Orofacial cold hyperalgesia due to infraorbital nerve constriction injury in rats: Reversal by endothelin receptor antagonists but not non-steroidal anti-inflammatory drugs

The susceptibility of changes in responsiveness to noxious cold stimulation of rats submitted to chronic constriction of the infraorbital nerve (CION) or carrageenan to drug inhibition was compared. Nocifensive responses were measured as total time rats engaged in bilateral facial grooming with both forepaws over the first 2 min following tetrafluoroethane spray application to the snout. Carrageenan (50 μg, s.c. into upper lip) caused short-lived ipsilateral cold hyperalgesia (peak at 3 h: vehicle 8.4 ± 1.3, carrageenan 21.2 ± 3.0 s) which was markedly suppressed by i.p. indomethacin (4 mg/kg), celecoxib (10 mg/kg) or s.c. dexamethasone (0.5 mg/kg), endothelin ET_A or ET_B receptor antagonists (BQ-123 and BQ-788, respectively; 10 nmol/lip). CION caused ipsilateral cold hyperalgesia between Days 2 and 12, which peaked on Days 4 (sham 15.3 ± 1.8, CION 32.4 ± 5.3 s) to 6. Established peak CION-induced cold hyperalgesia was unaffected by indomethacin and celecoxib, whereas dexamethasone, BQ-123, BQ-788, and i.v. injections of selective antagonists of ET_A (atrasentan, 3-10 mg/kg) or ET_B (A-192621, 5-20 mg/kg) receptors caused significant inhibitions lasting 1-2.5 h (peaks ∼65-90%). Bosentan (dual ET_A/ET_B receptor antagonist, 10 mg/kg, i.v.) abolished CION-induced cold hyperalgesia for up to 6 h. Thus, once established, CION-induced orofacial hyperalgesia to cold stimuli appears to lack an inflammatory component, but is alleviated by endothelin ET_A and/or ET_B receptor antagonists. If this CION injury model bears predictive value to trigeminal neuralgia (i.e., paroxysmal orofacial pain triggered by various stimuli), endothelin receptors might constitute new targets for treatment of this disorder.