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

Abstract

Several data suggest that melatonin may influence avian reproduction by acting at the level of the hypothalamic-hypophisial-gonadal axis, and/or on neural circuits controlling reproductive behaviours. The action of melatonin is exerted through specific receptors whose distribution and pharmacological 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 Japanese quail. Melatonin receptors are widely distributed in avian brain. They are mostly present in the visual pathways of all the investigated species and in the song controlling nuclei of oscine birds. Sexual dimorphism of melatonin binding sites (higher density in males than in females) was detected in some telencephalic nuclei of songbirds, in the visual pathways, and in the preoptic area of quail. The last region plays a key role in the activation of male quail copulatory behaviour and it hosts a large population of gonadotropin-releasing hormone-containing neurons. Sexual dimorphism of melatonin-binding sites in the above-mentioned regions suggests a differential role for this hormone in the modulation of visual perception, gonadotropin production, and seasonally activated behaviours in male and female quail. Further studies are necessary to understand interrelationships among photic cues, gonadal steroids, density, and sexually dimorphic distribution of melatonin receptors.

Sexually Dimorphic Effects of Melatonin on Brain Arginine Vasotocin Immunoreactivity in Green Treefrogs <b><i>(Hyla cinerea)</i></b>

“…For example, whether melatonin receptors are co-localized within AVT-ir neurons in these brain regions is unknown. Furthermore, it is unknown whether the distribution and/or expression of melatonin receptors in green treefrogs is sexually dimorphic, as has been reported for birds (reviewed in Aste et al [2001]). While it is possible that male testosterone concentrations regulate the regional distribution of melatonin receptors, available evidence in birds and mammals suggests that melatonin receptor expression is independent of gonadal androgens [Skene et al 1993; Bentley and Ball 2000].…”

Melatonin Receptor Expression in the Zebra Finch Brain and Peripheral Tissues

“…Studies on avian melatonin receptors mainly focused on the retina (Rada and Wiechmann, 2006), gonads (Aste et al, 2001) and song-control nuclei in the brain (Cassone et al, 2008;Whitfield-Rucker and Cassone, 1996) photoreception takes place in the retina only and there has been an evolutionary loss of two retinal cone classes (Barrett et al, 2003) and of certain opsins which are found in nonmammalian vertebrates including birds and fishes (Bellingham et al, 2003). These differences in circadian organisation are believed to be the consequence of a "nocturnal bottleneck" during early evolution of mammals (Foster et al, 1993).…”

Distribution of 2-[¹²⁵I]iodomelatonin Binding in the Brain of Mexican Free-Tailed Bats <i>(Tadarida brasiliensis)</i>

“…Our findings represent the first case of sexual dimorphism in IMEL binding in a mammalian brain region. The only other reported instances of sexual dimorphism in IMEL binding are in the song control system of birds and parts of the visual system and preoptic area in quail, which all displayed higher binding in males [Gahr and Kosar, 1996; Whitfield-Rucker and Cassone, 1996; Aste et al, 2001]. The reasons for these differences are unclear, but could potentially be the result of sexually dimorphic neurotransmitters, specifically GABA, which, when given at physiological doses, increases IMEL binding in several brain regions of the quail, including the preoptic area [Canonaco et al, 1994].…”