GABA Stimulation Research Articles

Altered behavioral and neurochemical response to stress in benzodiazepine-withdrawn rats

Rats chronically treated with diazepam (DZM) (2 mg/kg/day i.p.) for 21 days, were subsequently tested at 24, 48, 72 and 96 h after the last injection in the elevated plus-maze. When compared with their respective controls, withdrawn animals showed an anxiogenic response since they exhibited a significant decrease in the % of time spent in the open arms, and a reduction in open and total arms entries at 24 and 48 h following the last benzodiazepine administration. No other behavioral differences were observed in the remaining groups. Ninety-six hours following DZM withdrawal, when the anxiogenic response was no longer evident, withdrawn rats showed a significant decrease in immobility time when exposed to a forced-swim test (FST). GABA stimulation of chloride uptake in cortical tissue was measured in BDZ-withdrawn rats or vehicle-treated animals with or without exposure to the FST. An enhanced chloride uptake following GABA stimulation was observed in vehicle-treated rats following the swimming trial. However, similar values of chloride uptake were found among rats withdrawn from DZM at 96 h either exposed or not to the FST and vehicle-treated animals without prior stress exposure. These findings show that BDZ withdrawal alters the neurochemical and behavioral response to a subsequent stressful experience. These lines of evidence may indicate that BZD withdrawal reduced the ability to develop an adaptive response to stress.

Alterations in brain monoamines and GABA A receptors in transgenic mice overexpressing TGFα

This study investigated the possibility that overexpression of transforming growth factor α (TGFα) changes those neurotransmitter systems that have been associated with behaviors found to be altered in the transgenic TGFα CD-1 mice. The female TGFα mice showed elevated levels of norepinephrine (NE) in the hypothalamus and serotonin (5-HT) in the cortex and brain stem when compared with nontransgenic CD-1 females. The concentrations of monoamines were not altered in the male transgenic brain. The 5-hydroxyindoleacitic acid (5-HIAA) 5-HT ratio was significantly reduced in the brain stem of the male TGFα mice and frontal cortex in the female transgenics. The binding of the [ 3H]GBR 12935-labeled DA transporter was lower in the frontal cortex in the transgenic male TGFα mice than in the female TGFα mice. No gender difference in dopamine (DA) transporter binding was noted between the nontransgenic male and female mice. Serotonin and GABA A receptors were measured only in males. No differences in the number of 5-HT 1A and 5-HT 2 receptors were found in the cortex or hippocampus. Maximal GABA stimulation of [ 3H]flunitrazepam binding in the forebrain hemispheres and cerebellar binding of an imidazobenzodiazepine, [ 3H]Ro 15-4513, were not different between transgenic and nontransgenic male mice. However, forebrain [ 35S]TBPS binding in male TGFa mice was less affected by the blockade of the GABA agonist sites by the specific GABA A antagonists SR 95531 and bicuculline than the binding of the controls, suggesting either altered endogenous GABA concentrations or a change in receptor populations. Taken together, the previously reported behavioral alterations in male TGFa mice, including increased levels of aggressive behavior, locomotor activity, voluntary alcohol consumption, and immobility in the swim test, or the altered behavioral responses to alcohol and monoamine uptake inhibitors, may be due to a reduced 5-HIAA 5-HT ratio, [ 3H]GBR 12935-labeled DA transporter binding, or altered regulation of [ 35S]TBPS binding by endogenous GABA in the brain. Reduced aggressive behavior and shortened immobility in the swim test in the female TGFa mice, on the other hand, might reflect elevated levels of NE and 5-HT in the brain. It is possible that TGFα-induced increase in plasma estrogen levels in the transgenic mice is the common mechanism of action that causes gender-specific changes in certain neurotransmitter systems.

