Changes in Serum Immunoreactive Inhibin-α during Photoperiod-Induced Testicular Regression and Recrudescence in the Golden Hamster

BIOLOGY OF REPRODUCTION 49, 483-488 (1993) Changes in Serum Immunoreactive Inhibin-u during Photoperiod-Induced Testicular Regression and Recrudescence in the Golden Hamster1 JOHN D. KIRBY, 2 AMY E. JETTON, 3 JACQUELINE F. ACKLAND, FRED W. TUREK, and NEENA B. SCHWARTZ Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois, 60208 ABSTRACT INTRODUCTION The male golden hamster has been an invaluable model for studies of the control of reproduction by photoperiod. When hamsters are transferred from long days to short days (<12 h of light), a severe drop in testis weight, spermatogenesis, serum testosterone (T), and gonadotropins occurs [1, 2]. Conversely, hamsters with regressed testes resulting from prolonged exposure to short days demonstrate a rise in serum hormone levels and testicular recrudescence when placed in long-day lighting conditions [2, 3]. Photoperiod-induced changes in serum FSH are known to precede changes in serum LH in several species, including the hamster [2, 3]. This differential regulation of LH and FSH secretion suggests that changes in photoperiod may affect the hypothalamic-pituitary axis at more than one location. Furthermore, it is known that the negative feedback effects of T, which suppresses release of both LH and FSH, are more pronounced in short-day hamsters than in longday animals [4, 5]. These two sets of observations raise the possibility that gonadal inhibin, a specific inhibitor of pituitary FSH secretion [6, 7], may be dissociated from changes in steroid secretion during alterations in photoperiod and/ or that changes in negative feedback by inhibin may occur. Accepted April 12, 1993. Received November 20, 1992. 'Portions of this work were presented at the 21st annual meeting of the Society for Neuroscience, Nov. 11-15, New Orleans, LA, 1991. This research was supported by NIH grants R01-HD-07068, R01-HD-09885, P01 HD-21921, P30-HD-28048, and USDA 91-37203-6890. 2Correspondence and current address: Dr. John D. Kirby, Animal Sciences, B114, University of Arkansas, Fayetteville, AR 72701. 3Current address: Department of Zoology, Morrill Science Center, University of Massachusetts, Amherst, MA 01003. The validation of an inhibin assay for the golden hamster has made it possible to investigate whether changes in serum immunoreactive inhibin (irIa) levels accompany photoperiod-induced changes in the reproductive axis and to compare these changes to changes in FSH secretion. In the present study, hamsters with both active and photoperiodically regressed testes were castrated to ensure that any observed photoperiod-induced changes in irIa levels were caused by altered testis function. In subsequent experiments, the temporal aspects of the endocrine milieu during both photostimulation and photoperiodic gonadal regression were investigated. This study demonstrates that serum irIlo is dramatically altered in hamsters undergoing testicular collapse or recrudescence and that the changes in this hormone are temporally divergent from changes in serum FSH, T, and LH. MATERIALS AND METHODS Animals In each of the three experiments, male golden hamsters (Mesocricetus auretus), 9 wk old, were purchased from Charles River Lakeview (Wilmington, MA), and maintained on a 14L: 10D cycle for a 2-wk acclimatization period. At all times, hamsters were carefully monitored and maintained in accordance with the NIH Guide for the Care and Use of Laboratory animals. Experimental Procedures In the first experiment, hamsters (n = 36) were either placed in a 6L:18D photoperiod to induce maximal gonadal regression or maintained in 14L:10D for an addi- 483 Serum immunoreactive inhibin-a (irla), FSH, LH, and testosterone (T) were measured in male golden hamsters during shortday-induced testicular regression and during testicular recrudescence following the transfer from short to long days. Serum FSH levels were maximally suppressed within 2 wk of transfer to short days. In contrast to FSH, irla levels were not fully reduced until after 6 wk of exposure to short days, closely paralleling the timing of testicular regression. LH and T levels were also reduced within two 2k, with maximal suppression observed between 6 and 8 wk. Conversely, when males with regressed testes were transferred to long days, serum FSH rose to peak (25 ng/ml) levels by 3 wk and then declined to usual long-day levels. In contrast, serum irla levels rose gradudally, reaching adult' long-day levels following 10 wk of exposure. Serum LH and T levels rose to peak levels between 5 and 8 wk before declining to adult levels. FSH, LH, and irla levels were also measured after castration in male hamsters maintained on long or short days. Twenty-four hours after castration, levels of irIa were reduced in long-day males to levels comparable to those observed in castrated short-day males. Serum irIa levels respond slowly to abrupt changes in FSH levels after transfer to either long or short days, suggesting that testicular irloe secretion may not be directly and immediately influenced by circulating FSH levels in the hamster. Taken together, these results demonstrate that gradual changes in serum irla occur during photoperiod-induced changes in gonadal activity and that these changes are associated with gonadal function. 484 KIRBY ET AL. 100 80 60 0 m 40 0 100 10 GOLDEN HAMSTER SERUM (I/TUBE) PORCINE INHIBIN a-SUBUNIT (1-26)-Gly-Tyr (pg/TUBE) FIG. 1. Standard curve for inhibin RIA. The RIA standard curve is plotted as percent maximum binding against the quantity of porcine inhibin-a (1-26)-Gly-Tyr standard. Displacement of bound tracer by sera is plotted as the volume of serum assayed. Parallelism was assessed by regression analysis and comparison of the slopes by F test. tional 10 wk. One half of the males in each group were randomly selected and castrated under methoxyflurane anesthesia. Twenty-four hours later, blood was drawn by cardiac puncture from both castrated and intact males in each light treatment. After clotting at room temperature, sera were centrifuged, aliquoted, and stored at -20°C for detection of hormones by RIA. For the second experiment, hamsters (n = 18) were transferred from 14L: 10D to 6L: 18D for 10 wk. On the day of transfer, each animal was weighed and anesthetized with ketamine (100 mg/kg), and the right testis width was measured. The animals were then divided into 2 groups (9/ group), with right testis widths and blood samples, drawn by cardiac puncture under methyoxyflurane anesthesia, being taken on alternate weeks (i.e., group A: Weeks 1, 3, 5, 7, 9, and 10; group B: Weeks 0, 2, 4, 6, 8, and 10). Sera were prepared and stored as described above for subsequent RIA The third experiment required that another group of male hamsters (n = 18) be placed on short days (6L: 18D) for a period of 10 wk to induce testicular regression. After ensuring regression, the animals were divided into 2 groups (n = 9/group) and transferred to long days (...truncated)