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,KH = 2.8 × 10-8M, dissociation constant of low-affinity receptor,KL = 2.3 × 10-6M; forebrain: 41% high affinity,KH = 2.1 × 10-7M, KL = 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.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Clinical Neurology
- Developmental Biology