Barbiturate Anesthesia Blocks the Positive Feedback Effect of Progesterone, but not of Estrogen, on Luteinizing Hormone Release in Ovariectomized Guinea Pigs*

In female rats, pentobarbital anesthesia blocks estrogen- or progesterone-induced LH release. This experiment was designed to test the effects of barbiturates on the steroidinduced LH surge in the female guinea pig. Animals were housed in a light-controlled room (lights on from 0600–1800 h) and ovariectomized 2 weeks before the experiment. Blood samples were obtained through a chronic indwelling catheter. Plasma LH was estimated by RIA. Estradiol benzoate alone (EB; 10 fig injected sc at 1000 h) or EB (1.5 μg injected sc at 1000 h) folowed by progesterone (P4; 1.0 mg sc) 30 h later consistently induced an LH suige between 33–42 h after EB (10 μg) or between 6–12 h after P4. Multiple injections of pentobarbital (30 mg/kg), starting at 30 h after EB, or a single injection of phenobarbital (100 mg/kg), starting at 27 h after EB, were given before the expected LH surge in both EB alone and EB plus P4-treated animals. Animals slept for about 15 h under multiple injections of pentobarbital and for about 35 h under a single injection of phenobarbital. In all animals (n = 11), pentobarbital was effective in blocking the P4-induced LH surge completely. On the other hand, both pentobarbital and phenobarbital anesthesia failed to block the estrogen-induced LH surge; 7 of 14 and 6 of 9 animals, respectively, had LH surges during the expected period and 4 of 14 and 3 of 9 animals, respectively, had LH surges 24 h after the expected period. Thus, it is suggested that the mechanism of action by which P4 facilitates LH release differs from that of estrogen. P4 may act primarily on higher brain structures and for a limited period of time. However, estrogen (10 μg) may act primarily on the medial basal hypothalamus as well as on the pituitary and for a longer period so that LH release can occur during the time of anesthesia or 24 h after the expected period.