whether there are multiple regulatory neural pathways. Notably, when PPL1-DANs are inactivated, forgetting still occurs, albeit at a lower rate (7). This incomplete block suggests the existence of an additional pathway(s) that conveys forgetting signals to the MB. Third, other than memory decay over time, forgetting is also observed through interference (32, 33), when new learning or reversal learning is introduced after training (6, 34, 35). Time-based and interference-based forgetting shares a similar dependence on Rac and DAMB (6, 7). However, it is not known whether distinct circuits underlie forgetting in these different contexts.In the current study, we focus on the diverse set of MB extrinsic neurons (MBENs) that interconnect the MB lobes with other brain regions, which include 34 MB output neurons (MBONs) of 21 types and ∼130 dopaminergic neurons of 20 types in the PPL1 and protocerebral anterior medial (PAM) clusters (36, 37). These neurons have been intensively studied in olfactory memory formation, consolidation, and retrieval in recent years (e.g., 24, 28–30, 38–48); however, their roles in forgetting have not been characterized except for the aforementioned PPL1-DANs. In a functional screen, we unexpectedly found that several Gal4 driver lines of MBENs showed significantly better 3-h memory retention when the Gal4-expressing cells were inactivated. The screen has thus led us to identify two types of MBENs that are not involved in initial learning but play important and additive roles in mediating memory decay. Furthermore, neither of these MBEN types is required for reversal learning, supporting the notion that there is a diversity of neural circuits that drive different forms of forgetting.     

Optogenetic activation of cholinergic neurons in the PPT or LDT induces REM sleep

Rapid eye movement (REM) sleep is an important component of the natural sleep/wake cycle, yet the mechanisms that regulate REM sleep remain incompletely understood. Cholinergic neurons in the mesopontine tegmentum have been implicated in REM sleep regulation, but lesions of this area have had varying effects on REM sleep. Therefore, this study aimed to clarify the role of cholinergic neurons in the pedunculopontine tegmentum (PPT) and laterodorsal tegmentum (LDT) in REM sleep generation. Selective optogenetic activation of cholinergic neurons in the PPT or LDT during non-REM (NREM) sleep increased the number of REM sleep episodes and did not change REM sleep episode duration. Activation of cholinergic neurons in the PPT or LDT during NREM sleep was sufficient to induce REM sleep.Rapid eye movement (REM) sleep is tightly regulated, yet the mechanisms that control REM sleep remain incompletely understood. Early pharmacological and unit recording studies suggested that ACh was important for REM sleep regulation (1, 2). For example, injection of cholinergic drugs into the dorsal mesopontine tegmentum reliably induced a state very similar to natural REM sleep in cats (3–6). Unit recordings from the cholinergic areas of the mesopontine tegmentum revealed cells that were active during wakefulness and REM sleep, as well as neurons active only during REM sleep (7–13). Electrical stimulation of the laterodorsal tegmentum (LDT) in cats increased the percentage of time spent in REM sleep (14), and activation of the pedunculopontine tegmentum (PPT) in rats induced wakefulness and REM sleep (15). If cholinergic PPT and LDT neurons are necessary for REM sleep to occur, as the early studies suggest, then lesioning the PPT or LDT should decrease REM sleep. In cats, lesions of the PPT and LDT do disrupt REM sleep (16, 17), but lesions in rodents have had little effect on REM sleep or increased REM sleep (18–22). Additionally, c-fos studies have found very few cholinergic cells activated under high-REM sleep conditions. When c-fos–positive cholinergic neurons in the PPT and LDT are found to correlate with the percentage of REM sleep, they still account for only a few of the total cholinergic cells in the area (23). Juxtacellular recordings of identified cholinergic neurons in the LDT found these cells had wake and REM active firing profiles, with the majority firing the highest during REM sleep (13). These discrepancies have led to alternative theories of REM sleep regulation, where cholinergic neurons do not play a key role (18, 19, 23, 24 and reviewed in 25, 26).The PPT and LDT are made up of heterogeneous populations of cells, including distinct populations of cholinergic, GABAergic, and glutamatergic neurons (27–29). Many GABAergic neurons are active during REM sleep, as indicated by c-fos (23), and both GABAergic and glutamatergic neurons have been found with maximal firing rates during REM sleep in the LDT and medial PPT (13). To distinguish the differential roles of each cell type in REM sleep regulation, a method that can modulate specific cell types in the behaving animal is needed. Optogenetics now provides this ability to target specific subpopulations of neurons and control them with millisecond temporal resolution (30). Therefore, we aimed to determine the role of cholinergic neurons in the PPT and LDT in REM sleep regulation using optogenetics.     

