Developmental changes in short-term facilitation are opposite at temporoammonic synapses compared to Schaffer collateral synapses onto CA1 pyramidal cells

Labile glutamate synaptic transmission in the adult CA1 stratum-lacunosum-moleculare region

The excitatory monosynaptic activation of hippocampal CA1 pyramidal cells is spatially segregated such that the proximal part of the apical dendritic tree in stratum radiatum (SR) receives input from the hippocampal CA3 region while the distal part in the stratum-lacunosum-moleculare (SLM) receives input mainly from the entorhinal cortex. The AMPA receptor-mediated (AMPA) signalling of SLM synapses in slices from neonatal rats was previously found to considerably differ from that of the SR synapses. In the present study, AMPA signalling of SLM synapses in 1-month-old rats has been examined, that is, when the hippocampus is essentially functionally mature. For the SR synapses, this time is characterized by a facilitatory shift in short-term plasticity, in the disappearance of labile postsynaptic AMPA signalling, a property thought to be important for early activity-dependent organization of neural circuits, and the expression of an adult form of long-term potentiation. We found that the SLM synapses alter their short-term plasticity similarly to that of the SR synapses. However, the labile postsynaptic AMPA signalling was not only maintained but substantially enhanced in the SLM synapses. The long-term potentiation observed was not of the adult form but like that of the neonatal SR synapses based on unsilencing of AMPA labile synapses. We propose that these features of the SLM synapses in the mature hippocampus will help to produce a flexible map of the multimodal sensory input reaching the SLM required for its conjunctive operation with the SR input to generate a proper functional output from the CA1 region.

LTP is Absent in the CA1 Region of the Hippocampus of Male and Female Rett Syndrome Mouse Models

Rett syndrome (RTT) is a debilitating neurodevelopmental disorder caused by mutations in the X-linked methyl-CpG-binding protein 2 (MeCP2) gene, resulting in severe deficits in learning and memory. Alterations in synaptic plasticity have been reported in RTT, however most electrophysiological studies have been performed in male mice only, despite the fact that RTT is primarily found in females. In addition, most studies have focused on excitation, despite the emerging evidence for the important role of inhibition in learning and memory. Here, we performed an electrophysiological characterization in the CA1 region of the hippocampus in both males and females of RTT mouse models with a focus on neurogliaform (NGF) interneurons, given that they are the most abundant dendrite-targeting interneuron subtype in the hippocampus. We found that theta-burst stimulation (TBS) failed to induce long-term potentiation (LTP) in either pyramidal neurons or NGF interneurons in male or female RTT mice, with no apparent changes in short-term plasticity (STP). This failure to induce LTP was accompanied by excitation/inhibition (E/I) imbalances and altered excitability, in a sex- and cell-type specific manner. Specifically, NGF interneurons of male RTT mice displayed increased intrinsic excitability, a depolarized resting membrane potential, and decreased E/I balance, while in female RTT mice, the resting membrane potential was depolarized. Understanding the role of NGF interneurons in RTT animal models is crucial for developing targeted treatments to improve cognition in individuals with this disorder.

Dendritic distribution of autophagosomes underlies pathway-selective induction of LTD

The mechanism of long-term depression (LTD), a cellular substrate for learning, memory, and behavioral flexibility, is extensively studied in Schaffer collateral (SC) synapses, with inhibition of autophagy identified as a key factor. SC inputs terminate at basal and proximal apical dendrites, whereas distal apical dendrites receive inputs from the temporoammonic pathway (TAP). Here, we demonstrate that TAP and SC synapses have a shared LTD mechanism reliant on NMDA receptors, caspase-3, and autophagy inhibition. Despite this shared LTD mechanism, proximal apical dendrites contain more autophagosomes than distal apical dendrites. Additionally, unlike SC LTD, which diminishes with age, TAP LTD persists into adulthood. Our previous study shows that the high autophagy in adulthood disallows SC LTD induction. The reduction of autophagosomes from proximal to distal dendrites, combined with distinct LTD inducibility at SC and TAP synapses, suggests a model where the differential distribution of autophagosomes in dendrites gates LTD inducibility at specific circuits.

Hippocampus: Molecular, Cellular, and Circuit Features in Anxiety

Anxiety disorders are currently a major psychiatric and social problem, the mechanisms of which have been only partially elucidated. The hippocampus serves as a major target of stress mediators and is closely related to anxiety modulation. Yet so far, its complex anatomy has been a challenge for research on the mechanisms of anxiety regulation. Recent advances in imaging, virus tracking, and optogenetics/chemogenetics have permitted elucidation of the activity, connectivity, and function of specific cell types within the hippocampus and its connected brain regions, providing mechanistic insights into the elaborate organization of the hippocampal circuitry underlying anxiety. Studies of hippocampal neurotransmitter systems, including glutamatergic, GABAergic, cholinergic, dopaminergic, and serotonergic systems, have contributed to the interpretation of the underlying neural mechanisms of anxiety. Neuropeptides and neuroinflammatory factors are also involved in anxiety modulation. This review comprehensively summarizes the hippocampal mechanisms associated with anxiety modulation, based on molecular, cellular, and circuit properties, to provide tailored targets for future anxiety treatment.

Maturation of nucleus accumbens synaptic transmission signals a critical period for the rescue of social deficits in a mouse model of autism spectrum disorder

Social behavior emerges early in development, a time marked by the onset of neurodevelopmental disorders featuring social deficits, including autism spectrum disorder (ASD). Although social deficits are at the core of the clinical diagnosis of ASD, very little is known about their neural correlates at the time of clinical onset. The nucleus accumbens (NAc), a brain region extensively implicated in social behavior, undergoes synaptic, cellular and molecular alterations in early life, and is particularly affected in ASD mouse models. To explore a link between the maturation of the NAc and neurodevelopmental deficits in social behavior, we compared spontaneous synaptic transmission in NAc shell medium spiny neurons (MSNs) between the highly social C57BL/6J and the idiopathic ASD mouse model BTBR T⁺Itpr3^tf/J at postnatal day (P) 4, P6, P8, P12, P15, P21 and P30. BTBR NAc MSNs display increased spontaneous excitatory transmission during the first postnatal week, and increased inhibition across the first, second and fourth postnatal weeks, suggesting accelerated maturation of excitatory and inhibitory synaptic inputs compared to C57BL/6J mice. BTBR mice also show increased optically evoked medial prefrontal cortex-NAc paired pulse ratios at P15 and P30. These early changes in synaptic transmission are consistent with a potential critical period, which could maximize the efficacy of rescue interventions. To test this, we treated BTBR mice in either early life (P4-P8) or adulthood (P60-P64) with the mTORC1 antagonist rapamycin, a well-established intervention for ASD-like behavior. Rapamycin treatment rescued social interaction deficits in BTBR mice when injected in infancy, but did not affect social interaction in adulthood.

The properties of long-term potentiation at SC-CA1/ TA-CA1 hippocampal synaptic pathways depends upon their input pathway activation patterns

Long-term potentiation (LTP) has been considered as a cellular mechanism of memory. Since the Schaffer collateral (SC) and temporoammonic (TA) inputs to CA1 are distinct synaptic pathways that could mediate different cognitive functions, this study was therefore aimed to separately study and compare the properties of LTP of these two synaptic pathways. In the current study we used slice electrophysiological methods to compare various properties of these two synaptic pathways in response to single, paired pulse stimulation, and to three standard protocols for inducing LTP: the high frequency electrical stimulation (HFS), theta-burst (TBS), and primed burst (PBs) stimulation. We found that the SC-CA1 synapses could produce bigger maximum synaptic responses than TA-CA1 synapses. In addition, we showed that paired-pulse ratios of the SC-CA1 synapses were higher than TA-CA1 synapses at certain inter-pulses intervals. Finally, we showed a higher LTP% was induced by PBs or TBS at the SC-CA1 synapse than the TA-CA1 synapse. Briefly, our findings suggest the differential basal synaptic transmission, paired-pulse evoked synaptic responses, and LTP exhibition of the hippocampal SC-CA1/ TA-CA1 synaptic pathways, which may rely on spontaneous and evoked activity pattern at the local circuit level.

