Heroin-induced locomotion and mesolimbic dopamine release is unchanged in mice lacking the ORL.1 receptor gene

We sought evidence for a role of endogenous nociceptin in modulating opiate effects on locomotion and mesolimbic dopamine release. Heroin administration (1, 3 and 10 mg/kg) induced dose-dependent increases in locomotion and mesolimbic dopamine release. However, no differences were identified between wild-type and nociceptin receptor-deficient mice, suggesting that either these systems are not influenced by an endogenous nociceptin tone, or that compensatory mechanisms activated during development normalize the response.