Role Of GABA Research Articles

γ-Aminobutyric Acid Alleviates Programmed Cell Death in Two Brassica Species Under Cadmium Stress

Previous studies have demonstrated that γ-Aminobutyric acid (GABA) effectively alleviates heavy metal stresses by maintaining the redox balance and reducing the accumulation of reactive oxygen species (ROS). However, little is known about the role of GABA on programmed cell death (PCD) under Cd treatments in plants. The present study investigated the effects of GABA on Cd-induced PCD in two Brassica species, oilseed rape (Brassica napus, Bn), and black mustard (Brassica juncea, Bj). We observed that GABA significantly alleviated Cd-induced PCD by enhancing antioxidant systems, inhibiting chromatin condensation in the nucleus, and reducing DNA fragmentation under Cd stress. Moreover, GABA may not only reduce caspase-3-like activity by repressing gene expression, but also regulate transcription of PCD-related genes. Bn showed lower Cd accumulation and lower tolerance, with more pronounced PCD, compared with Bj. Our results provide new insights into the mechanism that GABA enhances Cd tolerance in plants.

Short term GABA antagonist treatment improves long term sleep quality, memory, and decision-making in a Down syndrome mouse model

Abstract Down syndrome (DS) is a common genetic condition affecting people worldwide. It involves cognitive disabilities for which there are no drug therapies. The Ts65Dn mouse model of DS shows cognitive impairment due to a reduction in neuron number and connectivity as well as excessive neuronal activity, as GABA antagonist treatment restores memory in these mice. Our study showed the effects of GABA antagonist treatment on sleep and decision-making in Ts65Dn mice. We administered a daily, low oral dose of pentylenetetrazol (PTZ) in milk to Ts65Dn mice for 17 days. Decision-making was tested with and without PTZ treatment. Short and long-term memories were tested before, immediately after, and one month following PTZ treatment. Electro-encephalography (EEG) was also recorded at these three time points to study the effect of the treatment on sleep. We showed that PTZ treatment improved long-term recognition, but not short-term, memory and led to more Ts65Dn mice showing safer decision-making behavior. PTZ treatment showed a moderate and only global beneficial effect on sleep by decreasing the global amount of wake and increasing NREM sleep in the Ts65Dn mice, which may explain the observed cognitive improvements. These results bring new knowledge on the role of GABA in sleep, memory consolidation, and decision-making abilities in DS.

Analysis of the functional role and mRNA expression of GABA B R in the nucleus accumbens of cocaine-addicted rats.

Drug addiction is a social and medical problem that must be urgently addressed. The nucleus accumbens (NAc) is closely related to addiction-related learning memory, and γ-aminobutyric acid type B receptor (GABA B R) is a potential target for the treatment of drug addiction. However, the role of GABA B R activity levels in the NAc in cocaine addiction is unclear. In this study, we established an animal model of cocaine dependence, modulated the level of GABA B R activity, applied a conditioned place preference assay (CPP) to assess the role of the NAc in reconsolidation of addiction memory, evaluated learning and memory functions by behavioral experiments, examined the expression of GB1, GB2, cyclic adenosine monophosphate response element binding protein (CREB), p-CREB, protein kinase A (PKA), protein kinase (ERK), and Brain-derived neurotrophic factor (BDNF) in the NAc by molecular biology experiments, and screened differentially significantly expressed genes by transcriptome sequencing. Our study showed that the GABA B receptor agonist baclofen (BLF) had a significant effect on locomotor distance in rats, promoted an increase in GABA levels and significantly inhibited the PKA and ERK1/2/CREB/BDNF signaling pathways. Moreover, transcriptome sequencing showed that GABA B R antagonist intervention identified a total of 21 upregulated mRNAs and 21 downregulated mRNAs. The differentially expressed (DE) mRNA genes were mainly enriched in tyrosine metabolism; however, further study is needed. GABA B R activity in the NAc is involved in the regulation of cocaine addiction and may play an important role through key mRNA pathways.

