Further characterization of the ORL1 receptor-mediated inhibition of noradrenaline release in the mouse brain in vitro.

Mouse brain slices preincubated with [3H]-noradrenaline or [3H]-serotonin were superfused with medium containing naloxone 10 microM; we studied whether nociceptin (the endogenous ligand at ORL1 receptors) affects monoamine release. Furthermore, the affinities of ORL1 ligands were determined using [3H]-nociceptin binding. The electrically (0.3 Hz) evoked tritium overflow in mouse cortex slices preincubated with [3H]-noradrenaline was inhibited by nociceptin and [Tyr14]-nociceptin (maximally by 80%; pEC50 7.52 and 8.28) but not affected by [des-Phe1]-nociceptin (pEC50<6). The ORL1 antagonist naloxone benzoylhydrazone antagonized the effect of nociceptin and [Tyr14]-nociceptin. The effect of nociceptin did not desensitize, was not affected by blockade of NO synthase, cyclooxygenase and P1-purinoceptors and was decreased by the alpha2-adrenoceptor agonist talipexole. Nociceptin also inhibited the evoked overflow in mouse cerebellar, hippocampal and hypothalamic slices in a manner sensitive to naloxone benzoylhydrazone. The electrically (3 Hz) evoked tritium overflow in mouse cortex slices preincubated with [3H]-serotonin was inhibited by nociceptin; naloxone benzoylhydrazone antagonized this effect. The affinities (pKi) for [3H]-nociceptin binding to mouse cortex membranes were: nociceptin, 8.71; [Tyr14]-nociceptin, 9.82; [des-Phe1]-nociceptin, <5.5; naloxone benzoylhydrazone, 5.85; naloxone, <4.5. In conclusion, nociceptin inhibits noradrenaline release in the mouse cortex via ORL1 receptors, which interact with presynaptic alpha2-autoreceptors on noradrenergic neurones. The effect of nociceptin does not desensitize nor does it involve NO, prostanoids or adenosine. Nociceptin also attenuates noradrenaline release from several subcortical regions and serotonin release from cortical slices by a naloxone benzoylhydrazone-sensitive mechanism.