Characteristics of the increased response to electrical stimulation of ileum preparations from morphine pretreated guinea pigs.

The relationship between electrical stimulus voltage or pulse duration and the tension generated in the resulting contractions has been studied in control and morphine tolerant guinea pig ileum preparations. A greater maximal tension was observed in the morphine tolerant preparations. Since the response of the preparations to exogenous applied acetylcholine was unchanged, the increase in tension probably reflected an increase in the amount of acetylcholine released by each stimulus. No change in threshold voltage for stimulation was observed. A neuronally mediated, opiate insensitive, component of the response to electrical stimulation has been demonstrated. This component was unchanged in morphine tolerant preparations. Naloxone did not affect the relationship between stimulus pulse duration and response tension in control or morphine tolerant preparations. These results provide further evidence that morphine tolerance is associated with general changes in the properties of opiate sensitive neurons which can be demonstrated in the absence of opiate drugs.