Structure and function of a pentapeptide isolated from the gut of the eel

Post- and pre-synaptic action of isotocin in the upper esophageal sphincter muscle of the eel: its role in water drinking

Isotocin is a fish analogue of the mammalian hormone oxytocin. To elucidate sites of action of isotocin (IT) in the upper esophageal sphincter (UES) muscle, a key muscle in swallowing, IT was applied after treatment with tetrodotoxin (TTX). Even after blocking nerve activity with TTX, IT relaxes the UES muscle in a concentration-dependent manner, suggesting that IT receptor(s) is present on the muscle cells. Similar relaxation was also obtained by application of 3-isobutyl-1-methylxanthine (IBMX), forskolin (FSK) and 8-bromo-adenosine, 3',5'-cyclic monophosphate (8BrcAMP) after pretreatment with TTX, suggesting that the relaxing effect (postsynaptic action) of IT may be mediated by cAMP. In contrast to such relaxing effect, IT enhanced the UES contraction induced by repetitive electrical field stimulation (EFS). Such enhancement was blocked by an IT receptor antagonist, suggesting that this effect is also mediated by IT receptor(s). Similar enhancement was also induced by IBMX, FSK and 8BrcAMP, suggesting the enhancing effect is also mediated by cAMP. However, no enhancing effect of IT was observed when the muscle was stimulated by carbachol, or after treatment with curare or TTX, denying the postsynaptic modulatory action of IT and suggesting presynaptic action for IT, i.e., accelerating acetylcholine release. Summarizing these results, role of IT in precisely regulating the drinking rate in the seawater eel is discussed.

Antagonistic effects of vasotocin and isotocin on the upper esophageal sphincter muscle of the eel acclimated to seawater

The effects of isotocin (IT) and vasotocin (VT), which are fish analogues of mammalian oxytocin and vasopressin respectively, were examined in the isolated upper esophageal sphincter (UES) muscle. IT relaxed and VT constricted the UES muscle in a concentration-dependent manner. The relaxation by IT and the contraction by VT were completely blocked by H-9405 (an oxytocin receptor antagonist) and by H-5350 (a V(1)-receptor antagonist), respectively, suggesting that the eel UES possesses both IT and VT receptors. Truncated fragments of VT did not show any significant effects, indicating that all nine residues are essential for the VT and IT actions. IT may relax the UES muscle through enhancing cAMP production, since similar relaxation was also observed after treatment with 3-isobutyl-1-methylxantine, forskolin and 8-bromoadenosine, 3', 5'-cyclic mono-phosphate (8BrcAMP). Although 8-bromoguanosine, 3', 5'-cyclic monophosphate also relaxed the UES, its effect was less than 1/3 of that 8BrcAMP, suggesting minor contribution of nitric oxide (NO) in the relaxation of the UES muscle. Both peptides seem to act directly on the UES muscle, not through release of other substances from the epithelial cells, since similar relaxation and contraction were observed even in the scraped UES preparations. When IT and VT were intravenously administrated (in vivo experiments), the drinking rate of the seawater eel was enhanced by IT and was inhibited by VT. These effects correspond to the in vitro results described above, relaxation by IT and contraction by VT in the UES muscle. The significance of the relaxing effect by IT is discussed with respect to controlling the drinking behavior of the eel.

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.