1 Segments of ileum, incubated for 2-24 h at 22 degrees C with
normorphine (0.01 - 1.0 muM), in the presence of
hexamethonium, contracted when challenged with
naloxone (0.03 muM). No response to this dose of
naloxone was induced either by incubation in control
solution without
opiate for 2-24 h or by exposure of the preparation to
opiate for 30 min at 37 degrees C.2 When segments were incubated for 24 h, the size of the response to
naloxone was directly related both to the
normorphine concentration in the incubation fluid (0.01 to 0.1 muM), and to the concentration of
naloxone applied (0.03 to 0.1 muM).3 A spontaneous withdrawal
contracture was elicited in ilea that had been incubated with
normorphine (1.0 muM), when the
normorphine-containing bathing fluid was exchanged for one without
opiate.4
Normorphine restored to resting level the tension of the withdrawal
contracture, whether it had been elicited spontaneously or by
naloxone challenge.5 Addition of
naloxone (1.0 muM) to
normorphine (1.0 muM) in the incubation fluid abolished the withdrawal
contracture to subsequent challenge with
naloxone.6
Naloxone elicited a
contracture from segments incubated for 24 h at 22 degrees C with
levorphanol (0.1 muM) but not from those incubated with
dextrorphan.7 Application of (+)-
naloxone (0.03 muM) to segments previously incubated with
normorphine (0.1 muM) did not elicit a
contracture.8 The
contracture elicited by
naloxone in preparations incubated with
morphine (10 muM) was associated with a reduction in sensitivity to the acute inhibitory effect of
morphine on the electrically-evoked response.9 Addition of
hyoscine (0.5 muM) immediately after challenge with
naloxone restored the tension of the withdrawal
contracture to resting level.10
Tetrodotoxin (3.0 muM) given before challenge, prevented
naloxone from eliciting a withdrawal
contracture.11 The inclusion of
5-hydroxytryptamine (10 muM) with
morphine (10 muM) inhibited the induction of tolerance to
morphine.12 These experiments, together with those described earlier, indicate that incubation with
opiate induces a dependence in the final
cholinergic motor neurones of the myenteric plexus, manifested as a
contracture of the longitudinal muscle on removal of
opiate or administration of an antagonist. This dependence is associated with tolerance, expressed as a decrease in sensitivity to inhibition by
morphine of the electrically-evoked
contracture. Tolerance and dependence are induced and withdrawal precipitated through specific and stereospecific
opiate receptors.