Characterization of cutaneous primary afferent fibers excited by acetic acid in a model of nociception in frogs

Acetic acid applied to the hind limb of a frog evokes nocifensive behaviors, including a vigorous wiping of the exposed skin, referred to as the wiping response. The aim of this study was to examine the responses of cutaneous primary afferent fibers in frogs to acetic acid (pH 2.84-1.42) applied topically to the skin. Conventional electrophysiological methods were used to record neuronal activity from single identified primary afferent fibers with cutaneous receptive fields on the hind limb. Fibers were classified according to their conduction velocities and responses evoked by mechanical and thermal (heat and cold) stimuli. One hundred and twenty-two mechanosensitive afferent fibers were studied (44 Aβ, 60 Aδ, and 18 C fibers). Thirty-nine percent of all fibers were excited by acetic acid, but a greater percentage of Aδ (52%) and C fibers (44%) were excited than Aβ fibers (20%). Evoked responses of fibers increased with increasingly more acidic pH until the greatest responses were evoked by acetic acid at pH 2.59-2.41. Application of acetic acid at pHs <2.41 evoked less excitation, suggesting that fibers became desensitized. Similar percentages of nociceptors and low-threshold mechanoreceptors were excited by acetic acid. Thus primary afferent fibers were excited by acetic acid at pHs that have been shown to evoke the wiping response in our previous study. The results of the present study suggest that the model of acetic acid-induced nociception in frogs may be useful for studying the mechanisms by which tissue acidosis produces pain.