Rapid presynaptic maturation in naturally regenerating axons of the adult mouse olfactory nerve

Successful neuronal regeneration requires the reestablishment of synaptic connectivity. This process requires the reconstitution of presynaptic neurotransmitter release, which we investigate here in a model of entirely natural regeneration. After toxin-induced injury, olfactory sensory neurons in the adult mouse olfactory epithelium can regenerate fully, sending axons via the olfactory nerve to reestablish synaptic contact with postsynaptic partners in the olfactory bulb. Using electrophysiological recordings in acute slices, we find that, after initial recontact, functional connectivity in this system is rapidly established. Reconnecting presynaptic terminals have almost mature functional properties, including high release probability and strong capacity for presynaptic inhibition. Release probability then matures quickly, rendering reestablished terminals functionally indistinguishable from controls just 1 week after initial contact. These data show that successful synaptic regeneration in the adult mammalian brain is almost a "plug-and-play" process, with presynaptic terminals undergoing a rapid phase of functional maturation as they reintegrate into target networks.

Bicuculline restores frequency-dependent hippocampal I/E ratio and circuit function in PGC-1ɑ null mice

Altered inhibition/excitation (I/E) balance contributes to various brain disorders. Dysfunctional GABAergic interneurons enhance or reduce inhibition, resulting in I/E imbalances. Differences in short-term plasticity between excitation and inhibition cause frequency-dependence of the I/E ratio, which can be altered by GABAergic dysfunction. However, it is unknown whether I/E imbalances can be rescued pharmacologically using a single dose when the imbalance magnitude is frequency-dependent. Loss of PGC-1α (peroxisome proliferator activated receptor γ coactivator 1α) causes transcriptional dysregulation in hippocampal GABAergic interneurons. PGC-1α-/- slices have enhanced baseline inhibition onto CA1 pyramidal cells, causing increased I/E ratio and impaired circuit function. High frequency stimulation reduces the I/E ratio and recovers circuit function in PGC-1α-/- slices. Here we tested if using a low dose of bicuculline that can restore baseline I/E ratio can also rescue the frequency-dependent I/E imbalances in these mice. Remarkably, bicuculline did not reduce the I/E ratio below that of wild type during high frequency stimulation. Interestingly, bicuculline enhanced the paired-pulse ratio (PPR) of disynaptic inhibition without changing the monosynaptic inhibition PPR, suggesting that bicuculline modifies interneuron recruitment and not GABA release. Bicuculline improved CA1 output in PGC-1α-/- slices, enhancing EPSP-spike coupling to wild type levels at high and low frequencies. Our results show that it is possible to rescue frequency-dependent I/E imbalances in an animal model of transcriptional dysregulation with a single treatment.

Leptin regulation of synaptic function at hippocampal TA-CA1 and SC-CA1 synapses

Increasing evidence indicates that the metabolic hormone, leptin markedly influences the functioning of the hippocampus. In particular, exposure to leptin results in persistent changes in synaptic efficacy at both temporoammonic (TA) and Schaffer Collateral (SC) inputs to hippocampal CA1 neurons. The ability of leptin to regulate TA-CA1 and SC-CA1 synapses has important functional implications, as both synaptic connections play important roles in hippocampal-dependent learning and memory. Here we review the modulatory actions of the hormone leptin at these hippocampal CA1 synapses and explore the impact on learning and memory processes.

Homosynaptic frequency-dependent depression by release site inactivation at neonatal hippocampal synapses in the stratum lacunosum-moleculare

When activated at low frequencies (0.1-1 Hz), second postnatal week synapses onto the most distal part of the apical dendritic tree (stratum lacunosum-moleculare) of rat hippocampal CA1 pyramidal cells display a frequency-dependent synaptic depression not observed for the more proximal (stratum radiatum) synapses. Depression in this frequency range is thought of as a possible contributor to behavioural habituation. In fact, in contrast to the proximal synapses, the distal synapses provide more direct sensory information from the entorhinal cortex as well as from thalamic nuclei. The use of antagonists showed that the activation of GABA_A , GABA_B , NMDA, mGlu, kainate, adenosine, or endocannabinoid receptors was not directly involved in the depression, indicating it to be intrinsic to the synapses themselves. While the depression affected paired-pulse plasticity in a manner indicating a decrease in vesicle release probability, the depression could not be explained by a stimulus-dependent decrease in calcium influx. Despite affecting the synaptic response evoked by brief high-frequency stimulation (10 impulses, 20 Hz) in a manner indicating vesicle depletion, the depression was unaffected by large variations in release probability. The depression was found not only to affect the synaptic transmission at low frequencies (0.1-1 Hz) but also to contribute to the depression evolving during brief high-frequency stimulation (10 impulses, 20 Hz). We propose that a release-independent process directly inactivating release sites with a fast onset (ms) and long duration (up to 20 s) underlies this synaptic depression.

Frequency-dependent layer-specific differences in short-term synaptic plasticity in the dorsal and ventral CA1 hippocampal field

Information from the entorhinal cortex arrives to the hippocampal CA1 microcircuit directly through the temporoammonic path (TA) that terminates in the stratum lacunosum-moleculare (SLM), and indirectly through Schaffer collateral pathway (SC) that terminates in the stratum radiatum (SR). By virtue of this input convergence, CA1 circuitry may act to compare and integrate incoming cortical information. Although a remarkable dorsal-ventral difference in short-term plasticity (STP) has been recently described at SC-CA1 synapses, the corresponding properties at TA-CA1 synapses have not been examined. Here, we report that stimulation of TA in the dorsal hippocampus produces significant facilitation of all conditioned responses evoked by 1-30 Hz, peaking at 20-30 Hz, and significant depression of steady-state responses to 50-100 Hz. Dorsal SC-CA1 synapses display a similar pattern of responses, yet, facilitation peaked at 10 Hz and depression (at 75-100 Hz) is weaker. Strikingly, stimulation of TA in the ventral hippocampus produces facilitation of steady-state responses to 1-30 Hz and highly contrasts with the depression of SC-CA1 synapses. Steady-state responses to 40-100 Hz in the ventral hippocampus depress in both layers similarly. High-frequency TA input (40-100 Hz) to the dorsal hippocampus depresses more in proximal than in distal SLM, while low-frequency (1-3 Hz) TA input to the ventral hippocampus facilitates more in distal than in proximal SLM. The present evidence suggests that direct and indirect entorhinal cortical inputs across the septotemporal extent of hippocampal CA1 field display frequency selectivity both in the radial and transverse axes, and that a rapid information processing may take place through direct ventral hippocampal CA1-EC circuit interactions independently of trisynaptic circuit.

