Effects of tank colour and stress on melanin-concentrating hormone gene expression in the rainbow trout

Abstract

Using in situ hybridization histochemistry, we have studied the influence of environmental colour and stress on the melanin-concentrating hormone (MCH) gene expression in the rainbow trout. Although MCH is encoded by two homologous genes (MCH1 and 2) in this fish, only oligoprobes against MCH2 hybridized to brain sections. The hybridization signal was four-fold greater in white-reared trout than in their black counterparts. This observation applied equally to magnocellular neurons in the nucleus lateralis tuberis (NLT), and parvocellular neurons above the lateral ventricular recess (LVR). Repeated moderate stress (5 min low water, 3 times daily) had no effect after 1 day, but enhanced MCH gene expression throughout the nucleus lateralis tuberis after 5 days (P<0.05). MCH neurons in the lateral ventricular recess showed no response to this stress. During early development, the response of the MHC neurons in the nucleus lateralis tuberis to tank colour was observed within 1 week after hatching, but MCH mRNA in these cells was elevated by a chronic stress (confinement and 1 min iced water daily) only 3 weeks posthatch. On the other hand, MHC neurons in the lateral ventricular recess did not respond to tank colour until 4 weeks posthatch, but MCH signal was enhanced by the stress from 2 weeks after hatching onwards. These results indicate a difference in sensitivity of the two groups of MCH neurons to background colour and stress.