THE INHIBITORY EFFECTS OF PRESYNAPTIC α‐ADRENOCEPTOR AGONISTS ON CONTRACTIONS OF GUINEA‐PIG ILEUM AND MOUSE VAS DEFERENS IN THE MORPHINE‐DEPENDENT AND WITHDRAWN STATES PRODUCED in vitro

Abstract

1 Isolated ilea from guinea-pigs implanted with morphine pellets were stimulated coaxially, either with or without morphine present in the bath fluid, and the longitudinal contractions recorded. 2 In the absence of morphine the inhibitory effects of the presynaptic α-adrenoceptor agonists, clonidine and oxymetazoline were much reduced and the dose-response curve was flat. This state of ‘withdrawal’ was readily reversed by morphine and levorphanol but not its inactive (+)-isomer, dextrorphan. 3 The κ-agonists, ketazocine and ethylketazocine, also restored the effects of clonidine as did the opioid peptides Tyr-d-Ala-Gly-Phe-d-Leu, acting preferentially on δ-receptors, and Tyr-d-Ala-Gly-MePhe-Met(O)-ol, acting mainly on μ-receptors. 4 The inhibitory effects of adrenaline and adenosine 3′,5′-diphosphate were reduced at low but not at high concentrations. 5 In contrast, the inhibitory effect of clonidine on the electrically evoked contractions of vasa deferentia from mice implanted with morphine pellets was not abolished by the lack of morphine in the bath fluid or by addition of naloxone. 6 A possible explanation is suggested for the loss of the inhibitory effects of presynaptic α-adrenoceptor agonists in the withdrawn state of the dependent ileum.