Presence of functional neuronal nicotinic acetylcholine receptors in brainstem motoneurons of the rat

In mammals, nicotinic acetylcholine receptors (nAChRs) play a crucial role in motor control. Muscle-type nAChRs mediate synaptic excitation of skeleta l muscle by motoneurons, and nAChRs are present on Renshaw cells, where the y produce recurrent inhibition of spinal motoneurons. We asked whether nACh Rs are also present in motoneurons. Whole-cell recordings were performed on various motor nuclei in brainstem slices of young rats. Neurons were visua lized using infrared (IR) videomicroscopy. Acetylcholine (ACh) or the nicot inic agonist, epibatidine, were delivered by pressure microinjection. Facia l (VII), hypoglossal (XII) and vagal (X) motoneurons responded to ACh by ge nerating a fast inward current. In VII motoneurons, the ACh effect was mimi cked by epibatidine, and nicotine induced a slow inward current and desensi tized the ACh-evoked current. In VII and XII motoneurons, the ACh-evoked cu rrent was blocked by the nicotinic antagonist dihydro-beta-erythroidine (DH bE), but was unaffected by methyllycaconitine (MLA), an alpha 7-specific an tagonist. By contrast, the ACh-induced current in X motoneurons was sensiti ve to MLA. Current-voltage relationships indicated that the currents mediat ed by either alpha 7-containing (X) or non-alpha 7-containing (VII, XII) nA ChRs displayed inward rectification. In accordance with the electrophysiolo gical data, autoradiography revealed that VII, X and XII nuclei of young ra ts contained binding sites for [H-3]epibatidine; binding sites for [I-125]a lpha-bungarotoxin, a selective ligand of alpha 7-containing nAChRs, were pr esent in X nucleus but were almost undetectable in VII and XII nuclei. Thus , brainstem motoneurons of young rats possess functional nAChRs. They could promote fast synaptic coupling between motoneurons, and thus play a role i n somatic and visceral motor functions.