Cu2+ suppresses GABA(A) receptor-mediated responses in rat sacral dorsal commissural neurons

The effect of copper ions (Cu(2+)) on gamma-aminobutyric acid (GABA)-induced responses in acutely dissociated neurons from the rat sacral dorsal commissural nucleus (SDCN) was investigated using a nystatin-perforated patch recording configuration under voltage clamp conditions. The application of Cu(2+) to SDCN neurons reversibly suppressed the GABA (10 microM)-activated Cl(-) current (I(GABA)) in a concentration-dependent manner (1-1000 microM; IC(50) = 24.5 microM). In the presence of Cu(2+) (30 microM), the concentration-response curve of GABA was shifted rightward without reducing I(GABA) recorded under the maximally effective concentration of GABA, thus indicating a dependence of Cu(2+) action on GABA concentration. Inhibition of GABA (10 microM) responses by 30 microM Cu(2+) was essentially voltage independent and was not accompanied by a shift in the reversal potential of the currents. Cu(2+) antagonized the suppressive effect of Zn(2+) in a concentration-dependent manner, suggesting competition between Cu(2+) and Zn(2+) for similar binding sites. These data demonstrate that Cu(2+) is a potent inhibitor of GABA(A) receptor-mediated responses, implying a possible modulatory effect of Cu(2+) on GABAergic synaptic transmission in the mammalian SDCN.     

Characterization of the interaction between fenamates and hippocampal neuron GABA(A) receptors

Fenamate NSAIDs have several central effects, including anti-epileptic and neuroprotective actions. The underlying mechanism(s) of these actions are not presently understood. In this study, the effects of five members of the fenamate NSAID group were investigated on native ligand-gated ion channels expressed in cultured rat hippocampal neurons. All fenamates tested (1-100 microM) dose-dependently potentiated GABA-evoked currents; mefenamic acid (MFA) was the most potent and efficacious and was found to shift the GABA dose-response curve to the left without effect on the maximum amplitude or the GABA Hill Slope. The modulation of GABA receptors by MFA was not reduced in the presence of the benzodiazepine antagonist, flumazenil (10 microM) and was moderately voltage-dependent. MFA at concentrations >or=10 microM evoked dose-dependent currents in the absence of GABA. These currents were potentiated by diazepam (1 microM) and blocked by bicuculline (10 microM). The MFA (50 microM) current-voltage relationship and reversal potential were similar to that evoked by GABA. MFA (1-100 microM) had no effects on sub-maximal glycine, glutamate or NMDA evoked currents. These data show that fenamate NSAIDs are a highly effective class of GABA(A) receptor modulator and activators.     

Suppression of kainate-evoked AMPA receptor mediated responses by lanthanum in rat sacral dorsal commissural neurons

The effect of lanthanum (La) on kainate (KA) responses in neurons acutely dissociated from the rat sacral dorsal commissural nucleus (SDCN) was investigated using the nystatin-perforated patch-recording configuration under voltage-clamp conditions. The responses to KA were mediated by activation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in SDCN neurons. La(3+) reversibly inhibited KA (100 microM) activated currents (I(KA)) in a concentration-dependent manner over the range from 30 microM to 30 mM, with IC(50) values of 0.64 +/- 0.06 mM at a holding potential (V(H)) of -40 mV. Our further study indicated that the effects of La(3+) on I(KA) were voltage independent. Moreover, the inhibition was not use dependent and was not overcome by increasing the concentration of agonist. These findings indicate that La(3+) is an efficacious inhibitor of AMPA receptor mediated responses which may contribute to its cytotoxic effect.     

GABAergic disinhibition facilitates polysynaptic excitatory transmission in rat anterior cingulate cortex

Various studies implicate the anterior cingulate cortex (ACC) in processing pain. Combining whole-cell patch clamp recordings in rat ACC slices and a formalin-induced conditioned place avoidance (F-CPA) behavioral model, the present study was to address the effect of GABA(A) receptors on excitatory transmission to ACC layer V neurons and its possible functional significance related to pain. Removal of GABA(A) inhibition by bicuculline (10 microM) induced a novel long-lasting response in layer V neurons, which could be blocked by high divalent extracellular solution and was sensitive to relatively higher rate stimuli. Co-application of NMDA receptor antagonist APV (50 microM) and non-NMDA receptor antagonist DNQX (10 microM) completely blocked the responses. Enhancement of inhibition by intra-ACC microinjection of muscimol abolished the acquisition of F-CPA without affecting formalin-induced acute nociceptive responses. These results suggest that GABA(A) inhibition may be involved in pain-related aversion by modulating glutamate-mediated excitatory transmission in the ACC.     

