GABA Release Research Articles

Adenosine-Dependent Arousal Induced by Astrocytes in a Brainstem Circuit.

Astrocytes play a crucial role in regulating sleep-wake behavior. However, how astrocytes govern a specific sleep-arousal circuit remains unknown. Here, the authors show that parafacial zone (PZ) astrocytes responded to sleep-wake cycles with state-differential Ca2+ activity, peaking during transitions from sleep to wakefulness. Using chemogenetic and optogenetic approaches, they find that activating PZ astrocytes elicited and sustained wakefulness by prolonging arousal episodes while impeding transitions from wakefulness to non-rapid eye movement (NREM) sleep. Activation of PZ astrocytes specially induced the elevation of extracellular adenosine through the ATP hydrolysis pathway but not equilibrative nucleoside transporter (ENT) mediated transportation. Strikingly, the rise in adenosine levels induced arousal by activating A1 receptors, suggesting a distinct role for adenosine in the PZ beyond its conventional sleep homeostasis modulation observed in the basal forebrain (BF) and cortex. Moreover, at the circuit level, PZ astrocyte activation induced arousal by suppressing the GABA release from the PZGABA neurons, which promote NREM sleep and project to the parabrachial nucleus (PB). Thus, their study unveils a distinctive arousal-promoting effect of astrocytes within the PZ through extracellular adenosine and elucidates the underlying mechanism at the neural circuit level.

Dialectics of perisomatic inhibition-The unity and conflict of opposites.

Over the past three decades, a great deal of attention has been paid to the study of perisomatic inhibition and perisomatic inhibitory basket cells. A growing body of experimental evidence points to the leading role of perisomatic inhibitory cells in the generation of oscillatory activity in various frequency ranges. Recently the link between the activity of basket cells and complex behavior has been demonstrated in several laboratories. However, all this is true only for one type of perisomatic inhibitory interneuron-parvalbumin-positive basket cells. Nevertheless, where parvalbumin-positive basket cells are found, there is another type of basket cell, cholecystokinin-positive interneurons. These two types of interneurons share a number of common features: they innervate the same compartments of target neurons and they often receive excitation from the same sources, but they also differ from each other in the synchrony of their GABA release and expression of receptors. The functional role of cholecystokinin-positive basket cells in oscillatory activity is not so obvious. They were thought to be involved in theta oscillations, however recent measurements in free moving animals have put some doubts on this hypothesis. Therefore, an important question is, whether these two types of basket cells work synergistically or perform opposing actions in functional networks? In this mini-review, we attempt to answer this question by putting forward the idea that these two types of basket cells are functionally united as two entities of the same network, and their opposing actions are necessary to maintain rhythmogenesis in a "healthy", physiological range.

KCN Channels "Cue" Up GABA Release from Astrocytes.

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HCN1 hyperpolarization-activated cyclic nucleotide–gated channels enhance evoked GABA release from parvalbumin-positive interneurons

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels generate the cationic Ih current in neurons and regulate the excitability of neuronal networks. The function of HCN channels depends, in part, on their subcellular localization. Of the four HCN isoforms (HCN1-4), HCN1 is strongly expressed in the dendrites of pyramidal neurons (PNs) in hippocampal area CA1 but also in presynaptic terminals of parvalbumin-positive interneurons (PV+ INs), which provide strong inhibitory control over hippocampal activity. Yet, little is known about how HCN1 channels in these cells regulate the evoked release of the inhibitory transmitter GABA from their axon terminals. Here, we used genetic, optogenetic, electrophysiological, and imaging techniques to investigate how the electrophysiological properties of PV+ INs are regulated by HCN1, including how HCN1 activity at presynaptic terminals regulates the release of GABA onto PNs in CA1. We found that application of HCN1 pharmacological blockers reduced the amplitude of the inhibitory postsynaptic potential recorded from CA1 PNs in response to selective optogenetic stimulation of PV+ INs. Homozygous HCN1 knockout mice also show reduced IPSCs in postsynaptic cells. Finally, two-photon imaging using genetically encoded fluorescent calcium indicators revealed that HCN1 blockers reduced the probability that an extracellular electrical stimulating pulse evoked a Ca2+ response in individual PV+ IN presynaptic boutons. Taken together, our results show that HCN1 channels in the axon terminals of PV+ interneurons facilitate GABAergic transmission in the hippocampal CA1 region.

