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. 2017 Aug 1;158(11):2222–2232. doi: 10.1097/j.pain.0000000000001027

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Copyright © 2017 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of The International Association for the Study of Pain.

This is an open access article distributed under the Creative Commons Attribution License 4.0 (CCBY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Figure 6. — Effect of intraperiaqueductal grey (PAG) and rostroventral medulla (RVM) microinjection of AM251 (CB1 receptor inverse agonist, GPR55 receptor agonist, 2.77 and 1.35 μg/animal) and lysophosphatidylinositol (LPI) (endogenous GPR55 agonist, 12 μg/animal) on withdrawal reflexes to mechanical von Frey hair stimulation in P10, P21, and P40 rats. (A) Intra-PAG AM251 and LPI only decreased spinal reflex excitability in P10 and P21 rats (P < 0.0001, 2-way analysis of variance [ANOVA]). There are significant age-related differences in AM251 and LPI responses between P40 and younger animals (P < 0.0001, 2-way ANOVA). (B) Intra-PAG AM251 and LPI did not change the mechanical thresholds in either P10 or P40 rats, but significantly increased it in P21 animals. There are significantly age-related differences in AM251 and LPI-mediated changes in mechanical withdrawal thresholds (P < 0.001, 2-way ANOVA). (C) Intra-RVM AM251 did not have an effect in P40 rats, but significantly reduced spinal reflex excitabilities in P10 and P21 animals. Lysophosphatidylinositol increased spinal reflex excitability in P40 rats, but decreased it in P10 and P21 animals. These age-related differences were significant (P < 0.0001, 2-way ANOVA). (D) Intra-RVM AM251 and LPI significantly increased mechanical thresholds in P10 and P21 rats, but had no effect in P40 animals. This age-related difference was significant (P < 0.0001, 2-way ANOVA). (E) Intra-PAG AM251 (2.77 and 1.35 μg) did not have an effect in P40, but significantly inhibited spinal reflex excitability in P21 animals (P < 0.0001, 1-way ANOVA). Coadministration of AM251 (1.35 μg) with the GPR55 receptor–specific antagonist ML193 (1 μg) reversed AM251-mediated inhibition of spinal reflex excitability in P21 animals. **, ***, and **** = P < 0.01, P < 0.001, and P < 0.0001, respectively, between drug comparisons, 2-way ANOVA and 1-way ANOVA with Bonferroni multiple comparisons. #, ### and #### = P < 0.05, P < 0.001 and P < 0.0001, respectively, between age comparison, 2-way ANOVA and 1-way ANOVA with Bonferroni multiple comparisons.