Effects of the conformationally restricted GABA analogues,Cis‐and Trans‐4‐aminocrotonic acid,on GABA neurotransmission in primary neuronal cultures

The effects of the GABA analogues, cis‐ and trans‐4‐aminocrotonic acid (ACA) on GABA_A receptor function and GABA uptake, together with the presence of ρ‐1 subunit mRNA and putative GABA_C receptors, were studied in primary cultures of neocortical neurons and cerebellar granule cells. Both isomers induced a Cl^‐ influx, which was inhibited by bicuculline, t‐butylbicyclophosphorothionate (TBPS), picrotoxinin (PTX), and γ‐hexachlorocyclohexane (γ‐HCH or lindane). [³H]‐flunitrazepam binding was also increased by both isomers and this increase was inhibited by bicuculline. In neocortical neurons, the trans‐isomer completely inhibited the [³H]GABA uptake, whereas the cis‐isomer produced only a 25% inhibition at the highest concentration used. The possible presence of GABA_C receptors was investigated only in neocortical cultures by using RT‐PCR in order to detect the presence of the mRNA encoding the ρ‐1 subunit which assembles to form homooligomeric Cl^‐ channels. The results presented here show that ρ‐1 subunits, and thus GABA_C receptors, may represent a very minor population of GABA receptors in these neuronal preparations. We conclude that both GABA analogues may act as agonists at the GABA_A receptors, although with very different potencies. J. Neurosci. Res. 57:95–105, 1999. © 1999 Wiley‐Liss, Inc.     

Attenuated Benzodiazepine‐Sensitive Tonic GABAA Currents of Supraoptic Magnocellular Neuroendocrine Cells in 24‐h Water‐Deprived Rats

In supraoptic nucleus (SON) magnocellular neurosecretory cells (MNCs), γ‐GABA, via activation of GABA_A receptors (GABA_ARs), mediates persistent tonic inhibitory currents (I_tonic), as well as conventional inhibitory postsynaptic currents (IPSCs, I_phasic). In the present study, we examined the functional significance of I_tonic in SON MNCs challenged by 24‐h water deprivation (24WD). Although the main characteristics of spontaneous IPSCs were similar in 24WD compared to euhydrated (EU) rats, I_tonic, measured by bicuculline (BIC)‐induced I_holding shifts, was significantly smaller in 24WD compared to EU rats (P < 0.05). Propofol and diazepam prolonged IPSC decay time to a similar extent in both groups but induced less I_tonic in 24WD compared to EU rats, suggesting a selective decrease in GABA_A receptors mediating I_tonic over I_phasic in 24WD rats. THIP (4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridin‐3‐ol), a preferential δ subunit agonist, and L‐655,708, a GABA_A receptor α₅ subunit selective imidazobenzodiazepine, caused a significantly smaller inward and outward shift in I_holding, respectively, in 24WD compared to EU rats (P < 0.05 in both cases), suggesting an overall decrease in the α₅ subunit‐containing GABA_ARs and the δ subunit‐containing receptors mediating I_tonic in 24WD animals. Consistent with a decrease in 24WD I_tonic, bath application of GABA induced significantly less inhibition of the neuronal firing activity in 24WD compared to EU SON MNCs (P < 0.05). Taken together, the results of the present study indicate a selective decrease in GABA_ARs functions mediating I_tonic as opposed to those mediating I_phasic in SON MNCs, demonstrating the functional significance of I_tonic with respect to increasing neuronal excitability and hormone secretion in 24WD rats.     

γ‐Aminobutyric acid‐ρ expression in ependymal glial cells of the mouse cerebellum

The ependymal glial cells (EGCs) from the periventricular zone of the cerebellum were studied to determine their distribution and the functional properties of their γ‐aminobutyric acid type A (GABA_A) receptors. EGCs were identified by the presence of ciliated structures on their ventricular surface and their expression of glial fibrillary acidic protein (GFAP). Interestingly, diverse cell types, including neurons, astrocytes, and other types of glia, were identified in the subventricular zone by their current profiles. Electron microscopy showed ciliated cells and myelinated axons in this zone, but we found no collateral connections to suggest the presence of functional synapses. GABA‐mediated currents were recorded from EGCs in cerebellar slices from postnatal days 13 to 35 (PN13–PN35). These currents were blocked by TPMPA (a highly specific GABA_Aρ subunit antagonist) and bicuculline (a selective antagonist for classic GABA_A receptors). Pentobarbital failed to modulate GABA_A‐mediated currents despite the expression of GABAα1 and GABAγ2 subunits. In situ hybridization, RT‐PCR, and immunofluorescence studies confirmed GABAρ1 expression in EGCs of the cerebellum. We conclude that cerebellar EGCs express GABAρ1, which is functionally involved in GABA_A receptor‐mediated responses that are unique among glial cells of the brain. © 2013 Wiley Periodicals, Inc.     

