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

1 Mouse brain slices preincubated with [H-3]-noradrenaline or [H-3]-seroton in were superfused with medium containing naloxone 10 mu M; we studied whet her nociceptin (the endogenous ligand at ORL1 receptors) affects monoamine release. Furthermore, the affinities of ORL1 ligands were determined using [H-3]-nociceptin binding. 2 The electrically (0.3 Hz) evoked tritium overflow in mouse cortex slices preincubated with [H-3]-noradrenaline was inhibited by nociceptin and [Tyr( 14)]-nociceptin (maximally by 80%; pEC(50) 7.52 and 8.28) but not affected by [des-Phe(1)]-nociceptin (pEC(50)<6). The ORL1 antagonist naloxone benzoy lhydrazone antagonized the effect of nociceptin and [Tyr(14)]-nociceptin. 3 The effect of nociceptin did not desensitize, was not affected by blockad e of NO synthase, cyclooxygenase and P-1-purinoceptors and was decreased by the alpha(2)-adrenoceptor agonist talipexole. Nociceptin also inhibited th e evoked overflow in mouse cerebellar, hippocampal and hypothalamic slices in a manner sensitive to naloxone benzoylhydrazone. 4 The electrically (3 Hz) evoked tritium overflow in mouse cortex slices pr eincubated with [H-3]-serotonin was inhibited by nociceptin; naloxone benzo ylhydrazone antagonized this effect. 5 The affinities (pK(1)) for [H-3]-nociceptin binding to mouse cortex membr anes were: nociceptin, 8.71; [Tyr(14)]-nociceptin, 9.82; [des-Phe(1)]-nocic eptin, <5.5; naloxone benzoylhydrazone, 5.85; naloxone, <4.5. 6 In conclusion, nociceptin inhibits noradrenaline release in the mouse cor tex via ORL1 receptors, which interact with presynaptic alpha(2)-autorecept ors on noradrenergic neurones. The effect of nociceptin does not desensitiz e nor does it involve NO, prostanoids or adenosine. Nociceptin also attenua tes noradrenaline release from several subcortical regions and serotonin re lease from cortical slices by a naloxone benzoylhydrazone-sensitive mechani sm.