GABA inhibition of lateral hypothalamic neurons: Role of reticular thalamic afferents

GABA and reticular thalamic (RT) stimulation induced inhibition of lateral hypothalamic (LH) neuronal activity was studied to determine if RT inhibitory input to the LH is mediated by stimulation of GABA receptors. Seven barrel electrodes were utilized to record simultaneously from the LH during microiontophoretic application of GABA, glycine, bicuculline, picrotoxin, and electrical stimulation of the RT. GABA produced an ejection current-related decrease in LH neuronal activity that was antagonized, in a dose-related manner, by the simultaneous administration of picrotoxin or bicuculline. LH neurons were much less sensitive to glycine inhibition which also was relatively insensitive to the GABA receptor antagonists. RT electrical stimulation provided a short latency inhibitory input to LH neurons that was mimicked by the microiontophoretic administration of GABA. In addition, the microiontophoretic application of picrotoxin or bicuculline blocked the RT stimulation induced inhibition of LH neuronal activity. These data indicate that the inhibitory effects of both GABA and RT synaptic inputs onto LH neurons are mediated by stimulation of GABA A receptors and substantiates the involvement of GABA as the inhibitory transmitter of thalamic to hypothalamic projections.

The increase in Cl- permeation across the Deiters' neuron membrane by GABA on its cytoplasmic side is abolished by protein kinase C (PKC) activators.

1. Cl- ion outward permeation across microdissected Deiters' neuron plasma membranes is augmented by GABA on the membrane cytoplasmic side. When these neurons are preincubated with a PKC activator, phorbol-12,13-dibutyrate (PdBu), there is a complex pattern of effects on basal and GABA-activated 36Cl- in-->out permeation. A distinct fact is an increase in basal Cl- passage and a disappearance of the 10(-6) M GABA effect at [PdBu] = 0.1 microM. 2. Likewise, 0.1 microM oleylacetylglycerol (OAG) treatment erases the effect completely, further supporting a role for PKC in modulating GABA-stimulated Cl- in-->out permeation. 3. The inactive ester, phorbol-12,13-didecanoate (Pdd), at 0.1 microM, does not affect GABA stimulation of Cl- passage. 4. High concentration (15-20 microM) of OAG and PdBu block the "intracellular" GABA efefct. However, the 20 microM PdBu effect is reversed by 30 microM H7. 5. These results indicate a role of endogenous PKC in Cl- extrusion by GABAA receptors on the cytoplasmic side of the Deiters' neuron membrane.

GABA induces Ca 2+ transients in astrocytes

By using the Ca 2+-sensitive indictor Fura-2/AM, the cytosolic Ca 2+ levels [Ca 2+] i were measured in type 1 astrocytes in rat cortical astroglial primary cultures, after stimulation with GABA, muscimol (GABA A agonist), or baclofen (GABA B agonist). We report the first evidence that stimulation of both GABA A and GABBA B receptors evokes Ca 2+ transients in type l astrocytes. Two types of Ca 2+ responses were seen: the single-phase curve, which was the most common, and the biphasic, which consisted of an initial rise that persisted at the maximal or submaximal level. Both types of Ca 2+ responses appeared with some latency. The responses were obtained in astrocytes grown for 12–16 days in culture and the response frequencies for all three agonists were 18% of the total number of examined cells. However, when the astrocytes were grown in a mixed astroglial/neuronal culture the response frequencies for all three agonists increased to 35% of the total number of examined cells. In some cells, the responses after GABA stimulation were blocked to baseline levels after exposure to bicuculline (GABA A antagonist). In other cells, bicuculline only slightly reduced the GABA-evoked responses, and the addition of phaclofen (GABA B antagonist) did not potentiate this partial inhibition. However, the muscimol-evoked rises in [Ca 2+] i were completely inhibited after exposure to bicuculline, while the responses after baclofen could only be partly blocked by phaclofen. GABA evoked rises in [Ca 2+] i which alternatively were inhibited (mostly) or persisted in Ca 2+-free buffer. The rises in [Ca 2+] i persisted, but were reduced, in Ca 2+-free buffer after stimulation with muscimol, but were inhibited after baclofen stimulation. The GABA uptake blockers guvacine, 4,5,6,7-tetrahydroisoxazolo(4,5-c)pyridin-3-ol and nipecotic acid were also able to reduce the GABA-evoked rises in [Ca 2+] i However, the L-type Ca 2+ chennel antagonist nifedipine failed to influence on the GABA-evoked Ca 2+ transients. The results suggest that type 1 astrocytes in primary culture express GABA receptors which can elevate [Ca 2+] i directly or indirectly via Ca 2+ channels and/or via release from internai Ca 2+ stores. The results also suggest that GABA can have intracellular Ca 2+-mobilizing sites since the GABA-evoked responses were reduced after incubation with GABA uptake blockers.

