Expression of orexin receptors 1 (OX1R) and 2 (OX2R) in the porcine pituitary during the oestrous cycle

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 (p < 0.05). In NP the OX1R mRNA level was elevated on days 10-12 compared to the remaining stages (p < 0.05). OX2R gene expression in AP was the lowest on days 10-12 (p < 0.05 compared to days 2-3 and 17-19) and the expression peak occurred on days 17-19 (p < 0.05 vs. the all studied stages). In NP the highest (p < 0.05) 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 (p < 0.05), whereas in NP it was greatest on days 2-3 and 14-16 (p < 0.05 vs. days 10-12 and 17-19). In both cases the lowest OX1R protein expression was observed during follicular phase (p < 0.05 in relation to three remaining studied stages). OX2R protein in AP was lower (p < 0.05) on days 2-3 and 14-16 compared to days 10-12 and 17-19. In NP the lowest (p < 0.05) expression of this protein was on days 17-19 and the highest on days 10-12 (p < 0.05 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.