Modulation of 5-hydroxytryptamine efflux from rat cortical synaptosomes by opioids and nociceptin

1 The modulation of [H-3]-5-hydroxytryptamine ([H-3]-5-HT) efflux from superfused rat cortical synaptosomes by delta, kappa, mu and ORL1 opioid receptor agonists and antagonists was studied. 2 Spontaneous [H-3]-5-HT efflux was reduced (20% inhibition) by either 0.5 mu M tetrodotoxin or Ca2+-omission. Ten mM K+-evoked [H-3]-5-HT overflow was largely Ca2+-dependent (90%) and tetrodotoxin-sensitive (50%). 3 The delta receptor agonist, deltorphin-I, failed to modulate the K+-evoked neurotransmitter efflux up to 0.3 mu M. The kappa and the mu receptor agonists, U-50,488 and endomorphin-1, inhibited K+-evoked [H-3]-5-HT overflow (EC50=112 and 7 nM, respectively; E-max=28 and 29% inhibition, respectively) in a norBinaltorphimine- (0.3 mu M) and naloxone- (1 mu M) sensitive manner, respectively. None of these agonists significantly affected spontaneous [H-3]-5-HT efflux. 4 The ORL1 receptor agonist nociceptin inhibited both spontaneous (EC50=67 nM) and K+-evoked (EC50=13 nM; E-max=52% inhibition) [H-3]-5-HT efflux. The effect of NC was insensitive to naloxone (up to 10 mu M), but was antagonized by [Nphe(1)]nociceptin(1-13)NH2 (a novel selective ORL1 receptor antagonist; pA(2)=6.7) and by naloxone benzoylhydrazone (pA(2)=6.3). The ORL1 ligand [Phe(1)phi(CH2-NH)Gly(2)]nociceptin(1-13)NH2 also inhibited K+ stimulated [H-3]-5-HT overflow (EC50=64nM; E-max=31% inhibition), but its effect was partially antagonized by 10 mu M naloxone. 5 It is concluded that the ORL1 receptor is the most important presynaptic modulator of neocortical 5-HT release within the opioid receptor family. This suggests that the ORL1/nociceptin system may have a powerful role in the control of cerebral 5-MT-mediated biological functions.