P2 × 7-NLRP3-Caspase-1 Signaling Mediates Activity-induced Muscle Pain in Male but Not Female Mice

Brain-derived neurotrophic factor contributes to activity-induced muscle pain in male but not female mice

“…Fatiguing muscle contractions release ATP and decrease pH [52–54] which can activate purinergic and acid sensing ion channels (ASICs) resulting in hyperalgesia. We previously showed that these fatiguing metabolites activate P2X4 and P2X7 receptors on resident macrophages in the muscle and subsequently increase the release of IL-1β and increase pain [8–10], while blockade of ASIC3 or ASIC1a in muscle prevents activity-induced pain [8, 55]. We have also shown an increase in IL-1β in the gastrocnemius muscle after induction of the activity-induced pain model [10], while blockade of IL-1β prevents the hyperalgesia [9, 10].…”

“…We previously showed that these fatiguing metabolites activate P2X4 and P2X7 receptors on resident macrophages in the muscle and subsequently increase the release of IL-1β and increase pain [8–10], while blockade of ASIC3 or ASIC1a in muscle prevents activity-induced pain [8, 55]. We have also shown an increase in IL-1β in the gastrocnemius muscle after induction of the activity-induced pain model [10], while blockade of IL-1β prevents the hyperalgesia [9, 10]. Sensory neurons also respond to decreases in pH and ATP through activation of ASICs and P2X receptors on nociceptors [12].…”

“…The acidic saline model directly activates ASIC3 on muscle afferents to produce pain [62, 63]. Acidic pH 6.0 alone (a similar pH observed in the acid-saline model in muscle after pH 4 injection), but not pH 6.5, induces release of IL-1β from cultured peritoneal macrophages [9, 10, 63]. In the current study we did not show changes in BDNF expression in the acidic saline model suggesting that BDNF expression and release requires activation of multiple receptors.…”

“…To test whether BDNF mediated the development and maintenance of activity-induced muscle pain, selective inhibitors were given intrathecally (5μl volume) or intramuscularly (20μl volume) prior to or after (24hr or 1wk) induction of the activity-induced muscle pain model and were compared to vehicle injections ( Table 1 ). For the pre-treatment protocol, intrathecal injections were administered 30 min prior to fatiguing contractions and intramuscular injections were administered 5 min prior to fatiguing contractions [10, 37]. We used two inhibitors of BDNF-TrkB signaling, ANA-12 and TrkB-Fc, and compared them to vehicle [38, 39].…”

“…For measurement of MWT, power analysis determined the minimal sample size for withdrawal thresholds was 8 mice per group at 80% power and p<0.05, comparing means for two independent groups (mean 1, 98%; mean 2, 60%; SD1, 25; SD2, 25) [9, 10]. For qPCR and protein analysis, we used the effect size from preliminary results (mean 1, 3,000; mean 2, 300; SD1, 2,450; SD2, 10) to estimate a sample size of 8 per group.…”