Abstract

For identification and characterization of muscarinic cholinergic receptors, a radioligand binding assay using [3H]quinuclidinyl benzilate (QNB) was carried out using normal and preganglionically denervated cat superior cervical ganglion (SCG). Specific [3H]QNB binding to the cat SCG homogenates was saturable, reversible and of high affinity (KD = 13.9 +/- 4.0 pM) with a Bmax of 54.1 +/- 5.8 fmol/mg of protein. From a kinetic analysis of the data, we calculated the association and dissociation rate constants of 1.47 X 10(9) M-1 min-1 and 0.025 min-1, respectively. The dissociation constant calculated from the rate constants (KD = K-1/K+1 = 17.0 pM) was similar to the value determined from Scatchard analysis. Values of K1 for muscarinic cholinergic drugs that inhibited specific binding were: QNB, 0.10 pM; atropine, 0.57 nM; acetylcholine, 145 nM; and oxotremorine, 100 nM. Nicotinic cholinergic agents such as alpha-bungarotoxin, nicotine and hexamethonium were ineffective in displacing [3H]QNB binding, at a concentration of 10 microM. After preganglionic denervation, the specific [3H]QNB binding to the SCG significantly increased, compared to the nonoperated side of SCG. The Bmax value was significantly increased at 2 weeks without a change in the KD, thereby suggesting an increase in the density of muscarinic cholinergic receptors. The present study suggests that the existence of muscarinic cholinergic receptors in the cat SCG and preganglionic denervation may lead to an increase in the density of muscarinic cholinergic receptors.