Release-enhancing pre-synaptic muscarinic and nicotinic receptors co-exist and interact on dopaminergic nerve endings of rat nucleus accumbens

Dopaminergic nerve endings in the corpus striatum possess nicotinic (nAChRs) and muscarinic cholinergic receptors (mAChRs) mediating release of dopamine (DA). Whether nAChRs and mAChRs co-exist and interact on the same nerve endings is unknown. We here investigate on these possibilities using rat nucleus accumbens synaptosomes pre-labeled with [(3)H]DA and exposed in superfusion to cholinergic receptor ligands. The mixed nAChR-mAChR agonists acetylcholine (ACh) and carbachol provoked [(3)H]DA release partially sensitive to the mAChR antagonist atropine but totally blocked by the nAChR antagonist mecamylamine. Addition of the mAChR agonist oxotremorine at the minimally effective concentration of 30 mu mol/L, together with 3, 10, or 100 mu mol/L (-)nicotine provoked synergistic effect on [(3)H]DA overflow. The [(3)H]DA overflow elicited by 100 mu mol/L (-)nicotine plus 30 mu mol/L oxotremorine was reduced by atropine down to the release produced by (-)nicotine alone and it was abolished by mecamylamine. The ryanodine receptor blockers dantrolene or 8-bromo-cADP-ribose, but not the inositol 1,4,5-trisphosphate receptor blocker xestospongin C inhibited the (-)nicotine/oxotremorine evoked [(3)H]DA overflow similarly to atropine. This overflow was partly sensitive to 100 nmol/L methyllycaconitine which did not prevent the synergistic effect of (-)nicotine/oxotremorine. Similarly to (-)nicotine, the selective alpha 4 beta 2 nAChR agonist RJR2403 exhibited synergism when added together with oxotremorine. To conclude, in rat nucleus accumbens, alpha 4 beta 2 nAChRs exert a permissive role on the releasing function of reportedly M(5) mAChRs co-existing on the same dopaminergic nerve endings.