Agonist-Dependent Internalization of γ-Aminobutyric AcidA/Benzodiazepine Receptors in Chick Cortical Neurons

An impermeant benzodiazepine receptor ligand was prepared by derivatization of the aminobenzodiazepine 1012-S with 4-sulfophenylisothiocyanate. The resulting N-(4-sulfophenyl)-thiocarbamoyl derivative of 1012-S (SPTC-1012S) was purified by reverse-phase HPLC, and the predicted structure was verified by mass spectrometry. The apparent affinity of SPTC-1012S (IC50 = 9.8 +/- 2.9 nM) for displacement of [3H]flunitrazepam from intact chick cortical neurons was similar to that of 1012-S (IC50 = 4.0 +/- 0.3 nM). However, at concentrations from 0.1 to 10 microM, 1012-S was consistently more efficacious than SPTC-1012S, a finding indicating that 6-8% of the benzodiazepine receptor pool was not accessible to the impermeant compound. This inaccessible pool was eliminated by permeabilization of the cells with saponin or Triton X-100, a result suggesting that approximately 7% of neuronal benzodiazepine receptors are intracellular. Acute treatment (1-4 h at 37 degrees C) of neurons with 100 microM gamma-aminobutyric acid (GABA) or 100 nM clonazepam had little effect on the level of [3H]flunitrazepam binding but increased the proportion of intracellular receptors by 61 and 74%, respectively, compared with untreated controls. Similar treatment with 1 mM GABA increased the level of intracellular sites by 154-176%. The effect of GABA on receptor internalization was blocked by cotreatment with the GABAA receptor antagonist R 5135. The results suggest that SPTC-1012S can be used as a probe to study the internalization of the GABAA/benzodiazepine receptor complex under normal conditions or following acute or chronic treatment with agonists.     

Mediation by calcium/calmodulin-dependent protein kinase II of suppression of GABAA receptors by NMDA

Using nystatin-perforated whole-cell recording configuration, the modulatory effect of N-methyl-D-aspartate (NMDA) on -aminobutyric acid (GABA)-activated whole-cell currents was investigated in neurons freshly dissociated from the rat sacral dorsal commissural nucleus (SDCN). The results showed that: (I) NMDA suppressed GABA- and muscimol (Mus)-activated currents (IGABA and Imus), respectively in the Mg2+-free external solution containing 1 mol/L glycine at a holding potential (VH) of 40 mV in SDCN neurons. The selective NMDA receptor antagonist, D-2-amino-5-phosphonovaleric acid (APV, 100 mol/L), inhibited the NMDA-evoked currents and blocked the NMDA-induced suppression of IGABA; (ii) when the neurons were incubated in a Ca2+-free bath or pre-loaded with a membrane-permeable Ca2+ chelator, BAPTA AM (10 mol/L), the inhibitory effect of NMDA on IGABA disappeared. Cd2+ (10 mol/L) or La3+ (30 mol/L), the non-selective blockers of voltage-dependent calcium channels, did not affect the suppression of IGABA by NMDA application; (iii) the suppression of IGABA by NMDA was inhibited by KN-62, a calcium/calmodulin-dependent protein kinase II (CaMKII) inhibitor. These results indicated that the inhibition of GABA response by NMDA is Ca2+-dependent and CaMKII is involved in the process of the Ca2+-dependent inhibition.     