Altered stressor-induced changes in GABA A receptor function in the cerebral cortex of adult rats exposed in utero to diazepam

Prenetal administration of the anxiolytic drug diazepam (DZP), 2.5 mg/kg) to the pregnant rat over gestational days 14–20 altered function and stressor-induced responsiveness of the GABA A receptor in the cerebral cortex of exposed animals as adults. In Experiment 1, the impact of 15 min of restraint on chloride-facilitated benzodiazepine binding was evaluated in male and female rats at 70–90 days of age. Early exposure to DZP led to an enhanced potency of chloride on binding in both males and females. In Experiment 2, GABA stimulation of 36chloride uptake was measured in male rats at 35 or 70 days of age following 10 min of forced swimming at ambient temperature. In control animals, stressor-induced changes in receptor function were not evident until 70 days, and in DZP-exposed rats the stressor had no effect on receptor function at either age. These changes in GABA A receptor responsiveness induced by early exposure to DZP may underlie the disrupted behavioral responses to environmental challenge that have been previously reported.

Regional differences in the enhancement by GABA of [ 3H]zolpidem binding to ω 1 sites in rat brain membranes and sections

The potential heterogeneity in the allosteric coupling between GABA and ω 1 binding sites within the native GABA A receptor complex has been evaluated in the rat by measuring the enhancement by GABA of [ 3H]zolpidem binding to ω 1 site in membranes from three rat brain structures (neocortex, cerebellum and hippocampus) and brain sections. The maximal stimulatory effect of GABA was significantly higher (265 ± 47%) in cortical membranes than in cerebellar (165 ± 48%) or in hippocampal (118 ± 17%) membranes. These differences are not due either to the presence of different amounts of residual GABA in the membrane preparations or to the labeling, in presence of GABA, of binding sites other than ω 1 since: (1) the pharmacological properties of the [ 3H]zolpidem binding sites were similar in the three regions; (2) the degree of allosteric enhancement was unrelated to the relative proportion of ω 1 sites in each structure; and (3) GABA did not increase the B max for [ 3H]zolpidem. Regional differences in the effect of 100 μM GABA on [ 3H]zolpidem binding were also observed by quantitative autoradiography. Regions where the strongest (3–4-fold) effects of GABA in [ 3H]zolpidem binding were observed included the substantia nigra, lateral geniculate body, olfactory tubercule and red nucleus. A large increase in [ 3H]zolpidem binding was also demonstrated in the cingulate and frontoparietal cortices with higher effects in deep (4.2-fold) rather than in superficial layers (3.3-fold). Heterogeneous subregional increases in [ 3H]zolpidem binding in the presence of GABA were quantified within the cerebellum, hippocampus and superior colliculus. In the cerebellum the effect of this neurotransmitter was larger in the molecular (3.8-fold) than in the granular (2.2-fold) layer. In the hippocampus the effect of GABA was also heterogeneous with larger increases in CA1 and CA2 fields (3.5-fold) than in CA3 field (2.2-fold) and dentate gyrus (2.5-fold). Finally in the deep layers of the superior colliculus GABA stimulation of [ 3H]zolpidem binding was greater than the superficial layer. In the other structures examined the GABA-induced increase in [ 3H]zolpidem binding was less than 3-fold. The smallest stimulations were quantified in the entorhinal cortex (2.1-fold), amygdala (2.4-fold) and nucleus accumbens (1.7-fold). These results suggest that [ 3H]zolpidem sites are associated to, at least, two GABA A receptor subtypes that can be differentiated by their allosteric interaction between GABA and [ 3H]zolpidem sites.

