Epidermal Growth Factor Suppresses the Development of GABAergic Neurons Via the Modulation of Perineuronal Net Formation in the Neocortex of Developing Rodent Brains

Previously, we reported that epidermal growth factor (EGF) suppresses GABAergic neuronal development in the rodent cortex. Parvalbumin-positive GABAergic neurons (PV neurons) have a unique extracellular structure, perineuronal nets (PNNs). PNNs are formed during the development of PV neurons and are mainly formed from chondroitin sulfate (CS) proteoglycans (CSPGs). We examined the effect of EGF on CSPG production and PNN formation as a potential molecular mechanism for the inhibition of inhibiting GABAergic neuronal development by EGF. In EGF-overexpressing transgenic (EGF-Tg) mice, the number of PNN-positive PV neurons was decreased in the cortex compared with that in wild-type mice, as in our previous report. The amount of CS and neurocan was also lower in the cortex of EGF-Tg mice, with a similar decrease observed in EGF-treated cultured cortical neurons. PD153035, an EGF receptor (ErbB1) kinase inhibitor, prevented those mentioned above excess EGF-induced reduction in PNN. We explored the molecular mechanism underlying the effect of EGF on PNNs using fluorescent substrates for matrix metalloproteinases (MMPs) and a disintegrin and metalloproteinases (ADAMs). EGF increased the enzyme activity of MMPs and ADAMs in cultured neurons. These enzyme activities were also increased in the EGF-Tg mice cortex. GM6001, a broad inhibitor of MMPs and ADAMs, also blocked EGF-induced PNN reductions. Therefore, EGF/EGF receptor signals may regulate PNN formation in the developing cortex.

LRP1 in GABAergic neurons is a key link between obesity and memory function

OBJECTIVE

Low-density lipoprotein receptor-related protein-1 (LRP1) regulates energy homeostasis, blood-brain barrier integrity, and metabolic signaling in the brain. Deficiency of LRP1 in inhibitory gamma-aminobutyric acid (GABA)ergic neurons causes severe obesity in mice. However, the impact of LRP1 in inhibitory neurons on memory function and cognition in the context of obesity is poorly understood.

METHODS

Mice lacking LRP1 in GABAergic neurons (Vgat-Cre; LRP1loxP/loxP) underwent behavioral tests for locomotor activity and motor coordination, short/long-term and spatial memory, and fear learning/memory. This study evaluated the relationships between behavior and metabolic risk factors and followed the mice at 16 and 32 weeks of age.

RESULTS

Deletion of LRP1 in GABAergic neurons caused a significant impairment in memory function in 32-week-old mice. In the spatial Y-maze test, Vgat-Cre; LRP1loxP/loxP mice exhibited decreased travel distance and duration in the novel arm compared with controls (LRP1loxP/loxP mice). In addition, GABAergic neuron-specific LRP1-deficient mice showed a diminished capacity for performing learning and memory tasks during the water T-maze test. Moreover, reduced freezing time was observed in these mice during the contextual and cued fear conditioning tests. These effects were accompanied by increased neuronal necrosis and satellitosis in the hippocampus. Importantly, the distance and duration in the novel arm, as well as the performance of the reversal water T-maze test, negatively correlated with metabolic risk parameters, including body weight, serum leptin, insulin, and apolipoprotein J. However, in 16-week-old Vgat-Cre; LRP1loxP/loxP mice, there were no differences in the behavioral tests or correlations between metabolic parameters and cognition.

CONCLUSIONS

Our findings demonstrate that LRP1 from GABAergic neurons is important in regulating normal learning and memory. Metabolically, obesity caused by GABAergic LRP1 deletion negatively regulates memory and cognitive function in an age-dependent manner. Thus, LRP1 in GABAergic neurons may play a crucial role in maintaining normal excitatory/inhibitory balance, impacting memory function, and reinforcing the potential importance of LRP1 in neural system integrity.

TGR5-mediated lateral hypothalamus-dCA3-dorsolateral septum circuit regulates depressive-like behavior in male mice

Although bile acids play a notable role in depression, the pathological significance of the bile acid TGR5 membrane-type receptor in this disorder remains elusive. Using depression models of chronic social defeat stress and chronic restraint stress in male mice, we found that TGR5 in the lateral hypothalamic area (LHA) predominantly decreased in GABAergic neurons, the excitability of which increased in depressive-like mice. Upregulation of TGR5 or inhibition of GABAergic excitability in LHA markedly alleviated depressive-like behavior, whereas down-regulation of TGR5 or enhancement of GABAergic excitability facilitated stress-induced depressive-like behavior. TGR5 also bidirectionally regulated excitability of LHA GABAergic neurons via extracellular regulated protein kinases-dependent Kv4.2 channels. Notably, LHA GABAergic neurons specifically innervated dorsal CA3 (dCA3) CaMKIIα neurons for mediation of depressive-like behavior. LHA GABAergic TGR5 exerted antidepressant-like effects by disinhibiting dCA3 CaMKIIα neurons projecting to the dorsolateral septum (DLS). These findings advance our understanding of TGR5 and the LHAGABA→dCA3CaMKIIα→DLSGABA circuit for the development of potential therapeutic strategies in depression.

Preferential expression of SCN1A in GABAergic neurons improves survival and epileptic phenotype in a mouse model of Dravet syndrome

The SCN1A gene encodes the alpha subunit of a voltage-gated sodium channel (Nav1.1), which is essential for the function of inhibitory neurons in the brain. Mutations in this gene cause severe encephalopathies such as Dravet syndrome (DS). Upregulation of SCN1A expression by different approaches has demonstrated promising therapeutic effects in preclinical models of DS. Limiting the effect to inhibitory neurons may contribute to the restoration of brain homeostasis, increasing the safety and efficacy of the treatment. In this work, we have evaluated different approaches to obtain preferential expression of the full SCN1A cDNA (6 Kb) in GABAergic neurons, using high-capacity adenoviral vectors (HC-AdV). In order to favour infection of these cells, we considered ErbB4 as a surface target. Incorporation of the EGF-like domain from neuregulin 1 alpha (NRG1α) in the fiber of adenovirus capsid allowed preferential infection in cells lines expressing ErbB4. However, it had no impact on the infectivity of the vector in primary cultures or in vivo. For transcriptional control of transgene expression, we developed a regulatory sequence (DP3V) based on the Distal-less homolog enhancer (Dlx), the vesicular GABA transporter (VGAT) promoter, and a portion of the SCN1A gene. The hybrid DP3V promoter allowed preferential expression of transgenes in GABAergic neurons both in vitro and in vivo. A new HC-AdV expressing SCN1A under the control of this promoter showed improved survival and amelioration of the epileptic phenotype in a DS mouse model. These results increase the repertoire of gene therapy vectors for the treatment of DS and indicate a new avenue for the refinement of gene supplementation in this disease. KEY MESSAGES: Adenoviral vectors can deliver the SCN1A cDNA and are amenable for targeting. An adenoviral vector displaying an ErbB4 ligand in the capsid does not target GABAergic neurons. A hybrid promoter allows preferential expression of transgenes in GABAergic neurons. Preferential expression of SCN1A in GABAergic cells is therapeutic in a Dravet syndrome model.

Dopaminergic Projections from the Hypothalamic A11 Nucleus to the Spinal Trigeminal Nucleus Are Involved in Bidirectional Migraine Modulation

Clinical imaging studies have revealed that the hypothalamus is activated in migraine patients prior to the onset of and during headache and have also shown that the hypothalamus has increased functional connectivity with the spinal trigeminal nucleus. The dopaminergic system of the hypothalamus plays an important role, and the dopamine-rich A11 nucleus may play an important role in migraine pathogenesis. We used intraperitoneal injections of glyceryl trinitrate to establish a model of acute migraine attack and chronicity in mice, which was verified by photophobia experiments and von Frey experiments. We explored the A11 nucleus and its downstream pathway using immunohistochemical staining and neuronal tracing techniques. During acute migraine attack and chronification, c-fos expression in GABAergic neurons in the A11 nucleus was significantly increased, and inhibition of DA neurons was achieved by binding to GABA A-type receptors on the surface of dopaminergic neurons in the A11 nucleus. However, the expression of tyrosine hydroxylase and glutamic acid decarboxylase proteins in the A11 nucleus of the hypothalamus did not change significantly. Specific destruction of dopaminergic neurons in the A11 nucleus of mice resulted in severe nociceptive sensitization and photophobic behavior. The expression levels of the D1 dopamine receptor and D2 dopamine receptor in the caudal part of the spinal trigeminal nucleus candalis of the chronic migraine model were increased. Skin nociceptive sensitization of mice was slowed by activation of the D2 dopamine receptor in SP5C, and activation of the D1 dopamine receptor reversed this behavioral change. GABAergic neurons in the A11 nucleus were activated and exerted postsynaptic inhibitory effects, which led to a decrease in the amount of DA secreted by the A11 nucleus in the spinal trigeminal nucleus candalis. The reduced DA bound preferentially to the D2 dopamine receptor, thus exerting a defensive effect against headache.

Stress induces behavioral abnormalities by increasing expression of phagocytic receptor MERTK in astrocytes to promote synapse phagocytosis

Childhood neglect and/or abuse can induce mental health conditions with unknown mechanisms. Here, we identified stress hormones as strong inducers of astrocyte-mediated synapse phagocytosis. Using in vitro, in vivo, and human brain organoid experiments, we showed that stress hormones increased the expression of the Mertk phagocytic receptor in astrocytes through glucocorticoid receptor (GR). In post-natal mice, exposure to early social deprivation (ESD) specifically activated the GR-MERTK pathway in astrocytes, but not in microglia. The excitatory post-synaptic density in cortical regions was reduced in ESD mice, and there was an increase in the astrocytic engulfment of these synapses. The loss of excitatory synapses, abnormal neuronal network activities, and behavioral abnormalities in ESD mice were largely prevented by ablating GR or MERTK in astrocytes. Our work reveals the critical roles of astrocytic GR-MERTK activation in evoking stress-induced abnormal behaviors in mice, suggesting GR-MERTK signaling as a therapeutic target for stress-induced mental health conditions.

