Enhanced sensitivity to β-carboline inverse agonists in rats chronically treated with FG 7142

The biochemical and behavioural effects of the chronic administration of the beta-carboline inverse agonist FG 7142 were studied in the rat. Repeated administration of FG 7142 (15 mg/kg IP, twice daily for 10 consecutive days) induced sensitization to the effects of this drug, which from proconvulsant became a full convulsant. Thus, myoclonic seizures were observed in 30% and 80% of the animals by the third and the eighth day of treatment, respectively. The sensitization to the convulsant effect of FG 7142 persisted for up to 50 days after withdrawal and was completely prevented by the concurrent administration of the benzodiazepine receptor antagonist Ro15-1788 (15 mg/kg IP, twice a day for 10 days). Moreover, four to twelve days after withdrawal from chronic treatment with FG 7142, an increased sensitivity to the proconvulsant beta CCE and to the convulsant DMCM was observed. In addition, convulsions induced by isoniazid (350 mg/kg, SC) were potentiated in rats chronically treated with FG 7142 at 5 and 20 days after withdrawal. These pharmacological effects were paralleled by a decrease in the density of low affinity GABA receptors in the cerebral cortex and cerebellum. These results are consistent with the view that repeated administration of FG 7142 induces a long-lasting down-regulation of the GABAergic function which results in an increased sensitivity to beta-carboline inverse agonists and isoniazid. The possibility that a concomitant decrease in the responsiveness to benzodiazepines and Ro15-1788 takes place after chronic treatment with FG 7142 is also discussed.