Capsaicin did not evoke pain from human hand vein segments but did so after injections into the paravascular tissue

Abstract

1. To see if pain from veins is mediated by C fibre endings, the C fibre stimulant capsaicin was applied intravenously, and, for comparison, paravenously and intracutaneously. 2. Capsaicin, dissolved in the fat emulsion Intralipid, was applied intravenously by continuous perfusion of vascularly isolated hand vein segments as well as by injections into occluded finger veins. Using the latter approach chemicals reach the paravascular space. 3. Pain intensities were recorded continuously with an electronically controlled visual analogue scale for deriving capsaicin concentration-pain intensity relations and the time course of pain (latencies, pain durations). 4. Capsaicin always evoked pain upon injection into skin and paravenous tissue (0.3-6.5 microM) and into occluded finger veins (3.3-33 microM), whereas it had no effect whatsoever when perfused through hand vein segments even at a concentration of 650 microM. 5. Pain intensity increased with concentration and usually reached the tolerance maximum at the fivefold threshold concentration, so that the concentration-pain intensity relations were congruent for the various routes of drug application. 6. The latencies and pain durations were independent of the capsaicin concentration, but were substantially longer with injections into occluded finger veins (latency 10-30 s, pain duration 60-120 s) than with intradermal or paravenous injections (2-9 s, 10-28 s). 7. These observations show for the first time a functional similarity between the nociceptive C fibre system of the skin and the paravascular tissues, and by inference, they dismiss the possibility that C fibre endings mediate pain in cutaneous veins.