GABABR-Dependent Long-Term Depression at Hippocampal Synapses between CB1-Positive Interneurons and CA1 Pyramidal Cells

Ventral hippocampal cholecystokinin interneurons gate contextual reward memory

Associating contexts with rewards depends on hippocampal circuits, with local inhibitory interneurons positioned to play an important role in shaping activity. Here, we demonstrate that the encoding of context-reward memory requires a ventral hippocampus (vHPC) to nucleus accumbens (NAc) circuit that is gated by cholecystokinin (CCK) interneurons. In a sucrose conditioned place preference (CPP) task, optogenetically inhibiting vHPC-NAc terminals impaired the acquisition of place preference. Transsynaptic rabies tracing revealed vHPC-NAc neurons were monosynaptically innervated by CCK interneurons. Using intersectional genetic targeting of CCK interneurons, ex vivo optogenetic activation of CCK interneurons increased GABAergic transmission onto vHPC-NAc neurons, while in vivo optogenetic inhibition of CCK interneurons increased cFos in these projection neurons. Notably, CCK interneuron inhibition during sucrose CPP learning increased time spent in the sucrose-associated location, suggesting enhanced place-reward memory. Our findings reveal a previously unknown hippocampal microcircuit crucial for modulating the strength of contextual reward learning.

Ethanol blocks a novel form of iLTD, but not iLTP of inhibitory inputs to VTA GABA neurons

The ventral tegmental area (VTA) is an essential component of the mesocorticolimbic dopamine (DA) circuit that processes reward and motivated behaviors. The VTA contains DA neurons essential in this process, as well as GABAergic inhibitory cells that regulate DA cell activity. In response to drug exposure, synaptic connections of the VTA circuit can be rewired via synaptic plasticity-a phenomenon thought to be responsible for the pathology of drug dependence. While synaptic plasticity to VTA DA neurons as well as prefrontal cortex to nucleus accumbens GABA neurons are well studied, VTA GABA cell plasticity, specifically inhibitory inputs to VTA GABA neurons, is less understood. Therefore, we investigated the plasticity of these inhibitory inputs. Using whole cell electrophysiology in GAD67-GFP mice to identify GABA cells, we observed that these VTA GABA cells experience either inhibitory GABAergic long-term potentiation (iLTP) or inhibitory long-term depression (iLTD) in response to a 5 Hz stimulus. Paired pulse ratios, coefficient of variance, and failure rates suggest a presynaptic mechanism for both plasticity types, where iLTP is NMDA receptor-dependent and iLTD is GABAB receptor-dependent-this being the first report of iLTD onto VTA GABA cells. As illicit drug exposure can alter VTA plasticity, we employed chronic intermittent exposure (CIE) to ethanol (EtOH) vapor in male and female mice to examine its potential impact on VTA GABA input plasticity. Chronic EtOH vapor exposure produced measurable behavioral changes illustrating dependence and concomitantly prevented previously observed iLTD, which continued in air-exposed controls, illustrating the impact of EtOH on VTA neurocircuitry and suggesting physiologic mechanisms at play in alcohol use disorder and withdrawal states. Taken together, these novel findings of unique GABAergic synapses exhibiting either iLTP or iLTD within the mesolimbic circuit, and EtOH blockade specifically of iLTD, characterize inhibitory VTA plasticity as a malleable, experience-dependent system modified by EtOH.

A critical period for learning and plastic changes at hippocampal CA1 synapses

Postnatal development of hippocampal function has been reported in many mammalian species, including humans. To obtain synaptic evidence, we analyzed developmental changes in plasticity after an inhibitory avoidance task in rats. Learning performance was low in infants (postnatal 2 weeks) but clearly improved from the juvenile period (3–4 weeks) to adulthood (8 weeks). One hour after the training, we prepared brain slices and sequentially recorded miniature excitatory postsynaptic currents (mEPSCs) and inhibitory postsynaptic currents (mIPSCs) from the same hippocampal CA1 neuron. Although the training failed to affect the amplitude of either mEPSCs or mIPSCs at 2 weeks, it increased mEPSC, but not mIPSC, amplitude at 3 weeks. At 4 weeks, the training had increased the amplitude of both mEPSCs and mIPSCs, whereas mIPSC, but not mEPSC, amplitude was increased at 8 weeks. Because early-life physiological functions can affect performance, we also evaluated sensory–motor functions together with emotional state and found adequate sensory/motor functions from infancy to adulthood. Moreover, by analyzing performance of rats in multiple hippocampal-dependent tasks, we found that the developmental changes in the performance are task dependent. Taken together, these findings delineate a critical period for learning and plastic changes at hippocampal CA1 synapses.

