Actions of ATP on the soma of bullfrog primary afferent neurons and its modulating action on the GABA-induced response

Adenosine 5′-triphosphate (ATP) produced a long-lasting depolarization inbullfrog spinal ganglion cells. Since the ATP-induced slow depolarization was associated with an increase in membrane resistance and a reverse in polarity (about—90mV) which was most likely brought about by an inactivation of membrane potassium conductance. In some cells, a rapid and transient depolarization followed by the long-lasting depolarization was produced by ATP and it was markedly reduced in sodium-free solution. ATP reversibly augmented the GABA-induced depolarization which was caused by ionophoresis of GABA. These observations were confirmed using a voltage clamp method. Dose-response analysis of the action of ATP on the GABA-induced response suggests that the facilitatory action of ATP on the GABA response is effected on the GABA receptor channel complexes without changing the GABA affinity.     

gamma-Aminobutyric acid (GABA) causes consistent depolarization of neurons in the guinea pig supraoptic nucleus due to an absence of GABAB recognition sites

The action of gamma-aminobutyric acid (GABA) in the supraoptic nucleus was investigated using guinea pig brain slices. GABA produced a membrane depolarization accompanied by a decrease in the input resistance. The action of GABA was concentration-dependent throughout a wide range of concentrations (10(-7)-10(-3) M). In none of the cells examined, a membrane hyperpolarization was observed. The reversal potential for the depolarization induced by GABA was about 25 mV positive to the resting membrane potential. The amplitude of the GABA-induced depolarization was increased to 1.5 X the control by reducing the external Cl- from 134.2 mM to 10.2 mM. The action of GABA was readily antagonized by relatively low concentrations of bicuculline (10(-5) M). The action of GABA in the hippocampus or in the anterior hypothalamus was markedly different from that in the supraoptic nucleus, i.e. GABA produced both depolarizing and hyperpolarizing responses in the hippocampus and consistently a hyperpolarization in the anterior hypothalamus. The depolarizing but not the hyperpolarizing response in the hippocampus was selectively blocked by picrotoxin (2 X 10(-5) M) or by bicuculline (10(-5) M). The depolarizing component was dependent on the external Cl- concentration and had a reversal potential similar to that of the depolarization induced by GABA in the supraoptic nucleus. The hyperpolarizing component was resistant to bicuculline and had a reversal potential about 30 mV negative to the resting membrane potential.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electrophysiological evidence for the existence of GABAA receptors in cultured frog melanotrophs

The neurotransmitter GABA exerts a biphasic effect on alpha-melanocyte-stimulating hormone (alpha-MSH) secretion from pars intermedia cells: GABA induces a rapid and transient stimulation followed by a sustained inhibition of alpha-MSH release. In the present study, we have investigated the effect of GABA on the electrophysiological properties of frog melanotrophs in primary culture using the patch-clamp technique in the whole cell configuration. In all cells tested, GABA stimulated an inward current and induced depolarization. A transient period of intense firing was consistently observed at the onset of GABA administration. During the depolarization phase, the membrane potential reached a plateau corresponding to the Cl- equilibrium potential. When repeated hyperpolarizing pulses were applied, an increase of membrane conductance was observed throughout the response evoked by GABA. The effect of GABA was abolished by the chloride channel blocker picrotoxin, and by antagonists of GABAA receptors (bicuculline and SR 95531). The depolarizing action of GABA was mimicked by muscimol, an agonist of GABAA receptors. Taken together, our results indicate that the rapid and transient stimulation of alpha-MSH release induced by GABA can be accounted for by activation of a chloride conductance which causes membrane depolarization. These data support the notion that the transient stimulation of alpha-MSH secretion induced by GABA can be accounted for by membrane depolarization which provokes activation of voltage-operated calcium channels. Since no evidence was found for GABA-induced hyperpolarization, the intracellular mechanisms leading to the strong inhibitory effect of GABA on alpha-MSH secretion remain to be elucidated.     

