The maturation of GABAergic transmission in visual cortex requires endocannabinoid-mediated LTD of inhibitory inputs during a critical period

Inhibitory maturation and ocular dominance plasticity in mouse visual cortex require astrocyte CB1 receptors

Endocannabinoids, signaling through the cannabinoid CB1 receptor (CB1R), regulate several forms of neuronal plasticity. CB1Rs in the developing primary visual cortex (V1) play a key role in the maturation of inhibitory circuits. Although CB1Rs were originally thought to reside mainly on presynaptic axon terminals, several studies have highlighted an unexpected role for astrocytic CB1Rs in endocannabinoid mediated plasticity. Here, we investigate the impact of cell-type-specific removal of CB1Rs from interneurons or astrocytes on development of inhibitory synapses and network plasticity in mouse V1. We show that removing CB1Rs from astrocytes interferes with maturation of inhibitory synaptic transmission. In addition, it strongly reduces ocular dominance (OD) plasticity during the critical period. In contrast, removing interneuron CB1Rs leaves these processes intact. Our results reveal an unexpected role of astrocytic CB1Rs in critical period plasticity in V1 and highlight the involvement of glial cells in plasticity and synaptic maturation of sensory circuits.

Layer specific regulation of critical period timing and maturation of mouse visual cortex by endocannabinoids

Plasticity during the critical period is important for the functional maturation of cortical neurons. While characteristics of plasticity are diverse among cortical layers, it is unknown whether critical period timing is controlled by a common or unique molecular mechanism among them. We here clarified layer-specific regulation of the critical period timing of ocular dominance plasticity in the primary visual cortex. Mice lacking the endocannabinoid synthesis enzyme diacylglycerol lipase-α exhibited precocious critical period timing, earlier maturation of inhibitory synaptic function in layers 2/3 and 4, and impaired development of the binocular matching of orientation selectivity exclusively in layer 2/3. Activation of cannabinoid receptor restored ocular dominance plasticity at the normal critical period in layer 2/3. Suppression of GABAA receptor rescued precocious ocular dominance plasticity in layer 4. Therefore, endocannabinoids regulate critical period timing and maturation of visual function partly through the development of inhibitory synaptic functions in a layer-dependent manner.

Neonatal GABAergic transmission primes vestibular gating of output for adult spatial navigation

GABAergic interneurons are poised with the capacity to shape circuit output via inhibitory gating. How early in the development of medial vestibular nucleus (MVN) are GABAergic neurons recruited for feedforward shaping of outputs to higher centers for spatial navigation? The role of early GABAergic transmission in assembling vestibular circuits for spatial navigation was explored by neonatal perturbation. Immunohistochemistry and confocal imaging were utilized to reveal the expression of parvalbumin (PV)-expressing MVN neurons and their perineuronal nets. Whole-cell patch-clamp recording, coupled with optogenetics, was conducted in vitro to examine the synaptic function of MVN circuitry. Chemogenetic targeting strategy was also employed in vivo to manipulate neuronal activity during navigational tests. We found in rats a neonatal critical period before postnatal day (P) 8 in which competitive antagonization of GABAergic transmission in the MVN retarded maturation of inhibitory neurotransmission, as evidenced by deranged developmental trajectory for excitation/inhibition ratio and an extended period of critical period-like plasticity in GABAergic transmission. Despite increased number of PV-expressing GABAergic interneurons in the MVN, optogenetic-coupled patch-clamp recording indicated null-recruitment of these neurons in tuning outputs along the ascending vestibular pathway. Such perturbation not only offset output dynamics of ascending MVN output neurons, but was further accompanied by impaired vestibular-dependent navigation in adulthood. The same perturbations were however non-consequential when applied after P8. Results highlight neonatal GABAergic transmission as key to establishing feedforward output dynamics to higher brain centers for spatial cognition and navigation.

Serotonergic Modulation of the Excitation/Inhibition Balance in the Visual Cortex

Serotonergic neurons constitute one of the main systems of neuromodulators, whose diffuse projections regulate the functions of the cerebral cortex. Serotonin (5-HT) is known to play a crucial role in the differential modulation of cortical activity related to behavioral contexts. Some features of the 5-HT signaling organization suggest its possible participation as a modulator of activity-dependent synaptic changes during the critical period of the primary visual cortex (V1). Cells of the serotonergic system are among the first neurons to differentiate and operate. During postnatal development, ramifications from raphe nuclei become massively distributed in the visual cortical area, remarkably increasing the availability of 5-HT for the regulation of excitatory and inhibitory synaptic activity. A substantial amount of evidence has demonstrated that synaptic plasticity at pyramidal neurons of the superficial layers of V1 critically depends on a fine regulation of the balance between excitation and inhibition (E/I). 5-HT could therefore play an important role in controlling this balance, providing the appropriate excitability conditions that favor synaptic modifications. In order to explore this possibility, the present work used in vitro intracellular electrophysiological recording techniques to study the effects of 5-HT on the E/I balance of V1 layer 2/3 neurons, during the critical period. Serotonergic action on the E/I balance has been analyzed on spontaneous activity, evoked synaptic responses, and long-term depression (LTD). Our results pointed out that the predominant action of 5-HT implies a reduction in the E/I balance. 5-HT promoted LTD at excitatory synapses while blocking it at inhibitory synaptic sites, thus shifting the Hebbian alterations of synaptic strength towards lower levels of E/I balance.

Investigation of the efficacy and safety of retinal inactivation as a treatment for amblyopia in cats

Introduction

Deprivation of normal vision early in postnatal development elicits modifications of neural circuitry within the primary visual pathway that can cause a severe and intractable vision impairment (amblyopia). In cats, amblyopia is often modeled with monocular deprivation (MD), a procedure that involves temporarily closing the lids of one eye. Following long-term MD, brief inactivation of the dominant eye's retina can promote recovery from the anatomical and physiological effects of MD. In consideration of retinal inactivation as a viable treatment for amblyopia it is imperative to compare its efficacy against conventional therapy, as well as assess the safety of its administration.

Methods

In the current study we compared the respective efficacies of retinal inactivation and occlusion of the dominant eye (reverse occlusion) to elicit physiological recovery from a prior long-term MD in cats. Because deprivation of form vision has been associated with development of myopia, we also examined whether ocular axial length or refractive error were altered by a period of retinal inactivation.

Results

The results of this study demonstrate that after a period of MD, inactivation of the dominant eye for up to 10 days elicited significant recovery of visually-evoked potentials that was superior to the recovery measured after a comparable duration of reverse occlusion. After monocular retinal inactivation, measurements of ocular axial length and refractive error were not significantly altered from their pre-inactivation values. The rate of body weight gain also was not changed during the period of inactivation, indicating that general well-being was not affected.

Discussion

These results provide evidence that inactivation of the dominant eye after a period of amblyogenic rearing promotes better recovery than eye occlusion, and this recovery was achieved without development of form-deprivation myopia.

The plasticitome of cortical interneurons

Hebb postulated that, to store information in the brain, assemblies of excitatory neurons coding for a percept are bound together via associative long-term synaptic plasticity. In this view, it is unclear what role, if any, is carried out by inhibitory interneurons. Indeed, some have argued that inhibitory interneurons are not plastic. Yet numerous recent studies have demonstrated that, similar to excitatory neurons, inhibitory interneurons also undergo long-term plasticity. Here, we discuss the many diverse forms of long-term plasticity that are found at inputs to and outputs from several types of cortical inhibitory interneuron, including their plasticity of intrinsic excitability and their homeostatic plasticity. We explain key plasticity terminology, highlight key interneuron plasticity mechanisms, extract overarching principles and point out implications for healthy brain functionality as well as for neuropathology. We introduce the concept of the plasticitome - the synaptic plasticity counterpart to the genome or the connectome - as well as nomenclature and definitions for dealing with this rich diversity of plasticity. We argue that the great diversity of interneuron plasticity rules is best understood at the circuit level, for example as a way of elucidating how the credit-assignment problem is solved in deep biological neural networks.

