Photoperiod-dependent regulation of inhibin in Siberian hamsters: I. Ovarian inhibin production and secretion

Abstract

Follicle-stimulating hormone (FSH) stimulates ovarian follicle development and the production of protein hormones including inhibin A and inhibin B. The inhibins are dimeric proteins ( -A or -B ) secreted by growing follicles that suppress FSH in a classical endocrine negative feedback loop. Siberian hamsters, Phodopus sungorus, exhibit seasonal variation in FSH levels. Given the role of inhibin in FSH regulation, we hypothesized that ovarian inhibin expression differs between animals reared in long (16 h light:8 h darkness) and short (6 h light:18 h darkness) photoperiods. To examine inhibin expression in animals housed under long or short photoperiods, hamster inhibin -, A -, and B -subunits were cloned and used to detect and localize inhibin subunit mRNA in developing follicles. Ovarian inhibin -subunit mRNA levels were significantly higher in long day-exposed (LD) than in short day-exposed (SD) hamsters. In addition, dimeric inhibin, as well as inhibin -, A -, and B -subunit protein levels were higher in the LD than in the SD hamster ovaries.

Photoperiod-dependent regulation of inhibin in Siberian hamsters: II. Regulation of inhibin production and secretion by pregnant mare serum gonadotropin

“…FSH stimulates the production of gonadal inhibins which, in turn, feedback to the pituitary to inhibit FSH synthesis and secretion (Woodruff et al 1996, Bernard et al 2001. Because serum FSH levels are higher in LD than in SD hamsters as shown in the companion paper (Kenny et al 2002), we hypothesized that inhibin may be differentially regulated in different photoperiods. Several important and novel observations were made including the ability of young (three-week-old), immature female hamsters reared in long day lengths to immediately respond to PMSG by synthesizing and secreting inhibin A and inhibin B. Conversely, older Siberian hamsters in which reproductive maturation had been suppressed by exposure to short photoperiods do not secrete significant quantities of inhibin A or inhibin B following treatment with PMSG.…”

“…The processing and cleavage of inhibin is not understood in vivo or in vitro. In the companion paper, we demonstrate that both the quantity and kind of inhibin is reduced in Siberian hamster (Phodopus sungorus) ovaries when the animals are housed in short rather than long days (Kenny et al 2002). Little or no mature, fully processed inhibin is detected in ovaries of females exposed to long day (LD) or short day (SD) photoperiods and the overall levels of unprocessed inhibin in the ovaries of SD females are approximately half of the unprocessed inhibin in ovaries of LD females.…”

“…Animals were housed as described in the companion paper (Kenny et al 2002). Two animal groups were analyzed.…”

“…The in situ hybridization, protein extraction, Western blot analysis, and hormone assay procedures were conducted as described in the companion paper (Kenny et al 2002).…”

“…While not originally appreciated, it is now clear that the differential regulation of the -subunits is critically important to the control of the normal reproductive cycle in polyovular as well as mono-ovular species (Fraser et al 1995, Woodruff et al 1996. Thus, the finding of differential compartmentalization of the mRNAs encoding the two -subunits (companion paper Kenny et al 2002) may be a first step towards an understanding of these mechanisms. Inhibin A and inhibin B are synthesized as prohormones that are cleaved in order to release the biologically active C-termini of their constitutive -and -subunits (Mason et al 1996).…”