Voltage-clamp studies of the inhibition of gamma-aminobutyric acid response by glucocorticoids in bullfrog primary afferent neurons

Acute effects of glucocorticoids on the response to gamma-aminobutyric acid (GABA) were examined in primary afferent neurons in bullfrog spinal ganglia, using intracellular and voltage-clamp recording techniques. Prednisolone and hydrocortisone (5 microM to 1 mM) caused a dose-dependent decrease in the amplitude of GABA-induced depolarization, while having no effect on the membrane potential and resistance of the neuron. Prednisolone depressed the muscimol-induced depolarization. Nipecotic acid, a blocker of GABA uptake, did not influence the inhibitory action of prednisolone. Voltage-clamp analyses showed that the inward current induced by an iontophoretic application of GABA (GABA current) was suppressed by prednisolone and hydrocortisone. The depression of the GABA current is neither due to a blockage of open channels nor a facilitation of the desensitization of GABA receptors. Prednisolone shifted the dose-response curve of the GABA current downward. The double-reciprocal (Lineweaver-Burk) plot showed that the maximum GABA current was reduced by prednisolone, suggesting a non-competitive antagonism. These results suggest that glucocorticoids suppress the GABA-induced chloride current, decreasing the number of functional channels associated with GABAA receptor.     

GABA-activated Cl- channels in astrocytes of hippocampal slices

We used kainic acid-lesioned hippocampal slices to examine glial responses to the inhibitory neurotransmitter GABA in a neuron-free environment. Slices were prepared from rats which received intracerebroventricular injections of kainic acid 1 month prior to experiments. Astrocytes (membrane potential averaged 81.4 +/- 5.5 mV; n = 46; mean +/- SD) were impaled in the CA3 region of the slice, which was completely depleted of neurons. GABA (1 mM) application by bath perfusion depolarized membrane potential from 1 to 5 mV. The GABA-induced depolarization was not affected by a tetrodotoxin (1 microM)/high-Mg2+/low-Ca2+ solution. Changing the Cl- equilibrium potential by reducing extracellular Cl- greatly increased the GABA-induced depolarization. Muscimol mimicked the GABA response, while picrotoxin (0.1 mM), an antagonist of the GABA-activated Cl- channel, resulted in a 60% blockade. The barbiturate, pentobarbital (0.1 mM), and the benzodiazepine agonist, flunitrazepam (1 mM), enhanced the depolarization by 60 and 40%, respectively. A blocker of glial GABA uptake, beta-alanine (1 mM), did not affect the GABA-induced membrane depolarization, indicating that the depolarization is not caused by electrogenic uptake of the amino acid. The pharmacological properties of the GABA response of astrocytes from the hippocampal slice is similar to that previously described for cultured astrocytes from rat cerebral hemispheres. Our data suggest that GABA receptors, which are coupled to Cl- channels, are also expressed by astrocytes in an intact tissue.     

GABA depolarizes neurons in the rat striatum: an in vivo study

GABA, applied by iontophoresis to striatal neurons of the rat in an in vivo preparation, depolarized the membrane potential and decreased the input resistance in a dose-dependent manner. The null potential of the GABA depolarization was about -50 mV. In addition, a fading of the GABA-induced response was observed for prolonged and relatively high amino acid application. We conclude that GABA has a depolarizing effect on striatal cells. This is in line with recent in vitro works describing a depolarizing effect of GABA in the rat neostriatum.     

Effects of GABA and baclofen on pyramidal cells in the developing rabbit hippocampus: an 'in vitro' study

Using the in vitro hippocampal slice preparation, we have investigated the effects of gamma-aminobutyric acid (GABA) and its analogue beta-(p-chlorophenyl)-GABA (baclofen) on CA1 and CA3 pyramidal cells in the developing rabbit hippocampus. Somatic applications: both GABA and baclofen, when applied to CA1 pyramidal cells from immature tissue, led to cell depolarization from resting membrane potential; this baclofen depolarization may be indirectly mediated. In contrast, CA3 pyramidal cells at the same age were primarily hyperpolarized by both drugs. In mature tissue, both GABA and baclofen applied at the soma induce cell hyperpolarizations. Dendritic applications: immature CA1 cells responded to dendritic GABA and baclofen application with depolarizations associated with increased cell excitability; here, too, the baclofen depolarization may be due to indirect 'disinhibition'. Both depolarizing and hyperpolarizing responses were recorded in immature tissue when GABA was applied to CA3 pyramidal cell dendrites: baclofen produced only hyperpolarizations. In mature CA1 cells, dendritic GABA application produced membrane depolarization, but dendritic baclofen application produced hyperpolarizations. In mature CA3 cells, dendritic GABA and baclofen application produced predominant hyperpolarizations. Mature CA1 pyramidal cells appear to retain some of the GABA-induced depolarizations characteristic of immature tissue. In contrast, mature CA3 neurons show only hyperpolarizing responses to GABA and baclofen application. In all cases, responses to GABA and baclofen are associated with a decrease in cell input resistance. We conclude that the GABAergic receptor/channel complexes mature differently in the CA1 and CA3 regions of the hippocampus.     

