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 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. The ion channels responsible for the longer duration action potentials may provide a target for the development of highly selective analgesic drugs.

The relationship between cutaneous C fibre type and antidromic vasodilatation in the rabbit and the rat.

1. Skin blood flow was monitored during antidromic stimulation of identified cutaneous C fibres in fine filaments dissected from the saphenous nerve of anaesthetized rabbits and rats. The techniques used to monitor skin blood flow were laser Doppler perfusion imaging and laser Doppler flowmetry. 2. In the rabbit filaments a total of thirty-three C fibres were tested for their ability to produce antidromic vasodilatation. The only C fibres found to have vasodilator actions were of the polymodal nociceptor afferent class, and fourteen (50%) of the twenty-eight polymodal nociceptor units tested were vasoactive. The afferent receptive fields of polymodal nociceptor afferents were mapped carefully using suprathreshold mechanical stimuli, and there was a good correlation between afferent receptive field area and area of vasodilatation. 3. In the rat, eleven of the fifty-four C fibres antidromically stimulated had vasodilator actions. All eleven vasoactive C fibres were nociceptive and comprised seven polymodal nociceptor units, two heat nociceptor units and two incompletely classified nociceptor units. The area of increased blood flow was always coincident with the afferent field of the stimulated unit. 4. In the rat the vasodilator units were not evenly distributed over the saphenous nerve receptive field. Nine of the eleven vasoactive C fibres had receptive fields located on the foot or the digits, and only two were on the ankle or lower leg. Overall, the population of nociceptive C fibres was evenly distributed over the saphenous nerve receptive field. 5. In both the rabbit and the rat, a subclass of polymodal nociceptor afferents form the majority of the vasoactive units and will make the main contribution to axon reflex flare and other neurogenic inflammatory responses involving vasodilatation. The vasoactive polymodal nociceptor units tend to have relatively low mechanical sensitivity, although they have typical heat thresholds. In the rat heat nociceptor units also have vasodilator actions. However, such heat nociceptor units form a minor functional class of afferent C fibre in the rat saphenous nerve, and are not found in the rabbit saphenous nerve. 6. The findings from this study in the rabbit and the rat are compared with the situation in pig skin. The close relationship between afferent receptive field area and spread of flare across species is noted, and the way these measures increase with body size is discussed.

Activity-dependent slowing of conduction velocity provides a method for identifying different functional classes of C-fibre in the rat saphenous nerve.

Repetitive firing of nerve fibres results in the slowing of their conduction velocity. The extent of conduction velocity slowing throughout a standard electrical stimulus (20 s, 20 Hz, 2 x electrical threshold) was examined in identified C-fibres dissected from the saphenous nerve of anaesthetized rats. The aim of this study was to establish whether the different functional classes of C-fibre could be identified on the basis of their activity-dependent slowing of conduction velocity. Following 20 s of stimulation at 20 Hz, nociceptive C-fibres showed a significantly greater slowing of conduction velocity (mean +/- S.E.; polymodal and heat nociceptors = 29.2% +/- 0.7, n = 53; mechanical nociceptors = 27.7% +/- 1.7, n =13) than cold thermoreceptive fibres (10.8% +/- 0.6, n = 10), mechanoreceptors (14.4% +/- 0.8, n = 17) and spontaneously active sympathetic efferent units (14.9% +/- 0.8, n = 24). The degree of conduction velocity slowing shown by a unit was not correlated with its resting conduction velocity. There was little overlap of the degree of conduction velocity slowing between the nociceptive and non-nociceptive fibres. Also, there was little overlap of conduction velocity slowing between the mechanoreceptors and the cold units, particularly after just 6 s of stimulation at 20 Hz. Units for which no receptive field to mechanical or thermal stimuli could be found showed a bimodal distribution of conduction velocity slowing. In the saphenous nerve, such inexcitable units will be of three main types--sympathetic efferent units, "sleeping" or "silent" nociceptors and non-cutaneous afferent fibres. Those inexcitable units slowing in conduction velocity by greater than 20% showed a similar distribution to the polymodal nociceptors and those inexcitable units slowing by less than 20% showed a similar distribution to the spontaneously active sympathetic units. Twenty-three of the 61 units without mechanical or thermal receptive fields were investigated using electrical skin stimulation and topical application of 5 or 10% mustard oil. Afferent fields could not be found for any of the nine units that slowed in conduction velocity by less than 20%. Afferent fields were detected for 11 of the remaining 14 insensitive units, which all showed a greater than 20% slowing from resting conduction velocity. Therefore, one can distinguish nociceptive and non-nociceptive afferent fibres simply by looking at the axonal property of activity-dependent slowing of conduction velocity. Moreover, it is possible to use this axonal property to separate the two classes of non-nociceptive afferent C-fibre (i.e. mechanoreceptors and cold thermoreceptors). In addition, one can also use this parameter to differentiate between the afferent and non-afferent populations of inexcitable C-fibres. The ability to identify a particular fibre type on the basis of an axonal property provides a useful tool for the functional classification of fibres in experiments where axons are separated from their terminals.