Exploring solute binding proteins in Pseudomonas aeruginosa that bind to γ-aminobutyrate and 5-aminovalerate and their role in activating sensor kinases.

The standard method of receptor activation involves the binding of signals or signal-loaded solute binding proteins (SBPs) to sensor domains. Many sensor histidine kinases (SHKs), which are activated by SBP binding, are encoded adjacent to their corresponding sbp gene. We examined three SBPs of Pseudomonas aeruginosa PAO1, encoded near the genes for the AgtS (PA0600) and AruS (PA4982) SHKs, to determine how common this arrangement is. Ligand screening and microcalorimetric studies revealed that the SBPs PA0602 and PA4985 preferentially bind to GABA (KD = 2.3 and 0.58 μM, respectively), followed by 5-aminovalerate (KD = 30 and 1.6 μM, respectively) and ethanoldiamine (KD = 2.3 and 0.58 μM, respectively). In contrast, AgtB (PA0604) exclusively recognizes 5-aminovaleric acid (KD = 2.9 μM). However, microcalorimetric titrations did not show any binding between the AgtS sensor domain and AgtB or PA0602, regardless of the presence of ligands. Similarly, bacterial two-hybrid assays did not demonstrate an interaction between PA4985 and the AruS sensor domain. Therefore, sbp and shk genes located nearby are not always functionally linked. We previously identified PA0222 as a GABA-specific SBP. The presence of three SBPs for GABA may be linked to GABA's role as a trigger for P. aeruginosa virulence.

The Role of GABA and Insulin Regulated Aminopeptidase on Insulin Resistance and GLUT4 in Prediabetes and Type 2 Diabetes Mellitus

Background: Different proposed mechanisms for insulin resistance have been put forth to understand the relationship and changes in insulin resistance. Looking at the levels of insulin, the neurotransmitter gamma-aminobutyric acid, glucose transport type 4, and the newly discovered enzyme insulin regulated aminopeptidase may provide an integrated perspective on insulin resistance. Objectives: To study the role of serum gamma-aminobutyric acid and insulin regulated aminopeptidase enzyme levels in pathogenesis of insulin resistance and translocation of glucose transport type 4 in patients with type2 diabetes mellitus and pre-diabetes ones. Subjects and methods: Ninety individuals were divided into three age- and BMI matched groups (diabetics, prediabetics, healthy) were assessed for glycated hemoglobin (turbidometry method), fasting serum glucose (colorimetric method), serum insulin (electrochemiluminescence immunoassay method), homeostatic model assessment for insulin resistance, serum gamma-aminobutyric acid, glucose transporter type 4, and insulin regulated aminopeptidase measured by the sandwich ELISA method. The cut-off value was adopted according to the World Health Organization, individuals with their fasting serum glucose lower than 110 mg/ml, and glycated hemoglobin level not more than 5.7 % are considered healthy individuals. Result: The mean (±SD) values of serum insulin regulated aminopeptidase, gamma-aminobutyric acid and glucose transport type 4 showed significantly lower levels with greater increases in insulin resistance, represented by the significant decrease in diabetics group who have high insulin resistance compared with pre-diabetics and healthy individuals’ groups (p < 0.05). They also significantly decrease in individuals with the pre-diabetic stage compared to healthy individuals without insulin resistance (p < 0.05). Significant negative correlations were found between HOMA-IR values and each of GABA, IRAP, and GLUT4. Conclusion: Decreasing gamma-aminobutyric acid may promote insulin resistance and prediabetes/diabetes onset. Decreased insulin regulated aminopeptidase levels suggest a role alongside glucose transport type 4 in glucose uptake, allowing screening for prediabetes risk.

