Pudendal motoneurons of the rat located in separated spinal nuclei possess nicotinic acetylcholine receptors having distinct pharmacological profiles

Abstract

Pudendal motoneurons are located in the ventral horn of the caudal lumbar spinal cord and innervate striated pelvic muscles implicated in sexual and eliminative functions. In rats they are distributed in the dorsomedial (DM) and dorsolateral (DL) nucleus. In male rats, dorsomedial motoneurons innervate the bulbocavernosus, the levator ani and the external anal sphincter, whereas dorsolateral motoneurons control the ischiocavernosus and external urethral sphincter. Using spinal cord slices of young male rats and whole-cell patch-clamp recordings, we investigated the sensitivity of pudendal motoneurons to nicotinic cholinergic agonists. Motoneurons were identified following 1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine, 4-chlorobenzenesulphonate retrograde labelling. In the presence of atropine, both dorsomedial and dorsolateral motoneurons responded to acetylcholine (ACh) by generating a rapidly activating inward current. By using selective nicotinic antagonists and a nicotinic positive allosteric modulator, we found that nicotinic ACh receptors present in dorsomedial and dorsolateral motoneurons display distinct pharmacological profiles. Whereas the former are of the heteromeric type, the latter are predominantly of the alpha7-containing type. These data were confirmed by light microscopic autoradiography. In young rats, a ligand for heteromeric nicotinic receptors labelled all laminae of the central grey matter, whereas in the ventral part of the central grey, a ligand selective for alpha7-containing nicotinic receptors labelled the DL but not the DM. Dorsolateral and dorsomedial motoneurons innervate two distinct groups of pelvic muscles. A difference in their nicotinic pharmacology may be clinically relevant, as it might allow a selective pharmacological intervention in view of influencing the activity of one or the other set of muscles.