1-Methyl-4-phenylpridinium (MPP+)-induced functional run-down of GABA(A) receptor-mediated currents in acutely dissociated dopaminergic neurons

We have evaluated GABA(A)receptor function during treatment of 1-methyl-4-phenylpridinium (MPP+) using patch-clamp perforated whole-cell recording techniques in acutely dissociated dopaminergic (DAergic) neurons from rat substantia nigra compacta (SNc). Gamma-aminobutyric acid (GABA), glutamate or glycine induced inward currents (I(GABA), I(Glu), I(Gly)) at a holding potential (VH) of -45 mV. The I(GABA) was reversibly blocked by the GABA(A) receptor antagonist, bicuculline, suggesting that I(GABA) is mediated through the activation of GABA(A) receptors. During extracellular perfusion of MPP+ (1-10 microm), I(GABA) , but neither I(Glu) nor I(Gly), declined (termed run-down) with repetitive agonist applications, indicating that the MPP+-induced I(GABA) run-down occurred earlier than I(Gly) or I(Glu) under our experimental conditions. The MPP+-induced I(GABA) run-down can be prevented by a DA transporter inhibitor, mazindol, and can be mimicked by a metabolic inhibitor, rotenone. Using conventional whole-cell recording with different concentrations of ATP in the pipette solution, I(GABA) run-down can be induced by decreasing intracellular ATP concentrations, or prevented by supplying intracellular ATP, indicating that I(GABA) run-down is dependent on intracellular ATP concentrations. A GABA(A) receptor positive modulator, pentobarbital (PB), potentiated the declined I(GABA) and eliminated I(GABA) run-down. Corresponding to these patch-clamp data, tyrosine hydroxylase (TH) immunohistochemical staining showed that TH-positive cell loss was protected by PB during MPP+ perfusion. It is concluded that extracellular perfusion of MPP+ induces a functional run-down of GABA(A) receptors, which may cause an imbalance of excitation and inhibition of DAergic neurons.     

(+)- and (-)-cis-2-aminomethylcyclopropanecarboxylic acids show opposite pharmacology at recombinant rho(1) and rho(2) GABA(C) receptors

The effects of the enantiomers of (+/-)-CAMP and (+/-)-TAMP [(+/-)-cis- and (+/-)-trans-2-aminomethylcyclopropanecarboxylic acids, respectively], which are cyclopropane analogues of GABA, were tested on GABA(A) and GABA(C) receptors expressed in Xenopus laevis oocytes using two-electrode voltage clamp methods. (+)-CAMP was found to be a potent and full agonist at homooligomeric GABA(C) receptors (K:(D) approximately 40 microM: and I:(max) approximately 100% at rho(1); K:(D) approximately 17 microM: and I:(max) approximately 100% at rho(2)) but a very weak antagonist at alpha(1)beta(2)gamma(2L) GABA(A) receptors. In contrast, (-)-CAMP was a very weak antagonist at both alpha(1)beta(2)gamma(2L) GABA(A) receptors and homooligomeric GABA(C) receptors (IC(50) approximately 900 microM: at rho(1) and approximately 400 microM: at rho(2)). Furthermore, (+)-CAMP appears to be a superior agonist to the widely used GABA(C) receptor partial agonist cis-4-aminocrotonic acid (K:(D) approximately 74 microM: and I:(max) approximately 78% at rho(1); K:(D) approximately 70 microM: and I:(max) approximately 82% at rho(2)). (-)-TAMP was the most potent of the cyclopropane analogues on GABA(C) receptors (K:(D) approximately 9 microM: and I:(max) approximately 40% at rho(1); K:(D) approximately 3 microM: and I:(max) approximately 50-60% at rho(2)), but it was also a moderately potent GABA(A) receptor partial agonist (K:(D) approximately 50-60 microM: and I:(max) approximately 50% at alpha(1)beta(2)gamma(2L) GABA(A) receptors). (+)-TAMP was a less potent partial agonist at GABA(C) receptors (K:(D) approximately 60 microM: and I:(max) approximately 40% at rho(1); K:(D) approximately 30 microM: and I:(max) approximately 60% at rho(2)) and a weak partial agonist at alpha(1)beta(2)gamma(2L) GABA(A) receptors (K:(D) approximately 500 micro: and I:(max) approximately 50%). None of the isomers of (+/-)-CAMP and (+/-)-TAMP displayed any interaction with GABA transport at the concentrations tested. Molecular modeling based on the present results provided new insights into the chiral preferences for either agonism or antagonism at GABA(C) receptors.     