GABA in plants: developmental and stress resilience perspective

AbstractGamma‐aminobutyric acid (GABA), a ubiquitously present non‐proteinogenic amino acid, has recently emerged as a key regulator of growth and development in plants during normal as well as challenging environmental conditions. GABA biosynthesis has been reported at multiple stages of plant development, particularly during vegetative and reproductive stages and in response to stress conditions. Accumulating evidence has highlighted the crucial roles of various cell cycle regulators such as type‐D cyclins and CDK;A1, transcription factors such as E2Fa, as well as Ca2+/Calmodulin proteins in GABA biosynthesis in plants. GABA is known to improve stress tolerance by improving photosynthetic activity, C/N metabolism, stomatal conductance, and stress‐induced reactive oxygen species (ROS) detoxification. Here, we have reviewed recent studies that have explored the novel roles of GABA in plants with a focus on plant development and stress resilience.

Integrated metabolomics and transcriptomics analysis reveals γ-aminobutyric acid enhances the ozone tolerance of wheat by accumulation of flavonoids

Wheat is susceptible to atmospheric ozone (O3) pollution, thus the increasing O3 is a serious threat to wheat production. γ-aminobutyric acid (GABA) is found to play key roles in the tolerance of plants to stress. However, few studies elaborated the function of GABA in response of wheat to O3. Here, we incorporated metabolome and transcriptome data to provide a more comprehensive insight on the role of GABA in enhancing the O3-tolerance of wheat. In our study, there were 31, 23, and 32 differentially accumulated flavonoids in the carbon-filtered air with GABA, elevated O3 with or without GABA treatments compared to the carbon-filtered air treatment, respectively. Elevated O3 triggered the accumulation of dihydroflavone, flavonols, and flavanols. Exogenous GABA enhanced dihydroflavone and dihydroflavonol, and also altered the expression of genes encoding some key enzymes in the flavonoid synthesis pathway. Additionally, GABA stimulated proline accumulation and antioxidant enzyme activities under elevated O3, resulting in the less accumulation of H2O2 and malondialdehyde. Consequently, GABA alleviated the grain yield loss from 19.6% to 9.6% induced by elevated O3. Our study provided comprehensive insight into the role of GABA in the alleviating the detrimental effects of elevated O3 on wheat, and a new avenue to mitigate O3 damage to the productivity of crops.

The GABA shunt contributes to ROS homeostasis in guard cells of Arabidopsis.

γ-Aminobutyric acid (GABA) accumulates rapidly under stress via the GABA shunt pathway, which has been implicated in reducing the accumulation of stress-induced reactive oxygen species (ROS) in plants. γ-Aminobutyric acid has been demonstrated to act as a guard-cell signal in Arabidopsis thaliana, modulating stomatal opening. Knockout of the major GABA synthesis enzyme Glutamate Decarboxylase 2 (GAD2) increases the aperture of gad2 mutants, which results in greater stomatal conductance and reduces water-use efficiency compared with wild-type plants. Here, we found that the additional loss of GAD1, GAD4, and GAD5 in gad2 leaves increased GABA deficiency but abolished the more open stomatal pore phenotype of gad2, which we link to increased cytosolic calcium (Ca2+ ) and ROS accumulation in gad1/2/4/5 guard cells. Compared with wild-type and gad2 plants, glutamate was ineffective in closing gad1/2/4/5 stomatal pores, whereas lowering apoplastic calcium, applying ROS inhibitors or complementation with GAD2 reduced gad1/2/4/5 guard-cell ROS, restored the gad2-like greater stomatal apertures of gad1/2/4/5 beyond that of wild-type. We conclude that GADs are important contributors to ROS homeostasis in guard cells likely via a Ca2+ -mediated pathway. As such, this study reveals greater complexity in GABA's role as a guard-cell signal and the interactions it has with other established signals.

New GABA-Targeting Therapies for the Treatment of Seizures and Epilepsy: I. Role of GABA as a Modulator of Seizure Activity and Recently Approved Medications Acting on the GABA System.

