EFFECTS OF PROGESTERONE AND PENTOBARBITONE ON HYPOTHALAMIC INDUCTION OF OVULATION IN THE 5-DAY CYCLIC RAT

Abstract

The involvement of the central nervous system in the inhibitory effects on ovulation of progesterone and pentobarbitone was studied in 5-day cyclic rats. The occurrence of ovulation following electrochemical stimulation of the median eminence or the preoptic area was the main technique used in this work. The following experiments are reported. Following pre-treatment with 10 mg progesterone on day 3 of the cycle stimulation of the preoptic area on day 4 was almost completely ineffective. However, stimulation of the median eminence was effective in most of these progesterone-treated rats (I). Following pretreatment with the much lower dose of 0.5 mg progesterone, spontaneous ovulation was still suppressed. However, stimulation of the preoptic area was now capable of inducing ovulations in most animals, but only if either anesthesia was used during the stimulation procedure. If combined pentobarbitone-ether anaesthesia was used, most animals did not ovulate (n). in other experiments stimulation of the median eminence or preoptic area was performed on day 3 (dioestrus) under ether anaesthesia or combined pentobarbitone-ether anaesthesia in normally cycling rats, which did not receive progesterone. Under either anaesthesia preoptic stimulation induced ovulation in most animals, but under combined pentobarbitone-ether anaesthesia none of the animals ovulated (III). Stimulation in the median eminence under otherwise identical experimental conditions, induced ovulation without exceptions under ether anaesthesia and in half of the cases operated under combined pentobarbitone-ether anaesthesia (IV). It is concluded that these findings support the concept that the inhibitory effect of progesterone on ovulation is due to an action on the central nervous system. Furthermore, consideration of the possible mechanisms of action of progesterone and of pentobarbitone in inhibiting ovulatlon led to 2 alternative hypotheses: the two substances exert their effect on ovulatlon at the same site, or, pentobarbitone acts at a level closer to the median eminence.