Ovarian sensitivity to exogenous gonadotropin stimulation (equine chorionic gonadotropin [eCG] and human chorionic gonadotropin [hCG]) following pre-treatment with a progestin (levonorgestrel) versus GnRH antagonist (antide) was studied in cats known to be induced versus spontaneous ovulators. Queens were assigned to one of three treatments: (1) levonorgestrel implants + eCG/hCG (n = 7 cats); (2) antide injections + eCG/hCG (n = 7) or (3) eCG/hCG alone (control; n = 7). Hormonal metabolites were assessed in fecal samples collected daily for 60 days before and during the 37 days inhibitory pre-treatment and for more than 60 days after eCG/hCG. Fecal metabolites of estradiol and progesterone were measured by radioimmunoassay. Females that maintained baseline progesterone were considered induced ovulators, whereas cats that exhibited a luteal phase before inhibition treatment were classified as spontaneous ovulators. Based on fecal hormone profiles, levonorgestrel thoroughly inhibited ovarian activity, whereas antide synchronized follicular phases but did not induce complete ovarian down-regulation. Both treatments prevented ovulation in spontaneous ovulators, but neither caused regression of existing corpora lutea (CL). Levonorgestrel, but not antide, pre-treatment resulted in a quiescent ovary at the time of eCG injection, yet endocrine responses to eCG/hCG were not different among treatments. Interestingly, spontaneously ovulating females exhibited a prolonged estradiol response to gonadotropin stimulation compared to induced ovulators, and this prolonged estradiol surge was replicated by levonorgestrel pre-treatment. Thus, the progestin levonorgestrel effectively suppresses follicular and luteal activity in the cat, resulting in a more consistent response to gonadotropin stimulation, even in females prone to spontaneous ovulation.
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We thank Megan Ross and Melissa Tafoya for technical assistance with fecal steroid assays. We also thank Michele Sommers for excellent care of the cat colony and for assisting in fecal sample collections. Harold Nash of the Population Council, New York, New York, kindly provided the levonorgestrel (Norplant ® ) implants. Antide was generously provided by Dr. Jean Rivier of the Salk Institute, La Jolla, California. Research was supported by the Smithsonian Institution Scholarly Studies Grant Program, the National Institutes of Health (1KO-01-RR17310-01) and a Graduate Fellowship awarded to K.M.P. by the University of Maryland.
- Fecal steroids
- Gonadotropin-releasing hormone