Microbes, metabolites and (synaptic) malleability, oh my! The effect of the microbiome on synaptic plasticity

The microbiome influences the emotional and cognitive phenotype of its host, as well as the neurodevelopment and pathophysiology of various brain processes and disorders, via the well‐established microbiome–gut–brain axis. Rapidly accumulating data link the microbiome to severe neuropsychiatric disorders in humans, including schizophrenia, Alzheimer's and Parkinson's. Moreover, preclinical work has shown that perturbation of the microbiome is closely associated with social, cognitive and behavioural deficits. The potential of the microbiome as a diagnostic and therapeutic tool is currently undercut by a lack of clear mechanistic understanding of the microbiome–gut–brain axis. This review establishes the hypothesis that the mechanism by which this influence is carried out is synaptic plasticity – long‐term changes to the physical and functional neuronal structures that enable the brain to undertake learning, memory formation, emotional regulation and more. By examining the different constituents of the microbiome–gut–brain axis through the lens of synaptic plasticity, this review explores the diverse aspects by which the microbiome shapes the behaviour and mental wellbeing of the host. Key elements of this complex bi‐directional relationship include neurotransmitters, neuronal electrophysiology, immune mediators that engage with both the central and enteric nervous systems and signalling cascades that trigger long‐term potentiation of synapses. The importance of establishing mechanistic correlations along the microbiome–gut–brain axis cannot be overstated as they hold the potential for furthering current understanding regarding the vast fields of neuroscience and neuropsychiatry. This review strives to elucidate the promising theory of microbiome‐driven synaptic plasticity in the hope of enlightening current researchers and inspiring future ones.

5-HT7 receptor activation rescues impaired synaptic plasticity in an autistic-like rat model induced by prenatal VPA exposure

Autism spectrum disorder (ASD) is a severe life-long neuropsychiatric disorder. Alterations and imbalance of several neurochemical systems may be involved in ASD pathophysiology, of them, serotonergic neurotransmission dysfunction and deficiency may underlie behavioral abnormalities associated with ASD. However, the functional importance of serotonergic receptors, particularly 5HT7 receptors in ASD pathology remains poorly defined. Serotonin receptor subtype 7 (5-HT7R) plays a direct regulatory role in the development and also for the mature function of the brain, therefore, further studies are necessary to elucidate the role of these receptors in the etiology of autism. To address this issue, we combined here behavioral, electrophysiological methods to further characterize the contribution of 5-HT7Rs in the prenatal valproic acid (VPA) exposure-induced impairment in synaptic plasticity and their impact on the associated behavioral changes. This may help to unravel the underlying cellular mechanisms involved in ASD and can lead to new treatment and/or prevention therapies based on the role of the serotonergic system for autism. Findings revealed that compared to control, autistic-like offspring showed increased anxiety-like behavior, reduced social interaction, decreased locomotor activity, and impaired identification of the novel object. However, administration of 5-HT7Rs agonist, LP-211, for 7 consecutive days before testing from postnatal day 21 to 27 reversed all behavioral deficits induced by prenatal exposure to VPA in offspring. Also, both short-term depression and long-term potentiation were impaired in the autistic-like pups, but activation of 5-HT7Rs rescued the LTP impairment in the autistic-like group so that there was no significant difference between the two groups. Blockade of 5-HT7Rs caused LTP impairment following HFS in the autistic-like group. Besides, there was a significant difference in LTD induction following SB-269970 application between the control and the autistic-like groups measured at first 10 min following TPS. Moreover, both the number and the size of retrograde fast blue-labelled neurons in the raphe nuclei were reduced. Overall, these results provide for the first time, as far as we know, functional evidence for the restorative role of 5-HT7Rs activation against prenatal VPA exposure induced behavioral deficits and hippocampal synaptic plasticity impairment. Therefore, these receptors could be a potential and promising pharmacotherapy target for the treatment of autism.