Visual Deprivation Selectively Reduces Thalamic Reticular Nucleus-Mediated Inhibition of the Auditory Thalamus in Adults

Sensory loss leads to widespread cross-modal plasticity across brain areas to allow the remaining senses to guide behavior. While multimodal sensory interactions are often attributed to higher-order sensory areas, cross-modal plasticity has been observed at the level of synaptic changes even across primary sensory cortices. In particular, vision loss leads to widespread circuit adaptation in the primary auditory cortex (A1) even in adults. Here we report using mice of both sexes in which cross-modal plasticity occurs even earlier in the sensory-processing pathway at the level of the thalamus in a modality-selective manner. A week of visual deprivation reduced inhibitory synaptic transmission from the thalamic reticular nucleus (TRN) to the primary auditory thalamus (MGBv) without changes to the primary visual thalamus (dLGN). The plasticity of TRN inhibition to MGBv was observed as a reduction in postsynaptic gain and short-term depression. There was no observable plasticity of the cortical feedback excitatory synaptic transmission from the primary visual cortex to dLGN or TRN and A1 to MGBv, which suggests that the visual deprivation-induced plasticity occurs predominantly at the level of thalamic inhibition. We provide evidence that visual deprivation-induced change in the short-term depression of TRN inhibition to MGBv involves endocannabinoid CB1 receptors. TRN inhibition is considered critical for sensory gating, selective attention, and multimodal performances; hence, its plasticity has implications for sensory processing. Our results suggest that selective disinhibition and altered short-term dynamics of TRN inhibition in the spared thalamic nucleus support cross-modal plasticity in the adult brain.SIGNIFICANCE STATEMENT Losing vision triggers adaptation of the brain to enhance the processing of the remaining senses, which can be observed as better auditory performance in blind subjects. We previously found that depriving vision of adult rodents produces widespread circuit reorganization in the primary auditory cortex and enhances auditory processing at a neural level. Here we report that visual deprivation-induced plasticity in adults occurs much earlier in the auditory pathway, at the level of thalamic inhibition. Sensory processing is largely gated at the level of the thalamus via strong cortical feedback inhibition mediated through the thalamic reticular nucleus (TRN). We found that TRN inhibition of the auditory thalamus is selectively reduced by visual deprivation, thus playing a role in adult cross-modal plasticity.

Visual-area-specific tonic modulation of GABA release by endocannabinoids sets the activity and coordination of neocortical principal neurons

Perisomatic inhibition of pyramidal neurons (PNs) coordinates cortical network activity during sensory processing, and this role is mainly attributed to parvalbumin-expressing basket cells (BCs). However, cannabinoid receptor type 1 (CB1)-expressing interneurons are also BCs, but the connectivity and function of these elusive but prominent neocortical inhibitory neurons are unclear. We find that their connectivity pattern is visual area specific. Persistently active CB1 signaling suppresses GABA release from CB1 BCs in the medial secondary visual cortex (V2M), but not in the primary visual cortex (V1). Accordingly, in vivo, tonic CB1 signaling is responsible for higher but less coordinated PN activity in the V2M than in the V1. These differential firing dynamics in the V1 and V2M can be captured by a computational network model that incorporates visual-area-specific properties. Our results indicate a differential CB1-mediated mechanism controlling PN activity, suggesting an alternative connectivity scheme of a specific GABAergic circuit in different cortical areas.

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CB1⁺ basket cells exhibit visual-area-specific morphology and connectivity patterns

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Tonic CB1 signaling underlies high pyramidal neurons (PN) activity in V2M but not V1

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Tonic CB1 signaling differentially modulates PN-correlated activity in V1 and V2M

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Numerical simulations capture specific CB1-dependent firing dynamics of V1 and V2M

Intrauterine Growth Restriction Disrupts the Postnatal Critical Period of Synaptic Plasticity in the Mouse Dorsal Hippocampus in a Model of Hypertensive Disease of Pregnancy

INTRODUCTION

Intrauterine growth restriction (IUGR) from hypertensive disease of pregnancy complicates up to 10% of all pregnancies. Significant hippocampal-dependent cognitive and memory impairments as well as neuropsychiatric disorders have been linked to IUGR. Because disturbance of the hippocampal critical period (CPd) of synaptic plasticity leads to impairments similar to those described in IUGR human offspring, we hypothesized that IUGR would perturb the CPd of synaptic plasticity in the mouse hippocampus in our model.

METHODS

IUGR was produced by a micro-osmotic pump infusion of the potent vasoconstrictor U-46619, a thromboxane A2-agonist, at embryonic day 12.5 in C57BL/6J mouse dams to precipitate hypertensive disease of pregnancy and IUGR. Sham-operated mice acted as controls. At P10, P18, and P40, we assessed astrogliosis using GFAP-IHC. In dorsal CA1 and CA3 subfields, we assessed the immunoreactivities (IR) (IF-IHC) to (i) parvalbumin (PV) and glutamate decarboxylase (GAD) 65/67, involved in CPd onset; (ii) PSA-NCAM that antagonizes CPd onset; (iii) NPTX2, necessary for excitatory synapse formation and engagement of CPd; and (iv) MBP and WFA, staining perineural nets (PNNs), marking CPd closure. ImageJ/Fiji and IMARIS were used for image processing and SPSS v24 for statistical analysis.

RESULTS

Although PV+ interneuron numbers and IR intensity were unchanged, development of GAD65/67+ synaptic boutons was accelerated at P18 IUGR mice and inversely correlated with decreased expression of PSA-NCAM in the CA of P18 IUGR mice at P18. NPTX2+ puncta and total volume were persistently decreased in the CA3 pyramidal and radiatum layers of IUGR mice from P18 to P40. At P40, axonal myelination (MBP+) in CA3 of IUGR mice was decreased and correlated with NPTX2 deficits. Lastly, the volume and integrity of the PNNs in the dorsal CA was disrupted in IUGR mice at P40.

DISCUSSION/CONCLUSION

IUGR disrupts the molecular and structural initiation, consolidation, and closure of the CPd of synaptic plasticity in the mouse hippocampus in our model, which may explain the learning and memory deficits observed in juvenile IUGR mice and the cognitive disorders seen in human IUGR offspring. The mechanistic links warrant further investigation, to identify therapeutic targets to prevent neurodevelopmental deficits in patients affected by IUGR.

A Practical Guide to Sparse k-Means Clustering for Studying Molecular Development of the Human Brain

Studying the molecular development of the human brain presents unique challenges for selecting a data analysis approach. The rare and valuable nature of human postmortem brain tissue, especially for developmental studies, means the sample sizes are small (n), but the use of high throughput genomic and proteomic methods measure the expression levels for hundreds or thousands of variables [e.g., genes or proteins (p)] for each sample. This leads to a data structure that is high dimensional (p ≫ n) and introduces the curse of dimensionality, which poses a challenge for traditional statistical approaches. In contrast, high dimensional analyses, especially cluster analyses developed for sparse data, have worked well for analyzing genomic datasets where p ≫ n. Here we explore applying a lasso-based clustering method developed for high dimensional genomic data with small sample sizes. Using protein and gene data from the developing human visual cortex, we compared clustering methods. We identified an application of sparse k-means clustering [robust sparse k-means clustering (RSKC)] that partitioned samples into age-related clusters that reflect lifespan stages from birth to aging. RSKC adaptively selects a subset of the genes or proteins contributing to partitioning samples into age-related clusters that progress across the lifespan. This approach addresses a problem in current studies that could not identify multiple postnatal clusters. Moreover, clusters encompassed a range of ages like a series of overlapping waves illustrating that chronological- and brain-age have a complex relationship. In addition, a recently developed workflow to create plasticity phenotypes (Balsor et al., 2020) was applied to the clusters and revealed neurobiologically relevant features that identified how the human visual cortex changes across the lifespan. These methods can help address the growing demand for multimodal integration, from molecular machinery to brain imaging signals, to understand the human brain’s development.

