Effects of N-ethylmaleimide on muscarinic acetylcholine receptor subtype autoradiography and inositide response in rat brain

Abstract

Chemical modification of brain muscarinic acetylcholine receptors (mAChr) with N-ethylmaleimide (NEM) has been employed to investigate mAChr-subtype distribution and mediation of the inositide response. 3 H-Pirenzepine and 3H-oxotremorine-M were used to autoradiographically localize the M1- and M2-AChr subtypes, respectively, in brain slices. M1- and M2-AChr distribution were observed to be distinct from each other. The presence of 1 mM NEM selectively reduced the labeling of M2-, but not of M1-AChr. These data support the notion that NEM converts the high-affinity M2-AChr to a lower affinity state, without affecting the affinity of the M1-AChr. Also, regional analysis indicated that the M1- and M2-AChr subtypes were not interconvertible by NEM. NEM at 30 μM enhanced the carbamylcholine stimulated labeling of phosphatidic acid from 32P i in nerve endings from rat forebrain, suggesting that the low affinity M2-AChr may mediate at least a part of the inositide response to cholinergic stimulation.