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

The posterior lobe hormones vasopressin and oxytocin are expressed in mutually-exclusive sets of magnocellular hypothamalic neurons. However, under certain functional conditions a partial coexpression has been observed. In the present study we subjected adult rats to long-term osmotic stress by water deprivation for up to 3 days.After 3 days, a marked reduction of vasopressin immunostaining was observed in the paraventricular and supraoptic nuclei as compared with controls. Coexistence of oxytocin and vasopressin occurred in a portion of the magnocellular neurons. Many fibers of the hypothalamic-neurohypophyseal tract contained both peptides. Rehydration for 24 h after 3 days of thirsting resulted in a light recovery of vasopressin immunoreactivity with almost none magnocellular neurons containing both nonapeptides. Our findings indicate that magnocellular hypothalamo neurohypophysial neurons are capable of oxytocin and vasopressin coexpression upon extended osmotic stress.