Obesity-induced astrocyte dysfunction impairs heterosynaptic plasticity in the orbitofrontal cortex

Overconsumption of highly palatable, energy-dense food is considered a key driver of the obesity pandemic. The orbitofrontal cortex (OFC) is critical for reward valuation of gustatory signals, yet how the OFC adapts to obesogenic diets is poorly understood. Here, we show that extended access to a cafeteria diet impairs astrocyte glutamate clearance, which leads to a heterosynaptic depression of GABA transmission onto pyramidal neurons of the OFC. This decrease in GABA tone is due to an increase in extrasynaptic glutamate, which acts via metabotropic glutamate receptors to liberate endocannabinoids. This impairs the induction of endocannabinoid-mediated long-term plasticity. The nutritional supplement, N-acetylcysteine rescues this cascade of synaptic impairments by restoring astrocytic glutamate transport. Together, our findings indicate that obesity targets astrocytes to disrupt the delicate balance between excitatory and inhibitory transmission in the OFC.

Spine dynamics of PSD-95-deficient neurons in the visual cortex link silent synapses to structural cortical plasticity

Critical periods (CPs) are time windows of heightened brain plasticity during which experience refines synaptic connections to achieve mature functionality. At glutamatergic synapses on dendritic spines of principal cortical neurons, the maturation is largely governed by postsynaptic density protein-95 (PSD-95)-dependent synaptic incorporation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors into nascent AMPA-receptor silent synapses. Consequently, in mouse primary visual cortex (V1), impaired silent synapse maturation in PSD-95-deficient neurons prevents the closure of the CP for juvenile ocular dominance plasticity (jODP). A structural hallmark of jODP is increased spine elimination, induced by brief monocular deprivation (MD). However, it is unknown whether impaired silent synapse maturation facilitates spine elimination and also preserves juvenile structural plasticity. Using two-photon microscopy, we assessed spine dynamics in apical dendrites of layer 2/3 pyramidal neurons (PNs) in binocular V1 during ODP in awake adult mice. Under basal conditions, spine formation and elimination ratios were similar between PSD-95 knockout (KO) and wild-type (WT) mice. However, a brief MD affected spine dynamics only in KO mice, where MD doubled spine elimination, primarily affecting newly formed spines, and caused a net reduction in spine density similar to what has been observed during jODP in WT mice. A similar increase in spine elimination after MD occurred if PSD-95 was knocked down in single PNs of layer 2/3. Thus, structural plasticity is dictated cell autonomously by PSD-95 in vivo in awake mice. Loss of PSD-95 preserves hallmark features of spine dynamics in jODP into adulthood, revealing a functional link of PSD-95 for experience-dependent synapse maturation and stabilization during CPs.

Critical aspects of neurodevelopment

Organisms have the unique ability to adapt to their environment by making use of external inputs. In the process, the brain is shaped by experiences that go hand-in-hand with optimisation of neural circuits. As such, there exists a time window for the development of different brain regions, each unique for a particular sensory modality, wherein the propensity of forming strong, irreversible connections are high, referred to as a critical period of development. Over the years, this domain of neurodevelopmental research has garnered considerable attention from many scientists, primarily because of the intensive activity-dependent nature of development. This review discusses the cellular, molecular, and neurophysiological bases of critical periods of different sensory modalities, and the disorders associated in cases the regulators of development are dysfunctional. Eventually, the neurobiological bases of the behavioural abnormalities related to developmental pathologies are discussed. A more in-depth insight into the development of the brain during the critical period of plasticity will eventually aid in developing potential therapeutics for several neurodevelopmental disorders that are categorised under critical period disorders.

Endocannabinoids and cortical plasticity: CB1R as a possible regulator of the excitation/inhibition balance in health and disease

The endocannabinoid system has been linked to neurological disorders in which the excitation inhibition (E/I) balance in the neocortex is dysregulated, such as schizophrenia. The main endocannabinoid receptor type 1 of the central nervous system-CB1R-is expressed on different cell types, that when activated, modulate the cortical E/I balance. Here we review how CB1R signalling contributes to phases of heightened plasticity of the neocortex. We review the major role of the CB1R in cortical plasticity throughout life, including the early life sensory critical periods, the later maturation phase of the association cortex in adolescence, and the adult phase of sensory deprivation-induced cortical plasticity. Endocannabinoid-mediated long-term potentiation and depression of synapse strength fine-tune the E/I balance in visual, somatosensory and association areas. We emphasize how a distinct set of key endocannabinoid-regulated elements such as GABA and glutamate release, basket parvalbumin interneurons, somatostatin interneurons and astrocytes, are essential for normal cortical plasticity and dysregulated in schizophrenia. Even though a lot of data has been gathered, mechanistic knowledge about the exact CB1R-based modulation of excitation and/or inhibition is still lacking depending on cortical area and maturation phase in life. We emphasize the importance of creating such detailed knowledge for a better comprehension of what underlies the dysregulation of the neocortex in schizophrenic patients in adulthood. We propose that taking age, brain area and cell type into consideration when modulating the cortical E/I imbalance via cannabinoid-based pharmacology may pave the way for better patient care.

Inhibitory neurotransmission drives endocannabinoid degradation to promote memory consolidation

Endocannabinoids retrogradely regulate synaptic transmission and their abundance is controlled by the fine balance between endocannabinoid synthesis and degradation. While the common assumption is that “on-demand” release determines endocannabinoid signaling, their rapid degradation is expected to control the temporal profile of endocannabinoid action and may impact neuronal signaling. Here we show that memory formation through fear conditioning selectively accelerates the degradation of endocannabinoids in the cerebellum. Learning induced a lasting increase in GABA release and this was responsible for driving the change in endocannabinoid degradation. Conversely, Gq-DREADD activation of cerebellar Purkinje cells enhanced endocannabinoid signaling and impaired memory consolidation. Our findings identify a previously unappreciated reciprocal interaction between GABA and the endocannabinoid system in which GABA signaling accelerates endocannabinoid degradation, and triggers a form of learning-induced metaplasticity.

Endocannabinoid levels are controlled by the fine balance between their synthesis and degradation. Here, the authors show that memory formation through fear conditioning selectively accelerates the degradation of endocannabinoids in the cerebellum via a lasting increase in GABA release.

Endocannabinoid Signaling Contributes to Experience-Induced Increase of Synaptic Release Sites From Parvalbumin Interneurons in Mouse Visual Cortex

During postnatal development of the visual cortex between eye-opening to puberty, visual experience promotes a gradual increase in the strength of inhibitory synaptic connections from parvalbumin-positive interneurons (PV-INs) onto layer 2/3 pyramidal cells. However, the detailed connectivity properties and molecular mechanisms underlying these developmental changes are not well understood. Using dual-patch clamp in brain slices from G42 mice, we revealed that both connection probability and the number of synaptic release sites contributed to the enhancement of synaptic strength. The increase of release site number was hindered by dark rearing from eye-opening and rescued by 3-days re-exposure to the normal visual environment. The effect of light re-exposure on restoring synaptic release sites in dark reared mice was mimicked by the agonist of cannabinoid-1 (CB1) receptors and blocked by an antagonist of these receptors, suggesting a role for endocannabinoid signaling in light-induced maturation of inhibitory connectivity from PV-INs to pyramidal cells during postnatal development.

Downregulation of the CB1-Mediated Endocannabinoid Signaling Underlies D-Galactose-Induced Memory Impairment

Imbalance in redox homeostasis is a major cause of age-related cognitive impairment. The endocannabinoid system (ECS) is a key player in regulating synaptic transmission, plasticity and memory. Increasing evidence indicates an important interplay between the two systems. However, how excessive oxidative stress could alter ECS and that, in turn, impairs its modulatory role in synaptic plasticity and cognitive function remains elusive. In the present study, we examined this causal link in D-galactose-induced oxidative rats. First, the reactive oxygen species generating enzymes, especially nitric oxide synthase (NOS), indeed show an elevated expression in D-galactose-treated rats, and this was correlated to an impaired hippocampal long-term potentiation (LTP) and spatial memory loss in animal behavioral tests. Second, the cannabinoid receptor type I (CB1)-mediated signaling is known to regulate synaptic plasticity. We show that a decrease in CB1 and increase in degradation enzymes for CB1 ligand endocannabinoid anandamide all occurred to D-galactose-treated rats. Surprisingly, application of low-dose anandamide, known to reduce LTP under physiological condition, now acted to enhance LTP in D-galactose-treated rats, most likely resulted from the inhibition of GABAergic synapses. Furthermore, this reversal behavior of CB1-signaling could be fully simulated by a NOS inhibitor, diphenyleneiodonium. These observations suggest that interaction between redox dysfunction and ECS should contribute significantly to the impaired synaptic plasticity and memory loss in D-galactose-treated rats. Therefore, therapies focusing on the balance of these two systems may shed lights on the treatment of age-related cognitive impairment in the future.

