Activity‐dependent plasticity of presynaptic GABAB receptors at parallel fiber synapses

Parallel fiber synapses in the cerebellum express a wide range of presynaptic receptors. However, presynaptic receptor expression at individual parallel fiber synapses is quite heterogeneous, suggesting physiological mechanisms regulate presynaptic receptor expression. We investigated changes in presynaptic GABA_B receptors at parallel fiber‐stellate cell synapses in acute cerebellar slices from juvenile mice. GABA_B receptor‐mediated inhibition of excitatory postsynaptic currents (EPSCs) is remarkably diverse at these synapses, with transmitter release at some synapses inhibited by >50% and little or no inhibition at others. GABA_B receptor‐mediated inhibition was significantly reduced following 4 Hz parallel fiber stimulation but not after stimulation at other frequencies. The reduction in GABA_B receptor‐mediated inhibition was replicated by bath application of forskolin and blocked by application of a PKA inhibitor, suggesting activation of adenylyl cyclase and PKA are required. Immunolabeling for an extracellular domain of the GABA_B2 subunit revealed reduced surface expression in the molecular layer after exposure to forskolin. GABA_B receptor‐mediated inhibition of action potential evoked calcium transients in parallel fiber varicosities was also reduced following bath application of forskolin, confirming presynaptic receptors are responsible for the reduced EPSC inhibition. These data demonstrate that presynaptic GABA_B receptor expression can be a plastic property of synapses, which may compliment other forms of synaptic plasticity. This opens the door to novel forms of receptor plasticity previously confined primarily to postsynaptic receptors.     

GABAA receptor subunit composition and competition at synapses are tuned by GABAB receptor activity

GABA_BRs have a well-established role in controlling neuronal excitability and presynaptic neurotransmitter release. We examined the role of GABA_BR activity in modulating the number and lateral diffusion of GABA_ARs at inhibitory synapses. Changes in diffusion of GABA_ARs at synapses were observed when subunit heterogeneity was taken into account. While α1-GABA_ARs were unaffected, α2- and α5-GABA_ARs showed inverse changes in enrichment and diffusion. The intracellular TM3-4 loop of α2 was sufficient to observe the changes in diffusion by GABA_BR activity, whereas the loop of α5 was not. The opposing effect on α2- and α5-GABA_ARs was caused by a competition between GABA_ARs for binding slots at synapses. Receptor immobilization by cross-linking revealed that α5-GABA_AR trapping at synapses is regulated by modulation of α2-GABA_AR mobility. Finally, PKC activity was determined to be part of the signaling pathway through which GABA_BR activity modulates α2-GABA_AR diffusion at synapses. These results outline a novel mechanism for tuning inhibitory transmission in a subunit-specific manner, and for the first time describe competition between GABA_ARs with different subunit compositions for binding slots at synapses.     

GABAB modulation of dopamine release in the nucleus accumbens core

Modulation of the concentration of dopamine (DA) released from dopaminergic terminals in the nucleus accumbens (NAc) influences behaviours such as the motivation to obtain drugs of abuse. γ‐Aminobutyric acid type B (GABA_B) receptors are expressed throughout the mesolimbic circuit, including in the NAc, and baclofen, an agonist of GABA_B receptors, can decrease drug‐seeking behaviours. However, the mechanism by which GABA_B receptors modulate terminal DA release has not been well studied. We explored how baclofen modulates the concentration of DA released from terminals in the NAc core using fast‐scan cyclic voltammetry in brain slices from adult male C57BL/6J mice. We found that baclofen concentration‐dependently decreased single pulse‐evoked DA release. This effect was blocked by the GABA_B antagonist, CGP 52432, but not by a nicotinic acetylcholine receptor antagonist. Suppression of DA release by a saturating concentration of baclofen was sustained for up to 1 h. The effect of baclofen was reduced with electrical stimulations mimicking burst firing of DA neurons. Similar to the D₂ receptor agonist, quinpirole, baclofen reduced the probability of DA release, supporting a mechanistic overlap with D₂ receptors. Baclofen‐mediated suppression of DA release persisted after a locomotor‐sensitizing cocaine treatment, indicating that GABA_B receptors on DA terminals were not altered by cocaine exposure. These data suggest that baclofen‐mediated suppression of terminal DA release is due to GABA_B activation on DA terminals to reduce the probability of DA release. This effect does not readily desensitize, and persists regardless of chronic cocaine treatment.     

