Amygdala stimulation modulates hippocampal synaptic plasticity

Experience-dependent synaptic plasticity is a fundamental feature of neural networks involved in the encoding of information, and the capability of synapses to express plasticity is itself activity-dependent. Here, we introduce a "low-frequency burst stimulation" protocol, which can readily induce both long-term potentiation (LTP) and long-term depression (LTD) at in vivo medial perforant path-dentate gyrus synapses. By varying stimulation parameters, we were able to build a stimulus-response map of synaptic plasticity as a LTP-LTD continuum. The response curve displayed a bidirectional shift toward LTP and LTD, depending on the degree and timing of neural activity of the basolateral amygdala. The range of this plastic modulation was also modified by past activity of the basolateral amygdala, suggesting that the amygdala can arrange its ability to regulate the dentate plastic responses. The effects of the BLA activation were replicated by stimulation of the lateral perforant path and, hence, BLA stimulation may recruit the lateral entorhinal cortex. These results represent a high-order dimension of heterosynaptic modulations of hippocampal synaptic plasticity.     

Regulation of excitability and plasticity by endocannabinoids and PKA in developing hippocampus

The activity-dependent strengthening and weakening of synaptic transmission are hypothesized to be the basis of not only memory and learning but also the refinement of neural circuits during development. Here we report that, in the developing CA1 area of the hippocampus, endocannabinoid (eCB)-mediated heterosynaptic long-term depression (LTD) of glutamatergic excitatory synaptic transmission is associated with PKA-mediated homosynaptic long-term potentiation (LTP). This form of LTD was dominant at postnatal days 2-10 (P2-P10), attenuated during development, and finally disappeared in the mature hippocampus. Heterosynaptic LTD of excitatory postsynaptic currents in the developing hippocampus was expressed presynaptically, spread to neighboring neurons, and was mediated by eCBs. Heterosynaptic LTD of field excitatory postsynaptic potentials was associated with a decrease in fiber volley amplitude with a similar time course. Depression of fiber volleys was blocked by K(+) channel blockers, suggesting the involvement of the decrease in presynaptic excitability in heterosynaptic LTD. In the P2-P5 hippocampus, eCBs also attenuate LTP and fiber volleys in homosynaptic pathways and help to prevent too much excitability in the neonatal hippocampus where the GABAergic system is poorly developed and even excitatory. In the hippocampus older than P6 (P > 6), however, LTP is protected from eCB-mediated depression by PKA activated at presynaptic sites by high-frequency stimulation, serving to highlight PKA-mediated LTP by weakening inactive synapses even in adjacent cells. Thus, eCBs and PKA make synapses plastic without changing excitability homeostasis in the developing hippocampus.     

Stress-induced alterations in hippocampal plasticity, place cells, and spatial memory

Acute, inescapable, and unpredictable stress can profoundly modify brain and cognition in humans and animals. The present study investigated the ensuing effects of 2-h variable "audiogenic" stress on three related levels of hippocampal functions in rats: long-term potentiation, place cell activity, and spatial memory. In agreement with prior findings, we observed that stress reduced the magnitude of Schaffer collateral/commissural-Cornu Ammonis field 1 long-term potentiation in vitro, and selectively impaired spatial memory on a hidden platform version of the Morris water maze task. We also observed that stress impaired the stability of firing rates (but not firing locations) of place cells recorded from dorsal Cornu Ammonis field 1 in rats foraging freely on a novel open-field platform located in a familiar surrounding room. These findings suggest that stress-induced modifications in synaptic plasticity may prevent the storage of stable "rate maps" by hippocampal place cells, which in turn may contribute to spatial memory impairments associated with stress.     

Synapse elimination accompanies functional plasticity in hippocampal neurons

A critical component of nervous system development is synapse elimination during early postnatal life, a process known to depend on neuronal activity. Changes in synaptic strength in the form of long-term potentiation (LTP) and long-term depression (LTD) correlate with dendritic spine enlargement or shrinkage, respectively, but whether LTD can lead to an actual separation of the synaptic structures when the spine shrinks or is lost remains unknown. Here, we addressed this issue by using concurrent imaging and electrophysiological recording of live synapses. Slices of rat hippocampus were cultured on multielectrode arrays, and the neurons were labeled with genes encoding red or green fluorescent proteins to visualize presynaptic and postsynaptic neuronal processes, respectively. LTD-inducing stimulation led to a reduction in the synaptic green and red colocalization, and, in many cases, it induced a complete separation of the presynaptic bouton from the dendritic spine. This type of synapse loss was associated with smaller initial spine size and greater synaptic depression but not spine shrinkage during LTD. All cases of synapse separation were observed without an accompanying loss of the spine during this period. We suggest that repeated low-frequency stimulation simultaneous with LTD induction is capable of restructuring synaptic contacts. Future work with this model will be able to provide critical insight into the molecular mechanisms of activity- and experience-dependent refinement of brain circuitry during development.     