From the Cover: PNAS Plus: Neuronal medium that supports basic synaptic functions and activity of human neurons in vitro

Human cell reprogramming technologies offer access to live human neurons from patients and provide a new alternative for modeling neurological disorders in vitro. Neural electrical activity is the essence of nervous system function in vivo. Therefore, we examined neuronal activity in media widely used to culture neurons. We found that classic basal media, as well as serum, impair action potential generation and synaptic communication. To overcome this problem, we designed a new neuronal medium (BrainPhys basal + serum-free supplements) in which we adjusted the concentrations of inorganic salts, neuroactive amino acids, and energetic substrates. We then tested that this medium adequately supports neuronal activity and survival of human neurons in culture. Long-term exposure to this physiological medium also improved the proportion of neurons that were synaptically active. The medium was designed to culture human neurons but also proved adequate for rodent neurons. The improvement in BrainPhys basal medium to support neurophysiological activity is an important step toward reducing the gap between brain physiological conditions in vivo and neuronal models in vitro.Induced pluripotent stem cell (iPSC) technology is currently being used to model human diseases in vitro and may contribute to the discovery and validation of new pharmacological treatments (1–3). In particular, neuroscientists have seized the opportunity to culture neurons from patients with neurological and psychiatric disorders and have demonstrated that phenotypes associated with particular disorders can be recapitulated in the dish (4–7). However, the basic culture conditions for growing neurons in vitro have not been updated to reflect fundamental principles of brain physiology. Currently, most human neuronal cultures are grown in media based on DMEM/F12 (4, 5, 7–24), Neurobasal (25–30), or a mixture of DMEM and Neurobasal (DN) (31–34). To promote neuronal differentiation and survival, a variety of supplements, such as serum, growth factors, hormones, proteins, and antioxidants, are typically added to these basal media. Although these media were designed and optimized to promote neuronal survival in vitro, they were not tested for their ability to support fundamental neuronal functions. Using several electrophysiological techniques such as patch clamping, calcium imaging, and multielectrode arrays, we found that widely used tissue culture media (e.g., DMEM basal, Neurobasal, serum) actually impaired neurophysiological functions.We identified several neuroactive components in these media that acutely interfered with neuronal function. To solve these issues, we designed a chemically defined basal medium: BrainPhys basal. We used human neurons in vitro to demonstrate in a series of experiments that this new medium, combined with the appropriate supplements, better supports important neuronal functions while sustaining cell survival in vitro. Notably, BrainPhys-based medium better mimics the environment present in healthy living brains, unlike previous media based on DMEM, Neurobasal or serum. Although BrainPhys basal was specifically designed for the culturing of mature human neurons, our studies also showed that BrainPhys provided a functional environment for ex vivo brain slices and for culturing rodent primary neurons.     

Distinct dopamine neurons mediate reward signals for short- and long-term memories

Drosophila melanogaster can acquire a stable appetitive olfactory memory when the presentation of a sugar reward and an odor are paired. However, the neuronal mechanisms by which a single training induces long-term memory are poorly understood. Here we show that two distinct subsets of dopamine neurons in the fly brain signal reward for short-term (STM) and long-term memories (LTM). One subset induces memory that decays within several hours, whereas the other induces memory that gradually develops after training. They convey reward signals to spatially segregated synaptic domains of the mushroom body (MB), a potential site for convergence. Furthermore, we identified a single type of dopamine neuron that conveys the reward signal to restricted subdomains of the mushroom body lobes and induces long-term memory. Constant appetitive memory retention after a single training session thus comprises two memory components triggered by distinct dopamine neurons.Memory of a momentous event persists for a long time. Whereas some forms of long-term memory (LTM) require repetitive training (1–3), a highly relevant stimulus such as food or poison is sufficient to induce LTM in a single training session (4–7). Recent studies have revealed aspects of the molecular and cellular mechanisms of LTM formation induced by repetitive training (8–11), but how a single training induces a stable LTM is poorly understood (12).Appetitive olfactory learning in fruit flies is suited to address the question, as a presentation of a sugar reward paired with odor induces robust short-term memory (STM) and LTM (6, 7). Odor is represented by a sparse ensemble of the 2,000 intrinsic neurons, the Kenyon cells (13). A current working model suggests that concomitant reward signals from sugar ingestion cause associative plasticity in Kenyon cells that might underlie memory formation (14–20). A single activation session of a specific cluster of dopamine neurons (PAM neurons) by sugar ingestion can induce appetitive memory that is stable over 24 h (19), underscoring the importance of sugar reward to the fly.The mushroom body (MB) is composed of the three different cell types, α/β, α′/β′, and γ, which have distinct roles in different phases of appetitive memories (11, 21–25). Similar to midbrain dopamine neurons in mammals (26, 27), the structure and function of PAM cluster neurons are heterogeneous, and distinct dopamine neurons intersect unique segments of the MB lobes (19, 28–34). Further circuit dissection is thus crucial to identify candidate synapses that undergo associative modulation.By activating distinct subsets of PAM neurons for reward signaling, we found that short- and long-term memories are independently formed by two complementary subsets of PAM cluster dopamine neurons. Conditioning flies with nutritious and nonnutritious sugars revealed that the two subsets could represent different reinforcing properties: sweet taste and nutritional value of sugar. Constant appetitive memory retention after a single training session thus comprises two memory components triggered by distinct reward signals.     

