High-Frequency Neuronal Bursting is Essential for Circadian and Sleep Behaviors in Drosophila

Circadian rhythms have been extensively studied in Drosophila; however, still little is known about how the electrical properties of clock neurons are specified. We have performed a behavioral genetic screen through the downregulation of candidate ion channels in the lateral ventral neurons (LNvs) and show that the hyperpolarization-activated cation current I_h is important for the behaviors that the LNvs influence: temporal organization of locomotor activity, analyzed in males, and sleep, analyzed in females. Using whole-cell patch clamp electrophysiology we demonstrate that small LNvs (sLNvs) are bursting neurons, and that I_h is necessary to achieve the high-frequency bursting firing pattern characteristic of both types of LNvs in females. Since firing in bursts has been associated to neuropeptide release, we hypothesized that I_h would be important for LNvs communication. Indeed, herein we demonstrate that I_h is fundamental for the recruitment of pigment dispersing factor (PDF) filled dense core vesicles (DCVs) to the terminals at the dorsal protocerebrum and for their timed release, and hence for the temporal coordination of circadian behaviors.SIGNIFICANCE STATEMENT Ion channels are transmembrane proteins with selective permeability to specific charged particles. The rich repertoire of parameters that may gate their opening state, such as voltage-sensitivity, modulation by second messengers and specific kinetics, make this protein family a determinant of neuronal identity. Ion channel structure is evolutionary conserved between vertebrates and invertebrates, making any discovery easily translatable. Through a screen to uncover ion channels with roles in circadian rhythms, we have identified the I_h channel as an important player in a subset of clock neurons of the fruit fly. We show that lateral ventral neurons (LNvs) need I_h to fire action potentials in a high-frequency bursting mode and that this is important for peptide transport and the control of behavior.     

Considerations arising from a complementary learning systems perspective on hippocampus and neocortex

We discuss a framework for the organization of learning systems in the mammalian brain, in which the hippocampus and related areas form a memory system complementary to learning mechanisms in neocortex and other areas. The hippocampal system stores new episodes and “replays” them to the neocortical system, interleaved with ongoing experience, allowing generalization as cortical memories form. The data to account for include: 1) neurophysiological findings concerning representations in hippocampal areas, 2) behavioral evidence demonstrating a spatial role for hippocampus, 3) and effects of surgical and pharmacological manipulations on neuronal firing in hippocampal regions in behaving animals. We hypothesize that the hippocampal memory system consists of three major modules: 1) an invertible encoder subsystem supported by the pathways between neocortex and entorhinal cortex, which provides a stable, compressed, invertible encoding in entorhinal cortex (EC) of cortical activity patterns, 2) a memory separation, storage, and retrieval subsystem, supported by pathways between EC, dentate gyrus and area CA3, including the CA3 recurrent collaterals, which facilitates encoding and storage in CA3 of individual EC patterns, and retrieval of those CA3 encodings, in a manner that minimizes interference, and 3) a memory decoding subsystem, supported by the Shaffer collaterals from area CA1 to area CA3 and the bi-directional pathways between EC and CA3, which provides the means by which a retrieved CA3 coding of an EC pattern can reinstate that pattern on EC. This model has shown that 1) there is a trade-off between the need for information-preserving, structure-extracting encoding of cortical traces and the need for effective storage and recall of arbitrary traces, 2) long-term depression of synaptic strength in the pathways subject to long-term potentiation is crucial in preserving information, 3) area CA1 must be able to exploit correlations in EC patterns in the direct perforant path synapses. © 1997 Wiley-Liss, Inc.     

