The relationship between axonal spike shape and functional modality in cutaneous C-fibres in the pig and rat

Axonal spike shape was examined in identified cutaneous C-fibres dissected from the saphenous nerves of anaesthetized pigs and rats, and was found to vary with functional class. In the pig, the action potential duration for h eal nociceptor units (duration at half peak amplitude, 1.25 +/- 0.16 ms, me an +/- S.E.M., n = 32) was significantly longer than the duration for polym odal nociceptive units (0.88 +/- 0.11 ms, n = 32). Both classes of nocicept ive C-fibre had action potentials of longer duration than the low-threshold mechanoreceptor units (0.49 +/- 0.04 ms. n = 24) and the inexcitable C-fib res (0.56 +/- 0.06 ms, n = 19). Undershoot durations were also longer in no ciceptive than non-nociceptive C-fibres. In contrast, spike amplitudes were similar in all classes of C-afferent. In the rat, as in the pig, the polym odal nociceptor units had action potentials of longer duration (0.75 +/- 0. 05 ms, n = 73) than the mechanoreceptor units (0.60 +/- 0.01 ms, n = 23). C -fibres identified as spontaneously active sympathetic efferent units had w ider action potentials (main initial peak: 1.01 +/- 0.12 ms, n = 22; unders hoot: 4.1 +/- 1.23 ms, n = 20) than the afferent C-fibres (main peak: 0.69 +/- 0.03 ms, n = 130; undershoot: 1.4 +/- 0.09 ms, n = 111). All rat C-fibr e types had action potentials with main initial peaks of a similar height. However, cold thermoreceptor units had spikes with significantly smaller un dershoots compared to nociceptive or inexcitable C-fibres. It is concluded that there are clear differences in axonal spike shape betw een the different functional classes of C-fibre and, in particular, that no ciceptive C-afferents tend to have axonal action potentials of longer durat ion than non-nociceptive afferents. The ion channels responsible for the lo nger duration action potentials may provide a target for the development of highly selective analgesic drugs. (C) 1999 IBRO. Published by Elsevier Sci ence Ltd.