Figure 1
SS rats exhibit higher nociceptive sensitivity. Mechanical nociceptive thresholds of untreated SS (full symbols), SD (empty symbols) and BN (gray symbols) rats were determined on the dorsum of the hindpaw (a) and the gastrocnemius muscle (b) by the Randall-Selitto test, or in the plantar surface, over the course of 30 weeks, starting at the 6th week post birth, using von Frey filaments (c). In all tests, SS rats showed significantly lower nociceptive thresholds than SD and BN rats (panels a and b, respectively: F(2,35) = 164.7 and F(2,37) = 293.2, ****p < 0.0001 for both strains, one-way ANOVA followed by Bonferroni posttest; (c): F(2,29) = 171.7, ****p < 0.0001 for both strains, two-way repeated measures ANOVA followed by Bonferroni posttest). (d): SS, SD and BN rats received an i.d. injection of CARR (1%) on the dorsum of the hindpaw, and the nociceptive thresholds were evaluated 4 h later. While significant mechanical hyperalgesia (decrease in nociceptive threshold) was observed in the SD (t4 = 9.411, p = 0.0007, when the thresholds 4 h after CARR were compared to the baseline, paired Student’s t-test) and BN (t4 = 8.595, p = 0.0010) groups (F(2,14) = 110.0, ****p < 0.0001 when the change in the thresholds induced by CARR in SS rats was compared to those in SD and BN rats, two-way repeated measures ANOVA followed by Bonferroni posttest), no significant change was observed in the SS group (t6 = 4.054, p = 0.1067, NS). (e) shows the percentage change in nociceptive threshold produced by CARR injection (SD = 31.84%; BN = 33.82%; SS = 7.6%), indicating the smaller effect of inflammation on this strain and that its baseline nociceptive threshold is naturally in the hyperalgesic range. (f): Formalin (1%) was injected subcutaneously on the dorsum of the hindpaw of SS, SD and BN rats. Nociceptive reaction (paw flinches) to formalin was followed for 90 min. Significant difference in the number of flinches in the SS group was observed in the first 10 min after injection (1st phase), when compared to the other 2 strains (F(1,10) = 24.98, ***p = 0.0005 compared to SD; F(1,10) = 47.91, ****p < 0.0001 when compared to BN group, two-way repeated measures ANOVA followed by Bonferroni posttest). However, the difference in the number of flinches was less robust when the SS rats were compared to the other strains from 15 to 90 min (2nd phase), with no significance compared to BN (F(1,10) = 1.973, p = 0.1904), but different from the SD group (F(1,10) = 7.839, p = 0.0188). (a and b): SD, n = 12; BN, n = 10; SS, n = 16. (c): SD, n = 12; BN, n = 12; SS, n = 8. (d and e): SD, n = 5; BN, n = 5; SS, n = 7. (f): n = 6 per group.
