Diurnal rhythms in neurohypophysial function

The neurohypophysial hormones oxytocin and vasopressin show daily rhythms o f secretion with elevated hormone release during the hours of sleep. This p attern can be modulated by ovarian steroids and alters with age. The patter n appears to be due in part to the nocturnal increase in melatonin secretio n, which stimulates hormone release in man, while being inhibitory in the r at. Pinealectomy alters both the 24 h pattern of neurohypophysial hormone r elease in the rat and the firing rate of magnocellular supraoptic nucleus n eurones. There is also a reduced hormone release in response to hypovolaemi a and raised plasma sodium concentration compared to sham operated animals, with a smaller increase in neuronal activity. as determined by immediate-e arly gene expression. The normal responses can be restored by nocturnal adm inistration of melatonin. Melatonin also influences the neurohypophysial ho rmone response in the human to known stimuli of release, such as raised pla sma osmolality, exercise and insulin-induced hypoglycaemia. Recent studies have revealed that not only does the release of vasopressin and oxytocin va ry over each 24 h, but the respective renal and pregnant uterine responses also show diurnal variations.