Activation of protein kinase C by phorbol dibutyrate modulates GABAA receptor binding in rat brain slices

Effects of protein kinase C (PKC) activation on the function of the GABA/benzodiazepine receptor-chloride complex were analyzed by quantitative autoradiography using [³H]muscimol, [³H]flunitrazepam and [³⁵S]TBPS in rat brain slices. The density of [³H]muscimol binding was highest in cerebellar granular layers and high in both the frontal cortex and thalamus, but binding levels in the hippocampus were low. After activation of PKC by 100 nM phorbol-12,13-dibutyrate (PDBu), [³H]muscimol binding was decreased in the frontal cortex, striatum and thalamus, but binding levels were not changed in the hippocampus or cerebellum. The density of [³H]flunitrazepam binding was high in the cortex, hippocampus and molecular layers of cerebellum but was low in thalamus. PDBu increased the [³H]flunitrazepam binding only in the striatum and in part of the cortex and thalamus after activation of PKC. After activation of PKC by PDBu, [³⁵S]TBPS binding was increased in most areas, but binding levels were not changed in the brainstem or cerebellum. The receptor binding was markedly decreased in almost all areas by the addition of 2.5 mM Mg²⁺. Elevated [³⁵S]TBPS binding produced by PDBu was significantly inhibited by the addition of Mg²⁺. These results suggest that the activation of PKC potentiates benzodiazepine and TBPS binding, but decreases muscimol binding in a region-specific manner in the rat brain.     

Changes of GABAA receptor binding and subunit mRNA level in rat brain by infusion of subtoxic dose of MK-801

In the present study, we have investigated the effects of prolonged inhibition of NMDA receptor by infusion of subtoxic dose of MK-801 to examine the modulation of GABA_A receptor binding and GABA_A receptor subunit mRNA level in rat brain. It has been reported that NMDA-selective glutamate receptor stimulation alters GABA_A receptor pharmacology in cerebellar granule neurons in vitro by altering the levels of selective subunit. However, we have investigated the effect of NMDA antagonist, MK-801, on GABA_A receptor binding characteristics in discrete brain regions by using autoradiographic and in situ hybridization techniques. The GABA_A receptor bindings were analyzed by quantitative autoradiography using [³H]muscimol, [³H]flunitrazepam, and [³⁵S]TBPS in rat brain slices. Rats were infused with MK-801 (1 pmol/10 μl per h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps (Alzet, model 2ML). The levels of [³H]muscimol binding were highly elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, and cerebellum. However, the [³H]flunitrazepam binding and [³⁵S]TBPS binding were increased only in specific regions; the former level was increased in parts of the cortex, thalamus, and hippocampus, while the latter binding sites were only slightly elevated in parts of thalamus. The levels of β2-subunit were elevated in the frontal cortex, thalamus, hippocampus, brainstem, and cerebellar granule layers while the levels of β3-subunit were significantly decreased in the cortex, hippocampus, and cerebellar granule layers in MK-801-infused rats. The levels of α6- and δ-subunits, which are highly localized in the cerebellum, were increased in the cerebellar granule layer after MK-801 treatment. These results show that the prolonged suppression of NMDA receptor function by MK-801-infusion strongly elevates [³H]muscimol binding throughout the brain, increases regional [³H]flunitrazepam and [³⁵S]TBPS binding, and alters GABA_A receptor subunit mRNA levels in different directions. The chronic MK-801 treatment has differential effect on various GABA_A receptor subunits, which suggests involvement of differential regulatory mechanisms in interaction of NMDA receptor with the GABA receptors.     

Age-related modifications on the GABAA receptor binding properties from Wistar rat prefrontal cortex

In the present communication we have investigated the pharmacological properties of the GABA_A receptor from adult (3 months old) and aged (24 months old) Wistar rat prefrontal cortex. The prefrontal cortex is implicated in cognitive functions and stress and both processes seem to be altered during aging. These changes could be mediated by modifications in the GABA_A receptor properties. Our results indicated the absence of generalized age-related modifications on the pharmacological properties of the GABA_A receptor from prefrontal cortical membranes. Saturation experiments using the non-selective benzodiazepine [³H]flunitrazepam revealed that neither the K_d values or the B_max were modified during aging. Moreover, Cl 218 872 displacement of [³H]flunitrazepam showed no age-related modifications on either the K_is or the relative proportion between the Type I and Type II benzodiazepine binding sites. Therefore, the benzodiazepine binding sites are well preserved in aged prefrontal cortex. On the other hand, saturation experiments using the GABA agonist [³H]muscimol demonstrated a decrease in the B_max of the low affinity [³H]muscimol binding sites in aged rats (4.3 ± 0.8 pmol/mg protein vs. 2.3 ± 0.2 pmol/mg protein in adult and aged rats, respectively). However, no age-dependent modifications were observed in the allosteric interaction between GABA and benzodiazepine binding sites. These results demonstrate that the benzodiazepine binding sites and the GABA binding sites of the GABA_A receptor complex from rat prefrontal cortical membranes are differentially affected by the aging process.     

