The development of tolerance to intrathecal morphine was studied in rats chronically implanted with intrathecal catheters connected to osmotic minipumps. Measures of cutaneous nociception were the hot plate (HP) and tail flick (TF) tests. Measures of visceral nociception were visceromotor (VM) responses to ramped colorectal distension (CRD) and cardiovascular (CV) responses to phasic colorectal distension. Tolerance to a continuous infusion of 6 or 20 nmol/h of morphine sulfate developed over 6 days. A significant reduction in the dose-dependent effects of intrathecal morphine in the TF and HP tests and VM and CV responses to CRD occurred in rats receiving continuous infusions of morphine. The development of tolerance to intrathecal morphine was similar in both cutaneous and visceral models.
- Morphine tolerance
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