Changes in Adrenocortical Function Precede the Onset of Persistent Estrus in Rats Exposed to Constant Light Before Puberty

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Abstract
The purpose of this study was to determine whether changes in adrenal function precede the onset of persistent estrus in rats undergoing exposure to constant light either from birth (LL) or after weaning age (S-LL-shift to LL from a 14 h light, 10 h dark photoperiod). Serum corticosterone shifts in response to a two minute ether stress administered at 0800 h at 25, 50 or 100 days of age were compared in LL, S-LL and LD (reared from birth in 14:10) rats. The patterns of serum corticosterone were comparable in all three groups at 25 days of age. At 50 days of age, the S-LL rats had higher basal corticosterone and showed an earlier stress response which persisted for 2 h while the other two groups showed a slower response that peaked at the 30 min sample period and then fell to baseline by 60 min. At 100 days of age, the LD rats continued to show a slower rise and more rapid fall while both LL exposed groups had higher basal levels, a more rapid response and persistent elevation past 60 min. This difference in the appearance of a deviant stress pattern was associated with the onset of persistent estrus which began at 100 days of age in the S-LL rats and at 150 days in the LL rats. It therefore appears that changes in adrenocortical function precede the loss of ovulatory cycles in both LL-reared groups and that these changes (elevated basal corticosterone and a prolonged stress response) emerge earlier in the S-LL groups which developed persistent estrus earlier.

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