Increased glutamate transmission onto dorsal striatum spiny projection neurons in Pink1 knockout rats

Loss-of-function PTEN Induced Kinase 1 (PINK1) mutations cause early-onset familial Parkinson's disease (PD) with similar clinical and neuropathological characteristics as idiopathic PD. While Pink1 knockout (KO) rats have mitochondrial dysfunction, locomotor deficits, and α-synuclein aggregates in several brain regions such as cerebral cortex, dorsal striatum, and substantia nigra, the functional ramifications on synaptic circuits are unknown. Using whole cell patch clamp recordings, we found a significant increase in the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) onto striatal spiny projection neurons (SPNs) in Pink1 KO rats at ages 4 and 6 months compared to wild-type (WT) littermates, suggesting increased excitability of presynaptic neurons. While sEPSC amplitudes were also increased at 2 and 4 months, no changes were observed in AMPAR/NMDAR ratio or receptor expression. Further analysis revealed increased glutamate release probability and decreased recovery of the synaptic vesicle pool following a train of stimulation in Pink1 KO rats. Ultrastructural analysis revealed increased excitatory and inhibitory synapse number and increased levels of presynaptic α-synuclein, while the number and structure of striatal mitochondria appeared normal. Lastly, we found that Pink1 KO rats have altered striatal dopamine tone, which together with the abnormal α- synuclein distribution and dysfunctional mitochondria, could contribute to the increase in excitatory transmission. Together, these studies show that PINK1 is necessary for normal glutamatergic transmission onto striatal SPNs and reveal possible mechanisms underlying striatal circuit dysfunction in PD.

Proximodistal Heterogeneity in Learning-promoted Pathway-specific Plasticity at Dorsal CA1 Synapses

Contextual learning requires the delivery of AMPA receptors to CA1 synapses in the dorsal hippocampus. However, proximodistal heterogeneity of pathway-specific plasticity remains unclear. Here, we examined the proximodistal heterogeneity in learning-induced plasticity at the CA1 synapses with inputs from the entorhinal cortex layer III (ECIII) or from CA3. We subjected male rats to an inhibitory avoidance task and prepared acute hippocampal slices for whole-cell patch clamp experiments, where we stimulated ECIII-CA1 or CA3-CA1 input fibers to analyze evoked excitatory postsynaptic currents (EPSCs). Compared to untrained controls, trained rats exhibited higher AMPA/NMDA current ratios at CA3-CA1 synapses of proximal and intermediate, but not distal CA1 neurons, which suggested that region-specific plasticity occurred after learning. Moreover, trained rats exhibited higher AMPA/NMDA current ratios at ECIII-CA1 synapses of intermediate and distal, but not proximal CA1 neurons. These findings suggested the presence of proximodistal heterogeneity in pathway-specific postsynaptic plasticity. Regarding presynaptic plasticity, training slightly, but significantly increased the paired-pulse ratios of CA3-CA1 synapses of proximal and intermediate, but not distal CA1 neurons. Moreover, trained rats exhibited higher paired-pulse ratios at ECIII-CA1 synapses of intermediate and distal, but not proximal CA1 neurons, which suggested region-specific presynaptic plasticity. Finally, learning was clearly prevented by the bilateral microinjection of a plasticity blocker in the proximal or intermediate, but not distal CA1 subfields, which suggested functional heterogeneity along the proximodistal axis. Understanding region- and pathway-specific plasticity at dorsal CA1 synapses could aid in controlling encoded memory.

Cellular and Molecular Changes in Hippocampal Glutamate Signaling and Alterations in Learning, Attention, and Impulsivity Following Prenatal Nicotine Exposure

Over 70 million European pregnant women are smokers during their child-bearing years. Consumption of tobacco-containing products during pregnancy is associated with several negative behavioral outcomes for the offspring, including a higher susceptibility for the development of attention-deficit/hyperactive disorder (ADHD). In efforts to minimize fetal exposure to tobacco smoke, many women around the world switch to nicotine replacement therapies (NRTs) during the gestational period; however, prenatal nicotine exposure (PNE) in any form has been associated with alterations in cognitive processes, including learning, memory, and attention. These processes are controlled by glutamatergic signaling of hippocampal pyramidal neurons within the CA1 region, suggesting actions of nicotine on glutamatergic transmission in this region if present prenatally. Accordingly, we aimed to investigate hippocampal glutamatergic function following PNE treatment in NMRI mice employing molecular, cellular electrophysiology, and pharmacological approaches, as well as to evaluate cognition in the rodent continuous performance task (rCPT), a recently developed mouse task allowing assessment of learning, attention, and impulsivity. PNE induced increases in the expression levels of mRNA coding for different glutamate receptors and subunits within the hippocampus. Functional alterations in AMPA and NMDA receptors on CA1 pyramidal neurons of PNE mice were suggestive of higher GluA2-lacking and lower GluN2A-containing receptors, respectively. Finally, PNE was associated with reduced learning, attention, and enhanced impulsivity in the rCPT. Alterations in glutamatergic functioning in CA1 neurons parallel changes seen in the spontaneously hypertensive rat ADHD model and likely contribute to the lower cognitive performance in the rCPT.

Differential expression of SV2A in hippocampal glutamatergic and GABAergic terminals during postnatal development

The function of synaptic vesicle protein 2A (SV2A) has not been clearly identified, although it has an essential role in normal neurotransmission. Changes in SV2A expression have been linked to several diseases that could implicate an imbalance between excitation and inhibition, such as epilepsy. Although it is known that SV2A expression is necessary for survival, SV2A expression and its relationship with γ-aminobutyric acid (GABA) and glutamate neurotransmitter systems along development has not been addressed. This report follows SV2A expression levels in the rat hippocampus and their association with glutamatergic and GABAergic terminals along postnatal development. Total SV2A expression was assessed by real time PCR and western blot, while immunofluorescence was used to identify SV2A protein in the different hippocampal layers and its co-localization with GABA or glutamate vesicular transporters. SV2A was dynamically regulated along development and its association with GABA or glutamate transporters varied in the different hippocampal layers. In the principal cells layers (granular and pyramidal), SV2A protein was preferentially localized to GABAergic terminals, while in the hilus and stratum lucidum SV2A was associated mainly to glutamatergic terminals. Although SV2A was ubiquitously expressed in the entire hippocampus, it established a differential association with excitatory or inhibitory terminals, which could contribute to the maturation of excitatory/inhibitory balance.

Leptin Regulation of Synaptic Function at Hippocampal TA-CA1 and SC-CA1 Synapses: Implications for Health and Disease

Growing evidence indicates that the endocrine hormone leptin regulates hippocampal synaptic function in addition to its established role as a hypothalamic satiety signal. Indeed, numerous studies show that leptin facilitates the cellular events that underlie hippocampal learning and memory including activity-dependent synaptic plasticity and glutamate receptor trafficking, indicating that leptin may be a potential cognitive enhancer. Although there has been extensive investigation into the modulatory role of leptin at hippocampal Schaffer collateral (SC)-CA1 synapses, recent evidence indicates that leptin also potently regulates excitatory synaptic transmission at the anatomically distinct temporoammonic (TA) input to hippocampal CA1 neurons. The cellular mechanisms underlying activity-dependent synaptic plasticity at TA-CA1 synapses differ from those at SC-CA1 synapses and the TA input is implicated in spatial and episodic memory formation. Furthermore, the TA input is an early target for neurodegeneration in Alzheimer's disease (AD) and aberrant leptin function is linked to AD. Here, we review the evidence that leptin regulates hippocampal synaptic function at both SC- and TA-CA1 synapses and discuss the consequences for neurodegenerative disorders like AD.

