OXYTOCIN RECEPTOR MESSENGER-RNA EXPRESSION IN THE VENTROMEDIAL HYPOTHALAMUS DURING THE ESTROUS-CYCLE

Changes in OR mRNA expression in the ventromedial hypothalamus (VMH) i n relation to the estrous cycle were measured by In situ hybridization with a rat oxytocin receptor (OR) probe. Binding studies have localiz ed ORs to various brain regions, and have detected a high density of r eceptors in the VMH, a nucleus containing large numbers of estrogen re sponsive neurons. Previous studies in this lab have reported a signifi cant increase in OR mRNA expression in the VMH in ovariectomized rats treated with estrogen. The present study was designed to determine whe ther changes in steroid hormone levels across the estrous cycle result in induction of OR mRNA expression. Autoradiographic studies revealed differences in OR mRNA expression in the rostral and caudal as well a s medial and lateral aspects of the VMH. OR mRNA levels were highest i n the caudal portion of the vIVMH on the afternoon (16:00 hr) of proes trus. The rostral region exhibited a high level of expression in the v entrolateral region of the VMH on the morning (9:00 hr) of proestrus a nd in the dorsomedial region of the VMH on the afternoon of proestrus. Little or no OR mRNA expression was evident in the rostral or caudal VMH on the morning or evening of diestrus. These results support previ ous findings which showed a regulation of OR binding by gonadal steroi ds and suggest that this may be due to altered expression of the OR ge ne. These effects suggest a possible role of ORs in the oxytocin stimu lated release of luteinizing hormone.