gamma-Aminobutyric acid increases intracellular Ca2+ concentration in cultured cortical neurons: role of Cl- transport

Calcium-engaged Mechanisms of Nongenomic Action of Neurosteroids

BACKGROUND

Neurosteroids form the unique group because of their dual mechanism of action. Classically, they bind to specific intracellular and/or nuclear receptors, and next modify genes transcription. Another mode of action is linked with the rapid effects induced at the plasma membrane level within seconds or milliseconds. The key molecules in neurotransmission are calcium ions, thereby we focus on the recent advances in understanding of complex signaling crosstalk between action of neurosteroids and calcium-engaged events.

METHODS

Short-time effects of neurosteroids action have been reviewed for GABAA receptor complex, glycine receptor, NMDA receptor, AMPA receptor, G protein-coupled receptors and sigma-1 receptor, as well as for several membrane ion channels and plasma membrane enzymes, based on available published research.

RESULTS

The physiological relevance of neurosteroids results from the fact that they can be synthesized and accumulated in the central nervous system, independently from peripheral sources. Fast action of neurosteroids is a prerequisite for genomic effects and these early events can significantly modify intracellular downstream signaling pathways. Since they may exert either positive or negative effects on calcium homeostasis, their role in monitoring of spatio-temporal Ca2+ dynamics, and subsequently, Ca2+-dependent physiological processes or initiation of pathological events, is evident.

CONCLUSION

Neurosteroids and calcium appear to be the integrated elements of signaling systems in neuronal cells under physiological and pathological conditions. A better understanding of cellular and molecular mechanisms of nongenomic, calcium-engaged neurosteroids action could open new ways for therapeutic interventions aimed to restore neuronal function in many neurological and psychiatric diseases.

Emerging themes in GABAergic synapse development

Glutamatergic synapse development has been rigorously investigated for the past two decades at both the molecular and cell biological level yet a comparable intensity of investigation into the cellular and molecular mechanisms of GABAergic synapse development has been lacking until relatively recently. This review will provide a detailed overview of the current understanding of GABAergic synapse development with a particular emphasis on assembly of synaptic components, molecular mechanisms of synaptic development, and a subset of human disorders which manifest when GABAergic synapse development is disrupted. An unexpected and emerging theme from these studies is that glutamatergic and GABAergic synapse development share a number of overlapping molecular and cell biological mechanisms that will be emphasized in this review.

Survey of ALS-associated factors potentially promoting Ca2+ overload of motor neurons

The deleterious consequences of Ca(2+) overload are thought to be a probable cause of motoneuronal death in ALS, although the overloading mechanism is currently unclear. In this paper some ALS-linked factors are analysed with regard to their influence on Ca(2+ )influx into neurons. Intensive cortex activity can render motor neurons susceptible to stimulation of calcium-permeable glutamate NMDA-receptors; increase in CSF concentrations of glutamate, glycine, and norepinephrine supposedly can intensify these receptors' activity. Elevated CSF levels of GABA and reduced levels of serotonin can promote Ca(2+ )influx through glutamate AMPA-receptors and voltage-gated channels of L-, N-, and P-type. Additionally, brain ischaemia can contribute to Ca(2+ )overload of motor neurons. Thus, ALS is characterized by the unique combination of factors potentially able to promote the overload of motor neurons with calcium.

Effects of calcium channel blockers on antidepressant action of Alprazolam and Imipramine

Alprazolam is effective as an anxiolytic and in the adjunct treatment of depression. In this study, the effects of calcium channel antagonists on the antidepressant action of alprazolam and imipramine were investigated. A forced swimming maze was used to study behavioral despair in albino mice. Mice were divided into nine groups (n = 7 per group). One group received a single dose of 1% Tween 80; two groups each received a single dose of the antidepressant alone (alprazolam or imipramine); two groups each received a single dose of the calcium channel blocker (nifedipine or verapamil); four groups each received a single dose of the calcium channel blocker followed by a single dose of the antidepressant (with same doses used for either in the previous four groups). Drug administration was performed concurrently on the nine groups. Our data confirmed the antidepressant action of alprazolam and imipramine. Both nifedipine and verapamil produced a significant antidepressant effect (delay the onset of immobility) when administered separately. Verapamil augmented the antidepressant effects of alprazolam and imipramine (additive antidepressant effect). This may be due to the possibility that verapamil might have antidepressant-like effect through different mechanism. Nifedipine and imipramine combined led to a delay in the onset of immobility greater than their single use but less than the sum of their independent administration. This may be due to the fact that nifedipine on its own might act as an antidepressant but blocks one imipramine mechanism that depends on L-type calcium channel activation. Combining nifedipine with alprazolam produced additional antidepressant effects, which indicates that they exert antidepressant effects through different mechanisms.

