Biomedical subjects

D M Chuang

Publications and source records attributed to D M Chuang.

At least 55 records · Page 3Linked to original sources

PubMed1995

Carbamazepine induction of apoptosis in cultured cerebellar neurons: effects of N-methyl-D-aspartate, aurintricarboxylic acid and cycloheximide.

We have previously demonstrated that carbamazepine (CBZ) at concentrations above the therapeutic range is toxic to cultured cerebellar granule cells. Here, we ask whether the effect of CBZ involves neuronal apoptosis or necrosis. Treatment of cultured cerebellar granule cells with CBZ for 3 days resulted in a concentration-dependent fragmentation of DNA revealed as a laddered pattern in agarose gel electrophoresis, a phenomenon characteristic of apoptosis. Pretreatment of cells with N-methyl-D-aspartate (NMDA) blocked CBZ-induced DNA fragmentation and neurotoxicity as assayed by loss of mitochondrial activity with MTT or by [3H]ouabain binding to Na+/K(+)-ATPase. Aurintricarboxylic acid (ATA), a polyanionic dye, also markedly suppressed DNA fragmentation and cell death detected by morphological examination. A considerable level of DNA ladder formation was detected in untreated cells and this basal DNA fragmentation was also blocked by NMDA and ATA. Moreover, NMDA and ATA prevented CBZ-induced chromatin condensation as revealed by DNA binding with the fluorescent dye Hoechst 33258. Pretreatment of cells with cycloheximide, a protein synthesis inhibitor, prevented CBZ-induced cell death detected morphologically and attenuated CBZ-induced neurotoxicity assessed by mitochondrial activity and [3H]ouabain binding assays. Taken together, our results suggest that CBZ-induces death of cerebellar granule cells by an apoptotic process that is sensitive to NMDA, ATA and cycloheximide.

Animals↗

PubMed1995

Glyceraldehyde-3-phosphate dehydrogenase is over-expressed during apoptotic death of neuronal cultures and is recognized by a monoclonal antibody against amyloid plaques from Alzheimer's brain.

The age-induced apoptotic death of cerebellar neurons in culture is associated with over-expression of a 38-kDa particulate protein identified as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Both the age-induced apoptosis and the 38-kDa protein overexpression were effectively suppressed by the presence of tetrahydroaminoacridine, an antidementia drug, or aurintricarboxylic acid. This over-expressed 38-kDa protein and purified GAPDH were found to react with a monoclonal antibody (mAb), Am-3, which was raised against amyloid plaques from Alzheimer's brain, but not with mAb, AmT-1, which was produced using synthetic amyloid beta peptide. These results raise the possibility that GAPDH is also involved in the neurodegeneration during the development of Alzheimer's disease.

Alzheimer Disease↗

PubMed1995

Chronic buspirone treatment differentially regulates 5-HT1A and 5-HT2A receptor mRNA and binding sites in various regions of the rat hippocampus.

Groups of rats were treated with buspirone (1 mg/kg/day) for 21 days using osmotic minipumps implanted subcutaneously. After buspirone treatment, the 5-HT1A receptor mRNA levels were significantly decreased in the CA1 and CA2 of the hippocampus, but were markedly increased in the dentate gyrus (DG), CA3 and CA4. The level of the 5-HT1A receptor binding sites was not significantly changed in these subhippocampal areas. Buspirone treatment markedly increased 5-HT2A receptor mRNA levels in the DG, CA2, CA3 and CA4. This was accompanied by a significant increase in the level of 5-HT2A receptor binding sites in all subhippocampal regions. These results demonstrate that chronic buspirone treatment differentially regulates 5-HT1A and 5-HT2A receptor mRNA as well as their expressed binding sites in various regions of the hippocampus.

Animals↗

PubMed1995

Role of second messengers in agonist up-regulation of 5-HT2A (5-HT2) receptor binding sites in cerebellar granule neurons: involvement of calcium influx and a calmodulin-dependent pathway.