Drosophila seminal protein ovulin mediates ovulation through female octopamine neuronal signaling

Across animal taxa, seminal proteins are important regulators of female reproductive physiology and behavior. However, little is understood about the physiological or molecular mechanisms by which seminal proteins effect these changes. To investigate this topic, we studied the increase in Drosophila melanogaster ovulation behavior induced by mating. Ovulation requires octopamine (OA) signaling from the central nervous system to coordinate an egg’s release from the ovary and its passage into the oviduct. The seminal protein ovulin increases ovulation rates after mating. We tested whether ovulin acts through OA to increase ovulation behavior. Increasing OA neuronal excitability compensated for a lack of ovulin received during mating. Moreover, we identified a mating-dependent relaxation of oviduct musculature, for which ovulin is a necessary and sufficient male contribution. We report further that oviduct muscle relaxation can be induced by activating OA neurons, requires normal metabolic production of OA, and reflects ovulin’s increasing of OA neuronal signaling. Finally, we showed that as a result of ovulin exposure, there is subsequent growth of OA synaptic sites at the oviduct, demonstrating that seminal proteins can contribute to synaptic plasticity. Together, these results demonstrate that ovulin increases ovulation through OA neuronal signaling and, by extension, that seminal proteins can alter reproductive physiology by modulating known female pathways regulating reproduction.Throughout internally fertilizing animals, seminal proteins play important roles in regulating female fertility by altering female physiology and, in some cases, behavior after mating (reviewed in refs. 1–3). Despite this, little is understood about the physiological mechanisms by which seminal proteins induce postmating changes and how their actions are linked with known networks regulating female reproductive physiology.In Drosophila melanogaster, the suite of seminal proteins has been identified, as have many seminal protein-dependent postmating responses, including changes in egg production and laying, remating behavior, locomotion, feeding, and in ovulation rate (reviewed in refs. 2 and 3). For example, the Drosophila seminal protein ovulin elevates ovulation rate to maximal levels during the 24 h following mating (4, 5), and the seminal protein sex peptide (SP) suppresses female mating receptivity and increases egg-laying behavior for several days after mating (6–10). However, although a receptor for SP has been identified (11), along with elements of the neural circuit in which it is required (12–14), SP’s mechanism of action has not yet been linked to regulatory networks known to control postmating behaviors. Thus, a crucial question remains: how do male-derived seminal proteins interact with regulatory networks in females to trigger postmating responses?We addressed this question by examining the stimulation of Drosophila ovulation by the seminal protein ovulin. In insects, ovulation, defined here as the release of an egg from the ovary to the uterus, is among the best understood reproductive processes in terms of its physiology and neurogenetics (15–27). In D. melanogaster, ovulation requires input from neurons in the abdominal ganglia that release the catecholaminergic neuromodulators octopamine (OA) and tyramine (17, 18, 28). Drosophila ovulation also requires an OA receptor, OA receptor in mushroom bodies (OAMB) (19, 20). Moreover, it has been proposed that OA may integrate extrinsic factors to regulate ovulation rates (17). Noradrenaline, the vertebrate structural and functional equivalent to OA (29, 30), is important for mammalian ovulation, and its dysregulation has been associated with ovulation disorders (31–38). In this paper we investigate the role of neurons that release OA and tyramine in ovulin’s action. For simplicity, we refer to these neurons as “OA neurons” to reflect the well-established role of OA in ovulation behavior (16–20, 22).We investigated how action of the seminal protein ovulin relates to the conserved canonical neuromodulatory pathway that regulates ovulation physiology (39–41). We found that ovulin increases ovulation and egg laying through OA neuronal signaling. We also found that ovulin relaxes oviduct muscle tonus, a postmating process that is also mediated by OA neuronal signaling. Finally, subsequent to these effects we detected an ovulin-dependent increase in synaptic sites between OA motor neurons and oviduct muscle, suggesting that ovulin’s stimulation of OA neurons could have increased their synaptic activity. These results suggest that ovulin affects ovulation by manipulating the gain of a neuromodulatory pathway regulating ovulation physiology.     

Loss of Miro1-directed mitochondrial movement results in a novel murine model for neuron disease

