Cholinergic innervation to the upper esophageal sphincter muscle in the eel, with special reference to drinking behavior.
To elucidate innervation in the upper esophageal sphincter (UES) muscle of the eel, a key muscle in swallowing, repetitive electrical field stimulation (EFS; 30 mA, 40 V, 300 micros, 10 Hz, 10 trains) was employed. Anatomically, the eel UES muscle consists of striated fibers. The EFS-induced contraction of the UES was completely blocked by tetrodotoxin and curare, and abolished in Ca2+ -free Ringer solution. These results suggest that the EFS stimulates nerve fibers specifically and releases acetylcholine as a neurotransmitter. In fact, acetylcholine and carbachol constricted the UES in a concentration-dependent manner. Even after blocking neuronal firing with tetrodotoxin, acetylcholine constricted the UES muscle, suggesting the existence of acetylcholine receptors on the UES muscle cells. Both EFS- and carbachol-evoked contractions of the UES were blocked by curare at a lower concentration than by atropine or hexamethonium, suggesting that the acetylcholine receptor is nicotinic. Even in Ca2+ -free Ringer solution, a direct current stimulus (2 s duration) constricted the UES muscle to an extent similar to that in the presence of Ca2+, indicating that the muscle contraction itself does not need extracellular Ca2+, i.e., the muscle can be constricted by a release of Ca2+ from the sarcoplasmic reticulum.
Kozaka
T
T
Laboratory of Integrative Physiology, Faculty of Integrated Arts and Sciences, Hiroshima University, 739-8521 Higashi-Hiroshima, Japan.
Ando
M
M
D000109:Acetylcholine / Q000502:physiology*; D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D018679:Cholinergic Agonists / Q000494:pharmacology; D018680:Cholinergic Antagonists / Q000494:pharmacology; D003472:Curare / Q000494:pharmacology; D003679:Deglutition / Q000502:physiology; D004327:Drinking Behavior; D004524:Eels / Q000502:physiology*; D004558:Electric Stimulation; D004943:Esophagogastric Junction / Q000187:drug effects / Q000294:innervation*; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects
D018679:Cholinergic Agonists; D018680:Cholinergic Antagonists; D003472:Curare; D002217:Carbachol; D000109:Acetylcholine