Ovulation in the PMSG-Treated Immature Mouse: Effect of Dose, Age, Weight, Puberty, Season and Strain (BALB/c, 129 and C129F1 Hybrid)

Ovulation occurred in 99% of 22-to 27-day-old (BALB/c X 129) F 1 , hybrid mice (C129F 1 ) 3 mornings after a single afternoon injection of pregnant mare’s serum gonadotropin (PMSG; 2.5 IU). By contrast, inbred mice of the 2 parental strains ovulated less regularly and the optimal age for treatment with PMSG was several days later than that for the F 1 hybrids. The following findings were based on studies using C129F 1 hybrid mice. The average number of ova ovulated was highest (approximately 40 ova) in females 19-23 days old when treated with 2.5 IU PMSG. Doses of 1.25 IU PMSG and lower resulted in numbers of ova equal to, or fewer than, those at spontaneous ovulation. PMSG doses of 0.63 IU and lower, or 5 IU and higher, were suboptimal in terms of frequency of mice ovulating. In mice over 4 weeks of age at treatment, the percentage which ovulated was significantly lower in postpubertal than in prepubertal mice. Other factors which were associated with a decreased incidence of PMSG-induced ovulation in immature mice were: loss of body weight and treatment during January (in contrast to February through October). It is proposed that PMSG-induced ovulation in the immature mouse (under readily controlled conditions, as described) is a particularly efficient and economical mammalian system either for studies of neurophysiological processes leading to ovulation or for bioassays of the ovulation-inhibiting effects of various pharmacological or environmental agents.