99 GLUCOCORTICOID RECEPTORS ARE EXPRESSED IN OVARIES OF NEWBORN AND ADULT FEMALE HORSES

Abstract

In contrast to other domestic animal species, in vitro maturation (IVM) of oocytes in the horse is still not successful. Oocytes for IVM are obtained either from slaughterhouse ovaries or via ovum pick-up from living mares. Both situations may be associated with a stress-induced glucocorticoid release. So far, neither an involvement of glucocorticoids in follicle and oocyte maturation nor the presence of glucocorticoid receptors (GCR) in ovarian tissue has been investigated in the horse. We hypothesised that GCR are expressed in equine ovarian tissue independent of the animal's age and stage of the oestrous cycle. Ovaries (n = 40) were collected from killed newborn female foals (n = 10) and killed or slaughtered adult mares (n = 10). For assessment of GCR mRNA expression, ovarian samples were fixed in Tissue-Tek O.C.T. Compound (Sakura Finetek, Zoeterwoude, the Netherlands) and stored at –80°C. Various cell populations were isolated using laser capture microdissection on cryosections. After RNA extraction, samples were analysed by qualitative RT-PCR and real time-PCR. For analysis of GCR protein, tissue was fixed in Bouin's solution and histological slides immunostained using a monoclonal antibody for GCR (Ab2768, Abcam, Cambridge, UK), followed by visualisation with diaminobenzidine. One tertiary follicle per slide (40×; light microscopy) was analysed and percentages of cells staining positive for GCR calculated. Statistical analysis was done with the SPSS Statistics 21 software (SPSS Inc., Chicago, IL, USA). Expression of mRNA for GCR was detected in oocytes, cumulus cells, granulosa, and theca cells, independent of age and stage of the oestrous cycle. In both neonates and adults, nuclei of the oocytes and cumulus cells stained positive for GCR regardless of stage of folliculogenesis. Also, GCR were constantly expressed in granulosa cells from both preantral and antral follicles. Percentage of granulosa cells staining positive for GCR (adult: 73.6 ± 3.2, fillies: 72.4 ± 1.9%) was higher (P < 0.001) than of theca cells (adult: 56.8 ± 3.9, fillies: 57.2 ± 1.9%), but not affected by age. GCR were lacking in ovarian stroma of adults but not of neonates. In periovulatory follicles from adult mares, GCR were abundant in developing luteal cells. GCR were also detected in the nuclei of luteal cells in corpora haemorrhagica and corpora lutea. Follicular atresia was associated with a decrease of GCR independent of cell type and age. This study describes for the first time the expression of GCR in horse ovaries, which are present independent of age of the animal, stage of folliculogenesis, and oestrous cycle stage. Results suggest that glucocorticoids are involved in follicular and oocyte maturation, ovulation, and luteal function in the horse. Presence of GCR in the ovaries of newborn horses suggests a role of glucocorticoids in ovarian tissue maturation. Nevertheless, detrimental effects of excess glucocorticoid secretion due to stress on follicular development, oocyte maturation, and luteal function cannot be excluded in the mare.