Exposure of intact female hamsters to light:dark cycles of 1 hr light alternating with 23 hr darkness (LD 1:23) led to involution of the reproductive organs, a drop in prolactin levels within the anterior pituitary gland, and hypertrophy of the intrascapular brown fat. All of these changes were prevented if the animals were pinealectomized. Pituitary prolactin levels in control hamsters maintained in LD 14:13 cycles also decreased during the 8 wk experimental period. Pituitaries of hamsters killed at the end of the experiment contained about 4/5 less prolactin than those necropsied at the beginning of the study.
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