Withdrawal from butorphanol dependence alters binding of [H]phorbol dibutyrate to protein kinase C, but not of [H]forskolin to adenylate cyclase

The time course of autoradiographic binding of major second messengers in the rat brain was studied at 2, 7, and 24 h after withdrawal from butorphanol infusion. [³H]Forskolin and [³H]phorbol 12,13-dibutyrate (PDBu) were used to label adenylate cyclase and protein kinase C (PKC), respectively. Rats were rendered dependent by intracerebroventricular infusion of butorphanol (26 nmol μl⁻¹ h⁻¹) via osmotic minipumps for 3 days. Withdrawal was initiated by abrupt cessation of the butorphanol infusion. The levels of [³H]forskolin binding were not changed at any time or in any brain area, except for an increase following 7 h of withdrawal in the brainstem only. The levels of [³H]PDBu binding were significantly increased (13–47%) in multiple areas of the rat brain following 7 h of withdrawal from butorphanol infusion. These findings suggest that the phosphoinositide cycle system is more susceptible to alteration during butorphanol dependence than is the adenylate cyclase system in the rat brain.     

Changes of GABA(A) receptor binding and subunit mRNA level in rat brain by infusion of subtoxic dose of MK-801

In the present study, we have investigated the effects of prolonged inhibition of NMDA receptor by infusion of subtoxic dose of MK-801 to examine the modulation of GABA_A receptor binding and GABA_A receptor subunit mRNA level in rat brain. It has been reported that NMDA-selective glutamate receptor stimulation alters GABA_A receptor pharmacology in cerebellar granule neurons in vitro by altering the levels of selective subunit. However, we have investigated the effect of NMDA antagonist, MK-801, on GABA_A receptor binding characteristics in discrete brain regions by using autoradiographic and in situ hybridization techniques. The GABA_A receptor bindings were analyzed by quantitative autoradiography using [³H]muscimol, [³H]flunitrazepam, and [³⁵S]TBPS in rat brain slices. Rats were infused with MK-801 (1 pmol/10 μl per h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps (Alzet, model 2ML). The levels of [³H]muscimol binding were highly elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, and cerebellum. However, the [³H]flunitrazepam binding and [³⁵S]TBPS binding were increased only in specific regions; the former level was increased in parts of the cortex, thalamus, and hippocampus, while the latter binding sites were only slightly elevated in parts of thalamus. The levels of β2-subunit were elevated in the frontal cortex, thalamus, hippocampus, brainstem, and cerebellar granule layers while the levels of β3-subunit were significantly decreased in the cortex, hippocampus, and cerebellar granule layers in MK-801-infused rats. The levels of 6- and δ-subunits, which are highly localized in the cerebellum, were increased in the cerebellar granule layer after MK-801 treatment. These results show that the prolonged suppression of NMDA receptor function by MK-801-infusion strongly elevates [³H]muscimol binding throughout the brain, increases regional [³H]flunitrazepam and [³⁵S]TBPS binding, and alters GABA_A receptor subunit mRNA levels in different directions. The chronic MK-801 treatment has differential effect on various GABA_A receptor subunits, which suggests involvement of differential regulatory mechanisms in interaction of NMDA receptor with the GABA receptors.     