Dopamine modulates synaptic plasticity in dendrites of rat and human dentate granule cells

The mechanisms underlying memory formation in the hippocampal network remain a major unanswered aspect of neuroscience. Although high-frequency activity appears essential for plasticity, salience for memory formation is also provided by activity in ventral tegmental area (VTA) dopamine projections. Here, we report that activation of dopamine D1 receptors in dentate granule cells (DGCs) can preferentially increase dendritic excitability to both high-frequency afferent activity and high-frequency trains of backpropagating action potentials. Using whole-cell patch clamp recordings, calcium imaging, and neuropeptide Y to inhibit postsynaptic calcium influx, we found that activation of dendritic voltage-dependent calcium channels (VDCCs) is essential for dopamine-induced long-term potentiation (LTP), both in rat and human dentate gyrus (DG). Moreover, we demonstrate previously unreported spike-timing-dependent plasticity in the human hippocampus. These results suggest that when dopamine is released in the dentate gyrus with concurrent high-frequency activity there is an increased probability that synapses will be strengthened and reward-associated spatial memories will be formed.     

Correlated memory defects and hippocampal dendritic spine loss after acute stress involve corticotropin-releasing hormone signaling

Stress affects the hippocampus, a brain region crucial for memory. In rodents, acute stress may reduce density of dendritic spines, the location of postsynaptic elements of excitatory synapses, and impair long-term potentiation and memory. Steroid stress hormones and neurotransmitters have been implicated in the underlying mechanisms, but the role of corticotropin-releasing hormone (CRH), a hypothalamic hormone also released during stress within hippocampus, has not been elucidated. In addition, the causal relationship of spine loss and memory defects after acute stress is unclear. We used transgenic mice that expressed YFP in hippocampal neurons and found that a 5-h stress resulted in profound loss of learning and memory. This deficit was associated with selective disruption of long-term potentiation and of dendritic spine integrity in commissural/associational pathways of hippocampal area CA3. The degree of memory deficit in individual mice correlated significantly with the reduced density of area CA3 apical dendritic spines in the same mice. Moreover, administration of the CRH receptor type 1 (CRFR₁) blocker NBI 30775 directly into the brain prevented the stress-induced spine loss and restored the stress-impaired cognitive functions. We conclude that acute, hours-long stress impairs learning and memory via mechanisms that disrupt the integrity of hippocampal dendritic spines. In addition, establishing the contribution of hippocampal CRH–CRFR₁ signaling to these processes highlights the complexity of the orchestrated mechanisms by which stress impacts hippocampal structure and function.     

Dorsal periaqueductal gray-amygdala pathway conveys both innate and learned fear responses in rats

The periaqueductal gray (PAG) and amygdala are known to be important for defensive responses, and many contemporary fear-conditioning models present the PAG as downstream of the amygdala, directing the appropriate behavior (i.e., freezing or fleeing). However, empirical studies of this circuitry are inconsistent and warrant further examination. Hence, the present study investigated the functional relationship between the PAG and amygdala in two different settings, fear conditioning and naturalistic foraging, in rats. In fear conditioning, electrical stimulation of the dorsal PAG (dPAG) produced unconditional responses (URs) composed of brief activity bursts followed by freezing and 22-kHz ultrasonic vocalization. In contrast, stimulation of ventral PAG and the basolateral amygdalar complex (BLA) evoked freezing and/or ultrasonic vocalization. Whereas dPAG stimulation served as an effective unconditional stimulus for fear conditioning to tone and context conditional stimuli, neither ventral PAG nor BLA stimulation supported fear conditioning. The conditioning effect of dPAG, however, was abolished by inactivation of the BLA. In a foraging task, dPAG and BLA stimulation evoked only fleeing toward the nest. Amygdalar lesion/inactivation blocked the UR of dPAG stimulation, but dPAG lesions did not block the UR of BLA stimulation. Furthermore, in vivo recordings demonstrated that electrical priming of the dPAG can modulate plasticity of subiculum–BLA synapses, providing additional evidence that the amygdala is downstream of the dPAG. These results suggest that the dPAG conveys unconditional stimulus information to the BLA, which directs both innate and learned fear responses, and that brain stimulation-evoked behaviors are modulated by context.     

Shortened tethering filaments stabilize presynaptic vesicles in support of elevated release probability during LTP in rat hippocampus

Long-term potentiation (LTP) is a cellular mechanism of learning and memory that results in a sustained increase in the probability of vesicular release of neurotransmitter. However, previous work in hippocampal area CA1 of the adult rat revealed that the total number of vesicles per synapse decreases following LTP, seemingly inconsistent with the elevated release probability. Here, electron-microscopic tomography (EMT) was used to assess whether changes in vesicle density or structure of vesicle tethering filaments at the active zone might explain the enhanced release probability following LTP. The spatial relationship of vesicles to the active zone varies with functional status. Tightly docked vesicles contact the presynaptic membrane, have partially formed SNARE complexes, and are primed for release of neurotransmitter upon the next action potential. Loosely docked vesicles are located within 8 nm of the presynaptic membrane where SNARE complexes begin to form. Nondocked vesicles comprise recycling and reserve pools. Vesicles are tethered to the active zone via filaments composed of molecules engaged in docking and release processes. The density of tightly docked vesicles was increased 2 h following LTP compared to control stimulation, whereas the densities of loosely docked or nondocked vesicles congregating within 45 nm above the active zones were unchanged. The tethering filaments on all vesicles were shorter and their attachment sites shifted closer to the active zone. These findings suggest that tethering filaments stabilize more vesicles in the primed state. Such changes would facilitate the long-lasting increase in release probability following LTP.