γ-Aminobutyric acid (GABA) is the most prevalent inhibitory neurotransmitter in the mammalian brain and has been found to play an important role in the pathogenesis or the expression of many neurological diseases, including epilepsy. Although GABA can act on different receptor subtypes, the component of the GABA system that is most critical to modulation of seizure activity is the GABAA-receptor-chloride (Cl-) channel complex, which controls the movement of Cl- ions across the neuronal membrane. In the mature brain, binding of GABA to GABAA receptors evokes a hyperpolarising (anticonvulsant) response, which is mediated by influx of Cl- into the cell driven by its concentration gradient between extracellular and intracellular fluid. However, in the immature brain and under certain pathological conditions, GABA can exert a paradoxical depolarising (proconvulsant) effect as a result of an efflux of chloride from high intracellular to lower extracellular Cl- levels. Extensive preclinical and clinical evidence indicates that alterations in GABAergic inhibition caused by drugs, toxins, gene defects or other disease states (including seizures themselves) play a causative or contributing role in facilitating or maintaning seizure activity. Conversely, enhancement of GABAergic transmission through pharmacological modulation of the GABA system is a major mechanism by which different antiseizure medications exert their therapeutic effect. In this article, we review the pharmacology and function of the GABA system and its perturbation in seizure disorders, and highlight how improved understanding of this system offers opportunities to develop more efficacious and better tolerated antiseizure medications. We also review the available data for the two most recently approved antiseizure medications that act, at least in part, through GABAergic mechanisms, namely cenobamate and ganaxolone. Differences in the mode of drug discovery, pharmacological profile, pharmacokinetic properties, drug-drug interaction potential, and clinical efficacy and tolerability of these agents are discussed.

Astrocytic control of extracellular GABA drives circadian timekeeping in the suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus (SCN) is the master mammalian circadian clock. Its cell-autonomous timing mechanism, a transcriptional/translational feedback loop (TTFL), drives daily peaks of neuronal electrical activity, which in turn control circadian behavior. Intercellular signals, mediated by neuropeptides, synchronize and amplify TTFL and electrical rhythms across the circuit. SCN neurons are GABAergic, but the role of GABA in circuit-level timekeeping is unclear. How can a GABAergic circuit sustain circadian cycles of electrical activity, when such increased neuronal firing should become inhibitory to the network? To explore this paradox, we show that SCN slices expressing the GABA sensor iGABASnFR demonstrate a circadian oscillation of extracellular GABA ([GABA]e) that, counterintuitively, runs in antiphase to neuronal activity, with a prolonged peak in circadian night and a pronounced trough in circadian day. Resolving this unexpected relationship, we found that [GABA]e is regulated by GABA transporters (GATs), with uptake peaking during circadian day, hence the daytime trough and nighttime peak. This uptake is mediated by the astrocytically expressed transporter GAT3 (Slc6a11), expression of which is circadian-regulated, being elevated in daytime. Clearance of [GABA]e in circadian day facilitates neuronal firing and is necessary for circadian release of the neuropeptide vasoactive intestinal peptide, a critical regulator of TTFL and circuit-level rhythmicity. Finally, we show that genetic complementation of the astrocytic TTFL alone, in otherwise clockless SCN, is sufficient to drive [GABA]e rhythms and control network timekeeping. Thus, astrocytic clocks maintain the SCN circadian clockwork by temporally controlling GABAergic inhibition of SCN neurons.

Short-Term, Voluntary Exercise Affects Morpho-Functional Maturation of Adult-Generated Neurons in Rat Hippocampus.