Involvement of GABAA and GABAB receptors in afterdischarge generation in rat hippocampal slices

It has been hypothesized that a disruption of γ-aminobutyric acid (GABA) receptor-mediated processes may be involved in the pathophysiology of focal epilepsy. This disinhibition hypothesis has been postulated from the results of in vitro experiments of the interictal activity of focal epilepsy. Less is known, however, about how disinhibition may be involved in the production of the ictal activity. We therefore examined the pharmacological effects of selective agonists and antagonists of GABA_A and GABA_B receptors on ictal-like afterdischarges (ADs) induced following repetitive high-frequency electrical stimulation in the CA1 region of rat hippocampal slices. The GABA_A receptor antagonist bicuculline (5 μM) fully blocked AD generation, as did the GABA_A receptor agonist muscimol (2 μM), which is thought to produce a tonic inhibition during application. However, the benzodiazepine receptor agonist diazepam (5 μM), which enhances the inhibitory postsynaptic potential induced by synaptically released GABA, increased the number of spikes in the AD to 148.3% of the control value. On the other hand, the GABA_B receptor antagonist phaclofen (1 mM) increased the number of spikes in the AD to 234.7% of the control value, while the GABA_B receptor agonist baclofen (5 μM) reduced it to 46.9%. We therefore conclude that synaptic, but not tonic, activation of GABA_A receptors appears to be necessary for ictal-like AD generation, while GABA_B receptor activation plays a protective role. We therefore propose a modification to the simple disinhibition hypothesis.     

Heterogeneous localization of protein kinase C in rat brain: autoradiographic analysis of phorbol ester receptor binding

Protein kinase C is a calcium- and phospholipid-stimulated enzyme present in high concentration in the brain. Phorbol esters are potent tumor promoters that bind to specific receptors with high affinity. Several lines of evidence indicate that the phorbol ester receptor is identical to protein kinase C. To determine the distribution of protein kinase C, we have localized phorbol ester receptors in the rat brain by autoradiography, using [3H]phorbol 12,13-dibutyrate ([3H]PDBu) and have performed a variety of lesions to assess the nature of the cellular elements possessing the binding sites. The [3H]PDBu binding sites in the rat brain are discretely localized and primarily associated with neurons. Evidence is presented for localization to intrinsic neurons of the cortex and hippocampus, terminals of the striatonigral projection, a projection to the molecular layer of the dentate gyrus, and to dendrites of Purkinje cells.     

Modulation of GABAergic receptor binding by activation of calcium and calmodulin-dependent kinase II membrane phosphorylation

γ-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system (CNS). Because of the important role that GABA plays in the CNS, alteration of GABA_A receptor function would significantly affect neuronal excitability. Protein phosphorylation is a major mechanism for regulating receptor function in the brain and has been implicated in modulating GABA_A receptor function. Therefore, this study was initiated to determine the role of calmodulin-dependent kinase II (CaM kinase II) membrane phosphorylation on GABA_A receptor binding. Synaptosomal membrane fractions were tested for CaM kinase II activity towards endogenous substrates. In addition, muscimol binding was evaluated under equilibrium conditions in synaptosomal membrane fractions subjected to either basal (Mg²⁺ alone) or maximal CaM kinase II-dependent phosphorylation. Activation of endogenous CaM kinase II-dependent phosphorylation resulted in a significant enhancement of the apparent B_max for muscimol binding without significantly altering the apparent binding affinity. The enhanced muscimol binding could be increased further by the addition of exogenous CaM kinase II to synaptosomal membrane fractions. Co-incubation with inhibitors of kinase activity during the phosphorylation reactions blocked the CaM kinase II-dependent increase in muscimol binding. The data support the hypothesis that activation of CaM kinase II-dependent phosphorylation caused an increased GABA_A receptor binding and may play an important role in modulating the function of this inhibitory receptor/chloride ion channel complex.     

Odor increases [3H]phorbol dibutyrate binding to protein kinase C in olfactory structures of rat brain. Effect of entorhinal cortex lesion

Since protein kinase C (PKC) is known to be activated in the olfactory bulb and in several limbic areas related to odor processing, we determined whether an olfactory stimulus was able to modulate the activity of PKC in animals with bilateral entorhinal cortex lesion. The translocation of PKC from the cytosol to the membrane was studied using the phorbol ester 12,13-dibutyrate ([3H]PDBu) binding in control and bilateral entorhinal cortex (EC) lesioned rats. The lesion of EC per se did not significantly affect [3H]PDBu binding in any of the brain structures analyzed, while odor stimulation induced it in both control and EC-lesioned groups in the external plexiform layer of the olfactory bulb. In contrast, an odor-induced increase of [3H]PDBu binding in internal glomerular layer of the olfactory bulb was only observed in EC lesioned animals. Similar results were obtained in the piriform cortex. In both CA1 and CA3 hippocampal subfields, odor stimulation induced an increase of [3H]PDBu binding in both control and EC-lesioned animals, the increase being potentiated only in CA1 of lesioned rats. The dentate gyrus and the amygdala exhibited a similar pattern of [3H]PDBu binding, showing a significant increase exclusively in EC-lesioned animals after odor stimulation. The results strongly suggest that the EC plays a key role in odor processing. PKC appears to play an important role in responding to the activation of lipid second messengers, which have been described to be involved in the processing of odor stimuli in several structures of the olfactory pathway.     

