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Orexins A and B are hypothalamic neuropeptides associated with homeostasis and the reproductive system. The aim of the study was to compare the expression of the prepro-orexin gene and the intensity of orexins immunoreactivity in the porcine ovary (corpora lutea, granulosa and theca interna cells) during four different stages of the oestrous cycle (days: 2-3, 10-12, 14-16 and 17-19) and to examine the in vitro effect of orexins on the secretion of steroid hormones by porcine luteal, granulosa and theca interna cells. The highest expression of prepro-orexin mRNA was observed in theca interna cells on days 17-19 of the oestrous cycle. The highest content of immunoreactive orexin A was noted in corpora lutea on days 10-12 and the highest level of immunoreactive orexin B on days 14-16 of the cycle. Immunoreactive orexin A concentrations were higher in theca interna cells than in granulosa cells, whereas similar levels of immunoreactive orexin B were observed in both cell types. Under in vitro conditions, at the concentration of 10 nM, orexins A and B inhibited FSH-induced oestradiol secretion by granulosa cells. The obtained results suggest that the pattern of orexin peptide expression in the porcine ovary is related to the animals' hormonal status. Our findings imply that orexins can affect porcine reproductive functions through modulation of ovarian steroidogenesis.

Orexin A and B, also termed hypocretin 1 and 2, are associated with the stimulation of food intake and arousal. The biological actions of the hormones are mediated via two distinct G protein-coupled receptors, termed orexin receptor 1 (OX1R) and orexin receptor 2 (OX2R). OX1R is selective for orexin A and OX2R binds orexin A and orexin B with similar affinity. The present study analyzed mRNA and protein expressions of OX1R and OX2R in adenohypophysis (AP) and neurohypophysis (NP) of cycling pigs. The tissue samples were harvested on days 2-3, 10-12, 14-16, and 17-19 of the oestrous cycle. Using quantitative real-time PCR higher OX1R gene expression was detected in AP on days 2-3 relative to days 10-12, 14-16 and 17-19 (p0.05). In NP the OX1R mRNA level was elevated on days 10-12 compared to the remaining stages (p0.05). OX2R gene expression in AP was the lowest on days 10-12 (p0.05 compared to days 2-3 and 17-19) and the expression peak occurred on days 17-19 (p0.05 vs. the all studied stages). In NP the highest (p0.05) expression of OX2R mRNA was noted on days 17-19 in relation to the remaining periods. OX1R protein content in AP was greatest on days 10-12 (p0.05), whereas in NP it was greatest on days 2-3 and 14-16 (p0.05 vs. days 10-12 and 17-19). In both cases the lowest OX1R protein expression was observed during follicular phase (p0.05 in relation to three remaining studied stages). OX2R protein in AP was lower (p0.05) on days 2-3 and 14-16 compared to days 10-12 and 17-19. In NP the lowest (p0.05) expression of this protein was on days 17-19 and the highest on days 10-12 (p0.05 compared to days 2-3 and 17-19). In summary, the present findings provide the first evidence that OX1R and OX2R mRNAs and proteins occur in the pituitary of the pig and indicate the dependence of orexin receptor expression on the endocrine reproductive state.