The ins and outs of inhibitory synaptic plasticity: Neuron types, molecular mechanisms and functional roles

GABAergic interneurons are highly diverse, and their synaptic outputs express various forms of plasticity. Compelling evidence indicates that activity-dependent changes of inhibitory synaptic transmission play a significant role in regulating neural circuits critically involved in learning and memory and circuit refinement. Here, we provide an updated overview of inhibitory synaptic plasticity with a focus on the hippocampus and neocortex. To illustrate the diversity of inhibitory interneurons, we discuss the case of two highly divergent interneuron types, parvalbumin-expressing basket cells and neurogliaform cells, which support unique roles on circuit dynamics. We also present recent progress on the molecular mechanisms underlying long-term, activity-dependent plasticity of fast inhibitory transmission. Lastly, we discuss the role of inhibitory synaptic plasticity in neuronal circuits' function. The emerging picture is that inhibitory synaptic transmission in the CNS is extremely diverse, undergoes various mechanistically distinct forms of plasticity and contributes to a much more refined computational role than initially thought. Both the remarkable diversity of inhibitory interneurons and the various forms of plasticity expressed by GABAergic synapses provide an amazingly rich inhibitory repertoire that is central to a variety of complex neural circuit functions, including memory.

Targeting the endocannabinoid system: a predictive, preventive, and personalized medicine-directed approach to the management of brain pathologies

Cannabis-inspired medical products are garnering increasing attention from the scientific community, general public, and health policy makers. A plethora of scientific literature demonstrates intricate engagement of the endocannabinoid system with human immunology, psychology, developmental processes, neuronal plasticity, signal transduction, and metabolic regulation. Despite the therapeutic potential, the adverse psychoactive effects and historical stigma, cannabinoids have limited widespread clinical application. Therefore, it is plausible to weigh carefully the beneficial effects of cannabinoids against the potential adverse impacts for every individual. This is where the concept of "personalized medicine" as a promising approach for disease prediction and prevention may take into the account. The goal of this review is to provide an outline of the endocannabinoid system, including endocannabinoid metabolizing pathways, and will progress to a more in-depth discussion of the therapeutic interventions by endocannabinoids in various neurological disorders.

Inhibitory Plasticity: From Molecules to Computation and Beyond

Synaptic plasticity is the cellular and molecular counterpart of learning and memory and, since its first discovery, the analysis of the mechanisms underlying long-term changes of synaptic strength has been almost exclusively focused on excitatory connections. Conversely, inhibition was considered as a fixed controller of circuit excitability. Only recently, inhibitory networks were shown to be finely regulated by a wide number of mechanisms residing in their synaptic connections. Here, we review recent findings on the forms of inhibitory plasticity (IP) that have been discovered and characterized in different brain areas. In particular, we focus our attention on the molecular pathways involved in the induction and expression mechanisms leading to changes in synaptic efficacy, and we discuss, from the computational perspective, how IP can contribute to the emergence of functional properties of brain circuits.

Postsynaptic GluN2B-containing NMDA receptors contribute to long-term depression induction in medial vestibular nucleus neurons of juvenile rats

Medial vestibular nucleus (MVN) neurons are involved in the regulation of eye movements to endure the stability of the image during head movement, and play a critical role in plasticity of the vestibulo-ocular reflex (VOR) during the juvenile period. We have previously shown that the long-term depression (LTD) of synaptic transmission was induced by high frequency stimulation (HFS) and blocked by N-methyl-D-aspartate (NMDA) receptor antagonist D-APV at the vestibular afferent synapses of type-B MVN neurons. In the present study, we used whole-cell patch-clamp recordings in vitro to investigate the subunit composition of these NMDA receptors in the induction of LTD in MVN slices from postnatal 13-16 day rats. We found that LTD induced in type-B neurons of the rat MVN with HFS was blocked by Ro 25-6981, a specific antagonist for GluN2B-containing NMDA receptors. Moreover, the other selective GluN2B-containing NMDA receptor antagonist (ifenprodil) also prevented the induction of LTD. However, bath application of the GluN2A-containing NMDA receptor antagonists (Zn²⁺ and TCN 201) had no influence on the induction of LTD. Similar results were obtained by exogenously applied two GluN2C/GluN2D-preferring NMDA receptor antagonists (PPDA and UBP 141). Furthermore, presynaptic NMDA receptor subunits are not necessary for vestibular LTD. These results suggest that the induction of LTD by HFS in vestibular afferent synapses of type-B MVN neurons requires postsynaptic GluN2B-containing NMDA receptors, but not GluN2A-containing NMDA receptors or GluN2C/GluN2D-containing NMDA receptors.

Association of mGluR-Dependent LTD of Excitatory Synapses with Endocannabinoid-Dependent LTD of Inhibitory Synapses Leads to EPSP to Spike Potentiation in CA1 Pyramidal Neurons

The input-output relationships in neural circuits are determined not only by synaptic efficacy but also by neuronal excitability. Activity-dependent alterations of synaptic efficacy have been extensively investigated, but relatively less is known about how the neuronal output is modulated when synaptic efficacy changes are associated with neuronal excitability changes. In this study, we demonstrate that paired pulses of low-frequency stimulation (PP-LFS) induced metabotropic glutamate receptor (mGluR)-dependent LTD at Schaffer collateral (SC)-CA1 synapses in Sprague Dawley rats (both sexes), and this LTD was associated with EPSP to spike (E-S) potentiation, leading to the increase in action potential (AP) outputs. Threshold voltage (V_th) for APs evoked by synaptic stimulation and that by somatic current injection were hyperpolarized significantly after PP-LFS. Blockers of GABA receptors mimicked and occluded PP-LFS effects on E-S potentiation and V_th hyperpolarization, suggesting that suppression of GABAergic mechanisms is involved in E-S potentiation after PP-LFS. Indeed, IPSCs and tonic inhibitory currents were reduced after PP-LFS. The IPSC reduction was accompanied by increased paired-pulse ratio, and abolished by AM251, a blocker for Type 1 cannabinoid receptors, suggesting that PP-LFS suppresses presynaptic GABA release by mGluR-dependent endocannabinoids signaling. By contrast, a Group 1 mGluR agonist, 3, 5-dihydroxyphenylglycine, induced LTD at SC-CA1 synapses but failed to induce significant IPSC reduction and AP output increase. We propose that mGluR signaling that induces LTD coexpression at excitatory and inhibitory synapses regulates an excitation-inhibition balance to increase neuronal output in CA1 neurons.SIGNIFICANCE STATEMENT Long-lasting forms of synaptic plasticity are usually associated with excitability changes, the ability to fire action potentials. However, excitability changes have been regarded to play subsidiary roles to synaptic plasticity in modifying neuronal output. We demonstrate that, when metabotropic glutamate receptor-dependent LTD is induced by paired pulses of low-frequency stimulation, the action potential output in response to a given input paradoxically increases, indicating that increased excitability is more powerful than synaptic depression. This increase is mediated by the suppression of a presynaptic GABA release via metabotropic glutamate receptor-dependent endocannabinoid signaling. Our study shows that neuronal output changes do not always follow the direction of synaptic plasticity at excitatory synapses, highlighting the importance of regulating inhibitory tone via endocannabinoid signaling.

