Pharmacological Research, Vol. 23, No. 1, 1991 33 STUDY OF THE MUSCARINIC RECEPTOR SUBTYPES IN N1E 115 MOUSE NEUROBLASTOMA CELLS NORBERT H. FRAEYMAN*, MARIE-ANGE M. BUYSEt and ROMA1N A. LEFEBVRE~: Heymans Institute of Pharmacology, Universityof Ghent, Medical School, De Pintelaan 185, B-9000 Ghent Received in final form 18 May 1990 SUMMARY Muscarinic receptors in N1E 115 mouse neuroblastoma cells were characterized by competition binding experiments using three agonists and five antagonists, including 4-DAMP and AF-DX 116, and by studying the effect of agonist stimulation on the cellular cAMP and cGMP content. The results of the binding studies with the antagonists suggest that only one single homogeneous binding site of the M1 muscarinic receptor subtype is present. For the binding with the agonists, two binding sites were detected, one with high affinity for the ligand (between 53 and 77% of the total binding sites depending on the agonist) and one with low affinity. In contrast to the results obtained with the binding experiments using antagonists, the study of the cellular cyclic nucleotide response upon carbachol stimulation suggested the presence of both the M 1 and M 2 subtypes as there was an increase in cyclic GMP concentration while at the same time, the prostaglandin- stimulated synthesis of cyclic AMP was inhibited. Considering both binding and functional data we suggest that in NIE 115 cells a majority of M1 and a minority of M2 muscarinic receptors are present; there is no evidence for the presence of M 3 muscarinic receptors. KEYWORDS: muscarinic receptors, N 1E 115 neuroblastoma cells, receptor subtypes. INTRODUCTION The number and pharmacological characteristics of the muscarinic receptors in N1E 115 mouse neuroblastoma cells using the classical agonists and antagonists have been described [1, 2]. The main conclusion from the experiments with agonists was that both high and low affinity binding sites are present, each one *To whom correspondence should be sent. tBursary of the IWONL. ~Senior Research Associate of the National Fund for Scientific Research. 1043-6618/91/010033-08/S03.00/0 © 1991 The Italian Pharmacological Society