Target-cell-specific Short-term Plasticity Reduces the Excitatory Drive onto CA1 Interneurons Relative to Pyramidal Cells During Physiologically-derived Spike Trains

Short-term plasticity enables synaptic strength to be dynamically regulated by input timing. Excitatory synapses arising from the same axon can have profoundly different presynaptic forms of short-term plasticity onto inhibitory and excitatory neurons. We previously showed that Schaffer collateral synapses onto most hippocampal CA1 stratum radiatum interneurons have less paired-pulse facilitation than synapses onto CA1 pyramidal cells, but little difference in steady-state short-term depression. However, less is known about how synapses onto interneurons respond to temporally complex patterns that occur in vivo. Here we compared Schaffer collateral synapses onto stratum radiatum interneurons and pyramidal cells in acute hippocampal slices in response to physiologically-derived spike trains. We find that synapses onto interneurons have less short-term facilitation than synapses onto pyramidal cells, and a subset expresses only short-term depression. Mathematical modeling predicts this target cell-specific short-term plasticity occurs through differences in initial release probability. All three groups have more short-term facilitation during physiologically-derived train stimulation than during constant-frequency stimulation at the same frequency, indicating that variability in stimulus timing is important. These target-cell specific differences in short-term plasticity reduce the strength of excitatory input onto interneurons relative to pyramidal cells, and of depression interneurons relative to facilitation interneurons, during high frequency portions of the train. This occurs to a similar extent at 25 °C and at 33 °C, and is even greater at physiological extracellular calcium. Target-cell specific differences in short-term plasticity enable synapses to have different temporal filtering characteristics, which may help to dynamically regulate the balance of inhibition and excitation in CA1.

Prominent facilitation at beta and gamma frequency range revealed with physiological calcium concentration in adult mouse piriform cortex in vitro

Neuronal activity is characterized by a diversity of oscillatory phenomena that are associated with multiple behavioral and cognitive processes, yet the functional consequences of these oscillations are not fully understood. Our aim was to determine whether and how these different oscillatory activities affect short-term synaptic plasticity (STP), using the olfactory system as a model. In response to odorant stimuli, the olfactory bulb displays a slow breathing rhythm as well as beta and gamma oscillations. Since the firing of olfactory bulb projecting neurons is phase-locked with beta and gamma oscillations, structures downstream from the olfactory bulb should be driven preferentially at these frequencies. We examined STP exhibited by olfactory bulb inputs in slices of adult mouse piriform cortex maintained in vitro in an in vivo-like ACSF (calcium concentration: 1.1 mM). We replaced the presynaptic neuronal firing rate by repeated electrical stimulation (frequency between 3.125 and 100 Hz) applied to the lateral olfactory tract. Our results revealed a considerable enhancement of postsynaptic response amplitude for stimulation frequencies in the beta and gamma range. A phenomenological model of STP fitted to the data suggests that the experimental results can be explained by the interplay between three mechanisms: a short-term facilitation mechanism (time constant ≈160 msec), and two short-term depression mechanisms (recovery time constants <20 msec and ≈140 msec). Increasing calcium concentration (2.2 mM) resulted in an increase in the time constant of facilitation and in a strengthening of the slowest depression mechanism. As a result, response enhancement was reduced and its peak shifted toward the low beta and alpha ranges while depression became predominant in the gamma band. Using environmental conditions corresponding to those that prevail in vivo, our study shows that STP in the lateral olfactory tract to layer Ia synapse allows amplification of olfactory bulb inputs at beta and gamma frequencies.

SNAP-25 phosphorylation at Ser187 regulates synaptic facilitation and short-term plasticity in an age-dependent manner

Neurotransmitter release is mediated by the SNARE complex, but the role of its phosphorylation has scarcely been elucidated. Although PKC activators are known to facilitate synaptic transmission, there has been a heated debate on whether PKC mediates facilitation of neurotransmitter release through phosphorylation. One of the SNARE proteins, SNAP-25, is phosphorylated at the residue serine-187 by PKC, but its physiological significance has been unclear. To examine these issues, we analyzed mutant mice lacking the phosphorylation of SNAP-25 serine-187 and found that they exhibited reduced release probability and enhanced presynaptic short-term plasticity, suggesting that not only the release process, but also the dynamics of synaptic vesicles was regulated by the phosphorylation. Furthermore, it has been known that the release probability changes with development, but the precise mechanism has been unclear, and we found that developmental changes in release probability of neurotransmitters were regulated by the phosphorylation. These results indicate that SNAP-25 phosphorylation developmentally facilitates neurotransmitter release but strongly inhibits presynaptic short-term plasticity via modification of the dynamics of synaptic vesicles in presynaptic terminals.

Endogenously Released Neuropeptide Y Suppresses Hippocampal Short-Term Facilitation and Is Impaired by Stress-Induced Anxiety

Neuropeptide Y (NPY) has robust anxiolytic properties and is reduced in patients with anxiety disorders. However, the mechanisms by which NPY modulates circuit function to reduce anxiety behavior are not known. Anxiolytic effects of NPY are mediated in the CA1 region of hippocampus, and NPY injection into hippocampus alleviates anxiety symptoms in the predator scent stress model of stress-induced anxiety. The mechanisms that regulate NPY release, and its effects on CA1 synaptic function, are not fully understood. Here we show in acute hippocampal slices from mice that endogenous NPY, released in response to optogenetic stimulation or synaptically evoked spiking of NPY+ cells, suppresses both of the feedforward pathways to CA1. Stimulation of temporoammonic synapses with a physiologically derived spike train causes NPY release that reduces short-term facilitation, whereas the release of NPY that modulates Schaffer collateral synapses requires integration of both the Schaffer collateral and temporoammonic pathways. Pathway specificity of NPY release is conferred by three functionally distinct NPY+ cell types, with differences in intrinsic excitability and short-term plasticity of their inputs. Predator scent stress abolishes the release of endogenous NPY onto temporoammonic synapses, a stress-sensitive pathway, thereby causing enhanced short-term facilitation. Our results demonstrate how stress alters CA1 circuit function through the impairment of endogenous NPY release, potentially contributing to heightened anxiety.

SIGNIFICANCE STATEMENT

Neuropeptide Y (NPY) has robust anxiolytic properties, and its levels are reduced in patients with post-traumatic stress disorder. The effects of endogenously released NPY during physiologically relevant stimulation, and the impact of stress-induced reductions in NPY on circuit function, are unknown. By demonstrating that NPY release modulates hippocampal synaptic plasticity and is impaired by predator scent stress, our results provide a novel mechanism by which stress-induced anxiety alters circuit function. These studies fill an important gap in knowledge between the molecular and behavioral effects of NPY. This article also advances the understanding of NPY+ cells and the factors that regulate their spiking, which could pave the way for new therapeutic targets to increase endogenous NPY release in patients in a spatially and temporally appropriate manner.

Klotho regulates CA1 hippocampal synaptic plasticity

Global klotho overexpression extends lifespan while global klotho-deficiency shortens it. As well, klotho protein manipulations inversely regulate cognitive function. Mice without klotho develop rapid onset cognitive impairment before they are 2months old. Meanwhile, adult mice overexpressing klotho show enhanced cognitive function, particularly in hippocampal-dependent tasks. The cognitive enhancing effects of klotho extend to humans with a klotho polymorphism that increases circulating klotho and executive function. To affect cognitive function, klotho could act in or on the synapse to modulate synaptic transmission or plasticity. However, it is not yet known if klotho is located at synapses, and little is known about its effects on synaptic function. To test this, we fractionated hippocampi and detected klotho expression in both pre and post-synaptic compartments. We find that loss of klotho enhances both pre and post-synaptic measures of CA1 hippocampal synaptic plasticity at 5weeks of age. However, a rapid loss of synaptic enhancement occurs such that by 7weeks, when mice are cognitively impaired, there is no difference from wild-type controls. Klotho overexpressing mice show no early life effects on synaptic plasticity, but decreased CA1 hippocampal long-term potentiation was measured at 6months of age. Together these data suggest that klotho affects cognition, at least in part, by regulating hippocampal synaptic plasticity.