Functional Relevance of Endocannabinoid-Dependent Synaptic Plasticity in the Central Nervous System

The endocannabinoid (eCB) signaling system plays a key role in short-term and long-term synaptic plasticity in brain regions involved in various neural functions ranging from action selection to appetite control. This review will explore the role of eCBs in shaping neural circuit function to regulate behaviors. In particular, we will discuss the behavioral consequences of eCB mediated long-term synaptic plasticity in different brain regions. This review brings together evidence from in vitro and ex vivo studies and points out the need for more in vivo studies.

Synaptic and circuit development of the primary sensory cortex

Animals, including humans, optimize their primary sensory cortex through the use of input signals, which allow them to adapt to the external environment and survive. The time window at the beginning of life in which external input signals are connected sensitively and strongly to neural circuit optimization is called the critical period. The critical period has attracted the attention of many neuroscientists due to the rapid activity-/experience-dependent circuit development that occurs, which is clearly differentiated from other developmental time periods and brain areas. This process involves various types of GABAergic inhibitory neurons, the extracellular matrix, neuromodulators, transcription factors, and neurodevelopmental factors. In this review, I discuss recent progress regarding the biological nature of the critical period that contribute to a better understanding of brain development.

Experience-dependent homeostasis of 'noise' at inhibitory synapses preserves information coding in adult visual cortex

Synapses are intrinsically 'noisy' in that neurotransmitter is occasionally released in the absence of an action potential. At inhibitory synapses, the frequency of action potential-independent release is orders of magnitude higher than that at excitatory synapses raising speculations that it may serve a function. Here we report that the frequency of action potential-independent inhibitory synaptic 'noise' (i.e. miniature inhibitory postsynaptic currents, mIPSCs) is highly regulated by sensory experience in visual cortex. Importantly, regulation of mIPSC frequency is so far the predominant form of functional plasticity at inhibitory synapses in adults during the refractory period for plasticity and is a locus of rapid non-genomic actions of oestrogen. Models predict that regulating the frequency of mIPSCs, together with the previously characterized synaptic scaling of miniature excitatory PSCs, allows homeostatic maintenance of both the mean and variance of inputs to a neuron, a necessary feature of probabilistic population codes. Furthermore, mIPSC frequency regulation allows preservation of the temporal profile of neural responses while homeostatically regulating the overall firing rate. Our results suggest that the control of inhibitory 'noise' allows adaptive maintenance of adult cortical function in tune with the sensory environment.This article is part of the themed issue 'Integrating Hebbian and homeostatic plasticity'.

Emerging Mechanisms Underlying Dynamics of GABAergic Synapses

Inhibitory circuits are diverse, yet with a poorly understood cell biology. Functional characterization of distinct inhibitory neuron subtypes has not been sufficient to explain how GABAergic neurotransmission sculpts principal cell activity in a relevant fashion. Our Mini-Symposium brings together several emerging mechanisms that modulate GABAergic neurotransmission dynamically from either the presynaptic or the postsynaptic site. The first two talks discuss novel developmental and neuronal subtype-specific contributions to the excitatory/inhibitory balance and circuit maturation. The next three talks examine how interactions between cellular pathways, lateral diffusion of proteins between synapses, and chloride transporter function at excitatory and inhibitory synapses and facilitate inhibitory synapse adaptations. Finally, we address functional differences within GABAergic interneurons to highlight the importance of diverse, flexible, and versatile inputs that shape network function. Together, the selection of topics demonstrates how developmental and activity-dependent mechanisms coordinate inhibition in relation to the excitatory inputs and vice versa.

Inhibition of Cdk5 rejuvenates inhibitory circuits and restores experience-dependent plasticity in adult visual cortex

Cyclin-dependent kinase 5 (Cdk5) acts as an essential modulator for neural development and neurological disorders. Here we show that Cdk5 plays a pivotal role in modulating GABAergic signaling and the maturation of visual system. In adult mouse primary visual cortex, Cdk5 formed complex with the GABA synthetic enzyme glutamate decarboxylase GAD67, but not with GAD65. In addition to enhancement in the surface level of NR2B-containing NMDA receptors, inhibition of Cdk5 reduced the protein levels of GADs and Otx2, while leaving intact the expression of vesicular GABA transporter and subunits of GABA_A or AMPA receptors. Whole-cell patch-clamp recording in layer II/III pyramidal neurons revealed a decrease in the frequency of miniature inhibitory postsynaptic current (mIPSC). Consequently, pharmacological inhibition and genetic knockdown of Cdk5 in adult mice led to a restoration of juvenile-like ocular dominance plasticity in vivo and long-term synaptic potential in layer II/III induced by white matter stimulation in vitro. Interestingly, we did not observe an alteration of perineuronal nets of extracellular matrix, but a reinstatement of the capability to evoke long-term depression at inhibitory synapses (iLTD), which depended on presynaptic endocannabinoid receptors and was a sign of the rejuvenated GABAergic synapses. Enhancement of GABA signaling by diazepam impeded ocular dominance plasticity rescued by Cdk5 inhibition. These results thus suggest that a physiological role of Cdk5 in visual cortex is to consolidate and stabilize neural circuits through controlling GABAergic signaling.

Versatility and Flexibility of Cortical Circuits

Cortical circuits are known to be plastic and adaptable, as shown by an impressive body of evidence demonstrating the ability of cortical circuits to adapt to changes in environmental stimuli, development, learning, and insults. In this review, we will discuss some of the features of cortical circuits that are thought to facilitate cortical circuit versatility and flexibility. Throughout life, cortical circuits can be extensively shaped and refined by experience while preserving their overall organization, suggesting that mechanisms are in place to favor change but also to stabilize some aspects of the circuit. First, we will describe the basic organization and some of the common features of cortical circuits. We will then discuss how this underlying cortical structure provides a substrate for the experience- and learning-dependent processes that contribute to cortical flexibility.

Developmental Disruption of GABAAR-Meditated Inhibition in Cntnap2 KO Mice

GABA released from presynaptic sites induces short-lived phasic inhibition mediated by synaptic GABA_A receptors (GABA_ARs) and longer-duration tonic inhibition mediated by extrasynaptic GABA_A or GABA_B receptors (GABA_BRs). A number of studies have found that contactin-associated protein 2 (Cntnap2) knockout (KO) mice, a well-established mouse model of autism, exhibit reduced interneuron numbers and aberrant phasic inhibition. However, little is known about whether tonic inhibition is disrupted in Cntnap2 KO mice and when the disruption of inhibition begins to occur during postnatal development. We examined tonic and phasic inhibition in layer 2/3 pyramidal cells of primary visual cortex of Cntnap2 KO at two different developmental stages, three to four and six to eight weeks of age. We found that both phasic inhibition and GABA_AR but not GABA_BR-mediated tonic inhibition was reduced in pyramidal cells from six- to eight-week-old Cntnap2 KO mice, while in three- to four-week-old mice, no significant effects of genotype on tonic or phasic inhibition was observed. We further found that activation of tonic currents mediated by δ-subunit-containing GABA_ARs reduced neural excitability, an effect that was attenuated by loss of Cntnap2. While the relative contribution of tonic versus phasic inhibition to autism-related symptoms remains unclear, our data suggest that reduced tonic inhibition may play an important role, and δ-subunit-containing GABA_ARs may be a useful target for therapeutic intervention in autism.

Molecular mechanisms of experience-dependent maturation in cortical GABAergic inhibition

Critical periods (CP) in early post-natal life are periods of plasticity during which the neuronal circuitry is most receptive to environmental stimuli. These early experiences translate to a more permanent and sophisticated neuronal connection in the adult brain systems. Multiple studies have pointed to the development of inhibitory circuitry as one of the central factors for the onset of critical periods. We discuss several molecular mechanisms regulating inhibitory circuit maturation and CP, from gene transcription level to protein signaling level. Also, beyond the level of gene sequences, we briefly consider recent information on dynamic epigenetic regulation of gene expression through histone methylation and acetylation and their implication on timed development of the inhibitory circuitry for the onset of CP.

