Comparison of opioid and alpha-2 adrenergic receptor location and density in the horse and dog using radioligand binding

Ketamine, a noncompetitive NMDA receptor antagonist, has been shown to provide analgesia in some species. To target the NMDA receptor specifically and to potentially minimize some untoward side-effects, ketamine had been used epidurally. The objective of this study was to determine the analgesic effect of epidurally administered ketamine in dogs with chemically induced synovitis.
Sixteen healthy dogs were used. Dogs were anesthetized with propofol (4 mg kg^?1 IV). Synovitis was induced by injecting 1 mL of sodium urate crystal solution (10 mg mL^?1) into the right stifle. Dogs were allowed to recover and the synovitis was allowed to develop for 12 hours. The dogs were then anesthetized again using propofol (4 mg kg^?1 IV). Lumbosacral epidural injections were performed with each dog receiving either 2 mg kg^?1 of ketamine (20 mg mL^?1) or an equal volume of placebo (sterile water containing not more than 0.1 mg mL^?1 benzethonium chloride). Analgesia was assessed at baseline and then at 12, 14, 16, 18, 20, and 24 hours after induction of synovitis. Ground reaction forces (peak vertical force and impulse area) and overall pain were measured using a force platform and a pain scoring system (numerical rating scale).
Analysis of the data by Repeated Measures anova showed that the dogs developed a significant lameness between the baseline and 12 hours. However, no significant difference in ground reaction forces or total pain score was demonstrated between the treatment and control groups at any other time.
In conclusion, ketamine administered epidurally at a dose of 2 mg kg^?1 did not provide significant analgesia in dogs with chemically induced synovitis.