Seasonal time measurement during reproduction.

Most species living outside the tropical zone undergo physiological adaptations to seasonal environmental changes and changing day length (photoperiod); this phenomenon is called photoperiodism. It is well known that the circadian clock is involved in the regulation of photoperiodism such as seasonal reproduction, but the mechanism underlying circadian clock regulation of photoperiodism remains unclear. Recent molecular analysis have revealed that, in mammals and birds, the pars tuberalis (PT) of the pituitary gland acts as the relay point from light receptors, which receive information about the photoperiod, to the endocrine responses. Long-day (LD)-induced thyroid-stimulating hormone (TSH) in the PT acts as a master regulator of seasonal reproduction in the ependymal cells (ECs) within the mediobasal hypothalamus (MBH) and activates thyroid hormone (TH) by inducing the expression of type 2 deiodinase in both LD and short-day (SD) breeding animals. Furthermore, the circadian clock has been found to be localized in the PT and ECs as well as in the circadian pacemaker(s). This review purposes to summarize the current knowledge concerning the involvement of the neuroendocrine system and circadian clock in seasonal reproduction.