Methyllycaconitine is a potent antagonist of alpha-conotoxin-MII-sensitive presynaptic nicotinic acetylcholine receptors in rat striatum

The plant alkaloid methyllycaconitine (MLA) is considered to be a selective antagonist of the alpha7 subtype of neuronal nicotinic acetylcholine receptor (nAChR). However, 50 nM MLA partially inhibited (by 16%) [H-3] dopamine release from rat striatal synaptosomes stimulated with 10 muM nicotine. Other alpha7-selective antagonists had no effect. Similarly, MLA (50 nM) inhibited [H-3] dopamine release evoked by the partial agonist (2-chloro-5-pyridyl)- 9-azabicyclo[4.2.1] non-2-ene (UB-165) (0.2 muM) by 37%. In both cases, inhibition by MLA was surmountable with higher agonist concentrations, indicative of a competitive interaction. At least two subtypes of presynaptic nAChR can modulate dopamine release in the striatum, and these nAChR are distinguished by their differential sensitivity to alpha-conotoxin-MII (alpha-CTx-MII). MLA was not additive with a maximally effective concentration of alpha-CTx-MII (100 nM) in inhibiting [H-3] dopamine release elicited by 10 muM nicotine or 0.2 muM UB-165, suggesting that both toxins act at the same site. This was confirmed in quantitative binding assays with I-125-alpha-CTx-MII, which displayed saturable specific binding to rat striatum and nucleus accumbens with B-max values of 9.8 and 16.5 fmol/mg of protein, and K-d values of 0.63 and 0.83 nM, respectively. MLA fully inhibited I-125-alpha-CTx-MII binding to striatum and nucleus accumbens with a K-i value of 33 nM, consistent with the potency observed in the functional assays. We speculate that MLA and alpha-CTx-MII interact with a presynaptic nAChR of subunit composition alpha3/alpha6beta2beta3* on dopamine neurons. The use of MLA as an alpha7-selective antagonist should be exercised with caution, especially in studies of nAChR in basal ganglia.