Immunocytochemical identification of melanin-concentrating hormone in the brain and pituitary gland of the teleost fishes Oncorhynchus keta and Salmo gairdneri

Summary Melanin-concentrating hormone (MCH) has been purified from the chum salmon pituitary. Its complete amino acid sequence has recently been established. To identify the precise site of origin of MCH, immunostaining was performed in the brain and pituitary gland of the chum salmon and the rainbow trout using a highly sensitive and specific antiserum raised against synthetic MCH. In these two salmonid species immunoreactivity for MCH was detected in neurons and neuronal processes in the pars lateralis of the nucleus lateralis tuberis (NLT) in the basal hypothalamus. Numerous positive-staining processes of these MCH-neurons project to the pituitary gland, extending into neurohypophysial tissues within the pars intermedia and, to a lesser extent, into the pars distalis. No pituitary cells showed cross-reactivity. These results suggest that MCH is biosynthesized in the neurons of the NLT/pars lateralis and released in the neurohypophysis. On the other hand, prominent but less numerous MCH-positive processes could be traced to the pretectal area in which projection of both optic and pineal fibers has been detected using tracers. This observation suggests that the synthesis and/or release of MCH might be under the influence of either of these photosensory neurons. Moreover, the existence of an extrahypothalamic projection from MCH-positive neurons suggests that, in addition to melanin-concentration, MCH might be involved in other neuronal functions, perhaps serving as neuromodulator in the brain.