Glucocorticoids are modulators of GABAA receptors in brain

Modulation of binding of [³H]muscimol, a GABA_A receptor agonist, by natural and synthetic glucocorticoids was investigated in crude synaptosomal membranes and in brain sections of rat. In adrenalectomized (Adx) rats, muscimol binding was reduced by 30–50% in cerebral cortex, cerebellum, thalamus and hippocampus, as compared to sham-operated controls. This decrease was due to reduced binding affinities of GABA receptors for muscimol. In contrast muscimol binding was increased by 38% in the hypothalamus and did not change in the pons-medulla after Adx. Nanomolar concentrations of corticosterone and pregnenolone-sulfate, but not dexamethasone, enhanced muscimol binding in brain regions that were characterized by reduced binding following Adx. This steroid-induced increase in muscimol binding was due to enhanced affinities of GABA receptors.     

Lower GABAA receptor binding in the amygdala and hypothalamus of spontaneously hypertensive rats

The central GABAergic system is associated with normal blood pressure regulation, but the role of GAGA receptors in genetic hypertension remains unclear. This study was conducted to investigate GABA_A receptor binding in several brain regions of spontaneously hypertensive (SHR) rats during development of hypertension. GAGA_A receptor binding was labeled with [³⁵S]TBPS and was assessed by quantitative autoradiography with the aid of a computer-assisted image analysis system. Densities of GABA_A receptor binding sites were significantly lower in all hypothalamic and amygdaloid nuclei evaluated in 4-week-old SHR rats, when compared with their age-matched normotensive Wistar-Kyoto rats. At 12 weeks of age, GABA_A receptor binding remained significantly lower in the central amygdaloid nucleus and paraventricular hypothalamic nucleus of SHR rats. Collectively, the results suggest that GABA_A receptors in these nuclei are likely to be involved in the initiation and maintenance of hypertension. In conclusion, this study supports a notion that downregulation of GABA_A receptor binding occurs in the hypothalamus and amygdala of SHR rats and may play a role in genetic hypertension.     

Effects of acute pentobarbital administration on GABAA receptor-regulated chloride uptake in rat brain synaptoneurosomes

Effects of acute pentobarbital administration on GABA_A receptor-regulated muscimol-stimulated, pentobarbital-stimulated, or flunitrazepam-enhanced, muscimol-stimulated chloride uptake were studied in the brains of Sprague-Dawley rats. Animals received sodium pentobarbital, 60 mg/kg IP and cerebral cortical and cerebellar Synaptoneurosomes were isolated at 10 min, l h and when animals had awakened. The basal uptake of chloride was not changed in either cerebral cortex or cerebellum at different time periods after pentobarbital administration. Ten minutes after sodium pentobarbital administration, muscimol-stimulated chloride uptake was significantly reduced in cerebellum when the muscimol concentration was 2.5, 5, or 20 μM and in cerebral cortex when the concentration of muscimol was 5 or 10 μM (p < 0.05, Duncan multiple-range test). One hour after pentobarbital administration or after animals had awakened, chloride uptake in brains from pentobarbital-treated animals was less at low concentration of muscimol (2.5 μM). No significant difference was found in either cerebral cortex or cerebellum in pentobarbital-(125-1,000 μM) stimulated or flunitrazepam-(2.5–20 μM) enhanced, muscimol-(3 μM) stimulated chloride uptake at different time periods after pentobarbital administration. Saline treatment had no effects on the basal or muscimol-stimulated chloride uptake in cerebellar Synaptoneurosomes when compared with naive animals. The results demonstrate that GABA_A receptor-regulated chloride uptake is decreased after acute pentobarbital administration, an effect that is reversible.     

