Regulation of putative muscarinic cholinergic receptor subtypes in rat brain

Transection of the fimbria/fornix, producing a 75% reduction in the activity of the cholinergic marker choline-o-acetyltransferase (CAT EC. 2.3.1.6) in rat hippocampus, did not change the binding characteristics of the non-subtype selective, muscarinic cholinergic receptor antagonist ligand [(3)H](?)quinuclidinyl benzilate {[(3)H](?)QNB}. Pirenzepine competition for [(3)H](?)QNB binding in the hippocampus was best described by a computer derived model assuming two binding sites of high affinity (putative M(1) receptors) and low affinity (putative M(2) receptors). There was no change in the proportion of high and low affinity pirenzepine binding sites in the hippocampus following cholinergic deafferentation. Thus, these data provide no evidence for a discrete localization of either putative subtype of muscarinic receptor discriminated by pirenzepine restricted to the terminals of CAT containing neurons innervating the rat hippocampus. Chronic scopolamine treatment produced a 48% increase in the B(max) of [(3)H](?)QNB binding in the hippocampus, but again there was no change in the proportions of the sites discriminated by pirenzepine demonstrating that both putative subtypes were regulated identically. Similarly, carbachol competition for [(3)H](?)QNB was unaltered following cholinergic deafferentation or chronic scopolamine treatment. Furthermore, similar guanylyl-5?-imidodiphosphate [Gpp(NH)p] modulation of the proportions of high and low affinity carbachol binding sites was found in the hippocampus following transection of the fimbria/fornix or chronic scopolamine treatment. Thus there is no adaptation of receptor-effector coupling following these treatments that is reflected by changes in receptor recognition site characteristics. Carbachol competition for [(3)H]pirenzepine binding to putative M(1) receptors in the hippocampus was biphasic and was also modulated by Gpp(NH)p. In the brainstem, there was a homogeneous population of putative M(2) [(3)H](?)QNB binding sites having low affinity for pirenzepine. Carbachol competition for these binding sites was also biphasic and modulated by guanine nucleotides. Thus, both putative M(1) and M(2) muscarinic receptors, as defined by high or low affinity for pirenzepine respectively, may mediate their effects in rat brain via a guanine nucleotide regulatory subunit.