The present study was undertaken to examine the involvement of 5-HT2A receptor-mediated second messengers in the agonist-induced up-regulation of 5-HT2A receptors in cerebellar granule cells. Stimulation of these cells with a 5-HT2A/5-HT2C agonist, (+/-)-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane for 16 hr resulted in a marked increase in [3H]ketanserin binding to 5-HT2A receptors in intact cells. (+/-)-(2, 5-dimethoxy-4-iodophenyl)-2- aminopropane up-regulated, but not basal levels of 5-HT2A binding sites, were largely attenuated by actinomycin D and cycloheximide, suggesting an essential role for de novo RNA and protein synthesis in this up-regulation process. This receptor up-regulation was 5-HT2A receptor-mediated but was unaffected by short-term pretreatment with phorbol dibutyrate to attenuate (+/-)-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane-induced phosphoinositide hydrolysis or by treatment with staurosporine to inhibit protein kinase C. In contrast, blockade of Ca2+ channels by LaCl3 or SK&F 96365 and depletion of extracellular Ca2+ by EGTA preferentially decreased the up-regulated 5-HT2A receptor levels, suggesting a role of Ca2+ influx in the receptor up-regulation. The (+/-)-(2, 5-dimethoxy-4- iodophenyl)-2-aminopropane up-regulation was also prevented by inhibitors of calmodulin, calmidazolium and W-7. Moreover, the effect of (+/-)-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane was blocked by KN-62, a selective Ca2+/calmodulin kinase inhibitor and by H-7 and staurosporine at concentrations high enough to be nonselective for a specific type of protein kinase.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1995

Modulation of opioid binding associated with nuclear matrix and nuclear membranes of NG108-15 cells.

Opioid binding sites were found in nuclear matrix preparations from NG108-15 neurohybrid cells. Binding parameters of delta-specific radioligands indicated that high-affinity binding sites discovered in purified nuclei were present in nuclear membranes and nuclear matrix fractions. Agonists bind with low affinity, if at all, to nuclear matrix preparations. Neither sensitivity of agonist binding to the GTP analog 5-guanylylimidodiphosphate nor adenylyl cyclase activity were detected in this fraction, suggesting the presence of guanine nucleotide binding regulatory protein/effector uncoupled sites. Opioid inhibition of basal and forskolin-stimulated adenylyl cyclase activity was found in nuclear membrane preparations. Cycloheximide treatment of cells inhibited opioid binding to nuclear membrane fractions to a greater extent than that associated with membranes sedimenting at 20,000 x g (P20) or nuclear matrix. Colchicine, a microtubule disrupter and inhibitor of receptor internalization, caused up-regulation of nuclear membrane and P20 opioid receptors and a loss in nuclear matrix associated sites. Taxol, a microtubule stabilizing agent, prevented the effect of colchicine. Etorphine-elicited down-regulation increased nuclear matrix associated binding while diminishing that in nuclear membranes and P20 fractions. Agonist-induced desensitization completely abolished nuclear matrix binding. In vitro preincubation of nuclear matrix preparations with protein kinase A catalytic subunit mimicked the desensitization effect. Forskolin treatment of cells potentiated nuclear matrix and P20 binding. These data suggest that nuclear membrane opioid receptors represent newly synthesized molecules en route to the cell surface, whereas nuclear matrix contains internalized delta sites.

Analgesics↗

PubMed1994

Endothelin-1 increases the levels of mRNA and protein of muscarinic acetylcholine receptors and c-fos mRNA in cerebellar granule cells.

Endothelin-1 (ET-1) induced a time- and dose-dependent increase in the levels of mRNA of m2- and m3-muscarinic acetylcholine receptors (mAChRs) in cultured cerebellar granule cells. The levels of immunoprecipitable m3-mAChR protein and total mAChR binding sites were also increased by ET-1 treatment. The up-regulation of m2- and m3-mAChR was blocked by phorbol ester pretreatment to inhibit ET-1-stimulated phosphoinositide hydrolysis and was preceded by an increase in c-fos mRNA levels. Treatments that prevented ET-1-induced c-fos mRNA increase also abolished the subsequent m2- and m3-mAChR mRNA up-regulation, suggesting that c-Fos protein is involved in the ET-1-induced mAChR expression.

2-Aminopurine↗

PubMed1994

Programmed cell death: implications for neuropsychiatric disorders.

Programmed cell death, sometimes referred to as apoptosis, occurs through an active process requiring new gene transcription, in contrast to the passive cell death produced by metabolic toxins. Programmed cell death is an essential part of normal development, particularly in the nervous system. Spatial, temporal, or quantitative errors in the stimuli that initiate programmed cell death, or errors within the programmed cell death pathway itself, can result in an abnormal number of neurons and pathological neural development. Excesses and deficits in neuronal numbers have now been observed not only in typical neurodegenerative disorders such as Alzheimer's and Huntington's diseases, but also in several neurodevelopmental disorders, including schizophrenia and autism. Recent investigations into the mechanisms of cell death during C. elegans neurodevelopment thymocyte negative selection, and withdrawal of sympathetic ganglion cells trophic support provides intriguing clues to the etiology and pathophysiology of these neuropsychiatric disorders.

