The effects of acute and chronic treatment with ethanol on the function of A1 adenosine receptor in the rat cerebellar cortex were investigated. Acute administration of ethanol (0.5–5 g/kg) had no effect on the binding of the A1-receptor agonist [3H]2-chloro-N6-cyclopentyladenosine ([3H]CCPA) or that of the antagonist [3H]8-cyclopentyl-1-3-dipropylxanthine ([3H]DPCPX) in rat cerebellar cortical membranes. Rats were rendered ethanol dependent by repeated forced oral administration of ethanol (12–18 g/kg per day) for 6 days. [3H]CCPA binding was increased by 23% in cerebellar cortical membranes prepared from rats killed 3 h after ethanol withdrawal compared with saline-treated animals. The increase in [3H]CCPA binding was still apparent 12–24 h after the last ethanol administration, but was no longer detectable 3–6 days after ethanol withdrawal. In contrast, the binding of [3H]DPCPX was not modified in the cerebellar cortex of rats killed at various times after ethanol withdrawal. The acute administration of CCPA [0.25–1 mg/kg, intraperitoneally (IP)] suppressed the tremors and audiogenic seizures apparent 24 h after ethanol withdrawal. Moreover, repeated coadministration of CCPA (0.5 mg/kg, IP, four times daily) and ethanol did not prevent the generation of audiogenic seizures during withdrawal but completely prevented mortality. Finally, CCPA antagonized with similar potencies and efficacies the isoniazid-induced convulsions observed in control and ethanolwithdrawn rats. These results indicate that long-term treatment with intoxicating doses of ethanol enhances [3H]CCPA binding but does not reduce the anticonvulsant efficacy of CCPA or the function of A1 adenosine receptors.

The effect of brief inhalation of carbon dioxide (CO2) was studied in a conflict situation (Vogel test) in the rat. This treatment, which inhibits γ-aminobutyric acid (GABA)-mediated transmission in rat brain and induces anxiety and panic attacks in humans, elicited a proconflict effect. Exposure of rats for l min to CO2 decreased by ~ 40% the number of licking periods in the test. This effect was abolished by prior administration of alprazolam (0.5 mg per kilogram of body Mass, I.p.). Although these results may support a role for GABA-mediated transmission in the anxiogenic effect of CO2 inhalation, the possibility that different neurotransmitters other than GABA are involved in the action of CO2 can not be ruled out.

The imidazobenzodiazepine 6-(2-bromophenyl)-8-fluoro-4H-imidazo-[1,5-a] [1,4]benzodiazepine-3-carboxamide (imidazenil) is a new anxiolytic and anticonvulsant ligand of the benzodiazepine recognition site that possesses the characteristics of a partial allosteric modulator of the gamma-aminobutyric acid (GABA) type A receptor. The effects of imidazenil on GABAA receptor function were examined both in vitro and in vivo. Imidazenil inhibited [3H] flumazenil binding to mouse cerebral cortical membranes in vitro with an IC50 of 0.9 nM, showing that this compound binds with high affinity to benzodiazepine receptors. However, imidazenil failed to modify t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to washed or unwashed mouse cortical membrane preparations. Furthermore, imidazenil injected i.p. into mice failed to affect [35S]TBPS binding subsequently measured in unwashed cortical membranes. In contrast, imidazenil reduced in a dose-dependent manner the increase in [35S]TBPS binding elicited by isoniazid (200 mg/kg s.c.), an effect mimicked by lorazepam and abecarnil++ but not by bretazenil. As expected, i.p. administration of lorazepam or abecarnil induced within 30 min a marked reduction in [35S]TBPS binding subsequently measured in unwashed cortical membranes of control mice. Moreover, imidazenil at a dose as low as 0.05 mg/kg (i.p.) delayed the onset of convulsions and death elicited by isoniazid and reduced significantly the number of mice exhibiting seizures. Accordingly, imidazenil also showed great potency in antagonizing the convulsions induced by pentylenetetrazole in rats. Imidazenil also completely abolished the increase in [35S]TBPS binding induced by foot-shock or exposure to carbon dioxide. Finally, imidazenil antagonized both in vitro and in vivo the effects of bretazenil or lorazepam on GABAA receptor function.

The ability of an anticonvulsant dose (0.1 mg/kg i.p.) of imidazenil, a new partial agonist of benzodiazepine receptors, to antagonize the convulsions and the increase in t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to the gamma-aminobutyric acid type A (GABAA) receptor elicited by isoniazid, an inhibitor of central GABAergic function, was evaluated in mice chronically treated (3 times daily for 30 days) with the same dose of imidazenil. The challenge dose of imidazenil, administered 36 h after the last injection of the chronic treatment protocol, reduced both isoniazid-induced convulsions and the isoniazid-induced increase in [35S]TBPS binding to the same marked extent as in control mice. These results indicate that long-term treatment with a pharmacologically effective dose of imidazenil failed to induce tolerance to both the anticonvulsant effect and the positive modulatory action on GABAA receptor function of this drug in mouse brain.

The ability of 2-chloro-N6-cyclopentyladenosine (CCPA) to suppress ethanol withdrawal syndrome was tested in male rats rendered physically dependent on ethanol by intragastric administrations of ethanol (12-18 g/kg daily for 6 days). CCPA administered 24 hr after the last ethanol dose produced a dose-dependent inhibition of withdrawal signs such as tremors and audiogenically induced seizures, an effect prevented by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX).

In rats habituated to the manipulation that precedes killing (handling-habituated) the cerebral cortical concentrations of pregnenolone and progesterone were significantly lower (-57% and -69%, respectively) than in naive animals. An acute stress, induced by CO2 inhalation, elicited a marked increase in the concentrations of pregnenolone, progesterone and deoxycorticosterone in the brain cortex and hippocampus of handling-habituated rats. An accepted stress, such as foot shock, also enhanced the brain cortical levels of pregnenolone, progesterone and deoxycorticosterone in handling-habituated rats. These data show that the rat brain cortical and hippocampal steroid content is related to the 'emotional state' of the animal.

