5°C or 49°C), then quantified latency to flick the tail. At both temperatures, check details there was a significant (∼2-fold) increase in the latency to
flick in DTX-treated males and females (Table 1). Next, we placed the mice on a hot plate heated to 52°C and measured the latency to flick, lick, or shake a hindpaw. DTX-treated mice of both sexes exhibited over a 2-fold increase in withdrawal latency (Table 1). Finally, we pharmacologically activated the thermosensor TRPV1 by injecting 0.1 μg/μl capsaicin into the left hindpaw. We found that the DTX-treated male and female mice showed a 2-fold reduction in the time spent licking the capsaicin-injected hindpaw. Collectively, these experiments revealed that CGRPα DRG neurons were required to sense and behaviorally respond to noxious heat and capsaicin. Heat and mechanical hypersensitivity are two common symptoms of inflammatory pain and neuropathic pain (Basbaum et al., 2009). To determine whether CGRPα DRG neuron ablation impaired heat and mechanical hypersensitivity, we studied saline- and DTX-treated CGRPα-DTR+/− mice in the complete Freund’s adjuvant (CFA) model of inflammatory pain (Figures 4A–4F) and in the spared nerve injury (SNI) model
of neuropathic pain (Figures 4G and 4H). We monitored heat and mechanical sensitivity before, during, and after saline/DTX treatment. We also monitored plasma extravasation in the noninflamed (contralateral) and buy Small molecule library CFA-inflamed hindpaw with Evans Blue dye. We found that plasma extravasation was increased in both groups of mice after inflammation of the hindpaw; however, plasma extravasation was significantly lower in the inflamed hindpaw of DTX-treated male and female mice (when compared to the inflamed paw of saline-treated mice; Figures 4A and 4B). This reduction supports a role for peptidergic, because CGRP+ afferents in neurogenic inflammation (Basbaum et al., 2009). In both chronic pain models, heat withdrawal latencies increased to the cutoff time (20 s) after the second DTX injection and remained at this level for at least 2 weeks (Figures 4C, 4D, and 4G). Moreover, DTX-treated animals showed
no sign of heat hyperalgesia after inflammation (CFA) or nerve injury (SNI). In contrast, mechanical sensitivity and hypersensitivity were not impaired in DTX-treated animals in either chronic pain model (Figures 4E, 4F, and 4H). Likewise, noxious (tail clip) and innocuous (cotton swab) mechanical sensitivity was not impaired in DTX-treated animals (Table 1). Taken together, these behavioral experiments provide direct evidence that CGRPα DRG neurons are required for noxious thermosensation but are not required for noxious or innocuous mechanosensation in vivo. Capsaicin-responsive DRG neurons respond to the pruritogens histamine and chloroquine (Imamachi et al., 2009; Liu et al., 2009; Schmelz et al., 2003; Sikand et al., 2011).