Muscarinic receptors in the prenatal mouse embryo. Comparison of M35-immunohistochemistry with [3H]quinuclidinyl benzylate autoradiography.

Muscarinic cholinergic receptors are widespread in nervous tissue and smooth muscle or paracrine epithelial cells of various organs. In the embryo, muscarinic receptors are transitorily expressed in the early blastoderm and later on in blastemic tissues during morphogenesis. Recently, a monoclonal antibody (M35) against muscarinic receptor from calf brain became available. In the present study the use of M35-immunohistochemistry is compared to autoradiographic localization of muscarinic binding sites in the mouse embryo. The aim of the study is to test the suitability of the antibody for localization of muscarinic receptors in embryonic tissues. For autoradiography whole-body sagittal cryostat sections of the 17- and 18-day mouse embryo were covered with LKB-Ultrofilm after incubation with the radioactive ligand [3H] quinuclidinyl benzylate (QNB). For immunohistochemistry cryostat sections of formalin fixed tissues were used. In general, all tissues exhibiting ligand binding were also recognized by the antibody. M35-immunohistochemistry resulted in higher spatial resolution of receptor localization than [3H]QNB autoradiography. Definitive muscarinic receptors were observed in smooth muscle and the epithelial lining of the vasuclar, intestinal, respiratory and urinary system, in the brain, spinal cord and peripheral nerves. The embryonic type of the muscarinic receptor was detected in the mesothelium of lung and liver, in the nephrogenic blastema of the metanephros, and in lung mesenchyme. A large amount of embryonic muscarinic receptors was found in the remnants of the notochord and in the nucleus pulposus of the developing vertebral column. A function in morphogenesis is discussed of the embryonic muscarinic receptor.