Comparative Immune-reactivity Patterns of Arginine Vasotocin (AVT) and Melatonin Receptors (Mel1a & Mel1b) in Hypothalamic Regions of Male Japanese Quail Coturnix coturnix japonica : Possible Role in Water-Electrolyte Balance

Abstract

Photoperiod influences circulatory Melatonin (Mel) and hypothalamic functions via retino-hypothalamic tract. However, interrelation between Mel receptors and Arginine Vasotocin (AVT), a water-electrolyte balancing hormone receptor expression in hypothalamic regions of avian brain has never been explored. We noted the expression pattern of two Mel receptors (Mel1a & Mel1b) along with AVT, in terms of neuronal immuno-positivity, in hypothalamic region of Japanese quail under different photoperiodic conditions with/without melatonin treatment. This is interesting and equally important because both Mel and AVT levels are regulated by light/dark cycle. Confocal imaging revealed specific regional localization of Mel1a/Mel1b in Supra-Chiasmatic Nucleus (SCN), Supra-Optic Nucleus (SON) and Para-Ventricular Nucleus (PVN), the intensity of which was dependent on the photoperiodic condition (long day, LD or short day, SD) and melatonin treatment. Mel1a/Mel1b was mostly co-localized along with AVT. Mel1b was abundant in hypothalamic regions in contrast to the Mel1a as reported in mammals. Mel1a immune-positivity was detected in SCN and SON regions of brain. Compared to control birds, a high intensity of Mel1a immunoreactivity was found in hypothalamic regions of birds under short photoperiod (SD, 8h L: 16h D) after Mel treatment. Further, Mel1b immunopositivity was high only in birds exposed to long days (LD, 16h L: 8h D). In SCN, abundant Mel1a and AVT immunoreative cells were found in Mel pretreated and SDexposed birds compared to LD-exposed ones. Mel1a and AVT immunoreative cells were less in PVN of both SD and LD exposed birds. Our data of co-localization of Mel receptor(s) along with AVT in hypothalamic regions (exposed to short or long days with/without melatonin administration) strongly suggest a role for Mel along with AVT in water-electrolyte balance of birds which is important during long duration nuptial migration/flight.