Timed daily injections of neurotransmitter precursors alter the gonad and body weights of spotted munia, <i>Lonchura punctulata</i>, maintained under short daily photoperiods

Abstract

Abstract The present study was designed to examine the role of circadian organization in the control of seasonality in the seasonally breeding spotted munia, Lonchura punctulata , which does not utilize photoperiod to time its reproduction under natural day‐length conditions. Daily injections of L‐dihydroxyphenylalanine (L‐DOPA, precursor of dopamine) given 12 hr and 8 hr after administration of 5‐hydroxytryptophan (5‐HTP, precursor of serotonin) alter the gonadal and body‐weight responses of the spotted munia, when it is maintained under short‐day conditions (LD 6:18). The 12‐hr interval was stimulatory and the 8‐hr interval was inhibitory for both gonadal and body‐weight responses while other intervals (0, 4, and 16 hr) had no effect. A 20‐hr interval was stimulatory only for gonadal growth and had no effect on body weight. These findings support the suggestion that the endogenous seasonal mechanism is determined by circadian organization (i.e., by changes in the phase relationships of two neural oscillations and their many other circadian expressions). The results also indicate that interaction between these two systems does not produce any pharmacological effect but actually affects the seasonality of the birds, as indicated by the different gonadal and metabolic conditions that followed differently timed daily injections of drugs that influence serotonergic and dopaminergic activities. Furthermore, the mechanism of regulation of seasonality seems to be similar in species that do and do not utilize day length as the primary regulator with which to time their seasonal reproductive and metabolic cycles.