Defective mitochondrial distribution in neurons is proposed to cause ATP depletion and calcium-buffering deficiencies that compromise cell function. However, it is unclear whether aberrant mitochondrial motility and distribution alone are sufficient to cause neurological disease. Calcium-binding mitochondrial Rho (Miro) GTPases attach mitochondria to motor proteins for anterograde and retrograde transport in neurons. Using two new KO mouse models, we demonstrate that Miro1 is essential for development of cranial motor nuclei required for respiratory control and maintenance of upper motor neurons required for ambulation. Neuron-specific loss of Miro1 causes depletion of mitochondria from corticospinal tract axons and progressive neurological deficits mirroring human upper motor neuron disease. Although Miro1-deficient neurons exhibit defects in retrograde axonal mitochondrial transport, mitochondrial respiratory function continues. Moreover, Miro1 is not essential for calcium-mediated inhibition of mitochondrial movement or mitochondrial calcium buffering. Our findings indicate that defects in mitochondrial motility and distribution are sufficient to cause neurological disease.Motor neuron diseases (MNDs), including ALS and spastic paraplegia (SP), are characterized by the progressive, length-dependent degeneration of motor neurons, leading to muscle atrophy, paralysis, and, in some cases, premature death. There are both inherited and sporadic forms of MNDs, which can affect upper motor neurons, lower motor neurons, or both. Although the molecular and cellular causes of most MNDs are unknown, many are associated with defects in axonal transport of cellular components required for neuron function and maintenance (1–6).A subset of MNDs is associated with impaired mitochondrial respiration and mitochondrial distribution. This observation has led to the hypothesis that neurodegeneration results from defects in mitochondrial motility and distribution, which, in turn, cause subcellular ATP depletion and interfere with mitochondrial calcium ([Ca²⁺]_m) buffering at sites of high synaptic activity (reviewed in ref. 7). It is not known, however, whether mitochondrial motility defects are a primary cause or a secondary consequence of MND progression. In addition, it has been difficult to isolate the primary effect of mitochondrial motility defects in MNDs because most mutations that impair mitochondrial motility in neurons also affect transport of other organelles and vesicles (1, 8–11).In mammals, the movement of neuronal mitochondria between the cell body and the synapse is controlled by adaptors called trafficking kinesin proteins (Trak1 and Trak2) and molecular motors (kinesin heavy chain and dynein), which transport the organelle in the anterograde or retrograde direction along axonal microtubule tracks (7, 12–24). Mitochondrial Rho (Miro) GTPase proteins are critical for transport because they are the only known surface receptors that attach mitochondria to these adaptors and motors (12–15, 18, 25, 26). Miro proteins are tail-anchored in the outer mitochondrial membrane with two GTPase domains and two predicted calcium-binding embryonic fibroblast (EF) hand motifs facing the cytoplasm (12, 13, 25, 27, 28). A recent Miro structure revealed two additional EF hands that were not predicted from the primary sequence (29). Studies in cultured cells suggest that Miro proteins also function as calcium sensors (via their EF hands) to regulate kinesin-mediated mitochondrial “stopping” in axons (15, 16, 26). Miro-mediated movement appears to be inhibited when cytoplasmic calcium is elevated in active synapses, effectively recruiting mitochondria to regions where calcium buffering and energy are needed. Despite this progress, the physiological relevance of these findings has not yet been tested in a mammalian animal model. In addition, mammals ubiquitously express two Miro orthologs, Miro1 and Miro2, which are 60% identical (12, 13). However, the individual roles of Miro1 and Miro2 in neuronal development, maintenance, and survival have no been evaluated.We describe two new mouse models that establish the importance of Miro1-mediated mitochondrial motility and distribution in mammalian neuronal function and maintenance. We show that Miro1 is essential for development/maintenance of specific cranial neurons, function of postmitotic motor neurons, and retrograde mitochondrial motility in axons. Loss of Miro1-directed retrograde mitochondrial transport is sufficient to cause MND phenotypes in mice without abrogating mitochondrial respiratory function. Furthermore, Miro1 is not essential for calcium-mediated inhibition of mitochondrial movement or [Ca²⁺]_m buffering. These findings have an impact on current models for Miro1 function and introduce a specific and rapidly progressing mouse model for MND.     

From the Cover: Neurons selective to the number of visual items in the corvid songbird endbrain

It is unknown whether anatomical specializations in the endbrains of different vertebrates determine the neuronal code to represent numerical quantity. Therefore, we recorded single-neuron activity from the endbrain of crows trained to judge the number of items in displays. Many neurons were tuned for numerosities irrespective of the physical appearance of the items, and their activity correlated with performance outcome. Comparison of both behavioral and neuronal representations of numerosity revealed that the data are best described by a logarithmically compressed scaling of numerical information, as postulated by the Weber–Fechner law. The behavioral and neuronal numerosity representations in the crow reflect surprisingly well those found in the primate association cortex. This finding suggests that distantly related vertebrates with independently developed endbrains adopted similar neuronal solutions to process quantity.Birds show elaborate quantification skills (1–3) that are of adaptive value in naturalistic situations like nest parasitism (4), food caching (5), or communication (6). The neuronal correlates of numerosity representations have only been explored in humans (7–9) and primates (10–18), and they have been found to reside in the prefrontal and posterior parietal neocortices. In contrast to primates, birds lack a six-layered neocortex. The birds’ lineage diverged from mammals 300 Mya (19), at a time when the neocortex had not yet developed from the pallium of the endbrain. Instead, birds developed different pallial parts as dominant endbrain structures (20, 21) based on convergent evolution, with the nidopallium caudolaterale (NCL) as a high-level association area (22–26). Where and how numerosity is encoded in vertebrates lacking a neocortex is unknown. Here, we show that neurons in the telencephalic NCL of corvid songbirds respond to numerosity and show a specific code for numerical information.     

