| Abstract: | Localization of muscarinic receptors in rabbit superior cervical ganglion (SCG) neurons was investigated by means of light microscopy autoradiographic study of binding sites for the labeled selective muscarinic antagonist atropine, and intracellular recordings were made from single ganglionic neurons to determine their response to local iontophoretic application of the selective muscarinic agonist--5-methylfurmethide (MF). Autoradiography showed that muscarinic receptors are located predominantly on the dendrites of ganglion neurons and on their soma near large processes. The remaining soma areas are usually devoid of muscarinic receptors or their density is much lower than in dendritic areas. MF-elicited depolarization was seen in 34% of neurons studied; an initial hyperpolarization was followed by depolarization in 23% of the neurons and no responses occurred in the remaining 43%. Lowering Ca2+ and increasing Mg2+ content in the external medium abolished MF-hyperpolarization but not MF-depolarization. The following properties of MF-depolarization support the concept of dendritic localization of muscarinic receptors: (i) the time-course of MF-depolarization varies widely in different cells; (ii) its rise time increases with the increase of the MF dose; (iii) MF is more effective in inducing a MF-depolarization if it is applied to cell processes than if it is applied to cell soma. The origin of MF-hyperpolarization is discussed. |
