Influence of the antidepressants desipramine and fluoxetine on tryptophan-2,3-dioxygenase in the presence of exogenous melatonin

The effects of desipramine and fluoxetine were examined on rat hepatic tryptophan-2,3-dioxygenase activity in the presence or absence of exogenous melatonin. Male Wistar rats were treated intraperitoneally over 8 days. The antidepressants were also administered individually and their impact on the enzyme noted. Desipramine reduced basal hepatic tryptophan-2,3-dioxygenase activity in the liver while fluoxetine had no observable effect. However, fluoxetine also prevented the hemin induced elevation in total enzyme activity. Exogenous melatonin (0.25mg/kg/day) for 8 days counteracted the inhibitory effects of both desipramine and fluoxetine at concentrations of 2.5mg/kg/day. Total enzyme activity was restored when melatonin was administered in combination with either drug. The experimental findings indicate that there is chemical antagonism at the surface of the enzyme between melatonin and the antidepressants that abolishes the observed inhibitory effects of the administered compounds.