Orexins (A & B) are derived from a common 130 amino acid precursor peptide, prepro-orexin, by proteolytic cleavage, and orchestrate their actions by binding and activating two types of G-protein coupled receptors, orexin-1 receptor (OX1R) and orexin-2 receptor (OX2R). The OX1R preferentially binds orexin-A, whilst OX2R binds both orexin-A and ?B, with similar affinity. Besides playing a role in the regulation of feeding and energy homeostasis, orexins appear to increase sexual arousal as well as the copulatory performance in rats. Furthermore, orexin-A and -B immunoreactivity has been detected in rat testis. In view of these findings we investigated the expression of orexin receptors and their signalling characteristics in the human male reproductive system.
Using RT-PCR analysis, we were able to demonstrate for first time that both OX1R and OX2R are expressed at the testis, epididymis, penis and seminal vesicle. Western blotting analysis revealed that both orexin receptors are expressed in human testis with apparent molecular weights of 52kDa for OX1R and 37kDa for OX2R. Immunofluorescent studies revealed intense staining for OX2R and ?to a lesser effect- for OX1R in Leydig cells and in the myoid cells of the seminiferous tubules. To test the orexin ability to activate testicular phospholipase C, we determined the effect of orexin-A and Orexin-B on inositol triphosphate (IP3) production. When membranes from testicular cells were incubated with Orexin-A or Orexin-B there was a rapid IP3 turnover produced by orexin-B but not by orexin-A. This effect appeared to be dose-dependent. This orexin-B effect has a threshold of 1nM and a maximum response at 100nM (47±13% of basal).
These data provide a novel insight into the expression and signalling characteristics of orexin receptors in the human male reproductive system and implicate further these peptides, acting in an autocrine/paracrine manner, into the regulation of arousal mechanisms in humans.
Funded by the City of Coventry General Charities
03 - 05 Nov 2003
Society for Endocrinology