| Abstract: | Plasma luteinizing hormone (LH) levels were determined at monthly intervals in intact and orchidectomized ground squirrels maintained in a constant 14L:10D photoperiod at a temperature of 23 +/- 2 degrees C. Animals were orchidectomized or sham operated at different ages and at different stages of the annual reproductive cycle. LH levels were elevated in orchidectomized as compared to intact males within 1 month, and often within 48 h, after orchidectomies performed either before or after the normal breeding season. LH levels tended to remain chronically elevated in long-term orchidectomized males. Although negative feedback regulation of LH by the testes was evident at most stages of the reproductive cycle, effects of orchidectomy on LH were most pronounced during the nonbreeding season when plasma LH titers of intact males were at their annual nadirs, and less pronounced or absent during the breeding season when levels were normally elevated in intact males. For most of the year male gonadal hormones exert negative feedback on LH release. We hypothesize that under the influence of a circannual timing mechanism, sensitivity to steroid feedback is reduced as the breeding season approaches and LH secretion increases despite the attendant rise in plasma androgen levels. These results contrast with data for female squirrels in which there is a seasonally restricted and characteristically brief elevation of plasma LH after ovariectomy; in females, but not in males, onset and termination of LH secretion appear to be regulated by a circannual clock that functions independently of gonadal secretions. |
