Fig. 1. Mean ± SEM altered behavioral activity and response profile of dorsal horn wide dynamic range spinal neurons after sustained morphine exposure in rats. After 5–8 days of chronic morphine (n = 20) or saline (n = 16) treatment, hind paw withdrawal frequencies to von Frey filaments of increasing force were recorded (A ). After 7–10 days, evoked neuronal response to electrical (B ), mechanical (C ), and thermal (D ) stimuli were recorded in naive animals (n = 27) as well as those treated with saline (n = 12) and morphine (n = 17). Sustained morphine exposure increased the excitability of spinal neurons to peripherally applied natural stimuli. Based on latency measurements, neuronal responses were subdivided into Aβ-, Aδ-, and C-fibers, or postdischarge. *P < 0.05 or **P < 0.01 or ***P < 0.001 versus saline group baseline responses.