Naltrexone induces arcuate nucleus neuropeptide Y gene expression in the rat.

Neuropeptide Y (NPY) has potent effects on several components of energy metabolism, including increased feeding and decreased brown fat thermogenesis. Negative energy balance, such as food deprivation, increases NPY mRNA in hypothalamic arcuate nucleus (ARC). Naltrexone (NLTX), an opioid receptor antagonist, decreases NPY-induced feeding. We hypothesized that NLTX would alter ARC NPY mRNA and change NPY effects on brown fat. Osmotic minipumps prefilled with either saline or NLTX (70 micrograms/h) were implanted subcutaneously in 32 male Sprague-Dawley rats. One-half of the rats were food deprived and one-half were allowed food ad libitum for 48 h. Food intake was measured at 24 and 48 h. At 48 h, ARC NPY mRNA and brown fat uncoupling protein (UCP) mRNA levels were determined using cDNA probes. Forty-eight-hour food intake was significantly decreased by 24% after NLTX infusion. Food deprivation and NLTX treatment significantly and independently increased ARC NPY mRNA and decreased UCP mRNA levels in brown fat, suggesting a complex interaction between hypothalamic NPY and endogenous opioids in the regulation of energy balance.