One half ml of sheep anti LHRH serum (No.772) completely blocked ovulation when administered iv or sc at any stage of the estrous cycle in cycling hamsters. This anti ovulatory activity lasted 12-13 days. The minimal effective iv dose (MED) of the anti LHRH serum for completely blocking the preovulatory LH surge and ovulation when administered at 1200 h on proestrus was 0.2 ml. Injection of LHRH on the afternoon of P (proestrus) induced ovulation in the antiserum treated hamsters when the antiserum was injected on P, but not when injected on D1 (diestrus 1) or D2 (diestrus 2). This suggests that the anti LHRH serum acts differently in blocking ovulation during D and P, by suppressing follicular development and inhibiting preovulatory surge of gonadotropins, respectively. Serum estradiol levels, measured by radioimmunoassay, were significantly reduced but not completely suppressed after injection of 0.5 ml. of anti LHRH serum. Histological examination of the ovaries revealed that an arrest of follicular maturation resulted 3 days after antiserum injection. 17 β Estradiol administered 22 h before the presumptive preovulatory LH surge improved significantly (P<0.05) the LH response to LHRH in the antiserum blocked hamsters. This suggests a direct modulation of pituitary LH responsiveness by estradiol in the absence of endogenous LHRH activity. When the integrated levels of serum LH following an injection of a minimum effective dose of exogenous LHRH to induce full ovulation in antiserum blocked hamsters were compared with those during the physiological preovulatory LH surge, it was found that only 11% of the amount of LH released on the afternoon of proestrus was sufficient for inducing full ovulation.
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