DEVELOPMENTAL-CHANGES IN MELANIN-CONCENTRATING HORMONE IN RANA-TEMPORARIA

Melanin-concentrating hormone (MCH) is a vertebrate neuropeptide produ ced in hypothalamic neurons. In bony fish, such as trout, MCH acts as a neurohypophysial hormone which, once released into the circulation, acts on pigmented skin cells with the result that the fish turns pale to camouflage itself against a light colored background. In other anim als the role of MCH is not clearly established but it appears to be a neuromodulator/transmitter within the central nervous system rather th an a hormone. The present study examines MCH function in the grass fro g, Rana temporaria. Using immunocytochemistry the location and morphol ogy of irMCH neurons were followed, from tadpole to adult frog. In adu lt R. temporaria a group of MCH neurons appeared to comprise small and large-celled populations located in the ventral and dorsal infundibul ar regions, respectively. A group of MCH neurons in the preoptic area is proposed, although the perikarya were rarely immunostainable. Immun oreactive fibers were seen in various areas of the brain, including th e olfactory lobes, optic tecta, habenular nucleus, and spinal cord. Im munoreactive MCH cells were only visible in midmetamorphic climax stag es, and cellular morphology suggested low secretory activity until the animal first emerged onto land at which time nuclear size and granula tion increased significantly. No such increase was observed in equival ent stages of the South African clawed toad, Xenopus laevis, an animal which is fully aquatic throughout its life in contrast to R, temporar ia which is terrestrial. These results suggest that increased neuronal activity in Rana emerging from water may reflect a role of MCH in osm otic balance. (C) 1995 Academic Press, Inc.