Impaired excitability of somatostatin- and parvalbumin-expressing cortical interneurons in a mouse model of Dravet syndrome

Haploinsufficiency of the voltage-gated sodium channel Na_V1.1 causes Dravet syndrome, an intractable developmental epilepsy syndrome with seizure onset in the first year of life. Specific heterozygous deletion of Na_V1.1 in forebrain GABAergic-inhibitory neurons is sufficient to cause all the manifestations of Dravet syndrome in mice, but the physiological roles of specific subtypes of GABAergic interneurons in the cerebral cortex in this disease are unknown. Voltage-clamp studies of dissociated interneurons from cerebral cortex did not detect a significant effect of the Dravet syndrome mutation on sodium currents in cell bodies. However, current-clamp recordings of intact interneurons in layer V of neocortical slices from mice with haploinsufficiency in the gene encoding the Na_V1.1 sodium channel, Scn1a, revealed substantial reduction of excitability in fast-spiking, parvalbumin-expressing interneurons and somatostatin-expressing interneurons. The threshold and rheobase for action potential generation were increased, the frequency of action potentials within trains was decreased, and action-potential firing within trains failed more frequently. Furthermore, the deficit in excitability of somatostatin-expressing interneurons caused significant reduction in frequency-dependent disynaptic inhibition between neighboring layer V pyramidal neurons mediated by somatostatin-expressing Martinotti cells, which would lead to substantial disinhibition of the output of cortical circuits. In contrast to these deficits in interneurons, pyramidal cells showed no differences in excitability. These results reveal that the two major subtypes of interneurons in layer V of the neocortex, parvalbumin-expressing and somatostatin-expressing, both have impaired excitability, resulting in disinhibition of the cortical network. These major functional deficits are likely to contribute synergistically to the pathophysiology of Dravet syndrome.Dravet syndrome (DS), also referred to as “severe myoclonic epilepsy in infancy,” is a rare genetic epileptic encephalopathy characterized by frequent intractable seizures, severe cognitive deficits, and premature death (1–3). DS is caused by loss-of-function mutations in SCN1A, the gene encoding type I voltage-gated sodium channel Na_V1.1, which usually arise de novo in the affected individuals (4–7). Like DS patients, mice with heterozygous loss-of-function mutations in Scn1a exhibit ataxia, sleep disorder, cognitive deficit, autistic-like behavior, and premature death (8–14). Like DS patients, DS mice first become susceptible to seizures caused by elevation of body temperature and subsequently experience spontaneous myoclonic and generalized tonic-clonic seizures (11). Global deletion of Na_V1.1 impairs Na⁺ currents and action potential (AP) firing in GABAergic-inhibitory interneurons (8–10), and specific deletion of Na_V1.1 in forebrain interneurons is sufficient to cause DS in mice (13, 15). These data suggest that the loss of interneuron excitability and resulting disinhibition of neural circuits cause DS, but the functional role of different subtypes of interneurons in the cerebral cortex in DS remains unknown.Neocortical GABAergic interneurons shape cortical output and display great diversity in morphology and function (16, 17). The expression of parvalbumin (PV) and somatostatin (SST) defines two large, nonoverlapping groups of interneurons (16, 18, 19). In layer V of the cerebral cortex, PV-expressing fast-spiking interneurons and SST-expressing Martinotti cells each account for ∼40% of interneurons, and these interneurons are the major inhibitory regulators of cortical network activity (17, 20). Layer V PV interneurons make synapses on the soma and proximal dendrites of pyramidal neurons (18, 19), where they mediate fast and powerful inhibition (21, 22). Selective heterozygous deletion of Scn1a in neocortical PV interneurons increases susceptibility to chemically induced seizures (23), spontaneous seizures, and premature death (24), indicating that this cell type may contribute to Scn1a deficits. However, selective deletion of Scn1a in neocortical PV interneurons failed to reproduce the effects of DS fully, suggesting the involvement of other subtypes of interneurons in this disease (23, 24). Layer V Martinotti cells have ascending axons that arborize in layer I and spread horizontally to neighboring cortical columns, making synapses on apical dendrites of pyramidal neurons (17, 25, 26). They generate frequency-dependent disynaptic inhibition (FDDI) that dampens excitability of neighboring layer V pyramidal cells (27–29), contributing to maintenance of the balance of excitation and inhibition in the neocortex. However, the functional roles of Martinotti cells and FDDI in DS are unknown.Because layer V forms the principal output pathway of the neocortex, reduction in inhibitory input to layer V pyramidal cells would have major functional consequences by increasing excitatory output from all cortical circuits. However, the effects of the DS mutation on interneurons and neural circuits that provide inhibitory input to layer V pyramidal cells have not been determined. Here we show that the intrinsic excitability of layer V fast-spiking PV interneurons and SST Martinotti cells and the FDDI mediated by Martinotti cells are reduced dramatically in DS mice, leading to an imbalance in the excitation/inhibition ratio. Our results suggest that loss of Na_V1.1 in these two major types of interneurons may contribute synergistically to increased cortical excitability, epileptogenesis, and cognitive deficits in DS.     

