Studies on the “cobalt” Variant of Rainbow Trout

The “cobalt” variant of rainbow trout Oncorhynchus mykis lacks most of the pars intermedia of the pituitary in which melanocyte-stimulating hormone (MSH) cells are distributed. An excessive accumulation of fat in the abdominal cavity is also observed in variant. In this study, the role of MSH on lipid metabolism of the trout and the possible relation to fat accumulation in the cobalt variant were investigated. In the light muscle, dark muscle, liver, and mesenteric fat, the cobalt variant showed higher contents of triacylglycerol (TG) than the normal trout. When α-MSH with an unacetylated –terminus (N-Des-Ac-α-MSH) was administered to the normal trout, circulating levels of fatty acid and lipolytic activities in the liver increased. N-Des-Ac-α-MSH also stimulated TG lipolysis in the cultured liver slices from both the normal and the cobalt variant of the trout. Those findings indicate the importance of MSH as a lipotropic hormone in the trout, and the lack of MSH cells in the pituitary remnant of the cobalt variant appears to be related to the abnormal accumulation of fat. Studies of the cobalt variant of trout may provide valuable information for pituitary research, for example, in studies of the functions of hormones produced by the pars intermedia and the early development of the pituitary.