Experimental early‐life febrile seizures induce changes in GABAAR‐mediated neurotransmission in the dentate gyrus

Purpose: Febrile seizures (FS), the most frequent seizure type during childhood, have been linked to temporal lobe epilepsy (TLE) in adulthood. Yet, underlying mechanisms are still largely unknown. Altered γ‐aminobutyric acid (GABA)ergic neurotransmission in the dentate gyrus (DG) circuit has been hypothesized to be involved. This study aims at analyzing whether experimental FS change inhibitory synaptic input and postsynaptic GABA_AR function in dentate granule cells. Methods: We applied an immature rat model of hyperthermia (HT)–induced FS. GABA_AR‐mediated neurotransmission was studied using whole‐cell patch‐clamp recordings from dentate granule neurons in hippocampal slices within 6–9 days post‐HT. Key Findings: Frequencies of spontaneous inhibitory postsynaptic currents (sIPSCs) were reduced in HT rats that had experienced seizures, whereas sIPSC amplitudes were enhanced. Whole‐cell GABA responses revealed a doubled GABA_AR sensitivity in dentate granule cells from HT animals, compared to that of normothermic (NT) controls. Analysis of sIPSCs and whole‐cell GABA responses showed similar kinetics in postsynaptic GABA_ARs of HT and NT rats. quantitative real‐time polymerase chain reaction (qPCR) experiments indicated changes in DG GABA_AR subunit expression, which was most pronounced for the α3 subunit. Significance: The data support the hypothesis that FS persistently alter neuronal excitability.     

GABA receptors and prepulse inhibition of acoustic startle in mice and rats

The acoustic startle reflex is strongly inhibited by a moderate‐intensity acoustic stimulus that precedes the startling stimulus by roughly 10–1000 ms (prepulse inhibition, PPI). At long interstimulus intervals (ISIs) of 100–1000 ms, PPI in rats is reduced by the muscarinic receptor antagonist scopolamine. Here, we studied the role of GABA receptors in PPI at full ISI ranges in both mice and rats. In B6 mice, PPI begins and ends at shorter ISIs (4 and 1000 ms, respectively) than in Wistar rats (8 and 5000 ms). The GABA_A antagonist bicuculline (1 mg/kg i.p.) reduced PPI at ISIs near the peak of PPI in both rats and mice. The GABA_B antagonist phaclofen (10 or 30 mg/kg i.p. in rats or mice, respectively) reduced PPI only at long ISIs, similar to the effects of the muscarinic antagonist scopolamine (1 mg/kg i.p.). The effects of phaclofen and scopolamine were additive in rats, suggesting independent effects of GABA_B and muscarinic receptors. Patch‐clamp recordings of startle‐mediating PnC (nucleus reticularis pontis caudalis) giant neurons in rat slices show that EPSCs evoked by either trigeminal or auditory fiber stimulation were inhibited by the GABA_A/C agonist muscimol or the GABA_B agonist baclofen via postsynaptic mechanisms. Hyperpolarization of PnC neurons by muscimol was reversed with bicuculline, indicating that postsynaptic GABA_A receptors strongly inhibit PnC giant neurons needed for startle. Therefore, GABA receptors on PnC giant neurons mediate a substantial part of PPI, with GABA_A receptors contributing at the peak of PPI, and GABA_B receptors adding to muscarinic effects on PPI at long ISIs.     

