Circadian expression of melatonin receptor subtypes MEL1a 1.4 and MEL1a 1.7 in central and peripheral locations in goldfish (Carassius auratus)

Abstract

Melatonin is considered to act as an important neuroendocrine transducer of environmental information. Four subtypes of melatonin receptors (Mel1a1. 4, Mel1a1. 7, Mel1b and Mel1c) are thought to be mediating physiological functions of this hormone in non-mammalian vertebrates. The aim of this study was to characterize possible daily variations in the expression of some of these melatoninergic receptors under different light and feeding conditions using goldfish (Carassius auratus) as a teleost model. Fish were acclimated for 15 days under a 12 h of light and 12 h of darkness (12L: 12D) photoperiod and fed once daily at ZT2. Then, three experimental groups were established: 12L: 12D and scheduled feeding at ZT2 (Group I), 12L: 12D and random schedule feeding (Group II), and 24L with scheduled feeding at CT2 (Group III). After two weeks, central and peripheral tissue samples were collected at ZT1, ZT7, ZT13, ZT19 and ZT1b throughout a 24 h cycle. Mel1a1. 4 and Mel1a1. 7 gene expression was determined using real-time RT-PCR. Results show that both receptor subtypes are expressed in central locations, with significant daily variations found only in the optic tectum for Mel1a1. 4 under constant light conditions, and in the retina for both receptors in Group I, in all cases maximum levels were detected at ZT/CT13. As for peripheral tissues, Mel1a1. 4 and Mel1a1. 7 seem to be synchronously expressed in the foregut with a peak at ZT7 in Group I, which is consistent with low levels of circulating melatonin. In the hindgut and head kidney, neither Mel1a1. 4 nor Mel1a1. 7 show a rhythmic expression under a 12L: 12D photoperiod and fed once daily at ZT2. Then, three experimental groups were established: 12L: 12D and scheduled feeding at ZT2 (Group I), 12L: 12D and random schedule feeding (Group II), and 24L with scheduled feeding at CT2 (Group III). After two weeks, central and peripheral tissue samples were collected at ZT1, ZT7, ZT13, ZT19 and ZT1b throughout a 24 h cycle. Mel1a1. 4 and Mel1a1. 7 gene expression was determined using real-time RT-PCR. Results show that both receptor subtypes are expressed in central locations, with significant daily variations found only in the optic tectum for Mel1a1. 4 under constant light conditions, and in the retina for both receptors in Group I, in all cases maximum levels were detected at ZT/CT13. As for peripheral tissues, Mel1a1. 4 and Mel1a1. 7 seem to be synchronously expressed in the foregut with a peak at ZT7 in Group I, which is consistent with low levels of circulating melatonin. In the hindgut and head kidney, neither Mel1a1. 4 nor Mel1a1. 7 show a rhythmic expression under a 12L: 12D photoperiod and a scheduled feeding at ZT2. These results demonstrate the presence of melatonin receptors and their daily expression not only in central, but also in peripheral tissues of goldfish.