RESUMO
Isatin (indole-2, 3-dione) is an endogenous compound with anxiogenic properties, which occur within a narrow dose range (15-20 mg/kg, i.p.). Dose increment beyond 50 mg/kg, i.p. leads to the loss of anxiogenesis. Since a link has been postulated between anxiogenic and convulsant activity, the effect of a range of doses of isatin (20-80 mg/kg, i.p.) was investigated on subconvulsant and convulsant doses of two seizure-inducing agents, namely, pentylenetetrazole (PTZ) and 3-mercapto-propionic acid (3MPA) in rats. Isatin was found to induce a dose-related effect on PTZ and 3MPA convulsions. The lower dose (20 mg/kg, i.p.) potentiated PTZ and 3MPA convulsions, a median dose (40 mg/kg, i.p.) had insignificant effect, whereas higher doses (60 and 80 mg/kg, i.p.) of isatin exhibited significant anticonvulsant effect against both PTZ and 3MPA induced clonic convulsions. The investigation, thus, supports the contention that anxiogenic agents increase the susceptibility to chemical seizures. The proconvulsant effect of isatin, may be due to its inhibitory effect on central atrial natriuretic peptide receptors and stimulation of 5-hydroxytryptamine3 (5-HT3) rather than its monoamine oxidase (MAO) B inhibitory action. The anticonvulsant effect on higher doses of isatin, on the contrary, may be induced by its metabolites, including 5-hydroxyisatin.