SEARCH FOR PROGESTIN RECEPTORS (PR) IN PROLACTIN-RELEASING PEPTIDERGIC NEURONS - OXYTOCIN NEURONS LACK PR, BUT RESPOND TO GONADAL-STEROIDS IN MONKEYS

Progesterone (P) increases PRL secretion in estrogen (E)-primed primat es, but not by a direct action on lactotropes. Oxytocin is one of seve ral hypothalamic hormones that stimulate PRL secretion. This study was conducted to determine whether oxytocin neurons directly. mediate the action of P on PRL secretion. Hypothalamic sections from steroid-mani pulated macaques were double immunolabeled for oxytocin and progestin receptors (PR). In addition, serum levels of oxytocin were measured in steroid-treated macaques, and hypothalamic levels of oxytocin were me asured in monkeys under various physiological conditions. E treatment (28 days) of spayed monkeys caused a significant increase in the numbe r of PR-positive neurons in the preoptic area, ventromedial nucleus, a rcuate nucleus, and median eminence. Addition of P to the E treatment for the last 14 of 28 days did not change the number of PR-positive ne urons in these areas. The number of PR-positive neurons was low and wa s unchanged by steroid treatment in the supraoptic and rostral paraven tricular nuclei. Oxytocin neurons rarely contained PR regardless of an atomical location, steroid treatment, or fixation protocol. Serum oxyt ocin levels increased with E treatment and increased further with supp lemental P treatment. The rostral and medial basal hypothalamic conten t of oxytocin was significantly higher in macaques with mature gonads. In conclusion, oxytocin neurons do not express nuclear PR and probabl y do not transcriptionally respond to P. However, gonadal steroids app arently affect the production and release of oxytocin in. vivo. Thus, it is possible that oxytocin neurons tranduce the action of P on PRL s ecretion via stimulatory neurotransmission from another PR-containing neural system.