Activation of Cannabinoid Receptor 1 in GABAergic Neurons in the Rostral Anterior Insular Cortex Contributes to the Analgesia Following Common Peroneal Nerve Ligation

The rostral agranular insular cortex (RAIC) has been associated with pain modulation. Although the endogenous cannabinoid system (eCB) has been shown to regulate chronic pain, the roles of eCBs in the RAIC remain elusive under the neuropathic pain state. Neuropathic pain was induced in C57BL/6 mice by common peroneal nerve (CPN) ligation. The roles of the eCB were tested in the RAIC of ligated CPN C57BL/6J mice, glutamatergic, or GABAergic neuron cannabinoid receptor 1 (CB1R) knockdown mice with the whole-cell patch-clamp and pain behavioral methods. The E/I ratio (amplitude ratio between mEPSCs and mIPSCs) was significantly increased in layer V pyramidal neurons of the RAIC in CPN-ligated mice. Depolarization-induced suppression of inhibition but not depolarization-induced suppression of excitation in RAIC layer V pyramidal neurons were significantly increased in CPN-ligated mice. The analgesic effect of ACEA (a CB1R agonist) was alleviated along with bilateral dorsolateral funiculus lesions, with the administration of AM251 (a CB1R antagonist), and in CB1R knockdown mice in GABAergic neurons, but not glutamatergic neurons of the RAIC. Our results suggest that CB1R activation reinforces the function of the descending pain inhibitory pathway via reducing the inhibition of glutamatergic layer V neurons by GABAergic neurons in the RAIC to induce an analgesic effect in neuropathic pain.

Insulin facilitates synaptic transmission via gap junctions between fast-spiking interneurons in the rat insular cortex

PURPOSE

Inhibitory synaptic currents from fast-spiking neurons (FSNs), a typical gamma-aminobutyric acid (GABA)ergic interneuron in the cerebral cortex, to pyramidal neurons are facilitated by insulin. FSNs frequently show electrical synapses to FSNs, however, the effect of insulin on these electrical synapses is unknown. The aim of this study was to evaluate effects of insulin on electrical synaptic potentials between FSNs.

METHODS

Electrical synaptic potentials via gap junctions between FSNs were recorded to examine how insulin modulates these potentials in the rat insular cortex (IC).

RESULTS

Bath application of insulin (10 nM), which increases the spike firing rate of pyramidal neurons and unitary inhibitory postsynaptic currents recorded from FSN to pyramidal neuron connections, slightly but significantly increased electrical synaptic currents. The mean ratio of electrical synapses, the coupling coefficient that is obtained by postsynaptic voltage responses divided by presynaptic voltage amplitude, was 8.3 ± 1.1% in control and 9.2 ± 1.1% (n = 14) during 10 nM insulin application. Input resistance and voltage responses to large hyperpolarizing currents (-140 pA) were not changed by insulin.

CONCLUSION

These results suggest that insulin facilitates spike synchronization by increasing electrical synaptic currents via gap junctions of GABAergic FSNs in the IC.

GABAergic neurons maturation is regulated by a delicate network

Gamma-aminobutyric acid-expressing (GABAergic) neurons are implicated in a variety of neuropsychiatric disorders, such as epilepsy, anxiety, autism, and other pathological processes, including cerebral ischemia injury and drug addiction. Therefore, GABAergic neuronal processes warrant further research. The development of GABAergic neurons is a tightly controlled process involving the activity of multiple transcription and growth factors. Here, we focus on the gene expression pathways and the molecular modulatory networks that are engaged during the development of GABAergic neurons with the goal of exploring regulatory mechanisms that influence GABAergic neuron fate (i.e., maturation). Overall, we hope to provide a basis for clarifying the pathogenesis of neurodegenerative disorders.

Effects of retinoic acid receptor α modulators on developmental ethanol-induced neurodegeneration and neuroinflammation

Ethanol exposure in neonatal mice induces acute neurodegeneration followed by long-lasting glial activation and GABAergic cell deficits along with behavioral abnormalities, providing a third trimester model of fetal alcohol spectrum disorders (FASD). Retinoic acid (RA), the active form of vitamin A, regulates transcription of RA-responsive genes and plays essential roles in the development of embryos and their CNS. Ethanol has been shown to disturb RA metabolism and signaling in the developing brain, which may be a cause of ethanol toxicity leading to FASD. Using an agonist and an antagonist specific to RA receptor α (RARα), we studied how RA/RARα signaling affects acute and long-lasting neurodegeneration and activation of phagocytic cells and astrocytes caused by ethanol administered to neonatal mice. We found that an RARα antagonist (BT382) administered 30 min before ethanol injection into postnatal day 7 (P7) mice partially blocked acute neurodegeneration as well as elevation of CD68-positive phagocytic cells in the same brain area. While an RARα agonist (BT75) did not affect acute neurodegeneration, BT75 given either before or after ethanol administration ameliorated long-lasting astrocyte activation and GABAergic cell deficits in certain brain regions. Our studies using Nkx2.1-Cre;Ai9 mice, in which major GABAergic neurons and their progenitors in the cortex and the hippocampus are labeled with constitutively expressed tdTomato fluorescent protein, indicate that the long-lasting GABAergic cell deficits are mainly caused by P7 ethanol-induced initial neurodegeneration. However, the partial reduction of prolonged GABAergic cell deficits and glial activation by post-ethanol BT75 treatment suggests that, in addition to the initial cell death, there may be delayed cell death or disturbed development of GABAergic cells, which is partially rescued by BT75. Since RARα agonists including BT75 have been shown to exert anti-inflammatory effects, BT75 may rescue GABAergic cell deficits by reducing glial activation/neuroinflammation.

Cell proliferation and neurogenesis in the adult telencephalon of the newt Cynops pyrrhogaster

Urodele amphibians have the ability to regenerate several organs, including the brain. For this reason, the research on neurogenesis in these species after ablation of some parts of the brain has markedly progressed. However, detailed information on the characteristics and fate of proliferated cells as well as the function of newly generated neurons under normal conditions is still limited. In this study, we focused on investigating the proliferative and neurogenic zones as well as the fate of proliferated cells in the adult brain of the Japanese red-bellied newt to clarify the significance of neurogenesis in adulthood. We found that the proximal region of the lateral ventricles in the telencephalon and the preoptic area in the diencephalon were the main sites for continuous cell proliferation in the adult brain. Furthermore, we characterized proliferative cells and analyzed neurogenesis through a combination of 5-ethynyl-2'-deoxyuridine (EdU) labeling and immunohistochemistry using antibodies against the stem cell marker Sox2 and neuronal marker NeuN. Twenty-four hours after EdU injection, most of the EdU-positive cells were Sox2-immunopositive, whereas, EdU-positive signals and NeuN-immunoreactivities were not colocalized. Two months after EdU injection, the colocalization ratio of EdU-positive signals with Sox2-immunoreactivities decreased to approximately 10%, whereas the ratio of colocalization of EdU-positive signals with NeuN-immunoreactivities increased to approximately 60%. Furthermore, a portion of the EdU-incorporated cells developed into γ-aminobutyric acid-producing cells, which are assumed to function as interneurons. On the basis of these results, the significance of newly generated neurons was discussed with special reference to their reproductive behavior.

miR-124- and let-7-Mediated Reprogram of Human Fibroblasts into SST Interneurons

Many neurological disorders stem from defects in or the loss of specific neurons. Dysfunction of γ-aminobutyric acid (GABA)ergic interneurons may cause a variety of neurological and psychiatric disorders such as epilepsy, autism, Alzheimer's disease, and depression. Unlike other types of neurons, which can be generated relatively easily by direct reprogramming, it is difficult to generate GABAergic neurons by traditional methods. Neuronal transdifferentiation of fibroblasts mediated by nongenomic-integrated adenovirus has many advantages, but the efficiency is low, and there is a lack of studies using human cells as the initial materials. In this study, we explored the feasibility of the conversion of human fibroblasts into neurons through adenovirus-mediated gene expression and found that by introducing two microRNAs, miR-124 and let-7, together with several small chemical compounds, they can effectively generate GABAergic neuron-like cells from human neonatal fibroblasts without reverting to a progenitor cell stage. Most of these cells expressed neuronal markers and were all somatostatin (SST)-positive cells. Therefore, our study provides a relatively safe and efficient method to generate SST interneurons.

Transient Receptor Potential Melastatin 2 Thermosensitive Neurons in the Preoptic Area Involved in Menopausal Hot Flashes in Ovariectomized Mice

INTRODUCTION

Menopausal hot flashes are related to hypothalamic preoptic area (POA) dysfunction. Thermosensitive transient receptor potential channels (ThermoTRPs) are involved in temperature sensing and regulation of thermosensitive neurons (TSNs) in the POA. Whether ThermoTRP-TSNs in the POA, particularly the non-noxious thermoreceptor, transient receptor potential melastatin 2 (TRPM2), are involved in the occurrence of hot flashes is still unclear.

