Enhanced potency of intravenous, but not intrathecal, morphine and morphine-6-glucuronide after burn trauma.

We examined the analgesic effect of morphine (M) and its metabolite morphine-6-glucuronide (M6G) in a rat model of acute thermal trauma. M or M6G were given by intrathecal (IT) or intravenous (i.v.) routes after brief burn or sham burn delivered during inhalational anesthesia. In the sham group, M6G was significantly less potent than M when given i.v., yet tended to be more potent than M when given IT. For both drugs, thermal injury increased i.v. potency, yet decreased (for M) or displayed a trend to decrease (for M6G) It potency. The increased potency seen with i.v. but not IT opioid administration may reflect pharmacokinetic (e.g., diminished clearance) and/or pharmacodynamic responses (e.g., activation of peripheral opioid receptors) after thermal injury.