Fig. 1. ( Left ) Withdrawal threshold to von Frey filament application to animals before spinal nerve ligation (time before 0) and after injection, at time 0 of 0.25–5.0 μg intrathecal morphine. Morphine significantly increased withdrawal thresholds for all doses except 0.25 μg. Each symbol represents the mean ± SE of six to eight rats. ( Right ) Dose response for intrathecal morphine to alleviate hypersensitivity after spinal nerve ligation, calculated as the percent maximum possible effect (%MPE), which is the percent return to presurgical threshold. The line represents the sigmoidal fit of the data ( R = 0.97, P < 0.001).