Opioid pathways exert a tonic restraint in the guinea-pig isolated colon: changes after chronic sympathetic denervation.
We have studied the effects of naloxone on acetylcholine and noradrenaline release in the guinea-pig isolated distal colon, and have assessed the effect of naloxone on electrically-induced contractions of the longitudinal muscle and non-adrenergic, non-cholinergic (NANC) relaxations of the circular muscle coat. Naloxone dose-dependently increased resting and electrically-evoked acetylcholine release and electrically-evoked noradrenaline release. Naloxone was more potent in increasing resting acetylcholine release in colonic specimens obtained after chronic sympathetic denervation. Naloxone (1 microM) did not affect electrically-induced contractions of the longitudinal muscle, while it enhanced NANC relaxations of the circular muscle. The effects observed with naloxone in the present experiments suggest that opioid pathways exert a tonic restraint on neurotransmission in the guinea-pig colon. After suppression of the adrenergic inhibitory tone, the functional relevance of opioid pathways seems to be increased.
Marino
F
F
Department of Internal Medicine and Therapeutics, University of Pavia, 2nd Faculty of Medicine, Varese, Italy.
Creta
F
F
de Ponti
F
F
Giaroni
C
C
Lecchini
S
S
Frigo
G M
GM
D000109:Acetylcholine / Q000378:metabolism*; D000818:Animals; D003106:Colon / Q000187:drug effects* / Q000294:innervation / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009126:Muscle Relaxation / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects* / Q000294:innervation; D009270:Naloxone / Q000494:pharmacology*; D009638:Norepinephrine / Q000378:metabolism*; D013562:Sympathectomy; D009435:Synaptic Transmission / Q000187:drug effects*
D009270:Naloxone; D000109:Acetylcholine; D009638:Norepinephrine