METHODS

Twenty wild-type and 50 Trpm2-Cre adult female mice were randomly divided into sham (SHAM) and ovariectomy (OVX) groups. In the POA, ERα, ERβ, GPR30, TRPA1, TRPM8, TRPM2, and TRPV1 expression was detected by Western blot or/and quantitative real-time polymerase chain reaction and the number of TSNs expressing TRPM2 (TRPM2-TSNs) by immunofluorescence. Before and after TRPM2-TSN activation/inhibition, back (BST) and tail skin temperature (TST) and the proportion of glutamatergic and GABAergic neurons among TRPM2-TSNs were recorded.

RESULTS

Compared with SHAM, the expression of ERα, ERβ, TRPM2, and TRPM8 in the POA of the OVX group decreased, with a significantly larger change range for TRPM2 than TRPM8. In addition, the number of TRPM2-TSNs showing TRPA1, TRPM8, and TRPV1 expression in the OVX group decreased, and the proportion of glutamatergic and GABAergic neurons in TRPM2-TSNs decreased and increased, respectively. Meanwhile, BST and TST increased. After activating or inhibiting TRPM2-TSNs, the proportions of glutamatergic and GABAergic neurons in TRPM2-TSNs changed, along with the BST and TST.

CONCLUSION

In menopause, the abnormal quantity and function of TRPM2-TSNs in the POA is key for the development of hot flashes, characterized by an imbalance in heat dissipation and production due to the corresponding imbalance in glutamatergic and GABAergic neurons.

Neuroregenerative gene therapy to treat temporal lobe epilepsy in a rat model

Temporal lobe epilepsy (TLE) is a common drug-resistant epilepsy associated with abundant cell death in the hippocampus. Here, we develop a novel gene therapy-mediated cell therapy that regenerates GABAergic neurons using internal hippocampal astrocytes to suppress seizure activity in a rat TLE model. We discovered that TLE-induced reactive astrocytes in the hippocampal CA1 region can be efficiently converted into GABAergic neurons after overexpressing a neural transcription factor NeuroD1. The astrocyte-converted neurons showed typical markers of GABAergic interneurons, fired action potentials, and formed functional synaptic connections with other neurons. Following NeuroD1-mediated astrocyte-to-neuron conversion, the number of hippocampal interneurons was significantly increased, and the spontaneous recurrent seizure (SRS) activity was significantly decreased. Moreover, NeuroD1 gene therapy treatment rescued total neuronal loss in the CA1 region and ameliorated the cognitive and mood dysfunctions in the TLE rat model. These results suggest that regeneration of GABAergic interneurons through gene therapy approach may provide a novel therapeutic intervention to treat drug-resistant TLE.

Effect and mechanism of BDNF/TrkB signaling on vestibular compensation

Brain-derived neurotrophic factor (BDNF) regulates neuronal plasticity by targeting the tyrosine kinase B receptor (TrkB) receptor, but limited researches concentrate on the role of BDNF/TrkB signaling in vestibular compensation. In this study, rats with unilateral vestibular dysfunction were established by unilateral labyrinthectomy (UL) and infusion with siBDNF or 7, 8-Dihydroxyflavone (7,8-DHF, a TrkB receptor agonist). The behavioral scores of rats with vestibular deficits were determined and the rotarod test was performed after UL. BDNF and TrkB levels after UL were determined by western blot and quantitative reverse transcription PCR (qRT-PCR). 5-bromo-2'-deoxyuridine (BrdU)-positive cells (newly generated cells) and GAD67-positive cells (GABAergic neurons) were identified by immunohistochemistry. Glial fibrillary acidic protein (GFAP) (astrocyte marker)-positive cells were identified and GABA type A receptor (GABAAR) expression was detected by immunofluorescence. We found that after UL, BDNF and TrkB levels were up-regulated with a maximum value at 4 h, and then progressively down-regulated during 4 h ~ 7 d. Blocking BDNF/TrkB signaling inhibited the recovery from vestibular deficits, decreased the numbers of newly generated cells and astrocytes in medial vestibular nucleus (MVN), inferior vestibular nerve (IVN), superior vestibular nerve (SVN) and lateral vestibular nucleus (LVN), and disrupted the balances of GABAergic neurons and GABAAR expressions in the left (lesioned) side and right (intact) side of MVN, whereas activation of BDNF/TrkB signaling caused opposite results. The current study indicated that BDNF/TrkB signaling avails vestibular compensation, depending on the number of newly generated cells and astrocytes, the rebalance of GABAergic neurons, and GABAAR expression in bilateral MVN.

Whole-brain connectivity atlas of glutamatergic and GABAergic neurons in the mouse dorsal and median raphe nuclei

The dorsal raphe nucleus (DR) and median raphe nucleus (MR) contain populations of glutamatergic and GABAergic neurons that regulate diverse behavioral functions. However, their whole-brain input-output circuits remain incompletely elucidated. We used viral tracing combined with fluorescence micro-optical sectioning tomography to generate a comprehensive whole-brain atlas of inputs and outputs of glutamatergic and GABAergic neurons in the DR and MR. We found that these neurons received inputs from similar upstream brain regions. The glutamatergic and GABAergic neurons in the same raphe nucleus had divergent projection patterns with differences in critical brain regions. Specifically, MR glutamatergic neurons projected to the lateral habenula through multiple pathways. Correlation and cluster analysis revealed that glutamatergic and GABAergic neurons in the same raphe nucleus received heterogeneous inputs and sent different collateral projections. This connectivity atlas further elucidates the anatomical architecture of the raphe nuclei, which could facilitate better understanding of their behavioral functions.

Acupuncture reduces cocaine psychomotor responses by activating the rostromedial tegmental nucleus

The rostromedial tegmental nucleus (RMTg), a GABAergic afferent to midbrain dopamine (DA) neurons, has emerged as an integral player in both rewarding and nociceptive responses. While previous studies have demonstrated that acupuncture modulates DA transmission in the mesolimbic reward system originating in the ventral tegmental area (VTA) and projecting to the nucleus accumbens (NAc) and can reduce drug self-administration, the central links between peripheral acupuncture signals and brain reward systems are not well-characterized. Thus, we hypothesised that acupuncture would elicit inhibitory signals from RMTg neurons to brain reward systems. Acupuncture reduced acute cocaine-induced locomotor activity and DA release in a point-specific manner, which was blocked by optogenetic silencing or chemical lesion of the RMTg. The acupuncture effect was mimicked by chemical activation of the RMTg. Acupuncture activated RMTg GABA neurons. In addition, the inhibitory effects of acupuncture on acute cocaine-induced locomotor activity were prevented by electrolytic lesions of the lateral habenula (LHb) or fasciculus retroflexus (FR), areas known to project to the RMTg. These findings suggest that acupuncture recruits the RMTg to reduce the psychomotor responses enhanced by acute cocaine.

ApoE4 increases susceptibility to stress-induced age-dependent depression-like behavior and cognitive impairment

Though apolipoprotein E ε4 (APOE ε4) is a major genetic risk factor for late-onset Alzheimer's disease, its association with depression remains controversial. In present study, 3-month-old and 8-month-old apoE-targeted replacement (TR) mice were both subjected to chronic unpredictable mild stress (CUMS) for six weeks. The results showed that 8-month apoE4-TR mice were more susceptible to the CUMS-induced depression-like behaviors and cognitive impairment than age-matched apoE3-TR mice. Stress induced a loss of GABAergic neurons and decline of Reelin level in the prefrontal cortex (PFC) and in the dentate gyrus (DG) of the hippocampus in both 3-month-old and 8-month-old apoE-TR mice, which were more pronounced in the 8-month-old apoE4-TR mice. Of note, stress decreased the level of PSD95 in the hippocampal synaptosome and increased the phosphorylation of N-methyl-D-aspartate receptor subunit GluN2B in the hippocampus of 8-month-old apoE4-TR mice. However, the expressions of apoE and apoE receptor 2 (apoER2) were not affected by stress. The study provides rodent evidence that APOE ε4 may increase the risk of depression and dementia in the elderly population by impairing the GABAergic signaling pathway and enhancing the GluN2B phosphorylation, which signifies that GluN2B inhibitors in clinical settings may be effective for elderly depression patients with APOE4 carriers.

An Inhibitory Medial Preoptic Circuit Mediates Innate Exploration

Animals have an innate motivation to explore objects and environments with unknown values. To this end, they need to activate neural pathways that enable exploration. Here, we reveal that photostimulation of a subset of medial preoptic area (MPA) neurons expressing the vesicular-GABA transporter gene (vgat+) and sending axonal projections to the ventrolateral periaqueductal gray (vPAG) increases exploration in a chamber but causes no place preference when tested there without photostimulation. Photoinhibition of MPA^vgat–vPAG projections leads to no emotional changes as measured by normal activity in an open field assay. Electrophysiological recordings revealed that most GABAergic vPAG neurons are inhibited by MPA^vgat neurons. In contrast to a previous report that suggested that MPA^vgat–vPAG neurons may impart positive valence to induce place preference, our results suggest that these neurons can increase innate exploration.

