Sensitization of nociceptive cutaneous nerve fibers from the rat's tail by noxious mechanical stimulation

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Abstract
This single fiber study on rat tail nerve afferents attempts to establish a peripheral neural correlate for the hyperalgesia to mechanical stimulation which follows injury to the skin. Mechano-heat sensitive C fibers (MH-C or “polymodal” nociceptors) and high-threshold mechanoreceptive A delta fibers (HTM-A delta) were examined with a series of constant noxious pressure stimulations (4-6-8-4 N on 25 mm2, 120 s each, 5 min intervals). These injurious stimuli were either directed to the most sensitive spot of the receptive fields (central stimulation) or closely outside their borders (1–5 mm). With this protocol no clear sensitization was seen in MH-C fibers apart from a stronger dynamic response to central stimulation in some of them. In contrast, most HTM-A delta units, irrespective of the site of noxious stimulation, developed spontaneous activity, lowering of their von Frey thresholds and expansion of their receptive fields. All HTM-A delta units responded to outside stimulation: upon the first stimulus (4 N) there was a delayed discharge of continuously increasing frequency (“recruited response”), but the onset of the last stimulation (4 N repeated) evoked vigorous dynamic responses in many fibers. The recruitment of HTM-A delta nociceptor activity may contribute to post-injury hyperalgesia to mechanical stimulation and it may counteract adaptation of the single afferent fiber during prolonged noxious influence.