Apoptosis↗

PubMed1994

Effect of chronic haloperidol treatment on dopamine-induced inositol phosphate formation in rat brain slices.

The effects of chronic haloperidol administration on the accumulation of inositol phosphates were examined in rat brain slices pre-labeled with [3H]myo-inositol and incubated with various dopaminergic drugs. Rats were treated with haloperidol-decanoate or its vehicle (sesame oil) for two, four or six weeks. Dopamine and the selective D1 agonist, SKF38393, induced a significant increase in lithium-dependent accumulation of [3H]inositol monophosphate (IP1) in the frontal cortex, hippocampus and striatum of vehicle-treated animals, while the selective D2 agonist quinpirole did not show any effect on IP1 accumulation. The actions of dopamine and SKF38393 were blocked by the D1 antagonist, SCH23390, but not by the D2 antagonist, spiperone, in all three brain regions. Haloperidol treatment did not affect basal phosphoinositide turnover in the three brain regions. Four or six weeks of haloperidol treatment significantly decreased dopamine-induced IP1 accumulation in the striatum (by 30% and 25%, respectively), but not in the frontal cortex and the hippocampus. Four weeks of treatment with haloperidol significantly decreased IP1 levels in the striatal slices when measured in the presence of quinpirole. However, the accumulation of IP1 measured in the presence of SKF38393 was not significantly altered after haloperidol treatment. The loss of dopamine-sensitive IP accumulation was not observed in the presence of spiperone after haloperidol treatment. The number, but not the affinity, of [3H]sulpiride binding sites in the striatum was significantly increased (by 34-46%) after chronic haloperidol treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗

PubMed1994

Regulation of beta-adrenergic receptor mRNA in rat C6 glioma cells is sensitive to the state of microtubule assembly.

Microtubule disrupter, colchicine, and microtubule stabilizer, taxol, were used to determine whether microtubules play a role in beta-adrenergic receptor mRNA homeostasis and agonist-induced down-regulation in C6 glioma cells. Colchicine treatment had significant, differential, time-dependent effects on constitutive beta 1- and beta 2-adrenergic receptor mRNA levels. These effects stemmed from the action of colchicine on microtubules, because beta-lumicolchicine, an inactive isomer, had no effect, and nocodazole, a structurally unrelated microtubule disrupter, had similar effects. Colchicine treatment had little effect on the total number of beta-adrenergic receptor binding sites as measured by (-)-[125I]iodopindolol binding, but did alter the relative proportion of beta 1- and beta 2-adrenergic receptor subtypes. Colchicine also had no effect on basal cyclic AMP levels. In contrast to colchicine, taxol treatment had little long-term effect on either beta 1- or beta 2-adrenergic receptor mRNA levels. Taxol antagonized the effects of colchicine on total binding and mRNA levels. Taxol treatment increased basal cyclic AMP levels fourfold and potentiated (-)-isoproterenol-induced cyclic AMP production. Colchicine pretreatment completely inhibited (-)-isoproterenol-induced down-regulation of beta 1-adrenergic receptor mRNA, but not that of beta 2-adrenergic receptor mRNA. Taxol pretreatment had little effect on isoproterenol-induced beta-adrenergic receptor mRNA down-regulation. Colchicine pretreatment also attenuated isoproterenol-induced receptor down-regulation and inhibited agonist-stimulated cyclic AMP production. These effects of colchicine were antagonized by taxol. Whereas the effects of taxol and colchicine on isoproterenol-induced down-regulation of beta-adrenergic receptor mRNA are consistent with their effects on cyclic AMP production, those of colchicine in the absence of stimulation must involve other mechanisms. The data demonstrate that the state of microtubule assembly can affect cyclic AMP levels, beta 1- and beta 2-adrenergic receptor mRNA, and binding site levels in C6 glioma cells.

Animals↗

PubMed1994

Tryptamine induces phosphoinositide turnover and modulates adrenergic and muscarinic cholinergic receptor function in cultured cerebellar granule cells.