Evidence for an extended duration of GABA-mediated excitation in the developing male versus female hippocampus

Gamma-aminobutyric acid (GABA) is as an excitatory neurotransmitter during brain development. Activation of GABA(A) receptors in neonatal rat hippocampus results in chloride efflux and membrane depolarization sufficient to open voltage sensitive calcium channels. As development progresses, there is a decline in the magnitude of calcium influx subsequent to GABA(A) receptor activation and the number of cells that respond to GABA with excitation. By the second postnatal week in the rat, GABA action in the hippocampus is predominantly inhibitory. The functional consequences and endogenous regulation of developmental GABA-mediated excitation remains under-explored. Hippocampal neurons in the newborn male and female rat respond to GABA(A) receptor activation with increased intracellular calcium and are susceptible to GABA-mediated damage -- both being indicative of the excitatory nature of GABA. In the present study we observed that by postnatal day 7, only males are susceptible to GABA(A) agonist-induced damage and respond to GABA(A) agonist administration with elevated levels of intracellular calcium in cultured hippocampal neurons. By postnatal day 14, GABA(A) agonist administration was without effect on intracellular calcium in both males and females. The age-related sex difference in the impact of GABA(A) receptor activation correlates with a sex difference in chloride co-transporter expression. Males have elevated protein levels of pNKCC1 on PN0 and PN7, with no sex difference by PN14. In contrast, females displayed elevated levels of KCC2 on PN7. This converging evidence infers that sex affects the duration of GABA(A) receptor-mediated excitation during normal hippocampal development, and provides a mechanism by which the effect is mediated.

GABAA-receptor-mediated increase in intracellular Ca2+ concentration in the regenerating retina of adult newt

We used optical recording with the Ca(2+)-sensitive dye, fura-2, in living slice preparations from the newt retina at different stages of regeneration. gamma-Aminobutyric acid (GABA) induced pronounced [Ca(2+)](i) rise in progenitor cells and differentiating ganglion cells in the 'intermediate' stage of retinal regeneration. This [Ca(2+)](i) rise became less pronounced at the beginning of synapse formation in the late regenerating retina. At the late period of the late regenerating retina with the IPL thickness comparable to that of the control retina, GABA-induced [Ca(2+)](i) rise became undetectable or sometimes a small decrease in [Ca(2+)](i) was observed in regenerated ganglion cells. In contrast, N-methyl-d-aspartate (NMDA)-induced [Ca(2+)](i) rise appeared in premature ganglion cells and became prominent gradually as the regeneration proceeded. The [Ca(2+)](i) rise to GABA was mediated by GABA(A) receptors. This was shown by inhibition of GABA-induced Ca(2+) response with the preincubation of the GABA(A) receptor antagonist, bicuculline. The [Ca(2+)](i) rise due to GABA was suppressed in the absence of extracellular Ca(2+) or in the presence of the L-type voltage-gated Ca(2+) channel blocker, verapamil, suggesting that Ca(2+) may be entered through L-type Ca(2+) channels. Transient appearance of [Ca(2+)](i) rise to GABA during regeneration and origin of GABA-induced [Ca(2+)](i) rise were similar to those in the developing retina [J. Neurobiol. 24 (1993) 1600]. These similarities may suggest that common mechanisms may control neurogenesis and/or synaptogenesis during development and regeneration.

