Sexual differences and effect of photoperiod on melatonin receptor in avian brain

Several data suggest that melatonin may influence avian reproduction by act ing at the level of the hypothalamic-hypophisial-gonadal axis, and/or on ne ural circuits controlling reproductive behaviours. The action of melatonin is exerted through specific receptors whose distribution and pharmacologica l properties have been extensively investigated. This review will focus on the distribution, sexual dimorphism, and dependence upon the photoperiod of melatonin binding sites in avian species with a special emphasis on Japane se quail. Melatonin receptors are widely distributed in avian brain. They a re mostly present in the visual pathways of all the investigated species an d in the song controlling nuclei of oscine birds. Sexual dimorphism of mela tonin binding sites (higher density in males than in females) was detected in some telencephalic nuclei of songbirds, in the visual pathways, and in t he preoptic area of quail. The last region plays a key role in the activati on of male quail copulatory behaviour and it hosts a large population of go nadotropin-releasing hormone-containing neurons. Sexual dimorphism of melat onin-binding sites in the above-mentioned regions suggests a differential r ole for this hormone in the modulation of visual perception, gonadotropin p roduction, and seasonally activated behaviours in male and female quail. Fu rther studies are necessary to understand interrelationships among photic c ues, gonadal steroids, density, and sexually dimorphic distribution of mela tonin receptors. Microsc. Res. Tech. 55: 37-47, 2001. (C) 2001 Wiley-Liss, Inc.