Age-related alterations in the central thermoregulatory responsiveness to alpha-MSH

Alpha-melanocyte-stimulating-hormone (alpha-MSH) is a neuropeptide that induces weight loss via its anorexigenic and hypermetabolic/hyperthermic effects. Two major public health problems of the human population involving energy balance (i.e. middle-aged obesity and aging cachexia) also appear in other mammals, therefore age-related regulatory alterations may also be assumed in the background. Previous studies demonstrated characteristic age-related shifts in the anorexigenic effects of centrally applied alpha-MSH with strong effects in young adult, diminished efficacy in middle-aged and very pronounced responsiveness in old rats. The present study aimed to investigate age-related changes in the acute central thermoregulatory responsiveness to an alpha-MSH injection in rats and to compare them with those of food intake-related responsiveness. Oxygen consumption (VO2), core (Tc) and tail skin temperatures (Ts, indicating heat loss) of male Wistar rats of different age groups (from 2 to 24 months of age), were recorded in an indirect calorimeter complemented by thermocouples upon intracerebroventricular alpha-MSH administration (0, 5 µg) at a slightly subthermoneutral environment (25-26 °C). Acute alpha-MSH-induced rises in VO2 and Tc were most pronounced in the young adult age-group. In these rats the hyperthemic effects were somewhat diminished by an activation of heat loss. Juvenile animals showed weaker hyperthermic responses, middle-aged rats none at all. Alpha-MSH-induced hyperthermia became significant again in old rats. Acute thermoregulatory (hypermetabolic/hyperthermic) responsiveness to alpha-MSH shows a distinct age-related pattern similar to that of acute anorexigenic responsiveness. Thus, our results may also contribute to the explanation of both middle-aged obesity and aging cachexia.