Role of sex hormones in the sexually dimorphic expression of KCC2 in rat substantia nigra

KCC2 is a neuronal-specific potassium chloride cotransporter. The level of KCC2 expression is a factor determining whether GABA(A) receptor agonists depolarize or hyperpolarize neurons. Substantia nigra reticulata (SNR) neurons of male postnatal day 15 (PN15) rats have low KCC2 mRNA expression and respond to GABA(A) receptor activation with depolarization and activation of calcium-regulated gene expression. Female PN15 SNR neurons have high KCC2 mRNA expression and GABA(A) receptor agonists cannot activate calcium-dependent signaling processes. We investigate whether sex hormones regulate KCC2 mRNA expression in PN15 rat SNR. Using in situ hybridization, we studied the effects of acute (4 h) or prolonged (52 h) subcutaneous (s.c.) administration of testosterone (100 microg), dihydrotestosterone (180 microg) or 17beta-estradiol benzoate (5 microg) on KCC2 mRNA expression in male and female PN15 rat SNR. Different doses of estradiol (1 and 10 microg s.c., 4 h) were also acutely administered in female PN15 rats. Controls received oil injections. Separate groups of PN15 male rats were pretreated with antagonists of L-type voltage-sensitive calcium channels (L-VSCCs) [nifedipine, 100 mg/kg s.c.] or GABA(A) receptors [bicuculline, 2 mg/kg intraperitoneally (i.p.)] or their vehicles, 30 min before estradiol (5 microg s.c., 4 h). Testosterone and dihydrotestosterone upregulated KCC2 mRNA in both sexes. Estradiol downregulated KCC2 mRNA in males but not in females. Both acute and prolonged hormonal administration had similar effects. In male PN15 SNR, nifedipine and bicuculline decreased KCC2 mRNA acutely and prevented further downregulation of KCC2 mRNA by estradiol. Estradiol therefore downregulates KCC2 mRNA in male PN15 SNR, by interacting with the GABA(A) receptor and L-VSCC signaling pathway.

Functional cooperation between neurosteroids and D2 dopamine antagonists on KCl-evoked [3H]noradrenaline release: modulation by calcium channel blockers

It has recently been proposed that neurosteroids, such as dehydroepiandrosterone sulphate and pregnenolone sulphate, interfere with the dopamine system in the central nervous system. According to our previous report showing that the butyrophenone, spiperone, slightly enhances the evoked release of [3H]-noradrenaline ([3H]NA) in the presence of these sulphated steroids, the present study was carried out to document the putative interplay between steroids and spiperone, which is known to be a prototypic D2 dopamine antagonist and also a 5-HT2 serotonin antagonist. For this purpose, the paradigm of KCl-evoked [3H]NA release from preloaded rat hippocampal slices was used to investigate the interactions between neurosteroids, spiperone and the voltage-sensitive calcium channels (VSCCs). The selective 5-HT2 serotonin antagonist ritanserine was ineffective, whereas sulpiride, a selective D2 dopamine antagonist mimicked the action of spiperone, thus suggesting that the blockade of D2 dopamine receptors accounted for the modulatory effect of spiperone on neurosteroid-induced modulation of evoked [3H]NA release. In addition, this facilitation of KCl-evoked [3H]NA release by the combination of a steroid and a D2 dopamine antagonist was partially inhibited by the L- and N-type VSCC blockers nifedipine and omega-conotoxin GVIA, respectively. The present results provide in-vitro functional evidence for the putative role of VSCCs in the interplay between steroids and D2 dopamine receptors.

Effect of beta-estradiol on voltage-gated Ca(2+) channels in rat hippocampal neurons: a comparison with dehydroepiandrosterone

