Suppression of the Preovulatory Surge of Luteinizing Hormone and Subsequent Ovulation in the Rat by Arginine Vasotocin

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Abstract
To characterize a possible relationship between 8-arginine vasotocin (AVT) and fertility, the effects of administration of this mammalian brain peptide on the preovulatory surge of luteinizing hormone (LH) and subsequent ovulation were observed. In 32 unanesthetized proestrous rats, AVT (1.0 .mu.g i.v. hourly from 1300-1600 h) inhibited the LH surge in 94% and suppressed ovulation in 78%. A lower dosage of AVT (0.1 .mu.g every hour) was less effective, and 0.001 .mu.g had no effect. Saline solution, 0.1 .mu.g antidiuretic hormone (ADH), and as much as 5 .mu.g oxytocin caused no response, but 1.0 .mu.g ADH had some inhibitory effect. Approximately 17% of the AVT-treated animals exhibited a delayed LH surge occurring between 1800 and 2000 h only. 8-Arginine vasotocin failed to affect tonic levels of blood LH during diestrus or the elevated, pulsatile blood LH values 30 days after ovariectomy. Similarly, AVT had no effect on the LH surge induced with LH-releasing hormone in ovariectomized animals. 8-Arginine vasotocin thus demonstrates a striking ability to inhibit the preovulatory surge of LH. The finding that it does not alter tonic levels of blood LH supports the concept that different mechanisms control cyclic and tonic release of pituitary LH. AVT may exert anovulatory effects by acting on efferent neuronal mechanisms rather than directly on the hypophysial-gonadal axis.