The suprachiasmastic nucleus: The biological clock of all seasons

The suprachiasmatic nucleus (SCN) plays an essential role in the generation and maintenance of circadian rhythms in mammals. The intrinsic pacemaker activity of the SCN is autonomous in nature and is entrained to the daily 24-hour period primarily by the light/dark cycle. Due to its connections with the pineal gland, the SCN is also known to be indirectly involved in the control of seasonal rhythms. The data presented in this chapter demonstrates that SCN activity is not only cued by the light/dark cycle but also strongly influenced by the photoperiod. Moreover, it appears that integration of the photoperiodic message through the rhythmic secretion of Mel implicates a Mel driven circadian rhythm of sensitivity to Mel. This clearly indicates that the SCN is a biological clock for all seasons regulating photoperiodic changes observed at the physiological and behavioral level. It also indicates that the currently accepted separation between the circadian and seasonal photo-timing system is perhaps not justified on the physiological grounds. © 1996 Academic Press Inc.