Altered behavioral responses to gamma‐aminobutyric acid pharmacological agents in a mouse model of Huntington's disease

Background : Disruptions in gamma‐aminobutyric (GABA) acid signaling are believed to be involved in Huntington's disease pathogenesis, but the regulation of GABAergic signaling remains elusive. Here we evaluated GABAergic signaling by examining the function of GABAergic drugs in Huntington's disease and the expression of GABAergic molecules using mouse models and human brain tissues from Huntington's disease. Methods : We treated wild‐type and R6/2 mice (a transgenic Huntington's disease mouse model) acutely with vehicle, diazepam, or gaboxadol (drugs that selectively target synaptic or extrasynaptic GABA_A receptors) and monitored their locomotor activity. The expression levels of GABA_A receptors and a major neuron‐specific chloride extruder (potassium‐chloride cotransporter‐2) were analyzed by real‐time quantitative polymerase chain reaction, Western blot, and immunocytochemistry. Results : The R6/2 mice were less sensitive to the sedative effects of both drugs, suggesting reduced function of GABA_A receptors. Consistently, the expression levels of α1/α2 and δ subunits were lower in the cortex and striatum of R6/2 mice. Similar results were also found in 2 other mouse models of Huntington's disease and in Huntington's disease patients. Moreover, the interaction and expression levels of potassium‐chloride cotransporter‐2 and its activator (brain‐type creatine kinase) were decreased in Huntington's disease neurons. These findings collectively suggest impaired chloride homeostasis, which further dampens GABA_A receptor‐mediated inhibitory signaling in Huntington's disease brains. Conclusions : The dysregulated GABAergic responses and altered expression levels of GABA_A receptors and potassium‐chloride cotransporter‐2 in Huntington's disease mice appear to be authentic and may contribute to the clinical manifestations of Huntington's disease patients. © 2017 International Parkinson and Movement Disorder Society     

GABAA and GABAB receptor subunit localization on neurochemically identified neurons of the human subthalamic nucleus

The subthalamic nucleus (STN) is a critical excitatory signaling center within the basal ganglia circuitry. The activity of subthalamic neurons is tightly controlled by upstream inhibitory signaling centers in the basal ganglia. In this study, we used immunohistochemical techniques to firstly, visualize and quantify the STN neurochemical organization based on neuronal markers including parvalbumin (PV), calretinin (CR), SMI‐32, and GAD_65/67. Secondly, we characterized the detailed regional, cellular and subcellular expression of GABA_A (α₁, α₂, α₃, β_2/3, and γ₂) and GABA_B (R1 and R2) receptor subunits within the normal human STN. Overall, we found seven neurochemically distinct populations of principal neurons in the human STN. The three main populations detected were: (a) triple‐labeled PV⁺/CR⁺/SMI32⁺; (b) double‐labeled PV⁺/CR⁺; and (c) single‐labeled CR⁺ neurons. Subthalamic principal neurons were found to express GABA_A receptor subunits α₁, α₃, β_2/3, γ₂, and GABA_B receptor subunits R1 and R2. However, no expression of GABA_A receptor α₂ subunit was detected. We also found a trend of increasing regional staining intensity for all positive GABA_A receptor subunits from the dorsolateral pole to ventromedial extremities. The GAD⁺ interneurons showed relatively low expression of GABA_A receptor subunits. These results provide the morphological basis of GABAergic transmission within the normal human subthalamic nucleus and evidence of GABA innervation through both GABA_A and GABA_B receptors on subthalamic principal neurons.     