Zinc modulates GABAB binding in rat brain

The effects of ZnCl₂ on [³H]GABA binding to GABA_A and GABA_B binding sites were investigated using receptor autoradiography. At concentrations exceeding 100 μM, zinc non-competitively inhibited GABA_B binding in a dose dependent fashion. GABA_A binding was not inhibited significantly by zinc eliminating the possibility of a non-specific effect of zinc. Increased calcium concentrations up to 10 MM enhanced total GABA_B binding but did not prevent zinc induced inhibition of GABA_B binding, indicating a separate site of action for these cations at the GABA_B binding site. In some regions, zinc modulates GABA_B binding in a biphasic manner as concentrations of 10–100 μM zinc significantly enhanced GABA_B binding in the hippocampus and the molecular layer of the cerebellum but not in the thalamus. These results provide further evidence for a neuromodulatory role for zinc in the central nervous system.     

Bihemispheric reduction of GABAA receptor binding following focal cortical photothrombotic lesions in the rat brain

Focal brain lesions may lead to neuronal dysfunctions in remote (exofocal) brain regions. In the present study, focal lesions were induced in the hindlimb representation area of the parietal cerebral cortex in rats using the technique of photothrombosis. Photothrombosis occurs after illumination of the brain through the intact skull following intravenous injection of the photosensitive dye Rose Bengal. This resulted in cortical lesions with a diameter of about 2 mm. Quantitative receptor autoradiography was used to study alterations in the density of []muscimol binding sites to GABA_A receptors seven days after lesion induction. A reduced GABA_A receptor binding (−13 to −27% of the control value) was found in layers II and III of remote exofocal regions in the ipsi- and contralateral cortex. The reduction was consistently more intense in the ipsilateral cortical areas than in those of the contralateral hemisphere. Using extracellular recordings, significant correlations between GABA_A receptor binding and paired pulse inhibition could be demonstrated. The present investigation demonstrates that focal brain lesions cause a widespread, functionally effective down-regulation of GABA_A receptors. These postlesional changes may result from lesion-induced alterations in cortical connectivity.     

Immunohistochemical studies on distribution of GABAA receptor complex in the rat brain using antibody against purified GABAA receptor complex

The distribution of the γ-aminobutyric acid (GABA)_A receptor/benzodizepine receptor/CI⁻ channel complex in the rat brain was examined immunohistochemically using the specific antibody against purified GABA_A receptor complex. The immunization of white albino rabbit with purified GABA_A receptor complex resulted in the formation of specific antibody as indicated by the immunoprecipitation test. Immunohistochemical examinations using the antiserum on rat brain slices by the peroxidase-antiperoxidase method revealed the presence of the following immunoreactive sites which coincided with a previous report using antibody againstl-glutamic acid decar☐ylase; ventromedial nucleus of hypothalamus, red nucleus, globus pallidus, zona compacta and zona reticulata of substantia nigra, layers of Purkinje cells and granular cells of cerebellum, layers III–V of cerebral cortex and stratum radiatum of hippocampus. These results strongly suggest that immunohistochemical application of the antibody against the purified GABA_A receptor complex is a useful tool for identifying GABAergic neurons having GABA_A receptor complex-mediated synapses.     

Activation of protein kinase C by phorbol dibutyrate potentiates [

By the use of combined in vitro radioreceptor binding and autoradiographic techniques with [³H]muscimol as a ligand, we analyzed the distribution of GABA_A receptor sites in the arteries of the circle of Willis as well as in the arteries and arterioles of the pial-arachnoid membrane in the rat. [³H]Muscimol was bound by sections of rat cerebral vessels in a manner consistent with the existence of GABA_A receptors, with K_d and B_max values of 46 nM and 0.60 pmol/mg tissue respectively. [³H]Muscimol was bound by the medial layer of cerebral arteries, while no specific binding was observed in the intima, the adventitia and the adventitial-medial border. These findings suggest that the vasodilatory action of GABA on in vitro preparations of cerebral vessels is mediated by muscular receptor sites. The posterior cerebral arteries are richer in [³H]muscimol binding sites than the anterior ones. Pial-arachnoid arterioles, which are of critical importance in controlling local cerebral blood flow, did not exhibit any significant binding of [³H]muscimol. These results may explain the difficulty in manipulating pharmacologically the cerebral tissue perfusion in intact animals using GABAergic agonists.     