Grading the gradient: Evidence for time-dependent memory reorganization in experimental animals

In humans, damage limited to the hippocampus produces temporally graded retrograde amnesia, with relative sparing of remote compared to recent memory. This observation forms the cornerstone of the idea that as memories mature they are reorganized in a time-dependent manner. In this paper, we evaluate evidence for similar time-dependent reorganization in experimental animals. In the majority of behavioral paradigms examined, these studies provide evidence that memories are gradually reorganized over time, with the hippocampus and cortex playing preferential (although not necessarily mutually exclusive) roles in the expression of recent and remote memory, respectively. This pattern is not observed in all tasks, however. For example, in the water maze hippocampal activity is always necessary for memory expression. Identifying situations when the hippocampus is, and is not, required for remote memory expression will help us to understand hippocampal contributions to memory, and, more generally, whether changes in memory organization lead to qualitative changes in the nature of memory.     

New learning and remote memory in atypical Alzheimer's disease

This paper presents the case of BB, an individual with an atypical posterior cortical presentation of Alzheimer's disease (AD). The severity of BB's visuo-spatial impairment far outweighed impairment of other cognitive functions. BB's case is also unusual in that despite a long history of progressive impairment, his cognitive symptoms remain relatively circumscribed. More specifically, BB's pattern of memory impairment was striking, since his impairment on formal psychometric tests of memory contrasted with his performance at clinical interview, where he talked lucidly about events in his past, and displayed remarkably well-preserved general semantic knowledge. On the basis of BB's clinical profile, it was hypothesised that his pattern of cognitive performance reflected an impairment of anterograde memory in the context of relative preservation of remote memory. Further investigations revealed that while BB's anterograde memory function was comparable to that of other AD patients, his remote memory was well preserved relative to other AD ‘controls'. These findings are discussed in terms of typical and atypical presentations of Alzheimer's disease, and in terms of the possible fractionation of different aspects of long-term memory. The implications of these findings for our understanding of the neural bases of different types of retrograde memory (i.e. ‘old' versus ‘recent') are considered, with particular reference to the contrasting theoretical frameworks that have recently been advanced by Squire and Moscovitch.     

Retrograde amnesia for spatial information: a dissociation between intra and extramaze cues following hippocampus lesions in rats

Several recent studies have shown a flat retrograde amnesia for spatial information following lesions to the hippocampus in rats and mice. However, the results of the present investigation demonstrate that in rats that presurgically learned a spatial reference memory task based on extramaze cues, a temporally graded retrograde amnesia is evident following lesions to the hippocampus (1, 16, 32 or 64 days after learning) if two conditions are met. First, that a wide range of retention intervals is used, and second, that independent groups of rats are tested, not a single group that learns different spatial discrimination tasks at different times (expt 1). The results of expt 2 show that the hippocampus does not serve as a consolidating mechanism when the spatial task learned presurgically is based on intramaze cues. Taken together, these results indicate that the hippocampus is critical for the storage and/or retrieval of spatial reference information that was learned up to 1 month before hippocampus damage; however, in the absence of the hippocampus, efficient retention can still occur provided that the spatial knowledge was learned in a simple associative manner.     

MK-801 Blockade of Fos and Jun Expression Following Passive Avoidance Training in the Chick

Training chicks on a one-trial passive avoidance task results in transient up-regulation of the N -methyl- d -aspartate (NMDA) receptor in the left intermediate medial hyperstriatum ventrale (IMHV) of the forebrain 30 min post-training. Injection of the non-competitive NMDA receptor inhibitor, (+)-5-methyl-10, 11-dihydro-5H-dibenzo- (a.d)-cyclohepten 5, 10-imine maleate (MK-801), around the time of training renders chicks amnesic for the task. Training also results in enhanced expression of the immediate early gene (IEG) c- fos in the IMHV. To determine the relationship between NMDA receptor up-regulation and IEG induction during memory formation we have examined the expression of Fos, Jun and their related proteins 1 h following training in the presence/absence of the putative amnestic agent MK-801. Western blotting of IMHV samples revealed two protein bands with immunoreactivity to the Fos antibody at 47 and 54 kDa. Using an antibody to Jun, two immunoreactive bands were revealed at 39 and 54 kDa. All bands were enhanced in the left IMHV following passive avoidance training. Post-training intraperitoneal injections of MK-801 (75 mM) produced amnesia in ∼50% of the birds when tested 1 h after training. Injection of MK-801 significantly attenuated expression of these proteins in birds rendered amnesic, but not in those that recalled the task. We conclude that NMDA receptor activation precedes immediate early gene expression in the memory formation cascade.     

