Melanin-concentrating hormone (MCH) stimulates the activity of rainbow trout (Oncorhynchus mykiss) head kidney phagocytes in vitro

Abstract

Melanin-concentrating hormone (MCH) is a widespread vertebrate neuropeptide first discovered in teleost fish by virtue of its role in the control of skin pigmentation. In rainbow trout, its release from the pituitary gland causes aggregation of melanin granules in dermal melanophores, thus lightening the colour of the skin (Rance & Baker, 1979). Another hormone, alpha-melanocyte stimulating hormone (a-MSH) has the opposite e#ect and results in skin darkening, a response common to teleosts, amphibians and reptiles (Baker, 1993). In vertebrates other than fish, MCH does not circulate as a hormone, but probably acts centrally as a neuromodulator (Baker, 1994). The peptide is known to be involved in the stress response and modulates the release of corticotrophin-releasing factor (CRF) from the hypothalamus of both fish and mammals (Baker, 1994). Evidence also suggests that MCH inhibits the release of a-MSH and adrenocorticotrophic hormone (ACTH) from the trout pituitary (Baker et al., 1985; Green & Baker, 1991). In mammals, the peptide stimulates the production and release of arginine-vasopressin (AVP) and oxytocin from the hypothalamus (Parkes & Vale, 1993). Other functions of MCH have yet to be elucidated, but it has been shown to antagonise the stimulatory e#ects of a-MSH on both the response to auditory stimuli (Miller et al., 1993) and grooming behaviour (Eberle, 1988) in rats. Whilst a-MSH is known to exert a variety of e#ects on the mammalian immune system (Lipton & Catania, 1997), the possibility that MCH may be involved in vertebrate immune responses has received little attention so far. It has been demonstrated that MCH antagonises the stimulatory e#ects of a-MSH on fish head kidney leucocytes in vitro (Harris & Bird, 1997). Here, it is shown that MCH can also directly influence the activity of rainbow trout (Oncorhynchus mykiss) phagocytes in vitro.