Opioid dependence prevents the action of pertussis toxin in the guinea-pig myenteric plexus.

The longitudinal muscle-myenteric plexus preparation of the guinea-pig ileum has been employed for the study of the effect of pertussis toxin (IAP) on opioid dependence. Guinea-pigs were treated with IAP (120 micrograms/kg, i.p.) either prior to chronic administration of an opioid or after opioid dependence had been established. The isolated preparations were tested in vitro for dependence; that is, the naloxone-precipitated withdrawal contracture. Naloxone almost failed to evoke a sign of dependence in preparations treated with IAP prior to chronic exposure to an opioid. In contrast, IAP failed to affect the withdrawal contracture when applied to an animal after dependence has been established. It is concluded that the Ni-unit, the substrate for IAP, plays a critical function in the development of dependence. The continuous activation of the opioid receptor associated with the development of dependence may induce changes in Ni which in turn prevent the interaction of IAP with its substrate.