Heterogeneity is a Fact of Category-Specific Semantic Deficits. An Issue Worth Considering. Comments on Bradford Z. Mahon and Alfonso Caramazza (2003)

Anterograde amnesia caused by bilateral hippocampal or diencephalon damage manifests in characteristic symptoms of preserved intellect and implicit learning, and short span of awareness with complete and rapid forgetting of episodic material. A new case, WO, 38-year-old male with anterograde amnesia, in the absence of structural brain changes or psychological explanation is presented, along with four comparison cases from the extant literature that share commonalities between them including preserved intellect, span of awareness greater than working memory, and complete forgetting within hours or days following successful learning, including notably for both explicit and implicit material. WO’s amnesia onset coincided with anesthetic injection and root canal procedure, with extended vasovagal-like incident. The commonalities between the five cases presented may suggest a shared biological mechanism involving the breakdown of intermediate-to-late-stage consolidation that does not depend on the structural integrity of the hippocampi. Speculation on the mechanism of consolidation breakdown and diagnostic implications are discussed.     

Traumatic brain injury produces impairments in long-term and recent memory

Traumatic brain injury (TBI) in humans typically produces neurological suppression and a longer lasting impairment of memory clinically defined as post-traumatic amnesia. An animal model that reliably reproduces the physiological changes associated with TBI was used to assess the memory deficits following brain injury. Prior to TBI, rats were trained to perform one of four tasks that assessed either motor performance, long-term or recent memory. Rats were randomly assigned to one of three groups (anesthesia only, sham operation or fluid percussion). Following fluid percussion, used to produce TBI, rats were tested for 6 test sessions. The first session occurred 1–2 min after the experimental manipulation. The next 5 sessions followed the training schedule maintained prior to experimental manipulation. Differences in long-term memory occurred only in the first post-operative test session. Differences in recent memory performance were found across all 6 test sessions. The memory deficits were clearly dissociated from motor deficits. The similar memory deficits observed following human head injury and the experimentally produced TBI injury demonstrate that fluid percussion is a useful approach to examine underlying neurobiological mechanisms involved in head injury and possible clinical interventions.     

Muscarinic receptor blockade in ventral hippocampus and prelimbic cortex impairs memory for socially transmitted food preference

Acetylcholine is involved in learning and memory and, particularly, in olfactory tasks, but reports on its specific role in consolidation processes are somewhat controversial. The present experiment sought to determine the effects of blocking muscarinic cholinergic receptors in the ventral hippocampus (vHPC) and the prelimbic cortex (PLC) on the consolidation of social transmission of food preference, an odor‐guided relational task that depends on such brain areas. Adult male Wistar rats were bilaterally infused with scopolamine (20 μg/site) immediately after social training and showed impairment, relative to vehicle‐injected controls, in the expression of the task measured 24 h after learning. Results indicated that scopolamine in the PLC completely abolished memory, suggesting that muscarinic transmission in this cortical region is crucial for consolidation of recent socially acquired information. Muscarinic receptors in the vHPC contribute in some way to task consolidation, as the rats injected with scopolamine in the vHPC showed significantly lower trained food preference than control rats, but higher than both chance level and that of the PLC‐injected rats. Behavioral measures such as social interaction, motivation to eat, neophobia, or exploration did not differ between rats infused with scopolamine or vehicle. Such data suggest a possible differential role of muscarinic receptors in the PLC and the vHPC in the initial consolidation of a naturalistic form of nonspatial relational memory. © 2008 Wiley‐Liss, Inc.     