Tryptamine dose-dependently increased phosphoinositide (PI) hydrolysis by approximately fourfold in primary cultures of rat cerebellar granule cells (EC50 = 56 microM). The PI response stimulated by tryptamine was dependent on the presence of extracellular Ca2+ and Na+. Tryptamine-induced PI breakdown could be partially inhibited by pretreatment with 4 beta-phorbol 12-myristate 13-acetate but not pertussis toxin. The presence of tryptamine markedly attenuated PI responses induced by norepinephrine (NE) and carbachol, with no apparent effect on the responses to 5-hydroxytryptamine and glutamate. The inhibition of NE- and carbachol-induced PI turnover by tryptamine was dose dependent with IC50 values of approximately 0.4 and approximately 2.5 mM, respectively. Pretreatment of cells with tryptamine (0.5 mM) also attenuated NE- and carbachol-induced PI turnover, but failed to affect 5-hydroxytryptamine- and glutamate-induced responses. Furthermore, ketanserin, atropine, and prazosin did not have any effect on inositol phosphate formation induced by tryptamine. These observations indicate that tryptamine markedly increased Ca(2+)- and Na(+)-dependent PI turnover in cerebellar neurons and selectively inhibited NE- and carbachol-induced PI hydrolysis.

Animals↗

PubMed1994

Different signal transduction pathways are coupled to the nucleotide receptor and the P2Y receptor in C6 glioma cells.

In C6 glioma cells, extracellular ATP and other nucleotide analogs stimulated phosphoinositide (PI) breakdown and inhibited isoproterenol-induced cyclic AMP (cAMP) accumulation. The rank orders of potencies of 15 nucleotide analogs for both responses were clearly different. ATP and adenosine 5'-O-(3-thiotriphosphate) are the most potent agonists for stimulating PI hydrolysis; 2-methylthio-ATP (2-MeSATP) is the most potent agonist for inhibiting cAMP accumulation. P1-mediated responses of PI turnover and cAMP formation are not present in C6 glioma cells. Pertussis toxin (PTX) blocked the nucleotide-induced inhibition of cAMP accumulation but exerted no effect on inositol phosphate formation. Short-term treatment with phorbol 12-myristate 13-acetate inhibited both signal transduction pathways. The effects of three P2 purinergic antagonists, suramin, reactive blue and 4,4'-diisothiocyanatostilbene sulfonic acid (DIDS), on ATP- and 2-MeSATP-induced stimulation of PI turnover and inhibition of cAMP formation, respectively, were compared. For stimulating PI turnover, suramin is a competitive antagonist (pA2, 4.4); reactive blue and DIDS are noncompetitive antagonists at 30 microM and 100 microM, respectively. For the inhibition of cAMP formation, reactive blue and DIDS competitively antagonized the response of 2-MeSATP (pA2 values, 6.3 for reactive blue and 5.7 for DIDS); suramin was only slightly effective at 100 microM. It was concluded that the nucleotide receptor is linked to phospholipase C by a PTX-insensitive Gp protein and the P2Y receptor is linked to adenylyl cyclase by a PTX-sensitive Gi protein. Suramin is a competitive antagonist for the nucleotide receptor; reactive blue and DIDS are more selective antagonists for the P2Y receptor.

Adenylate Cyclase Toxin↗

PubMed1993

Tetrahydroaminoacridine increases m3-, but not m2-, muscarinic acetylcholine receptor mRNA levels in differentiating cerebellar granule cells.

We used Northern blot hybridization to determine whether 9-amino-1,2,3,4-tetrahydroacridine (THA), a potential antidementia drug, selectively altered the levels of muscarinic acetylcholine receptor (mAChR) mRNA in differentiating cerebellar granule cells. Granule cells were cultured for 8 days in media containing 15 mM K+, 25 mM K+ or 15 mM K+ plus 30 microM THA. High K+ markedly increased the levels of m2- and m3-mAChR mRNA in the surviving cells. In contrast, THA increased the levels of m3-mAChR mRNA, but had little or no effect on m2-mAChR mRNA levels. These results suggest that THA selectively up-regulates the synthesis of m3-mAChR mRNA.

Animals↗

PubMed1993

Effect of cocaine, lidocaine kindling and carbamazepine on batrachotoxin-induced phosphoinositide hydrolysis in rat brain slices.

