Daily injection of L-dihydroxyphenylalanine (L-DOPA, a dopamine precursor) given 12 hr after 5-hydroxytryptophan (5-HTP, a serotonin precursor) stimulated testicular growth and body weight increase in the spotted munia Lonchura punctulata. Daily injections of the same drugs given 8 hr apart had the opposite effect, inhibiting both testicular growth and body weight gain. These responses were not observed when either of the drugs were given alone, indicating that the effect was due to temporal synergism between serotonergic and dopaminergic functions. The inhibitory effect of daily injections of thyroxine on testicular and body growth were negated by injections of 5-HTP and L-DOPA separated by 12 hr. Conversely, the stimulatory effects of thyroidectomy in maintaining full testicular size and body weight after the breeding seasons were negated by daily injections of 5-HTP and L-DOPA separated by 8 hr. It is concluded that the inverse relationship between seasonal changes in thyroidal and reproductive functions may involve seasonal changes in phase relationships between daily rhythm in serotonergic and dopaminergic activity in the central nervous system.
5-Hydroxytryptophan Animals