Physical exercise is a well-proven neurogenic stimulus, promoting neuronal progenitor proliferation and affecting newborn cell survival. Besides, it has beneficial effects on brain health and cognition. Previously, we found that three days of physical activity in a very precocious period of adult-generated granule cell life is able to antedate the appearance of the first GABAergic synaptic contacts and increase T-type Ca2+ channel expression. Considering the role of GABA and Ca2+ in fostering neuronal maturation, in this study, we used short-term, voluntary exercise on a running wheel to investigate if it is able to induce long-term morphological and synaptic changes in newborn neurons. Using adult male rats, we found that: (i) three days of voluntary physical exercise can definitively influence the morpho-functional maturation process of newborn granule neurons when applied very early during their development; (ii) a significant percentage of new neurons show more mature morphological characteristics far from the end of exercise protocol; (iii) the long-term morphological effects result in enhanced synaptic plasticity. Present findings demonstrate that the morpho-functional changes induced by exercise on very immature adult-generated neurons are permanent, affecting the neuron maturation and integration in hippocampal circuitry. Our data contribute to underpinning the beneficial potential of physical activity on brain health, also performed for short times.

Diffuse noxious inhibitory controls and conditioned pain modulation: a shared neurobiology within the descending pain inhibitory system?

Diffuse noxious inhibitory controls and conditioned pain modulation: a shared neurobiology within the descending pain inhibitory system?

Targeted metabolomic analysis identifies increased serum levels of GABA and branched chain amino acids in canine diabetes.

Dogs with naturally occurring diabetes mellitus represent a potential model for human type 1 diabetes, yet significant knowledge voids exist in terms of the pathogenic mechanisms underlying the canine disorder. Untargeted metabolomic studies from a limited number of diabetic dogs identified similarities to humans with the disease. To expand and validate earlier metabolomic studies, identify metabolites that differ consistently between diabetic and healthy dogs, and address whether certain metabolites might serve as disease biomarkers. Untargeted metabolomic analysis via liquid chromatography-mass spectrometry was performed on serum from diabetic (n = 15) and control (n = 15) dogs. Results were combined with those of our previously published studies using identical methods (12 diabetic and 12 control dogs) to identify metabolites consistently different between the groups in all 54 dogs. Thirty-two candidate biomarkers were quantified using targeted metabolomics. Biomarker concentrations were compared between the groups using multiple linear regression (corrected P < 0.0051 considered significant). Untargeted metabolomics identified multiple persistent differences in serum metabolites in diabetic dogs compared with previous studies. Targeted metabolomics showed increases in gamma amino butyric acid, valine, leucine, isoleucine, citramalate, and 2-hydroxyisobutyric acid in diabetic versus control dogs while indoxyl sulfate, N-acetyl-L-aspartic acid, kynurenine, anthranilic acid, tyrosine, glutamine, and tauroursodeoxycholic acid were decreased. Several of these findings parallel metabolomic studies in both human diabetes and other animal models of this disease. Given recent studies on the role of GABA and branched chain amino acids in human diabetes, the increase in serum concentrations in canine diabetes warrants further study of these metabolites as potential biomarkers, and to identify similarity in mechanisms underlying this disease in humans and dogs.

Neurophysiological modulations in the (pre)motor-motor network and the role of GABA+ levels underlying age-related reaction time slowing

Neurophysiological modulations in the (pre)motor-motor network and the role of GABA+ levels underlying age-related reaction time slowing

Ketogenic diet for anxiety disorders: Dietary regimen for relapse prevention?

IntroductionAnxiety disorders are a mental disorder that is widespread all over the world and are associated with a high burden of the disease. While poor dietary habits are common in depression, they can lead to worsening of the illness or prolong the treatment, which also leads to a higher risk of developing chronic diseases.ObjectivesWith the array of the treatment modalities including monoaminergic antidepressants, benzodiazepines, psychotherapy, there is the unmet need for the effective treatment of anxiety disorders resulting in full remission and relapse prevention. Benzodiazepines can quickly resolve anxiety due to their positive allosteric GABA modulation mechanism of action. Although, they are not recommended for chronic use.Methods Ketogenic diet(KD) may be hypothesized as the promising strategy impacting treatment strategies, in particularly facilitating full remission, recovery and preventing relapses. In this popular high-fat diet, where daily calories intake is consists in at least 70% from fat, up to 25 % from protein and as little sugar as possible is mainly known for its helpful role in drug resistant epilepsy treatment, glucose levels balance or fast way for weight-loss.ResultsCould be effective in anxiety disorders treatment due to its possible role in GABA:glutamate balance change in favor of GABA levels, which may enhance the anxiolytic effect in sustaining remission and preventing relapse.ConclusionsKD in some anxiety disorders may provide a rewarding outcome, but more research is needed. The evidence mentioned in this paper should encourage psychiatrists to recommend KD as advice somewhere between psychotherapy, pharmacology or as an add-on to those two.

