INFLUENCE OF ENVIRONMENTAL COLOR AND DIURNAL PHASE ON MCH GENE-EXPRESSION IN THE TROUT

Melanin-concentrating hormone (MCH) gene expression in the brain of ra inbow trout, reared and maintained in either pale or black-coloured ta nks, was studied using in situ hybridization histochemistry. MCH trans cripts were most prevalent in the magnocellular neurones of the nucleu s lateralis tuberis (NLT), which project to the pituitary gland. They were also present, although at much lower levels, in dorsally projecti ng parvocellular neurones, sited more posteriorly above the lateral ve ntricular recess (LVR). In the NLT the most intense hybridization sign al was seen over the pituitary stalk; above the LVR, the most active n eurones were located caudally, In both the NLT and above the LVR, MCH hybridization signal was 4-fold stronger in white-reared fish than in black-reared fish. There was also a marked diurnal variation in MCH ex pression in both sites, with high levels at 16.00 h and lower levels a t 04.00 h, The results show that gene activity in both hormonal (NLT) and neuromodulator/neurotransmitter (LVR) MCH neurones is induced by p ale environmental colour and that MCH gene activity is subject to pron ounced diurnal variation.