Brief Novel Visual Experience Fundamentally Changes Synaptic Plasticity in the Mouse Visual Cortex

LTP has been known to be a mechanism by which experience modifies synaptic responses in the neocortex. Visual deprivation in the form of dark exposure or dark rearing from birth enhances NMDAR-dependent LTP in layer 2/3 of visual cortex, a process often termed metaplasticity, which may involve changes in NMDAR subunit composition and function. However, the effects of reexposure to light after dark rearing from birth on LTP induction have not been explored. Here, we showed that the light exposure after dark rearing revealed a novel NMDAR independent form of LTP in the layer 2/3 pyramidal cells in visual cortex of mice of both sexes, which is dependent on mGluR5 activation and is associated with intracellular Ca²⁺ rise, CaMKII activity, PKC activity, and intact protein synthesis. Moreover, the capacity to induce mGluR-dependent LTP is transient: it only occurs when mice of both sexes reared in the dark from birth are exposed to light for 10-12 h, and it does not occur in vision-experienced, male mice, even after prolonged exposure to dark. Thus, the mGluR5-LTP unmasked by short visual experience can only be observed after dark rearing but not after dark exposure. These results suggested that, as in hippocampus, in layer 2/3 of visual cortex, there is coexistence of two distinct activity-dependent systems of synaptic plasticity, NMDAR-LTP, and mGluR5-LTP. The mGluR5-LTP unmasked by short visual experience may play a critical role in the faster establishment of normal receptive field properties.SIGNIFICANCE STATEMENT LTP has been known to be a mechanism by which experience modifies synaptic responses in the neocortex. Visual deprivation in the form of dark exposure or dark rearing from birth enhances NMDAR-dependent LTP in layer 2/3 of visual cortex, a process often termed metaplasticity. NMDAR-dependent form of LTP in visual cortex has been well characterized. Here, we report that an NMDAR-independent form of LTP can be promoted by novel visual experience on dark-reared mice, characterized as dependent on intracellular Ca²⁺ rise, PKC activity, and intact protein synthesis and also requires the activation of mGluR5. These findings suggest that, in layer 2/3 of visual cortex, as in hippocampus, there is coexistence of two distinct activity-dependent systems of synaptic plasticity.

GABAB receptor-dependent bidirectional regulation of critical period ocular dominance plasticity in cats

Gama amino butyric acid (GABA) inhibition plays an important role in the onset and offset of the critical period for ocular dominance (OD) plasticity in the primary visual cortex. Previous studies have focused on the involvement of GABA_A receptors, while the potential contribution of GABA_B receptors to OD plasticity has been neglected. In this study, the GABA_B receptor antagonist SCH50911 or agonist baclofen was infused into the primary visual cortex of cats concurrently with a period of monocular deprivation (MD). Using single-unit recordings we found that the OD shift induced by four days of MD during the critical period was impaired by infusion of the antagonist SCH50911, but enhanced by infusion of the agonist baclofen. In contrast, seven days of MD in adult cats did not induce any significant OD shift, even when combined with the infusion of SCH50911 or baclofen. Together, these findings indicate that an endogenous GABA_B receptor-mediated inhibition contributes to juvenile, but not adult, OD plasticity.

Activation of 5-HT7 receptors reverses NMDA-R-dependent LTD by activating PKA in medial vestibular neurons

The medial vestibular nucleus (MVN) is a major output station for neurons that project to the vestibulo-spinal pathway. MVN neurons show capacity for long-term depression (LTD) during the juvenile period. We investigated LTD of MVN neurons using whole-cell patch-clamp recordings. High frequency stimulation (HFS) robustly induced LTD in 90% of type B neurons in the MVN, while only 10% of type A neurons were responsive, indicating that type B neurons are the major contributors to LTD in the MVN. The neuromodulator serotonin (5-HT) is known to modulate LTD in neural circuits of the cerebral cortex and the hippocampus. We therefore aim to determine the action of 5-HT on the LTD of type B MVN neurons and elucidate the relevant 5-HT receptor subtypes responsible for its action. Using specific agonists and antagonists of 5-HT receptors, we found that selective activation of 5-HT₇ receptor in type B neurons in the MVN of juvenile (P13-16) rats completely abolished NMDA-receptor-mediated LTD in a protein kinase A (PKA)-dependent manner. Our finding that 5-HT restricts plasticity of type B MVN neurons via 5-HT₇ receptors offers a mechanism whereby vestibular tuning contributes to the maturation of the vestibulo-spinal circuit and highlights the role of 5-HT in postural control.

Integrating Hebbian and homeostatic plasticity: introduction

Hebbian plasticity is widely considered to be the mechanism by which information can be coded and retained in neurons in the brain. Homeostatic plasticity moves the neuron back towards its original state following a perturbation, including perturbations produced by Hebbian plasticity. How then does homeostatic plasticity avoid erasing the Hebbian coded information? To understand how plasticity works in the brain, and therefore to understand learning, memory, sensory adaptation, development and recovery from injury, requires development of a theory of plasticity that integrates both forms of plasticity into a whole. In April 2016, a group of computational and experimental neuroscientists met in London at a discussion meeting hosted by the Royal Society to identify the critical questions in the field and to frame the research agenda for the next steps. Here, we provide a brief introduction to the papers arising from the meeting and highlight some of the themes to have emerged from the discussions.This article is part of the themed issue 'Integrating Hebbian and homeostatic plasticity'.

Modulation of Synaptic Plasticity in the Cortex Needs to Understand All the Players

The prefrontal cortex (PFC) is involved in cognitive tasks such as working memory, decision making, risk assessment and regulation of attention. These functions performed by the PFC are supposed to rely on rhythmic electrical activity generated by neuronal network oscillations determined by a precise balance between excitation and inhibition balance (E/I balance) resulting from the coordinated activities of recurrent excitation and feedback and feedforward inhibition. Functional alterations in PFC functions have been associated with cognitive deficits in several pathologies such as major depression, anxiety and schizophrenia. These pathological situations are correlated with alterations of different neurotransmitter systems (i.e., serotonin (5-HT), dopamine (DA), acetylcholine…) that result in alterations of the E/I balance. The aim of this review article is to cover the basic aspects of the regulation of the E/I balance as well as to highlight the importance of the complementarity role of several neurotransmitters in the modulation of the plasticity of excitatory and inhibitory synapses. We illustrate our purpose by recent findings that demonstrate that 5-HT and DA cooperate to regulate the plasticity of excitatory and inhibitory synapses targeting layer 5 pyramidal neurons (L5PyNs) of the PFC and to fine tune the E/I balance. Using a method based on the decomposition of the synaptic conductance into its excitatory and inhibitory components, we show that concomitant activation of D1-like receptors (D1Rs) and 5-HT_1ARs, through a modulation of NMDA receptors, favors long term potentiation (LTP) of both excitation and inhibition and consequently does not modify the E/I balance. We also demonstrate that activation of D2-receptors requires functional 5-HT_1ARs to shift the E-I balance towards more inhibition and to favor long term depression (LTD) of excitatory synapses through the activation of glycogen synthase kinase 3β (GSK3β). This cooperation between different neurotransmitters is particularly relevant in view of pathological situations in which alterations of one neurotransmitter system will also have consequences on the regulation of synaptic efficacy by other neurotransmitters. This opens up new perspectives in the development of therapeutic strategies for the pharmacological treatment of neuronal disorders.

Electrophysiological Measurement of Cannabinoid-Mediated Synaptic Modulation in Acute Mouse Brain Slices

Endocannabinoids (eCBs) are a class of bioactive lipids that mediate retrograde synaptic modulation at central and peripheral synapses. The highly lipophilic nature of eCBs and the pharmacological tools available to interrogate this system require unique methodological consideration, especially when applied to ex vivo systems such as electrophysiological analysis in acute brain slices. This unit provides protocols for measuring cannabinoid and eCB-mediated synaptic signaling in mouse brain slices, including analysis of short-term, long-term, and tonic eCB signaling modes, and the unique considerations for working with eCBs and TRPV1/cannabinoid ligands in acute brain slices.

