Sites of action of morphine on the ascending excitatory reflex in the guinea-pig small intestine.
The effect of morphine on the ascending excitatory reflex of the circular muscle elicited by radial distension of the gut wall was studied in the isolated guinea-pig small intestine. A three compartment bath, in which an intermediate compartment divided the site of intraluminal stimulation (caudal compartment) from the site of reflex contraction recording (oral compartment), was used. Morphine (0.01-10 microM) applied independently to each compartment, caused a concentration-dependent depression (up to 90%) of the amplitude of distension-evoked reflex contractions. Concentration-response curves to morphine were shifted to the right by naloxone (30 nM) with an apparent pA2 value of about 8.5, which suggests an interaction with opioid mu-receptor subtypes. Our results indicate that morphine not only depressed transmission from excitatory motor neurons to the circular muscle but also neuro-neuronal transmission along the ascending excitatory reflex pathway.
Tonini
M
M
Department of Internal Medicine and Therapeutics, University of Pavia, Italy.
Waterman
S A
SA
Candura
S M
SM
Coccini
T
T
Costa
M
M
D000818:Animals; D004525:Efferent Pathways / Q000187:drug effects; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007421:Intestine, Small / Q000187:drug effects*; D009020:Morphine / Q000494:pharmacology*; D009046:Motor Neurons / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects / Q000294:innervation; D009270:Naloxone / Q000494:pharmacology; D017450:Receptors, Opioid, mu / Q000187:drug effects; D012018:Reflex / Q000187:drug effects*
D017450:Receptors, Opioid, mu; D009270:Naloxone; D009020:Morphine