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

Abstract

Nicotinic acetylcholine receptors are key molecules in cholinergic transmission in the nervous system. Because of their structural complexity, only a limited number of subtype-specific agonists and antagonists are available to study nicotinic receptor functions. To overcome this limitation, we used voltageclamp recordings to examine the effects of several frog skin alkaloids on acetylcholine-elicited currents in Xenopus laevis oocytes expressing major types of neuronal nicotinic receptors (␣4␤2, ␣7, ␣3␤2, ␣3␤4, and ␣4␤4). We found that the 5,8-disubstituted indolizidine (Ϫ)-235B acted as a potent noncompetitive blocker of ␣4␤2 nicotinic receptors (IC 50 ϭ 74 nM). This effect was highly selective for ␣4␤2 receptors compared with ␣3␤2, ␣3␤4, and ␣4␤4 receptors. The inhibition of ␣4␤2 currents by (Ϫ)-235B was more pronounced as the acetylcholine concentration increased (from 10 nM to 100 M). Moreover, the blockade of ␣4␤2 currents by (Ϫ)-235B was voltage-dependent (more pronounced at hyperpolarized potentials) and usedependent, indicating that (Ϫ)-235B behaves as an openchannel blocker of this receptor. Several other 5,8-disubstituted indolizidines (5-n-propyl-8-n-butylindolizidines), two 5,6,8-trisubstituted indolizidines ((Ϫ)-223A and (ϩ)-6-epi-223A), and a 1,4-disubstituted quinolizidine ((ϩ)-207I) were less potent than (Ϫ)-235B, and none showed selectivity for ␣4␤2 receptors. The quinolizidine (Ϫ)-1-epi-207I and the tricyclic (ϩ)-205B had 8.7-and 5.4-fold higher sensitivity, respectively, for inhibition of the ␣7 nicotinic receptor than for inhibition of the ␣4␤2 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.

Synthesis of Alkaloid (−)-205B via Stereoselective Reductive Cross-Coupling and Intramolecular [3+2] Cycloaddition

“…In this regard, the azatricyclododecene (−)- 205B, isolated from Dendrobated pumilio , 2 has risen as a target of interest due to its intriguing structure and the synthesis-driven discovery that the unnatural enantiomer blocks the α7 nicotinic acetylcholine receptor in a selective fashion (Figure 1). 3 Since Toyooka’s pioneering synthesis of (+)-205B, 4 Smith and Comins have independently reported syntheses of the natural antipode through chemical pathways that have secured the utility of anion relay chemistry and dihyropyridone-based functionalization processes to access the heterocyclic core of this target. 5 Here, we report a conceptually unique synthesis of (−)-205B that proceeds by two stereoselective synthetic methods that have recently emerged from our laboratories: (1) Ti-mediated reductive crosscoupling between an aldehyde and an allylic alcohol (via the intermediacy of a TMS-imine), 6 and (2) path-selective intramolecular [3+2] annulation of a glyoxylate-based homoallylic nitrone.…”

Metallacycle-mediated cross-coupling in natural product synthesis

“…130 It is unique among the natural product class in its ring structure and substitution and, while little is known about the biological activity of this agent, the unnatural antipode (prepared by Toyooka) 131 was found to inhibit the α7 nicotinic acetylcholine receptor in a selective fashion. 132 In a program that began with the development of metallacycle-mediated alkene–imine coupling chemistry, 124, 125, 133 the Micalizio laboratory embraced (−)-205B as a target for synthesis. 134 As illustrated in Figure 34B, their efforts focused on chemoselective union of the chiral allylic alcohol 80 with a TMS-imine generated in situ from aldehyde 79 (by treatment with LiHMDS at low temperature).…”

Novel G423S Mutation of Human α7 Nicotinic Receptor Promotes Agonist-Induced Desensitization by a Protein Kinase C-Dependent Mechanism

“…X. laevis oocytes expressing recombinant nicotinic receptors were prepared as described previously (Tsuneki et al, 2004). In brief, oocytes were surgically removed from X. laevis frogs under anesthesia and enzymatically isolated.…”

“…␣7 Nicotinic current evoked by acetylcholine (ACh) or choline was recorded as described previously (Tsuneki et al, 2004). An oocyte was placed in a tube-like chamber in which low-Ca 2ϩ Ringer's solution (82.5 mM NaCl, 2.5 mM KCl, 0.5 mM CaCl 2 , 2 mM MgCl 2 , and 5 mM HEPES, pH 7.4) was perfused by gravity (15 ml/min) to minimize the activation of endogenous Ca 2ϩ -activated chloride channels in the oocytes.…”