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.
Department of Physiology, Veterinary Faculty, Zaragoza, Spain.
M A
MA
Plaza
M P
MP
Arruebo
M D
MD
Murillo
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