Zinc modulation of glycine receptors in acutely isolated rat CA3 neurons

Glycine and GABA are the primary inhibitory neurotransmitters in the spinal cord and brain stem, with glycine exerting its physiological roles by activating strychnine-sensitive ionotropic receptors. Glycine receptors are also expressed in the brain, including the cortex and hippocampus, but their physiological roles and pharmacological properties are largely unknown. Here, we report the pharmacological properties of functional glycine receptors in acutely isolated rat CA3 neurons using conventional whole-cell patch clamp techniques. Both glycine and taurine, which are endogenous agonists of glycine receptors, elicited Cl(-) currents in a concentration-dependent manner. The glycine-induced current (I(Gly)) was inhibited by strychnine, picrotoxin or cyclothiazide in a concentration-dependent manner. At lower concentrations (0.01-1 microM), ICS-205,930 potentiated I(Gly), but at higher concentrations (>10 microM) it inhibited I(Gly). These pharmacological properties strongly suggest that CA3 neurons express functional strychnine-sensitive glycine receptors containing alpha2 subunits. Furthermore, at lower concentrations (1-30 microM), Zn(2+) potentiated I(Gly), but at higher concentrations (>100 microM) it inhibited I(Gly). Considering that Zn(2+) is synaptically co-released with glutamate from mossy fiber terminals that make excitatory synapses onto CA3 neurons, these results suggest that endogenous Zn(2+) modulation of these glycine receptors may have an important role in the excitability of CA3 neurons.     

Taurine-evoked chloride current and its potentiation by intracellular Ca2+ in immature rat hippocampal CA1 neurons

Taurine is one of the most abundant free amino acids in the immature mammalian central nervous system. In the present study, whole-cell patch-clamp recordings were made to examine taurine-evoked currents ( I(Tau)) in acutely dissociated immature rat hippocampal CA1 neurons. Taurine at low concentrations (/=3 mM) activated both glycine and GABA(A) receptors. Moreover, elevation of intracellular Ca(2+) via non-NMDA receptor activation enhanced I(Tau) reversibly. The results indicate that taurine may act as a native ligand of glycine receptors and modulate neurotransmissions in the immature hippocampus, and under certain conditions it can also activate GABA(A) receptors. The potentiation of I(Tau) by intracellular Ca(2+) may contribute to the protection effect of taurine under some cell-damaging conditions.     

Kindling alters neurosteroid-induced modulation of phasic and tonic GABAA receptor-mediated currents: role of phosphorylation

We have previously shown that after kindling (a model of temporal lobe epilepsy), the neuroactive steroid tetrahydrodeoxycorticosterone (THDOC) was unable to augment GABA type A receptor (GABA(A))-mediated synaptic currents occurring on pyramidal cells of the piriform cortex. Phosphorylation of GABA(A) receptors has been shown previously to alter the activity of THDOC, so we tested the hypothesis that kindling induces changes in the phosphorylation of GABA(A) receptors and this accounts for the loss in efficacy. To assay whether GABA(A) receptors are more phosphorylated after kindling, we examined the phosphorylation state of the β3 subunit and found that it was increased. Incubation of brain slices with the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) (100 nM) also increased phosphorylation in the same assay. In patch clamp, recordings from non-kindled rat brain slices PMA also reduced the activity of THDOC in a manner that was identical to what is observed after kindling. We also found that the tonic current was no longer augmented by THODC after kindling and PMA treatment. The protein kinase C (PKC) antagonist bisindolylmaleimide I blocked the effects PMA on the synaptic but not the tonic currents. However, the broad spectrum PKC antagonist staurosporine blocked the effects of PMA on the tonic currents, implying that different PKC isoforms phosphorylate GABA(A) receptors responsible for phasic and tonic currents. The phosphatase activator Li(+) palmitate restored the 'normal' activity of THDOC on synaptic currents in kindled brain slices but not the tonic currents. These data demonstrate that kindling enhances the phosphorylation state of GABA(A) receptors expressed in pyramidal neurons reducing THDOC efficacy.     