Effects of GABAA receptors in nucleus cuneiformis on the cannabinoid antinociception using the formalin test.

Nucleus cuneiformis (NC), a reticular nucleus of the midbrain, is a part of the descending pain modulatory system and therefore has an important role in pain perception. Considering the abundance of GABAA and cannabinoid receptors in the NC and also the bidirectional roles for GABA in controlling nociception, the present study examined the effects of bilateral intra-NC microinjection of different doses of the GABAA receptor agonist, muscimol, and the GABAA receptor antagonist, bicuculline, on pain modulation using formalin test. We also assessed interaction between canabinergic and GABAergic systems in the NC during this test. Rats were exposed to intra-NC microinjection of bicuculline (50,100, and 200 ng/side) or muscimol (60, 120, and 240 ng/side) and then subjected to the formalin test. In another set of experiments, the effects of muscimol (60 ng/side) or bicuculline (50 ng/side) administration 5 min before a cannabinoid receptor agonist WIN 55,212-2 (5, 10, and 20 μg/side) microinjection into NC on the formalin test were evaluated. Microinjection of bicuculline and muscimol into the NC decreased and increased pain responses, respectively, in a dose-dependent manner during both phases of the test. Microinjection of WIN 55,212-2 into the NC significantly reduced pain responses in a dose-dependent manner. Microinjection of bicuculline or muscimol in combination with WIN 55,212-2 into the NC respectively potentiated and attenuated WIN 55,212-2-induced antinociception in the formalin test. This study shows that GABA in the NC is involved in pain modulation and suggests the existence of a GABAA-mediated inhibitory system in the NC on pain control. Furthermore, it seems that the antinociceptive effect of WIN 55,212-2 in the formalin test is mediated partly by the activity of local GABAA receptors in the NC.

Presynaptic Inhibition of Pain and Touch in the Spinal Cord: From Receptors to Circuits.

Sensory primary afferent fibers, conveying touch, pain, itch, and proprioception, synapse onto spinal cord dorsal horn neurons. Primary afferent central terminals express a wide variety of receptors that modulate glutamate and peptide release. Regulation of the amount and timing of neurotransmitter release critically affects the integration of postsynaptic responses and the coding of sensory information. The role of GABA (γ-aminobutyric acid) receptors expressed on afferent central terminals is particularly important in sensory processing, both in physiological conditions and in sensitized states induced by chronic pain. During the last decade, techniques of opto- and chemogenetic stimulation and neuronal selective labeling have provided interesting insights on this topic. This review focused on the recent advances about the modulatory effects of presynaptic GABAergic receptors in spinal cord dorsal horn and the neural circuits involved in these mechanisms.