The effects of the cyclopyrrolones zopiclone and suriclone on the function of the central γ-aminobutyric acid type A (GABAAA) receptor complex in mouse brain were evaluated both in vitro and in vivo. Added in vitro to mouse cerebral cortical membranes, these compounds potently inhibited [3H]flumazenil binding with IC50 (50% inhibitory concentration) values of 35.8 nM (zopiclone) and 1.1 nM (suriclone). Similar results were obtained with cerebellar membranes, indicating that these drugs do not discriminate between putative type I and type II benzodiazepine receptors. The interaction of cyclopyrrolones with recognition sites present at the level of the GABA receptor complex appears to be competitive, because zopiclone decreased the affinity of the receptors for [3H]flumazenil without affecting the maximal number of binding sites. Moreover, zopiclone and suriclone did not affect the rate of dissociation of [3H]flumazenil from benzodiazepine receptors. The in vitro efficacy of zopiclone appeared different from that of suriclone and the benzodiazepines diazepam and flunitrazepam. Thus, zopiclone failed to affect muscimol-stimulated 36Cl− uptake and only slightly inhibited t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding. In contrast, like diazepam and flunitrazepam, suriclone increased muscimol-stimulated 36Cl− uptake and markedly inhibited [35S]TBPS binding. On the other hand, suriclone, like zopiclone, did not modify [3H]muscimol binding to mouse cerebral cortical membranes. Moreover, zopiclone antagonized the reduction in [35S]TBPS binding elicited by the benzodiazepine receptor full agonist diazepam. Consistent with its low efficacy in vitro, oral administration of zopiclone (2.5 to 100 mg/kg, p.o.) in mice failed to modify [35S]TBPS binding subsequently measured in cerebral cortical membranes “ex vivo”. In contrast, suriclone (10 to 20 mg/kg, p.o.), like diazepam, decreased [35S]TBPS binding measured ex vivo. Moreover, both zopiclone (50 to 100 mg/kg, p.o.) and suriclone (1 to 10 mg/kg, p.o.) abolished the increase in [35S]TBPS binding induced by isoniazid (200 mg/kg, s.c.). These results suggest that suriclone may enhance GABAergic transmission with an efficacy similar to that of diazepam. In contrast, the low efficacy of zopiclone both in vitro and in vivo suggests that this drug may act as a partial agonist at benzodiazepine receptors.

1. The effect of different stressful stimuli on the function of the GABAA-ionophore receptor complex was evaluated by measuring the binding of 35S-TBPS to the chloride channel associated recognition sites. 2. Foot-shock stress enhanced 35S-TBPS binding in membrane preparation from rat cerebral cortex. The effect of foot-shock on 35S-TBPS binding was mimicked by the anxiogenic and proconvulsant beta-carboline FG 7142 and antagonized by anxiolytic benzodiazepines and by the novel anxiolytic and anticonvulsant beta-carboline, abecarnil. 3. A brief exposure of rats to CO2 inhalation produced, like foot-shock and FG 7142, a marked increase of 35S-TBPS binding in the cerebral cortex, cerebellum and hippocampus. The effect of CO2 inhalation was maximal 10 min after treatment and return to control value in 2 hours. Previous administration of anxiolytic drugs (alprazolam and abecarnil) completely prevented the CO2 inhalation-induced increase of 35S-TBPS binding. 4. All together these data strongly suggest that carbon dioxide inhalation, like stress and anxiogenic drugs, decreases the function of the GABAA receptor complex.

The effect of long-term treatment with ethanol was investigated on the function of gamma-aminobutyric acid A (GABAA) and N-methyl-d-aspartic acid (NMDA) receptors. Rats were rendered ethanol-dependent by repeated forced administration of a 20% ethanol solution (12 to 18 g/kg/day po) for 6 days and tested while still intoxicated or at different time intervals after withdrawal. t-[35S]Butylbicyclophosphorothionate (35S-TBPS) binding was increased by 30% in cortical homogenates of rats killed 1 to 3 hr after last ethanol administration, when compared with saline-treated animals. However, GABA-stimulated 36Cl- uptake and its enhancement by flunitrazepam was decreased in the ethanol-treated animals. 35S-TBPS binding and 36Cl- influx measured 9 to 24 hr following the last ethanol injection, when withdrawal signs were present, were unmodified with respect to saline-treated rats. Moreover, the effects of both isoniazid and FG 7142 on 35S-TBPS binding were unchanged in ethanol-dependent rats tested at 1 to 3 and 9 to 24 hr, compared with controls. In contrast, ethanol-withdrawn rats tested at 9 to 24 hr showed a dramatic enhancement in their sensitivity to the convulsant action of isoniazid (50 to 250 mg/kg, sc). The same animals were also more susceptible to the convulsant action of NMDA (0.5 to 5 micrograms/5 microliters/rat intracerebroventricularly) and kainic acid (12 mg/kg, ip), and this effect was paralleled by an enhancement (+25%) in the density of 3H-MK 801 recognition sites in the hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of CO2 inhalation on the function of the GABA(A)-coupled chloride channel was evaluated in rat brain. This treatment decreased the capability of GABA to stimulate 36Cl- uptake and produced a significant increase of [35S]-TBPS ([35S]t-butylbicyclophosphorothionate) binding in the cerebral cortex, cerebellum and hippocampus. These results demonstrate that a brief exposure of rats to CO2 inhalation reduces the function of the GABA(A)-ionophore receptor complex in rat brain.