Measurement of CFA-Induced Hyperalgesia and Morphine-Induced Analgesia in Rats: Dorsal vs Plantar Mechanical Stimulation of the Hindpaw

Objective. To compare the sensitivity of stimulating the plantar and dorsal hindpaw surfaces in the detection of mechanical allodynia and morphine analgesia. Background. Several approaches are used to assess nociceptive reactivity to mechanical stimulation in animal models of pain. Although certain techniques seem to be favored for studying specific nociceptive conditions, the differences between techniques have not been directly compared and characterized. We chose to compare methods employing stimulation applied to the dorsum of the paw with stimulation of the plantar surface to demonstrate the utility of each approach in determining baseline nociceptive thresholds, changes in those thresholds after injury, and analgesic efficacy. Methods. Withdrawal thresholds from mechanical stimulation applied to the dorsal and plantar surface of the hindpaw were measured in rats treated with morphine after receiving subcutaneous injections of complete Freund's adjuvant (CFA) using Semmes-Weinstein (S-W) monofilaments and electro von Frey (EVF) stimulation. Results. In contrast to stimulation of the dorsal surface, plantar hindpaw stimulation seldom elicited an aversive withdrawal response. Differences in withdrawal response from baseline were only detectible within the first 5 hours post-CFA and only with EVF stimulation. No significant differences in stimulation techniques were observed after the initial 5-hour window. Effective dose 50 (ED50) for analgesic efficacy was consistently lower using dorsal stimulation. Conclusions. Stimulation of the plantar surface of the paw is superior for detecting small changes in paw sensitivity at very low stimulus intensities, whereas stimulation of the dorsal surface is superior for delineating baseline pain thresholds and for detecting robust analgesia. Clinical Relevance. Reliable and sensitive assessment of animal pain behaviors is critical to translational pain research. This study demonstrates the importance of using proper test protocols in animal studies and its implication in preclinical screening of potential analgesics.