Sphingosine 1-phosphate lyase ablation disrupts presynaptic architecture and function via an ubiquitin- proteasome mediated mechanism

The bioactive lipid sphingosine 1-phosphate (S1P) is a degradation product of sphingolipids that are particularly abundant in neurons. We have shown previously that neuronal S1P accumulation is toxic leading to ER-stress and an increase in intracellular calcium. To clarify the neuronal function of S1P, we generated brain-specific knockout mouse models in which S1P-lyase (SPL), the enzyme responsible for irreversible S1P cleavage was inactivated. Constitutive ablation of SPL in the brain (SPL^fl/fl/Nes) but not postnatal neuronal forebrain-restricted SPL deletion (SPL^fl/fl/CaMK) caused marked accumulation of S1P. Hence, altered presynaptic architecture including a significant decrease in number and density of synaptic vesicles, decreased expression of several presynaptic proteins, and impaired synaptic short term plasticity were observed in hippocampal neurons from SPL^fl/fl/Nes mice. Accordingly, these mice displayed cognitive deficits. At the molecular level, an activation of the ubiquitin-proteasome system (UPS) was detected which resulted in a decreased expression of the deubiquitinating enzyme USP14 and several presynaptic proteins. Upon inhibition of proteasomal activity, USP14 levels, expression of presynaptic proteins and synaptic function were restored. These findings identify S1P metabolism as a novel player in modulating synaptic architecture and plasticity.

Sex Differences in Long-Term Potentiation at Temporoammonic-CA1 Synapses: Potential Implications for Memory Consolidation

Sex differences in spatial memory have long been observed in humans, non-human primates and rodents, but the underlying cellular and molecular mechanisms responsible for these differences remain obscure. In the present study we found that adolescent male rats outperformed female rats in 7 d and 28 d retention probes, but not in learning trials and immediate probes, in the Morris water maze task. Male rats also had larger long-term potentiation (LTP) at hippocampal temproammonic-CA1 (TA-CA1) synapses, which have been implicated to play a key role in place field and memory consolidation, when protocols designed to elicit late-stage LTP (LLTP) were used. Interestingly, the ratio of evoked AMPA/NMDA currents was found to be smaller at TA-CA1 synapses in male rats compared to female rats. Protein biotinylation experiments showed that male rats expressed more surface GluN1 receptors in hippocampal CA1 stratum lacunosum-moleculare (SLM) than female rats, although GluA1 expression was also slightly higher in male rats. Taken together, our results suggest that differences in the expression of AMPA and NMDA receptors may affect LTP expression at TA-CA1 synapses in adolescent male and female rats, and thus possibly contribute to the observed sex difference in spatial memory.

Long-term Potentiation at Temporoammonic Path-CA1 Synapses in Freely Moving Rats

Hippocampal area CA1 receives direct entorhinal layer III input via the temporoammonic path (TAP) and recent studies implicate TAP-CA1 synapses are important for some aspects of hippocampal memory function. Nonetheless, as few studies have examined TAP-CA1 synaptic plasticity in vivo, the induction and longevity of TAP-CA1 long-term potentiation (LTP) has not been fully characterized. We analyzed CA1 responses following stimulation of the medial aspect of the angular bundle and investigated LTP at medial temporoammonic path (mTAP)-CA1 synapses in freely moving rats. We demonstrate monosynaptic mTAP-CA1 responses can be isolated in vivo as evidenced by observations of independent current sinks in the stratum lacunosum moleculare of both areas CA1 and CA3 following angular bundle stimulation. Contrasting prior indications that TAP input rarely elicits CA1 discharge, we observed mTAP-CA1 responses that appeared to contain putative population spikes in 40% of our behaving animals. Theta burst high frequency stimulation of mTAP afferents resulted in an input specific and N-methyl-D-aspartate (NMDA) receptor-dependent LTP of mTAP-CA1 responses in behaving animals. LTP of mTAP-CA1 responses decayed as a function of two exponential decay curves with time constants (τ) of 2.7 and 148 days to decay 63.2% of maximal LTP. In contrast, mTAP-CA1 population spike potentiation longevity demonstrated a τ of 9.6 days. To our knowledge, these studies provide the first description of mTAP-CA1 LTP longevity in vivo. These data indicate TAP input to area CA1 is a physiologically relevant afferent system that displays robust synaptic plasticity.

Short-term plasticity regulates the excitation/inhibition ratio and the temporal window for spike integration in CA1 pyramidal cells

Many neurodevelopmental and neuropsychiatric disorders involve an imbalance between excitation and inhibition caused by synaptic alterations. The proper excitation/inhibition (E/I) balance is especially critical in CA1 pyramidal cells, because they control hippocampal output. Activation of Schaffer collateral axons causes direct excitation of CA1 pyramidal cells, quickly followed by disynaptic feedforward inhibition, stemming from synaptically induced firing of GABAergic interneurons. Both excitatory and inhibitory synapses are modulated by short-term plasticity, potentially causing dynamic tuning of the E/I ratio. However, the effects of short-term plasticity on the E/I ratio in CA1 pyramidal cells are not yet known. To determine this, we recorded disynaptic inhibitory postsynaptic currents and the E/I ratio in CA1 pyramidal cells in acute hippocampal slices from juvenile mice. We found that, whereas inhibitory synapses had paired-pulse depression, disynaptic inhibition instead had paired-pulse facilitation (≤ 200-ms intervals), caused by increased recruitment of feedforward interneurons. Although enhanced disynaptic inhibition helped to constrain paired-pulse facilitation of excitation, the E/I ratio was still larger on the second pulse, increasing pyramidal cell spiking. Surprisingly, this occurred without compromising the precision of spike timing. The E/I balance regulates the temporal spike integration window from multiple inputs; here, we showed that paired-pulse stimulation can broaden the spike integration window. Together, our findings show that the combined effects of short-term plasticity of disynaptic inhibition and monosynaptic excitation alter the E/I balance in CA1 pyramidal cells, leading to dynamic modulation of spike probability and the spike integration window. Short-term plasticity is therefore an important mechanism for modulating signal processing of hippocampal output.

Mice lacking the transcriptional coactivator PGC-1α exhibit alterations in inhibitory synaptic transmission in the motor cortex

Peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) is a transcriptional coactivator known to regulate gene programs in a cell-specific manner in energy-demanding tissues, and its dysfunction has been implicated in numerous neurological and psychiatric disorders. Previous work from the Cowell laboratory indicates that PGC-1α is concentrated in inhibitory interneurons and is required for the expression of the calcium buffer parvalbumin (PV) in the cortex; however, the impact of PGC-1α deficiency on inhibitory neurotransmission in the motor cortex is not known. Here, we show that mice lacking PGC-1α exhibit increased amplitudes and decreased frequency of spontaneous inhibitory postsynaptic currents in layer V pyramidal neurons. Upon repetitive train stimulation at the gamma frequency, decreased GABA release is observed. Furthermore, PV-positive interneurons in PGC-1α -/- mice display reductions in intrinsic excitability and excitatory input without changes in gross interneuron morphology. Taken together, these data show that PGC-1α is required for normal inhibitory neurotransmission and cortical PV-positive interneuron function. Given the pronounced motor dysfunction in PGC-1α -/- mice and the essential role of PV-positive interneurons in maintenance of cortical excitatory:inhibitory balance, it is possible that deficiencies in PGC-1α expression could contribute to cortical hyperexcitability and motor abnormalities in multiple neurological disorders.