P物质对大鼠三叉神经节神经元GABA-和5-HT-激活电流的反向调制作用

实验采用全细胞膜片钳技术观察P 物质(SP)对大鼠同一三叉神经节(TG)神经元γ-氨基丁酸激活电流(IGABA)和5-羟色胺激活电流(I5-HT)的调制作用。在受检的47 个 TG 细胞中,多数情况下可在同一细胞记录到IGABA 和 I5-HT 两种电流(63.8%,30/47)。在 30 个同时对 GABA 和 5-HT 敏感的细胞,其中 22 个细胞预加 SP(0.01 μmol/L)后,IGABA 减小(35.7 ± 6.1)%,而I5-HT 增加(65.2 ±8.7)%。此种调制作用可被SP 受体拮抗剂GR82334 及胞内透析GDP-β -S 或GF109203X 所阻断。以上结果表明:SP 受体激活后经G 蛋白耦联,通过相同的PLC-DAG-PKC 转导途径对同一感觉神经元共存的GABAA 受体和5-HT3 受体产生相反的调制效应。     

Pharmacologic and therapeutic aspects of the developmentally regulated expression of GABA(A) and GABA(B) receptors: cerebellar granule cells as a model system.

Cerebellar granule neurons can be conveniently kept in culture. They constitute a useful model to study regulation of glutamatergic activity, in particular the inhibitory action of GABA (7-aminobutyrate). GABA exerts an inhibitory action on evoked transmitter release acting on both GABA(A) and GABA(B) receptors. The functional properties of these receptors are dependent upon the environment of the neurons during early development in culture as the expression of both receptor subtypes is enhanced by exposure of the neurons to GABA(A) receptor agonists. Thus, the inducible GABA(A) receptors are of low affinity and lack benzodiazepine sensitivity, and the G-protein coupling differs among the native and the inducible GABA(B) receptors. Moreover, the GABA(A) and the GABA(B) receptors are functionally coupled, leading to a disinhibitory action of GABA. Therefore drugs exhibiting selective agonist or antagonist action on subclasses of GABA(A) and GABA(B) may be of potential use as regulators of glutamatergic excitatory activity.     

An ionotropic GABA receptor with novel pharmacology at bullfrog cone photoreceptor terminals

Characteristics of ionotropic gamma-aminobutyric acid (GABA) receptors at bullfrog cone terminals were studied by patch clamp techniques in isolated cell and retinal slice preparations. GABA-induced inward currents from isolated cones reversed in polarity at a potential, very close to the chloride equilibrium potential, and they were completely suppressed by picrotoxin. Unexpectedly, the GABA current was dose-dependently potentiated by the well-known GABA(A) receptor antagonist bicuculline (BIC), but was suppressed by gabazine, another GABA(A) antagonist, and imidazole-4-acetic acid (I4AA), a GABA(C) receptor antagonist. Similarly, currents induced by both GABA(A) agonist muscimol and GABA(C) agonist cis-4-aminocrotonic acid (CACA) were also potentiated by BIC. Furthermore, currents induced from cones by GABA and kainate-caused depolarization of horizontal cells in retinal slice preparations were both potentiated by BIC. All these results suggest that the ionotropic GABA receptor at the bullfrog cone terminal exhibits novel pharmacology, distinct from both traditional GABA(A) and GABA(C) receptors.     

Effects of cimetidine-like drugs on recombinant GABAA receptors

Even though conventional systemic doses of cimetidine and other histamine H(2) antagonists display minimal brain penetration, central nervous system (CNS) effects (including seizures and analgesia) have been reported after administration of these drugs in animals and man. To test the hypothesis that cimetidine-like drugs produce these CNS effects via inhibition of GABA(A) receptors, the actions of these drugs were studied on seven different, precisely-defined rat recombinant GABA(A) receptors using whole-cell patch clamp recordings. The H(2) antagonists famotidine and tiotidine produced competitive and reversible inhibition of GABA-evoked currents in HEK293 cells transfected with various GABA(A) receptor subunits (IC(50) values were between 10-50 microM). In contrast, the H(2) antagonist ranitidine and the cimetidine congener improgan had very weak (if any) effects (IC(50) > 50 microM). Since the concentrations of cimetidine-like drugs required to inhibit GABA(A) receptors in vitro (greater than 50 microM) are considerably higher than those found during analgesia and/or seizures (1-2 microM), the present results suggest that cimetidine-like drugs do not appear to produce seizures or analgesia by directly inhibiting GABA(A) receptors.     

