Analysis of regional variations in the affinities of muscarinic agonists in the rat brain

The brain stem of the rat has a higher affinity toward muscarinic agonists than does the forebrain. Receptor occupancy curves of both regions of the brain deviate from simple mass-action binding. The characteristics of the binding in each region are compatible with the existence of two non-interacting binding sites, and are not attributable to desensitization or to negatively cooperative binding within a small oligomer; however, the possibility of large oligomers remains to be excluded. The agonist binding data were analyzed by a linear transformation of Scatchard-like inhibition curves of the binding of the antagonist [³H]quinuclidinyl benzilate (QNB). Such analysis, based on a model of two subpopulations of receptors in each area, shows the subpopulations of the brain stem and the forebrain to be distinct. Brain stem: 44% of receptors possess high affinity with dissociation constant for carbachol,K_H = 2.8 × 10^-8M, dissociation constant of low-affinity receptor,K_L = 2.3 × 10^-6M; forebrain: 41% high affinity,K_H = 2.1 × 10^-7M, K_L = 1.7 × 10^-5M. The data suggest that whole brain contains at least three major muscarinic receptors, which can be distinguished on the basis of their affinities for agonists.