NICOTINIC RECEPTOR-BINDING OF [H-3] CYTISINE, [H-3] NICOTINE AND [H-3] METHYLCARBAMYLCHOLINE IN RAT-BRAIN

Three radiolabeled nicotinic receptor agonists were examined for their binding characteristics and for inhibition by cholinergic compounds in order to distinguish possible differential affinities for subtypes of neuronal nicotinic acetylcholine receptors. K-D and B-max values for [H-3]cytisine, [H-3]methylcarbamylcholine and [H-3]nicotine were determined from Scatchard analysis using an enriched whole-brain membrane fraction from male Sprague-Dawley rats. Respective K-D values were 0.15, 1.07 and 0.89 nM while B-max values were 99, 64 and 115 fmol/mg protein respectively. All three ligands fit a one-site model of receptor-ligand interaction. Concentration-inhibition curves were used to determine K-i values for 16 cholinergic compounds. The rank order of potencies for displacement of the three ligands was: (-)-cytisine > (-)-nicotine > (-)-lobeline = methylcarbamylcholine > 1,1-dimethyl-4-phenylpiperazinium, (+)-nicotine, dihydro-beta-erythroidine, (+/-)-nornicotine > carbachol > arecoline >> oxotremorine, tetrahydroaminoacridine, AF102B >> (-)-cotinine > RS86 = heptylphysostigmine. Correlations of the affinities of these compounds determined with the three ligands were very near to unity. In contrast, there was a negative correlation of affinities for [H-3]cytisine compared to affinities for the muscarinic receptor agonist, [H-3]oxotremorine-M, and the muscarinic receptor antagonist, [H-3]quinuclidinyl benzoate. Using membranes from whole rat brain yields data suggesting that all three nicotinic ligands bind to the same nicotinic acetylcholine receptor subtype, and are unable to distinguish subtypes of neuronal nicotinic acetylcholine receptor at the level examined.