Role of the Neutrophil in Tnfα-induced Thermal Hyperalgesia

Thermal hyperalgesia is the heightened sensitivity to heat and occurs during inflammation. NGF-induced thermal hyperalgesia is neutrophil-dependent (Foster et al., 2003). This group previously found that the TRPV1 receptor mediates a bilateral thermal hyperalgesia in TNFα-induced inflammation (Russell et al., 2005), but it is not yet known how TNFα mediates the response. The aim of this study was to deplete circulating neutrophils in a mouse model of TNFα-induced inflammation to elucidate the role of the neutrophil in the thermal hyperalgesia. Female C57BL6 mice (20-30g) were pretreated with either rabbit anti-mouse neutrophil serum (ANS) or normal rabbit serum as control (Accurate Chemicals; diluted 1:10 in saline, 20mg/ml i.p.; Boyer et al., 2005). Total leukocyte counts and differential cell counts were carried out before treatment and 24h post treatment (Pitchford et al., 2003). At 24h mice were isoflurane (2%) anaesthetised and given intraplantar injections (i.pl) of TNFα (10pmol/50μl) and Tyrode (contralateral paw; 50μl). Thermal nociceptive thresholds were measured using the Hargreaves technique before injection and over a 4h time period after injection (Hargreaves et al., 1988). The mean of triplicate values was taken as paw withdrawal latency. Results are expressed as mean of group ± s.e.m and statistical analysis performed using Student’s t-test. ANS caused a >90% depletion of circulating neutrophils. Vehicle-treated mice that received TNFα into the ipsilateral paw exhibited significant hyperalgesia, that was also observed in the Tyrode-injected contralateral paw (Figure 1), evidence of a bilateral hyperalgesia, as previously described (Russell et al., 2005).