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Submitted on October 20, 2003
Accepted on January 26, 2004
Department of Cell Biology, Physiology and Immunology (M.L.B., R.P., V.M.N., L.P. E.A., M.T.-S.), University of Córdoba, 14004 Córdoba, Spain; Department of Physiology (M.L., R.S., C.D.), University of Santiago de Compostela, 15705 Santiago de Compostela, Spain; and Departments of Physiology and Pediatrics (J.S.S., J.T.), University of Turku, 20520 Turku, Finland
* To whom correspondence should be addressed. E-mail: fi1tesem{at}uco.es.
Orexins are hypothalamic neuropeptides primarily involved in the regulation of food-intake and arousal states. In addition, a role for orexins as central neuronedocrine modulators of reproductive function has recently emerged. Prepro-orexin and orexin type-1 receptor mRNAs have been detected in the rat testis. This raises the possibility of additional peripheral actions of orexins in the control of reproductive axis, which remains so far unexplored. To analyze the biological effects and mechanisms of action of orexins in the male gonad, we evaluated testicular expression of orexin type-1 (OX1) and type-2 (OX2) receptor mRNAs in different experimental settings, and the effect of orexin-A on testicular testosterone secretion. Persistent expression of OX1 receptor mRNA was demonstrated in the rat testis throughout post-natal development. In contrast, OX2 receptor transcript was not detected at any developmental stage. Expression of OX1 receptor mRNA persisted after selective elimination of mature Leydig cells, and was detected in isolated seminiferous tubules at defined stages of the seminiferous epithelial cycle. In addition, testicular OX1 receptor mRNA expression appeared to be under hormonal regulation: it was reduced by long-term hypophysectomy and partially restored by FSH replacement, whereas down-regulation was observed after exposure to increasing doses of the ligand in vitro. Moreover, OX1 receptor mRNA expression was sensitive to neonatal imprinting by estrogen. Finally, orexin-A, in a dose-dependent manner, significantly increased basal, but not human choriogonadotropin-stimulated, testosterone secretion in vitro. A similar stimulatory effect was observed in vivo following intratesticular administration of orexin-A. In conclusion, our present results provide the first evidence for the regulated expression of OX1 receptor mRNA and functional role of orexin-A in the rat testis. Overall, our data are suggestive of a novel site of action of orexins in the control of male reproductive axis.
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