Photoperiodism in higher vertebrates: an adaptive strategy in temporal environment.

Life on earth is subjected to strict regimen of cyclical changes. Of all geophysical changes, the most prominent are the daily changes between day and night and the regular succession of annual seasons. Synchronization to environmental day-night and seasonal cycles is key to survival. Night-time and day-time environments differ in illumination, temperature, food supplies and predators. Organisms have, therefore, developed highly specialized temporal programmes to get better adapted to activity either during night or during the day. Many species use annual cycle of changes in daylength as "calendar" to synchronize and/or to time their daily and seasonal physiological and behavioral functions. This is described as photoperiodism. The minimum daylength (photoperiod) that will induce a physiological response is the "critical daylength" (CD). CD requirement is quite stringent. CD is species-specific; CD may be response-specific as well. Switching 'on' and 'off' the seasonal responses in many species is regulated by the development of photorefractoriness-a phenomenon when organism remains no longer capable to continue its stimulatory response to a photoperiod. Photorefractoriness may be qualitatively different between species. Broadly, two categories of photorefractoriness are described: absolute refractoriness and relative refractoriness. What causes photorefractoriness and at what level in the brain photorefractoriness occurs are unknown. A photoperiodic response system has three principal components: a photoreceptor that interprets photic input, a clock that measures photic signal, and a neurosecretory system that translates photic signal into endocrine secretions. Both retinal and extra-retinal structures are involved in the photoperiodic photoreception. In insects, mollusks, crustaceans, fishes, amphibians, reptiles and birds, the photoreception occurs largely through extra-retinal photoreceptors (ERRs) localized in the hypothalamus. In adult mammals, brain photoreceptors are apparently absent and light input is only through eyes. Endogenous circa-rhythms believed to be involved in photoperiodism are synchronized to changes in illumination of the environment with either the time of day (circadian rhythm) or the season of the year (circannual rhythm). The Suprachiasmatic nuclei (SCN) of the hypothalamus in mammals function as the clock. The SCN monitors the photoperiodic message and decodes it by dictating the changes in rhythm in melatonin secretion in the pineal gland. The hypothalamic paraventricular nuclei (PVN) are also an integral component of the neural network mediating photoperiodism in mammals. The location of SCN homologue and the role of PVN in mediating photoperiodism in non-mammalian vertebrates is still unclear. Furthermore, how various components of photo-neuroendocrine circuitry are functionally linked together is unknown.