Abstract
The plant alkaloid methyllycaconitine (MLA) is considered to be a selective antagonist of the α7 subtype of neuronal nicotinic acetylcholine receptor (nAChR). However, 50 nM MLA partially inhibited (by 16%) [3H]dopamine release from rat striatal synaptosomes stimulated with 10 μM nicotine. Other α7-selective antagonists had no effect. Similarly, MLA (50 nM) inhibited [3H]dopamine release evoked by the partial agonist (2-chloro-5-pyridyl)-9-azabicyclo[4.2.1]non-2-ene (UB-165) (0.2 μM) 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 α-conotoxin-MII (α-CTx-MII). MLA was not additive with a maximally effective concentration of α-CTx-MII (100 nM) in inhibiting [3H]dopamine release elicited by 10 μM nicotine or 0.2 μM UB-165, suggesting that both toxins act at the same site. This was confirmed in quantitative binding assays with 125I-α-CTx-MII, which displayed saturable specific binding to rat striatum and nucleus accumbens withBmax values of 9.8 and 16.5 fmol/mg of protein, and Kd values of 0.63 and 0.83 nM, respectively. MLA fully inhibited 125I-α-CTx-MII binding to striatum and nucleus accumbens with a Kivalue of 33 nM, consistent with the potency observed in the functional assays. We speculate that MLA and α-CTx-MII interact with a presynaptic nAChR of subunit composition α3/α6β2β3* on dopamine neurons. The use of MLA as an α7-selective antagonist should be exercised with caution, especially in studies of nAChR in basal ganglia.
Footnotes
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This study was supported financially by the Biological and Biotechnological Sciences Research Council Project Grant 86/B11785 (to S.W.), National Institute on Drug Abuse Grant DA-12242 (to P.W., M.M., and J.M.M.), National Institute of Mental Health Grant MH-53631 (to J.M.M.), and a Biological and Biotechnological Sciences Research Council Cooperative Award in Science and Engineering Studentship in conjunction with GlaxoSmithKline (to A.J.M.). A.C.C. is supported, in part, by the National Institute on Drug Abuse Research Scientist Award DA-00197.
- Abbreviations:
- nAChR
- nicotinic acetylcholine receptor(s)
- α-CTx-MII
- α-conotoxin-MII
- αBgt
- α-bungarotoxin
- α-CTx-ImI
- α-conotoxin-ImI
- MLA
- methyllycaconitine
- ANOVA
- analysis of variance
- DHβE
- dihydro-β-erythroidine
- UB-165
- (2-chloro-5-pyridyl)-9-azabicyclo[4.2.1]non-2-ene
- Received February 15, 2002.
- Accepted March 14, 2002.
- The American Society for Pharmacology and Experimental Therapeutics
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