Cholecystokinin-Mediated Neuromodulation of Anxiety and Schizophrenia: A "Dimmer-Switch" Hypothesis

Cholecystokinin (CCK), the most abundant brain neuropeptide, is involved in relevant behavioral functions like memory, cognition, and reward through its interactions with the opioid and dopaminergic systems in the limbic system. CCK excites neurons by binding two receptors, CCK1 and CCK2, expressed at low and high levels in the brain, respectively. Historically, CCK2 receptors have been related to the induction of panic attacks in humans. Disturbances in brain CCK expression also underlie the physiopathology of schizophrenia, which is attributed to the modulation by CCK1 receptors of the dopamine flux in the basal striatum. Despite this evidence, neither CCK2 receptor antagonists ameliorate human anxiety nor CCK agonists have consistently shown neuroleptic effects in clinical trials. A neglected aspect of the function of brain CCK is its neuromodulatory role in mental disorders. Interestingly, CCK is expressed in pivotal inhibitory interneurons that sculpt cortical dynamics and the flux of nerve impulses across corticolimbic areas and the excitatory projections to mesolimbic pathways. At the basal striatum, CCK modulates the excitability of glutamate, the release of inhibitory GABA, and the discharge of dopamine. Here we focus on how CCK may reduce rather than trigger anxiety by regulating its cognitive component. Adequate levels of CCK release in the basal striatum may control the interplay between cognition and reward circuitry, which is critical in schizophrenia. Hence, it is proposed that disturbances in the excitatory/ inhibitory interplay modulated by CCK may contribute to the imbalanced interaction between corticolimbic and mesolimbic neural activity found in anxiety and schizophrenia.

The interaction effect of sleep deprivation and cannabinoid type 1 receptor in the CA1 hippocampal region on passive avoidance memory, depressive-like behavior and locomotor activity in rats

Increasing evidence shows the interaction effect of cannabinoids and sleep on cognitive functions. In the present study, we aimed to investigate the interaction effect of cannabinoids type 1 receptor (CB1r) in the CA1 hippocampal region and sleep deprivation (SD) on passive avoidance memory and depressive-like behavior in male Wistar rats. We used water box apparatus to induce total SD (TSD) for 24 h. The shuttle-box was applied to assess passive avoidance memory and locomotion apparatus was applied to assess locomotor activity. Forced swim test (FST) was used to evaluate rat's behavior. ACPA (CB1r agonist) at the doses of 0.01, 0.001 and 0.0001 μg/rat, and AM251 (CB1r antagonist) at the doses of 100, 10 and 1 ng/rat were injected intra-CA1, five minutes after training via stereotaxic surgery. Results showed SD impaired memory. ACPA at the doses of 0.01 and 0.001 μg/rat impaired memory and at all doses did not alter the effect of SD on memory. AM251 by itself did not alter memory, while at lowest dose (1 ng/rat) restored SD-induced memory deficit. Both drugs induced depressive-like behavior in a dose-dependent manner. Furthermore, both drugs decreased swimming at some doses (ACPA at 0.0001 μg/rat, AM251 at 0.001 and 0.01 ng/rat). Also, ACPA at the highest dose increased climbing of SD rats. In conclusion, we suggest CB1r may interact with the effect of SD on memory. Additionally, cannabinoids may show a dose-dependent manner in modulating mood and behavior. Interestingly, CB1r agonists and antagonists may exhibit a similar effect in some behavioral assessments.

A comprehensive knowledge base of synaptic electrophysiology in the rodent hippocampal formation

The cellular and synaptic architecture of the rodent hippocampus has been described in thousands of peer-reviewed publications. However, no human- or machine-readable public catalog of synaptic electrophysiology data exists for this or any other neural system. Harnessing state-of-the-art information technology, we have developed a cloud-based toolset for identifying empirical evidence from the scientific literature pertaining to synaptic electrophysiology, for extracting the experimental data of interest, and for linking each entry to relevant text or figure excerpts. Mining more than 1,200 published journal articles, we have identified eight different signal modalities quantified by 90 different methods to measure synaptic amplitude, kinetics, and plasticity in hippocampal neurons. We have designed a data structure that both reflects the differences and maintains the existing relations among experimental modalities. Moreover, we mapped every annotated experiment to identified potential connections, that is, specific pairs of presynaptic and postsynaptic neuron types. To this aim, we leveraged Hippocampome.org, an open-access knowledge base of morphologically, electrophysiologically, and molecularly characterized neuron types in the rodent hippocampal formation. Specifically, we have implemented a computational pipeline to systematically translate neuron type properties into formal queries in order to find all compatible potential connections. With this system, we have collected nearly 40,000 synaptic data entities covering 88% of the 3,120 potential connections in Hippocampome.org. Correcting membrane potentials with respect to liquid junction potentials significantly reduced the difference between theoretical and experimental reversal potentials, thereby enabling the accurate conversion of all synaptic amplitudes to conductance. This data set allows for large-scale hypothesis testing of the general rules governing synaptic signals. To illustrate these applications, we confirmed several expected correlations between synaptic measurements and their covariates while suggesting previously unreported ones. We release all data open-source at Hippocampome.org in order to further research across disciplines.