Orexin/hypocretin system modulates amygdala-dependent threat learning through the locus coeruleus

Survival in a dangerous environment requires learning about stimuli that predict harm. Although recent work has focused on the amygdala as the locus of aversive memory formation, the hypothalamus has long been implicated in emotional regulation, and the hypothalamic neuropeptide orexin (hypocretin) is involved in anxiety states and arousal. Nevertheless, little is known about the role of orexin in aversive memory formation. Using a combination of behavioral pharmacology, slice physiology, and optogenetic techniques, we show that orexin acts upstream of the amygdala via the noradrenergic locus coeruleus to enable threat (fear) learning, specifically during the aversive event. Our results are consistent with clinical studies linking orexin levels to aversive learning and anxiety in humans and dysregulation of the orexin system may contribute to the etiology of fear and anxiety disorders.Hess and Akert demonstrated that electrical stimulation of the perifornical (PFH) region of the hypothalamus elicits defensive or aggressive responses in cats (1). Others showed that hypothalamic stimulation can serve as the aversive unconditioned stimulus (US) (2), indicating that the hypothalamus processes threat information important for aversive learning. One possibility is that orexin neurons, which populate these hypothalamic areas, may mediate these observed responses, as these neurons project to and modulate brain areas critical for threat processing, reward, and memory.Orexins are neuropeptides produced in the PFH and lateral regions of the hypothalamus (LH) (3, 4). Two orexin peptides (Orexin-A and Orexin-B) are processed from one peptide precursor (prepro-orexin) and bind two distinct G protein–coupled receptors (OrxR1 and OrxR2) in the brain (3, 4). Activation of either receptor commonly increases excitability in target neurons by reducing potassium channel conductance, enhancing presynaptic glutamate release, or increasing postsynaptic NMDA receptor (NMDAR) conductance (5, 6). Orexin receptors are differentially distributed in the brain and may serve differing roles in stress, arousal, vigilance, feeding, reward processing, and drug addiction (7–10). Evidence suggests that, in general, OrxR2 is involved in maintenance of arousal or wakefulness (11, 12), whereas OrxR1 mediates responses to environmental stimuli (13, 14).Recent reports point to a role for the orexin system in emotional regulation. Overactivity in orexin neurons can exacerbate panic-like episodes and lead to an anxiety-like phenotype in rats (15, 16). Conversely, administration of the dual orexin receptor antagonist almorexant blunts autonomic and behavioral responses affiliated with heightened stress levels (17, 18). Although orexin system activity is linked to general states of hyperarousal, the precise role of orexin in these and other aversive states remains unknown.Hypothalamic orexin neurons send a dense output to the locus coeruelus (LC) and depolarize neurons in vitro and in vivo (19–21). In line with their connectivity, LC neurons respond to phasic stimuli in a manner comparable to orexin neurons (22), suggesting that orexin neurons modulate LC responses to salient sensory events. Interestingly, orexin and LC neurons are both activated by aversive stimuli such as shock (23, 24). Thus, orexin could contribute to aversive learning by way of LC, given the importance of norepinephrine to aversive memory processes in amygdala (25–27).Pavlovian threat (fear) conditioning is a well-established behavioral paradigm to assess the formation, storage, and expression of aversive memories (28). During training, animals learn to associate an aversive US, such as a footshock, with a neutral conditioned stimulus (CS), such as a tone, when both occur in close temporal proximity. Here, we tested the hypothesis that orexin neurons phasically activate locus coeruleus neurons during an aversive event to enable threat learning. Using a combination of behavioral pharmacology, electrophysiology, and optogenetic approaches, we show that orexin neurons, via activation of OrxR1 in the LC, facilitate the acquisition of amygdala-dependent threat memory.     