Memory for relations in the short term and the long term after medial temporal lobe damage

A central idea about the organization of declarative memory and the function of the hippocampus is that the hippocampus provides for the coding of relationships between items. A question arises whether this idea refers to the process of forming long‐term memory or whether, as some studies have suggested, memory for relations might depend on the hippocampus even at short retention intervals and even when the task falls within the province of short‐term (working) memory. The latter formulation appears to place the operation of relational memory into conflict with the idea that working memory is independent of medial temporal lobe (MTL) structures. In this report, the concepts of relational memory and working memory are discussed in the light of a simple demonstration experiment. Patients with MTL lesions successfully learned and recalled two word pairs when tested directly after learning but failed altogether when tested after a delay. The results do not contradict the idea that the hippocampus has a fundamental role in relational memory. However, there is a need for further elaboration and specification of the idea in order to explain why patients with MTL lesions can establish relational memory in the short term but not in long‐term memory. © 2017 Wiley Periodicals, Inc.     

Hippocampal lesions produce both nongraded and temporally graded retrograde amnesia in the same rat

Rats were administered contextual fear conditioning and trained on a water‐maze, spatial memory task 28 days or 24 h before undergoing hippocampal lesion or control surgery. When tested postoperatively on both tasks, rats with hippocampal lesions exhibited retrograde amnesia for spatial memory at both delays but temporally graded retrograde amnesia for the contextual fear response. In demonstrating both types of retrograde amnesia in the same animals, the results parallel similar observations in human amnesics with hippocampal damage and provide compelling evidence that the nature of the task and the type of information being accessed are crucial factors in determining the pattern of retrograde memory loss associated with hippocampal damage. The results are interpreted as consistent with our transformation hypothesis (Winocur et al. (2010a) Neuropsychologia 48:2339–2356; Winocur and Moscovitch (2011) J Int Neuropsychol Soc 17:766–780) and at variance with standard consolidation theory and other theoretical models of memory. © 2013 Wiley Periodicals, Inc.     

Novelty causes time-dependent retrograde amnesia for one-trial avoidance in rats through NMDA receptor- and CaMKII-dependent mechanisms in the hippocampus

Exposure to a novel environment (an open field) for 2 min, 1 h after one-trial inhibitory avoidance training, hindered memory of the avoidance task measured 24 h later. The effect was seen regardless of the intensity of the avoidance training footshock. The effect was not seen if the exposure to novelty was carried out 5 min before, or 6 h after, the avoidance training, or if the animals did not perceive the open field as new and react accordingly. The amnesic effect of the novelty presented 1 h after avoidance training was blocked by the intrahippocampal infusion of D-2-amino-5-phosphono-pentanoic acid (AP5, 25 nmoles per side) or 1-(N, O-bis-[5-isoquinolinylsulphonyl]-N-methyl-L-tyrosyl)-4- phenylpiperazine (KN62, 100 micromoles per side) but not by that of C32H25N3O6 (KT5720, 90 micromoles per side) given 5 min before the novelty. In the open field there was habituation, measured by the decrease in exploration between the first and second minute. AP5 and KN62 impaired this habituation, but not KT5720. Exploration of the open field was similar in the groups exposed to the avoidance task 5 min later, or 1 h or 6 h before. Therefore, there was no reciprocity between the effect of the two tasks: novelty was amnesic for the one-trial avoidance task, but the opposite was not true. The amnesic effect of novelty appears to rely on N-methyl-D-aspartate (NMDA) receptor- and calcium/calmodulin-dependent protein kinase II (CaMKII)-dependent, but not on PKA-dependent, aspects of its habituation learning.     

Specialization in the medial temporal lobe for processing of objects and scenes

There has been considerable debate as to whether the hippocampus and perirhinal cortex may subserve both memory and perception. We administered a series of oddity tasks, in which subjects selected the odd stimulus from a visual array, to amnesic patients with either selective hippocampal damage (HC group) or more extensive medial temporal damage, including the perirhinal cortex (MTL group). All patients performed normally when the stimuli could be discriminated using simple visual features, even if faces or complex virtual reality scenes were presented. Both patient groups were, however, severely impaired at scene discrimination when a significant demand was placed on processing spatial information across viewpoint independent representations, while only the MTL group showed a significant deficit in oddity judgments of faces and objects when object viewpoint independent perception was emphasized. These observations provide compelling evidence that the human hippocampus and perirhinal cortex are critical to processes beyond long-term declarative memory and may subserve spatial and object perception, respectively.     