The actions of capsaicin applied topically to the skin of the rat on C-fibre afferents, antidromic vasodilatation and substance P levels.

1. Single applications of solutions of capsaicin were made to the intact skin of anaesthetized rats and the effects on cutaneous blood flow and the firing of C-nociceptor afferents determined. Blood flow was measured by laser-Doppler flowmetry. C-fibre activity was recorded from filaments dissected from the saphenous nerve. 2. Following the application of a capsaicin solution (concentration > or = 1 mM) to rat saphenous skin, low frequency firing occurred in C-polymodal nociceptors that sometimes continued for > 10 min. At the some time, large increases in skin blood flow occurred exceeding 300% in some instances. 3. After the initial excitation, some C-polymodal nociceptors lost their sensitivity to pressure whilst their sensitivity to heat was lost or enhanced depending on the vehicle used. 4. Sensitivity of C-polymodal nociceptors to heat recovered in < 1 day following a single application of 33 mM capsaicin. Thresholds to mechanical pressure, however, were still significantly elevated by 123% on day 1, but had recovered on day 2. 5. Vasodilatation in response to saphenous nerve stimulation ('antidromic vasodilatation') was significantly reduced by 35%, 2 days after a single application of 33 mM capsaicin, but was normal at 4 days. 6. Following a single application of 33 mM capsaicin, skin substance P levels fell to only half the normal value at day 1 and remained at this level throughout the 4 day period examined. 7. It is suggested that the ability of relatively low concentrations of capsaicin to desensitize C-fibre nociceptors may underlie the analgesic action of topical capsaicin in man.

Blood flow increases in the skin of the anaesthetized rat that follow antidromic sensory nerve stimulation and strong mechanical stimulation.

In anaesthetized rats, punctate pressure using forces greater than or equal to 20 mN caused small transient rises in skin blood flow that were similar in normally innervated and chronically denervated skin. A force of 11 mN, sufficient to excite most C-fibres of the polymodal nociceptor class, failed to cause vasodilatation. Following short periods of low frequency electrical stimulation of the saphenous nerve at C-fibre strength, larger increases in blood flow ('antidromic vasodilatation') were seen. Antidromic vasodilation was unaffected by high frequency stimulation of A alpha beta axons or by simultaneous innocuous mechanical stimulation. The failure of pressure at levels suprathreshold for C-fibre nociceptors to cause neurogenic vasodilatation may mean that antidromic vasodilation in rat skin is due to activity restricted to a mechanically insensitive sub-population of C-fibres.

The delay in onset of vasodilator flare in human skin at increasing distances from a localized noxious stimulus.

Flare was measured on the arm of human subjects at 8, 16, and 24 mm from localized areas of skin heating, using laser-Doppler flowmetry. Vasodilatation started after a delay that averaged 3.2 sec at 8 mm and increased significantly by 0.4 sec at 16 mm and by 1.1 sec at 24 mm. In contrast, there were no significant changes in onset delay associated with changes in the amplitude of the heat stimulus. Flare appears to spread more slowly than would be expected if rate of spread were determined only by conduction delays in unmyelinated nerve terminals. This finding is discussed in relation to models of flare that involve coupling between adjacent nerve terminals.