Dorsal root reflexes and cutaneous neurogenic inflammation after intradermal injection of capsaicin in rats

Dorsal root reflexes and cutaneous neurogenic inflammation after intraderma l injection of capsaicin in rats. J. Neurophysiol. 82: 2602-2611, 1999, The role of dorsal root reflexes (DRRs) in acute cutaneous neurogenic inflamma tion induced by intradermal injection of capsaicin (CAP) was examined in an esthetized rats. Changes in cutaneous blood flow (flare) on the planter sur face of the foot were measured using a laser Doppler flowmeter, and neuroge nic edema was examined by measurements of paw thickness. To implicate DRRs in neurogenic inflammation after CAP injection, the ipsilateral sciatic and femoral nerves were sectioned, dorsal rhizotomies were performed at L3--S- 1, and antagonists of GABA or excitatory amino acid receptors were administ ered intrathecally. Intradermal injection of CAP evoked a flare response th at was largest at 15-20 mm from the injection site and that spread >30 mm. Acute transection of the sciatic and femoral nerves or dorsal rhizotomies n early completely abolished the blood flow changes 15-20 mm from the CAP inj ection site, although there was only a minimal effect on blood how near the injection site. These procedures also significantly reduced neurogenic ede ma. Intrathecal bicuculline, 6-cyano-7-nitroquinoxaline-2,3-dione, (CNQX) o r D(-)-2-amino-7-phosphonoheptanoic acid (AP7), but not phaclofen, also red uced dramatically the increases in blood flow 15-20 mm from the CAP injecti on site, but had only a minimal effect on blood how near the injection site . Neurogenic edema was reduced by the same agents that reduced blood flow. Multiunit DRRs recorded from the central stumps of cut dorsal rootlets in t he lumbar spinal cord were enhanced after CAP injection. This enhanced DRR activity could be reduced significantly by posttreatment of the spinal cord with bicuculline, CNQX or AW, but not phaclofen. It is concluded that peri pheral cutaneous inflammation induced by intradermal injection of CAP invol ves central nervous mechanisms. DRRs play a major role in the development o f neurogenic cutaneous inflammation, although a direct action of CAP on per ipheral nerve terminals or the generation of axon reflexes also may contrib ute to changes in the skin near the injection site.