Autoradiographic localization of the GABAA receptor agonist [H]muscimol in rat cerebral vessels

By the use of combined in vitro radioreceptor binding and autoradiographic techniques with [³H]muscimol as a ligand, we analyzed the distribution of GABA_A receptor sites in the arteries of the circle of Willis as well as in the arteries and arterioles of the pial-arachnoid membrane in the rat. [³H]Muscimol was bound by sections of rat cerebral vessels in a manner consistent with the existence of GABA_A receptors, with K_d and B_max values of 46 nM and 0.60 pmol/mg tissue respectively. [³H]Muscimol was bound by the medial layer of cerebral arteries, while no specific binding was observed in the intima, the adventitia and the adventitial-medial border. These findings suggest that the vasodilatory action of GABA on in vitro preparations of cerebral vessels is mediated by muscular receptor sites. The posterior cerebral arteries are richer in [³H]muscimol binding sites than the anterior ones. Pial-arachnoid arterioles, which are of critical importance in controlling local cerebral blood flow, did not exhibit any significant binding of [³H]muscimol. These results may explain the difficulty in manipulating pharmacologically the cerebral tissue perfusion in intact animals using GABAergic agonists.     

The GABAA/benzodiazepine receptor complex in rat brain neuronal cultures. Characterization by immunoprecipitation

The expression of the GABA_A/benzodiazepine receptor (GABAR/BZDR) complex in primary neuronal cultures from rat brain embryos has been investigated. The GABAR/BZDR complex was photoaffinity labeled with [³H]flunitrazepam [³H]FNZ and immunoprecipitated with subunit specific antibodies. These were the mAb 62-3G1 which is specific for the 57-kDa GABA binding subunit, and the rabbit antiserum A which recognizes the 51-kDa [³H]FNZ binding subunit. The results indicate that the cultured neurons express 5 different peptides of 51, 53, 54, 57 and 59 kDa that can be photoaffinity labeled with [³H]FNZ and that all of them are physically coupled to the GABA_A receptor. Most of the [³H]FNZ photolabeled peptides have similar mobilities to those found in the brain of the newborn rat. Nevertheless, some of the quantitative changes in the photolabeled peptides observed during the normal development of the rat brain were not observed or occurred at much slower pace in the cultured neurons.     

Propofol modulation of binding to GABAA receptors in guinea pig cerebral cortex

Binding of the radioligand to membranes prepared from the cerebral cortex of adult, male guinea pigs under equilibrium and non-equilibrium conditions was used to investigate the allosteric interaction between the intravenous general anesthetic propofol and the benzodiazepine site of the GABA_A receptor. Propofol induced a potentiation of binding with an EC₅₀ of 9±4 μM. Propofol increased the affinity for binding with no change in maximal binding. Propofol did not change the rate constant of association for binding to cerebral cortical membranes. In contrast, the rate constant for dissociation of was significantly decreased in the presence of propofol. These data demonstrate that propofol increases the affinity of the benzodiazepine site of the GABA_A receptor via a selective decrease in the rate constant for dissociation, and suggest a mechanism for the allosteric interaction between propofol and benzodiazepines at the GABA_A receptor.     

Zinc modulation of GABAA receptor-mediated chloride flux in rat hippocampal slices

We studied the effect of ZnCl₂ application on GABA_A receptor-mediated ³⁶Cl⁻ flux in microsacs prepared from whole rat hippocampus and in region-specific hippocampal slices. Slices were obtained from the dentate gyrus (DG), which contains the zinc-enriched hilar region, and from the CA1 region which contains lower levels of endogenous zinc. Muscimol (10 μM)-evoked ³⁶Cl⁻ flux was significantly reduced by ZnCl₂ (100 μM) in hippocampal microsacs. In hippocampal slices, muscimol (50 μM)-evoked ³⁶Cl⁻ efflux was higher in CA1 (112.5 ± 27.9% above basal efflux rate) than in DG slices (29.7 ± 5.6%). In the presence of ZnCl₂, the muscimol effect on efflux rate in CA1 and DG regions was decreased to 10.6 ± 5.4% and 6.9 ± 4.9%, respectively. Preincubation with the zinc chelator, tetrakis(2-pyridylmethyl)ethylenediamine (TPEN, 20 μM), caused a significant increase in muscimol-evoked ³⁶Cl⁻ efflux only in DG slices (57.2 ± 7.0%), suggesting that GABA_A receptors in the DG of rat hippocampus under physiological conditions may function under the inhibitory influence of endogenous chelatable zinc. In intracellular recordings, ZnCl₂ (100 μM) application had no effect on the responses to GABA applied perisomatically or in the dendritic region of CA1 neurons. The lack of Zn²⁺ effect on the postsynaptic GABA_A receptor-mediated responses suggests that the decreases of the ³⁶Cl⁻ efflux observed in the biochemical assays may be due to zinc action on neurons other than the principal pyramidal CA1 cells, and possibly the non-neuronal cell populations.