Medullary motor neurones associated with drinking behaviour of Japanese eels

A fluorescent dye, Evans blue (EB), was injected into the following seven drinking‐associated muscles of the Japanese eel Anguilla japonica : the sternohyoid, third branchial, fourth branchial, opercular, pharyngeal, upper oesophageal sphincter and oesophageal body muscles. The sternohyoid muscle promotes ‘ingestion’, and the remaining muscles contribute to ‘swallowing’. All neurones stained by EB were located ipsilaterally in the caudal medulla oblongata (MO) of the Japanese eel. Neurones projecting into the sternohyoid muscle were identified as those in the spino‐occipital motor nucleus (NSO), and neurones projecting into the remaining muscles as those in the glossopharyngeal–vagal motor complex (GVC). Within the GVC, the neuronal arrangement was topological, and hence, ‘swallowing’ will be completed if the GVC neurones ‘fire’ progressively from rostral to caudal. These neurones in the NSO and GVC may use acetylcholine (ACh) as a neurotransmitter, as the EB‐positive neurones in both nuclei were immunoreactive against anticholine acetyltransferase (anti‐ChAT) antibody. Besides the MO, some somata in a ganglion of the vagal nerve were also stained by EB injected into the pharyngeal, the upper oesophageal sphincter and the oesophageal body muscles. The localization and the shape of the somata suggest that they are sensory neurones. These sensory neurones were not ChAT‐immunoreactive. Combining these results, based on a model for ‘swallowing’ in mammals, a plausible model for central organization of ‘drinking’ in the Japanese eel is proposed, which suggests that ‘drinking’ in the fishes is regulated by the neuronal circuit for ‘swallowing’ in mammals.