Opposite responses to aversive stimuli in lateral habenula neurons

Appropriate behavioural strategies to cope with unexpected salient stimuli require synergistic neuronal responses in diverse brain regions. Among them, the epithalamic lateral habenula (LHb) plays a pivotal role in processing salient stimuli of aversive valence. Integrated in the complex motivational circuit, LHb neurons are indeed excited by aversive stimuli, including footshock (Fs). However, whether such excitation is a common feature represented throughout the LHb remains unclear. Here, we combined single‐unit extracellular recordings in anaesthetized mice with juxtacellular labelling to describe the nature, location and pharmacological properties of Fs‐driven responses within the LHb. We find that, along with Fs‐excited cells, about 10% of LHb neurons display Fs‐mediated inhibitory responses. Such inhibited neuronal population, in contrast to Fs‐excited neurons, display regular and high frequency activity at baseline and is clustered in the medial portion of the LHb. Juxtacellular labelling of Fs‐excited and inhibited neurons unravels that both populations are of glutamatergic type, as they co‐localized with the EAAC1 glutamatergic transporter but not with the GAD67 GABAergic marker. Moreover, while the excitatory responses to Fs require both AMPA and NMDA receptors, the inhibitory responses rely instead on GABA_A channels. Taken together, our results indicate that two functionally and partly segregated LHb neuronal ensembles encode Fs in an opposite fashion. This highlights the neuronal complexity in the LHb for processing aversive external stimuli.     

α4βδ‐GABAA receptors in dorsal hippocampal CA1 of adolescent female rats traffic to the plasma membrane of dendritic spines following voluntary exercise and contribute to protection of animals from activity‐based anorexia through localization at excitatory synapses

In hippocampal CA1 of adolescent female rodents, α4βδ‐GABA_A receptors (α4βδ‐GABA_ARs) suppress excitability of pyramidal neurons through shunting inhibition at excitatory synapses. This contributes to anxiolysis of stressed animals. Socially isolated adolescent female rats with 8 days of wheel access, the last 4 days of which entail restricted food access, have been shown to exhibit excessive exercise, choosing to run instead of eat (activity‐based anorexia [ABA]). Upregulation of α4βδ‐GABA_ARs in the dorsal hippocampal CA1 (DH), seen among some ABA animals, correlates with suppression of excessive exercise. We used electron microscopic immunocytochemistry to show that exercise alone (EX), but not food restriction alone (FR), also augments α4βδ‐GABA_AR expression at axospinous excitatory synapses of the DH (67%, P = 0.027), relative to socially isolated controls without exercise or food restriction (CON). Relative to CON, ABA animals' synaptic α4βδ‐GABA_AR elevation was modestly elevated (37%), but this level correlated strongly and negatively with individual differences in ABA vulnerability—i.e., food restriction–evoked hyperactivity (Pearson R = ?0.902, P = 0.002) and weight changes (R = 0.822, P = 0.012). These correlations were absent from FR and EX brains or ventral hippocampus of ABA brains. Comparison to CON of α4βδ‐GABA_AR location in the DH indicated that ABA induces trafficking of α4βδ‐GABA_AR from reserve pools in spine cytoplasm to excitatory synapses. Pair‐housing CON animals reduced cytoplasmic α4βδ‐GABA_AR without reducing synaptic α4βδ‐GABA_AR. Thus, exercise induces trafficking of α4βδ‐GABA_ARs to excitatory synapses, while individual differences in ABA vulnerability are linked most strongly to trafficking of α4βδ‐GABA_ARs in the reverse direction—from excitatory synapses to the reserve pool during co‐occurring food restriction. © 2017 Wiley Periodicals, Inc.     

Roles for γ‐aminobutyric acid in the development of the paraventricular nucleus of the hypothalamus

The development of the hypothalamic paraventricular nucleus (PVN) involves several factors that work together to establish a cell group that regulates neuroendocrine functions and behaviors. Several molecular markers were noted within the developing PVN, including estrogen receptors (ER), neuronal nitric oxide synthase (nNOS), and brain‐derived neurotrophic factor (BDNF). By contrast, immunoreactive γ‐aminobutyric acid (GABA) was found in cells and fibers surrounding the PVN. Two animal models were used to test the hypothesis that GABA works through GABA_A and GABA_B receptors to influence the development of the PVN. Treatment with bicuculline to decrease GABA_A receptor signaling from embryonic day (E) 10 to E17 resulted in fewer cells containing immunoreactive (ir) ERα in the region of the PVN vs. control. GABA_BR1 receptor subunit knockout mice were used to examine the PVN at P0 without GABA_B signaling. In female but not male GABA_BR1 subunit knockout mice, the positions of cells containing ir ERα shifted from medial to lateral compared with wild‐type controls, whereas the total number of ir ERα‐containing cells was unchanged. In E17 knockout mice, ir nNOS cells and fibers were spread over a greater area. There was also a significant decrease in ir BDNF in the knockout mice in a region‐dependent manner. Changes in cell position and protein expression subsequent to disruption of GABA signaling may be due, in part, to changes in nNOS and BDNF signaling. Based on the current study, the PVN can be added as another site where GABA exerts morphogenetic actions in development. J. Comp. Neurol. 518:2710–2728, 2010. © 2010 Wiley‐Liss, Inc.     