Vasopressin regulates social play behavior in sex-specific ways through glutamate modulation in the lateral septum.

Understanding the neural basis of social play in juvenile rats may ultimately help restore social play deficits in autistic children. We previously found that administration of a vasopressin (AVP) V1a receptor (V1aR) antagonist into the lateral septum (LS) increased social play behavior in male juvenile rats and decreased it in females. Here, we demonstrate that glutamate, but not GABA, is involved in this sex-specific regulation. First, we found a sex difference in extracellular LS glutamate/GABA ratio (lower in females) that was eliminated by V1aR antagonist infusion in the LS that caused an increase in glutamate release in females only. Second, infusion of the glutamate receptor agonist L-glutamic acid into the LS mimicked the V1aR antagonist-induced decrease in female social play while preventing the increase in male social play. Third, infusion of the glutamate receptor antagonists AP-5 and CNQX into the LS prevented the V1aR antagonist-induced decrease in female social play. Fourth, there were no sex differences in extracellular GABA release in the LS upon either V1aR antagonist infusion or in social play expression upon infusion of the GABA-A receptor agonist muscimol into the LS, suggesting that GABA is not involved in the sex-specific regulation of social play by the LS-AVP system. Last, we found no sex differences in the type (GAD1/2, somatostatin, calbindin 1, Sox9) of V1aR-expressing LS cells, suggesting other cellular mechanisms mediating the sex-specific effects on glutamate release in the LS by the LS-AVP system. In conclusion, we demonstrate that the LS-AVP system regulates social play sex-specifically via glutamatergic neurotransmission. These findings have relevance for potential sex-specific effects of AVP-based treatment of social deficits in children.

μ-Opioid Receptor Modulation of the Glutamatergic/GABAergic Midbrain Inputs to the Mouse Dorsal Hippocampus.

We used virus-mediated anterograde and retrograde tracing, optogenetic modulation, immunostaining, in situ hybridization, and patch-clamp recordings in acute brain slices to study the release mechanism and μ-opioid modulation of the dual glutamatergic/GABAergic inputs from the ventral tegmental area and supramammillary nucleus to the granule cells of the dorsal hippocampus of male and female mice. In keeping with previous reports showing that the two transmitters are released by separate active zones within the same terminals, we found that the short-term plasticity and pharmacological modulation of the glutamatergic and GABAergic currents are indistinguishable. We further found that glutamate and GABA release at these synapses are both virtually completely mediated by N- and P/Q-type calcium channels. We then investigated μ-opioid modulation of these synapses and found that activation of μ-opioid receptors (MORs) strongly inhibits the glutamate and GABA release, mostly through inhibition of presynaptic N-type channels. However, the modulation by MORs of these dual synapses is complex, as it likely includes also a disinhibition due to downmodulation of local GABAergic interneurons which make direct axo-axonic contacts with the dual glutamatergic/GABAergic terminals. We discuss how this opioid modulation may enhance LTP at the perforant path inputs, potentially contributing to reinforce memories of drug-associated contexts.

Sleep-improving effect and the potential mechanism of Morus alba L. on mice

As insufficient sleep has become a widespread concern in modern society, potential sleep-improving effect of mulberry (Morus alba L.) leaf ethanol extract (MLE) and the related mechanism were investigated in the present study. According to the results, MLE could significantly shorten sleep latency by 33 %, extend sleep duration by 56 % and increase sleep ratio of mice through increasing 5-HT and GABA release in serum, hypothalamus and hippocampus. Metabonomic analysis showed that phenylalanine metabolism, arginine and proline metabolism might be the potential pathways of MLE to improve sleep. Network pharmacological and LC-MS analysis suggested that the key sleep-improving active ingredients in MLE might be luteolin, kaempferol, naringenin, morin, stigmasterol and β-sitosterol. Further molecular docking and qRT-PCR results demonstrated that the key targets for MLE to improve sleep might be MAOA, GABRA1 and GABRA2. In conclusion, MLE showed outstanding sleep-improving effect and great potential for the application as novel sleep-improving functional food.