An Introduction to the Endogenous Cannabinoid System

The endocannabinoid system (ECS) is a widespread neuromodulatory system that plays important roles in central nervous system development, synaptic plasticity, and the response to endogenous and environmental insults. The ECS comprises cannabinoid receptors, endogenous cannabinoids (endocannabinoids), and the enzymes responsible for the synthesis and degradation of the endocannabinoids. The most abundant cannabinoid receptors are the CB1 cannabinoid receptors; however, CB2 cannabinoid receptors, transient receptor potential channels, and peroxisome proliferator activated receptors are also engaged by some cannabinoids. Exogenous cannabinoids, such as tetrahydrocannabinol, produce their biological effects through their interactions with cannabinoid receptors. The best-studied endogenous cannabinoids are 2-arachidonoyl glycerol and arachidonoyl ethanolamide (anandamide). Despite similarities in chemical structure, 2-arachidonoyl glycerol and anandamide are synthesized and degraded by distinct enzymatic pathways, which impart fundamentally different physiologic and pathophysiologic roles to these two endocannabinoids. As a result of the pervasive social use of cannabis and the involvement of endocannabinoids in a multitude of biological processes, much has been learned about the physiologic and pathophysiologic roles of the ECS. This review provides an introduction to the ECS with an emphasis on its role in synaptic plasticity and how the ECS is perturbed in schizophrenia.

Brief Dark Exposure Reduces Tonic Inhibition in Visual Cortex

Tonic inhibition mediated by extrasynaptic GABA(A) receptors (GABARs) sensing ambient levels of GABA can profoundly alter the membrane input resistance to affect cellular excitability. Therefore, regulation of tonic inhibition is an attractive mechanism to control the levels of cortical firing. In cortical pyramidal cells, tonic inhibition is regulated by age and several neurotransmitters and is affected by stroke and epilepsy. However, the possible role of sensory experience has not been examined. Here, we report that a brief 2-day exposure to dark reduces by 1/3 the inhibitory tonic conductance recorded in layer II/III pyramidal cells of the mouse juvenile (postnatal day 12-27) visual cortex. In these cells, tonic inhibition is carried primarily by GABARs containing the δ subunit. Consistently, the dark exposure reduction in conductance was associated with a reduction in δ subunit levels, which were not affected in control frontal cortex. We propose that a deprivation-induced reduction in tonic inhibition might serve a homeostatic function by increasing the firing levels of cells in deprived cortical circuits.

The Endocannabinoid System in the Retina: From Physiology to Practical and Therapeutic Applications

Cannabis is one of the most prevalent drugs used in industrialized countries. The main effects of Cannabis are mediated by two major exogenous cannabinoids: ∆9-tetrahydroxycannabinol and cannabidiol. They act on specific endocannabinoid receptors, especially types 1 and 2. Mammals are endowed with a functional cannabinoid system including cannabinoid receptors, ligands, and enzymes. This endocannabinoid signaling pathway is involved in both physiological and pathophysiological conditions with a main role in the biology of the central nervous system. As the retina is a part of the central nervous system due to its embryonic origin, we aim at providing the relevance of studying the endocannabinoid system in the retina. Here, we review the distribution of the cannabinoid receptors, ligands, and enzymes in the retina and focus on the role of the cannabinoid system in retinal neurobiology. This review describes the presence of the cannabinoid system in critical stages of retinal processing and its broad involvement in retinal neurotransmission, neuroplasticity, and neuroprotection. Accordingly, we support the use of synthetic cannabinoids as new neuroprotective drugs to prevent and treat retinal diseases. Finally, we argue for the relevance of functional retinal measures in cannabis users to evaluate the impact of cannabis use on human retinal processing.

Presynaptic GluN2D receptors detect glutamate spillover and regulate cerebellar GABA release

Glutamate directly activates N-methyl-d-aspartate (NMDA) receptors on presynaptic inhibitory interneurons and enhances GABA release, altering the excitatory-inhibitory balance within a neuronal circuit. However, which class of NMDA receptors is involved in the detection of glutamate spillover is not known. GluN2D subunit-containing NMDA receptors are ideal candidates as they exhibit a high affinity for glutamate. We now show that cerebellar stellate cells express both GluN2B and GluN2D NMDA receptor subunits. Genetic deletion of GluN2D subunits prevented a physiologically relevant, stimulation-induced, lasting increase in GABA release from stellate cells [long-term potentiation of inhibitory transmission (I-LTP)]. NMDA receptors are tetramers composed of two GluN1 subunits associated to either two identical subunits (di-heteromeric receptors) or to two different subunits (tri-heteromeric receptors). To determine whether tri-heteromeric GluN2B/2D NMDA receptors mediate I-LTP, we tested the prediction that deletion of GluN2D converts tri-heteromeric GluN2B/2D to di-heteromeric GluN2B NMDA receptors. We find that prolonged stimulation rescued I-LTP in GluN2D knockout mice, and this was abolished by GluN2B receptor blockers that failed to prevent I-LTP in wild-type mice. Therefore, NMDA receptors that contain both GluN2D and GluN2B mediate the induction of I-LTP. Because these receptors are not present in the soma and dendrites, presynaptic tri-heteromeric GluN2B/2D NMDA receptors in inhibitory interneurons are likely to mediate the cross talk between excitatory and inhibitory transmission.

Expression and Function of the Endocannabinoid System in the Retina and the Visual Brain

Endocannabinoids are important retrograde modulators of synaptic transmission throughout the nervous system. Cannabinoid receptors are seven transmembrane G-protein coupled receptors favoring G_i/o protein. They are known to play an important role in various processes, including metabolic regulation, craving, pain, anxiety, and immune function. In the last decade, there has been a growing interest for endocannabinoids in the retina and their role in visual processing. The purpose of this review is to characterize the expression and physiological functions of the endocannabinoid system in the visual system, from the retina to the primary visual cortex, with a main interest regarding the retina, which is the best-described area in this system so far. It will show that the endocannabinoid system is widely present in the retina, mostly in the through pathway where it can modulate neurotransmitter release and ion channel activity, although some evidence also indicates possible mechanisms via amacrine, horizontal, and Müller cells. The presence of multiple endocannabinoid ligands, synthesizing and catabolizing enzymes, and receptors highlights various pharmacological targets for novel therapeutic application to retinal diseases.

The neuronal identity bias behind neocortical GABAergic plasticity

In the neocortex, different types of excitatory and inhibitory neurons connect to one another following a detailed blueprint, defining functionally-distinct subnetworks, whose activity and modulation underlie complex cognitive functions. We review the cell-autonomous plasticity of perisomatic inhibition onto principal excitatory neurons. We propose that the tendency of different cortical layers to exhibit depression or potentiation of perisomatic inhibition is dictated by the specific identities of principal neurons (PNs). These are mainly defined by their projection targets and by their preference to be innervated by specific perisomatic-targeting basket cell types. Therefore, principal neurons responsible for relaying information to subcortical nuclei are differentially inhibited and show specific forms of plasticity compared to other PNs that are specialized in more associative functions.

Layer-specific endocannabinoid-mediated long-term depression of GABAergic neurotransmission onto principal neurons in mouse visual cortex

Visually induced endocannabinoid-mediated long-term depression of GABAergic neurotransmission (iLTD) mediates the maturation of GABAergic release in layer 2/3 of visual cortex. Here we examined whether the maturation of GABAergic transmission in other layers of visual cortex also requires endocannabinoids. The developmental plasticity of GABAergic neurotransmission onto the principal neurons in different layers of mouse visual cortex was examined in cortical slices by whole-cell recordings of inhibitory postsynaptic currents evoked by presynaptic inhibitory inputs. Theta burst stimulation of GABAergic inputs induced an endocannabinoid-mediated long-term depression of GABAergic neurotransmission onto pyramidal cells in layer 2/3 from postnatal day (P)10 to 30 and in layer 5 from P10 to 40, whereas that of GABAergic inputs did not induce iLTD onto star pyramidal neurons in layer 4 at any time postnatally, indicating that this plasticity is laminar-specific. The developmental loss of iLTD paralleled the maturation of GABAergic inhibition in both layer 2/3 and layer 5. Visual deprivation delayed the developmental loss of iLTD in layers 3 and 5 during a critical period, while 2 days of light exposure eliminated iLTD in both layers. Furthermore, the GABAergic synapses in layers 2/3 and 5 did not normally mature in the type 1 cannabinoid receptor knock-out mice, whereas those in layer 4 did not require endocannabinoid receptor for maturation. These results suggest that visually induced endocannabinoid-dependent iLTD mediates the maturation of GABAergic release in extragranular layer rather than in granular layer of mouse visual cortex.

