Effect of chronic morphine with or without ULD naloxone enantiomers on mechanical withdrawal thresholds.

Control (C3H/HeOuJ) and TLR4 mutant (C3H/HeJ) mice were treated with vehicle (saline, SAL), escalating morphine (10–40 mg/kg, MS), MS and (−)naloxone (1 ng/kg, NLX), or MS and (+)naloxone (1 ng/kg) twice daily (i.p) for 7 days. Mechanical withdrawal thresholds were measured prior to the morning injection and are expressed as mean number of withdrawals to 10 repeated stimulations ± s.e.m.*significantly different compared to morphine-treated group of same genotype.#significantly different compared to baseline (Day 0) response. Responses were not different between genotypes for any treatment at any time point.