Influence of temporal relationships between serotonergic and dopaminergic precursors on the regulation of gonadal development in birds.

This article is focused on the effect of specific phase relation of serotonergic and dopaminergic oscillations on the gonadal responses of different avian species. These species include (i) summer breeding birds - Red headed bunting (exhibiting post-reproductive absolute photorefractoriness) and Indian Weaver bird (which lacks absolute photorefractoriness), (ii) autumn breeding - Spotted munia and Lal munia (photoperiodic responses of which are distinctly different from that of any long day birds described thus far) and (iii) domesticated Japanese quail (which under natural day length breeds in summer, exhibits relative photorefractoriness, but breeds continuously if maintained in long photoperiod). Previous experiments have shown that daily administration of serotonin precursor 5-hydroxytryptophan and dopamine precursor L-dihydroxyphenylalanine given 8h apart during progressive phase of gonadal cycle can lead to a significant decrease in gonadal activity. However, if given at an interval of 12h it leads to an increase in gonadal activity and the effect of other intervals (0-, 4-, 16- and 20-h) were not different from control. Similar effects were observed during regressive phase of the gonadal cycle of above species except in those which display absolute photorefractory phase in their breeding cycle. This shows that, gonad of such species not only develop post-reproductive photorefractoriness but also becomes insensitive to the stimulatory effects of 5-HTP and L-DOPA when given at the interval of 12h. It is suggested that, temporal phase relation of circadian serotonergic and dopaminergic oscillations is the basis of seasonality in birds and may alter the activity of neuroendocrine-gonadal axis not only under natural day length but also under experimental/different photoperiodic conditions. These results are in agreement with the internal coincidence model of photoperiodic time measurement and indicate that birds can detect specific phase relationship between the two neural oscillations for their reproductive regulations and circadian organization is intricately involved in the regulation of seasonality.