Generation of Astrocyte-specific BEST1 Conditional Knockout Mouse with Reduced Tonic GABA Inhibition in the Brain.

Bestrophin-1 (BEST1) is a Ca2+-activated anion channel known for its role in astrocytes. Best1 is permeable to gliotransmitters, including GABA, to contribute to tonic GABA inhibition and modulate synaptic transmission in neighboring neurons. Despite the crucial functions of astrocytic BEST1, there is an absence of genetically engineered cell-type specific conditional mouse models addressing these roles. In this study, we developed an astrocyte-specific BEST1 conditional knock-out (BEST1 aKO) mouse line. Using the embryonic stem cell (ES cell) targeting method, we developed Best1 floxed mice (C57BL/6JCya-Best1em1flox/Cya), which have exon 3, 4, 5, and 6 of Best1 flanked by two loxP sites. By crossing with hGFAP-CreERT2 mice, we generated Best1 floxed/hGFAP-CreERT2 mice, which allowed for the tamoxifen-inducible deletion of Best1 under the human GFAP promoter. We characterized its features across various brain regions, including the striatum, hippocampal dentate gyrus (HpDG), and Parafascicular thalamic nucleus (Pf). Compared to the Cre-negative control, we observed significantly reduced BEST1 protein expression in immunohistochemistry (IHC) and tonic GABA inhibition in patch clamp recordings. The reduction in tonic GABA inhibition was 66.7% in the striatum, 46.4% in the HpDG, and 49.6% in the Pf. Our findings demonstrate that the BEST1 channel in astrocytes significantly contributes to tonic inhibition in the local brain areas. These mice will be valuable for future studies not only on tonic GABA release but also on tonic release of gliotransmitters mediated by astrocytic BEST1.

Regulation of Hippocamposeptal Synaptic Transmission by GABABRs Is Altered in 5XFAD Mice in a Sex- and Age-Dependent Manner

Hippocamposeptal (HS) neurons send GABAergic projections from the hippocampus to the medial septum/diagonal band of Broca (MS/DBB) as part of a reciprocal loop that is critical for memory. HS neurons are proposed to be particularly sensitive to the deleterious effects of pathological exposure to amyloid-β (Aβ), as would occur during Alzheimer’s disease (AD). However, it is not known how HS GABA release in the MS/DBB is altered during the progression of AD. To target HS neurons in a mouse model of AD, we crossed SST-Cre mice to 5XFAD mice and performed stereotaxic injections of Cre-dependent AAV containing mCherry/channelrhodopsin-2 (ChR2) into the hippocampus of offspring at 4, 6, 9, and 12 months. We used optogenetics to selectively stimulate HS terminals while performing whole-cell patch-clamp recordings from MS/DBB neurons in slices. There was a transient reduction in HS-inhibitory postsynaptic current (IPSC) amplitude in female 5XFAD mice at 6 months, but no difference in males at any age, and no difference in paired-pulse ratio in either sex at any age. When bath applying the GABABR agonist, baclofen, we found a larger decrease in HS-IPSC amplitude in 5XFAD females at 9 months and 5XFAD males at 12 months. In 12-month-old 5XFAD females, response to baclofen was significantly reduced. These data suggest that there is a transient increase in responsiveness to GABABR activation in 5XFAD mice that occurs earlier in females than in males. These sex-specific changes to HS function are likely to impact the relay of information between the hippocampus and MS/DBB.

Opposing actions of co-released GABA and neurotensin on the activity of preoptic neurons and on body temperature.

