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Abstract
To explore the part of molecular mechanism of rhodopsin family genes in the olive flounder (Paralichths olivaceus), we observed the change patterns of the photoreceptors, rhodopsin (RH) and extra-ocular rhodopsin (Exo-RH) genes. We isolated cDNAs of RH and Exo-RH genes from the retina and pineal gland, respectively, of the olive flounder and investigated the differential expression of these genes in these organs during a 28-h time period under the 12-h light:12-h dark (LD) photocycle, constant dark (DD) and constant light (LL). The expression of RH gene in retina showed clear circadian pattern under LD, with the highest level at Zeitgeber time (ZT) 4 and ZT28 and lowest at ZT16. The expression level of Exo-RH gene in pineal organ under LD showed opposite pattern to that of RH, with higher expression during the dark period, and the peak was at ZT20. This expression pattern was similar between in vivo and in vitro (incubated) pineal organs. Under DD and LL conditions, the expression patterns of RH and Exo-RH genes were similar to those under LD photocycle, but the circadian change patterns were less clear. The expression levels of Exo-RH were about 100-fold (LD), 30,000-fold (DD) and 10-fold (LL) lower in pineal organ treated with high concentration of melatonin compared to the untreated pineal organ. The melatonin concentration in plasma had similar circadian pattern with that of the expression of Exo-RH gene, with the peak at ZT20 for fish kept under all LD, DD and LL conditions. Our results suggest that circadian rhythm in the olive flounder is controlled and maintained by the environmental photocycle through the mediation of RH and Exo-RH, and the melatonin concentration in blood plasma.
Acknowledgements
This study was supported by the Technology Development Program for Fisheries, Ministry for Food, Agriculture, Forestry and Fisheries, Korea, and by the ITRC support program supervised by the National IT Industry Promotion Agency (NIPA-2012-C1090-1221-0015).