Rat brain concentrations of 5-hydroxytryptamine following acute and chronic administration of MAO-inhibiting antidepressants.

Groups of male Sprague-Dawley rats were injected once daily with phenelzine (15 mg/kg) or tranylcypromine (10 mg/kg) and killed 6 h after drug administration on days 1, 2, 8 and 19. Brains were analyzed for MAO activity by a radiochemical procedure and for 5-HT concentrations by an electron-capture gas chromatographic procedure. Both drugs affected 5-HT concentrations in a similar manner--a significant increase of 5-HT over control levels by day 1, with levels still increasing between days 8 and 19, having attained concentrations approximately 5 times control values by day 19.