GABA-induced responses in Purkinje cell dendrites of the rabbit cerebellar slice.

Pressure applications of GABA localized to Purkinje cell somas in a rabbit cerebellar slice produced uniphasic hyperpolarizing responses, whereas applications of GABA that were directed at the Purkinje cell dendrites produced complex, triphasic responses with hyperpolarizing and depolarizing components. Both somatic and dendritic application of GABA elicited fast hyperpolarization (GABAhf), but dendritic application also elicited a slower depolarization (GABAd) and a later, long-lasting hyperpolarization (GABAhl). All three types of responses were accompanied by increased conductance. Use of either GABA antagonist, bicuculline or picrotoxin, eliminated the GABAhf and GABAd responses but left the GABAhl response intact. Pressure delivery of the GABA agonist, baclofen, to the dendrites but not the soma elicited a GABAhl response. Application of baclofen paired with membrane depolarization sufficient to elicit local, calcium-dependent dendritic spiking produced a persistent reduction in the GABAhl response, whereas alternating presentations of baclofen and membrane depolarization or presentations of baclofen alone could not. The fact that GABA and baclofen inhibited Purkinje cell activity in the rabbit cerebellar slice and that picrotoxin and bicuculline eliminated some, but not all of the components of the GABA response suggests the presence of both GABAA and GABAB receptors. The ability of baclofen to inhibit Purkinje cells if it was applied to the dendrites but not if applied to the soma suggests that GABAB receptors are located predominantly on Purkinje cell dendrites. The pairing-specific change in the baclofen response suggests the existence of GABAB-mediated modifiability of Purkinje cell dendrites.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of neuroleptics on the GABA receptor of cat primary afferent neurons

Butyrophenones (haloperidol and pimozide) at low concentrations (0.05-1.0 micro M) inhibited the GABA-induced depolarization of cat primary afferent neurons, while at high concentrations (greater than 10 micro M) they enhanced the GABA-depolarization. These actions of the butyrophenones were not accompanied by any measurable change in the dissociation constant of the GABA-GABA receptor interaction, whereas their inhibitory and facilitatory influences on the GABA-depolarization were associated with reduction and increase in the cell membrane resistance, respectively. Further analysis showed that the reduction of membrane resistance by low concentrations of butyrophenones was brought about by an increased sodium and potassium conductance and that the increase in membrane resistance by a high concentration of these drugs was caused by a reduced sodium and potassium conductance. In contrast to the butyrophenones, a typical phenothiazine derivative (chlorpromazine) at concentrations of 0.1-100 micro M did not affect the GABA-depolarization. The results suggest that butyrophenones do not mimic the action of GABA as originally proposed by Janssen, but alter the GABA-induced depolarization indirectly by modifying the electrically excitable portion of the cell membrane. Chlorpromazine, a phenothiazine, on the other hand, has no recognizable effects on either the GABA-receptor membrane or the electrically excitable membrane.     

A depolarizing inhibitory response to GABA in brainstem auditory neurons of the chick

Neurons in the brainstem auditory nuclei, n. magnocellularis and n. laminaris, of the chick are contacted by terminals containing the inhibitory neurotransmitter γ-aminobutyric acid (GABA). In this report we describe the physiological response of these neurons to GABA using an in vitro slice preparation. In brainstem auditory neurons, GABA produced a depolarization of up to 20 mV and an associated decrease in input resistance. This depolarization was inhibitory; action potentials generated by orthodromic synaptic drive, antidromic stimulation and intracellular current injection were prevented by GABA application. The GABA response still occurred when synaptic transmission was prevented by perfusing the slice with a medium containing low Ca²⁺ and high Mg²⁺ concentrations. Thus, the effects of GABA were directly on the postsynaptic neuron and not via an interneuron. Whole-cell voltage clamp of neurons revealed that the reversal potential of the inward current was approximately −45 mV, suggesting that the channel responsible for this response is not selective for Cl⁻ or K⁺. Pharmacological analyses suggest that this GABA receptor has properties distinct from those typical of either GABA_a or GABA_b receptors. Although a similar response was observed with the GABA_a agonist, muscimol, it was not blocked by the GABA_a antagonist, bicuculline. The response was not evoked by the GABA_b agonist, baclofen, and was not blocked by the GABA_b antagonist phaclofen. This unusual depolarizing response is not a common feature of all brainstem neurons. Neurons located in the neighboring medial vestibular nucleus show a more traditional response to GABA application. At resting potential, these neurons show a hyperpolarizing or biphasic response associated with a decrease in input resistance and inhibition of their spontaneous activity. GABA-induced responses in the medial vestibular nucleus are blocked by bicuculline. These results suggest that an unusual form of the GABA receptor is present in the brainstem auditory system of the chick. It is possible that this form of GABA receptor provides an efficient mechanism for inhibiting the relatively powerful EPSPs received by brainstem auditory neurons, or it may play a trophic role in the afferent regulation of neuronal integrity in this system.     

