A brief exposure to thyroxine synchronizes the circannual testicular cycle and associated molt in the subtropical spotted munia (Lonchura punctulata)

The concept of the circannual clock has gained wide acceptance, but its genesis, physiology and mode of synchronisation in birds remain unknown. The subtropical spotted munia exhibits distinct endogenous rhythms of gonadal function, molt, fattening and food intake in ‘information-free’ conditions of continuous illumination (LL) or darkness, thus rendering it an ideal model for further investigation. The circannual reproductive rhythm in this bird is free-running, with a periodicity of approximately 10 months in LL. It appears that in order to be in harmony with the annual calendar, the endogenous rhythm must be adjusted by about 2 months every year. In our earlier experiments, daily increments in length of day just prior to the vernal equinox were found to synchronize the circannual reproductive rhythm and associated molt. The process of synchronization involves a phase delay and hence a gonado-inhibitory rather than conventional gonado-stimulatory response to increasing day length. Considering the anti-gonadal and pro-molt role of the thyroid gland (specifically thyroxine, T4) in this bird, we now examined the involvement of thyroid hormones in the process of photoperiodic synchronization. Free-running birds held in LL were administered physiological doses of T4 over 15 days during quiescence/recrudescence and regression phases, and the effects on reproductive and molt cycles were studied. The effect of LL on the thyroid hormone profile and its phase relationship with reproductive and molt cycles was also investigated. The brief exposure to thyroxine in winter synchronised the free-running reproductive and associated molt cycles in LL with the monsoon period through a phase delay. The neuroendocrine axis was shown to be sensitive to T4 during the window when photoperiodic synchronisation occurs. It appears that entrainment by daily increments of light hours may be mediated by thyroid hormones, effecting a temporary suppression of gonadotrophin secretion involving a photo-neuroendocrine locus of action.