Deficits in visceral pain and referred hyperalgesia in Nav1.8 (SNS/PN3)-null mice

Abstract

The tetrodotoxin-resistant sodium channel alpha subunit Nav1.8 is expressed exclusively in primary sensory neurons and is proposed to play an important role in sensitization of nociceptors. Here we compared visceral pain and referred hyperalgesia in Nav1.8-null mice and their wild-type littermates in five tests that differ in the degree to which behavior depends on spontaneous, ongoing firing in sensitized nociceptors. Nav1.8-null mice showed normal nociceptive behavior provoked by acute noxious stimulation of abdominal viscera (intracolonic saline or intraperitoneal acetylcholine). However, Nav1.8-null mutants showed weak pain and no referred hyperalgesia to intracolonic capsaicin, a model in which behavior is sustained by ongoing activity in nociceptors sensitized by the initial application. Nav1.8-null mice also showed blunted pain and hyperalgesia to intracolonic mustard oil, which sensitizes nociceptors but also provokes tissue damage. To distinguish between a possible role for Nav1.8 in ongoing activity per se and ongoing activity after sensitization in the absence of additional stimuli, we tried a visceral model of tonic noxious chemical stimulation, cyclophosphamide cystitis. Cyclophosphamide produces cystitis by gradual accumulation of toxic metabolites in the bladder. In this model, Nav1.8-null mice showed normal responses. There were no differences between null mutants and their normal littermates in tissue damage and inflammation evoked by any of the stimuli tested, suggesting that the behavioral differences are not secondary to impairment of inflammatory responses. We conclude that there is an essential role for Nav1.8 in mediating spontaneous activity in sensitized nociceptors.

Fig. 1.

Spontaneous visceral pain-related behavior in wild-type (+/+) and Nav1.8-null (−/−) mice. A, Mean ± SEM number of behaviors per 5 min observation period in the first 20 min after intracolonic instillation of 0.1% capsaicin (n = 6 per group) or isotonic saline (n = 10 per group). B, Mean ± SEM number of behaviors per 5 min observation period in the first 20 min after intracolonic instillation of 1% mustard oil (n = 6 per genotype). C, Time course of mean ± SEM behavioral scores assigned at 30 min intervals over the 4 hr observation period after intraperitoneal injection of 300 mg/kg cyclophosphamide (n = 6 per genotype). For details of scoring system, see Materials and Methods. ∗p < 0.05 and ∗∗p < 0.01 where a significant difference between +/+ and −/− mice was observed, respectively.

Fig. 2.

Referred visceral hyperalgesia in wild-type (+/+) and Nav1.8-null (−/−) mice. Referred visceral hyperalgesia was measured as responses to mechanical stimulation of the hindpaws with von Frey hairs of five intensities in wild-type and Nav1.8-null mice. Data are shown as mean percentage response frequency ± SEM before (baseline) and 20 min after intracolonic instillation of 0.1% capsaicin (A), 20 min after intracolonic instillation of 1% mustard oil (B), and 4 hr after 300 mg/kg intraperitoneal cyclophosphamide (C). ∗p < 0.05, indicates forces where a significant difference between +/+ and −/− mice was observed. ANOVA revealed a significant difference overall between genotypes 20 min after intracolonic capsaicin (p = 0.03) and mustard oil (p = 0.02) but not after cyclophosphamide.