Inflammatory pain in the rabbit: A new, efficient method for measuring mechanical hyperalgesia in the hind paw

The discovery of novel analgesic compounds that target some receptors can be challenging due to species differences in ligand pharmacology. If a putative analgesic compound has markedly lower affinity for rodent versus other mammalian orthologs of a receptor, the evaluation of antinociceptive efficacy in non-rodent species becomes necessary. Here, we describe a new, efficient method for measuring inflammation-associated nociception in conscious rabbits. An electronic von Frey device is used, consisting of a rigid plastic tip connected to a force transducer in a hand-held probe. The plastic tip is applied to the plantar surface of a hind paw with increasing force until a withdrawal response is observed. The maximum force (g) tolerated by the rabbit (i.e., withdrawal threshold) is recorded. In young, conscious rabbits (500–700 g), baseline hind paw withdrawal thresholds typically fell within the 60–80 g range. Three hours after injection of the inflammatory agent carrageenan (3%, 200 μL, intra-plantar), withdrawal thresholds dropped by ∼30–40 g, indicating the presence of punctate mechanical hyperalgesia. The development of hyperalgesia was dose dependently prevented by the NSAID indomethacin (ED50 = 2.56 mg/kg, p.o.) or the bradykinin B2 receptor peptide antagonist HOE 140 (intra-paw administration). An established hyperalgesia was dose dependently reversed by morphine sulfate (ED50 = 0.096 mg/kg, s.c.) or the bradykinin B1 receptor peptide antagonist [des-Arg10,Leu9]-kallidin (ED50 = 0.45 mg/kg, s.c.). Rabbits treated with the novel B1 receptor small molecule antagonist compound A also showed dose-dependent reversal of hyperalgesia (ED50 = 20.19 mg/kg, s.c.) and analysis of plasma samples taken from these rabbits showed that, unlike other rabbit pain models, the current method permits the evaluation of pharmacokinetic–pharmacodynamic (PK–PD) relationships (compound A plasma EC50 = 402.6 nM). We conclude that the Electrovonfrey® method can be used in rabbits with inflammatory pain to generate reliable dose- and plasma concentration-effect curves for different classes of analgesics.