Clustering of extrasynaptic GABA(A) receptors modulates tonic inhibition in cultured hippocampal neurons

Tonic inhibition plays a crucial role in regulating neuronal excitability because it sets the threshold for action potential generation and integrates excitatory signals. Tonic currents are known to be largely mediated by extrasynaptic gamma-aminobutyric acid type A (GABA(A)) receptors that are persistently activated by submicromolar concentrations of ambient GABA. We recently reported that, in cultured hippocampal neurons, the clustering of synaptic GABA(A) receptors significantly affects synaptic transmission. In this work, we demonstrated that the clustering of extrasynaptic GABA(A) receptors modulated tonic inhibition. Depolymerization of the cytoskeleton with nocodazole promoted the disassembly of extrasynaptic clusters of delta and gamma(2) subunit-containing GABA(A) receptors. This effect was associated with a reduction in the amplitude of tonic currents and diminished shunting inhibition. Moreover, diffuse GABA(A) receptors were less sensitive to the GAT-1 inhibitor NO-711 and to flurazepam. Quantitative analysis of GABA-evoked currents after prolonged exposure to submicromolar concentrations of GABA and model simulations suggest that clustering affects the gating properties of extrasynaptic GABA(A) receptors. In particular, a larger occupancy of the singly and doubly bound desensitized states can account for the modulation of tonic inhibition recorded after nocodazole treatment. Moreover, comparison of tonic currents recorded during spontaneous activity and those elicited by exogenously applied low agonist concentrations allows estimation of the concentration of ambient GABA. In conclusion, receptor clustering appears to be an additional regulating factor for tonic inhibition.     

Effects of benzodiazepines on frog ERG

Effect of flurazepam (water-soluble benzodiazepine) on the amplitude and time course of ERG waves was investigated in superfused frog eyecups (Rana ridibunda). Flurazepam (50 and 100 microM) had inhibitory effect on the b- and d-wave amplitude, which was not accompanied with significant changes in their implicit time. Flurazepam potentiated the depressant effect of GABA (2.5 and 5 mM) on the b- and d-wave amplitude. The inhibitory effect of flurazepam was not blocked by 50 microM bicuculline (BCC), (GABA(A) antagonist), although the blocker markedly potentiated the b- and d-wave amplitude. The suppressive effect of flurazepam on the b- but not d-wave amplitude was blocked by 100 microM BCC. Our results indicate existence of functional benzodiazepine regulatory sites on GABA(A) receptors in distal frog retina.     

Plasticity of pre- and postsynaptic GABAB receptor function in the paraventricular nucleus in spontaneously hypertensive rats

GABA(B) receptor function is upregulated in the paraventricular nucleus (PVN) of the hypothalamus in spontaneously hypertensive rats (SHR), but it is unclear whether this upregulation occurs pre- or postsynaptically. We therefore determined pre- and postsynaptic GABA(B) receptor function in retrogradely labeled spinally projecting PVN neurons using whole cell patch-clamp recording in brain slices in SHR and Wistar-Kyoto (WKY) rats. Bath application of the GABA(B) receptor agonist baclofen significantly decreased the spontaneous firing activity of labeled PVN neurons in both SHR and WKY rats. However, the magnitude of reduction in the firing rate was significantly greater in SHR than in WKY rats. Furthermore, baclofen produced larger membrane hyperpolarization and outward currents in labeled PVN neurons in SHR than in WKY rats. The baclofen-induced current was abolished by either including G protein inhibitor GDPbetaS in the pipette solution or bath application of the GABA(B) receptor antagonist in both SHR and WKY rats. Blocking N-methyl-d-aspartic acid receptors had no significant effect on baclofen-elicited outward currents in SHR. In addition, baclofen caused significantly greater inhibition of glutamatergic excitatory postsynaptic currents (EPSCs) in labeled PVN neurons in brain slices from SHR than WKY rats. By contrast, baclofen produced significantly less inhibition of GABAergic inhibitory postsynaptic currents (IPSCs) in labeled PVN neurons in SHR than in WKY rats. Although microinjection of the GABA(B) antagonist into the PVN increases sympathetic vasomotor tone in SHR, the GABA(B) antagonist did not affect EPSCs and IPSCs of the PVN neurons in vitro. These findings suggest that postsynaptic GABA(B) receptor function is upregulated in PVN presympathetic neurons in SHR. Whereas presynaptic GABA(B) receptor control of glutamatergic synaptic inputs is enhanced, presynaptic GABA(B) receptor control of GABAergic inputs in the PVN is attenuated in SHR. Changes in both pre- and postsynaptic GABA(B) receptors in the PVN may contribute to the control of sympathetic outflow in hypertension.     

