Resetting the annual cycle with timed daily injections of 5-hydroxytryptophan and L-dihydroxyphenylalanine in Syrian hamsters.

Daily injections of dihydroxyphenylalanine (DOPA), a precursor for dopamine synthesis, given 12 hr after daily injections of 5-hydroxytryptophan (5-HTP), a serotonin precursor, induced uterine growth and increased serum thyroxine and luteinizing hormone (LH) concentrations in scotosensitive female hamsters maintained on short daylengths. On the other hand, daily injections of DOPA given at the same time as daily injections of 5-HTP reduced uterine weights and serum concentrations of thyroxine and LH in scotorefractory female hamsters. These results indicate that the endogenous mechanism controlling seasonality (scotosensitivity and scotorefractoriness) in the Syrian hamster may be reset by drugs that influence serotonergic and catecholaminergic activity. This seasonal mechanism might involve two circadian systems that undergo seasonal changes in their phase relations.