[3H]N-methyl-carbamylcholine, a new radioligand specific for nicotinic acetylcholine receptors in brain.

The present study characterized the binding of [3H]N-methyl-carbamylcholine ([3H]methyl-carbachol), a new radioligand, to rat cerebral cortex membranes and demonstrated the autoradiographic distribution of these sites in rat brain. With atropine used to block muscarinic acetylcholine sites and nicotine to define non-specific binding, [3H]methyl-carbachol bound specifically, saturably and with high affinity (Kd = 11.0 nM, Bmax = 118.4 fmol/mg protein and Hill coefficient = 0.92) to a population of presumably nicotinic sites in cerebral cortex membranes. When nicotine was used to block nicotinic acetylcholine sites and atropine to define non-specific binding there was no specific binding of [3H]methyl-carbachol (concentrations up to 45 nM) to possible muscarinic sites in cerebral cortex membranes. The binding parameters under non-selective conditions (without blockade of either muscarinic or nicotinic acetylcholine sites) had very similar values to those obtained under nicotinic conditions (Kd = 8.0 nM, Bmax = 125.0 fmol/mg protein and Hill coefficient = 0.98). [3H]Methyl-carbachol binding was potently inhibited by nicotinic agonists and antagonists but only poorly displaced by muscarinic agents. Autoradiographic studies evidenced high densities of [3H]methyl-carbachol binding sites in the interpeduncular nucleus, various thalamic nuclei, superior colliculus and layers III/IV of the cortex. Such a distribution was very similar to those previously reported for nicotinic acetylcholine sites and other radioligands. These results suggest that [3H]methyl-carbachol is a specific radioligand of the neuronal nicotinic receptor. Its stability and high selectivity constitute distinct advantages over previously used nicotinic radioligands such as acetylcholine and nicotine.