Repeated administration of a subconvulsant dose of a local anesthetic will eventually induce seizures, a phenomenon similar to electrical kindling. We have investigated the effect of repeated lidocaine and cocaine administration on the phosphoinositide (PI) hydrolysis induced by batrachotoxin (BTX), a specific Na channel activator. Rats were injected with cocaine or saline daily for 6 days and PI hydrolysis was assayed in sliced frontal cortex. Cocaine treatment had no effect on BTX-induced PI hydrolysis while in vitro cocaine blocked the BTX effect. In a second experiment, rats received daily injections of lidocaine or saline. After a rat developed at least two seizures, it was sacrificed together with a rat receiving lidocaine injections which had never seized and a rat receiving saline injections. Basal, BTX and ibotenic acid (IBO; a glutamate receptor agonist)-stimulated PI hydrolysis did not differ among the three groups in slices of either hippocampus (HC) or piriform cortex (PC) though IBO-stimulated PI hydrolysis was much greater in the HC than in the PC. Neither in vitro nor in vivo carbamazepine altered the effect of cocaine on BTX-induced PI hydrolysis. These results demonstrate that local anesthetic kindling does not alter PI hydrolysis coupled to Na channel or IBO activation.

Animals↗

PubMed1993

Autoradiographic demonstration of an increase in muscarinic cholinergic receptors in cerebellar granule cells treated with tetrahydroaminoacridine.

The neurotrophic and neurosurviving effects of 9-amino-1,2,3,4-tetrahydroacridine (THA), a putative antidementia agent, were studied in cultured granule cells using biochemical and morphological methods. The addition of 30 microM THA to cultures grown in 15 mM K(+)-containing media markedly increased cell survival and enhanced [3H]N-methylscopolamine binding to muscarinic cholinergic receptors (mAChRs). Furthermore, receptor autoradiographic studies revealed that neuronal cells were labelled over both cell bodies and fibers by the [3H]receptor ligand. These observations provide direct evidence that THA promotes the expression of mAChR binding sites in differentiating cerebellar granule cells.

Animals↗

PubMed1993

Extracellular ATP stimulates inositol phospholipid turnover and calcium influx in C6 glioma cells.

Extracellular ATP caused a dose-dependent accumulation of inositol phosphates and a rise in cytosolic free Ca2+ ([Ca2+]i) in C6 glioma cells with an EC50 of 60 +/- 4 and 10 +/- 5 microM, respectively. The threshold concentration of ATP (3 microM) for increasing [Ca2+]i was approximately 10-fold less than that for stimulating phosphoinositide (PI) turnover. The PI response showed a preference for ATP; ADP was about 3-fold less potent than ATP but had a comparable maximal stimulation (11-fold of the control). AMP and adenosine were without effect at concentrations up to 1 mM. ATP-stimulated PI metabolism was found to be partially dependent on extracellular Ca2+ and Na+ but was resistant to tetrodotoxin, saxitoxin, amiloride, ouabain, and inorganic blockers of Ca2+ channels (Co2+, Mn2+, La3+, or Cd2+). In Ca(2+)-free medium, ATP caused only a transient increase in [Ca2+]i as opposed to a sustained [Ca2+]i increase in normal medium. The ATP-induced elevation of [Ca2+]i was resistant to Na+ depletion and treatment with saxitoxin, verapamil and nisoldipine, but was attenuated by La3+. The differences in the characteristics of ATP-caused P1 hydrolysis and [Ca2+]i rise suggest that ATP receptors are independently coupled to phospholipase C and receptor-gated Ca2+ channels. Because of the robust effect of ATP in stimulating PI turnover and the apparent absence of P1-purinergic receptors, the C6 glioma cell line provides a useful model for investigating the transmembrane signalling pathway induced by extracellular ATP. The mechanisms underlying the unexpected finding of [Na+]o dependency for ATP-induced PI turnover require further investigation.

Adenosine Triphosphate↗

PubMed1993

Long-term biphasic effects of lithium treatment on phospholipase C-coupled M3-muscarinic acetylcholine receptors in cultured cerebellar granule cells.

