Baranowski, Richard; (1990) Capsaicin treatment of cutaneous sensory nerves: Functional and anatomical consequences. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Brief exposure of adult mammalian cutaneous nerve trunks to capsaicin is shown to have severe species-specific consequences for polymodal nociceptors. In rat and ferret, capsaicin initially produces a rapid, profound and irreversible reduction in the C-fibre compound action potential (C-CAP) in a dose-dependent manner. With the aid of a C-CAP model this effect is interpreted as a capsaicin-induced axonal conduction block in many, but not all, C-fibre polymodal nociceptors. Reductions in the amplitude of A-fibre compound action potentials (A-CAPs) are small and reversible as are the effects on C- and A-CAPs of guinea-pig and rabbit. The model reconstructs the rat saphenous nerve C-CAP through summation of its temporally dispersed single fibre action potentials (C-SFAPs). C-SFAP spike rise time, fall time and amplitude vary with conduction velocity (CV) raised to the powers -0.34, -0.62 and 1.17 respectively. The C-SFAP CV distribution is assumed to be unimodal, ranging from 0.4-1.08 m/s with a mean of 0.74 m/s. In rat nerves examined up to 1 year after capsaicin treatment, C-CAP amplitude remains depressed both proximal and distal to the application site and electron microscopic examination of these regions reveals a considerable loss of C-fibre axons. The proportion of polymodal nociceptors in single unit surveys of these nerves is lower than normal. This evidence is consistent with a neurotoxic action of capsaicin directed to a sub-population of polymodal nociceptors in the adult rat and possibly the ferret. Reports of expansion in the receptive field of spinal cord cells supplied by myelinated afferents following nerve injury or capsaicin treatment of neonatal rats are controversial, being attributed to central changes. A possible peripheral contribution has been investigated through accurate measurement of hair-follicle unit receptive fields in rats treated with capsaicin as neonates. There is a trend for receptive fields to be larger in these afferent C-fibre depleted rats, but the expansion is small compared to the reported central changes.

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