Early structural and functional defects in synapses and myelinated axons in stratum lacunosum moleculare in two preclinical models for tauopathy

The stratum lacunosum moleculare (SLM) is the connection hub between entorhinal cortex and hippocampus, two brain regions that are most vulnerable in Alzheimer's disease. We recently identified a specific synaptic deficit of Nectin-3 in transgenic models for tauopathy. Here we defined cognitive impairment and electrophysiological problems in the SLM of Tau.P301L mice, which corroborated the structural defects in synapses and dendritic spines. Reduced diffusion of DiI from the ERC to the hippocampus indicated defective myelinated axonal pathways. Ultrastructurally, myelinated axons in the temporoammonic pathway (TA) that connects ERC to CA1 were damaged in Tau.P301L mice at young age. Unexpectedly, the myelin defects were even more severe in bigenic biGT mice that co-express GSK3β with Tau.P301L in neurons. Combined, our data demonstrate that neuronal expression of protein Tau profoundly affected the functional and structural organization of the entorhinal-hippocampal complex, in particular synapses and myelinated axons in the SLM. White matter pathology deserves further attention in patients suffering from tauopathy and Alzheimer's disease.

A catalytic independent function of the deubiquitinating enzyme USP14 regulates hippocampal synaptic short-term plasticity and vesicle number

The ubiquitin proteasome system is required for the rapid and precise control of protein abundance that is essential for synaptic function. USP14 is a proteasome-bound deubiquitinating enzyme that recycles ubiquitin and regulates synaptic short-term synaptic plasticity. We previously reported that loss of USP14 in ax(J) mice causes a deficit in paired pulse facilitation (PPF) at hippocampal synapses. Here we report that USP14 regulates synaptic function through a novel, deubiquitination-independent mechanism. Although PPF is usually inversely related to release probability, USP14 deficiency impairs PPF without altering basal release probability. Instead, the loss of USP14 causes a large reduction in the number of synaptic vesicles. Over-expression of a catalytically inactive form of USP14 rescues the PPF deficit and restores synaptic vesicle number, indicating that USP14 regulates presynaptic structure and function independently of its role in deubiquitination. Finally, the PPF deficit caused by loss of USP14 can be rescued by pharmacological inhibition of proteasome activity, suggesting that inappropriate protein degradation underlies the PPF impairment. Overall, we demonstrate a novel, deubiquitination-independent function for USP14 in influencing synaptic architecture and plasticity.

The temporoammonic input to the hippocampal CA1 region displays distinctly different synaptic plasticity compared to the Schaffer collateral input in vivo: significance for synaptic information processing

In terms of its sub-regional differentiation, the hippocampal CA1 region receives cortical information directly via the perforant (temporoammonic) path (pp-CA1 synapse) and indirectly via the tri-synaptic pathway where the last relay station is the Schaffer collateral-CA1 synapse (Sc-CA1 synapse). Research to date on pp-CA1 synapses has been conducted predominantly in vitro and never in awake animals, but these studies hint that information processing at this synapse might be distinct to processing at the Sc-CA1 synapse. Here, we characterized synaptic properties and synaptic plasticity at the pp-CA1 synapse of freely behaving adult rats. We observed that field excitatory postsynaptic potentials at the pp-CA1 synapse have longer onset latencies and a shorter time-to-peak compared to the Sc-CA1 synapse. LTP (>24 h) was successfully evoked by tetanic afferent stimulation of pp-CA1 synapses. Low frequency stimulation evoked synaptic depression at Sc-CA1 synapses, but did not elicit LTD at pp-CA1 synapses unless the Schaffer collateral afferents to the CA1 region had been severed. Paired-pulse responses also showed significant differences. Our data suggest that synaptic plasticity at the pp-CA1 synapse is distinct from the Sc-CA1 synapse and that this may reflect its specific role in hippocampal information processing.

Neurodevelopmental role for VGLUT2 in pyramidal neuron plasticity, dendritic refinement, and in spatial learning

The level and integrity of glutamate transmission during critical periods of postnatal development plays an important role in the refinement of pyramidal neuron dendritic arbor, synaptic plasticity, and cognition. Presently, it is not clear how excitatory transmission via the two predominant isoforms of the vesicular glutamate transporter (VGLUT1 and VGLUT2) participate in this process. To assess a neurodevelopmental role for VGLUT2 in pyramidal neuron maturation, we generated recombinant VGLUT2 knock-out mice and inactivated VGLUT2 throughout development using Emx1-Cre(+/+) knock-in mice. We show that VGLUT2 deficiency in corticolimbic circuits results in reduced evoked glutamate transmission, release probability, and LTD at hippocampal CA3-CA1 synapses during a formative developmental period (postnatal days 11-14). In adults, we find a marked reduction in the amount of dendritic arbor across the span of the dendritic tree of CA1 pyramidal neurons and reduced long-term potentiation and levels of synaptic markers spinophilin and VGLUT1. Loss of dendritic arbor is accompanied by corresponding reductions in the number of dendritic spines, suggesting widespread alterations in synaptic connectivity. Conditional VGLUT2 knock-out mice exhibit increased open-field exploratory activity yet impaired spatial learning and memory, endophenotypes similar to those of NMDA receptor knock-down mice. Remarkably, the impairment in learning can be partially restored by selectively increasing NMDA receptor-mediated glutamate transmission in adult mice by prolonged treatment with d-serine and a d-amino acid oxidase inhibitor. Our data indicate that VGLUT2 expression is pivotal to the proper development of mature pyramidal neuronal architecture and plasticity, and that such glutamatergic deficiency leads to cognitive malfunction as observed in several neurodevelopmental psychiatric disorders.

Activation of extrasynaptic NMDA receptors induces LTD in rat hippocampal CA1 neurons

In the adult rat hippocampus, activation of N-methyl-d-aspartate receptors (NMDARs) is required for the induction of certain forms of synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Several studies have indicated the opposing role of synaptic NMDARS (S-NMDARs) versus extrasynaptic NMDARs (ES-NMDARs) in CREB-dependent gene regulation and neuronal survival/death. The contribution of ES-NMDARs in synaptic plasticity, however, remains unclear. Here we investigated the contribution of ES-NMDARs on LTD induction in CA1 neurons of rat hippocampal slices. ES-NMDARs were selectively activated by theta burst stimulation (TBS) after selective blockade of S-NMDARs with pairing of 5 Hz stimulation and MK-801, an irreversible use-dependent antagonist of NMDARs. Application of TBS in naïve slices evoked a transient potentiation. In contrast, the activation of ES-NMDARs evoked a robust LTD. These results suggest the involvement of ES-NMDARs in LTD induction.