The impact of diazepam's discovery on the treatment and understanding of status epilepticus

The fortuitous discovery of the benzodiazepines and the subsequent application of these agents to the treatment of status epilepticus (SE) heralds in the modern age of treating this neurologic emergency. More than 50 years after their discovery, the benzodiazepines remain the drugs of first choice in the treatment of SE. However, the benzodiazepines can be ineffective, especially in those patients whose seizures are the most prolonged. The benzodiazepines act by increasing the affinity of γ‐aminobutyric acid (GABA) for GABA_A receptors. A receptor’s subunit composition affects its functional and pharmacologic properties, trafficking, and cellular localization. The GABA_A receptors that mediate synaptic inhibition typically contain a γ2 subunit and are diazepam‐sensitive. Among the GABA_A receptors that mediate tonic inhibition are the benzodiazepine‐insensitive δ subunit–containing receptors. The initial studies investigating the pathogenesis of SE demonstrated that a reduction in GABA‐mediated inhibition within the hippocampus was important in maintenance of SE, and this reduction correlated with a rapid modification in the postsynaptic GABA_A receptor population expressed on the surface of the hippocampal principal neurons. Subsequent studies found that this rapid modification is, in part, mediated by an activity‐dependent, subunit‐specific trafficking of the receptors that resulted in the reduction in the surface expression of the benzodiazepine‐sensitive γ2 subunit–containing receptors and the preserved surface expression of the benzodiazepine‐insensitive δ subunit‐containing receptors. This improved understanding of the changes in the trafficking of GABA_A receptors during SE partially accounts for the development of benzodiazepine‐pharmacoresistance and has implications for the current and future treatment of benzodiazepine‐refractory SE.     

Nucleus-specific modulation of phasic and tonic inhibition by endogenous neurosteroidogenesis in the murine thalamus

Neurosteroids are potent allosteric modulators of GABA_A receptors (GABA_ARs). Although the effects of exogenous neurosteroids on GABA_AR function are well documented, less is known about effects of neurosteroids produced by local endogenous biosynthesis. The neurosteroidogenic enzymes 5α-reductase and 3α-hydroxysteroid dehydrogenase are expressed in two nuclei of somatosensory thalamus, the thalamic reticular nucleus (nRT) and ventrobasal nucleus (VB). Here, the effects of acute blockade of neurosteroidogenesis by the 5α-reductase inhibitor finasteride on phasic and tonic GABA_AR-mediated currents were examined in nRT and VB of mice. In nRT, finasteride altered the decay and amplitude, but not the frequency, of phasic currents, with no effect on tonic inhibition. In VB neurons, by contrast, finasteride reduced both the size and frequency of phasic currents, and also reduced the degree of tonic inhibition. These studies thus provide novel evidence for endogenous modulation of GABA_AR function by 5α-reduced neurosteroids in the mature thalamus.     

The general anaesthetic etomidate inhibits the excitability of mouse thalamocortical relay neurons by modulating multiple modes of GABAA receptor‐mediated inhibition

Modulation of thalamocortical (TC) relay neuron function has been implicated in the sedative and hypnotic effects of general anaesthetics. Inhibition of TC neurons is mediated predominantly by a combination of phasic and tonic inhibition, together with a recently described ‘spillover’ mode of inhibition, generated by the dynamic recruitment of extrasynaptic γ‐aminobutyric acid (GABA)_A receptors (GABA_ARs). Previous studies demonstrated that the intravenous anaesthetic etomidate enhances tonic and phasic inhibition in TC relay neurons, but it is not known how etomidate may influence spillover inhibition. Moreover, it is unclear how etomidate influences the excitability of TC neurons. Thus, to investigate the relative contribution of synaptic (α1β2γ2) and extrasynaptic (α4β2δ) GABA_ARs to the thalamic effects of etomidate, we performed whole‐cell recordings from mouse TC neurons lacking synaptic (α1^0/0) or extrasynaptic (δ^0/0) GABA_ARs. Etomidate (3 μm ) significantly inhibited action‐potential discharge in a manner that was dependent on facilitation of both synaptic and extrasynaptic GABA_ARs, although enhanced tonic inhibition was dominant in this respect. Additionally, phasic inhibition evoked by stimulation of the nucleus reticularis exhibited a spillover component mediated by δ‐GABA_ARs, which was significantly prolonged in the presence of etomidate. Thus, etomidate greatly enhanced the transient suppression of TC spike trains by evoked inhibitory postsynaptic potentials. Collectively, these results suggest that the deactivation of thalamus observed during etomidate‐induced anaesthesia involves potentiation of tonic and phasic inhibition, and implicate amplification of spillover inhibition as a novel mechanism to regulate the gating of sensory information through the thalamus during anaesthetic states.     

