@article {WASTEK768, author = {GREGORY J. WASTEK and HENRY I. YAMAMURA}, title = {Biochemical Characterization of the Muscarinic Cholinergic Receptor in Human Brain: Alterations in Huntington{\textquoteright}s Disease}, volume = {14}, number = {5}, pages = {768--780}, year = {1978}, publisher = {American Society for Pharmacology and Experimental Therapeutics}, abstract = {[3H]3-Quinuclidinyl benzilate was used to characterize the muscarinic cholinergic receptor binding in three regions of the normal human brain and in rat brain. In each of these tissues, specific ligand binding was linear between 0.05 and 0.7 mg tissue protein/assay at [3H]QNB concentrations between 20 and 200 pM. For the human brain regions, the bimolecular rate constant of association was 6.9 x 108 M-1 min-1 and the dissociation rate constant was 14.3 x 10-3 min-1. The KD calculated from the ratio of these two values was 20.1 pM. The kinetic KD determined for rat brain, under these same conditions, was 25 pM. The dissociation rate constant and the kinetic KD for both the human brain regions and the rat brain were similar whether dissociation was initiated before or after the ligand-receptor interaction had achieved equilibrium. Scatchard analysis of saturation isotherms, using similar receptor concentrations for each of the human brain regions, gave KD values of 60-70 pM with binding capacities between 600 and 900 fmol/mg protein. Analysis of saturation data from rat brain using similar receptor concentrations gave a KD of 70 pM and a binding capacity of 900 fmol/mg protein. [3H]Quinuclidinyl benzilate binding was inhibited by muscarinic agonists and antagonists; the antagonists were 1000-fold more potent than were the agonists. Under the specified conditions the antagonists and agonists had IC50 values between 0.1-2 nM and 0.8-10 {\textmu}M, respectively. The muscarinic antagonists had Hill coefficients of 1.0 and the agonists had Hill coefficients between 0.70 and 0.85. Nicotinic and noncholinergic drugs were ineffective in inhibiting [3H]quinuclidinyl benzilate binding at concentrations up to 10 {\textmu}M. Huntington{\textquoteright}s diseased brains had significant decreases in muscarinic receptor binding in the caudate nucleus and putamen. Choline acetyltransferase activity was decreased significantly in the caudate nucleus, putamen and globus pallidus of Huntington{\textquoteright}s diseased brains. Saturation isotherms for [3H]quinuclidinyl benzilate binding were determined in each of the three control brain regions and in Huntington{\textquoteright}s diseased putamen at different receptor concentrations. Each of these brain regions had a "true" KD between 20 and 40 pM. These studies, plus inhibition studies using dexetimide, the biologically-active stereoisomer of benzetimide, revealed that the decreases in ligand binding were due to receptor loss rather than to changes in receptor affinity.}, issn = {0026-895X}, URL = {https://molpharm.aspetjournals.org/content/14/5/768}, eprint = {https://molpharm.aspetjournals.org/content/14/5/768.full.pdf}, journal = {Molecular Pharmacology} }