Long-term potentiation (LTP) is the persistent strengthening of synapses after a brief high-frequency stimulation and is widely accepted as a cellular correlate of learning and memory (1, 2). Within minutes after the induction of LTP, new receptors are inserted into the postsynaptic membrane. The resulting increase in the excitatory postsynaptic potential is immediate and can persist for hours in vitro or days to months in vivo (1, 3–8). Quantal content is also increased soon after LTP induction and reflects an increase in the number of presynaptic vesicles that release neurotransmitter (9–13). This increase in release probability is sustained several hours following LTP (14), concurrent with postsynaptic growth and spine enlargement (15). One might expect that the enhanced probability of release would involve increasing the number of vesicles docked and primed for neurotransmitter release. However, 2 h after induction of LTP, the total number of both docked and nondocked vesicles per presynaptic bouton are markedly decreased relative to control stimulation (16). These findings raise the question of whether an altered structure of docking and priming molecules leads to local clustering of vesicles that would elevate the probability of release following LTP.The proteins that connect synaptic vesicles to the plasma membrane can be visualized as filaments with electron-microscopic tomography (EMT) connecting vesicles to the presynaptic active zone (17–24). Studies suggest that the SNARE complex begins to form when a vesicle and presynaptic membrane are within 8 nm of each other rendering them loosely docked (25–27). Vesicles are then drawn toward the active zone, and the SNARE complex bundle is fully formed when the vesicle is within 2 nm of the presynaptic membrane (26, 28). Tightly docked vesicles are defined as being in contact with the presynaptic membrane and correspond to primed vesicles that comprise the readily releasable pool (27). Recent studies have suggested that docked vesicles can oscillate between loosely and tightly docked states (27, 29), providing a target mechanism for synaptic plasticity.To address the question of how changes in vesicle proximity and tethering might enhance the probability of release, we used EMT, which enabled us to acquire high-resolution structural data from small volumes of presynaptic boutons that were enriched in synaptic vesicles. We targeted active zones of hippocampal synapses, comparing their structure 2 h after LTP induction to control stimulation. The vesicle density and tethering filament dimensions were unchanged for the loosely docked and nondocked vesicles. In contrast, the density of tightly docked vesicles was increased, their tethering filaments were shorter, and the filament attachment sites on the vesicles were positioned closer to the side of the vesicle membrane facing the presynaptic membrane. Such alterations could contribute to the sustained increase in the probability of neurotransmitter release following LTP.     

Synaptic P-Rex1 signaling regulates hippocampal long-term depression and autism-like social behavior

Autism spectrum disorders (ASDs) are a group of highly inheritable mental disorders associated with synaptic dysfunction, but the underlying cellular and molecular mechanisms remain to be clarified. Here we report that autism in Chinese Han population is associated with genetic variations and copy number deletion of P-Rex1 (phosphatidylinositol-3,4,5-trisphosphate-dependent Rac exchange factor 1). Genetic deletion or knockdown of P-Rex1 in the CA1 region of the hippocampus in mice resulted in autism-like social behavior that was specifically linked to the defect of long-term depression (LTD) in the CA1 region through alteration of AMPA receptor endocytosis mediated by the postsynaptic PP1α (protein phosphase 1α)–P-Rex1–Rac1 (Ras-related C3 botulinum toxin substrate 1) signaling pathway. Rescue of the LTD in the CA1 region markedly alleviated autism-like social behavior. Together, our findings suggest a vital role of P-Rex1 signaling in CA1 LTD that is critical for social behavior and cognitive function and offer new insight into the etiology of ASDs.Deficits in social interaction and communication skills and repetitive behavior/restricted interests have been demonstrated in people diagnosed with autism spectrum disorders (ASDs) (1). Several studies have documented impairments of social recognition [e.g., such as deficits in recognizing unfamiliar faces (2)] and in behavioral flexibility [e.g., impaired reversal learning and difficulties in error correction (3, 4)] in autistic people. However, the neurobiological mechanism responsible for the symptoms of ASDs, and especially for the deficit in social recognition, is little known.Recent genetic studies have identified a large number of candidate genes for ASDs (5, 6), including many that code for synaptic proteins. Synaptic dysfunction may play a critical role in ASDs (7).Here we have identified a new autism-associated gene, Prex1, that codes for P-Rex1 (phosphatidylinositol-3,4,5-trisphosphate-dependent Rac exchange factor 1), a Rac-specific Rho GTPase guanine nucleotide exchange factor (GEF). This gene is known to be highly expressed in neutrophils and in the mouse brain (8). Mice with the Prex1 gene deleted (Prex1^−/−) exhibited Rac-dependent mild neutrophilia (9) and melanoblast migration defects (10). P-Rex1 influences neuronal cell motility (11) and neurite elongation (12) by regulating actin dynamics specifically at the growth cone. However, the role of P-Rex1 in regulating synaptic function and related behaviors remains unknown.In addition to identifying an association between PREX1 and autism in humans, we demonstrate that genetic disruption of P-Rex1 in mice leads to autism-like social behavior and to other features known to be associated with ASDs. Electrophysiological studies revealed a specific impairment of NMDA receptor (NMDAR)-dependent long-term depression (LTD) at Schaffer collateral– cornus ammonis region 1 (SC–CA1) synapses. Furthermore, these defects were associated with dysfunction in NMDA-induced AMPA receptor (AMPAR) endocytosis, because of defective PP1α (serine/threonine protein phosphase 1α)–P-Rex1–Rac1 (Ras-related C3 botulinum toxin substrate 1) signaling, and correcting the latter rectified the social recognition deficit of Prex1^−/− mice. Thus, we have elucidated a synaptic mechanism underlying the deficit in social recognition induced by P-Rex1 disruption and the cognitive dysfunction associated with ASDs.     