GABAB receptor‐dependent modulation of network activity in the rat prefrontal cortex in vitro

GABA (γ‐aminobutyric acid) can mediate inhibition via pre‐ and post/extrasynaptic GABA receptors. In this paper we demonstrate potentially post/extrasynaptic GABA_B receptor‐dependent tonic inhibition in L2/3 pyramidal cells of rat medial prefrontal cortex (mPFC) in vitro. First, we show via voltage‐clamp experiments the presence of a tonic GABA_B receptor‐dependent outward current in these neurons. This GABA_Bergic current could be induced by ambient GABA when present at sufficient concentrations. To increase ambient GABA levels in the usually silent slice preparation, we amplified network activity and hence synaptic GABA release with a modified artificial cerebrospinal fluid. The amplitude of tonic GABA_B current was similar at different temperatures. In addition to the tonic GABA_B current, we found presynaptic GABA_B effects, GABA_B‐mediated inhibitory postsynaptic currents and tonic GABA_A currents. Second, we performed current‐clamp experiments to evaluate the functional impact of GABA_B receptor‐mediated inhibition in the mPFC. Activating or inactivating GABA_B receptors led to rightward (reduction of excitability) or leftward (increase of excitability) shifts, respectively, of the input–output function of mPFC L2/3 pyramidal cells without effects on the slope. Finally, we showed in electrophysiological recordings and epifluorescence Ca²⁺‐imaging that GABA_B receptor‐mediated tonic inhibition is capable of regulating network activity. Blocking GABA_B receptors increased the frequency of excitatory postsynaptic currents impinging on a neuron and prolonged network upstates. These results show that ambient GABA via GABA_B receptors is powerful enough to modulate neuronal excitability and the activity of neural networks.     

Baicalein reduces β‐amyloid and promotes nonamyloidogenic amyloid precursor protein processing in an Alzheimer's disease transgenic mouse model

Baicalein, a flavonoid isolated from the roots of Scutellaria baicalensis, is known to modulate γ‐aminobutyric acid (GABA) type A receptors. Given prior reports demonstrating benefits of GABA_A modulation for Alzheimer's disease (AD) treatment, we wished to determine whether this agent might be beneficial for AD. CHO cells engineered to overexpress wild‐type amyloid precursor protein (APP), primary culture neuronal cells from AD mice (Tg2576) and AD mice were treated with baicalein. In the cell cultures, baicalein significantly reduced the production of β‐amyloid (Aβ) by increasing APP α‐processing. These effects were blocked by the GABA_A antagonist bicuculline. Likewise, AD mice treated daily with i.p. baicalein for 8 weeks showed enhanced APP α‐secretase processing, reduced Aβ production, and reduced AD‐like pathology together with improved cognitive performance. Our findings suggest that baicalein promotes nonamyloidogenic processing of APP, thereby reducing Aβ production and improving cognitive performance, by activating GABA_A receptors. © 2013 Wiley Periodicals, Inc.     