Effect of benzodiazepines and pentobarbital on the GABA-induced depolarization in cultured astrocytes

We have previously shown that cultured astrocytes from neonatal rat cerebral cortex are depolarized by GABA. The underlying ionic mechanism, activation of a Cl- conductance and responses to an agonist and antagonists were found to be similar to those of the neuronal GABAA receptor (Kettenmann et al.: Brain Research 404:1-9, 1987; Kettenmann and Schachner: Journal of Neuroscience 5:3295-3301, 1985). To characterize further the pharmacological properties of the GABA receptor we have tested the influence of pentobarbital and benzodiazepines on the GABA response. Pentobarbital potentiated and prolonged the GABA-induced depolarization and enhanced the velocity of the depolarization. Agonists of the neuronal benzodiazepine receptor, flunitrazepam, diazepam, and midazolam, increased the GABA-induced depolarization. As in neurons, an antagonist of the benzodiazepine receptor, Ro 15-1788, blocked the flunitrazepam-induced enhancement of the GABA response. In contrast to their effects on neurons, the inverse agonists Ro 22-7497 and DMCM increased the GABA-induced depolarization. The ligand of the putative peripheral benzodiazepine binding site, Ro 5-4864, did not show consistent effects on the GABA response. These studies confirm that cultured astrocytes express GABAA receptors. This receptor is similar to the neuronal GABAA receptor with regard to Cl- conductance and its pharmacological responses to muscimol, bicuculline, picrotoxin, pentobarbital, and benzodiazepine agonists and an antagonist, but it is different in its responses to inverse agonists of the benzodiazepine site. The physiological role of the glial GABAA receptor is at present unknown.     

gamma-Aminobutyric acid directly depolarizes cultured oligodendrocytes

gamma-Aminobutyric acid (GABA) depolarizes in a dose-dependent manner approximately one-third of all immunologically identified oligodendrocytes in cultures of mouse spinal cord. Measurements of [K+]o indicate that the response to GABA is not due to K+ released from active neurons. The depolarization is not accompanied by a change in cell input resistance. Replacement of sodium in the bathing solution abolishes the entire response, whereas ouabain only inhibits the repolarization phase. Current clamp experiments with two separate intracellular electrodes show that the depolarization increases at more positive potentials while the repolarization increases at more negative potentials. Bicuculline and picrotoxin but not nipecotic acid reduce the GABA effect. Pentobarbital and chlordiazepoxid also reduce the GABA-induced depolarization. Muscimol produces a depolarization similar to that of GABA. Heterogeneity in the oligodendrocyte population is indicated by the observation that some cells respond to both GABA and glutamate, while others respond only to one and some are not responsive to either.     

The dendritic response to GABA in CA1 of the hippocampal slice

Application of GABA in the dendritic region of pyramidal cells elicits a depolarization which, in fact, is the sum of a hyperpolarizing and a depolarizing process. At the reversal potential of the depolarizing response (-42 mV) the GABA-induced current fluctuations do not have a minimum. Consequently, a conductance change to more than one ion is involved. Cl- is in part responsible, Ca2+ is not because Mn2+ and Mg2+ do not change the response. Whether Na+ is involved is uncertain. Substitution with choline had no effect but choline may permeate through the membrane during the depolarizing response. Nipecotic acid inhibits a Na+-GABA uptake mechanism but does not change the dendritic response.     