We investigated the effects of beta-estradiol, dehydroepiandrosterone and dehydroepiandrosterone sulfate on intracellular calcium concentration ([Ca(2+)](i)) increases induced by gamma-aminobutyric acid (GABA), high K(+) and N-methyl-D-aspartate acid (NMDA) in cultured hippocampal neurons. Acute treatment with beta-estradiol, dehydroepiandrosterone and dehydroepiandrosterone sulfate inhibited the GABA-induced [Ca(2+)](i) increases to the similar extent. Tamoxifen, an estrogen receptor antagonist, did not block the inhibitory effects of beta-estradiol. On the other hand, GABA type A (GABA(A)) receptor antagonists, picrotoxin and bicuculline, blocked the GABA-induced [Ca(2+)](i) increases. Previously, we demonstrated that GABA- and high K(+)-induced [Ca(2+)](i) increases were commonly mediated by voltage-gated calcium channels (VGCCs). Therefore, we examined the effects of these steroids on the high K(+)-induced [Ca(2+)](i) increases. The inhibitory effect of beta-estradiol on the high K(+)-induced [Ca(2+)](i) increases was much greater than that of dehydroepiandrosterone and dehydroepiandrosterone sulfate. beta-Estradiol inhibited the NMDA-induced [Ca(2+)](i) increases with an IC(50) of 51.8 microM and NMDA responses were reduced to half in the presence of 10 micro M nifedipine, indicating that the NMDA-induced [Ca(2+)](i) increases also involved VGCCs. Further, we examined the inhibitory effect of beta-estradiol on the high K(+)-induced [Ca(2+)](i) increases in the presence of a N-type VGCCs antagonist, 1 microM omega-conotoxin, or a L-type VGCCs antagonist, 10 microM nifedipine. The IC(50) value of beta-estradiol alone (45.5 microM) was similar to that of omega-conotoxin (33.1 microM), while the value combined with nifedipine was reduced to 2.2 microM. beta-Estradiol also abolished the positive modulatory effect of L-type VGCCs agonist, 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]pyridine-3-carboxylic acid methyl ester (Bay K 8644). Our results showed that the inhibitory mechanism of beta-estradiol is different from that of dehydroepiandrosterone and dehydroepiandrosterone sulfate and beta-estradiol may act primarily at L-type VGCCs.

Potassium chloride depolarization mediates CREB phosphorylation in striatal neurons in an NMDA receptor-dependent manner

Potassium chloride (KCl)-depolarization has been used to study the properties of L-type Ca2+ channel-mediated signal transduction in hippocampal neurons. Calcium influx through L-type Ca2+ channels stimulates a second messenger pathway that transactivates genes under the regulatory control of the Ca2+-and cyclic AMP-responsive element (CRE). Here, we show that in striatal neurons, but not in hippocampal neurons, CRE binding protein (CREB) phosphorylation and CRE-mediated gene expression after KCl-depolarization depends on functional NMDA receptors. This difference in NMDA receptor dependence is not due to different properties of L-type Ca2+ channels in either neuronal type, but rather to different neuron-intrinsic properties. Despite this variation, the second messenger pathway activated by KCl requires Ca2+/calmodulin (CaM) kinase for CREB phosphorylation in both neuronal types. We conclude that depolarization by KCl works differently in striatal and hippocampal neurons.

Afferent ingrowth and onset of activity in the rat trigeminal nucleus

A novel in vitro preparation, consisting of the rat brainstem with the trigeminal ganglion attached, has been used to study the anatomical and functional development of the trigeminal nucleus from embryonic day (E)13 to postnatal day (P)6. Neurobiotin injections into the trigeminal ganglion showed that primary afferents had reached the trigeminal tract by E13 and had grown simple, mainly unbranched, collaterals into all levels of the nucleus by E15. By E17, these collaterals were extensively branched, with occasional boutons present. Patches of intense neurobiotin-labelled terminals, corresponding to whisker-related patterns, were first seen at E20 and became clearer over the next few days. Terminal arbours at this stage were relatively localized and densely branched, with many boutons. Responses from the trigeminal nucleus were recorded with suction electrodes, following stimulation of the trigeminal ganglion. Recordings from the main sensory nucleus showed a postsynaptic response was first present at E15. At E16, bath application of AP5 and DNQX showed that the response contained both NMDA and AMPA components, with NMDA predominating (75%). The NMDA : AMPA ratio remained high until P1, then gradually declined to 50% by P6. The postsynaptic response was also reduced by bath application of bicuculline, indicating the presence of a GABAA-mediated excitatory component. GABAergic excitation was present at all ages but was maximal from E20 to P1, the age at which whisker-related patterns are developing. It is hypothesized that both GABAergic excitation and NMDA receptor activation play a role in the consolidation of trigeminal connections, and are thus important in the development of whisker-related patterns.

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

It is well established that GABAA-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 GABAA-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 GABAA 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 GABAA receptor-mediated current (EGABA-A) switched from depolarizing to hyperpolarizing within the first postnatal week. EGABA-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 EGABA-A, indicating that a lack of Cl- extrusion during P0-3 is mainly responsible for early GABAA-ergic excitation. At the network level, blockade of GABAA 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 GABAA-mediated modulation of the respiratory rhythm. GABAA receptor-activated inhibition may therefore be necessary for frequency modulation in the respiratory network beginning on the fourth postnatal day in the mouse brainstem.