A comprehensive knowledge base of synaptic electrophysiology in the rodent hippocampal formation

The cellular and synaptic architecture of the rodent hippocampus has been described in thousands of peer-reviewed publications. However, no human- or machine-readable public catalog of synaptic electrophysiology data exists for this or any other neural system. Harnessing state-of-the-art information technology, we have developed a cloud-based toolset for identifying empirical evidence from the scientific literature pertaining to synaptic electrophysiology, for extracting the experimental data of interest, and for linking each entry to relevant text or figure excerpts. Mining more than 1,200 published journal articles, we have identified eight different signal modalities quantified by 90 different methods to measure synaptic amplitude, kinetics, and plasticity in hippocampal neurons. We have designed a data structure that both reflects the differences and maintains the existing relations among experimental modalities. Moreover, we mapped every annotated experiment to identified potential connections, that is, specific pairs of presynaptic and postsynaptic neuron types. To this aim, we leveraged Hippocampome.org, an open-access knowledge base of morphologically, electrophysiologically, and molecularly characterized neuron types in the rodent hippocampal formation. Specifically, we have implemented a computational pipeline to systematically translate neuron type properties into formal queries in order to find all compatible potential connections. With this system, we have collected nearly 40,000 synaptic data entities covering 88% of the 3,120 potential connections in Hippocampome.org. Correcting membrane potentials with respect to liquid junction potentials significantly reduced the difference between theoretical and experimental reversal potentials, thereby enabling the accurate conversion of all synaptic amplitudes to conductance. This data set allows for large-scale hypothesis testing of the general rules governing synaptic signals. To illustrate these applications, we confirmed several expected correlations between synaptic measurements and their covariates while suggesting previously unreported ones. We release all data open-source at Hippocampome.org in order to further research across disciplines.

Synergistic but not additive effect between ACPA and lithium in the dorsal hippocampal region on spatial learning and memory in rats: Isobolographic analyses

Lithium and cannabinoids can disrupt learning and memory performance. The goal of the present study is to investigate the additive or synergistic effect of lithium and cannabinoid combination doses on spatial learning and memory in rats by isobolographic analyses. Although several studies have suggested synergistic effects of cannabinoids or lithium in response to other compounds, in most of them isobolographic analyses were not used; Thus, there is a need for more detailed studies using isobolographic analyses. In this study, spatial memory was evaluated in the Morris Water Maze (MWM) apparatus by eight trials in the training day and one trial in the test day. Lithium was injected intraperitoneal and ACPA (cannabinoid type 1 receptor agonist) was injected into the dorsal hippocampal region (intra-CA1). For the isobolographic analyses, the ED50 of lithium (2.5 mg/kg) and ACPA (0.5 μg/rat) was measured by linear regression analysis, considering the doses were tested in our previous research. The results showed that, combinations of low, medium and high doses of lithium (0.312 mg/kg, 0.625 mg/kg and 1.25 mg/kg, respectively) and ACPA (0.0625 μg/rat, 0.125 μg/rat and 0.25 μg/rat, respectively) had synergistic but not additive effect on spatial learning and spatial memory. In conclusion, we suggest that combination doses of lithium and ACPA have synergistic but not additive effect on spatial learning and memory in the rat's dorsal hippocampal region.

The Ever-Growing Puzzle of Asynchronous Release

Invasion of an action potential (AP) to presynaptic terminals triggers calcium dependent vesicle fusion in a relatively short time window, about a millisecond, after the onset of the AP. This allows fast and precise information transfer from neuron to neuron by means of synaptic transmission and phasic mediator release. However, at some synapses a single AP or a short burst of APs can generate delayed or asynchronous synaptic release lasting for tens or hundreds of milliseconds. Understanding the mechanisms underlying asynchronous release (AR) is important, since AR can better recruit extrasynaptic metabotropic receptors and maintain a high level of neurotransmitter in the extracellular space for a substantially longer period of time after presynaptic activity. Over the last decade substantial work has been done to identify the presynaptic calcium sensor that may be involved in AR. Several models have been suggested which may explain the long lasting presynaptic calcium elevation a prerequisite for prolonged delayed release. However, the presynaptic mechanisms underlying asynchronous vesicle release are still not well understood. In this review article, we provide an overview of the current state of knowledge on the molecular components involved in delayed vesicle fusion and in the maintenance of sufficient calcium concentration to trigger AR. In addition, we discuss possible alternative models that may explain intraterminal calcium dynamics underlying AR.

