title: Excitatory effect of morphine and opioid peptides in the rat isolated colon.
abstract: Morphine and the opioid peptides cause isolated segments of rat colon to contract and relax rhythmically. This study re-examines two hypotheses to explain this phenomenon: Release of 5-hydroxytryptamine (5-HT)/acetylcholine by morphine or inhibition of a tonically active non-adrenergic, non-cholinergic (NANC) inhibitory mechanism. Rhythmic contractions induced by morphine (5 x 10(-6) M) were naloxone sensitive (10(-6) M) but unaffected by methysergide (10(-6) M), atropine (10(-6) M) or pretreatment of rats with p-chlorophenylalanine (200 mg kg-1 i.p. for four days) which lowered the 5-HT level in the colon from 3.73 +/- 0.83 mg g-1 in controls to 0.41 +/- 0.06 mg g-1 (P less than 0.001). The pattern of rhythmic contractions produced by morphine was unlike those produced by 5-HT (5 x 10(-6) M), acetylcholine (5 x 10(-6) M) or potassium chloride (30 mM). Tetrodotoxin (10(-6) M), apamin (10(-8) M), clonidine (2 x 10(-8) M), phentolamine (10(-5) M) or oxprenolol (10(-5) M) caused rhythmic contractions which were unaffected by naloxone. Clonidine contractions were inhibited by yohimbine (10(-7) M) but not by prazosin (10(-6) M). Electrical field stimulation at the peak of a contraction induced by morphine, apamin or clonidine, produced an inhibitory response which was unaffected by atropine, phentolamine, propranolol and guanethidine (all 10(-5) M). It persisted in colon segments from the rats with reserpine or 6-hydroxydopamine. These results suggest that neither the 5-HT/acetylcholine hypothesis nor inhibition of the NANC mechanism adequately explains the excitatory effect of morphine in the rat colon.
authors.affiliation: Department of Pharmacology, University of Glasgow, UK.
authors.forename: A
authors.initials: A
authors.lastname: Laniyonu
authors.forename: C
authors.initials: C
authors.lastname: McCulloch
authors.forename: D
authors.initials: D
authors.lastname: Pollock
mesh_terms: D000109:Acetylcholine / Q000096:biosynthesis / Q000494:pharmacology; D000818:Animals; D001030:Apamin / Q000494:pharmacology; D003106:Colon / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D004558:Electric Stimulation; D010134:Fenclonine / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009294:Narcotics / Q000494:pharmacology*; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012701:Serotonin / Q000096:biosynthesis; D013565:Sympatholytics / Q000494:pharmacology; D013566:Sympathomimetics / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D009294:Narcotics; D013565:Sympatholytics; D013566:Sympathomimetics; D001030:Apamin; D012701:Serotonin; D013779:Tetrodotoxin; D011189:Potassium Chloride; D009020:Morphine; D000109:Acetylcholine; D010134:Fenclonine