REVERSAL OF THE INHIBITORY ACTION OF AN ANTISERUM TO LUTEINIZING HORMONE RELEASING HORMONE (LH-RH) BY AN INACTIVE FRAGMENT OF LH-RH

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Abstract
This study was designed to investigate whether the inhibition of ovulation produced by an antiserum to LH-RH [luteinizing hormone-releasing hormone] injected into rats at 12.00 h on the day of proestrus could be prevented by subsequent injection of the 3-10 octapeptide fragment of LH-RH on the same day. Antiserum was raised in a rabbit to LH-RH conjugated to bovine serum albumin by carbodiimide. Antiserum (300 .mu.l) to LH-RH blocked normal preovulatory increases in gonadotropin in the circulation and ovulation. According to the capacity of the antibody to bind 125I-labeled LH-RH in vitro, approximately 50 .mu.g octapeptide would be required to saturate 300 .mu.l antiserum. That 50 .mu.g octapeptide did not itself induce ovulation was shown by administering it to rats in which ovulation was blocked by Nembutal. When 50 .mu.g 3-10 octapeptide were injected at 15.00 h on proestrus into rats pretreated with LH-RH antiserum at 12.00 h, increased levels of LH [luteinizing hormone] and FSH [follicle stimulating hormone] were observed at 17.30 h, and ovulation was not prevented. Reduction in the capacity of antibody in the circulation to bind LH-RH in these rats was shown by measuring the binding capacity of a 1.50 dilution of serum for 125I-labeled LH-RH in vitro. In rats receiving antibody only, 74 .+-. 4% was bound at 17.30 h of proestrus and 74 .+-. 3% at 0.900 h of estrus, while serum from rats receiving octapeptide bound 24 .+-. 2% at 17.30 h of proestrus and 35 .+-. 2% at 0.900 h of estrus (values .+-. SEM). To determine whether ovulation could occur in rats in which LH-RH had been inhibited during the critical period and up to 19.00 h the effect of delaying the injection of octapeptide until 19.00 h was tested. Of 17 rats, 12 ovulated. That this was a response to endogenous LH-RH was supported by the absence of ovulation when Nembutal was injected before the octapeptide. Secretion of LH-RH capable of stimulating LH release for ovulation can probably occur for several hours after the critical period.

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