ABSENCE OF RECEPTOR RESERVE AT HIPPOCAMPAL MUSCARINIC AUTORECEPTORS WHICH INHIBIT STIMULUS-DEPENDENT ACETYLCHOLINE-RELEASE

The extent of reserve among inhibitory muscarinic autoreceptors on hip pocampal cholinergic nerve terminals was examined in superfused calciu m-naive synaptosomes. The tissues were treated with the irreversible m uscarinic cholinergic receptor antagonist propylbenzilylcholine mustar d (PrBCM) and then used to assess the functional status of autorecepto rs through acetylcholine (ACh)-induced inhibition of calcium-evoked [H -3]Ach release. PrBCM treatment caused a marked reduction in the densi ty of high-affinity [H-3]quinuclidinyl benzilate binding sites (46%, 7 2% and 90% reductions after 3, 6 or 10 nM PrBCM, respectively) but had no apparent influence on the binding affinities or relative proportio ns of high- and low-affinity binding sites for the M1-selective antago nist pirenzepine or the agonist ACh. In vehicle-treated tissues, ACh w as a potent (EC50 = 240 nM) and efficacious (maximal inhibition of sti mulated [H-3]ACh release = 65%) agonist at muscarinic autoreceptors. H owever, after PrBCM treatment, the maximal inhibition for ACh was grea tly attenuated (35% and 17% for 3 and 6 nM PrBCM, respectively) with n o concurrent changes in the EC50 or slope factor. Comparisons of equie ffective agonist concentrations before and after receptor occlusion re vealed a direct linear relationship between autoreceptor occupancy and inhibition of [H-3]ACh release with close agreement between the calcu lated agonist dissociation constant (K(A) = 220 nM) and the EC50 for A Ch. Pretreatment with 100 nM atropine methylbromide completely prevent ed PrBCM-induced reductions in muscarinic cholinergic receptor binding and autoreceptor function. These results support the conclusion that muscarinic autoreceptors on hippocampal nerve endings exhibit little o r no reserve for inhibition of ACh release by the endogenous neurotran smitter.