Effects of Pentobarbital Anesthesia on Tonic Luteinizing Hormone Secretion in the Ewe: Evidence for Active Inhibition of Luteinizing Hormone in Anestrus 1

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Abstract
In the ewe, seasonal anestrus appears to result from 2 effects of inhibitory photoperiod. Estradiol gains the capacity to suppress luteinizing hormone (LH) pulse frequency and therefore becomes a potent inhibitor of tonic LH secretion and a steroid-independent decrease in LH pulse frequency occurs in ovariectomized ewes. Using pentobarbital anesthesia, it was evidenced that both these actions of photoperiod reflect the activation, in anestrus, of an inhibitory neural system. Administration of pentobarbital to intact anestrous ewes produced a dramatic, 3-fold increase in LH pulse frequency during the 6 h of anesthesia. During the breeding season, pentobarbital inhibited LH pulse frequency in luteal phase animals. There was also a seasonal variation in the effects of pentobarbital in ovariectomized ewes. During the breeding season this drug again suppressed LH secretion, inhibiting both LH pulse amplitude and frequency. In anestrus, pentobarbital also suppressed pulse amplitude, but it produced a transitory increase (lasting 3 h) in pulse frequency. To account for the stimulatory actions of pentobarbital, in anestrus, but not the breeding season, LH pulse frequency is held in check by a set of estradiol-sensitive inhibitory neurons. These neurons could be activated by inhibitory photoperiod and account for both the steroid-dependent and steroid-independent actions of photoperiod.