RESPONSES OF MELANIN-CONCENTRATING HORMONE MESSENGER-RNA TO SALT-WATER CHALLENGE IN THE RAINBOW-TROUT

Melanin-concentrating hormone (MCH) is a structurally conserved neurop eptide, produced in the hypothalamus of all vertebrates where it proba bly serves as a central neurotransmitter/neuromodulator. In teleost fi sh it is also a neurohypophysial hormone with peripheral effects on sk in colour but its central effects are less well understood. In mammals , MCH mRNA abundance changes in response to salt-loading or dehydratio n, suggesting an involvement in salt or water balance. The present stu dy has used in situ hybridization to investigate the response of the M CH neurons in the rainbow trout (Oncorhynchus mykiss) to progressive c hanges in ambient salinity. In trout, MCH perikarya are found in two h ypothalamic sites: predominantly in the nucleus lateralis tuberis (NLT ) and, to a lesser extent, in neurons above the lateral ventricular re cess (LVR). Immersion in 50% salt water (SW) for 24 h had no effects o n MCH transcripts, plasma osmotic pressure (OF) or cortisol concentrat ions, but after 24 h in 80% SW, plasma OP and cortisol were raised and MCH transcripts in the NLT were significantly increased (159% of cont rols, p < 0.01). LVR-MCH neurons remained unaffected. However, after 2 3 h in 100% SW, MCH mRNA was significantly reduced in both groups of n eurons (NLT -62% of controls, p < 0.001; LVR -33% of controls, p < 0.0 01). These responses were transient and were no longer apparent after 6 days in 100% SW, despite the fact that plasma OP and cortisol levels continued to rise. The relative importance of osmotic disturbance and stress on the differential responses of the 2 groups of MCH neurons t o changing salinity is discussed, together with a consideration of the potential role of MCH in osmoregulation.