Intracellular [Cl(-)] modulates synchronous electrical activity in rat neocortical neurons in culture by way of GABAergic inputs

The influence of GABAergic neurons on spontaneous electrical activities of neocortical neurons in culture, which was estimated to be about 9.5% of the total neurons by immunohistochemistry, was examined using dual whole-cell recording. Synchronized depolarization or hyperpolarization was observed in recorded neurons with pipettes containing low [Cl(-)] solution, while synchronized bursting of action potentials (APs) was observed with pipettes containing high [Cl(-)] solution. Spontaneous currents (SCs) were synchronous in all pairs tested with either pipettes containing low or high [Cl(-)] solution and spontaneous outward currents (SOCs) observed at around -30 mV were sensitive to the GABA-A receptor antagonist, bicuculline. Their reversal potential (V(rev)) was linearly related to the logarithm of Cl(-) activity in the pipette (-56.9 mV/decade). The intracellular chloride concentration was estimated from the V(rev) of SCs with gramicidin perforated-patch recordings and was between 5.9 and 28.1 mM (mean: 13.0 mM). These results suggest that GABA depolarized some neurons and hyperpolarized others, depending on the E(Cl). Bicuculline decreased the frequency of periodic depolarized potentials and increased their amplitudes. However, perfusion with low [Cl(-)] bath solution did not decrease the frequency. Our data indicate that recurrent subthreshold electrical activities by GABAergic inputs along with glutamatergic inputs take part in deterring synchronized bursting and that intracellular [Cl(-)] can modulate this bursting.

Neuroactive neurosteroids as endogenous effectors for the sigma1 (sigma1) receptor: pharmacological evidence and therapeutic opportunities

Neuroactive neurosteroids, including progesterone, allopregnanolone, pregnenolone and dehydroepiandrosterone, represent steroid hormones synthesized de novo in the brain and acting locally on nervous cells. Neurosteroids modulate several neurotransmitter systems such as gamma-aminobutyric acid type A (GABA(A)), N-methyl-D-aspartate (NMDA) and acetylcholine receptors. As physiologic consequences, they are involved in neuronal plasticity, learning and memory processes, aggression and epilepsy, and they modulate the responses to stress, anxiety and depression. The sigma1-receptor protein was recently purified and its cDNA was cloned in several species. The amino-acid sequences are structurally unrelated to known mammalian proteins, but shared homology with a fungal sterol C8-C7 isomerase. The sigma1-receptor ligands exert a potent neuromodulation on excitatory neurotransmitter systems, including the glutamate and cholinergic systems. Consequently, selective sigma1 agonists show neuroprotective properties and beneficial effects in memory processes, stress and depression. The evidence of a direct interaction between neurosteroids and sigma1 receptors was first suggested by the ability of several steroids to inhibit the binding of sigma1-receptor radioligands in vitro and in vivo. A crossed pharmacology between neurosteroids and sigma1-receptor ligands was described in several physiological tests and behavioral responses. This review will detail the recent evidence for a common mechanism of action between neurosteroids and sigma1-receptor ligands and focus on the potential therapeutic interests of such interaction in the physiopathology of learning and memory impairments, stress, depression and neuroprotection.

Nucleus accumbens dopamine release modulation by mesolimbic GABAA receptors-an in vivo electrochemical study

The role of GABA receptors in regulating the mesolimbic dopamine (DA) system and drug reinforced behaviors has not been well characterized. Using fast-cyclic voltammetry, the effects of specific GABA receptor modulation on DA release in the nucleus accumbens (NAcc) and heroin self-administration (SA) behavior was investigated. The GABAA agonist muscimol, administered either intravenously or directly into the ventral tegmental area (VTA), significantly increased DA release in the NAcc in 7 of the 10 rats tested. DA release decreased in the remaining three rats; both effects were blocked by pretreatment with the GABAA receptor antagonist bicuculline. In contrast, the GABAB agonist baclofen decreased, while 2-OH-saclofen (a GABAB antagonist) increased DA release in the NAcc. However, when VTA GABAB receptors were previously activated or inactivated by microinjections of baclofen or 2-OH-saclofen, systemic injections of muscimol caused an inhibition of NAcc DA release. These results suggest that GABAA receptors may be co-localized on both DA neurons and non-DA (GABAergic) interneurons in the VTA, with the effects of GABAA determined by the net effect of both direct inhibition and indirect disinhibition of DA neurons. Finally, although a DA releaser, muscimol was neither self-administered in drug naive rats, nor did it substitute for heroin in rats previously trained to self-administer heroin, suggesting that GABAA receptors appear to play a complex role in mediating drug reinforcement, depending upon the dynamic functional state of GABAA receptors on both tegmental DA and non-DA neurons.

