Ovariectomy permits progesterone to increase the binding of [3H]spiperone to the anterior pituitary gland in estrogen-primed rats.

The binding of the dopamine (DA) D2 antagonist spiperone to DA receptors in the anterior pituitary gland was evaluated in intact and ovariectomized (OVX) estrogen-primed rats in which circulating levels of progesterone (P) were varied. Nembutal (PB; 35 mg/kg, ip) was administered at 1130 h to intact proestrous animals to prevent the LH-stimulated release of ovarian P. Two hours later some of these intact rats were QVX to remove the endogenous ovarian steroids. Both intact and OVX rats were given exogenous P. All rats were killed on the following morning, and adenohypophysial binding of [3H]spiperone was evaluated at a saturating concentration. The binding in intact PB-blocked rats that received P was only 52% of that in PB-blocked rats that were OVX and received P. The binding of [3H]spiperone in PB-treated rats that were OVX and given P did not differ statistically from that in normal estrous rats. A parallel experiment was conducted on other rats that had been OVX 7 days before the sc implantation (on day 0) of Silastic capsules containing estradiol (E2). Two days later (day 2), some of these rats were injected with Nembutal at 1130 h. Two hours later, the E2 capsules of some rats were removed to simulate ovariectomy; Silastic capsules containing P were implanted into both E2-bearing and non-E2-bearing rats. All rats were killed the following morning (day 3), and the density of adenohypophysial DA receptors was assessed. Binding densities were greatest in rats bearing both E2 and P capsules and in rats from which E2 capsules were removed and replaced with P capsules. We conclude that treatment of OVX rats with the sequential combination of E2 and P increases the density of DA receptors in the anterior pituitary gland. However, in intact rats an additional ovarian product prevents the P-induced increase in the density of adenohypophysial DA receptors.