SKIN BLOOD-FLOW ABNORMALITIES IN A RAT MODEL OF NEUROPATHIC PAIN - RESULT OF DECREASED SYMPATHETIC VASOCONSTRICTOR OUTFLOW

Loose ligation of a sciatic nerve in rats provokes signs and symptoms like those observed in human conditions of neuropathic pain. Some of t hese have been associated with sympathetic dysfunction. Since the skin microcirculation in the rat is strongly influenced by sympathetic ton e, abnormalities in skin blood flow may be used as an indirect measure of sympathetic dysfunction. We measured, by means of laser Doppler fl owmetry, skin blood flow at the plantar surface of the rat hind paw be fore and after ipsilateral loose sciatic nerve ligation. We assessed b asal skin blood flow as well as the vasoconstrictor response which fol lows cooling of the rat abdomen. The effectiveness of this response ma y be used as a measure of sympathetic vasoconstrictor outflow. As comp ared to the values obtained before ligation (= 100%): (1) the vasocons trictor response was impaired (65%, P < 0.01) from day 1 onwards, wher eas (2) basal skin blood flow was increased (171%, P < 0.01) from day 3 until day 5, and decreased (51%, P < 0.0001) from day 7 until day 28 . At day 28, blockade of impulse propagation in the loosely ligated sc iatic nerve (by means of lidocaine) did not increase the lowered level of skin blood flow. These findings suggest that in the chronic constr uction injury model loose ligation of a sciatic nerve reduces sympathe tic vasoconstrictor outflow, which, in turn may induce supersensitivit y of skin microvessels to catecholamines.