Adolescent‐onset GABAAα1 silencing regulates reward‐related decision making

The GABA_A receptor mediates fast, inhibitory signaling, and cortical expression of the α1 subunit increases during postnatal development. Certain pathological stimuli such as stressors or prenatal cocaine exposure can interfere with this process, but causal relationships between GABA_Aα1 deficiency and complex behavioral outcomes remain unconfirmed. We chronically reduced GABA_Aα1 expression selectively in the medial prefrontal cortex (prelimbic subregion) of mice using viral‐mediated gene silencing of Gabra1. Adolescent‐onset Gabra1 knockdown delayed the acquisition of a cocaine‐reinforced instrumental response but spared cocaine seeking in extinction and in a cue‐induced reinstatement procedure. To determine whether response acquisition deficits could be associated with impairments in action–outcome associative learning and memory, we next assessed behavioral sensitivity to instrumental contingency degradation. In this case, the predictive relationship between familiar actions and their outcomes is violated. Adolescent‐onset knockdown, although not adult‐onset knockdown, delayed the expression of goal‐directed response strategies in this task, resulting instead in inflexible habit‐like modes of response. Thus, the maturation of medial prefrontal cortex GABA_Aα1 systems during adolescence appears necessary for goal‐directed reward‐related decision making in adulthood. These findings are discussed in the light of evidence that prolonged Gabra1 deficiency may impair synaptic plasticity.     

Partial inactivation of GABAA receptors containing the α5 subunit affects the development of adult‐born dentate gyrus granule cells

Alterations of neuronal activity due to changes in GABA_A receptors (GABA_AR) mediating tonic inhibition influence different hippocampal functions. Gabra5‐null mice and α5 subunit^(H105R) knock‐in mice exhibit signs of hippocampal dysfunction, but are capable of improved performance in several learning and memory tasks. Accordingly, alleviating abnormal GABAergic tonic inhibition in the hippocampal formation by selective α5‐GABA_AR modulators represents a possible therapeutic approach for several intellectual deficit disorders. Adult neurogenesis in the dentate gyrus is an important facet of hippocampal plasticity; it is regulated by tonic GABAergic transmission, as shown by deficits in proliferation, migration and dendritic development of adult‐born neurons in Gabra4‐null mice. Here, we investigated the contribution of α5‐GABA_ARs to granule cell development, using retroviral vectors expressing eGFP for labeling precursor cells in the subgranular zone. Global α5‐GABA_AR knockout (α5‐KO) mice showed no alterations in migration and morphological development of eGFP‐positive granule cells. However, upregulation of α1 subunit‐immunoreactivity was observed in the hippocampal formation and cerebral cortex. In contrast, partial gene inactivation in α5‐heterozygous (α5‐het) mice, as well as single‐cell deletion of Gabra5 in newborn granule cells from α5‐floxed mice, caused severe alterations of migration and dendrite development. In α5‐het mice, retrovirally mediated overexpression of Cdk5 resulted in normal migration and dendritic branching, suggesting that Cdk5 cooperates with α5‐GABA_ARs to regulate neuronal development. These results show that minor imbalance of α5‐GABA_AR‐mediated transmission may have major consequences for neuronal plasticity; and call for caution upon chronic therapeutic use of negative allosteric modulators acting at these receptors.     

