The occurrence of ovulation and serum levels of LH and FSH (measured by radioimmunoas- say) were determined in periovulatory hamsters injected with an ovulation-blocking dose of phenobarbital (Phen) combined with progesterone (P), estradiol-17β(E2), or testosterone (T). Proestroush amsters were treated at 1300 h with Phen plus oil, P, P plus E2 E2, T, or a second injection of Phen at 2000 h. Each treatment group was divided into 3 subgroups, each of which was serially bled 4 times at 6 h intervals beginning at 1200, 1400, and 1600h on proestrus. Phen blocked ovulation on the next morning in all animals, while treatments that included P (1 mg) restored the normal complement of ova in 65-75% of the animals. Neither E2 (1, 10 or 50 μg) nor T (0.1 or 1 mg) overcame the Phen block of ovulation. Control hamsters had peak levels of LH between 1400 and 1800 h and a biphasic release ofFSH consisting of a peak at 1600 h on proestrus, a return to basal levels at 2200 h, and a second more sustained surge between 2400 and 0800 h on the morning ofestrus. Phen completely depressed the proestrous surge of both gonadotropins but only partially inhibited the second FSH elevation on the morning of estrus. In ovulatory animals, P alone or combined with 1 or 10 μg E2 restored peak LH levels at 1600 h. FSH levels on proestrus in hamsters treated withPhen plus P peaked at 1800 h, while the addition of 1 fig E2 resulted in increased FSH levels at 1600 h; peak levels in both groups were about half of control values. No proestrous increase was detected in ovulatory animals treated with P and 10 μg E2. FSHl evels on esrrus in hamsters injected with P alone or in combination with E2 were intermediate between thoseof controls and animals given Phen only. Levels of LH and FSH in animals treated with a single or double dose of Phen or Phen plus E2 or T were not different during the periovulatory period.
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