Expression of clock gene in the brain of rainbow trout: Comparison with the distribution of melatonin receptors

To identify brain structures potentially acting as biological clocks in rai nbow trout (Oncorhynchus mykiss), the expression sites of a trout homolog o f the mouse clock gene were studied and compared with that of melatonin rec eptors (Mel-R). For this purpose, a partial sequence of the trout clock gen e, including a PAS domain, was obtained by reverse transcription-polymerase chain reaction and used to perform in situ hybridization. The highest dens ity of clock transcripts was observed in the periventricular layer (SPV) of the optic tectum, but a weaker expression was detected in some pretectal n uclei, such as the posterior pretectal nucleus (PO) and the periventricular regions of the diencephalon. Comparison of the hybridization signal in fis h sacrificed at 08:00 and 17:00 did not indicate major changes in clock exp ression levels. Comparison of adjacent sections alternatively treated with clock and Mel-R probes suggests that both messengers are probably expressed in the same cells in the SPV and PO. In addition, in situ hybridization wi th a glutamate decarboxylase 65 probe, demonstrates that cells expressing c lock and Mel-R in the optic tectum are gamma-aminobutyric acid neurons. The tight overlapping between the expression of Mel-R and clock transcripts in cells of the PO and SPV suggests a functional link between these two facto rs. These results indicate that the optic tectum and the pretectal area of the rainbow trout are major sites of integration of the melatonin signal, e xpress the clock gene, and may act as biological clocks to influence behavi oral and endocrine responses in trout. (C) 2000 Wiley-Liss, Inc.