Early postnatal treatment with muscimol transiently alters brain GABA A receptors and open-field behavior in rat

The long-term effects of treatment with muscimol, a GABA A agonist, and with ethanol, also shown to have a GABAergic profile of action, on subsequent behavior and on the binding parameters of GABA A receptors were studied in Wistar rats. Rats were given two daily injections of muscimol (0.1 mg/kg M1 group of 0.2 mg/kg M2 group) or ethanol (1.5 g/kg) between the 1st and 21st postnatal days, with saline-treated animals serving as controls. At the age of one month, the activity of the M2 rats was decreased in the open-field arena. At the age of four months, ethanol-treated rats consumed less 10% ethanol solution in a free choice situation. No changes were seen in the elevated plus-maze test. Maximal GABA stimulation of [ 3H]flunitrazepam binding was decreased in the cerebellum and hippocampus in M2 rats at the age of one month but not four months. A significant positive correlation was found in all rats between maximal GABA-stimulated [ 3H]flunitrazepam binding in the cerebellum and ambulation in the open-field arena at the age of one month. Ethanol intake correlated negatively with maximal GABA stimulation of [ 3H]flunitrazepam binding in the cerebral cortex. None of the treatments affected the cerebellar binding of an imidazobenzodiazepine, [ 3H]Ro 15-4513. There was an age-dependent decline in the efficacy and potency (EC 50) of the GABA enhancement of [ 3H]flunitrazepam binding in the cerebellum in all treatment groups. Basal binding of [ 3H]flunitrazepam to hippocampal and cerebellar membranes as well as binding of [ 3H]Ro 15-4513 to cerebellar membranes was also decreased as a function of age. The results suggest that muscimol exposure induces a transient change in GABA A receptor sensitivity. Also, open-field behavior, ethanol consumption and GABA A receptor mechanisms may be interrelated in the rat.

Neonatal administration of a GABA-T inhibitor alters central GABAA receptor mechanisms and alcohol drinking in adult rats.

Long-term effects of chronic treatment with a GABA-T (GABA-transaminase) inhibitor, ethanolamine O-sulphate (EOS) (200 mg/kg/day for the postnatal days 3-21) on the binding parameters of GABAA receptors, hypothalamic monoamines and subsequent behavior were studied in Wistar rats. At the age of 1 month, EOS-treated rats showed reduced activity in the open-field and, at the age of 4 months, their voluntary alcohol consumption was increased. No changes were seen in Porsolt's swim test or in the plus-maze test. Weight gain was significantly retarded in EOS-treated rats. Maximal stimulation of [3H] flunitrazepam binding by GABA was decreased in the cerebral cortex and the EC50-value for the GABA stimulation increased in the hippocampus in the EOS rats at the age of 4 months. EOS treatment did not alter the cerebellar diazepam sensitive and insensitive binding components of the imidazobenzodiazepine [3H]Ro 15-4513. No changes were observed in the hypothalamic monoamine concentrations. The results are in agreement with the idea that GABA-T inhibitor treatment permanently alters GABAA mechanisms. Moreover, altering the CNS GABA level during development increases adult alcohol intake in rat.

Alterations of GABA-A and dopamine D-2 brain receptors in dogs with portal-systemic encephalopathy

The binding characteristics of gamma-aminobutyric acid-A (GABA-A) receptors and the kinetic characteristics of the target enzyme of GABA synthesis in nerve terminals, glutamic acid decarboxylase (GAD), were studied in a dog model of portal-systemic encephalopathy obtained by porta-caval shunt performed in dimethylnitrosamine pretreated animals. Furthermore the properties of dopamine receptors and the levels of catecholamines of encephalopathic dogs were investigated. The mild stage of encephalopathy was characterized by an up-regulation of the inhibitory GABA-A receptors probably related to a decrease of GABA in nerve terminals since GAD was decreased and by a slight decrease of catecholamines and by an increased synthesis of octopamine associated with a decreased affinity of dopamine receptors. In the severe stage there was a selection of high affinity GABA-A receptors with an increased number of benzodiazepine recognition sites which were supersensitive to GABA stimulation, a decreased number of Dopamine D-2 receptors and a marked reduction of catecholamines. These data seem to suggest that the neurological disturbances of experimental portal-systemic encephalopathy might be the result of an imbalance between inhibitory and excitatory systems leading to a prevalence of the first one.