Usp14 deficiency increases tau phosphorylation without altering tau degradation or causing tau-dependent deficits

Regulated protein degradation by the proteasome plays an essential role in the enhancement and suppression of signaling pathways in the nervous system. Proteasome-associated factors are pivotal in ensuring appropriate protein degradation, and we have previously demonstrated that alterations in one of these factors, the proteasomal deubiquitinating enzyme ubiquitin-specific protease 14 (Usp14), can lead to proteasome dysfunction and neurological disease. Recent studies in cell culture have shown that Usp14 can also stabilize the expression of over-expressed, disease-associated proteins such as tau and ataxin-3. Using Usp14-deficient ax^J mice, we investigated if loss of Usp14 results in decreased levels of endogenous tau and ataxin-3 in the nervous system of mice. Although loss of Usp14 did not alter the overall neuronal levels of tau and ataxin-3, we found increased levels of phosphorylated tau that correlated with the onset of axonal varicosities in the Usp14-deficient mice. These changes in tau phosphorylation were accompanied by increased levels of activated phospho-Akt, phosphorylated MAPKs, and inactivated phospho-GSK3β. However, genetic ablation of tau did not alter any of the neurological deficits in the Usp14-deficient mice, demonstrating that increased levels of phosphorylated tau do not necessarily lead to neurological disease. Due to the widespread activation of intracellular signaling pathways induced by the loss of Usp14, a better understanding of the cellular pathways regulated by the proteasome is required before effective proteasomal-based therapies can be used to treat chronic neurological diseases.

Kainic Acid-induced neurotoxicity: targeting glial responses and glia-derived cytokines

Glutamate excitotoxicity contributes to a variety of disorders in the central nervous system, which is triggered primarily by excessive Ca²⁺ influx arising from overstimulation of glutamate receptors, followed by disintegration of the endoplasmic reticulum (ER) membrane and ER stress, the generation and detoxification of reactive oxygen species as well as mitochondrial dysfunction, leading to neuronal apoptosis and necrosis. Kainic acid (KA), a potent agonist to the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate class of glutamate receptors, is 30-fold more potent in neuro-toxicity than glutamate. In rodents, KA injection resulted in recurrent seizures, behavioral changes and subsequent degeneration of selective populations of neurons in the brain, which has been widely used as a model to study the mechanisms of neurodegenerative pathways induced by excitatory neurotransmitter. Microglial activation and astrocytes proliferation are the other characteristics of KA-induced neurodegeneration. The cytokines and other inflammatory molecules secreted by activated glia cells can modify the outcome of disease progression. Thus, anti-oxidant and anti-inflammatory treatment could attenuate or prevent KA-induced neurodegeneration. In this review, we summarized updated experimental data with regard to the KA-induced neurotoxicity in the brain and emphasized glial responses and glia-oriented cytokines, tumor necrosis factor-α, interleukin (IL)-1, IL-12 and IL-18.

Modulation by pregnenolone sulfate of filtering properties in the hippocampal trisynaptic circuit

Short-term synaptic plasticity alters synaptic efficacy on a timescale that is relevant to encoding information in spike trains. The dynamics of this plasticity, combined with that of the feedback and feedforward contributions of local interneurons, impose frequency-dependent properties on neuronal networks with implications for nervous system function. The trisynaptic network of the hippocampus is especially well suited to selectively filter components of frequency-dependent signals that are transmitted from the entorhinal cortex. We measured presynaptic [Ca(2+)](i) in perforant path, mossy fiber, or Schaffer collateral terminals while simultaneously measuring field potentials of principal cells of the dentate, CA3, or CA1 synaptic fields over a range of stimulus frequencies of 2 to 77 Hz. In all three synaptic fields, the average [Ca(2+)](i) during a 500 ms stimulus train rose monotonically with stimulus frequency. The average population spike amplitude during this stimulus train, however, exhibited a non-linear relationship to frequency that was distinct for each of the three synaptic fields. The dentate synaptic field exhibited the characteristics of a low pass filter, while both CA synaptic fields had bandpass filter characteristics with a gain that was greater than 1 in the passband frequencies. Importantly, alteration of the dynamic properties of this network could significantly impact information processing performed by the hippocampus. Pregnenolone sulfate (PregS), has frequency-dependent effects on paired- and multipulse plasticity in the dentate and CA1 synaptic fields of the hippocampal formation. We investigated the PregS-dependent modulation of the dynamic properties of transmission by the principal cells of the three hippocampal synaptic fields. Importantly, PregS is capable of altering the pattern separation capabilities that may underlie hippocampal information processing.

Dendritic position is a major determinant of presynaptic strength

Different regulatory principles influence synaptic coupling between neurons, including positional principles. In dendrites of pyramidal neurons, postsynaptic sensitivity depends on synapse location, with distal synapses having the highest gain. In this paper, we investigate whether similar rules exist for presynaptic terminals in mixed networks of pyramidal and dentate gyrus (DG) neurons. Unexpectedly, distal synapses had the lowest staining intensities for vesicular proteins vGlut, vGAT, Synaptotagmin, and VAMP and for many nonvesicular proteins, including Bassoon, Munc18, and Syntaxin. Concomitantly, distal synapses displayed less vesicle release upon stimulation. This dependence of presynaptic strength on dendritic position persisted after chronically blocking action potential firing and postsynaptic receptors but was markedly reduced on DG dendrites compared with pyramidal dendrites. These data reveal a novel rule, independent of neuronal activity, which regulates presynaptic strength according to dendritic position, with the strongest terminals closest to the soma. This gradient is opposite to postsynaptic gradients observed in pyramidal dendrites, and different cell types apply this rule to a different extent.

A critical role for GluN2B-containing NMDA receptors in cortical development and function

The subunit composition of N-methyl D-aspartate receptors (NMDARs) is tightly regulated during cortical development. NMDARs are initially dominated by GluN2B (NR2B), whereas GluN2A (NR2A) incorporation increases after birth. The function of GluN2B-containing NMDARs during development, however, is incompletely understood. We generated a mouse in which we genetically replaced GluN2B with GluN2A (2B→2A). Although this manipulation restored NMDAR-mediated currents at glutamatergic synapses, it did not rescue GluN2B loss of function. Protein translation-dependent homeostatic synaptic plasticity is occluded in the absence of GluN2B, and AMPA receptor contribution is enriched at excitatory cortical synapses. Our experiments indicate that specificity of GluN2B-mediated signaling is due to its unique interaction with the protein effector alpha calcium-calmodulin kinase II and the regulation of the mTOR pathway. Homozygous 2B→2A mice exhibited high rates of lethality, suppressed feeding, and depressed social exploratory behavior. These experiments indicate that GluN2B-containing NMDARs activate unique cellular processes that cannot be rescued by replacement with GluN2A.

Transplantation of rat synapsin-EGFP-labeled embryonic neurons into the intact and ischemic CA1 hippocampal region: distribution, phenotype, and axodendritic sprouting

A major limitation of neural transplantation studies is assessing the degree of host-graft interaction. In the present study, rat hippocampal/cortical embryonic neurons (E18) were infected with a lentivirus encoding enhanced green fluorescent protein (GFP) under control of the neuron-specific synapsin promoter, thus permitting robust identification of labeled neurons after in vivo grafting. Two weeks after transient forebrain ischemia or sham-surgery, GFP-expressing neurons were transplanted into CA1 hippocampal regions in immunosuppressed adult Wistar rats. The survival, distribution, phenotype, and axonal projections of GFP-immunoreactive (IR) positive transplanted neurons were evaluated in the sham-operated and ischemia- damaged CA1 hippocampal regions 4, 8, and 12 weeks after transplantation. In both experimental groups, we observed that the main phenotype of host-derived afferents projecting towards grafted GFP-IR neurons as well as transplant-derived GFP-IR efferents were glutamatergic in both animal groups. GFP axonal projections were seen in the nucleus accumbens, septal nuclei, and subiculum-known target areas of CA1 pyramidal neurons. Compared to sham-operated animals, GFP-IR neurons grafted into the ischemia-damaged CA1 migrated more extensively throughout a larger volume of host tissue, particularly in the stratum radiatum. Moreover, enhanced axonal sprouting and neuronal plasticity of grafted cells were evident in the hippocampus, subiculum, septal nuclei, and nucleus accumbens of the ischemia-damaged rats. Our study suggests that the adult rat brain retains some capacity to direct newly sprouting axons of transplanted embryonic neurons to the correct targets and that this capacity is enhanced in previously ischemia-injured forebrain.