Synaptic plasticity rules with physiological calcium levels

Spike-timing–dependent plasticity (STDP) is considered as a primary mechanism underlying formation of new memories during learning. Despite the growing interest in activity-dependent plasticity, it is still unclear whether synaptic plasticity rules inferred from in vitro experiments are correct in physiological conditions. The abnormally high calcium concentration used in in vitro studies of STDP suggests that in vivo plasticity rules may differ significantly from in vitro experiments, especially since STDP depends strongly on calcium for induction. We therefore studied here the influence of extracellular calcium on synaptic plasticity. Using a combination of experimental (patch-clamp recording and Ca²⁺ imaging at CA3-CA1 synapses) and theoretical approaches, we show here that the classic STDP rule in which pairs of single pre- and postsynaptic action potentials induce synaptic modifications is not valid in the physiological Ca²⁺ range. Rather, we found that these pairs of single stimuli are unable to induce any synaptic modification in 1.3 and 1.5 mM calcium and lead to depression in 1.8 mM. Plasticity can only be recovered when bursts of postsynaptic spikes are used, or when neurons fire at sufficiently high frequency. In conclusion, the STDP rule is profoundly altered in physiological Ca²⁺, but specific activity regimes restore a classical STDP profile.

Spike-timing–dependent plasticity (STDP) is a form of synaptic modification thought to constitute a mechanism underlying formation of new memories. The polarity of synaptic changes is controlled by the relative timing between pre- and postsynaptic activity and depends on intracellular Ca²⁺ signaling (review in refs. 1 and 2). In hippocampal and neocortical pyramidal neurons, timing-dependent long-term synaptic potentiation (t-LTP) is induced when synaptic activity is followed by one or more backpropagating action potentials in the postsynaptic cell (3–8). It involves postsynaptic Ca²⁺ influx through N-methyl-d-aspartate (NMDA) receptors that in turn activates protein kinases (3, 6, 8, 9). Timing-dependent long-term synaptic depression (t-LTD) is expressed when synaptic activity is repeatedly preceded by one or more backpropagating action potentials (4–7, 10). It depends on NMDA receptor activation, postsynaptic metabotropic glutamate receptors (mGluR), voltage-dependent calcium channels, protein phosphatases, cannabinoid receptor CB1, and astrocytic signaling (6, 10–16). Calcium therefore represents potentially a key factor in the induction of STDP. The intracellular Ca²⁺ dependence of STDP suggests that extracellular Ca²⁺ might play a critical role in shaping STDP. Yet, most if not all in vitro STDP studies (6–10, 17–19) used nonphysiological external Ca²⁺ concentrations ranging between 2 and 3 mM because elevated calcium is known to stabilize recording of synaptic transmission and to avoid intrinsic bursting that could obscure induction of STDP with single pre- and postsynaptic spikes (20, 21). In contrast, the physiological Ca²⁺ concentration is typically around 1.3 mM, with small (0.1–0.3 mM) variations between awake, sleep, and anesthesia, and with age, but in all cases concentrations are below 1.8 mM in rodent hippocampus (22–24).Calcium-based models of synaptic plasticity (25, 26) where Ca²⁺ transients result from backpropagating action potentials and excitatory postsynaptic potentials (EPSPs) predict that the sign, shape, and magnitude of STDP strongly depend on the amplitudes of calcium transients triggered by pre- and postsynaptic spikes and therefore on external Ca²⁺ concentration (26) (Fig. 1). These modeling studies suggest the possibility that plasticity rules at physiological concentrations might be very different from the ones inferred from currently available data. Several scenarios are possible: In the mildest one, high Ca²⁺ concentrations used in experimental studies would lead to an overestimate of the in vivo levels of plasticity; in the most extreme one, a complete lack of plasticity could be observed in physiological Ca²⁺. In addition, recent work shows that synaptic plasticity rules at a cerebellar synapse are profoundly altered in physiological calcium (27, 28). We therefore set out to determine STDP rules in physiological Ca²⁺ at the CA3-CA1 synapse of the hippocampus in vitro.Open in a separate windowFig. 1.Prediction of a calcium-based model of spike-timing–dependent plasticity. Cartoon showing qualitatively calcium transients induced by pairing a presynaptic spike with a postsynaptic spike with a delay ∆t, for three extracellular calcium concentrations (high on top, low on the bottom). Synaptic changes depend on two plasticity thresholds, one for LTP (blue) and one for LTD (red). The resulting ''STDP curves'' (change in synaptic stength ∆w as a function of ∆t) are shown on the right. At high extracellular calcium, the calcium transient exceeds LTP threshold in a range of positive ∆ts, and the STDP curves has a LTP window surrounded by two LTD windows. Decreasing extracellular calcium leads to a decrease in the amplitude of the calcium transient, which no longer cross the LTP threshold, resulting in a STDP curve with only LTD. Finally, a further reduction in extracellular calcium leads to no threshold crossing, and consequently no synaptic changes.We show here that the classical STDP rule (t-LTD for post-before-pre pairings, t-LTP for pre-before-post pairings) is obtained solely with a high external Ca²⁺ concentration (≥ 2.5 mM), whereas no plasticity could be induced for concentrations lower than 1.5 mM external Ca²⁺, and only t-LTD could be induced by positive or negative time delays in 1.8 mM external Ca²⁺. t-LTP could be restored only when bursts of three or four postsynaptic spikes were used instead of single spikes, or when the pairing frequency was increased from 0.33 to 5 or 10 Hz. We used two variants of a Ca²⁺-based plasticity model (26) in which both t-LTD and t-LTP depend on transient changes in postsynaptic Ca²⁺ (Fig. 1) to fit the data. We found that the nonlinearity of transient Ca²⁺ changes conferred by NMDA receptor activation is critical to quantitatively account for the entire experimental dataset. Our results indicate that the STDP rule is profoundly altered in physiological Ca²⁺, but that a classical STDP profile can be restored under specific activity regimes.     