Adenylate Cyclase 1 Links Calcium Signaling to CFTR-Dependent Cytosolic Chloride Elevations in Chick Amacrine Cells

The strength and sign of synapses involving ionotropic GABA and glycine receptors are dependent upon the Cl^- gradient. We have shown that nitric oxide (NO) elicits the release of Cl^- from internal acidic stores in retinal amacrine cells (ACs); temporarily altering the Cl^- gradient and the strength or even sign of incoming GABAergic or glycinergic synapses. The underlying mechanism for this effect of NO requires the cystic fibrosis transmembrane regulator (CFTR) but the link between NO and CFTR activation has not been determined. Here, we test the hypothesis that NO-dependent Ca²⁺ elevations activate the Ca²⁺-dependent adenylate cyclase 1 (AdC1) leading to activation of protein kinase A (PKA) whose activity is known to open the CFTR channel. Using the reversal potential of GABA-gated currents to monitor cytosolic Cl^-, we established the requirement for Ca²⁺ elevations. Inhibitors of AdC1 suppressed the NO-dependent increases in cytosolic Cl^- whereas inhibitors of other AdC subtypes were ineffective suggesting that AdC1 is involved. Inhibition of PKA also suppressed the action of NO. To address the sufficiency of this pathway in linking NO to elevations in cytosolic Cl^-, GABA-gated currents were measured under internal and external zero Cl^- conditions to isolate the internal Cl^- store. Activators of the cAMP pathway were less effective than NO in producing GABA-gated currents. However, coupling the cAMP pathway activators with the release of Ca²⁺ from stores produced GABA-gated currents indistinguishable from those stimulated with NO. Together, these results demonstrate that cytosolic Ca²⁺ links NO to the activation of CFTR and the elevation of cytosolic Cl^-.

Propofol differentially induces unconsciousness and respiratory depression through distinct interactions between GABAA receptor and GABAergic neuron in corresponding nuclei

Propofol is the most commonly used intravenous anesthetic worldwide. It can induce loss of consciousness prior to the occurrence of severe respiratory suppression, which is also a pharmacodynamic feature of all general anesthetics. However, the neural mechanisms underlying this natural phenomenon are controversial and highly related to patient safety. In the present study, we demonstrated that the pharmacodynamic effects of propofol (50 and 100 μM) on suppression of consciousness-related excitatory postsynaptic currents in the medial prefrontal cortex (mPFC) and centromedian nucleus of the thalamus (CMT) were lower than those in the kernel respiratory rhythmogenesis nucleus pre-Bötzinger complex (PrBo). Furthermore, we unexpectedly found that the GABAA receptor β3 subunit is the key target for propofol's action and that it is mutually and exclusively expressed in GABAergic neurons. It is also more abundant in the mPFC and CMT, but mainly co-localized with GABAergic neurons in the PrBo. As a result, the differentiated expression pattern should mediate more neuron suppression through the activation of GABAergic neurons in the mPFC and CMT at low doses of propofol (50 μM). However, PrBo GABAergic neurons were only activated by propofol at a high dose (100 μM). These results highlight the detailed pharmacodynamic effects of propofol on consciousness-related and respiration-related nuclei and provide the distinct interaction mechanism between the β3 subunit and GABAergic neurons in mediating the suppression of consciousness compared to the inhibition of respiration.

Single-cell alternative polyadenylation analysis delineates GABAergic neuron types

Background

Alternative polyadenylation (APA) is emerging as an important mechanism in the post-transcriptional regulation of gene expression across eukaryotic species. Recent studies have shown that APA plays key roles in biological processes, such as cell proliferation and differentiation. Single-cell RNA-seq technologies are widely used in gene expression heterogeneity studies; however, systematic studies of APA at the single-cell level are still lacking.

Results

Here, we described a novel computational framework, SAPAS, that utilizes 3′-tag-based scRNA-seq data to identify novel poly(A) sites and quantify APA at the single-cell level. Applying SAPAS to the scRNA-seq data of phenotype characterized GABAergic interneurons, we identified cell type-specific APA events for different GABAergic neuron types. Genes with cell type-specific APA events are enriched for synaptic architecture and communications. In further, we observed a strong enrichment of heritability for several psychiatric disorders and brain traits in altered 3′ UTRs and coding sequences of cell type-specific APA events. Finally, by exploring the modalities of APA, we discovered that the bimodal APA pattern of Pak3 could classify chandelier cells into different subpopulations that are from different laminar positions.

Conclusions

We established a method to characterize APA at the single-cell level. When applied to a scRNA-seq dataset of GABAergic interneurons, the single-cell APA analysis not only identified cell type-specific APA events but also revealed that the modality of APA could classify cell subpopulations. Thus, SAPAS will expand our understanding of cellular heterogeneity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01076-3.

Distinct Expression of SLM2 Underlies Splicing-Dependent Trans-Synaptic Signaling of Neurexin Across GABAergic Neuron Subtypes

The mammalian brain contains multiple types of neuronal cells with complex assemblies and distinct structural and functional properties encoded by divergent gene programs. There is increasing evidence that alternative splicing (AS) plays fundamental roles in transcriptomic diversity and specifying synaptic properties of each neuronal cell type. However, the mechanisms underlying AS regulation and whether it controls synapse formation across GABAergic interneurons have not been fully elucidated. Here we show the differential expression levels of Sam68-like molecule 2 (SLM2), a major splicing regulator of neurexin (NRX), in GABAergic neuronal subtypes and its contribution to GABAergic synapse specification. Cortical SLM2 is strongly expressed not only in excitatory neurons but also in a subpopulation of GABAergic interneurons, especially in VIP-positive neurons that are originated from late-born caudal ganglionic eminence (GE)- derived cells. Using artificial synapse formation assay, we found that GE containing cortices form a strong synapse with LRRTM2, a trans-synaptic receptor of the alternatively spliced segment 4 (AS4)(-) of NRX. SLM2 knock-down reduced the NRX AS4(-) isoform expression and hence weaken LRRTM2-induced synapse formation. The addition of NRX AS4(-) was sufficient to rescue the synaptic formation by LRRTM2 in SLM2 knock-down neurons. Thus, our findings suggest a novel function of SLM2 in modifying network formation of a specific population of GABAergic interneurons and contribute to a better understanding of the roles AS plays in regulating synapse specificity and neuronal molecular diversity.

Takeda G Protein-Coupled Receptor 5 Modulates Depression-like Behaviors via Hippocampal CA3 Pyramidal Neurons Afferent to Dorsolateral Septum

BACKGROUND

Takeda G protein-coupled receptor 5 (TGR5) is recognized as a promising target for type 2 diabetes and metabolic syndrome; its expression has been demonstrated in the brain and is thought to be neuroprotective. Here, we hypothesize that dysfunction of central TGR5 may contribute to the pathogenesis of depression.

METHODS

In well-established chronic social defeat stress (CSDS) and chronic restraint stress (CRS) models of depression, we investigated the functional roles of TGR5 in CA3 pyramidal neurons (PyNs) and underlying mechanisms of the neuronal circuit in depression (for in vivo studies, n = 10; for in vitro studies, n = 5-10) using fiber photometry; optogenetic, chemogenetic, pharmacological, and molecular profiling techniques; and behavioral tests.

RESULTS

Both CSDS and CRS most significantly reduced TGR5 expression of hippocampal CA3 PyNs. Genetic overexpression of TGR5 in CA3 PyNs or intra-CA3 infusion of INT-777, a specific agonist, protected against CSDS and CRS, exerting significant antidepressant-like effects that were mediated via CA3 PyN activation. Conversely, genetic knockout or TGR5 knockdown in CA3 facilitated stress-induced depression-like behaviors. Re-expression of TGR5 in CA3 PyNs rather than infusion of INT-777 significantly improved depression-like behaviors in Tgr5 knockout mice exposed to CSDS or CRS. Silencing and stimulation of CA3 PyNs→somatostatin-GABAergic (gamma-aminobutyric acidergic) neurons of the dorsolateral septum circuit bidirectionally regulated depression-like behaviors, and blockade of this circuit abrogated the antidepressant-like effects from TGR5 activation of CA3 PyNs.

CONCLUSIONS

These findings indicate that TGR5 can regulate depression via CA3 PyNs→somatostatin-GABAergic neurons of dorsolateral septum transmission, suggesting that TGR5 could be a novel target for developing antidepressants.

Endocannabinoid Signaling for GABAergic-Microglia (Mis)Communication in the Brain Aging

The aging brain seems to be characterized by neuronal loss leading to cognitive decline and progressively worsening symptoms related to neurodegeneration. Also, pro-inflammatory states, if prolonged, may increase neuronal vulnerability via excessive activation of microglia and their pro-inflammatory by-products, which is seen as individuals increase in age. Consequently, microglial activity is tightly regulated by neuron-microglia communications. The endocannabinoid system (ECS) is emerging as a regulator of microglia and the neuronal-microglia communication system. Recently, it has been demonstrated that cannabinoid 1 (CB1) receptor signaling on GABAergic interneurons plays a crucial role in regulating microglial activity. Interestingly, if endocannabinoid signaling on GABAergic neurons are disturbed, the phenotypes mimic central nervous system insult models by activating microglia and leading to accelerated brain aging. Investigating the endocannabinoid receptors, ligands, and genetic deletions yields the potential to understand the communication system and mechanism by which the ECS regulates glial cells and aspects of aging. While there remains much to discover with the ECS, the information gathered and identified already could lead to the development of cell-specific therapeutic interventions that help in reducing the effects of age-related pro-inflammatory states and neurodegeneration.

Direct In Vitro Reprogramming of Astrocytes into Induced Neurons

Spontaneous neuronal replacement is almost absent in the postnatal mammalian nervous system. However, several studies have shown that both early postnatal and adult astroglia can be reprogrammed in vitro or in vivo by forced expression of proneural transcription factors, such as Neurogenin-2 or Achaete-scute homolog 1 (Ascl1), to acquire a neuronal fate. The reprogramming process stably induces properties such as distinctly neuronal morphology, expression of neuron-specific proteins, and the gain of mature neuronal functional features. Direct conversion of astroglia into neurons thus possesses potential as a basis for cell-based strategies against neurological diseases. In this chapter, we describe a well-established protocol used for direct reprogramming of postnatal cortical astrocytes into functional neurons in vitro and discuss available tools and approaches to dissect molecular and cell biological mechanisms underlying the reprogramming process.

