STEREOSPECIFIC IN VlTRO BINDING OF [3H]DEXETIMIDE TO BRAIN MUSCARINIC RECEPTORS
Abstract—
A binding assay for muscarinic cholinergic receptors has been developed using labelled dexetimide as ligand and a filtration technique. The main features of this assay are its stereospecific nature, the very high affinity of the ligand for the specific receptors sitcs and its very low affinity for non-specific binding sites.
The latter point was further investigated using labelled levitimide, the inactive enantiomer. The binding was found to be neither stereospecific nor saturable and displacement by both enantiomers revealed a particular curve with a very flattened course.
Kinetic experiments with [3H]dexetimide suggest the occurrence of a heterogenous population of muscarinic receptors in the rat striatum.
A study of the regional distribution of muscarinic receptors in rat brain showed a high concentration in the dopaminergic areas, the cortex and the hippocampus, but practically none in the cerebellum.
The subcellular distribution pattern revealed a marked enrichment of [3H]dexetimide stereospecific binding sites in the microsomal fraction of rat striatum and hippocampus. Such a distribution was not found with [3H]levitimide. All the characteristics of this binding assay make dexetimide a very appropriate ligand for labelling muscarinic receptors in vitro as well as in vivo.
