Antagonists have a greater selectivity for muscarinic receptor subtypes in intact cerebellar granule cells than in membranes.
A comparison of muscarinic acetylcholine receptor (mAChR) antagonist binding properties was made between intact cerebellar granule cell cultures and membranes prepared from these cells. [3H]quinuclidinyl benzylate (QNB) binding displacement by four mAChR antagonists was measured and the selectivities for m2- or m3-mAChRs estimated by curve fitting. For each antagonist, the preparation of membranes caused a subtype selective decrease in receptor affinity, as compared to intact cell binding. The m2-selective antagonists had lower affinities in membranes for m2- but not for m3-mAChR, while the m3-selective antagonists had lower affinities for m3- but not for m2-mAChR. As a result, the m2-mAChR selectivity of AF-DX 116 and methoctramine in membranes was 66- and 1.7-fold less than in intact cells, and the m3-mAChR selectivity of 4-DAMP and pFHHSiD was 2.4- and 3.9-fold less in membranes than in intact cells. The m3-mAChR selectivity of 4-DAMP in intact cells was unaffected by cytoskeletal depolymerization with cytochalasins and colchicine. We suggest that the changes in selectivity seen with cell disruption may be due to a loss of cellular factors which regulate receptor properties. Antagonists binding to receptors on intact cells may cause subtype-specific changes in the interaction of the mAChR with these factors. These data suggest that mAChR antagonist binding selectivity needs to be re-examined in intact cell systems.
Saunders
P A
PA
Section on Molecular Neurobiology, National Institutes of Mental Health, National Institutes of Health, Bethesda, MD 20892-1272, USA.
Hough
C J
CJ
Chuang
D M
DM
D000818:Animals; D001667:Binding, Competitive; D002462:Cell Membrane / Q000187:drug effects*; D002478:Cells, Cultured / Q000187:drug effects; D002531:Cerebellum / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D018727:Muscarinic Antagonists / Q000494:pharmacology*; D010880:Piperidines / Q000494:pharmacology; D011813:Quinuclidinyl Benzilate / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011976:Receptors, Muscarinic / Q000187:drug effects*; D013997:Time Factors
D018727:Muscarinic Antagonists; D010880:Piperidines; D011976:Receptors, Muscarinic; D011813:Quinuclidinyl Benzilate; C042375:4-diphenylacetoxy-1,1-dimethylpiperidinium