Daily changes of neuropeptide Y-like immunoreactivity in the suprachiasmatic nucleus of the rat.

Most of the biochemical, physiological and behavioural events in living organisms show diurnal fluctuations, normally synchronized with 24-h environmental rhythms, such as the light-dark cycle. The suprachiasmatic nucleus (SCN) of the hypothalamus is considered to be a pacemaker of the circadian rhythms in several mammals. The light-dark cycle is the primary synchronizing agent for many of the circadian rhythms which are regulated by the SCN. The photic information reaches the SCN also through a neuropeptide Y(NPY)-like immunoreactive pathway from the ventro-lateral geniculate nucleus. We found that in 12-h-dark and 12-h-light housed rats the NPY-like immunoreactive innervation of the ventro-lateral part of the SCN shows a 24 h rhythm with values rising gradually during the light phase and falling during the dark phase. Besides this rhythm, we found two peaks corresponding to the switching on and switching off of the light. The average level of NPY-like immunoreactivity, as assessed by means of semiquantitative immunocytochemistry and expressed in 'arbitrary units', is reduced in rats housed in total darkness for 2 weeks. These results confirm the physiological role of NPY in the timing of the circadian activity of the SCN.