Metabotropic glutamate receptor signaling is required for NMDA receptor-dependent ocular dominance plasticity and LTD in visual cortex

A feature of early postnatal neocortical development is a transient peak in signaling via metabotropic glutamate receptor 5 (mGluR5). In visual cortex, this change coincides with increased sensitivity of excitatory synapses to monocular deprivation (MD). However, loss of visual responsiveness after MD occurs via mechanisms revealed by the study of long-term depression (LTD) of synaptic transmission, which in layer 4 is induced by acute activation of NMDA receptors (NMDARs) rather than mGluR5. Here we report that chronic postnatal down-regulation of mGluR5 signaling produces coordinated impairments in both NMDAR-dependent LTD in vitro and ocular dominance plasticity in vivo. The data suggest that ongoing mGluR5 signaling during a critical period of postnatal development establishes the biochemical conditions that are permissive for activity-dependent sculpting of excitatory synapses via the mechanism of NMDAR-dependent LTD.Temporary monocular deprivation (MD) sets in motion synaptic changes in visual cortex that result in impaired vision through the deprived eye. The primary cause of visual impairment is depression of excitatory thalamocortical synaptic transmission in layer 4 of visual cortex (1–3). The study of long-term depression (LTD) of synapses, elicited in vitro by electrical or chemical stimulation, has revealed many of the mechanisms involved in deprived-eye depression (4). In slices of visual cortex, LTD in layer 4 is induced by NMDA receptor (NMDAR) activation and expressed by posttranslational modification and internalization of AMPA receptors (AMPARs) (5, 6). MD induces identical NMDAR-dependent changes in AMPARs, and synaptic depression induced by deprivation in vivo occludes LTD in visual cortex ex vivo (6–8). Manipulations of NMDARs and AMPAR trafficking that interfere with LTD also prevent the effects of MD (7, 9–11).Although NMDAR-dependent LTD is widely expressed in the brain (12, 13), it is now understood that different circuits use different mechanisms for long-term homosynaptic depression (14). For example, in the CA1 region of hippocampus, synaptic activation of either NMDARs or metabotropic glutamate receptor 5 (mGluR5) induces LTD. In both cases, depression is expressed postsynaptically as a reduction in AMPARs, but these forms of LTD are not mutually occluding and have distinct signaling requirements (15). A defining feature of mGluR5-dependent postsynaptic LTD in CA1 is a requirement for the immediate translation of synaptic mRNAs (16). In visual cortex, there is evidence that induction of LTD in layers 2–4 requires NMDAR activation, whereas induction of LTD in layer 6 requires activation of mGluR5 (17, 18).The hypothesis that mGluRs, in addition to NMDARs, play a key role in visual cortical plasticity can be traced back more than 25 y to observations that glutamate-stimulated phosphoinositide turnover, mediated in visual cortex by mGluR5 coupled to phospholipase C, is elevated during the postnatal period of heightened sensitivity to MD (19). Early attempts to test this hypothesis were inconclusive owing to the use of weak and nonselective orthosteric compounds (20–22); however, subsequent experiments did confirm that NMDAR-dependent LTD occurs normally in layers 2/3 of visual cortex in Grm5 knockout mice (23).The idea that mGluR5 is critically involved in visual cortical plasticity in vivo was rekindled with the finding that deprived-eye depression fails to occur in layer 4 of Grm5^+/− mutant mice (24). This finding was unexpected because, as reviewed above, a considerable body of evidence has implicated the mechanism of NMDAR-dependent LTD in deprived-eye depression. In the present study, we reexamined the role of mGluR5 in LTD and ocular dominance plasticity in layer 4, using the Grm5^+/− mouse and a highly specific negative allosteric modulator, 2-chloro-4-((2,5-dimethyl-1-(4-(trifluoromethoxy)phenyl)-1H-imidazol-4-yl)ethynyl)pyridine (CTEP), that has proven suitable for chronic inhibition of mGluR5 (25, 26). Our data show that NMDAR-dependent LTD and deprived-eye depression in layer 4 require mGluR5 signaling during postnatal development.     