D-limonene Inhibits Pentylenetetrazole-Induced Seizure via Adenosine A2A Receptor Modulation on GABAergic Neuronal Activity

Background: Epilepsy is a chronic neurological disorder characterized by the recurrence of seizures. One-third of patients with epilepsy may not respond to antiseizure drugs. Purpose: We aimed to examine whether D-limonene, a cyclic monoterpene, exhibited any antiseizure activity in the pentylenetetrazole (PTZ)-induced kindling mouse model and in vitro. Methods: PTZ kindling mouse model was established by administering PTZ (30 mg/kg) intraperitoneally to mice once every 48 h. We performed immunoblot blots, immunohistochemistry (IHC), and high-performance liquid chromatography (HPLC) analysis after the behavioral study. Results: An acute injection of PTZ (60 mg/kg) induced seizure in mice, while pretreatment with D-limonene inhibited PTZ-induced seizure. Repeated administration of PTZ (30 mg/kg) increased the seizure score gradually in mice, which was reduced in D-limonene (10 mg/kg)-pretreated group. In addition, D-limonene treatment increased glutamate decarboxylase-67 (GAD-67) expression in the hippocampus. Axonal sprouting of hippocampal neurons after kindling was inhibited by D-limonene pretreatment. Moreover, D-limonene reduced the expression levels of Neuronal PAS Domain Protein 4 (Npas4)-induced by PTZ. Furthermore, the adenosine A2A antagonist SCH58261 and ZM241385 inhibited anticonvulsant activity and gamma-aminobutyric acid (GABA)ergic neurotransmission-induced by D-limonene. Conclusion: These results suggest that D-limonene exhibits anticonvulsant activity through modulation of adenosine A2A receptors on GABAergic neuronal function.

The Wnt/Ca2+ signaling pathway is essential for the regeneration of GABAergic neurons in planarian Dugesia japonica

The growth and differentiation of neurons are critical events in the establishment of proper neuron connectivity and function. Planarians have a remarkable ability to completely regenerate a functional nervous system from a pluripotent stem cell population. Thus, planarians provide a powerful model to identify genes required for neuronal differentiation in vivo. The Wnt/Ca²⁺ signaling pathway is crucial for cancer development, arousing inflammatory responses, and neurodegeneration. We analyzed the expression patterns and RNAi phenotypes for members of the Wnt/Ca²⁺ signaling pathway in the planarian, Dugesia japonica. The expression of DjWnt5a, DjPLC-β, DjCamKII, and DjCaln during regeneration was surprisingly similar and revealing in the regenerated brain. RNAi knockdown of DjWnt5a, DjPLC-β, DjCamKII, and DjCaln led to defects in regenerated brains including brain partial deletions, incompact phenotypes at the posterior of the new brain, and lateral branches, which could not regenerate. Furthermore, the expressions of GAD and the number of GABAergic neurons decreased. Together, these results suggest that the Wnt/Ca²⁺ signaling pathway is required for GABAergic neuron regeneration.

Mapping Brain-Wide Afferent Inputs of Parvalbumin-Expressing GABAergic Neurons in Barrel Cortex Reveals Local and Long-Range Circuit Motifs

Parvalbumin (PV)-expressing GABAergic neurons are the largest class of inhibitory neocortical cells. We visualize brain-wide, monosynaptic inputs to PV neurons in mouse barrel cortex. We develop intersectional rabies virus tracing to specifically target GABAergic PV cells and exclude a small fraction of excitatory PV cells from our starter population. Local inputs are mainly from layer (L) IV and excitatory cells. A small number of inhibitory inputs originate from LI neurons, which connect to LII/III PV neurons. Long-range inputs originate mainly from other sensory cortices and the thalamus. In visual cortex, most transsynaptically labeled neurons are located in LIV, which contains a molecularly mixed population of projection neurons with putative functional similarity to LIII neurons. This study expands our knowledge of the brain-wide circuits in which PV neurons are embedded and introduces intersectional rabies virus tracing as an applicable tool to dissect the circuitry of more clearly defined cell types.

Repeated maternal separation causes transient reduction in BDNF expression in the medial prefrontal cortex during early brain development, affecting inhibitory neuron development

It is widely accepted that maternal separation (MS) induces stress in children and disrupts neural circuit formation during early brain development. Even though such disruption occurs transiently early in life, its influence persists after maturation, and could lead to various neurodevelopmental disorders. Our recent study revealed that repeated MS reduces the number of inhibitory neurons and synapses in the medial prefrontal cortex (mPFC) and causes mPFC-related social deficits after maturation. However, how MS impedes mPFC development during early brain development remains poorly understood. Here, we focused on brain-derived neurotrophic factor (BDNF) involved in the development of inhibitory neurons, and examined time-dependent BDNF expression in the mPFC during the pre-weaning period in male rats exposed to MS. Our results show that MS attenuates BDNF expression only around the end of the first postnatal week. Likewise, mRNA expression of activity-regulated cytoskeleton-associated protein (Arc), an immediate-early gene whose expression is partly regulated by BDNF, also decreased in the MS group along with the reduction in BDNF expression. On the contrary, mRNA expression of tropomyosin-related kinase B (TrkB), which is a BDNF receptor, was scarcely altered, while its protein expression decreased in the MS group only during the weaning period. In addition, MS reduced mRNA levels of glutamic acid decarboxylase (GAD) 65, a GABA synthesizing enzyme, only during the weaning period. Our results suggest that repeated MS temporarily attenuates BDNF signaling in the mPFC during early brain development. BDNF plays a crucial role in the development of inhibitory neurons; therefore, transient attenuation of BDNF signaling may cause delays in GABAergic neuron development in the mPFC.

Disinhibition of PVN-projecting GABAergic neurons in AV region in BNST participates in visceral hypersensitivity in rats

Ample evidence suggests that early life stress (ELS) is a high-risk factor for the development of visceral pain disorders, whereas the mechanism underlying neuronal circuit remains elusive. Herein, we employed neonatal colorectal distension (CRD) to induce visceral hypersensitivity in rats. A combination of electrophysiology, pharmacology, behavioral test, molecular biology, chemogenetics and optogenetics confirmed that CRD in neonatal rats could predispose the elevated firing frequency of the parvocellular corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus of hypothalamus (PVN) in adulthood, with the CRH neurons activated and the frequency of spontaneous inhibitory postsynaptic currents (sIPSC) diminished, both contributing to chronic visceral hypersensitivity. Moreover, following administration of exogenous GABA (300 mM/0.5 μL) and GABA_A receptor agonist muscimol (3 mM/0.5 μL) in PVN, visceral hyperalgesia was abrogated. In addition, the PVN-projecting GABAergic neurons were mainly distributed in the anterior ventral (AV) region in the bed nucleus of stria terminalis (BNST), and the excitability of these GABAergic neurons was weakened in visceral hypersensitivity. Specific depletion of the GABAergic neurons in AV region precipitated visceral hyperalgesia. Moreover, chemogenetic activation of the PVN-projecting neurons alleviated the visceral hypersensitivity. Photoactivation of PVN-projecting GABAergic neurons abated the visceral hypersensitivity in neonatal-CRD rats, whereas photoinhibition evoked visceral hyperalgesia in naïve rats. Our findings demonstrated that disinhibition of the PVN-projecting GABAergic neurons in AV region contributed to the excitation of CRH neurons, thereby mediating visceral hypersensitivity. Our study might provide a novel insight into the neuronal circuits involved in the ELS-induced visceral hypersensitivity.

Remotely Produced and Axon-Derived Netrin-1 Instructs GABAergic Neuron Migration and Dopaminergic Substantia Nigra Development

The midbrain dopamine (mDA) system is composed of molecularly and functionally distinct neuron subtypes that mediate specific behaviors and show select disease vulnerability, including in Parkinson's disease. Despite progress in identifying mDA neuron subtypes, how these neuronal subsets develop and organize into functional brain structures remains poorly understood. Here we generate and use an intersectional genetic platform, Pitx3-ITC, to dissect the mechanisms of substantia nigra (SN) development and implicate the guidance molecule Netrin-1 in the migration and positioning of mDA neuron subtypes in the SN. Unexpectedly, we show that Netrin-1, produced in the forebrain and provided to the midbrain through axon projections, instructs the migration of GABAergic neurons into the ventral SN. This migration is required to confine mDA neurons to the dorsal SN. These data demonstrate that neuron migration can be controlled by remotely produced and axon-derived secreted guidance cues, a principle that is likely to apply more generally.

GABAergic dysfunction in excitatory and inhibitory (E/I) imbalance drives the pathogenesis of Alzheimer's disease

OBJECTIVE

To propose a new hypothesis that GABAergic dysfunction in excitatory and inhibitory (E/I) imbalance drives the pathogenesis of Alzheimer's disease (AD).

BACKGROUND

Synaptic dysfunction and E/I imbalance emerge decades before the appearance of cognitive decline in AD patients, which contribute to neurodegeneration. Initially, E/I imbalance was thought to occur first, due to dysfunction of the glutamatergic and cholinergic systems. However, new evidence has demonstrated that the GABAergic system, the counterpart of E/I balance and the major inhibitory neurotransmitter system in the central nervous system, is altered enormously and that this contributes to E/I imbalance and further AD pathogenesis.

NEW HYPOTHESIS

Alterations to the GABAergic system, induced by multiple AD pathogenic or risk factors, contribute to E/I imbalance and AD pathogenesis.

