Figure 9.
Role of the inhibitory G-protein αi subunit in type II priming. A, Rats received an intradermal injection of vehicle (control, black bars) or PTX (1 μg, white bars) on the dorsum of the hindpaw. Thirty minutes later, DAMGO (1 μg) was injected (hourly, ×4) in the same site. We observed significant mechanical hyperalgesia after the fourth injection of DAMGO, with no significant (NS) difference between groups (t(5) = 0.1075, p = 0.9186, control vs PTX groups, unpaired Student's t test), demonstrating that the αi subunit does not play a role in the hyperalgesia induced by repeated injection of DAMGO. Four days later, when the mechanical thresholds were not different from the prevehicle/PTX injection levels (t(5) = 0.3384; p = 0.7488 and t(5) = 0.5547; p = 0.6030, respectively, paired Student's t test), rats received intradermal injection of PGE2 (100 ng) in the same site and mechanical hyperalgesia was evaluated 30 min and 4 h later. No significant difference was observed in PGE2-induced mechanical hyperalgesia evaluated 30 min and 4 h after injection (F(1,20) = 0.17; p = 0.6859, when comparing both groups; NS, p > 0.05, two-way repeated-measures ANOVA followed by Bonferroni post hoc test), indicating that the αi subunit does not play a role in the prolongation of PGE2 hyperalgesia in type II priming; B, Rats received repeated (hourly, ×4) intradermal injections of the G-protein αi subunit peptide activator mastoparan (1 μg) on the dorsum of the hindpaw. The mechanical nociceptive threshold was evaluated before and 30 min after the first and fourth administrations. Unpaired Student's t test showed that the injection of mastoparan did not induce significant change in the mechanical nociceptive threshold (p = 1.0000) even after 4 hourly administrations compared with the control group. Four days later, PGE2 (100 ng) was injected at the same site and mechanical thresholds were evaluated after 30 min and 4 h. In both groups, we observed significant mechanical hyperalgesia induced by PGE2 30 min after injection (NS, p > 0.05) that was no longer present at the fourth hour (NS; F(1,20) = 2.14; p = 0.1747, when comparing both groups at the fourth hour after PGE2 injection; NS, p > 0.05), indicating that an activator of G-protein αi subunit does not induce priming. n = 6 paws per group.