Vision loss shifts the balance of feedforward and intracortical circuits in opposite directions in mouse primary auditory and visual cortices

Loss of a sensory modality leads to widespread changes in synaptic function across sensory cortices, which are thought to be the basis for cross-modal adaptation. Previous studies suggest that experience-dependent cross-modal regulation of the spared sensory cortices may be mediated by changes in cortical circuits. Here, we report that loss of vision, in the form of dark exposure (DE) for 1 week, produces laminar-specific changes in excitatory and inhibitory circuits in the primary auditory cortex (A1) of adult mice to promote feedforward (FF) processing and also strengthens intracortical inputs to primary visual cortex (V1). Specifically, DE potentiated FF excitatory synapses from layer 4 (L4) to L2/3 in A1 and recurrent excitatory inputs in A1-L4 in parallel with a reduction in the strength of lateral intracortical excitatory inputs to A1-L2/3. This suggests a shift in processing in favor of FF information at the expense of intracortical processing. Vision loss also strengthened inhibitory synaptic function in L4 and L2/3 of A1, but via laminar specific mechanisms. In A1-L4, DE specifically potentiated the evoked synaptic transmission from parvalbumin-positive inhibitory interneurons to principal neurons without changes in spontaneous miniature IPSCs (mIPSCs). In contrast, DE specifically increased the frequency of mIPSCs in A1-L2/3. In V1, FF excitatory inputs were unaltered by DE, whereas lateral intracortical connections in L2/3 were strengthened, suggesting a shift toward intracortical processing. Our results suggest that loss of vision produces distinct circuit changes in the spared and deprived sensory cortices to shift between FF and intracortical processing to allow adaptation.

Progressive maturation of silent synapses governs the duration of a critical period

During critical periods, all cortical neural circuits are refined to optimize their functional properties. The prevailing notion is that the balance between excitation and inhibition determines the onset and closure of critical periods. In contrast, we show that maturation of silent glutamatergic synapses onto principal neurons was sufficient to govern the duration of the critical period for ocular dominance plasticity in the visual cortex of mice. Specifically, postsynaptic density protein-95 (PSD-95) was absolutely required for experience-dependent maturation of silent synapses, and its absence before the onset of critical periods resulted in lifelong juvenile ocular dominance plasticity. Loss of PSD-95 in the visual cortex after the closure of the critical period reinstated silent synapses, resulting in reopening of juvenile-like ocular dominance plasticity. Additionally, silent synapse-based ocular dominance plasticity was largely independent of the inhibitory tone, whose developmental maturation was independent of PSD-95. Moreover, glutamatergic synaptic transmission onto parvalbumin-positive interneurons was unaltered in PSD-95 KO mice. These findings reveal not only that PSD-95-dependent silent synapse maturation in visual cortical principal neurons terminates the critical period for ocular dominance plasticity but also indicate that, in general, once silent synapses are consolidated in any neural circuit, initial experience-dependent functional optimization and critical periods end.

Role for Endogenous BDNF in Endocannabinoid-Mediated Long-Term Depression at Neocortical Inhibitory Synapses

The endogenous cannabinoid (endocannabinoid) system is an important regulator of synaptic function. Endocannabinoids acutely modulate inhibitory and excitatory transmission, and also mediate long-term depression at GABAergic and glutamatergic synapses. Typically, endocannabinoid synthesis and release is stimulated by depolarization-induced calcium influx and/or activation of phospholipase-C (PLC) signaling triggered by mGluR activation. Recently it has been shown that brain-derived neurotrophic factor (BDNF) can also induce endocannabinoid release. Although there is growing evidence for cross-talk between BDNF and endocannabinoid signaling, little is known about the functional relevance of these interactions. In the present studies, we examined BDNF - endocannabinoid interactions in regulating activity-dependent long-term depression at inhibitory synapses (iLTD). We found that theta burst stimulation (TBS) in layer 2/3 of mouse somatosensory cortical slices can induce a form of endocannabinoid-mediated iLTD that is independent of metabotropic glutamate receptor (mGluR) activation. This endocannabinoid-dependent iLTD, however, requires endogenous BDNF-trkB signaling, as it is blocked by a trk tyrosine kinase inhibitor and by a trkB receptor antagonist, and also requires activation of diacylglycerol lipase (DAG-lipase, DGL). In addition, endocannabinoid-mediated iLTD can be induced by combining a subthreshold concentration of exogenous BDNF with weak TBS stimulation that by itself is insufficient to induce iLTD. Taken together, our results suggest that TBS can induce the release of endogenous BDNF, which triggers DGL-dependent endocannabinoid mobilization and cannabinoid receptor-dependent iLTD at layer 2/3 cortical synapses.

Sleep slow wave-related homo and heterosynaptic LTD of intrathalamic GABAAergic synapses: involvement of T-type Ca2+ channels and metabotropic glutamate receptors

Slow waves of non-REM sleep are suggested to play a role in shaping synaptic connectivity to consolidate recently acquired memories and/or restore synaptic homeostasis. During sleep slow waves, both GABAergic neurons of the nucleus reticularis thalami (NRT) and thalamocortical (TC) neurons discharge high-frequency bursts of action potentials mediated by low-threshold calcium spikes due to T-type Ca(2+) channel activation. Although such activity of the intrathalamic network characterized by high-frequency firing and calcium influx is highly suited to modify synaptic efficacy, very little is still known about its consequences on intrathalamic synapse strength. Combining in vitro electrophysiological recordings and calcium imaging, here we show that the inhibitory GABAergic synapses between NRT and TC neurons of the rat somatosensory nucleus develop a long-term depression (I-LTD) when challenged by a stimulation paradigm that mimics the thalamic network activity occurring during sleep slow waves. The mechanism underlying this plasticity presents unique features as it is both heterosynaptic and homosynaptic in nature and requires Ca(2+) entry selectively through T-type Ca(2+) channels and activation of the Ca(2+)-activated phosphatase, calcineurin. We propose that during slow-wave sleep the tight functional coupling between GABAA receptors, calcineurin, and T-type Ca(2+) channels will elicit LTD of the activated GABAergic synapses when coupled with concomitant activation of metabotropic glutamate receptors postsynaptic to cortical afferences. This I-LTD may be a key element involved in the reshaping of the somatosensory information pathway during sleep.

Visual experience prevents dysregulation of GABAB receptor-dependent short-term depression in adult superior colliculus

Progressive loss of plasticity during development prevents refined circuits from regressing to an immature state and is thought to depend on maturation of GABAergic inhibition. For example, a gradual reduction in size of visual receptive fields (RFs) occurs in the superior colliculus (SC) during development. Maintenance of the refined state throughout adulthood requires early light exposure. Here we investigate the potential role of changes in long- or short-term plasticity in experience-dependent maintenance of refined RFs. Using an acute SC slice preparation, we found that long-term plasticity was not affected by visual deprivation, indicating that it does not underlie deprivation-induced RF enlargement. In contrast, visual deprivation altered short-term plasticity in an unexpected way. Specifically, GABAB receptor (GABABR)-mediated paired pulse depression was increased in slices from dark-reared animals. This increase was mimicked by GABAAR blockade in slices from normally reared animals, suggesting that experience-dependent maintenance of GABAAR function prevents an increase in probability of neurotransmitter release. GABABR-mediated short-term depression in response to strong stimulation (such as occurs during vision) was reduced in slices from dark-reared animals. This change was mimicked in slices from normal animals by reducing GABA release. These results are consistent with the hypothesis that early visual experience maintains GABAergic inhibition and prevents later deprivation-induced alterations of short-term depression in SC. Identifying how plasticity is restricted in mature circuits could guide therapies to enhance recovery of function in adults.