MAJOR CHALLENGES FOR THE HYPOTHESIS

This GABAergic hypothesis accounts for many critical questions and common challenges confronting a new hypothesis of AD pathogenesis. More specifically, it explains why amyloid beta (Aβ), β-secretase (BACE1), apolipoprotein E4 gene (APOE ε4), hyperactive glia cells, contributes to AD pathogenesis and why age and sex are the risk factors of AD. GABAergic dysfunction promotes the spread of Aβ pathology throughout the AD brain and associated cognitive impairments, and the induction of dysfunction induced by these varied risk factors shares this common neurobiology leading to E/I imbalance. In turn, some of these factors exacerbate GABAergic dysfunction and E/I imbalance. Moreover, the GABAergic system modulates various brain functions and thus, the GABAergic hypothesis accounts for nonamnestic manifestations. Furthermore, corrections of E/I balance through manipulation of GABAergic functions have shown positive outcomes in preclinical and clinical studies, suggesting the potential of the GABAergic system as a therapeutic target in AD.

LINKAGE TO OTHER MAJOR THEORIES

Dysfunction of the GABAergic system is induced by multiple critical signaling pathways, which include the existing major theories of AD pathogenesis, such as the Aβ and neuroinflammation hypotheses. In a new perspective, this GABAergic hypothesis accounts for the E/I imbalance and related excitotoxicity, which contribute to cognitive decline and AD pathogenesis. Therefore, the GABAergic system could be a key target to restore, at least partially, the E/I balance and cognitive function in AD patients.

Gap Junctions Interconnect Different Subtypes of Parvalbumin-Positive Interneurons in Barrels and Septa with Connectivity Unique to Each Subtype

Parvalbumin (PV)-positive interneurons form dendritic gap junctions with one another, but the connectivity among gap junction-coupled dendrites remains uninvestigated in most neocortical areas. We visualized gap junctions in layer 4 of the mouse barrel cortex and examined their structural details. PV neurons were divided into 4 types based on the location of soma and dendrites within or outside barrels. Type 1 neurons that had soma and all dendrites inside a barrel, considered most specific to single vibrissa-derived signals, unexpectedly formed gap junctions only with other types but never with each other. Type 2 neurons inside a barrel elongated dendrites outward, forming gap junctions within a column that contained the home barrel. Type 3 neurons located outside barrels established connections with all types including Type 4 neurons that were confined inside the inter-barrel septa. The majority (33/38, 86.8%) of dendritic gap junctions were within 75 μm from at least 1 of 2 paired somata. All types received vesicular glutamate transporter 2-positive axon terminals preferentially on somata and proximal dendrites, indicating the involvement of all types in thalamocortical feedforward regulation in which proximal gap junctions may also participate. These structural organizations provide a new morphological basis for regulatory mechanisms in barrel cortex.

Diabetic encephalopathy causes the imbalance of neural activities between hippocampal glutamatergic neurons and GABAergic neurons in mice

Diabetic encephalopathy is a severe diabetes-related complication in the central nervous system (CNS) that is characterized by the impairment of neurochemical and structural changes leading to cognitive dysfunction. Its cellular and molecular mechanisms are still unclear and clinical approaches are still lacking of promising therapies. In this study, we have investigated the changes of different hippocampal neurons during diabetic encephalopathy in mouse models of diabetes by simultaneously analyzing the activities and synaptic transmission of glutamatergic neurons and GABAergic neurons in brain slices. Compared with the data from a group of control, diabetic encephalopathy permanently impairs the excitability of GABAergic neurons and synaptic transmission mediated by γ-aminobutyric acid (GABA). However, glutamatergic neurons appear to be more excited. Our findings highlight the critical role of the dysfunction of GABAergic neurons and glutamatergic neurons during diabetic encephalopathy in hippocampus to neural impairment as well as a strategy to prevent the function of progress of diabetic encephalopathy by protecting central neurons.

A novel mouse model for the study of endogenous neural stem and progenitor cells after traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of death and disability in the US. Neural stem/progenitor cells (NSPCs) persist in the adult brain and represent a potential cell source for tissue regeneration and wound healing after injury. The Notch signaling pathway is critical for embryonic development and adult brain injury response. However, the specific role of Notch signaling in the injured brain is not well characterized. Our previous study has established a Notch1CR2-GFP reporter mouse line in which the Notch1CR2 enhancer directs GFP expression in NSPCs and their progeny. In this study, we performed closed head injury (CHI) in the Notch1CR2-GFP mice to study the response of injury-activated NSPCs. We show that CHI induces neuroinflammation, cell death, and the expression of typical TBI markers (e.g., ApoE, Il1b, and Tau), validating the animal model. In addition, CHI induces cell proliferation in GFP+ cells expressing NSPC markers, e.g., Notch1 and Nestin. A significant higher percentage of GFP+ astrocytes and GABAergic neurons was observed in the injured brain, with no significant change in oligodendrocyte lineage between the CHI and sham animal groups. Since injury is known to activate astrogliosis, our results suggest that injury-induced GFP+ NSPCs preferentially differentiate into GABAergic neurons. Our study establishes that Notch1CR2-GFP transgenic mouse is a useful tool for the study of NSPC behavior in vivo after TBI. Unveiling the potential of NSPCs response to TBI (e.g., proliferation and differentiation) will identify new therapeutic strategy for the treatment of brain trauma.

PSA-NCAM Colocalized with Cholecystokinin-Expressing Cells in the Hippocampus Is Involved in Mediating Antidepressant Efficacy

The extracellular glycan polysialic acid linked to neural cell adhesion molecule (PSA-NCAM) is principally expressed in the developing brain and the adult neurogenic regions. Although colocalization of PSA-NCAM with cholecystokinin (CCK) was found in the adult brain, the role of PSA-NCAM remains unclear. In this study, we aimed to elucidate the functional significance of PSA-NCAM in the CA1 region of the male mouse hippocampus. Combined fluorescence in situ hybridization and immunohistochemistry showed that few vesicular glutamate transporter 3-negative/CCK-positive (VGluT3^-/CCK⁺) cells were colocalized with PSA-NCAM, but most of the VGluT3⁺/CCK⁺ cells were colocalized with PSA-NCAM. The somata of PSA-NCAM⁺/CCK⁺ cells were highly innervated by serotonergic boutons than those of PSA-NCAM^-/CCK⁺ cells. The expression ratios of 5-HT_3A receptors and p11, a serotonin receptor-interacting protein, were higher in PSA-NCAM⁺/CCK⁺ cells than in PSA-NCAM^-/CCK⁺ cells. Pharmacological digestion of PSA-NCAM impaired the efficacy of antidepressant fluoxetine (FLX), a selective serotonin reuptake inhibitor, but not the efficacy of benzodiazepine anxiolytic diazepam. A Western blot showed that restraint stress decreased the expressions of p11 and mature brain-derived neurotrophic factor (BDNF), and FLX increased them. Interestingly, the FLX-induced elevation of expression of p11, but not mature BDNF, was impaired by the digestion of PSA-NCAM. Quantitative reverse transcription-polymerase chain reaction showed that restraint stress reduced the expression of polysialyltransferase ST8Sia IV and FLX elevated it. Collectively, PSA-NCAM colocalized with VGluT3⁺/CCK⁺ cells in the CA1 region of the hippocampus may play a unique role in the regulation of antidepressant efficacy via the serotonergic pathway.SIGNIFICANCE STATEMENT Polysialic acid (PSA) is composed of eight or more α2,8-linked sialic acids. Here, we examined the functional significance of polysialic acid linked to the neural cell adhesion molecule (PSA-NCAM) in the adult mouse hippocampus. Few vesicular glutamate transporter 3-negative/cholecystokinin-positive (VGluT3^-/CCK⁺) cells were colocalized with PSA-NCAM, but most of the VGluT3⁺/CCK⁺ cells were colocalized with PSA-NCAM. The expression ratios of 5-HT_3A receptors and p11, a serotonin receptor-interacting protein, were higher in PSA-NCAM⁺/CCK⁺ cells than in PSA-NCAM^-/CCK⁺ cells. The efficacy of antidepressants, but not anxiolytics, was impaired by the digestion of PSA-NCAM. The antidepressant-induced increase in p11 expression was inhibited following PSA-NCAM digestion. We hence hypothesize that PSA-NCAM colocalized with VGluT3⁺/CCK⁺ cells may play a unique role in regulating antidepressant efficacy.

Propofol decreases spike firing frequency with an increase in spike synchronization in the cerebral cortex

Little is known about how propofol modulates the spike firing correlation between excitatory and inhibitory cortical neurons in vivo. We performed extracellular unit recordings from rat insular cortical neurons, and classified neurons with high spontaneous firing frequency, bursting, and short spike width as high frequency with bursting neurons (HFB; pseudo fast-spiking GABAergic neurons) and other neurons with low spontaneous firing frequency and no bursting were classified as non-HFB. Intravenous administration of propofol (12 mg/kg) from the caudal vein reduced the firing frequency of HFB, whereas propofol initially increased (within 30 s) and then decreased the firing frequency of non-HFB. Both HFB and non-HFB spontaneous action potential discharge was depressed by propofol with a greater depression seen for HFB. Cross-correlograms and auto-correlograms demonstrated propofol-induced increases in the ratio of the peak, which were mostly observed around 0-10 ms divided to baseline amplitude. The analysis of interspike intervals showed a decrease in spike firing at 20-100 Hz and a relative increase at 8-15 Hz. These results suggest that propofol induces a larger suppression of firing frequency in HFB and an enhancement of synchronized neural activities in the α frequency band in the cerebral cortex (192 words).

