Serum FSH and LH concentrations were determined in normal adult female rats which were made persistent-estrous by exposure to constant illumination for periods of 2-200 days. The earliest discernible effect was a decreased incidence of the normal, late afternoon proestrous LH “surge,” even during the first vaginal smear cycle following exposure to constant illumination. In the 4th to 6th vaginal cycles following exposure, only 1 of 19 animals exhibited the late afternoon proestrous LH “surge.” Contrary to a report of others (8) which indicated that serum LH concentrations in persistent- estrous rats were as high as those of castrates, mean serum LH levels in persistentestrous rats never exceeded 200 ng NIAMD-Rat LH-RP-1/ml. These values are only slightly higher than those of the estrous stage of a normal estrous cycle, and markedly lower than those of ovariectomized animals. Mean serum FSH levels in persistent estrous rats were as high as 200 ng NIAMD-Rat FSH-RP-1/ml, values slightly elevated above those of the diestrous stage, but substantially below those of the estrous stage of the normal estrous cycle. The “persistent estrus” of rats exposed to constant illumination may be explained, in part, by an unremitting, but only slight, hypersecretion of both FSH and LH, rather than by a highly elevated hypersecretion of either, or both. (Endocrinology88: 964, 1971)
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Daane et al. (1971) studied this question.