Increased body weight in mice lacking mu-opioid receptors.

Opioids have been suggested to affect feeding behaviour. To clarify the role of mu-opioid receptors in feeding, we measured several parameters relating to food intake in mu-opioid receptor knockout mice. Here, we show that the knockout mice had increased body weight in adulthood, although the intake amount of standard food was similar between the wild-type and knockout littermates. Serum markers for energy homeostasis were not significantly altered in the knockout mice. Hypothalamic neuropeptide Y mRNA, however, was higher in knockouts than in wild-type mice. Our results suggest that the up-regulated expression of neuropeptide Y mRNA might contribute to the increased weights of adult mu-opioid receptor knockout mice.