GABA-stimulated chloride uptake and enhancement by diazepam in synaptoneurosomes from rat brain during prenatal and postnatal development

The present study was undertaken to evaluate the ability of diazepam (DZ) to elicit a response in the brains of developing animals. γ-Aminobutyric acid (GABA)-stimulated 36Cl − uptake in the presence and absence of DZ was measured in synaptoneurosomal preparations from whole brain of fetal rats at 20 and 21 days gestation and from cerebral cortex of rats at 7, 14, 21, 28 and 60–90 days postnatal age. The ability of GABA to stimulate 36Cl − uptake in a concentration-dependent manner was evident from gestational day 20. The EC 50 for GABA stimulation but not the maximum stimulation increased significantly from day 20 to day 21 of gestation. Postnatally, only moderate changes in the EC 50 were evident, but the maximum 36Cl − transported increased significantly from 7 to 14 days and remained stable thereafter. DZ enhanced GABA-mediated 36Cl − influx from 20 days gestation, and this enhancement was seen as a decrease in the EC 50 for GABA stimulation. However, DZ also significantly increased maximum GABA-stimulated 36Cl − transport in synaptoneurosomal preparations at 21 days gestation and at 7 days postnatal age, a response to DZ not seen at older ages. DZ had a less robust effect on GABA-mediated 36Cl − transport at 28 days postnatal age than at any other age, a result consistent with functional observations of decreased responsivity to DZ in late juvenile, early adolescent rats. The benzodiazepine antagonist Ro 15-1788 prevented the effect of DA on GABA-stimulated 36Cl − uptake in tissue from 21-day fetuses and from 60- to 90-day-old adults. These studies demonstrate the early appearance of functional GABA-gated Cl − channels as well as the early developmental appearance of a functional link between the DZ recognition site and GABA-mediated Cl − influx, thus providing evidence that long-term consequences of prenatal exposure to DZ could result from a direct action of the drug on the fetal brain.

Regional GABA/benzodiazepine receptor/chloride channel coupling after acute and chronic benzodiazepine treatment

GABA/benzodiazepine coupling was evaluated in 8 regions of rat brain by the ability of GABA to stimulate 0.5 nM [ 3H]flunitrazepam binding. Rats were treated acutely with diazepam (p.o) or chronically with flurazepam, offered in the drinking water for 4 weeks, and compared to a pair-handled vehicle-treated control group. Regional variations in GABA/benzodiazepine coupling were found in control membranes. GABA increased benzodiazepine binding maximally (40%) in cerebellum and medulla, and least (25%) in olfactory bulb. A significant decrease in the effect of GABA was found in cortex of chronically treated rats immediately after, but not 2 days following treatment. The E max for GABA stimulation of [ 3H]flunitrazepam binding was significantly increased in medulla after acute treatment but was not altered after acute or chronic treatment in other brain areas evaluated. Treatment had no effect on the ability of bicuculline to inhibit [ 3H]flunitrazepam binding in cortex. Benzodiazepine /Cl − couping in cortex or hippocampus of acutely and chronically treated rats, evaluated by the ability of Cl − to stimulate specific [ 3H]flunitrazepam binding, was not changed. The results support the hypothesis that a functional uncoupling of the benzodiazepine recognition site from the GABA receptor in cortex, but not from the anion recognition site, may play a role in tolerance development.

Studies on the GABAergic system in cardiovascular control in normotensive and in sinoaortic denervated rats.

The cardiovascular effects of i.v. gamma-amino-beta-hydroxybutyric acid (GABOB) were investigated in rats anaesthetized with urethane. GABOB produced a dose-dependent hypotensive response. Treatment with GABA-A receptor antagonists prevented the GABOB response while the GABA stimulation by diazepam enhanced this response. The beta 1-adrenoceptor antagonist reduced the GABOB-induced hypotension but beta 2-adrenoceptor antagonists did not affect it. Picrotoxin, bicuculline or diazepam produced an increase in basal blood pressure. Fifteen days after sinoaortic denervation in rats the glutamic acid decarboxylase and the aminobutyric acid transaminase (GABA-T) activities were significantly reduced in dorsal pons and in anterior hypothalamus whereas GABA-T activity was increased in ventral medulla oblongata. Our results demonstrate that GABOB stimulates GABA-A receptors in anaesthetized rats and thus exerts a neuromodulatory effect on cardiovascular function. GABAergic neurotransmission participates in the sinoaortic deafferentation in rats.

