Daily GnRH agonist treatment effectively delayed puberty in female rats without long-term effects on sexual behavior or estrous cyclicity.

The present study examined the long-term effects of suppressing puberty with a GnRH agonist on reproductive physiology and behavior in female rats. We have recently reported that administration of the GnRH agonist leuprolide acetate (25 µg/kg) daily between postnatal day (PD) 25-50 delayed puberty and disrupted the development of copulatory behavior and sexual motivation in male rats. However, pilot data from our lab suggest that this low dose of leuprolide acetate (25 µg/kg) was not high enough to significantly delay puberty in female rats. Therefore, we injected female Long-Evans rats with leuprolide acetate at a higher dose (50 µg/kg) or 0.9% sterile saline, daily , starting on PD 25 and ending on PD 50. Vaginal opening was monitored daily starting on PD 30 for signs of pubertal onset and first estrous cycle. In addition, we measured estrous cyclicity starting approximately 2 weeks after the last injection of leuprolide (∼PD 64). Immediately after monitoring estrous cyclicity, the female rats were mated on their first day in behavioral estrus using the partner-preference paradigm, with and without physical contact (PD 95-110). We found that this dose of leuprolide (50 µg/kg) significantly delayed puberty; however, neither estrous cyclicity nor sexual motivation was significantly affected by periadolescent exposure to leuprolide. Together with our findings in male rats, these results add to our understanding of the developmental effects of chemically suppressing puberty in rats.

Daily GnRH agonist treatment delays the development of reproductive physiology and behavior in male rats.

The present study was designed to examine the effects of suppressing pubertal onset with leuprolide acetate, a gonadotropin releasing hormone (GnRH) agonist. Starting on postnatal day (PD) 25, male Long-Evans rats were injected daily with either leuprolide acetate (25 µg/kg dissolved in 0.9% sterile physiological saline; n = 13) or sterile physiological saline (1.0 ml/kg 0.9% NaCl; n = 14) for a total of 25 days. Males were monitored daily for signs of puberty (i.e., preputial separation). On the last day of leuprolide treatment (PD 50), half of each treatment group was injected with 10.0 µg of estradiol benzoate (EB) daily for three consecutive days (PD 50-52) and 1.0 mg of progesterone (P) on the 4th day (PD 53), whereas the other half of each treatment group received oil injections. Four hours after P injections, all subjects were given the opportunity to interact with a gonadally-intact male and a sexually receptive female rat (i.e., a partner-preference test with and without physical contact). Copulatory behavior and sexual motivation were measured. Hormone injections and mating tests were repeated weekly for a total of 3 consecutive weeks. Results showed that leuprolide delayed puberty as well as the development of copulatory behavior and the expression of sexual motivation. By the last test, the leuprolide-treated subjects showed signs of catching up, however, many continued to be delayed. Estradiol and progesterone mildly feminized male physiology (e.g., decreased testes weight and serum testosterone) and behavior (e.g., increased lordosis), but did not interact with leuprolide treatment.