Central mediation of naloxone-induced anorexia in the ventral tegmental area

Central naloxone injections were used to show that endogenous opioids in the ventral tegmental area (VTA) regulate consumption of palatable foods. Peripheral injections of naloxone were more effective in reducing the consumption of a sweet solution in normally fed rats than in animals maintained at 85% of their free-feeding body weight. A dose of 10 micrograms/side naloxone injected into the VTA reduced consumption in normally fed rats, whereas a dose of 25 micrograms/side did the same in food-restricted animals. The inactive isomer, (+) naloxone, did not reduce consumption; the effect has chemical specificity. Naloxone injected 0.5, 1.0, or 2.0 mm above the VTA did not reduce consumption; the effect has anatomical specificity. Naloxone effectively decreases the eating of palatable foods, but not eating for survival. This has important implications for the use of opioid antagonists in weight-loss programs.