Colocalization of vasopressin and oxytocin in hypothalamic magnocellular neurons in water-deprived rats

The posterior lobe hormones vasopressin and oxytocin are expressed in mutua lly-exclusive sets of magnocellular hypothamalic neurons. However, under ce rtain functional conditions a partial coexpression has been observed. In th e present study we subjected adult rats to long-term osmotic stress by wate r deprivation for up to 3 days. After 3 days, a marked reduction of vasopre ssin immunostaining was observed in the paraventricular and supraoptic nucl ei as compared with controls. Coexistence of oxytocin and vasopressin occur red in a portion of the magnocellular neurons. Many fibers of the hypothala mio-neurohypophyseal tract contained both peptides. Rehydration for 24 h af ter 3 days of thirsting resulted in a light recovery of vasopressin immunor eactivity with almost no magnocellular neurons containing both nonapeptides . Our findings indicate that magnocellular hypothalamo-neurohypophysial neu rons are capable of oxytocin and vasopressin coexpression upon extended osm otic stress. (C) 2001 Harcourt Publishers Ltd.