Forebrain-specific loss of synaptic GABAA receptors results in altered neuronal excitability and synaptic plasticity in mice

Mutations that result in the defective trafficking of γ2 subunit containing GABA_A receptors (γ2-GABA_ARs) are known to reduce synaptic inhibition. Whether perturbed clustering of non-mutated GABA_ARs similarly reduces synaptic inhibition in vivo is less clear. In this study we provide evidence that the loss of postsynaptic γ2-GABA_ARs upon postnatal ablation of gephyrin, the major scaffolding protein of inhibitory postsynapses, from mature principal neurons within the forebrain results in reduced induction of long-term potentiation (LTP) and impaired network excitability within the hippocampal dentate gyrus. The preferential reduction in not only synaptic γ2-GABA_AR cluster number at dendritic sites but also the decrease in γ2-GABA_AR density within individual clusters at dendritic inhibitory synapses suggests that distal synapses are more sensitive to the loss of gephyrin expression than proximal synapses. The fact that these mice display behavioural features of anxiety and epilepsy emphasises the importance of postsynaptic γ2-GABA_AR clustering for synaptic inhibition.     

The CaMKII-dependent phosphorylation of GABAB receptors in the nucleus accumbens was involved in cocaine-induced behavioral sensitization in rats

Background

Previous studies have established that the regulation of prolonged, distal neuronal inhibition by the GABA_B heteroreceptor (GABA_BR) is determined by its stability, and hence residence time, on the plasma membrane.

Aims

Here, we show that GABA_BR in the nucleus accumbens (NAc) of rats affects the development of cocaine-induced behavioral sensitization by mediating its perinucleus internalization and membrane expression.

Materials & Methods

By immunofluorescent labeling, flow cytometry analysis, Co-immunoprecipitation and open field test, we measured the role of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) to the control of GABA_BR membrane anchoring and cocaine induced-behavioral sensitization.

Results

Repeated cocaine treatment in rats (15 mg/kg) significantly decreases membrane levels of GABA_B1R and GABA_B2R in the NAc after day 3, 5 and 7. The membrane fluorescence and protein levels of GABA_BR was also decreased in NAc GAD₆₇⁺ neurons post cocaine (1 μM) treatment after 5 min. Moreover, the majority of internalized GABA_B1Rs exhibited perinuclear localization, a decrease in GABA_B1R-pHluroin signals was observed in cocaine-treated NAc neurons. By contrast, membrane expression of phosphorylated CaMKII (pCaMKII) post cocaine treatment was significantly increased after day 1, 3, 5 and 7. Baclofen blocked the cocaine induced behavioral sensitization via inhibition of cocaine enhanced-pCaMKII-GABA_B1R interaction.

Conclusion

These findings reveal a new mechanism by which pCaMKII-GABA_BR signaling can promote psychostimulant-induced behavioral sensitization.     

Benzodiazepine-dependent stabilization of GABAA receptors at synapses

GABA_A receptors constitutively enter and exit synapses by lateral diffusion in the plane of the neuronal membrane. They are trapped at synapses through their interactions with gephyrin, the main scaffolding protein at inhibitory post-synaptic densities. Previous work has shown that the synaptic accumulation and diffusion dynamics of GABA_ARs are controlled via excitatory synaptic activity. However, it remains unknown whether GABA_AR activity can itself impact the surface trafficking of the receptors. Here we report the effects of GABA_AR agonists, antagonists and allosteric modulators on the receptor's surface dynamics. Using immunocytochemistry and single particle tracking experiments on mouse hippocampal neurons, we show that the agonist muscimol decreases GABA_AR and gephyrin levels at synapses and accelerates the receptor's lateral diffusion within 30–120 min of treatment. In contrast, the GABA_AR antagonist gabazine increased GABA_AR amounts and slowed down GABA_AR diffusion at synapses. The response to GABA_AR activation or inhibition appears to be an adaptative regulation of GABAergic synapses. Surprisingly, the positive allosteric modulator diazepam abolished the regulation induced by muscimol, and this effect was observed on α1, α2, α5 and γ2 GABA_AR subunits. Altogether these results indicate that diazepam stabilizes synaptic GABA_ARs and thus prevents the agonist-induced regulation of GABA_AR levels at synapses. This occurred independently of neuronal activity and intracellular calcium and involved GABA_AR–gephyrin interactions, suggesting that the changes in GABA_AR diffusion depend on conformational changes of the receptor. Our study provides a new molecular mechanism involved in the adaptative response to changes in GABA_AR activity and benzodiazepine treatments.     

