The role of the hypothalamic melanocortin system in behavioral appetitive processes

Much evidence suggests that the hypothalamic melanocortin (MC) system plays an important role in the control of food intake. However, investigations o f the potential behavioral mechanisms have been limited to measures of aver sion. The purpose of the present experiment was to assess whether other beh avioral consequences of administration of MC peptides were similar to those produced by 0- or 24-h food deprivation, respectively. Rats were first tra ined while food deprived that a tone predicted the delivery of peanut oil. They then received exposure to oil under food deprivation, satiation, intra -third-cerebroventricular (i3vt) infusion of MTII (a potent MC agonist) or SHU-9119 (a potent MC antagonist). All rats were then tested during extinct ion for levels of responding to the tone under food satiation. Previous res ults demonstrated that sated exposure reduces subsequent test responding to the tone. During the present extinction test, rats that received sated exp osure exhibited reduced responding to the tone, relative to rats that recei ved deprived exposure. Unlike satiation, rats that received exposure after MTII exhibited continued high levels of responding to the tone. Further, ra ts that received SHU-9119 exhibited a small reduction in responding. These data suggest that MTII and SHU-9119 do not influence intake via the same me chanisms as hunger and food satiation, respectively. (C) 2001 Elsevier Scie nce Inc. All rights reserved.