Effects of GABA on spontaneous [Ca]c dynamics and electrical properties of rat adrenal chromaffin cells

A large fraction of rat adrenal chromaffin cells (about 60%) shows spontaneous [Ca²⁺]_c oscillations and spontaneous action potentials. In the present study the effects of γ-aminobutyric acid (GABA) on the spontaneous [Ca²⁺]_c oscillations and electrical properties of rat adrenal chromaffin cells were investigated using Fura-2 [Ca²⁺]_c imaging and patch clamp techniques. GABA inhibited the spontaneous [Ca²⁺]_c oscillations in a reversible manner. The effect of GABA was mimicked by the GABA_A and GABA_C receptor agonist, muscimol, but not by the GABA_B receptor agonist, baclofen. Moreover, the effect was antagonized by the selective GABA_A receptor antagonist, bicuculline. The mode of the inhibition was all-or-none, and the threshold concentration at which the inhibition occurred varied widely (50 μM to over 1 μM) from cell to cell. GABA (100 μM) elicited a transient burst of action potentials of diminished amplitude, which was followed by arrest of action potentials. Further analysis showed that GABA (100 μM) induced inward whole-cell currents in voltage-clamp experiments and produced depolarization and membrane conductance increase in current-clamp experiments. The effects appear to be due to an increase in chloride ion conductance since the degree of GABA-induced depolarization depended on the pipette [Cl⁻]. These results suggest that GABA, acting through GABA_A receptor, may play a role in the physiological regulation of rat adrenal chromaffin cells by directly modifying the discharge of spontaneous action potentials and spontaneous [Ca²⁺]_c oscillations.     

Role of GABAB receptors in GABA and baclofen-induced inhibition of adult rat cerebellar interpositus nucleus neurons in vitro

Previous studies suggested that the postsynaptic GABA(B) receptors of deep cerebellar nuclear neurons of adult rats were not activated by selective GABA(B) receptor agonist baclofen or endogenous GABA released by cerebellar cortical Purkinje cells, although the receptors have been demonstrated to exist in the deep cerebellar nuclei. In this study, cerebellar slices of adult rats were prepared for testing effects of GABA, baclofen and muscimol (selective GABA(A) receptor agonist) on cerebellar interpositus nucleus (IN) neurons. Perfusing slices with GABA (10-1000 microM), baclofen (1-30 microM) and muscimol (1-100 microM) respectively produced a dose-dependent inhibitory response on the IN neurons (n = 39, 62 and 50), which was not blocked by low-Ca(2+)/high-Mg(2+) medium (n = 5, 6 and 6), supporting a direct postsynaptic action of these GABAergic agonists. Moreover, both selective GABA(B) receptor antagonist CGP35348 and selective GABA(A) receptor antagonist bicuculline were capable of partially blocking the inhibitory response of IN neurons to GABA (n = 14 and 11), suggesting that the GABA-induced inhibition may contain two components, a GABA(B) receptors-mediated component and a GABA(A) receptors-mediated one. Further experiments revealed that not only muscimol (n = 50) but also baclofen (n = 62) suppressed IN cells' activity. The baclofen-induced inhibition was selectively blocked by CGP35348 (n = 12) but not by bicuculline (n = 8), whereas the muscimol-induced inhibition was selectively antagonized by bicuculline (n = 8) instead of CGP35348 (n = 9). These results indicate that GABA(B) receptors in the IN neurons can be activated not only by GABA but also by baclofen, suggesting that besides GABA(A) receptors, GABA(B) receptors may also be involved in mediating the inhibitory effect of GABA on cerebellar IN neurons of adult rats.     

Depolarization- and transmitter-induced changes in intracellular Ca2+ of rat cerebellar granule cells in explant cultures

Digital imaging of the Ca indicator fura-2 has been used to study the responses of developing granule cells in culture to depolarization and transmitter action. Unstimulated cells bathed in Krebs saline exhibited cytoplasmic Ca ion concentrations, [Ca2+], that were generally in the 30-60 nM range. Exposure of cells to high-potassium (25 mM) saline depolarized the membrane potential and produced an immediate rise in [Ca2+] that recovered within 2-3 min in normal saline. The response grew progressively larger over the first 20 d in culture. Transient increases in [Ca2+] to levels greater than 1 microM were observed after 12-14 d in vitro, at which time the cells displayed intense electrical activity when exposed to high K. At this stage, the increases were attenuated by blocking action potential activity with TTX. In TTX-treated or immature cells, in which the transient phase of the Ca change was relatively small, a second exposure to high K typically produced a much larger Ca response that the initial exposure. The duration of this facilitation of the response persisted for periods longer than 5 min. Application of the neurotransmitter GABA induced a transient increase in membrane conductance, with a reversal potential near resting potential (approx. -60 mV), and caused an intracellular Ca2+ increase that outlasted the exposure to GABA by several minutes. Glutamate, or kainate, induced an increase in membrane conductance but with a reversal potential more positive than spike threshold. These agents also elevated intracellular Ca2+, but unlike the case with GABA, this Ca response reversed rapidly upon removal of the transmitter. The facilitatory effect of repeated exposures to high-K saline, as well as the persistent Ca elevation following a brief GABA application, suggests that granule cells possess the capability of displaying activity-dependent changes in Ca levels in culture.     

