Uncoupling of vaginal opening and the first ovulation--an indication of an alteration in the pituitary-gonadal axis.

Abstract

The onset of puberty in the female rat has been defined as vaginal opening and/or the first ovulation. The present study examines the effects of types of caging upon the events of puberty and the cycle. ARS Sprague Dawley rats were received at 18 days of age and housed in either plastic translucent cages with filter tops or metal cages and exposed to 14L:1OD. Plastic caged animals exhibited vaginal opening earlier (33.9 ± 0.5 days) than metal caged animals (36.6 ± 0.6), but the first vaginal estrus occurred at similar ages. Thus, the two events were coincident in 12 of 16 metal caged animals but only 8 of 20 plastic. Rats completed one full vaginal cycle and were autopsied at the second metestrus. Ovarian histology revealed two sets of corpora and a recent set of ova. Metal caged animals had 7.9 ± 0.3 ova per ovary while plastic caged had 5.8 ± 0.5. The decrease can be accounted for by a decrease in the total number of follicles matured and by the occurrence of 1.3 ± 0.5 ova trapped in stimulated or partially luteinized follicles. Serum hormone measurements revealed higher estrogen and lower progesterone, LH and FSH in the plastic caged animals. Thus, the uncoupling of the events of puberty may be an indication of an alteration in the pituitary-gonadal axis which continues through the second cycle.