Luteotropic Hormone Receptors in the Ovary of the Mink (Mustela vison) during Delayed Implantation and Early-Postimplantation Gestation

Abstract

The reproductive cycle of the mink displays rigid seasonality and obligate embryonic diapause. After ovulation, the corpus luteum (CL) involutes, and it secretes basal progesterone until activated prior to implantation. To study changes in the relative abundance of luteal prolactin and LH receptor mRNA through gestation, ovaries and serum were collected from pregnant female mink at 2-day intervals (n = 3 per date) through embryonic diapause and CL activation (March 19-31) and at 5-day intervals through implantation and early-postimplantation gestation (March 31-April 15). To determine the effect of endogenous prolactin, mink received Alzet osmotic minipumps releasing 2 mg/day 2-bromo-alpha-ergocryptine (bromocriptine) or saline on March 19. Ovaries and serum were taken from 3 animals every 2 days until March 31. Prolactin receptor mRNA in ovaries was low during CL activation but increased 3-fold through embryo implantation. Its abundance correlated with prolactin binding to ovarian membranes and with circulating prolactin. Bromocriptine suppressed endogenous prolactin levels and prevented the increase in prolactin receptor mRNA. There was a transient peak in LH receptor mRNA in the ovaries at March 19-23, which declined to basal levels by March 25 and remained constant through midgestation. Bromocriptine prevented the preimplantation peak in LH receptor mRNA and reduced its abundance below pretreatment levels. The results suggest that prolactin up-regulates its receptor and maintains the LH receptor in the mink CL. The pattern of LH receptor mRNA argues for a role for LH in CL reactivation and termination of embryonic diapause in mink.

Cloning and sequence analysis of the extracellular region of the polar bear (Ursus maritimus) luteinizing hormone receptor (LHr), follicle stimulating hormone receptor (FSHr), and prolactin receptor (PRLr) genes and their expression in the testis of the black bear (Ursus americanus)

“…This observation agrees with studies in numerous species that reported multiple LHr transcripts variable in size, number, and relative abundance in ovaries and testes (Hu et al, 1990;LaPolt et al, 1990;Segaloff et al, 1990b;Shan and Hardy, 1992;Zhang et al, 1994;Chuzel et al, 1995). The major LHr mRNA transcript expressed in the bear testis was 2.5 kb which is similar to the 2.4-kb transcript found in a different carnivore, the mink (Douglas et al, 1998), but in contrast to that in the rat where the major transcript is 6.8 kb. The full length coding region of the LHr is 2.1 kb so that any transcript larger than 2.1 kb can potentially code for the functional form of the receptor (Segaloff et al, 1990a).…”

“…PRLr mRNA expression usually reflects PRLr protein expression (Douglas et al, 1998) and therefore we expected to see PRLr mRNA abundance increase from January to May in the bear testis. In contrast, we observed little difference in the expression of the major PRLr mRNA transcript in the bear testis between January and May.…”

“…Multiple PRLr mRNA transcripts have been identified in many species including the rat, rabbit, human, sheep, and mink Shirota et al, 1990;Dunsanter-Fourt et al, 1991;Douglas et al, 1998;Jabbour et al, 1999). Multiple mRNA transcripts for PRLr arise from alternative exon splicing of the primary PRLr mRNA transcript.…”

Embryonic diapause: development on hold

“…The corpora lutea in both the mink and skunk develop following ovulation, but cell volumes actually decline from their initial size at differentiation (Mead, 1981;Douglas et al, 1998a). Progesterone synthesis occurs, as reflected in circulating concentrations, but at a much reduced level relative to post-implantation gestation (Papke et al, 1980;Mead, 1981;Douglas et al, 1998a).…”

“…Progesterone synthesis occurs, as reflected in circulating concentrations, but at a much reduced level relative to post-implantation gestation (Papke et al, 1980;Mead, 1981;Douglas et al, 1998a). Increased circulating prolactin following the equinox induces expression of both prolactin and LH receptors in the mink corpus luteum, and these two hypophyseal factors then drive progesterone synthesis (Murphy and Rajkumar, 1985;Douglas et al, 1998a). A peculiarity of the mink corpus luteum of diapause is that the cells are not terminally differentiated by luteinization as in other species, but retain their capacity to divide (Douglas et al, 1998b).…”