CHARACTERIZATION OF MUSCARINIC RECEPTORS IN THE RAT NUCLEUS-TRACTUS-SOLITARIUS

Crude membrane preparations from the dorsomedial medulla and whole hea rt of adult rats were used to characterize muscarinic receptors (mChRs ) in the nucleus tractus solitarius (NTS). Isotherm binding assays wit h [H-3]quinuclidinyl benzilate ([H-3]QNB) indicated a uniform populati on of muscarinic cholinergic receptors in both tissues with a K-d of 6 0.3 pM and a B-max of 401 fmol/mg protein for NTS membranes while hear t membranes had a K-d of 56.6 pM and a B-max of 193 fmol/mg protein. C ompetitive binding assays with selective muscarinic antagonists on NTS membranes using 80 pM [H-3]QNB showed the following relative K-i valu es consistent with M2 mChRs: atropine, 1.9 nM; methoctramine, 7.7 nM; 4-diphenylacetoxy-N-methylpiperidine methobromide, 9.4 nM; AF-DX 116, 87.6 nM; and pirenzepine, 657 nM. Similarly, two agonists, carbachol a nd oxotremorine, showed K-i values of 1.82 mu M and 0.379 mu M, respec tively. All of the competitive binding assays exhibited Hill coefficie nts close to 1.0 and InPlot analysis showed a single binding site for each agonist and antagonist examined.