Differential antinociceptive effect of transcutaneous electrical stimulation on pain behavior sensitive or insensitive to phentolamine in neuropathicrats

The effects of transcutaneous electrical stimulation and systemic injection of phentolamine, a non-specific alpha-adrenergic antagonist, on the behavi oral signs of mechanical allodynia and cold hyperalgesia in rats with nerve injury were investigated. Mechanical allodynia and cold hyperalgesia were evaluated by measuring the paw withdrawal frequency (PWF) resulting from re petitive application of a von Frey hair and the paw lift duration (PLD) at a cold temperature, respectively. After a unilateral nerve injury, both PWF and PLD increased in the injured hind paw. Application of low-frequency, h igh-intensity transcutaneous electrical stimulation (LFHI-TES) to the injur ed hind paw depressed the injury-induced increased PWF, whereas it had no e ffect on the injury-induced increased PLD. Naloxone reversed the LFHI-TES p roduced depression of PWF. Intraperitoneal administration of phentolamine d epressed the injury-induced increased PLD without affecting the injury-indu ced increased PWF. Our results suggest that LFHI-TES, which activates the e ndogenous opioid systems, produces an antinociceptive effect that appears t o be related to whether or not the pain is mediated by sympathetic activity . (C) 2001 Published by Elsevier Science Ireland Ltd.