Intrinsic activation of GABAA receptors suppresses epileptiform activity in the cerebral cortex of immature mice

Purpose: Activation of ionotropic γ‐aminobutyric acid type A (GABA_A) receptors induces in immature neocortical neurons a membrane depolarization that may contribute to the higher epilepsy susceptibility in newborns. To elucidate whether depolarizing GABAergic responses enhance or attenuate epileptiform activity in the immature neocortex, we investigated the effect of agonists, antagonists, and positive modulators of GABA_A receptors on epileptiform activity. Methods: We performed in vitro field potential recordings on isolated whole neocortex preparations and whole cell recordings of identified pyramidal neurons in 400‐μm slices of immature (postnatal day 1–7) mice. Epileptiform activity was induced by low Mg²⁺ solutions with or without 50–100 μm 4‐aminopyridine. Results: Bath application of GABA (3–100 μm , in the presence of tiagabine) attenuated epileptiform activity. The GABA transporter isoform 1 (GAT‐1) inhibitor tiagabine (30 μm ) and the GAT‐2/3 specific inhibitor SNAP 5114 (40 μm ) reduced the frequency of epileptiform activity. The benzodiazepines midazolam (0.2 μm ) and zolpidem (0.5 μm ) as well as the barbiturate phenobarbital (30 μm ) slightly attenuated epileptiform activity. Continuous bath application of the GABAergic antagonist gabazine (SR‐95531, 2–3 μm ) or picrotoxin (15 μm ) induced epileptiform discharges. Discussion: These results demonstrate, that (1) the activation or positive modulation of GABA_A receptors attenuates epileptiform activity, (2) GABA_A antagonists mediate a disinhibition, and (3) GABA uptake contributes to the regulation of extracellular GABA in immature neocortex. We conclude from these findings that a constant inhibition via GABA_A receptors is required to suppress epileptiform activity already in the immature neocortex.     

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.     

Ca2+/calmodulin‐dependent protein kinase II in spinal dorsal horn contributes to the pain hypersensitivity induced by γ‐aminobutyric acid type a receptor inhibition

The fast inhibitory synaptic transmission mediated by the γ‐aminobutyric acid type A receptor (GABA_AR) within spinal dorsal horn exerts a gating control over the synaptic conveyance of nociceptive information from the periphery to higher brain regions. Although a large body of evidence has demonstrated that the impairment of GABAergic inhibition alone is sufficient to elicit pain hypersensitivity in intact animals, the underlying mechanisms remain to be characterized. The present study shows that Ca²⁺/calmodulin‐dependent protein kinase II (CaMKII) is an important signaling protein downstream of reduced GABAergic inhibition. We found that pharmacological removal of inhibition by intrathecal application of the GABA_AR antagonist bicuculline significantly enhanced the autophosphorylation of CaMKII at Thr286 in spinal dorsal horn of mice. In addition to increased CaMKII activity, bicuculline also promoted CaMKII interaction with N‐methyl‐D‐aspartate (NMDA)‐subtype glutamate receptors and induced the translocation of CaMKII from cytosolic compartments to the synaptosomal membrane fraction. Immunoblotting analysis revealed that the phosphorylation levels of NMDA receptor NR2B subunit at Ser1303 and of AMPA‐subtype glutamate receptor GluR1 subunit at Ser831, two important CaMKII phosphorylation sites, were substantially enhanced after bicuculline application. Behavioral tests illustrated that intrathecal administration of the CaMKII inhibitor KN‐93, NMDA receptor antagonist D‐APV, or AMPA receptor antagonist GYKI 52466 effectively ameliorated the mechanical allodynia evoked by bicuculline. These data thus indicate that CaMKII signaling is critical for the reduced inhibition to evoke spinal sensitization. © 2013 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.     

γ-Aminobutyric acid type A/benzodiazepine receptors regulate rat retina neurosteroidogenesis

It has been previously shown that retinal ganglion cells have the ability to synthesize steroids including neuroactive steroids such as pregnenolone sulfate. Since ganglion cells possess GABA_A/benzodiazepine (BZ) receptors and neurosteroids modulate retinal GABA_A receptor function, we investigated the role of these receptors in isolated rat retina neurosteroidogenesis. Ligands for central-type BZ receptors stimulated retinal pregnenolone synthesis. Clonazepam was the most potent ligand examined acting at nanomolar concentrations. Moreover, the effective steroidogenesis stimulatory dose (ED₅₀) for these ligands and theK_i to inhibit [³-H]flunitrazepam binding showed a coefficient of correlation ofr = 0.87, suggesting the involvement of the central-type BZ receptors in this event. Ro 5-4864, which preferentially binds to peripheral-type BZ receptors, was less efficacious and potent whereas PK 11195 did not affect the basal pregnenolone formation and did not antagonize the Ro 5-4864 stimulated steroid synthesis. The GABAergic agonist muscimol, stimulated neurosteroid synthesis and this effect was reversed by the GABAergic antagonists bicuculline and picrotoxinin. In addition, these antagonists decreased basal pregnenolone formation, suggesting a tonic GABAergic control of the steroidogenic pathway, and reduced clonazepam-stimulated steroidogenesis. These results, together with the reported ability of neurosteroids to modulate GABA_A receptor function, suggest a novel regulatory mechanism to control the inhibitory transmission.     