We have studied the long-term effects of lithium on neuronal morphology and the functional expression of phospholipase C-coupled m3-muscarinic acetylcholine receptors (mAChRs) in cerebellar granule cells. There was a biphasic dose-dependent effect on cell morphology following treatment with lithium for 7 days. At low concentrations (< or = 2 mM), this drug elicited an increase in the number and thickness of connecting nerve fibers, and the size of neuronal aggregates. At high concentrations (5-10 mM), lithium induced a severe deterioration of cell morphology, which ultimately resulted in neuronal death. Carbachol-induced phosphoinositide (PI) turnover was similarly affected by lithium treatment with a significant potentiation at concentrations up to 2 mM and a marked inhibition at doses higher than 5 mM due to lithium-induced neurotoxicity. The biphasic effect on mAChR-mediated PI hydrolysis was associated with corresponding changes in the maximal extent of carbachol-induced inositol phosphate accumulation, and was accompanied by similar changes in [3H]N-methyl-scopolamine binding to mAChRs and the levels of mRNAs for m3-mAChR and c-Fos. The up-regulation of m3-mAChR mRNA induced by low concentrations of lithium was associated with a down-regulation of m2-mAChR mRNA and no change in either total RNA or beta-actin mRNA. Lithium's effects on m2- and m3-mAChR mRNAs were time-dependent, requiring a pretreatment time of > or = 3 days. The biphasic effect was also demonstrated by the binding of [3H]ouabain to Na+, K(+)-ATPase, which was shown to be a convenient method for quantifying viable neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Agonist-induced desensitization of ATP receptor-mediated phosphoinositide turnover in C6 glioma cells: comparison with the negative-feedback regulation by protein kinase C.

In C6 glioma cells, ATP increased 3H-inositol phosphate (IP) accumulation in a dose-dependent manner. Preincubation of cells with ATP (100 microM or 1 mM) resulted in a time-dependent loss of the ability of ATP to stimulate phosphoinositide (PI) hydrolysis. The agonist-induced desensitization of ATP-stimulated PI hydrolysis developed rapidly, and appeared to be independent on the activation of protein kinase C (PKC). Thus, PKC inhibitors (staurosporine, H-7 and polymyxin B), depletion of PKC and diacylglycerol (DG) kinase inhibitors (R59002, R59949) had no effect on the homologous desensitization. ATP-induced PI breakdown was inhibited by a 10 min pretreatment with the PKC activator, phorbol 12-myristate 13-acetate (PMA) or octylindolactam V, with a comparable IC50 of 5 nM, but was unaffected by the biologically inactive 4-alpha-phorbol 12,13-didecanoate (4 alpha-PDD). The inhibition caused by PMA and octylindolactam V was completely prevented by staurosporine (1 microM) and partially prevented by H-7 (300 microM), H-8 (300 microM) and polymyxin B (300 micrograms/ml). In addition, PKC activator-induced inhibition was unchanged after ATP pretreatment, but disappeared after PKC depletion. The IP formation elicited by NaF was inhibited by PMA and octylindolactam V with a comparable IC50 value of 7.5 nM while was unchanged after ATP pretreatment. These results indicate that ATP receptors are present in the C6 glioma cells, and that these receptors are coupled to PI turnover and undergo homologous desensitization. The agonist-induced desensitization, unlike the negative-feedback regulation caused by PMA and octylindolactam V, does not seem to involve PKC activation.

Adenosine Triphosphate↗

PubMed1993

Long-term GABA treatment elicits supersensitivity of quisqualate-preferring metabotropic glutamate receptor in cultured rat cerebellar neurons.

In primary cultures of rat cerebellar granule neurons, GABA treatment (50 microM, 7 days) caused a withdrawal supersensitivity selective for the metabotropic glutamate receptors that mainly prefer L-glutamate, quisqualate and, to a lesser extent, kainate. The withdrawal supersensitivity was absent when 10 microM SR-95531 was coadministered with GABA during the treatment period, an event that suggests the GABAA receptors primarily produced the GABA treatment effect. This was supported further by the inability of baclofen treatment to mimic completely the treatment effect of GABA. Withdrawal from 7 days of baclofen treatment only produced a slight increase in the metabotropic effect of L-glutamate and carbachol. In addition, in untreated neurons, baclofen had no acute effect, whereas GABA inhibited the effect of L-glutamate and carbachol. The inhibitory effect of GABA was reversed by SR-95531 and was absent in neurons treated with GABA. These observations suggest the involvement of GABAA receptors and the apparent development of tolerance to GABA, respectively. Also, dependence on GABA may have occurred; the metabotropic effects of glutamate, kainate, and quisqualate were not altered in neurons maintained with GABA treatment.

Animals↗