Vasomotor response to cold stimulation in human capsaicin-induced hyperalgesic area

Cooling the skin induces sympathetically driven vasoconstriction, with some vasoparalytic dilatation at the lowest temperatures. Neurogenic inflammation, on the other hand, entails vasodilatation. In this study we investigated the balance between vasoconstriction and vasodilatation in an area of experimentally induced secondary hyperalgesia (2° HA), in response to low-temperature stimulations. Fourteen healthy volunteers were exposed to three 30-s long cold stimuli (20, 10, and 0°C) applied, at three adjacent sites, before (baseline) and 8 min after intradermal injection of 50 g capsaicin to the volar forearm. The cold stimuli were applied distally to the injection site within the 2° HA. Blood flux (BF) and skin temperatures were measured at four different regions (proximally, and distally to the capsaicin injection and at the 0, 10, and 20°C thermode sites) all within the 2° HA. The vascular measurements were conducted five times. Results showed a marked increase in BF after baseline cold stimulation (P<0.001) at the 0°C compared with the three other sites. In addition, vasodilatory effect (elevated BF) was found following the capsaicin injection compared with baseline for all regions (P<0.001): the non-cooled area was dilated by 450±5.1%; The vasoconstrictive effect for the 10 and 20°C did not overcome the capsaicin vasodilatation, but did reduce it, with dilatation of 364±7.0% and 329±7.3%, respectively. For 0°C, a dilatation of 407±6.5% was seen. It is concluded that in this experimental model, and potentially in the equivalent clinical syndromes, vasodilatation induced by the inflammation is only slightly reduced by cold stimulation such that it is still dominant, despite some cold-induced vasoconstriction.