Day night changes in responses to osmotic stress

Plasma volume and osmolality show daily variation in the rat, and there is a cyclical pattern of activity in the suprachiasmatic nucleus (SCN). The secretory cells of the supraoptic nucleus (SON) receive input from the SCN, and show a daily rhythm of firing rate. They respond to osmotic stimuli but their responsiveness is influenced by the light-dark cycle and by the pineal. The projection from SCN to SON is different for oxytocin (OT) and arginine vasopressin (AVP) cells, and also changes with the light-dark cycle. The firing of neurosecretory cells is not related in a simple linear fashion to plasma osmolality and plasma volume nor to peptide secretion. The responsiveness of the neurohypophysial system as a whole, and the capacity of mammals to respond to osmotic stress is thus very dependent on the time of day.