Effect of dantrolene on KCl- or NMDA-induced intracellular Ca2+ changes and spontaneous Ca2+ oscillation in cultured rat frontal cortical neurons

Dantrolene has been known to affect intracellular Ca2+ concentration ([Ca2+]i) by inhibiting Ca2+ release from intracellular stores in cultured neurons. We were interested in examining this property of dantrolene in influencing the [Ca2+]i affected by the NMDA receptor ligands, KCl, L-type Ca2+ channel blocker nifedipine, and two other intracellular Ca2(+)-mobilizing agents caffeine and bradykinin. Effect of dantrolene on the spontaneous oscillation of [Ca2+]i was also examined. Dantrolene in microM concentrations dose-dependently inhibited the increase in [Ca2+]i elicited by NMDA and KCl. AP-5, MK-801 (NMDA antagonists), and nifedipine respectively reduced the NMDA and KCl-induced increase in [Ca2+]i. Dantrolene, added to the buffer solution together with the antagonists or nifedipine, caused a further reduction in [Ca2+]i to a degree similar to that seen with dantrolene alone inhibiting the increase in [Ca2+]i caused by NMDA or KCl. At 30 microM, dantrolene partially inhibited caffeine-induced increase in [Ca2+]i whereas it has no effect on the bradykinin-induced change in [Ca2+]i. The spontaneous oscillation of [Ca2+]i in frontal cortical neurons was reduced both in amplitude and in base line concentration in the presence of 10 microM dantrolene. Our results indicate that dantrolene's mobilizing effects on intracellular Ca2+ stores operate independently from the influxed Ca2+ and that a component of the apparent increase in [Ca2+]i elicited by NMDA or KCl represents a dantrolene-sensitive Ca2+ release from intracellular stores. Results also suggest that dantrolene does not affect the IP3-gated release of intracellular Ca2+ and that the spontaneous Ca2+ oscillation is, at least partially, under the control of Ca2+ mobilization from internal stores.

Differential regulation by pregnenolone sulfate of intracellular Ca2+ increase by amino acids in primary cultured rat cortical neurons

We investigated the effects of pregnenolone sulfate (PS) on the [Ca2+]i increase induced by gamma-aminobutyric acid (GABA) and N-methyl-D-aspartate (NMDA) using fluorescence imaging. PS inhibited the 50 microM GABA-induced increase in [Ca2+]i in a dose-dependent manner with an IC50 of 30 microM. The inhibitory effect of PS was apparent within 5 min and was in a non-competitive manner, suggesting that PS may act directly to the membrane level but indirectly to the GABA binding sites. Our previous study has already shown that the GABA-induced Ca2+ increase involves GABAA receptors and the similar pathway to a high K(+)-induced Ca2+ response (Takebayashi et al., 1996). Because 50 microM of PS could not inhibit a 25 mM K(+)-induced Ca2+ increase, it seems likely that the site of the inhibitory action of PS on the GABA-induced Ca2+ increase may be independent of the pathway of the high K(+)-induced Ca2+ response, but rather at GABAA receptor complex. In contrast, PS potentiated the 50 microM NMDA-induced increase in [Ca2+]i in a dose-dependent manner. The magnitude of the NMDA response was approximately doubled in the presence of 100 microM of PS. However, PS did not affect the acetylcholine(Ach)-induced increase in [Ca2+]i. Furthermore, corticosterone had little effect on the GABA- and NMDA-induced Ca2+ increases, indicating that the alteration of the Ca2+ response is specific for PS. In conclusion, it is suggested that PS modulates differentially [Ca2+]i increase induced by GABA and NMDA.