Optogenetic neuronal stimulation promotes functional recovery after stroke

Clinical and research efforts have focused on promoting functional recovery after stroke. Brain stimulation strategies are particularly promising because they allow direct manipulation of the target area’s excitability. However, elucidating the cell type and mechanisms mediating recovery has been difficult because existing stimulation techniques nonspecifically target all cell types near the stimulated site. To circumvent these barriers, we used optogenetics to selectively activate neurons that express channelrhodopsin 2 and demonstrated that selective neuronal stimulations in the ipsilesional primary motor cortex (iM1) can promote functional recovery. Stroke mice that received repeated neuronal stimulations exhibited significant improvement in cerebral blood flow and the neurovascular coupling response, as well as increased expression of activity-dependent neurotrophins in the contralesional cortex, including brain-derived neurotrophic factor, nerve growth factor, and neurotrophin 3. Western analysis also indicated that stimulated mice exhibited a significant increase in the expression of a plasticity marker growth-associated protein 43. Moreover, iM1 neuronal stimulations promoted functional recovery, as stimulated stroke mice showed faster weight gain and performed significantly better in sensory-motor behavior tests. Interestingly, stimulations in normal nonstroke mice did not alter motor behavior or neurotrophin expression, suggesting that the prorecovery effect of selective neuronal stimulations is dependent on the poststroke environment. These results demonstrate that stimulation of neurons in the stroke hemisphere is sufficient to promote recovery.Stroke is a major acute neurological insult that disrupts brain function and causes neuron death. Functional recovery after stroke has been observed and is currently attributed to both brain remodeling and plasticity (1–4). Structural and functional remodeling of areas next to an infarct or remote regions can alter signaling within bilateral neuronal networks and thus contribute to functional recovery (3–7). Rewiring of neural connections is mediated by electrical activity, which can activate a number of plasticity mechanisms, including the release of activity-dependent neurotrophins such as brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) (8–10). Both BDNF and NGF have been shown to improve recovery by enhancing axonal and dendritic sprouting (10–12).Tremendous effort has been focused on promoting recovery after stroke, including pharmacological treatment, rehabilitation (e.g., constraint-induced therapy), stem cell transplantation, and brain stimulation (1, 4, 13). Brain stimulation is a promising area of research because it allows direct activation and manipulation of the target area’s excitability (14–16). The primary motor cortex (M1) is a commonly stimulated area as it directly innervates the corticospinal tract to initiate movement (1, 7). Although electrical stimulation and transcranial magnetic stimulation show promise in promoting recovery (17, 18), these techniques are limited by imprecision and indiscriminate activation or inhibition of all cell types near the stimulated site; thus, they can produce undesired effects such as psychiatric and motor/speech problems (19–21). In addition, it has been difficult to elucidate the cell type and mechanisms driving recovery, as multiple cell types such as neurons, astrocytes, and oligodendrocytes have been shown to contribute to remodeling and recovery processes after stroke (5, 22–27).To elucidate whether activation of neurons alone can promote recovery, we used optogenetics to selectively manipulate the excitability of specific cell groups with millisecond-scale temporal precision in a manner more similar to endogenous neuronal firing patterns (21, 28, 29). This technique uses light-activated microbial proteins such as Channelrhodopsin 2 (ChR2), which depolarizes neurons when illuminated with blue light, or Halorhodopsin (NpHR), which hyperpolarizes neurons (21, 28, 29). Optogenetic approaches have been used in rodents to probe neuronal circuits for several neurological/neurodegenerative diseases, including Parkinson disease (30) and epilepsy (31). Recent studies have also used optogenetics to map functional organization after stroke (32–35). The safety and efficacy of using optogenetics in nonhuman primates has also been characterized (29, 36).In this study, we used optogenetics to selectively stimulate neurons in layer V of the ipsilesional primary motor cortex (iM1) and examine the effects of repeated neuronal stimulations in normal and stroke mice. Sensory-motor behavior tests were used to evaluate functional recovery after stroke, and plasticity-associated mechanisms, such as cerebral blood flow (CBF)/neurovascular coupling responses and activity-dependent neurotrophin expression, were investigated.     

From the Cover: PNAS Plus: Whole-brain calcium imaging with cellular resolution in freely behaving Caenorhabditis elegans

