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Abstract
The natural toxin anatoxin-a (AnTx) is a potent nicotinic agonist that is valuable
for the study of nicotinic receptors. We have synthesized 2-(propan-1-oxo-1-yl)-9-azabicyclo[4.2.1]non-2-ene,
the homologue of AnTx in which the side-chain is extended by one methylene unit from
a methyl to an ethyl ketone. This chemistry would allow the generation of a tritiated
product and the homologue, designated homoanatoxin (HomoAnTx), has been characterized
here with that aim in mind. In competition binding assays at neuronal nicotinic ligand
binding sites characterized by [3H]nicotine and [125I]-alpha bungarotoxin, HomoAnTx
retained the same potency as the parent molecule, with Ki values of 7.5 nM and 1.1
microM, respectively. In contrast, it showed little inhibition of muscarinic binding
defined by [3H]-quinuclidinyl benzilate. HomoAnTx is a potent nicotinic agonist in
frog muscle contracture assays, having four times the potency of carbamylcholine and
one tenth of the activity of AnTx itself. The N-methylated version of HomoAnTx was
more than two orders of magnitude weaker in both functional and binding assays. The
successful synthesis of HomoAnTx with retention of high nicotinic potency offers a
route for the generation of novel, potent radiolabelled nicotinic ligands.