Comparison of [3H]pirenzepine and [3H]quinuclidinylbenzilate binding to muscarinic cholinergic receptors in rat brain.

The properties of [3H]quinuclidinylbenzilate ( [3H]QNB) binding and [3H]pirenzepine ( [3H]PZ) binding to various regions of rat brain were compared. [3H]PZ appeared to bind with high affinity to a single site, with a Kd value of approximately 15 nM in the cerebral cortex. The rank order of potencies of muscarinic drugs to inhibit binding of either [3H]QNB or [3H]PZ was QNB greater than atropine = scopolamine greater than pirenzepine greater than oxotremorine greater than bethanechol. Muscarinic antagonists (except PZ) inhibited both [3H]PZ and [3H]QNB binding with Hill coefficients of approximately 1. PZ inhibited [3H]QNB binding in cortex with a Hill coefficient of 0.7, but inhibited [3H]PZ binding with a Hill coefficient of 1.0. Hill coefficients for agonists were less than 1. The density of [3H]PZ binding sites was approximately half the density of [3H]QNB binding sites in cortex, striatum and hippocampus. In pons-medulla and cerebellum, the densities of [3H]PZ binding sites were 20 and 0%, respectively, relative to the densities of [3H]QNB binding sites. When unlabeled PZ was used to compete for [3H]QNB binding, the relative number of high-affinity PZ binding sites in cortex, pons and cerebellum agreed with the relative number of [3H]PZ binding sites in those regions. The binding of [3H]PZ and [3H]QNB was nonadditive in cortex. GTP inhibited high-affinity oxotremorine binding, but not PZ binding. Together, these data suggest that [3H]PZ binds to a subset of [3H]QNB binding sites. Whether this subset reflects the existence of subtypes of muscarinic receptors or is a consequence of coupling to another membrane protein remains to be seen.