Modifications in food intake and energy metabolism in rats as a function of chronic naltrexone infusions

The effects of chronic naltrexone infusions on food intake and energy balance were examined in male rats. Animals were fed either Purina Chow, or chow plus a 32% sucrose solution. After one week of being maintained on these diets, animals were implanted (intrascapularly) with osmotic minipumps infusing either 200 micrograms/kg/hr naltrexone hydrochloride or saline. Sucrose + chow-fed animals exhibited increased O2 consumption, increased CO2 production and an elevation in the respiratory quotient (RQ) relative to chow-fed controls. When infused with naltrexone, sucrose + chow-fed animals decreased food intake and body weight gain. While chow-fed animals also suppressed food intake and body weight gain, these decreases were not as great as those observed in sucrose + chow-fed animals. As a function of naltrexone administration, both chow-fed and sucrose + chow-fed animals altered their metabolism as reflected by decreased RQ and adiposity as determined by skinfold measurements. In addition, sucrose feeding led to a hyperthermia which was reversed by naltrexone infusions. Thus, chronic naltrexone administration depressed appetite, reduced energy production and induced hypothermia in rats. As naltrexone is thought to block the endogenous opioid system, this suggests that the endorphins are involved in the regulation of food intake and thermogenesis.