Protein kinase C regulates tonic GABAA receptor‐mediated inhibition in the hippocampus and thalamus

Tonic inhibition mediated by extrasynaptic GABA_A receptors (GABA_ARs) is an important regulator of neuronal excitability. Phosphorylation by protein kinase C (PKC) provides a key mode of regulation for synaptic GABA_ARs underlying phasic inhibition; however, less attention has been focused on the plasticity of tonic inhibition and whether this can also be modulated by receptor phosphorylation. To address this issue, we used whole‐cell patch clamp recording in acute murine brain slices at both room and physiological temperatures to examine the effects of PKC‐mediated phosphorylation on tonic inhibition. Recordings from dentate gyrus granule cells in the hippocampus and dorsal lateral geniculate relay neurons in the thalamus demonstrated that PKC activation caused downregulation of tonic GABA_AR‐mediated inhibition. Conversely, inhibition of PKC resulted in an increase in tonic GABA_AR activity. These findings were corroborated by experiments on human embryonic kidney 293 cells expressing recombinant α4β2δ GABA_ARs, which represent a key extrasynaptic GABA_AR isoform in the hippocampus and thalamus. Using bath application of low GABA concentrations to mimic activation by ambient neurotransmitter, we demonstrated a similar inhibition of receptor function following PKC activation at physiological temperature. Live cell imaging revealed that this was correlated with a loss of cell surface GABA_ARs. The inhibitory effects of PKC activation on α4β2δ GABA_AR activity appeared to be mediated by direct phosphorylation at a previously identified site on the β2 subunit, serine 410. These results indicate that PKC‐mediated phosphorylation can be an important physiological regulator of tonic GABA_AR‐mediated inhibition.     

GABAergic inhibition gates excitatory LTP in perirhinal cortex

The perirhinal cortex (PRh) is a key region downstream of auditory cortex (ACx) that processes familiarity linked mnemonic signaling. In gerbils, ACx‐driven EPSPs recorded in PRh neurons are largely shunted by GABAergic inhibition (Kotak et al., 2015, Frontiers in Neural Circuits, 9). To determine whether inhibitory shunting prevents the induction of excitatory long‐term potentiation (e‐LTP), we stimulated ACx‐recipient PRh in a brain slice preparation using theta burst stimulation (TBS). Under control conditions, without GABA blockers, the majority of PRh neurons exhibited long‐term depression. A very low concentration of bicuculline increased EPSP amplitude, but under this condition TBS did not significantly increase e‐LTP induction. Since PRh synaptic inhibition included a GABA_B receptor‐mediated component, we added a GABA_B receptor antagonist. When both GABA_A and GABA_B receptors were blocked, TBS reliably induced e‐LTP in a majority of PRh neurons. We conclude that GABAergic transmission is a vital mechanism regulating e‐LTP induction in the PRh, and may be associated with auditory learning.     

