Studies on PMSG-Induced Persistent Estrus in Rats -The Primary factors responsible for This phenomenon, Its Characteristics and Restoration to the Normal Cycle-

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We previously reported that estrus can be made to continue indefinitely in many rats by administration of PMSG at a specified period during its cycle. In the present study, partial clarification was made of the primary factors responsible for this phenomenon, its characteristics and restoration to the normal cycle by various treatments.Serum concentrations of gonadotropins and prolactin and sex steroids were measured on each of the 4 following days after PMSG administration. LH concentration was found to have significantly increased from day 1 to day 3 in PMSG treated rats compared with the control. Their FSH levels, however, remained low throughout the experimental period. The serum levels of testosterone and estradiol increased greatly on days 2 and 3 as a result of this hormone. During the persistent estrus, the LH level was lower and FSH higher than in the control. LH and FSH secretions were delayed as a negative feedback response in rats following ovariectomy. In these rats, the secretion of LH due to estradiol implantation was noted as a low positive feedback response. HCG and progeterone injections induced ovulation and then the restoration of the cycle. However, no ovulation occurred when they were paired with male rats for a week. The normal cycle was not recovered by stimulation of the uterine cervix.Based on the results obtained, the following conclusions were drawn: 1)hormone imbalance due to PMSG administration inhibits LH surge for ovulation and prolongs or causes estrus to continue indefinitely; 2) during persistent estrus, low negative and positive feedback response is indicative of abnormal levels of LH and FSH and there is dysfunction of the central nervous system in relation to mating and 3) and the regular estrous cycle can be restored through appropriate treatment.