The ability to acquire large-scale recordings of neuronal activity in awake and unrestrained animals is needed to provide new insights into how populations of neurons generate animal behavior. We present an instrument capable of recording intracellular calcium transients from the majority of neurons in the head of a freely behaving Caenorhabditis elegans with cellular resolution while simultaneously recording the animal’s position, posture, and locomotion. This instrument provides whole-brain imaging with cellular resolution in an unrestrained and behaving animal. We use spinning-disk confocal microscopy to capture 3D volumetric fluorescent images of neurons expressing the calcium indicator GCaMP6s at 6 head-volumes/s. A suite of three cameras monitor neuronal fluorescence and the animal’s position and orientation. Custom software tracks the 3D position of the animal’s head in real time and two feedback loops adjust a motorized stage and objective to keep the animal’s head within the field of view as the animal roams freely. We observe calcium transients from up to 77 neurons for over 4 min and correlate this activity with the animal’s behavior. We characterize noise in the system due to animal motion and show that, across worms, multiple neurons show significant correlations with modes of behavior corresponding to forward, backward, and turning locomotion.How do patterns of neural activity generate an animal’s behavior? To answer this question, it is important to develop new methods for recording from large populations of neurons in animals as they move and behave freely. The collective activity of many individual neurons appears to be critical for generating behaviors including arm reach in primates (1), song production in zebrafinch (2), the choice between swimming or crawling in leech (3), and decision-making in mice during navigation (4). New methods for recording from larger populations of neurons in unrestrained animals are needed to better understand neural coding of these behaviors and neural control of behavior more generally.Calcium imaging has emerged as a promising technique for recording dynamics from populations of neurons. Calcium-sensitive proteins are used to visualize changes in intracellular calcium levels in neurons in vivo which serve as a proxy for neural activity (5). To resolve the often weak fluorescent signal of an individual neuron in a dense forest of other labeled cells requires a high magnification objective with a large numerical aperture that, consequently, can image only a small field of view. Calcium imaging has traditionally been performed on animals that are stationary from anesthetization or immobilization to avoid imaging artifacts induced by animal motion. As a result, calcium imaging studies have historically focused on small brain regions in immobile animals that exhibit little or no behavior (6).No previous neurophysiological study has attained whole-brain imaging with cellular resolution in a freely behaving unrestrained animal. Previous whole-brain cellular resolution calcium imaging studies of populations of neurons that involve behavior investigate either fictive locomotion (3, 7), or behaviors that can be performed in restrained animals, such as eye movements (8) or navigation of a virtual environment (9). One exception has been the development of fluorescence endoscopy, which allows recording from rodents during unrestrained behavior, although imaging is restricted to even smaller subbrain regions (10).Investigating neural activity in small transparent organisms allows one to move beyond studying subbrain regions to record dynamics from entire brains with cellular resolution. Whole-brain imaging was performed first in larval zebrafish using two-photon microscopy (7). More recently, whole-brain imaging was performed in Caenorhabditis elegans using two-photon (11) and light-field microscopy (12). Animals in these studies were immobilized, anesthetized, or both and thus exhibited no gross behavior.C. elegans’ compact nervous system of only 302 neurons and small size of only 1 mm make it ideally suited for the development of new whole-brain imaging techniques for studying behavior. There is a long and rich history of studying and quantifying the behavior of freely moving C. elegans dating back to the mid-1970s (13, 14). Many of these works involved quantifying animal body posture as the worm moved, for example as in ref. 15. To facilitate higher-throughput recordings of behavior, a number of tracking microscopes (16–18) or multiworm imagers were developed (19, 20) along with sophisticated behavioral analysis software and analytical tools (21, 22). Motivated in part to understand these behaviors, calcium imaging systems were also developed that could probe neural activity in at first partially immobilized (23) and then freely moving animals, beginning with ref. 24 and and then developing rapidly (17, 18, 25–29). One limitation of these freely moving calcium imaging systems is that they are limited to imaging only a very small subset of neurons and lack the ability to distinguish neurons that lie atop one another in the axial direction of the microscope. Despite this limitation, these studies, combined with laser-ablation experiments, have identified a number of neurons that correlate or affect changes in particular behaviors including the AVB neuron pair and VB-type motor neurons for forward locomotion; the AVA, AIB, and AVE neuron pairs and VA-type motor neurons for backward locomotion; and the RIV, RIB, and SMD neurons and the DD-type motor neurons for turning behaviors (17, 18, 25, 26, 28, 30, 31). To move beyond these largely single-cell studies, we sought to record simultaneously from the entire brain of C. elegans with cellular resolution and record its behavior as it moved about unrestrained.     

High-speed odor transduction and pulse tracking by insect olfactory receptor neurons

Sensory systems encode both the static quality of a stimulus (e.g., color or shape) and its kinetics (e.g., speed and direction). The limits with which stimulus kinetics can be resolved are well understood in vision, audition, and somatosensation. However, the maximum temporal resolution of olfactory systems has not been accurately determined. Here, we probe the limits of temporal resolution in insect olfaction by delivering high frequency odor pulses and measuring sensory responses in the antennae. We show that transduction times and pulse tracking capabilities of olfactory receptor neurons are faster than previously reported. Once an odorant arrives at the boundary layer of the antenna, odor transduction can occur within less than 2 ms and fluctuating odor stimuli can be resolved at frequencies more than 100 Hz. Thus, insect olfactory receptor neurons can track stimuli of very short duration, as occur when their antennae encounter narrow filaments in an odor plume. These results provide a new upper bound to the kinetics of odor tracking in insect olfactory receptor neurons and to the latency of initial transduction events in olfaction.Odors carried in air plumes quickly break up into thin filaments that spread out across short distances from an odor source (1). The ability to track the temporal structure of filaments in an odor plume is essential for insects to segregate concurrent odors that arise from different sources (2–6). However, it is not clear whether signal transduction times and tracking rates of olfactory receptor neurons (ORNs) are fast enough to allow animals to use the higher frequency components of information present in odor plumes. Insect odor-guided behavior is remarkably robust against the spatial and temporal variability inherent in olfactory stimuli. For example, moths and beetles use temporal stimulus cues to segregate concurrent odors from closely spaced sources (2–5), and honey bees can detect 6-ms asynchrony in the onset of concurrent odor stimuli and use this onset asynchrony to segregate concurrent odors (6, 7). These observations of fast temporal resolution challenge the frequent notion that olfaction has slow integration times relative to other senses. Olfactory transduction speed has never been measured directly, and estimates range from 10 to 30 ms (8–11). Previous studies suggest that the maximum pulse tracking frequency of ORNs is species specific and ranges from 5 to 50 Hz (12–19). However, these numbers do not match the high temporal resolution observed in behavioral studies (2–7).We tested the limits of olfactory transduction speed and pulse tracking in five different insect species by measuring ORN population responses using electroantennogram (EAG) recordings. The amplitude and dynamics of EAG signals are proportional to the number of sensilla stimulated (20). They are also affected by receptor current amplitude, positions of the neurons relative to the recording electrode, and electrical properties of the antenna itself (21). With appropriate odorless controls for electrical and mechanical artifacts, measuring EAG signals provides reliable estimates of transduction latencies and pulse tracking ability of ORNs.