GABA transmission, but not benzodiazepine receptor stimulation, modulates ethanol intake by rats

Adult male Long Evans were selected as ethanol preferring rats (DR) during 28 days. After this period, they were daily IP injected during 14 days with one of the next drugs: diazepam 1 mg·kg −1, alprazolam 1 mg·kg −1 (benzodiazepines), progabide 25 mg·kg −1 (GABA A and B agonist), nipecotic acid 150 mg·kg −1 (GABA uptake inhibitor), muscimol 0.2 mg·kg −1 (GABA A agonist), AOAA 10 mg·kg −1 (GABA decarboxylase inhibitor), baclofen 3 mg·kg −1 (GABA B agonist), or NaCl 0.9% (1 ml/200 g). During treatments, rats were isolated, had free access to food, and free choice between ethanol (12%) and water whose respective consumption were daily noted. Among treatments, only AOAA and baclofen were able to decrease significantly ethanol intake, without modifying total liquid intake. The action of these different drugs on GABA transmission and on ethanol intake was discussed. It was concluded that GABA A and benzodiazepine receptors were not implicated in ethanol intake, but that modulation of voluntary ethanol intake could be associated with a modification of GABA metabolism and/or stimulation of GABA B receptors. An intervention of GABA B receptors on noradrenergic pathways was also evoked.

Cyclodiene insecticides inhibit GABA A receptor-regulated chloride transport

The function of GABA A receptors in rat brain was assayed by GABA stimulation of 36Cl − influx into membrane microsacs. The evidence that the measured flux resulted from GABA Aa receptor activation was supported by the higher potency of muscimol than of GABA in inducing 36Cl − influx with Hill coefficients of 1.71 and 1.87, respectively, suggesting that two agonist molecules were binding to each receptor. Also, the GABA-induced 36Cl − influx was inhibited by the convulsants picrotoxinin and t-butylbicyclophosphorothionate (TBPS), and the competitive inhibitor (+)-bicuculline was more potent than (−)-bicuculline in inhibiting this 36Cl − influx. Furthermore, preincubation of the membranes with pentobarbital or diazepam increased the GABA A receptor's affinity for GABA as judged by the increased 36Cl − influx induced by the same GABA concentration without increasing maximal 36Cl − influx. The GABA A receptor was desensitized as shown by the dose-dependent reduction in GABA-induced 36Cl − influx that resulted from preincubation of the membranes with GABA. Seven cyclodienes inhibited the GABA-induced 36Cl − influx, with endosulfan I and endrin being more potent than their less toxic isomers endosulfan II and dieldrin, and the epoxides of heptachlor and aldrin (i.e., dieldrin) were more potent than their less toxic parent compounds. There was good correlation ( r = 0.9) between inhibition of [ 35S]TBPS binding to rat brain membranes by these cyclodienes and their inhibition of GABA-induced 36Cl − influx.

Actions of taurine on the GABA-benzodiazepine receptor complex solubilized from rat brain

The actions of taurine on the solubilized GABA-benzodiazepine receptor complex were investigated, and the results compared to those obtained with detergent-treated membrane-bound receptors. The receptor complex of adult rat brain was solubilized with Triton X-100 or CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulphonate). The properties of the solubilized GABA and flunitrazepam binding sites were similar to those in washed brain membranes. Taurine displaced GABA from its receptor sites and inhibited GABA stimulation of flunitrazepam binding to receptor complexes solubilized with Triton X-100. Thus the modulatory action of taurine on the receptor complex in washed membrane preparations was well preserved after this solubilization. No specific taurine binding to either Triton- or CHAPS-solubilized sample could be demonstrated.

Ranitidine potentiates ileum contractions caused by GABA and electrical stimulation

GABA-evoked contractions of the guinea pig ileum were significantly potentiated by the histamine H 2-receptor antagonist ranitidine in concentrations above 10 μM. To help define the mechanism of this interaction, the present study compared the effects of ranitidine on contractile responses of the guinea pig ileum to GABA, acetylcholine (ACh) and electrical stimulation of intrinsic cholinergic neurons. Ranitidine, at concentrations that potentiated responses to GABA, also potentiated contractions induced by transmural electrical stimulation. The ability of ranitidine to amplify these latter responses was antagonized by atropine. Contractile responses to exogenous A Ch, however, were unaffected by ranitidine at any concentration. These results suggest that prejunctional, rather than postjunctional mechanisms, are of primary importance in the interaction between ranitidine and GABA.