Enhanced contextual fear memory in central serotonin-deficient mice

Central serotonin (5-HT) dysregulation contributes to the susceptibility for mental disorders, including depression, anxiety, and posttraumatic stress disorder, and learning and memory deficits. We report that the formation of hippocampus-dependent spatial memory is compromised, but the acquisition and retrieval of contextual fear memory are enhanced, in central 5-HT-deficient mice. Genetic deletion of serotonin in the brain was achieved by inactivating Lmx1b selectively in the raphe nuclei of the brainstem, resulting in a near-complete loss of 5-HT throughout the brain. These 5-HT-deficient mice exhibited no gross abnormality in brain structures and had normal locomotor activity. Spatial learning in the Morris water maze was unaffected, but the retrieval of spatial memory was impaired. In contrast, contextual fear learning and memory induced by foot-shock conditioning was markedly enhanced, but this enhancement could be prevented by intracerebroventricular administration of 5-HT. Foot shock impaired long-term potentiation and facilitated long-term depression in hippocampal slices in WT mice but had no effect in 5-HT-deficient mice. Furthermore, bath application of 5-HT in 5-HT-deficient mice restored foot shock-induced alterations of hippocampal synaptic plasticity. Thus, central 5-HT regulates hippocampus-dependent contextual fear memory, and 5-HT modulation of hippocampal synaptic plasticity may be the underlying mechanism. The enhanced fear memory in 5-HT-deficient mice supports the notion that 5-HT deficiency confers susceptibility to posttraumatic stress disorder in humans.     

Plastic modifications induced by object recognition memory processing

Long-term potentiation (LTP) phenomenon is widely accepted as a cellular model of memory consolidation. Object recognition (OR) is a particularly useful way of studying declarative memory in rodents because it makes use of their innate preference for novel over familiar objects. In this study, mice had electrodes implanted in the hippocampal Schaffer collaterals–pyramidal CA1 pathway and were trained for OR. Field EPSPs evoked at the CA3-CA1 synapse were recorded at the moment of training and at different times thereafter. LTP-like synaptic enhancement was found 6 h posttraining. A testing session was conducted 24 h after training, in the presence of one familiar and one novel object. Hippocampal synaptic facilitation was observed during exploration of familiar and novel objects. A short depotentiation period was observed early after the test and was followed by a later phase of synaptic efficacy enhancement. Here, we show that OR memory consolidation is accompanied by transient potentiation in the hippocampal CA3-CA1 synapses, while reconsolidation of this memory requires a short-lasting phase of depotentiation that could account for its well described vulnerability. The late synaptic enhancement phase, on the other hand, would be a consequence of memory restabilization.     

Robust network oscillations during mammalian respiratory rhythm generation driven by synaptic dynamics

