Low intensity intra-epidermal electrical stimulation can activate Aδ-nociceptors selectively

In the past 30 years, the study of nociception has relied mostly on thermal stimulation to activate nociceptors selectively. However, thermal stimulation suffers from some important limitations. For this reason, investigators have proposed intra-epidermal electrical stimulation (IES) as an alternative method to activate nociceptors selectively. This method relies on the fact that nociceptors are located mainly in the epidermis, while non-nociceptive fibres terminate more deeply in the dermis. Therefore, provided that the difference in receptor depth is sufficient, electric currents spatially restricted to the epidermal layers might activate nociceptors selectively. Here, we examined whether or not IES provides a fully selective nociceptive input. In a first experiment, we used capsaicin to induce a selective denervation of capsaicin-sensitive nociceptors, and thereby test whether the responses to IES are mediated by this population of afferent fibres. We found that capsaicin abolishes both the behavioural and the electrophysiological responses to IES applied at twice the perceptual threshold. In a second experiment, we applied a nerve pressure block to the superficial radial nerve to induce a temporally dissociated impairment of Aβ-, Aδ- and C-fibre afferents, and thereby determine the fibre populations contributing to the responses elicited by IES. We found that the time course of the blockade of the responses to IES follows closely the time course of the blockade of Aδ-fibres, but not of Aβ-fibres. Taken together, our results provide converging evidence that Aδ-nociceptors can be activated selectively using IES, provided that low intensities of stimulation are used.