Neurotensin (Nts) is a neuropeptide acting as a neuromodulator in the brain. Pharmacological studies have identified Nts as a potent hypothermic agent. The medial preoptic area, a region that plays an important role in the control of thermoregulation, contains a high density of neurotensinergic neurons and Nts receptors. The conditions in which neurotensinergic neurons play a role in thermoregulation are not known. In this study, optogenetic stimulation of preoptic Nts neurons induced a small hyperthermia. In vitro, optogenetic stimulation of preoptic Nts neurons resulted in synaptic release of GABA and net inhibition of the preoptic pituitary adenylate cyclase-activating polypeptide (Adcyap1) neurons firing activity. GABA-A receptor antagonist or genetic deletion of Slc32a1 (VGAT) in Nts neurons unmasked also an excitatory effect that was blocked by a Nts receptor 1 antagonist. Stimulation of preoptic Nts neurons lacking Slc32a1 resulted in excitation of Adcyap1 neurons and hypothermia. Mice lacking Slc32a1 expression in Nts neurons presented changes in the fever response and in the responses to heat or cold exposure as well as an altered circadian rhythm of body temperature. Chemogenetic activation of all Nts neurons in the brain induced a 4-5°C hypothermia, which could be blocked by Nts receptor antagonists in the preoptic area. Chemogenetic activation of preoptic neurotensinergic projections resulted in robust excitation of preoptic Adcyap1 neurons. Taken together, our data demonstrate that endogenously released Nts can induce potent hypothermia and that excitation of preoptic Adcyap1 neurons is the cellular mechanism that triggers this response.

Basal forebrain-lateral habenula inputs and control of impulsive behavior.

Deficits in impulse control are observed in several neurocognitive disorders, including attention deficit hyperactivity (ADHD), substance use disorders (SUDs), and those following traumatic brain injury (TBI). Understanding brain circuits and mechanisms contributing to impulsive behavior may aid in identifying therapeutic interventions. We previously reported that intact lateral habenula (LHb) function is necessary to limit impulsivity defined by impaired response inhibition in rats. Here, we examine the involvement of a synaptic input to the LHb on response inhibition using cellular, circuit, and behavioral approaches. Retrograde fluorogold tracing identified basal forebrain (BF) inputs to LHb, primarily arising from ventral pallidum and nucleus accumbens shell (VP/NAcs). Glutamic acid decarboxylase and cannabinoid CB1 receptor (CB1R) mRNAs colocalized with fluorogold, suggesting a cannabinoid modulated GABAergic pathway. Optogenetic activation of these axons strongly inhibited LHb neuron action potentials and GABA release was tonically suppressed by an endogenous cannabinoid in vitro. Behavioral experiments showed that response inhibition during signaled reward omission was impaired when VP/NAcs inputs to LHb were optogenetically stimulated, whereas inhibition of this pathway did not alter LHb control of impulsivity. Systemic injection with the psychotropic phytocannabinoid, Δ9-tetrahydrocannabinol (Δ9-THC), also increased impulsivity in male, and not female rats, and this was blocked by LHb CB1R antagonism. However, as optogenetic VP/NAcs pathway inhibition did not alter impulse control, we conclude that the pro-impulsive effects of Δ9-THC likely do not occur via inhibition of this afferent. These results identify an inhibitory LHb afferent that is controlled by CB1Rs that can regulate impulsive behavior.

Postsynaptic GABAB-receptor mediated currents in diverse dentate gyrus interneuron types.

The processing of rich synaptic information in the dentate gyrus (DG) relies on a diverse population of inhibitory GABAergic interneurons to regulate cellular and circuit activity, in a layer-specific manner. Metabotropic GABAB-receptors (GABABRs) provide powerful inhibition to the DG circuit, on timescales consistent with behavior and learning, but their role in controlling the activity of interneurons is poorly understood with respect to identified cell types. We hypothesize that GABABRs display cell type-specific heterogeneity in signaling strength, which will have direct ramifications for signal processing in DG networks. To test this, we perform in vitro whole-cell patch-clamp recordings from identified DG principal cells and interneurons, followed by GABABR pharmacology, photolysis of caged GABA, and extracellular stimulation of endogenous GABA release to classify the cell type-specific inhibitory potential. Based on our previous classification of DG interneurons, we show that postsynaptic GABABR-mediated currents are present on all interneuron types albeit at different amplitudes, dependent largely on soma location and synaptic targets. GABABRs were coupled to inwardly-rectifying K+ channels that strongly reduced the excitability of those interneurons where large currents were observed. These data provide a systematic characterization of GABABR signaling in the rat DG to provide greater insight into circuit dynamics.

