Naloxone-precipitated contractions of the guinea pig ileum made tolerant to a synthetic opioid peptide: evidence against the involvement of capsaicin-sensitive neurones.
In segments of guinea pig ileum made tolerant-dependent to the synthetic opioid peptide [D-Met2,Pro5]enkephalinamide, naloxone (1 microM) caused half-maximal longitudinal contractions. These were strongly reduced by hyoscine or desensitization to serotonin but not by hexamethonium, desensitization to substance P or pretreatment with capsaicin. Contractions of similar size evoked by capsaicin (0.3 microM) in the presence of naloxone were abolished by capsaicin pretreatment, reduced by substance P desensitization or hyoscine but not by desensitization to serotonin. Thus, the two types of contraction clearly have different mechanisms; enteric capsaicin-sensitive neurones or substance P-containing motor neurones seem not to be involved in the naloxone-precipitated contraction while serotonin may play a role in this response.
Barthó
L
L
Department of Pharmacology, University Medical School of Pécs, Hungary.
D000818:Animals; D002211:Capsaicin / Q000494:pharmacology*; D004744:Enkephalin, Methionine / Q000008:administration & dosage / Q000031:analogs & derivatives*; D005260:Female; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects / Q000294:innervation / Q000502:physiology*; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects / Q000294:innervation / Q000502:physiology*; D009270:Naloxone / Q000494:pharmacology*
D009270:Naloxone; D004744:Enkephalin, Methionine; C021717:enkephalin, Met(2)-ProNH2(5)-; D002211:Capsaicin