Responses of melanin-concentrating hormone mRNA to salt water challenge in the rainbow trout.

Abstract

Melanin-concentrating hormone (MCH) is a structurally conserved neuropeptide, produced in the hypothalamus of all vertebrates where it probably serves as a central neurotransmitter/neuromodulator. In teleost fish it is also a neurohypophysial hormone with peripheral effects on skin colour but its central effects are less well understood. In mammals, MCH mRNA abundance changes in response to salt-loading or dehydration, suggesting an involvement in salt or water balance. The present study has used in situ hybridization to investigate the response of the MCH neurons in the rainbow trout (Oncorhynchus mykiss) to progressive changes in ambient salinity. In trout, MCH perikarya are found in two hypothalamic sites: predominantly in the nucleus lateralis tuberis (NLT) and, to a lesser extent, in neurons above the lateral ventricular recess (LVR). Immersion in 50% salt water (SW) for 24 h had no effects on MCH transcripts, plasma osmotic pressure (OP) or cortisol concentrations, but after 24 h in 80% SW, plasma OP and cortisol were raised and MCH transcripts in the NLT were significantly increased (159% of controls, p < 0.01). LVR-MCH neurons remained unaffected. However, after 24 h in 100% SW, MCH mRNA was significantly reduced in both groups of neurons (NLT -62% of controls, p < 0.001; LVR -33% of control, p < 0.001). 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 to changing salinity is discussed, together with a consideration of the potential role of MCH in osmoregulation.