Delta opioid receptors engage multiple signaling cascades to differentially modulate prefrontal GABA release with input and target specificity.

Opioids regulate circuits associated with motivation and reward across the brain. Of the opioid receptor types, delta opioid receptors (DORs) appear to have a unique role in regulating the activity of circuits related to reward without a liability for abuse. In neocortex, DORs are expressed primarily in interneurons, including parvalbumin- and somatostatin-expressing interneurons that inhibit somatic and dendritic compartments of excitatory pyramidal cells, respectively. But how DORs regulate transmission from these key interneuron classes is unclear. We found that DORs regulate inhibition from these interneuron classes using different G-protein signaling pathways that both converge on presynaptic calcium channels, but regulate distinct aspects of calcium channel function. This imposes different temporal filtering effects, via short-term plasticity, that depend on how calcium channels are regulated. Thus, DORs engage differential signaling cascades to regulate inhibition depending on the postsynaptic target compartment, with different effects on synaptic information transfer in somatic and dendritic domains.

Sedum kamtschaticum Exerts Hypnotic Effects via the Adenosine A2A Receptor in Mice.

Insomnia is a common sleep disorder with significant societal and economic impacts. Current pharmacotherapies for insomnia are often accompanied by side effects, necessitating the development of new therapeutic drugs. In this study, the hypnotic effects and mechanisms of Sedum kamtschaticum 30% ethanol extract (ESK) and one of its active compounds, myricitrin, were investigated using pentobarbital-induced sleep experiments, immunohistochemistry (IHC), receptor binding assays, and enzyme-linked immunosorbent assay (ELISA). The pentobarbital-induced sleep experiments revealed that ESK and myricitrin reduced sleep latency and prolonged total sleep time in a dose-dependent manner. Based on c-Fos immunostaining, ESK, and myricitrin enhanced the GABAergic neural activity in sleep-promoting ventrolateral preoptic nucleus (VLPO) GABAergic. By measuring the level of GABA released from VLPO GABAergic neurons, ESK and myricitrin were found to increase GABA release in the hypothalamus. These effects were significantly inhibited by SCH. Moreover, ESK exhibited a concentration-dependent binding affinity for the adenosine A2A receptors (A2AR). In conclusion, ESK and myricitrin have hypnotic effects, and their underlying mechanisms may be related to the activation of A2AR.

Modulation of sensory input to the spinal cord: Contribution of focal epidural polarization and of GABA released by interneurons and glial cells.

Modulation of input from primary afferent fibres has long been examined at the level of the first relays of these fibres. However, recent studies reveal that input to the spinal cord may also be modulated at the level of the very entry of afferent fibres to the spinal grey matter before action potentials in intraspinal collaterals of afferent fibres reach their target neurons. Such modulation greatly depends on the actions of GABA via extrasynaptic membrane receptors. In the reported study we hypothesized that the increase in excitability of afferent fibres following epidural polarization close to the site where collaterals of afferent fibres leave the dorsal columns is due to the release of GABA from two sources: not only GABAergic interneurons but also glial cells. We present evidence, primo, that GABA released from both these sources contributes to a long-lasting increase in the excitability and a shortening of the refractory period of epidurally stimulated afferent fibres and, secondo, that effects of epidural polarization on the release of GABA are more critical for these changes than direct effects of DC on the stimulated fibres. The experiments were carried out in deeply anaesthetized rats in which changes in compound action potentials evoked in hindlimb peripheral nerves by dorsal column stimulation were used as a measure of the excitability of afferent fibres. The study throws new light on the modulation of input to spinal networks but also on mechanisms underlying the restoration of spinal functions.