How might synaptic dynamics generate synchronous oscillations in neuronal networks? We address this question in the preBötzinger complex (preBötC), a brainstem neural network that paces robust, yet labile, inspiration in mammals. The preBötC is composed of a few hundred neurons that alternate bursting activity with silent periods, but the mechanism underlying this vital rhythm remains elusive. Using a computational approach to model a randomly connected neuronal network that relies on short-term synaptic facilitation (SF) and depression (SD), we show that synaptic fluctuations can initiate population activities through recurrent excitation. We also show that a two-step SD process allows activity in the network to synchronize (bursts) and generate a population refractory period (silence). The model was validated against an array of experimental conditions, which recapitulate several processes the preBötC may experience. Consistent with the modeling assumptions, we reveal, by electrophysiological recordings, that SF/SD can occur at preBötC synapses on timescales that influence rhythmic population activity. We conclude that nondeterministic neuronal spiking and dynamic synaptic strengths in a randomly connected network are sufficient to give rise to regular respiratory-like rhythmic network activity and lability, which may play an important role in generating the rhythm for breathing and other coordinated motor activities in mammals.Central pattern generators (CPGs) are neuronal circuits that generate coordinated activity in the absence of sensory input (1). One such mammalian CPG, the preBötzinger complex (preBötC), gives rise to the eupneic respiratory rhythm (2, 3). Located in the medulla, the preBötC preserves a spontaneous respiratory-like rhythm when isolated in transverse slices, but the precise nature of the cellular and synaptic mechanisms underlying rhythmogenesis remains elusive (3–7). An early hypothesis was that the neuronal activity is driven by intrinsically bursting pacemaker neurons synchronized via excitatory synaptic connections (2, 6, 8, 9). However, electrophysiological and modeling studies (7, 10–12) now suggest the rhythm emerges through stochastic activation of intrinsic currents conveyed by recurrent synaptic connections, without the need for pacemaker neurons (3, 4, 11, 13, 14). In either case, excitatory synapses are required for rhythm generation; the possibility that synaptic properties also underlie periodic burst initiation and termination is yet to be demonstrated.Synaptic transmission relies on the release of vesicles, which can be modulated at the presynaptic terminal. Synaptic depression (SD), based on vesicular release, consists of decaying release probability after sustained activity, which subsequently decreases excitability within the underlying connected network. Conversely, synaptic facilitation (SF) enhances vesicular release probability and promotes neuronal synchronization. These synaptic dynamics are critical for short-term synaptic plasticity, and here they are explored in the context of preBötC rhythm generation.We first consider a randomly connected network where each neuron is modeled using a generalized Hodgkin–Huxley system of equations and exhibits spontaneous spiking activity based on a random process, but the neurons do not have intrinsic bursting mechanisms. These neurons are sparsely connected within a realistically sized network by excitatory synapses. The distinction of this model, from previous preBötC models, is that synapses express SF and SD that is implemented using two separate pools of vesicles and creates dynamic synapses. The first pool is the readily releasable pool (RRP) and the other is the recycling pool (RP) (15), modeled with mass-action kinetics. Synaptic dynamics has been repeatedly used to describe changes in spike rates in neural network populations (16) and emergence of gamma oscillations (17). Furthermore network connectivity can also participate to define bursting or the oscillation frequency in neural networks (18, 19).We show here that random networks connected with these synaptic properties, with random spiking, are sufficient for periodic bursting and examine a variety of experimental scenarios testing this model. The present model shows that an ensemble of excitatory neurons driven by synaptic dynamics can generate population-wide rhythmic activity and behaves in a manner similar to the preBötC under different conditions observed in vitro. Finally, we show experimentally that excitatory inputs to preBötC neurons often exhibit dynamically changing excitatory postsynaptic currents (EPSCs), supporting the modeled concept that SF/SD occurs on a timescale relevant to influence respiratory periods.     

NMDA receptors inhibit synapse unsilencing during brain development

How the billions of synapses in the adult mammalian brain are precisely specified remains one of the fundamental questions of neuroscience. Although a genetic program is likely to encode the basic neural blueprint, much evidence suggests that experience-driven activity through NMDA receptors wires up neuronal circuits by inducing a process similar to long-term potentiation. To test this notion directly, we eliminated NMDA receptors before and during synaptogenesis in single cells in vitro and in vivo. Although the prevailing model would predict that NMDA receptor deletion should strongly inhibit the maturation of excitatory circuits, we find that genetic ablation of NMDA receptor function profoundly increases the number of functional synapses between neurons. Conversely, reintroduction of NMDA receptors into NR1-deficient neurons reduces the number of functional inputs, a process requiring network activity and NMDA receptor function. Although NMDA receptor deletion increases the strength of unitary connections, it does not alter neuronal morphology, suggesting that basal NMDA receptor activation blocks the recruitment of AMPA receptors to silent synapses. Based on these results we suggest a new model for the maturation of excitatory synapses in which ongoing activation of NMDA receptors prevents premature synaptic maturation by ensuring that only punctuated bursts of activity lead to the induction of a functional synapse for the activity-dependent wiring of neural circuitry.     

The neural cell adhesion molecule (NCAM) in development and plasticity of the nervous system

—The neural cell adhesion molecule (NCAM) is a member of the immunoglobulin superfamily and is strongly expressed in the nervous system. NCAM is found in three major forms, of which two—NCAM-140 and NCAM-180—are transmembrane proteins, while the third—NCAM-120—is attached to the membrane via a glycosylphosphatidyl inositol anchor. In addition, soluble NCAM forms exist in brain, cerebrospinal fluid, and plasma. NCAM mediates cell adhesion through homophilic as well as through heterophilic interactions. Following NCAM binding, transmembrane signalling is believed to be activated, resulting in increased intracellular calcium. By mediating cell adhesion to other cells and to the extracellular matrix and by activating intracellular signaling pathways, NCAM influences cell migration, neurite extension, and fasciculation, and possibly formation of synapses in the brain. From studies on NCAM knock-out mice, NCAM have been shown to be crucial for the formation of the olfactory bulb and the mossy fiber system in the hippocampus. In addition, NCAM is important for neuronal plasticity in the adult brain associated with learning and regeneration.     