Autobiographical memory and the self in a case of transient epileptic amnesia

Transient epileptic amnesia (TEA) is characterized by deficits in autobiographical memory (AM). One of the functions of AM is to maintain the self, suggesting that the self may undergo changes as a result of memory loss in temporal lobe epilepsy. To examine this, we used a modification of a task used to assess the relationship between self and memory (the IAM task) in a single case, E.B. Despite complaints of AM loss, E.B. had no difficulty in producing a range of self-images (e.g., I am a husband) and collections of self-defining AMs in support of these statements. E.B. produced fewer episodic memories at times of self-formation, but this did not seem to impact on the maintenance of self. The results support recent work suggesting the self may be maintained in the absence of episodic memory. The application of tasks such as that used here will further elucidate AM impairment in temporal lobe epilepsy.     

Severe tactual memory deficits in monkeys after combined removal of the amygdala and hippocampus

Monkeys with bilateral removals of both the amygdaloid complex and hippocampal formation were far more severely impaired on a tactual memory task than were monkeys with removal of either structure alone. These data parallel earlier findings on visual memory in monkeys and suggest that: (i) the memory deficit following combined ablations of the amygdala and hippocampus is multimodal; and (ii) the global anterograde amnesia observed in patients with medial temporal-lobe damage is also due to combined damage to these two structures.     

The effect of hippocampal damage in children on recalling the past and imagining new experiences

Compared to adults, relatively little is known about autobiographical memory and the ability to imagine fictitious and future scenarios in school-aged children, despite the importance of these functions for development and subsequent independent living. Even less is understood about the effect of early hippocampal damage on children's memory and imagination abilities. To bridge this gap, we devised a novel naturalistic autobiographical memory task that enabled us to formally assess the memory for recent autobiographical experiences in healthy school-aged children. Contemporaneous with the autobiographical memories being formed, the children also imagined and described fictitious scenarios. Having established the performance of healthy school-aged children on these tasks, we proceeded to make comparisons with children (n = 21) who had experienced neonatal hypoxia/ischaemia, and consequent bilateral hippocampal damage. Our results showed that healthy children could recall autobiographical events, including spatiotemporal information and specific episodic details. By contrast, children who had experienced neonatal hypoxia/ischaemia had impaired recall, with the specific details of episodes being lost. Despite this significant memory deficit they were able to construct fictitious scenarios. This is in clear contrast to adults with hippocampal damage, who typically have impaired autobiographical memory and deficits in the construction of fictitious and future scenarios. We speculate that the paediatric patients’ relatively intact semantic memory and/or some functionality in their residual hippocampi may underpin their scene construction ability.     

Stimulus Train-Induced Bursting in Hippocampal Slices: Difference between CA1 and CA2/3 Subfields

Abstract: We studied the relationship between CA2/3 and CA1 subfields in the generation of stimulus train-induced bursting using in vitro hippocampal slices. Repeated stimulation to the stratum radiatum induced three types of epileptiform activities: afterdis-charge bursts, spontaneous bursts and single stimulus-triggered bursts. In the intact slices, spontaneous bursts in CA3 preceded those in CA1, whereas afterdischarge bursts in CA3 followed those in CA1 when it was possible to distinguish differences in the burst onset. After transection between CA2/3 and CA1, neither spontaneous nor triggered bursts could be induced or maintained in isolated CA1, where only small afterdischarge bursts with a higher bursting frequency were observed. It was difficult to induce after-discharge bursts by the tetani to the mossy fibers. Some of the mechanisms underlying the generation of these stimulus train-induced bursts are discussed.