| Abstract: | 1 Compared with the response of preparations incubated in solutions without clonidine, a three to four fold increase in the magnitude of the contracture of the longitudinal muscle to challenge with phentolamine (1.0 μm) was induced by incubating the guinea-pig isolated ileum at 22°C for 24 h with clonidine (1.0 μm) in Krebs solution containing hexamethonium (70 μm). Incubation of the ileum with clondine (1.0 μm) for 0.5 h at 37°C did not increase responsiveness to phentolamine.2 The increase in responsiveness to phentolamine was directly related to the clonidine concentration in the incubation fluid over the range 0.01 to 1.0 μm.3 The magnitude of the contracture to phentolamine of ilea incubated with clonidine (1.0 μm) (withdrawal contracture) was directly related to the challenge dose of phentolamine over the range 0.3 to 1.0 μm.4 Yohimbine (1.0 μm) or piperoxane (1.0 μm) elicited a response comparable to that elicited by phentolamine but propranolol (1.0 μm) was inactive.5 Addition of phentolamine (1.0 μm) to clonidine (1.0 μm) in the incubation fluid abolished the increased response of the preparation to subsequent challenge with phentolamine.6 Addition of hyoscine (0.5 μm) immediately after challenge with phentolamine restored the tension of the withdrawal contracture to its resting level.7 Tetrodotoxin (3.0 μm) given before challenge, prevented phentolamine from eliciting a withdrawal contracture.8 Ileal segments incubated with clonidine (1.0 μm) were unresponsive to challenge with naloxone (100 nm); and segments incubated with normorphine (1.0 μm) were unresponsive to phentolamine (1.0 μm), although responsive to naloxone.9 Normorphine (1.0 μm) restored to resting level the tension of the clonidine withdrawal contracture; and clonidine (0.1 μm) restored to resting level the tension of the contracture to naloxone in ileal segments incubated with normorphine.10 These experiments indicate that incubation with clonidine induces, in the final cholinergic motor neurones of the myenteric plexus of the isolated ileum, a dependence the withdrawal from which is expressed as a contracture in response to α-adrenoceptor antagonists.11 Although opiate receptors are not involved in clonidine dependence nor α-adrenoceptors in opiate dependence, the findings that normorphine suppresses the clonidine withdrawal-contracture and that clonidine suppresses the contracture of opiate-dependent ileum to naloxone, suggest that the withdrawal effect studied in both clonidine and normorphine dependence in this preparation is mediated by release of acetylcholine from the final motor neurone. |