The role of CA1 CB1 receptors on lithium-induced spatial memory impairment in rats

Lithium, a glycogen synthase kinase-3β (GSK-3β) inhibitor, prevents cannabinoid withdrawal syndrome, but there is limited data exploring the interaction between lithium and cannabinoid system on memory processes. The present study aimed to test the interaction between dorsal hippocampal (CA1 region) cannabinoid system and lithium on spatial memory in rats. Spatial memory was assessed in Morris Water Maze (MWM) apparatus by a single training session of eight trials. The results showed that pre-training intra-CA1 microinjection of ACPA, the cannabinoid type 1 receptor (CB1r) agonist, at doses of 0.001, 0.01 or 1 µg/rat, or AM251, the cannabinoid type 1 receptor (CB1r) antagonist, at doses of 1, 10 or 100 ng/rat, increased escape latency and traveled distance to the platform, suggesting a spatial learning impairment, whereas intraperitoneal administration of lithium (0.5, 1 or 5 mg/kg) had no effect on spatial learning. Also, rats that received lithium plus a lower dose of ACPA (0.001 µg/rat) or AM251 (1 ng/rat) had successful performance in the MWM. In the probe test, the results showed that pre-training administration of lithium (5 mg/kg) and ACPA (0.01 or 1 µg/rat) but not AM251 (at all doses used) impaired spatial memory retrieval. Also, lower dose of ACPA (0.001 µg/rat) or AM251 (1 ng/rat) potentiated the effect of ineffective doses of lithium (0.5 and 1 mg/kg) on spatial memory retrieval, while restored the effect of effective dose of lithium (5 mg/kg). In conclusion, cannabinoids may have a dual effect on lithium-induced spatial memory impairment in rats.

Synaptic plasticity mechanisms common to learning and alcohol use disorder

Alcohol use disorders include drinking problems that span a range from binge drinking to alcohol abuse and dependence. Plastic changes in synaptic efficacy, such as long-term depression and long-term potentiation are widely recognized as mechanisms involved in learning and memory, responses to drugs of abuse, and addiction. In this review, we focus on the effects of chronic ethanol (EtOH) exposure on the induction of synaptic plasticity in different brain regions. We also review findings indicating that synaptic plasticity occurs in vivo during EtOH exposure, with a focus on ex vivo electrophysiological indices of plasticity. Evidence for effects of EtOH-induced or altered synaptic plasticity on learning and memory and EtOH-related behaviors is also reviewed. As this review indicates, there is much work needed to provide more information about the molecular, cellular, circuit, and behavioral consequences of EtOH interactions with synaptic plasticity mechanisms.

Differential surface density and modulatory effects of presynaptic GABAB receptors in hippocampal cholecystokinin and parvalbumin basket cells

The perisomatic domain of cortical neurons is under the control of two major GABAergic inhibitory interneuron types: regular-spiking cholecystokinin (CCK) basket cells (BCs) and fast-spiking parvalbumin (PV) BCs. CCK and PV BCs are different not only in their intrinsic physiological, anatomical and molecular characteristics, but also in their presynaptic modulation of their synaptic output. Most GABAergic terminals are known to contain GABA_B receptors (GABA_BR), but their role in presynaptic inhibition and surface expression have not been comparatively characterized in the two BC types. To address this, we performed whole-cell recordings from CCK and PV BCs and postsynaptic pyramidal cells (PCs), as well as freeze-fracture replica-based quantitative immunogold electron microscopy of their synapses in the rat hippocampal CA1 area. Our results demonstrate that while both CCK and PV BCs contain functional presynaptic GABA_BRs, their modulatory effects and relative abundance are markedly different at these two synapses: GABA release is dramatically inhibited by the agonist baclofen at CCK BC synapses, whereas a moderate reduction in inhibitory transmission is observed at PV BC synapses. Furthermore, GABA_BR activation has divergent effects on synaptic dynamics: paired-pulse depression (PPD) is enhanced at CCK BC synapses, but abolished at PV BC synapses. Consistent with the quantitative differences in presynaptic inhibition, virtually all CCK BC terminals were found to contain GABA_BRs at high densities, but only 40% of PV BC axon terminals contain GABA_BRs at detectable levels. These findings add to an increasing list of differences between these two interneuron types, with implications for their network functions.