Electrophysiological differences between nociceptive and non-nociceptive dorsal root ganglion neurones in the rat in vivo

Intracellular recordings were made from 1022 somatic lumbar dorsal root ganglion (DRG) neurones in anaesthetized adult rats, classified from dorsal root conduction velocities (CVs) as C, A delta or A alpha/beta, and according to their responses to mechanical and thermal stimuli as nociceptive (including high-threshold mechanoreceptive (HTM) units), and non-nociceptive (including low-threshold mechanoreceptive (LTM) and cooling units). Of these, 463 met electro-physiological criteria for analysis of action potentials (APs) evoked by dorsal root stimulation. These included 47 C-,71 A delta and 102 A alpha/beta-nociceptive, 10 C-, 8 A delta - and 178 A alpha/beta-LTM, 18 C- and 19 A delta unresponsive, and 4 C-cooling units. Medians of AP and afterhyperpolarization (AHP) durations and AP overshoots were significantly greater for nociceptive than LTM units in all CV groups. AP overshoots and AHP durations were similar in nociceptors; of all CV groups whereas AP durations were greater in slowly conducting, especially C-fibre, nociceptors. C-cooling units had faster CVs, smaller AP overshoots and shorter AP durations than C-HTM units. A subgroup of Ac alpha/beta-HTM, moderate pressure units, had faster CVs and AP kinetics than other A alpha/beta-HTM units. Of the A alpha/beta-LTM units, muscle spindle afferents had the fastest CV and AP kinetics, while rapidly adapting cutaneous units had the slowest AP kinetics. AP variables in unresponsive and nociceptive units were similar in both C- and A delta-fibre CV groups. The ability of fibres to follow rapid stimulus trains (fibre maximum following frequency) was correlated with CV but not sensory modality.These findings indicate both the usefulness and limitations of using electrophysiological criteria for identifying neurones acutely in vitro as nociceptive.