Sex differences in systemic morphine analgesic tolerance following intrathecal morphine injections.

Morphine analgesic potency following systemic administration was assessed in male and female mice undergoing prior and repeated intrathecal morphine injections. Although morphine ED50 values were significantly increased in both sexes relative to their respective saline-injected controls, the magnitude of tolerance was greater in females. Intrathecal injection alone had no effect on morphine analgesia.

Genetic variation in morphine analgesic tolerance: a survey of 11 inbred mouse strains.

The present study assessed the analgesic potency of morphine in 11 inbred mouse strains before and after chronic morphine treatment. Using the 49 degrees C tail-withdrawal test, significant strain differences in morphine AD(50) estimates derived from cumulative dose-response curves were noted prior to tolerance induction on Day 1. AD(50) estimates were reassessed on Day 4, after three daily systemic morphine injections for 3 days using an escalating dose schedule (10, 20, and 40 mg/kg sc).