Melanin-concentrating hormone (MCH) antagonizes the effects of alpha-MSH and neuropeptide E-I on grooming and locomotor activities in the rat.

The intraventricular (i.c.v.) administration of the neuropeptide melanocyte stimulating hormone (alpha-MSH) is known to elicit a series of behaviors in the rat which include excessive grooming and other motor activities. In bony fish, the pigmentary effects of alpha-MSH can be antagonized by the neuropeptide melanin-concentrating hormone (MCH). We therefore examined whether MCH or its sister peptide neuro-peptide E-I (NEI), derived from the same precursor molecule, would modulate the effect of alpha-MSH on grooming and motor activity in the rat, or perhaps elicit some responses of their own. Rats were injected i.c.v. with either artificial cerebrospinal fluid, alpha-MSH, MCH, NEI, or with two peptides together, and behavioral responses were monitored over the next 65 min. The i.c.v. injection of 1 microgram MSH significantly enhanced grooming behavior. NEI at the same dose increased grooming, rearing, and locomotor activities. MCH alone had no behavioral effects but it annulled the behavioral responses induced by either alpha-MSH or NEI. alpha-MSH also antagonized the locomotor and rearing behavior induced by NEI. The physiological significance of these observations is discussed.