RESUMO
In the present study, effect of SR 57227A, a selective 5-hydroxytryptamine-3 (5-HT(3)) receptor agonist, on the firing activity of pyramidal neurons in the medial prefrontal cortex (mPFC) was studied in normal rats and rats with 6-hydroxydopamine lesions of the substantia nigra pars compacta by using extracellular recording. Systemic administration of SR 57227A (40-640 µg/kg, i.v.) decreased the mean firing rate of pyramidal neurons in normal and the lesioned rats. This inhibition was significant only at doses higher than 320 µg/kg and 640 µg/kg in normal and the lesioned rats, respectively, and was reversed by i.v. administration of 5-HT(3) receptor antagonist tropisetron or GABA(A) receptor antagonist bicuculline. Furthermore, local application of SR 57227A (0.01 µg) in the mPFC inhibited the firing rate of pyramidal neurons in normal rats while having no effect on firing rate in the lesioned rats. The i.v. administration of bicuculline excited the pyramidal neurons in normal rats, and then local application of SR 57227A did not alter the mean firing rate of these neurons. However, these two drugs did not affect the activity of the pyramidal neurons in the lesioned rats. We conclude that activation of 5-HT(3) receptors inhibited pyramidal neurons in the mPFC of normal rats via GABAergic interneurons, and degeneration of the nigrostriatal pathway decreased response of the pyramidal neurons to SR 57227A, suggesting the dysfunction of 5-HT(3) receptors and/or down-regulation of the expression on GABAergic interneurons in the lesioned rats.