Regional distribution of muscarinic and nicotinic cholinergic receptor binding activities in the human brain.

The distribution of nicotinic and muscarinic receptor subtypes was examined in human brain tissue obtained at autopsy from neurologically normal adult (50-60 years) individuals. Membrane preparations from 15 brain regions were examined for nicotinic (L-[3H]nicotine) binding, both M1 and M2 muscarinic receptor binding (distinguished on the basis of pirenzepine affinity) and high (H) and low (L) affinity muscarinic agonist binding (distinguished on the basis of carbachol displacement). Total muscarinic receptor binding sites were relatively high in striatal and cortical areas, where both M1 subtype and agonist binding type L predominated, compared with thalamic, nigral and cerebellar regions and spinal cord, where the M2 subtype and agonist binding type H predominated. Nicotinic receptor binding sites (predominantly high affinity, measured at low ligand concentrations) did not parallel any of the muscarinic subtypes, being concentrated in thalamic, neocortical and striatal regions. Scatchard analysis indicated the presence of both high and low affinity nicotinic sites, the numbers of the latter generally exceeding the former by over one order of magnitude. Neither muscarinic nor nicotinic receptor binding sites were closely related to the distribution of the cholinergic neuronal marker, choline acetyltransferase, suggesting that individual cholinergic pathways may be distinguished by the relative proportion of the different types of cholinergic receptors present.