Effect of pentobarbital anaesthesia on luteinizing hormone levels in intact and ovariectomized ewes

In six intact ewes, deep anaesthesia lasting 1-2 h was induced with pentobarbital given intravenously at the onset of oestrus. In a second group of six ovariectomized ewes, light pentobarbital anaesthesia was induced following a control period of 4 h and then maintained for 2, 3, or 4 h. No drug was given to control animals (seven in the first group, three in the second). Serum levels of luteinizing hormone (LH) were determined by radioimmunoassay hourly in the first group, and at intervals of 5 or 10 min in the second. In the oestrous ewes, pentobarbital did not prevent or modify the expected LH surge. In the ovariectomized ewes, the drug, while not affecting the pulsatile release pattern of LH, reduced average LH levels to about half their control values (P smaller than 0.001) when given in a dosage maintaining 3 or 4 of light sleep; the 2-h dosage had no such effect. These differential effects of pentobarbital indicate that LH secretion in ewes is regulated by more than one mechanism. In particular, our findings are consistent with a hypothesis that there are, in ovariectomized ewes, independent central regulatory mechanisms for the amount of LH released, and for the time pattern of its release.