Excitation of cutaneous afferent nerve endings in vitro by a combination of inflammatory mediators and conditioning effect of substance P

Abstract

A broad mixture of inflammatory mediators ("inflammatory soup") was used to investigate the responsiveness of primary afferents from rat hairy skin in an in vitro skin-saphenous nerve preparation. In addition, a conditioning effect of the tachykinin substance P on chemosensitivity of nociceptors was examined. Inflammatory soup (IS) was made up in synthetic interstitial fluid from bradykinin, serotonin, histamin and prostaglandin E2 (all 10(-5) M). In addition, the potassium and the hydrogen ion concentration (7 mM, pH 7.0) and the temperature (39.5 degrees C) were elevated. The latter agents, in a control solution, did not excite nociceptors (n = 5). IS was repeatedly superfused over the receptive fields for 5 min at 10 min intervals; substance P (SP 10(-6) and 10(-5) M) was applied during the last 5 min of the interval and during the subsequent IS stimulation. IS excited more than 80% of the mechano-heat sensitive ("polymodal") afferents with slowly conducting nerve fibres (n = 72), but none of the low-threshold mechanoreceptive slow and fast conducting units (n = 17). Slow conducting afferents with high mechanical threshold (n = 35) were weakly, and less frequently (< 20%), driven by IS. A majority, but not all, of the responsive units showed tachyphylaxis upon repeated IS application. None, however, lost its responsiveness completely. Conditioning heat stimulation (32-46.5 degrees C in 20 s) did not enhance the subsequent IS response, which may indicate that sensitizing substances normally released by a noxious heat stimulus were already contained in IS. No sensitization to mechanical (von Frey) or heat stimulation could be established in the period after the IS response had subsided and after the washout was completed, respectively. A short-lived sensitization may have been overlooked under these temporal restrictions. Conditioning SP in 10(-5) M but not in 10(-6) M concentration significantly increased the IS response of polymodal C fibres, by 58% on average (n = 14). SP did not excite the units. Comparing with previous data, we conclude that there is a significant synergism between inflammatory mediators, acting to induce more intense and more sustained discharge via many nociceptors than single mediators alone could achieve. Conditioning substance P can further enhance this algogenic action. Mechanisms of interaction and relative contributions of single substances remain to be elucidated.