Crystal structure of the GluK1 ligand-binding domain with kainate and the full-spanning positive allosteric modulator BPAM538

Kainate receptors play an important role in the central nervous system by mediating postsynaptic excitatory neurotransmission and modulating the release of the inhibitory neurotransmitter GABA through a presynaptic mechanism. To date, only three structures of the ligand-binding domain (LBD) of the kainate receptor subunit GluK1 in complex with positive allosteric modulators have been determined by X-ray crystallography, all belonging to class II modulators. Here, we report a high-resolution structure of GluK1-LBD in complex with kainate and BPAM538, which belongs to the full-spanning class III. One BPAM538 molecule binds at the GluK1 dimer interface, thereby occupying two allosteric binding sites simultaneously. BPAM538 stabilizes the active receptor conformation with only minor conformational changes being introduced to the receptor. Using a calcium-sensitive fluorescence-based assay, a 5-fold potentiation of the kainate response (100 μM) was observed in presence of 100 μM BPAM538 at GluK1(Q)b, whereas no potentiation was observed at GluK2(VCQ)a. Using electrophysiology recordings of outside-out patches excised from HEK293 cells, BPAM538 increased the peak response of GluK1(Q)b co-expressed with NETO2 to rapid application of 10 mM L-glutamate with 130 ± 20 %, and decreased desensitization determined as the steady-state/peak response ratio from 23 ± 2 % to 90 ± 4 %. Based on dose–response relationship experiments on GluK1(Q)b the EC50 of BPAM538 was estimated to be 58 ± 29 μM.

Dysregulation of kappa opioid receptor neuromodulation of lateral habenula synaptic function following a repetitive mild traumatic brain injury

Mild traumatic brain injury (mTBI) increases the risk of affective disorders, anxiety and substance use disorder. The lateral habenula (LHb) plays an important role in pathophysiology of psychiatric disorders. Recently, we demonstrated a causal link between mTBI-induced LHb hyperactivity due to excitation/inhibition (E/I) imbalance and motivational deficits in male mice using a repetitive closed head injury mTBI model. A major neuromodulatory system that is responsive to traumatic brain injuries, influences affective states and also modulates LHb activity is the dynorphin/kappa opioid receptor (Dyn/KOR) system. However, the effects of mTBI on KOR neuromodulation of LHb function are unknown. Here, we first used retrograde tracing in male and female Cre mouse lines and identified several major KOR-expressing and two prominent Dyn-expressing inputs projecting to the mouse LHb, highlighting the medial prefrontal cortex (mPFC) and the ventromedial nucleus of the hypothalamus (VMH) as the main LHb-projecting Dyn inputs that regulate KOR signaling to the LHb. We then functionally evaluated the effects of in vitro KOR modulation of spontaneous synaptic activity within the LHb of male and female sham and mTBI mice at 4 week post-injury. We observed sex-specific differences in spontaneous release of glutamate and GABA from presynaptic terminals onto LHb neurons with higher levels of presynaptic glutamate and GABA release in females compared to male mice. However, KOR effects on the spontaneous E/I ratios and synaptic drive ratio within the LHb did not differ between male and female sham and mTBI mice. KOR activation generally suppressed spontaneous glutamatergic transmission without altering GABAergic transmission, resulting in a significant but sex-similar reduction in net spontaneous E/I and synaptic drive ratios in LHb neurons of sham mice. Following mTBI, while responses to KOR activation at LHb glutamatergic synapses remained intact, LHb GABAergic synapses acquired an additional sensitivity to KOR-mediated inhibition where we observed a reduction in GABA release probability in response to KOR stimulation in LHb neurons of mTBI mice. Further analysis of percent change in spontaneous synaptic ratios induced by KOR activation revealed that independent of sex mTBI switches KOR-driven synaptic inhibition of LHb neurons (normally observed in sham mice) in a subset of mTBI mice toward synaptic excitation resulting in mTBI-induced divergence of KOR actions within the LHb. Overall, we uncovered the sources of major Dyn/KOR-expressing synaptic inputs projecting to the mouse LHb. We demonstrate that an engagement of intra-LHb Dyn/KOR signaling provides a global KOR-driven synaptic inhibition within the mouse LHb independent of sex. The additional engagement of KOR-mediated action on LHb GABAergic transmission by mTBI could contribute to the E/I imbalance after mTBI, with Dyn/KOR signaling serving as a disinhibitory mechanism for LHb neurons of a subset of mTBI mice.