Optogenetic/Chemogenetic Activation of GABAergic Neurons in the Ventral Tegmental Area Facilitates General Anesthesia via Projections to the Lateral Hypothalamus in Mice

The ventral tegmental area (VTA) reportedly regulates sleep and wakefulness through communication with the lateral hypothalamus (LH). It has also been suggested that adequate anesthesia produced by administration of chloral hydrate, ketamine, or halothane significantly reduces the GABAergic neuronal firing rate within the VTA. However, the exact effects on GABAergic neurons in the VTA and the mechanisms through which these neurons modulate anesthesia through associated neural circuits is still unclear. Here, we used optogenetic and chemogenetic methods to specifically activate or inhibit GABAergic neuronal perikarya in the VTA or their projections to the LH in Vgat-Cre mice. Electroencephalogram (EEG) spectral analyses and burst suppression ratio (BSR) calculations were conducted following administration of 0.8 or 1.0% isoflurane, respectively; and loss of righting reflex (LORR), recovery of righting reflex (RORR), and anesthesia sensitivity were assessed under 1.4% isoflurane anesthesia. The results showed that activation of GABAergic neurons in the VTA increased delta wave power from 40.0 to 46.4% (P = 0.006) and decreased gamma wave power from 15.2 to 11.5% (P = 0.017) during anesthesia maintenance. BSR was increased from 51.8 to 68.3% (P = 0.017). Induction time (LORR) was reduced from 333 to 290 s (P = 0.019), whereas arousal time (RORR) was prolonged from 498 to 661 s (P = 0.007). Conversely, inhibition of VTA GABAergic neurons led to opposite effects. In contrast, optical activation of VTA-LH GABAergic projection neurons increased power of slow delta waves from 44.2 to 48.8% (P = 0.014) and decreased that of gamma oscillations from 10.2 to 8.0%. BSR was increased from 39.9 to 60.2% (P = 0.0002). LORR was reduced from 330 to 232 s (P = 0.002), and RORR increased from 396 to 565 s (P = 0.007). Optical inhibition of the projection neurons caused opposite effects in terms of both the EEG spectrum and the BSR, except that inhibition of this projection did not accelerate arousal time. These results indicate that VTA GABAergic neurons could facilitate the anesthetic effects of isoflurane during induction and maintenance while postponing anesthetic recovery, at least partially, through modulation of their projections to the LH.

Comparative analysis of cellular expression pattern of schizophrenia risk genes in human versus mouse cortex

Background

Schizophrenia is a common psychiatric disease with high hereditary. The identification of schizophrenia risk genes (SRG) has shed light on its pathophysiological mechanisms. Mouse genetic models have been widely used to study the function of SRG in the brain with a cell type specific fashion. However, whether the cellular expression pattern of SRG is conserved between human and mouse brain is not thoroughly studied.

Results

We analyzed the single-cell transcription of 180 SRG from human and mouse primary visual cortex (V1). We compared the percentage of glutamatergic, GABAergic and non-neuronal cells that express each SRG between mouse and human V1 cortex. Thirty percent (54/180) of SRG had significantly different expression rate in glutamatergic neurons between mouse and human V1 cortex. By contrast, only 5.6% (10/180) of SRG showed significantly different expression in GABAergic neurons, which is similar with the ratio of SRG (15/180) with species difference in total cell populations. Strikingly, the percentage of non-neuronal cells expressing all SRG are indistinguishable between human and mouse V1 cortex. We further analyzed the biological significance of differentially expressed SRG by gene ontology. The species-different SRG in glutamatergic neurons are highly expressed in dendrite and axon. They are enriched in the biological process of response to stimulus. However, the differentially expressed SRG in GABAergic neurons are enriched in the regulation of organelle organization.

Conclusion

GABAergic neurons are more conserved in the expression of SRG than glutamatergic neurons while the non-neuronal cells show the species conservation for the expression of all SRG. It should be cautious to use mouse models to study those SRG which show different cellular expression pattern between human and mouse cortex.

Long-range projections from sparse populations of GABAergic neurons in murine subplate

The murine subplate contains some of the earliest generated populations of neurons in the cerebral cortex, which play an important role in the maturation of cortical inhibition. Here we present multiple lines of evidence, that the subplate itself is only very sparsely populated with GABAergic neurons at postnatal day (P)8. We used three different transgenic mouse lines, each of which labels a subset of GABAergic, ganglionic eminence derived neurons. Dlx5/6-eGFP labels the most neurons in cortex (on average 11% of NEUN+ cells across all layers at P8) whereas CGE-derived Lhx6-Cre::Dlx1-Venusfl cells are the sparsest (2% of NEUN+ cells across all layers at P8). There is significant variability in the layer distribution of labeled interneurons, with Dlx5/6-eGFP and Lhx6-Cre::R26R-YFP being expressed most abundantly in Layer 5, whereas CGE-derived Lhx6-Cre::Dlx1-Venusfl cells are least abundant in that layer. All three lines label at most 3% of NEUN+ neurons in the subplate, in contrast to L5, in which up to 30% of neurons are GFP+ in Dlx5/6-eGFP. We assessed all three GABAergic populations for expression of the subplate neuron marker connective tissue growth factor (CTGF). CTGF labels up to two-thirds of NEUN+ cells in the subplate, but was never found to colocalize with labeled GABAergic neurons in any of the three transgenic strains. Despite the GABAergic neuronal population in the subplate being sparse, long-distance axonal connection tracing with carbocyanine dyes revealed that some Gad65-GFP+ subplate cells form long-range axonal projections to the internal capsule or callosum.

Inhibition of GABAergic Neurons and Excitation of Glutamatergic Neurons in the Ventrolateral Periaqueductal Gray Participate in Electroacupuncture Analgesia Mediated by Cannabinoid Receptor

Although electroacupuncture (EA) has become a worldwide practice, little is understood about its precise target in the central nervous system (CNS) and the cell type-specific analgesia mechanism. In the present study, we found that EA has significant antinociceptive effects both in inflammatory and neuropathic pain models. Chemogenetic inhibition of GABAergic neurons in the ventrolateral periaqueductal gray (vlPAG) replicated the effects of EA, whereas the combination of chemogenetic activation of GABAergic neurons and chemogenetic inhibition of glutamatergic neurons in the vlPAG was needed to reverse the effects of EA. Specifically knocking out CB1 receptors on GABAergic neurons in the vlPAG abolished the EA effect on pain hypersensitivity, while specifically knocking out CB1 receptors on glutamatergic neurons attenuated only a small portion of the EA effect. EA synchronously inhibits GABAergic neurons and activates glutamatergic neurons in the vlPAG through CB1 receptors to produce EA-induced analgesia. The CB1 receptors on GABAergic neurons localized in the vlPAG was the basis of the EA effect on pain hypersensitivity. This study provides new experimental evidence that EA can bidirectionally regulate GABAergic neurons and glutamatergic neurons via the CB1 receptors of the vlPAG to produce analgesia effects.

Distribution Patterns of Three Molecularly Defined Classes of GABAergic Neurons Across Columnar Compartments in Mouse Barrel Cortex

The mouse somatosensory cortex is an excellent model to study the structural basis of cortical information processing, since it possesses anatomically recognizable domains that receive different thalamic inputs, which indicates spatial segregation of different processing tasks. In this work we examined three genetically labeled, non-overlapping subpopulations of GABAergic neurons: parvalbumin- (PV+), somatostatin- (SST+), and vasoactive intestinal polypeptide-expressing (VIP+) cells. Each of these subpopulations displayed a unique cellular distribution pattern across layers. In terms of columnar localization, the distribution of these three populations was not quantitatively different between barrel-related versus septal compartments in most layers. However, in layer IV (LIV), SST+, and VIP+, but not PV+ neurons preferred the septal compartment over barrels. The examined cell types showed a tendency toward differential distribution in supragranular and infragranular barrel-related versus septal compartments, too. Our data suggests that the location of GABAergic neuron cell bodies correlates with the spatial pattern of cortical domains receiving different kinds of thalamic input. Thus, at least in LIV, lemniscal inputs present a close spatial relation preferentially to PV+ cells whereas paralemniscal inputs target compartments in which more SST+ and VIP+ cells are localized. Our findings suggest pathway-specific roles for neocortical GABAergic neurons.

Glutamatergic Innervation onto Striatal Neurons Potentiates GABAergic Synaptic Output

Striatal output pathways are known to play a crucial role in the control of movement. One possible component for shaping the synaptic output of striatal neuron is the glutamatergic input that originates from cortex and thalamus. Although reports focusing on quantifying glutamatergic-induced morphological changes in striatum exist, the role of glutamatergic input in regulating striatal function remains poorly understood. Using primary neurons from newborn mice of either sex in a reduced two-neuron microcircuit culture system, we examined whether glutamatergic input modulates the output of striatal neurons. We found that glutamatergic input enhanced striatal inhibition in vitro With a glutamatergic partner from either cortex or thalamus, we attributed this potentiation to an increase in the size of quantal IPSC, suggesting a strengthening of the postsynaptic response to GABAergic signaling. Additionally, a differential effect of cortical and thalamic innervation onto striatal GABAergic neurons output was revealed. We observed that cortical, but not thalamic input, enhanced the number of releasable GABAergic synaptic vesicles and morphological synapses. Importantly, these alterations were reverted by blockade of neuronal activity and glutamate receptors, as well as disruption of BDNF-TrkB signaling. Together, our data indicate, for first time, that GABAergic synapse formation in corticostriatal pairs depends on two parallel, but potentially intersecting, signaling pathways that involve glutamate receptor activation in striatal neurons, as well as BDNF signaling. Understanding how cortical and thalamic inputs refine striatal output will pave the way toward dissecting basal ganglia activity in both physiological and pathological conditions.SIGNIFICANCE STATEMENT Striatal GABAergic microcircuits are critical for motor function. However, the mechanisms controlling striatal output, particularly at the level of synaptic strength, are unclear. Using two-neuron culture system, we quantified the synaptic output of individual striatal GABAergic neurons paired with a glutamatergic partner and studied the influence of the excitatory connections that are known to be interregionally formed in vivo We found that glutamatergic input potentiated striatal inhibitory output, potentially involving an increased feedback and/or feedforward inhibition. Moreover, distinct components of glutamatergic innervation, such as firing activity or release of neurotrophic factors were shown to be required for the glutamatergic-induced phenotype. Investigation, therefore, of two-neuron in vitro microcircuits could be a powerful tool to explore synaptic mechanisms or disease pathophysiology.

