"Seasonal changes in the neuroendocrine system": some reflections.

This perspective considers first the general issue of seasonality and how it is shaped ecologically. It asks what is the relative importance of "strategic" (photoperiod-dependent) versus "tactical" (supplemental) cues in seasonality and what neural circuits are involved? It then considers recent developments as reflected in the Special Issue. What don't we understand about the photoperiodic clock and also the long-term timing mechanisms underlying refractoriness? Are these latter related to the endogenous annual rhythms? Can we finally identify the opsins involved in photodetection? What is the present position with regard to melatonin as "the" annual calendar? An exciting development has been the recognition of the involvement of thyroid hormones in seasonality but how does the Dio/TSH/thyroid hormone pathway integrate with downstream components of the photoperiodic response system? Finally, there are the seasonal changes within the central nervous system itself--perhaps the most exciting aspect of all.