Cholinergic receptors in the upper respiratory system of the rat

Radioligand receptor binding might give more detailed information on the innervation pattern of the nasal mucosa and the character of the various neuroreceptors involved. With respect to the cholinergic receptors, this technique reveals that specific binding of tritiated I-quinuclidinyl benzilate to rat nasal mucosa homogenates occurs to a homogeneous class of binding sites, with a dissociation constant of 0.06 +/- 0.02 nM and a receptor density of 8 +/- 2 pmole/g of tissue. Binding is stereoselectively inhibited by benzetimide hydrochloride enantiomers. Pirenzepine displacement (inhibition constant = 0.5 X 10(-6) M) classifies tritiated I-quinuclidinyl benzilate binding sites as M2-muscarinic receptors. Methylfurthrethonium inhibits tritiated I-quinuclidinyl benzilate binding at high concentrations, pointing to the presence of low-affinity agonist binding sites, probably admixed with a small proportion of high-affinity agonist binding sites. These data obtained in the rat open new perspectives for studying muscarinic receptors in the human nose to elucidate the supposed disturbance of autonomic nerve regulation in nasal hyperreactivity.