Effects of castration on entrainment of the peripheral corticosterone rhythm in rats before and after puberty.

Abstract

Rats reared in an 8 h light, 16 h dark photoperiod (8:16) exhibit an evening peak of serum corticosterone at 22.00 h colony time. When exposed to a new test cycle of 14:10, the animals first show a blunted peak at 18.00 h and 22.00 h and then a clear peak at 18.00 h with loss of the 22.00 h peak. In this study, rats 30 days of age were ovariectomized during the challenge of a new light-dark cycle (14:10) to determine whether the ovaries were necessary for the adaptation of the peripheral corticosterone rhythm prior to puberty in response to a photoperiod challenge. At 31 days of age, all three groups (intact, sham-operated and gonadectomized on day 30) exhibited elevated corticosterone levels at 14.00 through 22.00 h which fell to somewhat lower values in the morning of day 32. The intact groups showed a peak at 18.00 h on day 32 while the other two groups had a less defined elevation in corticosterone in the evening persisting through 22.00 h. All three groups showed a clearcut fall at 02.00 h and 06.00 h on day 33. On day 35, intact and gonadectomized (OVX) rats showed similar patterns and levels of corticosterone, but both higher amplitude and elevated daytime values were observed in the sham-operated group. The latter animals were undergoing puberty (vaginal opening, day 34–35) while the other two groups were still prepuberal. In the OVX rats, the peak of serum corticosterone was observed at 22.00 h 10 and 20 days after surgery. Intact and sham-operated controls retained a peak at 18.00 h. Adults ovariectomized after puberty showed a reduction in daily corticosterone levels but did not exhibit a phase shift once the peak emerged at 18.00 h. It can be concluded that either sham surgery or ovariectomy performed on day 30 can partially disrupt the rephasing of the serum corticosterone rhythm in prepuberal rats, but that the pattern quickly normalizes. This suggests that the ovaries are not necessary for the emergence of a phase shift in response to a change from a short day (8:16) to a long day (14:10) cycle. However, maintenance of the phase of the corticosterone rhythm in the new cycle does depend upon ovarian secretion in rats ovariectomized before puberty, but not in adult rats.