CLEM and Super-Resolution Microscopy Pipeline Demonstrate Single Co-Releasing GABA-Glutamate Axon Terminals Establishing Convergent Synapses for Glutamate or GABA Release in Mouse Brain

CLEM and Super-Resolution Microscopy Pipeline Demonstrate Single Co-Releasing GABA-Glutamate Axon Terminals Establishing Convergent Synapses for Glutamate or GABA Release in Mouse Brain

Regulation of GABAergic neurotransmission by purinergic receptors in brain physiology and disease.

Purinergic receptors regulate the processing of neural information in the hippocampus and cerebral cortex, structures related to cognitive functions. These receptors are activated when astrocytic and neuronal populations release adenosine triphosphate (ATP) in an autocrine and paracrine manner, following sustained patterns of neuronal activity. The modulation by these receptors of GABAergic transmission has only recently been studied. Through their ramifications, astrocytes and GABAergic interneurons reach large groups of excitatory pyramidal neurons. Their inhibitory effect establishes different synchronization patterns that determine gamma frequency rhythms, which characterize neural activities related to cognitive processes. During early life, GABAergic-mediated synchronization of excitatory signals directs the experience-driven maturation of cognitive development, and dysfunctions concerning this process have been associated with neurological and neuropsychiatric diseases. Purinergic receptors timely modulate GABAergic control over ongoing neural activity and deeply affect neural processing in the hippocampal and neocortical circuitry. Stimulation of A2 receptors increases GABA release from presynaptic terminals, leading to a considerable reduction in neuronal firing of pyramidal neurons. A1 receptors inhibit GABAergic activity but only act in the early postnatal period when GABA produces excitatory signals. P2X and P2Y receptors expressed in pyramidal neurons reduce the inhibitory tone by blocking GABAA receptors. Finally, P2Y receptor activation elicits depolarization of GABAergic neurons and increases GABA release, thus favoring the emergence of gamma oscillations. The present review provides an overall picture of purinergic influence on GABAergic transmission and its consequences on neural processing, extending the discussion to receptor subtypes and their involvement in the onset of brain disorders, including epilepsy and Alzheimer's disease.

Subthreshold activation of the melanocortin system causes generalized sensitization to anorectic agents in mice.

The melanocortin-3 receptor (MC3R) regulates GABA release from agouti-related protein (AgRP) nerve terminals and thus tonically suppresses multiple circuits involved in feeding behavior and energy homeostasis. Here, we examined the role of the MC3R and the melanocortin system in regulating the response to various anorexigenic agents. The genetic deletion or pharmacological inhibition of the MC3R, or subthreshold doses of an MC4R agonist, improved the dose responsiveness to glucagon-like peptide 1 (GLP1) agonists, as assayed by inhibition of food intake and weight loss. An enhanced anorectic response to the acute satiety factors peptide YY (PYY3-36) and cholecystokinin (CCK) and the long-term adipostatic factor leptin demonstrated that increased sensitivity to anorectic agents was a generalized result of MC3R antagonism. We observed enhanced neuronal activation in multiple hypothalamic nuclei using Fos IHC following low-dose liraglutide in MC3R-KO mice (Mc3r-/-), supporting the hypothesis that the MC3R is a negative regulator of circuits that control multiple aspects of feeding behavior. The enhanced anorectic response in Mc3r-/- mice after administration of GLP1 analogs was also independent of the incretin effects and malaise induced by GLP1 receptor (GLP1R) analogs, suggesting that MC3R antagonists or MC4R agonists may have value in enhancing the dose-response range of obesity therapeutics.