John L. Ellis

Competitive interaction of gallamine with multiple muscarinic receptors

Gallamine, a cholinergic antagonist at the (nicotinic) neuromuscular junction possesses antimuscarinic potency in several systems. We report here that gallamine inhibited the binding of [3H] quinuclidinyl benzilate (QNB) in a competitive manner in the brainstem and forebrain of the rat. The occupancy curves derived from these studies suggest that gallamine has widely varying affinities for different subpopulations of muscarinic receptors, a finding which sets gallamine apart from classical muscarinic antagonists such as atropine and QNB. The greatest difference in affinities for gallamine occurred in the brainstem, where the data could be satisfactorily fitted to a two-site model, with 77% of the receptors having high affinity (Kd = 25 nM) and 23% low affinity (93 microM). Further, these affinities displayed rank order correlation with those of carbachol (an agonist), although gallamine has not, so far, displayed agonist (or partial agonist) activity. The finding that antagonists as well as agonists can display multiple affinities for muscarinic receptors suggests that there are fundamental differences among subpopulations of these receptors.

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

Abstract 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 [ 3 H]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 −8 M , dissociation constant of low-affinity receptor, K L = 2.3 × 10 −6 M ; forebrain: 41% high affinity, K H = 2.1 × 10 −7 M, K L = 1.7 × 10 −5 M . 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.