Received for publication March 26, 2004.
Revised July 5, 2004.
Accepted for publication July 6, 2004.

Alkaloids Indolizidine 235B', Quinolizidine 1-epi-207I and the Tricyclic 205B are Potent and Selective Non-competitive Inhibitors of Nicotinic Acetylcholine Receptors

Abstract

Nicotinic acetylcholine receptors are key molecules in cholinergic transmission in the nervous system. Because of their structural complexity, there are only a limited number of subtype-specific agonists and antagonists available to study nicotinic receptor functions. To overcome this limitation, we used voltage-clamp recordings to examine the effects of several frog skin alkaloids on acetylcholine-elicited currents in Xenopus oocytes expressing major types of neuronal nicotinic receptors (42, 7, 32, 34, and 44). We found that the 5,8-disubstituted indolizidine, (-)-235B', acted as a potent noncompetitive blocker of 42 nicotinic receptors (IC₅₀ = 74 nM). This effect was highly selective for 42 receptors when compared to 32, 34, and 44 receptors. The inhibition of 42 currents by (-)-235B' was more pronounced as the acetylcholine concentration increased (from 10 nM to 100 µM). Moreover, the blockade of 42 currents by (-)-235B' was voltage-dependent (more pronounced at hyperpolarized potentials) and use-dependent, indicating that (-)-235B' behaves as an open-channel blocker of this receptor. Several other 5,8-disubstituted indolizidines (5-n-propyl-8-n-butylindolizidines), two 5,6,8-trisubstituted indolizidines ((-)-223A and (+)-6-epi-223A) and a 1,4-disubstituted quinolizidine ((+)-207I) were less potent than (-)-235B', and none showed selectivity for 42 receptors. The quinolizidine (-)-1-epi-207I and the tricyclic (+)-205B had 8.7-fold and 5.4-fold higher sensitivity respectively for inhibition of the 7 nicotinic receptor than for inhibition of the 42 receptor. These results show that frog alkaloids alter the function of nicotinic receptors in a subtype-selective manner, suggesting that an analysis of these alkaloids may aid in the development of selective drugs to alter nicotinic cholinergic functions.

Key words: Nicotinic cholinergic, Func. analysis receptor/ion channel mutants

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