Neostriatal evoked inhibition and effects of dopamine on globus pallidal neurons in rat slice preparations

Spontaneous unit discharges were recorded extracellularly from globus pallidal (GP) neurons in rat slice preparations. The firing rates of GP neurons ranged from 2.0 to 24.0 spikes/s and their firing patterns were predominantly of two types: regular and irregular. Stimulation of the neostriatum evoked two distinct types of inhibition which were dependent on GP neuronal firing patterns, a brief inhibition (about 75 ms) followed by resetting rhythmic neuronal activities and a relatively long-term inhibition (about 100 ms). These inhibitions evoked by neostriatal stimulation were attenuated or completely blocked by bath application of either bicuculline of strychnine (2 × 10 −5−10 −4M) but not by a naloxone. Bath application of dopamine (10 −4−10 −3M) produced slow increases in the firing rates by 30–65% in about a half of GP neurons tested. Iontophoretic application of dopamine (10–20 nA) attenuated inhibition in GP neurons by 40–55% induced by either iontophoretically applied GABA (5–30 nA) or neostriatal stimulation without affecting their spontaneous firings. These results suggest that dopamine may produce change in the firing patterns of GP neurons by either acting directly or attenuating GABAergic inhibitory transmission from the neostriatum.

γ-aminobutyric acid, benzodiazepine binding sites and γ-aminobutyric acid concentrations in epileptic El mouse brain

All El mice provoked by postural stimulation since the age of 4 weeks had convulsions between 22 and 24 weeks of age, the refractory period ranging from 20 to 30 min. As compared to ddY mice, the maximal number of high-affinity [ 3H]muscimol binding sites was larger and the affinity was lower in the brains of the El mice, which had or had not experienced repeated seizures caused by postural stimuli. The basal and γ-aminobutyric acid (GABA)-stimulated [ 3H]flunitrazepam binding sites, and GABA concentration in the brains of the El mice did not differ from those of the ddY mice. In El mice following provoked convulsions, there were no temporary changes in [ 3H]muscimol binding, or [ 3H]flunitrazepam binding with or without exogenous GABA stimulation. The GABA concentration in the brains of the El mice increased immediately after seizures, and returned to the control values within 60 min.

Evidence that GABAA receptors mediate relaxation of rat duodenum by activating intramural nonadrenergic-noncholinergic neurones.

GABA produced rapid and transient relaxation of rat duodenum. Homotaurine (3-aminopropansulphonic acid) but not (+/-)-baclofen had a GABA-like effect. GABA-induced relaxation was almost completely inhibited by tetrodotoxin but was unaffected by atropine. Cross desensitization developed between GABA and homotaurine but not between GABA and (+/-)-baclofen. The concentration response curve to the relaxant effects of GABA was shifted to the right by both bicuculline and picrotoxin. However maximal relaxation was still produced by GABA in the presence of bicuculline but not in the presence of picrotoxin. GABA-induced relaxation was not affected by prazosin, yohimbine, propranolol or reserpine pretreatment. Field stimulation (0.1 Hz) of rat isolated duodenum in the presence of atropine and guanethidine produced relaxation similar to that produced by GABA. The ganglionic stimulant DMPP produced a similar effect. Neither Met-enkephalin, noradrenaline, 5-HT, histamine, VIP or arachidonic acid could be held responsible for GABA-induced neurogenic relaxation of rat duodenum. ATP produced relaxations which closely mimicked those produced by either GABA or field stimulation. Exposure to ATP desensitized responses to both GABA and field stimulation to about the same extent. ATP, GABA and field stimulation-induced relaxation was unaffected by either theophylline or indomethacin, but was significantly and selectively antagonized by apamin. In conclusion, GABA-induced relaxation of rat isolated duodenum is largely dependent upon activation of intra-mural nonadrenergic-noncholinergic neurones. The GABA receptor involved appears to be of the GABAA subtype. Circumstantial evidence is provided indicating that ATP might be the endogenous substance released by GABA.