Effects of GABA on CA3 pyramidal cell dendrites in rabbit hippocampal slices

Using the in vitro rabbit hippocampal slice preparation, we have investigated the effects of gamma-aminobutyric acid (GABA) iontophoresis on CA3 pyramidal cell dendrites. The predominant response (70% of the cells tested) was a hyperpolarization associated with a 30% decrease in cell input resistance (Rm). These hyperpolarizations displayed a very pronounced voltage dependency: they were decreased by cell depolarization and flattened by hyperpolarization. Bicuculline methiodide (BMI, 50 microM) did not abolish this response, nor did intracellular iontophoresis of chloride ions. In 5% of the cells, an additional hyperpolarization was obtained with longer ejection times; it reversed close to the reversal potential of the early component of the IPSP. In 25% of the cells, dendritic GABA application produced a depolarization. This response was reversed with cell membrane depolarization and was associated with a large (80%) decrease in Rm. The depolarizations were abolished by BMI (50 microM) and greatly increased by increasing the intracellular chloride concentration. None of the responses to GABA were affected by blockade of synaptic transmission. We conclude that the predominant response of CA3 pyramidal cell dendrites to GABA application is a hyperpolarization mediated by GABAB receptors and probably carried by potassium ions. The depolarizing responses are mediated via GABAA receptors and depend on an increase in chloride permeability.     

Pharmacological properties of gamma-aminobutyric acid-, glutamate-, and aspartate-induced depolarizations in cultured astrocytes

Differentiated glial fibrillary acidic protein-positive astrocytes in homogeneous cultures of early postnatal rat cerebral hemispheres respond by membrane depolarization to gamma-aminobutyric acid (GABA), glutamate, and aspartate with a threshold concentration of approximately 10(-5) M. The GABA-induced depolarization is antagonized by two blockers of the neuronal GABAA receptor, picrotoxin and bicuculline, but is not affected by the uptake blockers beta-alanine or nipecotic acid. An agonist of the GABAA receptor, muscimol, produces a dose-response curve similar to that of GABA, whereas the agonist of the GABAB receptor, baclofen, did not alter the membrane potential. When repetitive pulses of GABA are given to one cell, its responsiveness depends on the time interval between pulses. Within 30 sec after termination of the first pulse the cell remains unresponsive to the second pulse. With increased time intervals between the pulses, reactivity toward GABA recovers. Five minutes after the first pulse the cell regains 75% of its initial depolarization peak. Aspartate results in a depolarization similar in size and time course to that induced by glutamate. The glutamate agonists, quisqualate and ibotenate, and kainate are less potent than glutamate. N-Methyl-D-aspartate has no effect on the membrane potential of astrocytes. The pharmacological features of the glutamate response are therefore similar to those of the receptor mediating neuronal glutamate transport.     

Heterogeneity of GABA(A) receptor-mediated responses in the human IMR-32 neuroblastoma cell line

The gamma-aminobutyric acid (GABA) response profiles of IMR-32 human neuroblastoma cells were examined using whole-cell patch clamp and RT-PCR techniques. GABA activated a concentration-dependent and bicuculline-sensitive current, and RT-PCR revealed the expression of multiple GABA(A) receptor subunit mRNAs (alpha(1), alpha(3), alpha(4), beta(1), beta(3), gamma(2), and delta). A pharmacological profile of the GABA-induced current was derived using several subunit-selective agents. Diazepam, which requires the presence of a gamma subunit in order to modulate GABA(A) receptor-mediated responses, potentiated GABA-induced currents in a subset of IMR-32 cells. Two populations of GABA-activated currents were also evident based on sensitivity to modulation by zinc. Comparison of zinc- and diazepam-induced modulation of GABA-induced current responses in the same cells revealed an inverse correlation between these two modulators. No differences, however, were observed with the GABA(A) receptor modulators loreclezole, allopregnanolone, and pentobarbital. Thus, IMR-32 cells maintained in culture are heterogeneous in terms of expression of GABA(A) receptor isoforms.