Effect of naltrexone on food intake and body weight in Syrian hamsters depends on metabolic status.

Opioids are a family of neuropeptides involved in the control of food intake and regulation of body weight. In general, nonselective opioid antagonists have inhibited food intake in a variety of paradigms in rodent species. Syrian hamsters may be an exception to the general findings. In a previous report, we showed that systemic administration of an opioid antagonist, naltrexone, for 2 days increased body weight in female Syrian hamsters. To confirm the extent of these finding we designed the present experiment testing the effect of a chronic 6-day infusion of naltrexone on food intake, water intake, and body weight in freely feeding male hamsters. In addition, we examined the effect of acute administration of naltrexone on food intake in both ad-libitum-fed and food-deprived hamsters. We found that chronic systemic administration of naltrexone caused a significant increase in food intake and body weight. Second, acute administration of naltrexone decreased food intake after a 48-h fast but had no effect in ad-libitum-fed hamsters. Water consumption was not altered in any experimental paradigm. Our results suggest that opioid circuits in Syrian hamsters may function tonically to suppress food intake and body weight when Syrian hamsters are in positive energy balance. Paradoxically, opioids may enhance food intake after a sustained fast.