Protons are a neurotransmitter that regulates synaptic plasticity in the lateral amygdala

Stimulating presynaptic terminals can increase the proton concentration in synapses. Potential receptors for protons are acid-sensing ion channels (ASICs), Na⁺- and Ca²⁺-permeable channels that are activated by extracellular acidosis. Those observations suggest that protons might be a neurotransmitter. We found that presynaptic stimulation transiently reduced extracellular pH in the amygdala. The protons activated ASICs in lateral amygdala pyramidal neurons, generating excitatory postsynaptic currents. Moreover, both protons and ASICs were required for synaptic plasticity in lateral amygdala neurons. The results identify protons as a neurotransmitter, and they establish ASICs as the postsynaptic receptor. They also indicate that protons and ASICs are a neurotransmitter/receptor pair critical for amygdala-dependent learning and memory.Although homeostatic mechanisms generally maintain the brain’s extracellular pH within narrow limits, neural activity can induce transient and localized pH fluctuations. For example, acidification may occur when synaptic vesicles, which have a pH of ∼5.2–5.7 (1–3), release their contents into the synapse. Studies of mammalian cone photoreceptors showed that synaptic vesicle exocytosis rapidly reduced synaptic cleft pH by an estimated 0.2–0.6 units (4–6). Transient synaptic cleft acidification also occurred with GABAergic transmission (7). Some, but not all, studies also reported that high-frequency stimulation (HFS) transiently acidified hippocampal brain slices, likely as a result of the release of synaptic vesicle contents (8, 9). Neurotransmission also induces a slower, more prolonged alkalinization (10, 11). In addition to release of synaptic vesicle protons, neuronal and glial H⁺ and HCO₃⁻ transporters, channels, H⁺-ATPases, and metabolism might influence extracellular pH (10–12).ASICs are potential targets of reduced extracellular pH. ASICs are Na⁺-permeable and, to a lesser extent, Ca²⁺-permeable channels that are activated by extracellular acidosis (13–19). In the brain, ASICs consist of homotrimeric and heterotrimeric complexes of ASIC1a, ASIC2a, and ASIC2b. The ASIC1a subunit is required for acid-activation in the physiological range (>pH 5.0) (20, 21). Several observations indicate that ASIC are located postsynaptically. ASICs are located on dendritic spines. Although similar to glutamate receptors, they are also present on dendrites and cell bodies (20, 22–24). ASIC subunits interact with postsynaptic scaffolding proteins, including postsynaptic density protein 95 and protein interacting with C-kinase-1 (20, 24–29). In addition, ASICs are enriched in synaptosome-containing brain fractions (20, 24, 30).Although these observations raised the possibility that protons might be a neurotransmitter, postsynaptic ASIC currents have not been detected in cultured hippocampal neurons (31, 32), and whether localized pH transients might play a signaling role in neuronal communication remains unclear. In previous studies of hippocampal brain slices, extracellular field potential recordings suggested impaired hippocampal long-term potentiation (LTP) in ASIC1a^−/− mice (20), although another study did not detect an effect of ASIC1a (33). Another study using microisland cultures of hippocampal neurons suggested that the probability of neurotransmitter release increased in ASIC1a^−/− mice (32).Here, we tested the hypothesis that protons are a neurotransmitter and that ASICs are the receptor. Criteria to identify substances as neurotransmitters have been proposed (34). Beg and colleagues (35) used these criteria to conclude that protons are a transmitter released from Caenorhabditis elegans intestine to cause muscle contraction. Key questions about whether protons meet criteria for a neurotransmitter are: Does presynaptic stimulation increase the extracellular proton concentration? Do protons activate currents in postsynaptic cells? Can exogenously applied protons reproduce effects of endogenous protons? What is the postsynaptic proton receptor? We studied lateral amygdala brain slices because amygdala-dependent fear-related behavior depends on a pH reduction (36). In addition, ASICs are abundantly expressed there, and ASIC1a^−/− mice have impaired fear-like behavior (36–38).     

Nogo receptor 1 regulates formation of lasting memories

Formation of lasting memories is believed to rely on structural alterations at the synaptic level. We had found that increased neuronal activity down-regulates Nogo receptor-1 (NgR1) in brain regions linked to memory formation and storage, and postulated this to be required for formation of lasting memories. We now show that mice with inducible overexpression of NgR1 in forebrain neurons have normal long-term potentiation and normal 24-h memory, but severely impaired month-long memory in both passive avoidance and swim maze tests. Blocking transgene expression normalizes these memory impairments. Nogo, Lingo-1, Troy, endogenous NgR1, and BDNF mRNA expression levels were not altered by transgene expression, suggesting that the impaired ability to form lasting memories is directly coupled to inability to down-regulate NgR1. Regulation of NgR1 may therefore serve as a key regulator of memory consolidation. Understanding the molecular underpinnings of synaptic rearrangements that carry lasting memories may facilitate development of treatments for memory dysfunction.     

Short-term plasticity constrains spatial organization of a hippocampal presynaptic terminal

Although the CA3-CA1 synapse is critically important for learning and memory, experimental limitations have to date prevented direct determination of the structural features that determine the response plasticity. Specifically, the local calcium influx responsible for vesicular release and short-term synaptic facilitation strongly depends on the distance between the voltage-dependent calcium channels (VDCCs) and the presynaptic active zone. Estimates for this distance range over two orders of magnitude. Here, we use a biophysically detailed computational model of the presynaptic bouton and demonstrate that available experimental data provide sufficient constraints to uniquely reconstruct the presynaptic architecture. We predict that for a typical CA3-CA1 synapse, there are ～70 VDCCs located 300 nm from the active zone. This result is surprising, because structural studies on other synapses in the hippocampus report much tighter spatial coupling. We demonstrate that the unusual structure of this synapse reflects its functional role in short-term plasticity (STP).