Different compartments of apical CA1 dendrites have different plasticity thresholds for expressing synaptic tagging and capture

The consolidation process from short- to long-term memory depends on the type of stimulation received from a specific neuronal network and on the cooperativity and associativity between different synaptic inputs converging onto a specific neuron. We show here that the plasticity thresholds for inducing LTP are different in proximal and distal compartments of apical dendrites. In addition, we show interactions between the proximal and distal compartments of the apical dendrites by providing evidence that even a subthreshold stimulus can activate plasticity-related proteins, such as PKMζ, enabling associativity between two distinct dendritic compartments in apical dendrites to occur.

Developmental changes in presynaptic Ca(2 +) clearance kinetics and synaptic plasticity in mouse Schaffer collateral terminals

Presynaptic Ca(2+) influx pathways, cytoplasmic Ca(2+) buffering proteins and Ca(2+) extrusion processes undergo considerable change during the first postnatal month in rodent neurons. These changes may be critical in establishing short-term plasticity at maturing presynaptic terminals where neurotransmitter release is directly dependent on the dynamics of cytoplasmic residual Ca(2+) ([Ca(2+)](res)). In particular, the robust paired-pulse facilitation characteristic of adult neurons is almost entirely lacking in newborns. To examine developmental changes in processes controlling [Ca(2+)](res), we measured the timecourse of [Ca(2+)](res) decay in presynaptic terminals of Schaffer collateral to CA1 synapses in acute hippocampal slices following single and paired orthodromic stimuli in the stratum radiatum. Developmental changes were observed in both the rise time and slow exponential decay components of the response to single stimuli such that this decay was larger and faster in the adult. Furthermore, we observed a greater caffeine-sensitive basal Ca(2+) store, which was differentially affected when active uptake into the endoplasmic reticulum was blocked, in the presynaptic fields of the Schaffer collateral to CA1 terminals of P6 and younger mice when compared to adults. These transitions in [Ca(2+)](res) dynamics occurred gradually over the first weeks of postnatal life and correlated with changes in short-term plasticity.

Recruitment of N-Type Ca(2+) channels during LTP enhances low release efficacy of hippocampal CA1 perforant path synapses

The entorhinal cortex provides both direct and indirect inputs to hippocampal CA1 neurons through the perforant path and Schaffer collateral synapses, respectively. Using both two-photon imaging of synaptic vesicle cycling and electrophysiological recordings, we found that the efficacy of transmitter release at perforant path synapses is lower than at Schaffer collateral inputs. This difference is due to the greater contribution to release by presynaptic N-type voltage-gated Ca(2+) channels at the Schaffer collateral than perforant path synapses. Induction of long-term potentiation that depends on activation of NMDA receptors and L-type voltage-gated Ca(2+) channels enhances the low efficacy of release at perforant path synapses by increasing the contribution of N-type channels to exocytosis. This represents a previously uncharacterized presynaptic mechanism for fine-tuning release properties of distinct classes of synapses onto a common postsynaptic neuron and for regulating synaptic function during long-term synaptic plasticity.

D1/D5 modulation of synaptic NMDA receptor currents

Converging evidence suggests that salience-associated modulation of behavior is mediated by the release of monoamines and that monoaminergic activation of D(1)/D(5) receptors is required for normal hippocampal-dependent learning and memory. However, it is not understood how D(1)/D(5) modulation of hippocampal circuits can affect salience-associated learning and memory. We have observed in CA1 pyramidal neurons that D(1)/D(5) receptor activation elicits a bidirectional long-term plasticity of NMDA receptor-mediated synaptic currents with the polarity of plasticity determined by NMDA receptor, NR2A/B subunit composition. This plasticity results in a decrease in the NR2A/NR2B ratio of subunit composition. Synaptic responses mediated by NMDA receptors that include NR2B subunits are potentiated by D(1)/D(5) receptor activation, whereas responses mediated by NMDA receptors that include NR2A subunits are depressed. Furthermore, these bidirectional, subunit-specific effects are mediated by distinctive intracellular signaling mechanisms. Because there is a predominance of NMDA receptors composed of NR2A subunits observed in entorhinal-CA1 inputs and a predominance of NMDA receptors composed of NR2B subunits in CA3-CA1 synapses, potentiation of synaptic NMDA currents predominates in the proximal CA3-CA1 synapses, whereas depression of synaptic NMDA currents predominates in the distal entorhinal-CA1 synapses. Finally, all of these effects are reproduced by the release of endogenous monoamines through activation of D(1)/D(5) receptors. Thus, endogenous D(1)/D(5) activation can (1) decrease the NR2A/NR2B ratio of NMDA receptor subunit composition at glutamatergic synapses, a rejuvenation to a composition similar to developmentally immature synapses, and, (2) in CA1, bias NMDA receptor responsiveness toward the more highly processed trisynaptic CA3-CA1 circuit and away from the direct entorhinal-CA1 input.

Calcium-permeable presynaptic kainate receptors involved in excitatory short-term facilitation onto somatostatin interneurons during natural stimulus patterns

Schaffer collateral synapses in hippocampus show target-cell specific short-term plasticity. Using GFP-expressing Inhibitory Neuron (GIN) transgenic mice that express enhanced green fluorescent protein (EGFP) in a subset of somatostatin-containing interneurons (SOM interneurons), we previously showed that Schaffer collateral synapses onto SOM interneurons in stratum (S.) radiatum have unusually large (up to 6-fold) paired-pulse facilitation. This results from a low initial release probability and the enhancement of facilitation by synaptic activation of presynaptic kainate receptors. Here we further investigate the properties of these kainate receptors and examine their effects on short-term facilitation during physiologically derived stimulation patterns, using excitatory postsynaptic currents recorded in S. radiatum interneurons during Schaffer collateral stimulation in acute slices from juvenile GIN mice. We find that GluR5 and GluR6 antagonists decrease short-term facilitation at Schaffer collateral synapses onto SOM interneurons with no additive effects, suggesting that the presynaptic kainate receptors are heteromers containing both GluR5 and GluR6 subunits. The calcium-permeable receptor antagonist 1-napthyl acetyl spermine (NASPM) both mimics and occludes the effect of the kainate receptor antagonists, indicating that the presynaptic kainate receptors are calcium permeable. Furthermore, Schaffer collateral synapses onto SOM interneurons show up to 11-fold short-term facilitation during physiologically derived stimulus patterns, in contrast to other interneurons that have less than 1.5-fold facilitation. Blocking the kainate receptors reduces facilitation in SOM interneurons by approximately 50% during the physiologically derived patterns and reduces the dynamic range. Activation of calcium-permeable kainate receptors containing GluR5/GluR6 causes a dramatic increase in short-term facilitation during physiologically derived stimulus patterns, a mechanism likely to be important in regulating the strength of Schaffer collateral synapses onto SOM interneurons in vivo.