Modality-specific hyper-responsivity of regenerated cat cutaneous nociceptors

1. Experiments were performed on anaesthetized cats to investigate the rece ptive properties of regenerated cutaneous tibial nerve nociceptors, and to obtain evidence for coupling between them and other afferent fibres as bein g possible peripheral mechanisms involved in neuropathic pain. These proper ties were studied 6-7 months after nerve section and repair. 2. Recordings were made from 25 regenerated nociceptors; 14 were A fibres a nd the remainder were C fibres. Their receptive field sizes and conduction velocities were similar to controls. There was no significant difference be tween their mechanical thresholds and those of a control population of noci ceptors. 3. Regenerated nociceptors were significantly more responsive to suprathres hold mechanical stimuli than were uninjured control fibres. This increase i n mechanical sensitivity occurred in both A and C fibres, although A fibres showed a greater increase in mechano-sensitivity than C fibres. Over half of the regenerated nociceptors (13/25) showed after-discharge to mechanical stimuli which was never seen in controls; the mean firing rate during this period of after-discharge was significantly related to both stimulus inten sity and stimulus area. 4. There was no significant difference between the heat encoding properties of regenerated nociceptors and control nociceptors. Cold sensitivity was s imilarly unchanged. Thus, abnormal peripheral sprouting was unlikely to acc ount for the increased mechanical sensitivity of the regenerated fibres.;No ne of the regenerated nociceptors were found to be coupled to other fibres. 5. These results suggest that the clinical observation of mechanical hypera lgesia in patients after nerve injury may have a peripheral basis. Based on this model, other signs of neuropathic pain (i.e. tactile or thermal allod ynia) are more likely to be due to altered central processing.