痙縮治療におけるバクロフェン持続髄注（ITB）療法の臨床―GABA B 受容体からの考察―

痙縮治療におけるバクロフェン持続髄注（ITB）療法の臨床―GABA B 受容体からの考察―

Effects of homotaurine on DLPFC disarray in MCI and the role of GABA

AbstractBackgroundHomotaurine has been studied for its capacity to interact with amyloid‐beta monomers preventing their aggregation to oligomers, which carry a high degree of synaptic toxicity and are thought to play an upstream role in the pathogenesis of Alzheimer’s disease (AD)1. At once, this compound directly modulates excitatory neurotransmission via its high affinity with GABA‐A receptors. A recent study showed that homotaurine improves cholinergic transmission, likely by modulating inhibitory cortical activity2. Its anti‐amyloid properties combined with the GABAergic profile make homotaurine a new potential drug for the treatment of AD. Since pathological changes long precede cognitive decline in AD, Mild Cognitive Impairment (MCI) should be looked upon as a favoured stage for interventions aimed at stopping or slowing the progression of the disease. Several neurophysiological studies also highlighted that, from the earliest stages of AD, cortical plasticity at the dorso‐lateral Pre‐Frontal Cortex (dlPFC) is impaired, with lost of physiological beta‐gamma oscillatory response to TMS. We decided to test the effects of homotaurine on dlPFC functionality in patients with MCI.MethodWe used TMS‐EEG to non‐invasively stimulate the left and right dlPFC of 5 patients that accessed to our Memory Clinic for subjective amnestic impairment (mean MMSE 28). Registration was performed at baseline and after 15 days of treatment with Tramiprosate 100 mg/day.ResultAt baseline, the response on left dlPFC was weak in frequency, with an anomalous spot on lower alpha frequencies. Instead, the stimulation on right dlPFC originated a powerful response, yet aspecific in frequency, outlining a prominent inter‐hemispheric asymmetry. Post treatment results showed (1) a spectrum of oscillation frequency more coherent with physiological registrations and (2) interhemispheric symmetry in response.ConclusionThese data suggest that Homotaurine increases frequency of oscillation in dlPFC, and helps re‐establishing, at least partially, interhemispheric balance in terms of oscillation in patients with amnestic impairment. This rapid effect likely depends on Homotaurine‐mediated changes of cortical GABA transmission, meaning that GABA might be implicated in cortical disarray from the earliest stages of AD.

Vesicular Release of GABA by Mammalian Horizontal Cells Mediates Inhibitory Output to Photoreceptors.

Feedback inhibition by horizontal cells regulates rod and cone photoreceptor calcium channels that control their release of the neurotransmitter glutamate. This inhibition contributes to synaptic gain control and the formation of the center-surround antagonistic receptive fields passed on to all downstream neurons, which is important for contrast sensitivity and color opponency in vision. In contrast to the plasmalemmal GABA transporter found in non-mammalian horizontal cells, there is evidence that the mechanism by which mammalian horizontal cells inhibit photoreceptors involves the vesicular release of the inhibitory neurotransmitter GABA. Historically, inconsistent findings of GABA and its biosynthetic enzyme, L-glutamate decarboxylase (GAD) in horizontal cells, and the apparent lack of surround response block by GABAergic agents diminished support for GABA's role in feedback inhibition. However, the immunolocalization of the vesicular GABA transporter (VGAT) in the dendritic and axonal endings of horizontal cells that innervate photoreceptor terminals suggested GABA was released via vesicular exocytosis. To test the idea that GABA is released from vesicles, we localized GABA and GAD, multiple SNARE complex proteins, synaptic vesicle proteins, and Cav channels that mediate exocytosis to horizontal cell dendritic tips and axonal terminals. To address the perceived relative paucity of synaptic vesicles in horizontal cell endings, we used conical electron tomography on mouse and guinea pig retinas that revealed small, clear-core vesicles, along with a few clathrin-coated vesicles and endosomes in horizontal cell processes within photoreceptor terminals. Some small-diameter vesicles were adjacent to the plasma membrane and plasma membrane specializations. To assess vesicular release, a functional assay involving incubation of retinal slices in luminal VGAT-C antibodies demonstrated vesicles fused with the membrane in a depolarization- and calcium-dependent manner, and these labeled vesicles can fuse multiple times. Finally, targeted elimination of VGAT in horizontal cells resulted in a loss of tonic, autaptic GABA currents, and of inhibitory feedback modulation of the cone photoreceptor Cai, consistent with the elimination of GABA release from horizontal cell endings. These results in mammalian retina identify the central role of vesicular release of GABA from horizontal cells in the feedback inhibition of photoreceptors.