Pain inhibition through transplantation of fetal neuronal progenitors into the injured spinal cord in rats

Neuropathic pain after spinal cord injury (SCI) is a complex condition that responds poorly to usual treatments. Cell transplantation represents a promising therapy; nevertheless, the ideal cell type in terms of neurogenic potential and effectiveness against pain remains largely controversial. Here, we evaluated the ability of fetal neural stem cells (fNSC) to relieve chronic pain and, secondarily, their effects on motor recovery. Adult Wistar rats with traumatic SCI were treated, 10 days after injury, with intra-spinal injections of culture medium (sham) or fNSCs extracted from telencephalic vesicles (TV group) or the ventral medulla (VM group) of E/14 embryos. Sensory (von Frey filaments and hot plate) and motor (the Basso, Beattie, Bresnahan locomotor rating scale and inclined plane test) assessments were performed during 8 weeks. Thereafter, spinal cords were processed for immunofluorescence and transplanted cells were quantified by stereology. The results showed improvement of thermal hyperalgesia in the TV and VM groups at 4 and 5 weeks after transplantation, respectively. Moreover, mechanical allodynia improved in both the TV and VM groups at 8 weeks. No significant motor recovery was observed in the TV or VM groups compared with sham. Stereological analyses showed that ~70% of TV and VM cells differentiated into NeuN⁺ neurons, with a high proportion of enkephalinergic and GABAergic cells in the TV group and enkephalinergic and serotoninergic cells in the VM group. Our study suggests that neuronal precursors from TV and VM, once implanted into the injured spinal cord, maturate into different neuronal subtypes, mainly GABAergic, serotoninergic, and enkephalinergic, and all subtypes alleviate pain, despite no significant motor recovery. The study was approved by the Animal Ethics Committee of the Medical School of the University of São Paulo (protocol number 033/14) on March 4, 2016.

Transforming Sensory Cues into Aversive Emotion via Septal-Habenular Pathway

Emotions evoked by environmental cues are important for animal survival and life quality. However, neural circuits responsible for transforming sensory signals to aversive emotion and behavioral avoidance remain unclear. Here, we found that medial septum (MS) mediates aversion induced by both auditory and somatosensory stimuli. Ablation of glutamatergic or GABAergic MS neurons results in impaired or strengthened aversion, respectively. Optogenetic activation of the two cell types results in place avoidance and preference, respectively. Cell-type-specific screening reveals that glutamatergic MS projections to the lateral habenula (LHb) are responsible for the induction of aversion, which can be antagonized by GABAergic MS projections to LHb. Additionally, the sensory-induced place avoidance is facilitated by enhanced locomotion mediated by glutamatergic MS projections to the preoptic area. Thus, MS can transmit innately aversive signals via a bottom-up multimodal sensory pathway and produce concurrent emotional and motional effects, allowing animals to efficiently avoid unfavorable environments.

Aucubin Promotes Differentiation of Neural Precursor Cells into GABAergic Neurons

Aucubin is a small compound naturally found in traditional medicinal herbs with primarily anti-inflammatory and protective effects. In the nervous system, aucubin is reported to be neuroprotective by enhancing neuronal survival and inhibiting apoptotic cell death in cultures and disease models. Our previous data, however, suggest that aucubin facilitates neurite elongation in cultured hippocampal neurons and axonal regrowth in regenerating sciatic nerves. Here, we investigated whether aucubin facilitates the differentiation of neural precursor cells (NPCs) into specific types of neurons. In NPCs cultured primarily from the rat embryonic hippocampus, aucubin significantly elevated the number of GAD65/67 immunoreactive cells and the expression of GAD65/67 proteins was upregulated dramatically by more than three-fold at relatively low concentrations of aucubin (0.01 µM to 10 µM). The expression of both NeuN and vGluT1 of NPCs, the markers for neurons and glutamatergic cells, respectively, and the number of vGluT1 immunoreactive cells also increased with higher concentrations of aucubin (1 µM and 10 µM), but the ratio of the increases was largely lower than GAD expression and GAD immunoreactive cells. The GABAergic differentiation of pax6-expressing late NPCs into GABA-producing cells was further supported in cortical NPCs primarily cultured from transgenic mouse brains, which express recombinant GFP under the control of pax6 promoter. The results suggest that aucubin can be developed as a therapeutic candidate for neurodegenerative disorders caused by the loss of inhibitory GABAergic neurons.

Optogenetic Inhibition of Striatal GABAergic Neuronal Activity Improves Outcomes After Ischemic Brain Injury

BACKGROUND AND PURPOSE

Striatal GABAergic neuron is known as a key regulator in adult neurogenesis. However, the specific role of striatal GABAergic neuronal activity in the promotion of neurological recovery after ischemic stroke remains unknown. Here, we used optogenetic approach to investigate these effects and mechanism.

METHODS

Laser stimulation was delivered via an implanted optical fiber to inhibit or activate the striatal GABAergic neurons in Gad2-Arch-GFP or Gad2-ChR2-tdTomato mice (n=80) 1 week after 60-minute transient middle cerebral artery occlusion. Neurological severity score, brain atrophy volume, microvessel density, and cell morphological changes were examined using immunohistochemistry. Gene expression and protein levels of related growth factors were further examined using real-time polymerase chain reaction and Western blotting.

RESULTS

Inhibiting striatal GABAergic neuronal activity improved functional recovery, reduced brain atrophy volume, and prohibited cell death compared with the control (P<0.05). Microvessel density and bFGF (basic fibroblast growth factor) expression in the inhibition group were also increased (P<0.05). In contrast, activation of striatal GABAergic neurons resulted in adverse effects compared with the control (P<0.05). Using cocultures of GABAergic neurons, astrocytes, and endothelial cells, we further demonstrated that the photoinhibition of GABAergic neuronal activity could upregulate bFGF expression in endothelial cells, depending on the presence of astrocytes. The conditioned medium from the aforementioned photoinhibited 3-cell coculture system protected cells from oxygen glucose deprivation injury.

CONCLUSIONS

After ischemic stroke, optogenetic inhibition of GABAergic neurons upregulated bFGF expression by endothelial cells and promoted neurobehavioral recovery, possibly orchestrated by astrocytes. Optogenetically inhibiting neuronal activity provides a novel approach to promote neurological recovery.

Neuroligin-induced presynaptic differentiation through SLM2-mediated splicing modifications of neurexin in cerebellar cultures

Neurexins (NRXs) and neuroligins (NLs) play important roles in synapse specification. The alternatively spliced segment 4 (AS4) of NRX genes (Nrxn) is a critical element in selective trans-synaptic interactions. However, the role of splicing of NRXs and NLs in synapse specification is not fully understood. To investigate the exact role of splice-dependent NRX-NL interaction in the specification of glutamatergic and gamma-aminobutyric acid (GABA)-ergic synapses in the cerebellum, we evaluated the synaptogenic receptor activity of NL1/2/3 isoforms in a neuron-fibroblast co-culture system, in which the Nrxn AS4 segments are manipulated using SLM2, a selective and dominant regulator of AS4 splicing. We show that ectopic SLM2 expression (SLM2 E/E) causes marked skipping of exon 20 of AS4 in cerebellar neuron culture. Whereas NLs can induce VAMP2⁺ presynaptic contacts from mainly glutamatergic neurons in both uninfected (control) and SLM2 E/E co-cultures, they induce VGAT⁺ GABAergic contacts in the control culture, but not properly in the SLM2 E/E culture. Furthermore, Nrxn3 is responsible for the NL-induced assembly of GABAergic synapses in co-culture. Importantly, lentivirus-based expression of Nrxn3 containing exon 20 restores the reduced NL-induced GABAergic contacts in the SLM2 E/E co-culture. Therefore, our findings may provide further insights into NRX-NL mediated synapse specification.

Morphine-induced inhibition of Ca2+ -dependent d-serine release from astrocytes suppresses excitability of GABAergic neurons in the nucleus accumbens

The nucleus accumbens (NAc) plays a critical role in addictive drug-induced behavioral changes. d-serine is present at high levels in the brain and is involved in the regulation of N-methyl-d-aspartate glutamate (NMDA)-dependent synaptic activity. In this study, we aimed to examine the involvement of d-serine in morphine addiction. Morphine decreased the NMDA receptor-mediated excitatory postsynaptic currents and excitability of GABAergic neurons in the NAc, while exogenous d-serine alleviated the effects of morphine. Morphine reduced extracellular d-serine levels in rat NAc or in primary culture of astrocytes through inhibition of intracellular Ca²⁺ signals and blockade of d-serine release from cell vesicles. Morphine induced robust internalization of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate acid receptor (AMPAR) in primary cultured astrocytes. Moreover, administration of exogenous d-serine to rats inhibited the development of locomotor sensitization to morphine, attenuated the morphine-induced potentiation on conditioned place preference and suppressed the morphine-enhanced expression of p-CREB and ΔFosB in the NAc. Overall, our results showed that morphine inhibited d-serine release from astrocytes through modulation of AMPAR-mediated Ca²⁺ influx, and led to the inhibition of postsynaptic excitability of GABAergic neurons in the NAc. This work may provide a new insight into the underlying mechanisms of morphine addiction.