title: Effect of delayed intrathecal administration of capsaicin on neuropathic pain induced by chronic constriction injury of the sciatic nerve in rats.
abstract: OBJECTIVE
  The current study was designed to examine the antinociceptive effect of intrathecally administered capsaicin, a transient receptor potential vanilloid 1 receptor agonist, in a rat model of neuropathic pain induced by unilateral sciatic nerve chronic constriction injury.
  
  
  METHODS
  Male adult Sprague Dawley rats were randomly assigned to six groups, and all rats underwent unilateral sciatic nerve chronic constriction injury. Two weeks after injury, five groups received intrathecal administration of either capsaicin in three different dosing regimens or equal volumes of vehicle. The other group received intrathecal capsaicin on the third day after nerve injury. The antinociceptive effect of capsaicin was assessed by measuring the capsaicin-induced change in thermal and mechanical response thresholds.
  
  
  RESULTS
  Capsaicin (150-300 μg/100-200 μL), when administered by fast infusion or chronic infusions at 8 μL/hour or 1 μL/hour, attenuated thermal hyperalgesia as indicated by significantly prolonging paw withdrawal latency to noxious thermal stimulation. The antinociceptive effect of capsaicin was more profound in the injured limb compared to that in the uninjured limb. When capsaicin was administered on the third day after nerve injury, it failed to attenuate thermal hyperalgesia. No significant effect on the mechanical response threshold was observed with intrathecally administered capsaicin.
  
  
  CONCLUSIONS
  Our data suggest that intrathecal capsaicin could significantly attenuate thermal hyperalgesia, depending on the time when the drug is given after nerve injury, and that the antinociceptive efficacy of intrathecal capsaicin positively correlates with the previously reported dynamic profile of spinal transient receptor potential vanilloid 1 activity after nerve injury.
authors.affiliation: Department of Anesthesiology, University of Texas Health Science Center at San Antonio, San Antonio, TX, USA.
authors.forename: Kun
authors.initials: K
authors.lastname: Zhang
authors.affiliation: Department of Anesthesiology, University of Texas Health Science Center at San Antonio, San Antonio, TX, USA.
authors.forename: Somayaji
authors.initials: S
authors.lastname: Ramamurthy
authors.affiliation: Department of Pathology, University of Texas Health Science Center at San Antonio, San Antonio, TX, USA.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Prihoda
authors.affiliation: Department of Anesthesiology, University of Texas Health Science Center at San Antonio, San Antonio, TX, USA.
authors.forename: Maxim S
authors.initials: MS
authors.lastname: Eckmann
keywords: TRPV1; hyperalgesia; paw withdrawal latency; paw withdrawal threshold