5-Hydroxytryptamine induces forestomach hypomotility in sheep through 5-HT4 receptors.
The effects evoked by 5-hydroxytryptamine (5-HT; serotonin) on forestomach myoelectric activity were investigated in conscious sheep. Myoelectric signals were recorded with electrodes chronically implanted in the reticulum, rumen (dorsal sac) and omasal body, and were analysed by a computer-based method. The 5-HT receptors and the neuronal pathways involved in these actions were studied. The intravenous (i.v.) infusion of 5-HT (8 micrograms kg-1 min-1 for 5 min) evoked an inhibition of activity of the whole forestomach. Methiothepin, injected i.v. at 0.1 mg kg-1, inhibited rumen secondary contractions and omasum activity. However, forestomach activity remained unchanged after the administration of 0.2 mg kg-1 of ketanserin, ondansetron, tropisetron, GR-113808, phentolamine, propranolol, domperidone and naloxone. Atropine (0.2 mg kg-1), hexamethonium (2 mg kg-1) or haloperidol (0.1 mg kg-1) abolished rumen secondary cycles and inhibited omasum activity. In addition, atropine also suppressed primary cycles. GR-113808 blocked all 5-HT-induced effects. Furthermore, atropine or hexamethonium prevented the 5-HT-evoked inhibition of reticulorumen primary cycles. In contrast, the remaining antagonists did not alter the 5-HT-evoked forestomach hypomotility. In conclusion, 5-HT induces inhibition of forestomach myoelectric activity through 5-HT4 receptors, these actions being mediated by cholinergic neural pathways involving muscarinic and nicotinic receptors. However, adrenergic, dopaminergic or opiate pathways are not implicated.
Plaza
M A
MA
Department of Physiology, Veterinary Faculty, Zaragoza, Spain.
Arruebo
M P
MP
Murillo
M D
MD
D000818:Animals; D001285:Atropine / Q000494:pharmacology; D005260:Female; D005769:Gastrointestinal Motility / Q000187:drug effects*; D018738:Hexamethonium / Q000494:pharmacology; D008719:Methiothepin / Q000494:pharmacology; D011985:Receptors, Serotonin / Q000378:metabolism*; D044422:Receptors, Serotonin, 5-HT4; D012701:Serotonin / Q000494:pharmacology*; D012756:Sheep; D013270:Stomach / Q000187:drug effects* / Q000502:physiology
D011985:Receptors, Serotonin; D044422:Receptors, Serotonin, 5-HT4; D012701:Serotonin; D018738:Hexamethonium; D008719:Methiothepin; D001285:Atropine