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

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