In vivo neurochemical evidence that stimulation of accumbal GABAA and GABAB receptors each reduce acetylcholine efflux without affecting dopamine efflux in the nucleus accumbens of freely moving rats

Cholinergic neurons in the nucleus accumbens contain GABA_A and GABA_B receptors that are thought to inhibit neural activity. We analyzed the roles of GABA_A and GABA_B receptors in regulating accumbal acetylcholine efflux of freely moving rats using in vivo microdialysis. The effects of GABA receptor ligands on the accumbal dopamine efflux were also analyzed because accumbal cholinergic and dopaminergic neurons could mutually interact. Drugs were applied intracerebrally through the dialysis probe. Doses of compounds indicate total amount administered (mol) during 30–60 min infusions. To monitor basal acetylcholine, a low concentration of physostigmine (50 nM) was added to the perfusate. GABA_A receptor agonist muscimol (3 and 30 pmol) induced a dose‐related decrease in accumbal acetylcholine. GABA_B receptor agonist baclofen (30 and 300 pmol) also produced a dose‐related decrease in acetylcholine. GABA_A receptor antagonist bicuculline (60 pmol) which failed to alter baseline acetylcholine counteracted the muscimol (30 pmol)‐induced decrease in acetylcholine. GABA_B receptor antagonist 2‐hydroxysaclofen (12 nmol) which failed to change baseline acetylcholine, counteracted the baclofen (300 pmol)‐induced decrease in acetylcholine. Neither muscimol (30 pmol) nor baclofen (300 pmol) which reduced accumbal acetylcholine altered baseline accumbal dopamine. Neither bicuculline (60 pmol) nor 2‐hydroxysaclofen (12 nmol) also affected the baseline dopamine. These results show that GABA_A and GABA_B receptors each exert inhibitory roles in the regulation of accumbal cholinergic neural activity. The present results also provides in vivo neurochemical evidence that stimulation of GABA_A and GABA_B receptors each reduce acetylcholine efflux without affecting dopamine efflux in the nucleus accumbens of freely moving rats.     

Influence of Gender and Gonadectomy on Bicuculline-Induced Convulsions and on GABAA Receptors

The response to IV administration of GABA_A receptor antagonist bicuculline was studied in young (30 days) and in adult gonad-intact or gonadectomized male and female rats. The properties of GABA_A receptors, obtained from cortex and cerebellum 30 days following gonadectomy, and the affinity of muscimol and bicuculline for cortical and cerebellar GABA binding sites were also studied. While young rats failed to show sex differences, the threshold doses of bicuculline producing the first myoclonic twitch and running/bouncing clonus (RB clonus) were lower in adult male than female rats. Fifteen days after gonadectomy or sham operation male rats needed less bicuculline to the onset of myoclonic twitch and RB clonus than identically treated females, while orchidectomized rats needed more bicuculline to the onset of tonic hindlimb extension than all other groups examined. All sex differences disappeared 30 days following gonadectomy. At the same time, in males gonadectomy decreased the affinity and enhanced the density of cortical ³H-muscimol binding sites. In female rats, gonadectomy only decreased the affinity of cortical GABA_A receptors. Only regional but not sex differences were observed in the affinity of muscimol and bicuculline for GABA_A receptors. Sex differences in the threshold doses of bicuculline-producing convulsions do not correlate either with the properties of cortical and cerebellar GABA_A receptors or with the affinity of bicuculline for the same binding sites.     

