Effects of bilateral ibotenate-induced lesions of the nucleus basalis magnocellularis upon selective cholinergic biochemical markers in the rat anterior cerebral cortex

The relationship of choline acetyltransferase (ChAT) activity and high affinity binding of the potent and selective sodium-dependent choline uptake inhibitor [³H]hemicholinium-3 ([³H]HC-3) to high-affinity binding of the muscarinic agonist [³H](+)-cis-methyl-dioxolane ([³H](+)CD), the putative M₁ selective antagonist [³H]pirenzepine ([³H]PZ) and the classical antagonist [³H](-)-quinuclidinyl benzilate ([³H](-)QNB) in homogenates of the rat neocortex was studied. ChAT activity was 42% lower in rats with ibotenate-induced lesions of the nucleus basalis magnocellularis (nbm) when compared to controls, and [³H]HC-3 binding was similarly reduced by 44%. However, equilibrium dissociation constants (K_d values) for [³H]HC-3 (0.8-1.0 nM), [³H](-)QNB (11-24 pM), [³H]PZ (4.0-4.3 nM) and [³H](+)CD (2.1-2.9 nM) were each unchanged. Mean B_max values (total binding site densities) for [³H](+)CD were significantly altered in both hemispheres of the anterior cerebral cortex, showing a 25% reduction in the number of sites which display the highest affinity conformation for this potent muscarinic agonist. The decreased ChAT activity and [³H]HC-3 binding after nbm lesions were associated with only slight reductions in putative M₁ muscarinic site density (14%) and [³H](-)QNB binding site density (13%). Thus, it appears that while [³H]PZ and [³H](-)QNB label predominantly postsynaptic muscarinic binding sites, a significant number of sites labeled by [³H](+)CD may be associated with presynaptic cholinergic nerve terminals. These data suggest that cholinergic input differentially regulates the drug binding sites of anterior cerebral cortical muscarinic receptors, exerting a substantial effect upon the highest affinity conformational state for agonists.