The vasopressin gene is expressed prior to regulation in the supraoptic nuclei of fetal rats.

The development of the regulation of vasopressin (prepropressophysin) mRNA in the supraoptic nuclei (SON) of rats was studied using quantitative in situ hybridization. On day 21 of gestation, vasopressin mRNA levels in the SON were increased by osmotic stimulation, with significant positive correlation between vasopressin mRNA levels and plasma osmolality. On day 19 of gestation, vasopressin mRNA levels in the SON were not significantly increased by osmotic stimulation, and no correlation between vasopressin mRNA levels and plasma osmolality was noted. The results suggest that the mechanisms for osmotic regulation of vasopressin mRNA levels develop between days 19 and 21 of gestation, a time course consistent with the initiation of afferent innervation of the SON.