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. Author manuscript; available in PMC: 2020 Sep 1.
Published in final edited form as: J Pain. 2019 Mar 7;20(9):1040–1047. doi: 10.1016/j.jpain.2019.03.002

Figure 3. Lack of cross-tolerance between vlPAG morphine, and fentanyl.

Figure 3.

Rats were injected twice daily for two days with saline (0.4 μL), morphine (5 μg/0.4 μL), or fentanyl (3 μg/0.4 μL) into the vlPAG. (a) The antinociceptive potency of morphine did not differ between rats pretreated with fentanyl (N = 8) or saline (N = 8). (b) Likewise, the antinociceptive potency of fentanyl did not differ between rats pretreated with morphine (N = 7) or saline (N = 8).