Exogenous, but not endogenous nociceptin modulates mesolimbic dopamine release in mice

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Abstract
The effect of nociceptin (an endogenous ligand of the ORL1 receptor) on mesolimbic dopamine release and simultaneous horizontal locomotion was studied in freely moving mice undergoing microdialysis of the nucleus accumbens. Intracerebroventricular (i.c.v.) administration of nociceptin (7 nmol) induced a long‐lasting suppression of mesolimbic dopamine release and horizontal locomotion in wild‐type but not ORL1 knockout mice. I.c.v. administration of the recently reported peptide nociceptin antagonist [Nphe1, Arg14, Lys15] nociceptin‐NH2(known also as UFP‐101, 5 nmol) completely abolished the suppressive effect of nociceptin on mesolimbic dopamine release. However, UFP‐101 administration alone induced a mild and lasting suppression of mesolimbic dopamine release in both wild‐type and ORL1 knockout mice that was magnified in ORL1 knockout mice by coadministration of nociceptin. UFP‐101 administration alone suppressed locomotion in both genotypes. These results confirm that the suppressive action of nociceptin on mesolimbic dopamine release is mediated entirely by the ORL1 receptor, and that UFP‐101 effectively antagonizes this action. However, the lack of a stimulatory effect of UFP‐101 in wild‐type mice indicates that despite being sensitive to exogenous nociceptin action, basal mesolimbic dopaminergic activity is not determined by endogenous nociceptin in mice.