Rapid Cessation of Estrous Cyclicity and Depressed Castration Response in Short Photoperiod-Treated, Inbred LSH/SsLak Hamsters1

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Abstract
This study examined the effects of transfer from long photoperiod (LP) to short photoperiod (SP) on the cessation of ovarian cyclicity and the castration response in inbred LSH/SsLak golden Syrian hamsters. Forty-six 8 to 10-wk-old female hamsters were acclimatized in LP (14L:10D; lights on at 0600 h) during which time animals were monitored for regular ovarian cyclicity. Twenty-six animals were transferred to SP (8L:16D; lights on at 0600 h) and examined daily for vaginal discharges. One day after the day of the first missed ovulation, individual SP-exposed animals were bilaterally ovariectomized; concomitantly, an LP control animal in diestrus I underwent the same procedure. Thirty days after ovariectomy, the hamsters were fitted with intra-atrial silastic cannulae. On the following two postoperative days, 0.6 ml blood samples were collected at 0700, 1200, 1400, and 1600 h for SP animals and at 0700, 1400, 1600 and 1800 h for LP controls. On the third day, the animals were decapitated and sera and pituitaries saved for determination of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL) by radioimmunoassay (RIA). All SP-exposed animals displayed their last estrous discharge 14-34 days after transfer to SP (mean = 23.0 .+-. 0.8 days). Their ovaries were characterized by the absence of corpora lutea, the presence of large atretic antral follicles, few growing follicles, and interstitium that was stimulated to varying degrees. Total and adjusted pituitary weights were decreased by SP exposure (p < 0.001). Compared with LP controls, SP-treated animals demonstrated significantly depressed tonic levels of serum LH and suppressed surges, which occurred at 1400 h (p < 0.1 on postoperative Day 1; p < 0.001 on Day 2). Levels of serum FSH in SP-treated animals were also significantly depressed at all times points (p < 0.001), although afternoon surges were still detectable. This contrasted with LP controls in which consistently high levels of FSH were observed. Pituitary gonadotropin contents, similar to serum levels, were reduced by SP treatment (p < 0.001), as were PRL serum and pituitary values (p < 0.001). The above results suggest that LSH/SsLak female hamsters respond rapidly and uniformly to SP treatment with cessation of ovarian cyclicity and depression of the castration response.