Elsevier

Neuroscience

Volume 90, Issue 2, 22 February 1999, Pages 509-518
Neuroscience

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

https://doi.org/10.1016/S0306-4522(98)00454-0Get rights and content

Abstract

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 heat nociceptor units (duration at half peak amplitude, 1.25±0.16 ms, mean±S.E.M., n=32) was significantly longer than the duration for polymodal nociceptive units (0.88±0.11 ms, n=32). Both classes of nociceptive 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-fibres (0.56±0.06 ms, n=19). Undershoot durations were also longer in nociceptive 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 polymodal 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 wider action potentials (main initial peak: 1.01±0.12 ms, n=22; undershoot: 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-fibre types had action potentials with main initial peaks of a similar height. However, cold thermoreceptor units had spikes with significantly smaller undershoots compared to nociceptive or inexcitable C-fibres.

It is concluded that there are clear differences in axonal spike shape between the different functional classes of C-fibre and, in particular, that nociceptive C-afferents tend to have axonal action potentials of longer duration than non-nociceptive afferents. However, there is too much overlap in the distributions of spike durations of different functional classes for this measure to be useful on its own for identifying the class of individual C-fibres. The ion channels responsible for the longer duration action potentials may provide a target for the development of highly selective analgesic drugs.

Section snippets

Anaesthesia and surgery

Pigs (Deutsches Landschwein, n=40; weights 12–35 kg) were premedicated [azaperone (Stresnil, Janssen, Belgium), 2 mg/kg, i.m.] and then anaesthetized with halothane (1.5–2%) in 60% N2O/40% O2. The electrocardiogram, rectal temperature and expired CO2 were monitored continuously. The room temperature was kept between 24 and 26°C, the pig was covered with paper drapes and a heating pad was switched on if necessary to prevent any fall in temperature. Under these conditions, rectal temperature was

Results

Recordings of axonal spike shape have been made from C-fibres dissected from the pig and rat saphenous nerve. The 109 C-fibres studied in the pig comprised 32 heat nociceptor units, 32 polymodal nociceptor units, 24 low-threshold mechanoreceptor units and 21 inexcitable C-fibres (see Table 1). The 180 C-fibres studied in the rat comprised 70 polymodal nociceptor units, 14 mechanical nociceptor units, three heat nociceptor units, 23 mechanoreceptor units, 20 cold thermoreceptor units, 28

Variability of spike shapes

Recording extracellularly from fine filaments is not the ideal way to get information about action potential shape. Many factors related to the recording conditions will affect the amplitude and time-course of the action potential recorded in this way. Comparing spikes recorded from the same filament at the same time reduced the variability considerably, but only small numbers of such recordings were achieved, and only for the more numerous classes. A major variable will be the extent to which

Conclusions

This study has shown that, in two different species, axonal spikes of nociceptive C-fibres are of longer duration than those of C-mechanoreceptors. In the pig, where two major sub-classes of C-fibre nociceptor are found, the heat nociceptors have wider spikes than the polymodal nociceptors. In the rat, sympathetic postganglionic axons have spikes of longer duration than afferent axons. None of the differences in spike duration can be explained by differences in conduction velocity. The

Acknowledgements

M.D.G. was funded by a Bayliss Starling studentship from University College London. Funding for B.L. to visit Bad Nauheim was provided by the Max-Planck-Gesellschaft.

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