Early postnatal maturation of GABAA-mediated inhibition in the brainstem respiratory rhythm-generating network of the mouse

It is well established that GABA_A‐mediated postsynaptic potentials are excitatory in many brain regions during embryonic and early postnatal life. The pre‐Bötzinger complex (PBC) in the brainstem is an essential component of the respiratory rhythm‐generating network, where GABA_A‐mediated inhibition plays a critical role in generating a stable respiratory rhythm in adult animals. In the present study, using the perforated patch technique, we investigated the maturation of GABA_A receptor‐mediated effects on rhythmically active PBC neurons and on the motor output in slice preparations from P0–15 neonatal mice. The reversal potential of GABA_A receptor‐mediated current (E_GABA‐A) switched from depolarizing to hyperpolarizing within the first postnatal week. E_GABA‐A was ?13.7 ± 9.8 mV at P0, then it changed to ?44.8 ± 7.0 mV at P2 and ?71.5 ± 6.8 mV at P4. Perfusion of bicarbonate‐free saline has no detectable influence on E_GABA‐A, indicating that a lack of Cl^– extrusion during P0–3 is mainly responsible for early GABA_A‐ergic excitation. At the network level, blockade of GABA_A receptors with bicuculline did not significantly change the frequency of rhythmic bursts recorded from hypoglossal nerve roots before P3, whereas it increased the coefficient of variation. After P3, bicuculline increased burst frequency with little effect on the coefficient of variation. Thus, chloride‐mediated inhibition, which appears in PBC neurons after P3, coincides with the appearance of GABA_A‐mediated modulation of the respiratory rhythm. GABA_A receptor‐activated inhibition may therefore be necessary for frequency modulation in the respiratory network beginning on the fourth postnatal day in the mouse brainstem.     

Tonic inhibition of striatal dopamine transmission: effects of benzodiazepine and GABAA receptor antagonists on extracellular dopamine levels

At present, it is unclear whether ligands which bind at the benzodiazepine/GABA receptor complex play a tonic modulatory role with regard to striatal dopamine (DA) transmission. The present study was designed to examine the effects of Ro15-1788, a benzodiazepine (BZ) receptor antagonist, and SR 95531, a GABA_A receptor antagonist, on striatal extracellular DA (DA_[e]) concentrations in anesthetized and awake rats using the technique of in vivo microdialysis. Local administration of Ro15-1788 resulted in a dose-dependent increase in DA_[e] in both anesthetized and awake animals. The Ro15-1788-induced increase in DA_[e] was blocked by coadministration of the BZ agonist diazepam, as well as GABA. Local administration of SR 95531 also resulted in a dose-dependent alteration in striatal DA levels in both anesthetized and awake animals. The SR 95531-induced increase in DA was blocked by coadministration of GABA. The results suggest that GABA may play a tonic inhibitory role with regard to striatal DA transmission.     

Reduced tonic inhibition in the dentate gyrus contributes to chronic stress‐induced impairments in learning and memory

It is well established that stress impacts the underlying processes of learning and memory. The effects of stress on memory are thought to involve, at least in part, effects on the hippocampus, which is particularly vulnerable to stress. Chronic stress induces hippocampal alterations, including but not limited to dendritic atrophy and decreased neurogenesis, which are thought to contribute to chronic stress‐induced hippocampal dysfunction and deficits in learning and memory. Changes in synaptic transmission, including changes in GABAergic inhibition, have been documented following chronic stress. Recently, our laboratory demonstrated shifts in E_GABA in CA1 pyramidal neurons following chronic stress, compromising GABAergic transmission and increasing excitability of these neurons. Interestingly, here we demonstrate that these alterations are unique to CA1 pyramidal neurons, since we do not observe shifts in E_GABA following chronic stress in dentate gyrus granule cells. Following chronic stress, there is a decrease in the expression of the GABA_A receptor (GABA_AR) δ subunit and tonic GABAergic inhibition in dentate gyrus granule cells, whereas there is an increase in the phasic component of GABAergic inhibition, evident by an increase in the peak amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs). Given the numerous changes observed in the hippocampus following stress, it is difficult to pinpoint the pertinent contributing pathophysiological factors. Here we directly assess the impact of a reduction in tonic GABAergic inhibition of dentate gyrus granule cells on learning and memory using a mouse model with a decrease in GABA_AR δ subunit expression specifically in dentate gyrus granule cells (Gabrd/Pomc mice). Reduced GABA_AR δ subunit expression and function in dentate gyrus granule cells is sufficient to induce deficits in learning and memory. Collectively, these findings suggest that the reduction in GABA_AR δ subunit‐mediated tonic inhibition in dentate gyrus granule cells contributes, at least in part, to deficits in learning and memory associated with chronic stress. These findings have significant implications regarding the pathophysiological mechanisms underlying impairments in learning and memory associated with stress and suggest a role for GABA_AR δ subunit containing receptors in dentate gyrus granule cells. © 2016 Wiley Periodicals, Inc.