Effects of gamma-aminobutyric acid on secretagogue-induced exocrine secretion of isolated, perfused rat pancreas

Because GABA and its related enzymes have been determined in beta-cells of pancreas islets, effects of GABA on pancreatic exocrine secretion were investigated in the isolated, perfused rat pancreas. GABA, given intra-arterially at concentrations of 3, 10, 30, and 100 microM, did not exert any influence on spontaneous or secretin (12 pM)-induced pancreatic exocrine secretion. However, GABA further elevated CCK (10 pM)-, gastrin-releasing peptide (100 pM)-, or electrical field stimulation-induced pancreatic secretions of fluid and amylase dose dependently. The GABA (30 microM)-enhanced CCK-induced pancreatic secretions were completely blocked by bicuculline (10 microM), a GABA(A) receptor antagonist, but were not affected by saclofen (10 microM), a GABA(B) receptor antagonist. The enhancing effects of GABA (30 microM) on CCK-induced pancreatic secretions were not changed by tetrodotoxin (1 microM) but were partially reduced by cyclo-(7-aminoheptanonyl-Phe-D-Trp-Lys-Thr[BZL]) (10 nM), a somatostatin antagonist. In conclusion, GABA enhances pancreatic exocrine secretion induced by secretagogues, which predominantly induce enzyme secretion, via GABA(A) receptors in the rat pancreas. The enhancing effect of GABA is partially mediated by inhibition of islet somatostatin release.     

Differential effects of zinc on native GABA(A) receptor function in rat hippocampus and cerebellum.

Hippocampal noradrenergic and cerebellar glutamatergic granule cell axon terminals possess GABA(A) receptors mediating enhancement of noradrenaline and glutamate release, respectively. The hippocampal receptor is benzodiazepine-sensitive, whereas the cerebellar one is not affected by benzodiazepine agonists, indicating the presence of an alpha6 subunit. We tested here the effects of Zn2+ on these two native GABA(A) receptor subtypes using superfused rat hippocampal and cerebellar synaptosomes. In the cerebellum, zinc ions strongly inhibited (IC50 approximately 1 microM) the potentiation of the K(+)-evoked [3H]D-aspartate release induced by GABA. In contrast, the GABA-evoked release of [3H]noradrenaline from hippocampal synaptosomes was much less sensitive to Zn2+ (IC50 > 30 microM). The effects of Zn2+ were then studied in two rat lines selected for high (ANT) and low (AT) alcohol sensitivity because granule cell GABA(A) receptors in ANT, but not AT, rats respond to benzodiazepine agonists due to a critical mutation in the alpha6 subunit. GABA increased the K(+)-evoked release of [3H]DCNS REGIONS-aspartate from cerebellar synaptosomes of AT and ANT rats, an effect prevented by the GABAA selective antagonist bicuculline. In AT rat cerebellum, the effect of GABA was strongly inhibited by Zn2+ (IC50 < or = 1 microM), whereas in ANT rats, the divalent cation was about 100-fold less potent. Thus, native benzodiazepine-sensitive GABAA receptors appear largely insensitive to functional inhibition by Zn2+ and vice versa. Changes in sensitivity to Zn2+ inhibition consequent to mutations in cerebellar granule cell GABA(A) receptor subunits may lead to changes in glutamate release from parallel fibers onto Purkinje cells and may play important roles in cerebellar dysfunctions.