Dexmedetomidine protects rats from postoperative cognitive dysfunction via regulating the GABABR‐mediated cAMP‐PKA‐CREB signaling pathway

This work attempts to discuss whether dexmedetomidine (Dex) can protect rats from postoperative cognitive dysfunction (POCD) through regulating the γ‐aminobutyric acid‐_B receptor (GABA_BR)‐mediated cyclic adenosine monophosphate (cAMP) – protein kinase A (PKA) – cAMP‐response element binding (cAMP‐PKA‐CREB) signaling pathway. Sprague‐Dawley rats were divided into a non‐surgical group (Control), a surgical group (Model), a surgical group treated with Dex (Model + Dex), a surgical group treated with GABA_BR antagonist (Model + CGP 35348) and a surgical group treated with Dex and GABA_BR agonist (Model + Dex + Baclofen). Cognitive and memory functions were evaluated by Y‐maze test and open‐field test. The neuronal morphology of the hippocampus was observed by hematoxylin and eosin staining and neuronal apoptosis was by terminal deoxynucleotidyl transferase‐mediated dUTP‐biotin nick‐end labeling method. Inflammatory factors and cAMP levels were detected by enzyme‐linked immunosorbent assay while expressions of GABA_BR and PKA‐CREB pathway‐related molecules by Western blot. Compared with control rats, the model rats exhibited reduced alternation rates with a prolonged time spent in the central zone; meanwhile, levels of tumor necrosis factor‐α and interleukin‐1β and the apoptotic index, as well as GABA_BR1 and GABA_BR2 expressions were increased in the model rats, but the cAMP‐PKA‐CREB pathway was inhibited (all P < 0.05). When treated with either Dex or CGP 35348, the surgical rats displayed an opposite tendency concerning the above factors as compared to the model rats (all P < 0.05). Furthermore, Baclofen, the agonist of GABA_BR, could reverse the protective effect of Dex against POCD in rats. Dex protects rats from POCD possibly via suppressing GABA_BR to up‐regulate the cAMP‐PKA‐CREB signaling pathway, thereby alleviating the hippocampal inflammation caused by surgical trauma.     

Subpopulations of neurons in the rat neostriatum display GABABR1 receptor immunoreactivity

Immunoreactivity for gamma aminobutyric acid BR1 receptor (GABA_BR1) was detected in the neuropilar elements as well as in the perikarya of neurons in the neostriatum. Many of the GABA_BR1-immunoreactive perikarya were medium-sized with a thin rim of cytoplasm. They resembled the morphology of medium spiny neurons, the projection neurons of the neostriatum. In addition, some GABA_BR1-immunoreactive neurons were densely labeled and were of medium to large in size. These neurons were characterized by double immunofluorescence using their neurochemicals as markers. Over 90% of the parvalbumin- and choline acetyltransferase-immunoreactive neurons and about 80% of the nitric oxide synthase-immunoreactive neurons displayed GABA_BR1 immunoreactivity. The present results show for the first time that the major four subpopulations of striatal neurons express GABA_BR1 receptor and may have a functional implication in the GABA neurotransmission in the microcircuitry of the neostriatum.     

Subpopulations of neurons in rat substantia nigra display GABABR2 receptor immunoreactivity

A functional gamma-aminobutyric acid (GABA) B receptor is the first metabotropic receptor known to be composed of two heteromeric subunits, GABA_BR1 and GABA_BR2. Our previous report [Neuroscience 99 (2000) 65] has demonstrated that subpopulations of neurons in the rat substantia nigra display distinct patterns of distribution of GABA_BR1 receptor immunoreactivity. A robust level of GABA_BR1 receptor is only found in the dopaminergic neurons of the substantia nigra pars compacta (SNc). The objective of the present study was to determine the precise cellular localization of GABA_BR2 subunit in the rat substantia nigra using double immunofluorescence. Neuropilar elements in the SNc and the substantia nigra pars reticulata (SNr) were found to display GABA_BR2 immunoreactivity. In addition, the tyrosine hydroxylase-immunoreactive dopaminergic neurons and the parvalbumin-immunoreactive GABAergic neurons in the SNr were also found to display GABA_BR2 immunoreactivity. The present results thus demonstrate that a functional GABA_B receptor may be expressed by the dopaminergic neurons in the SNc. It is less clear whether neurons in the SNr express a functional GABA_B receptor. The present findings have important functional implications in GABA neurotransmission in the substantia nigra.