Fig. 6.  Drug effects on paw withdrawal thresholds (PWTs) in spinal nerve-ligated (SNL) rats. Dose-effect curves were determined for morphine (A ), heroin (B ), and cocaine (C ) on PWT in SNL rats at the indicated times after an intraperitoneal injection (n = 6). *Significantly different from saline treatment (P ≤ 0.05).