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

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Abstract

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 [3H]DA and exposed in superfusion to cholinergic receptor ligands. The mixed nAChR–mAChR agonists acetylcholine (ACh) and carbachol provoked [3H]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 μmol/L, together with 3, 10, or 100 μmol/L (−)nicotine provoked synergistic effect on [3H]DA overflow. The [3H]DA overflow elicited by 100 μmol/L (−)nicotine plus 30 μmol/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 [3H]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 α4β2 nAChR agonist RJR2403 exhibited synergism when added together with oxotremorine. To conclude, in rat nucleus accumbens, α4β2 nAChRs exert a permissive role on the releasing function of reportedly M5 mAChRs co-existing on the same dopaminergic nerve endings.

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