Fig. 3. Morphine tolerance induced by intraperitoneal morphine injection. A indicates HWL in response to noxious thermal stimulation; B represents HWLs to noxious mechanical stimulation. Rats have received intraperitoneal morphine injection for 7 days, and the antinociceptive effects are evaluate 30 minutes after the first morphine administration on day 1 and 7. The data of the experiment are expressed as mean ± standard error of the mean. Two-tailed Student’s t-test was employed to assess the statistical difference among groups. Differences with ***P < 0.001 are deemed statistically significant. HWL: hindpaw withdrawal latency.
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