Figure 1.

Daily changes in hot plate response latency in rats treated with continuous intrathecal (IT) infusion of morphine (M) (20 nmol h⁻¹) or saline (S) and daily intraperitoneal (i.p.) injections of naloxone (N) (0.6 mg kg⁻¹) or saline (S). (IT-M+IP-N, rats with continuous IT infusion of morphine for 3 days and daily i.p. injection of naloxone; IT-M+IP-S, rats with continuous IT infusion of morphine for 3 days and daily i.p. injection of saline; IT-S+IP-N, rats with continuous IT infusion of saline and daily i.p. injection of naloxone; IT-S+IP-S, rats with continuous IT infusion of saline and daily i.p. injection of saline). Each test was performed prior to the daily injection of naloxone or saline. Each line represents the mean±s.e.mean of six rats. Both IT-M+IP-N and IT-M+IP-S, the withdrawal latency on day 1 was significantly increased from the baseline and then declined. The development of tolerance was more enhanced in IT-M+IP-N compared with IT-M+IP-S. One-way repeated measures ANOVA (P<0.05). Vertical arrows indicate differences of P<0.05.