Background: We evaluated the epidural administration of tezampanel, a non-N-methyl-d-aspartate receptor antagonist, in a rat model for postoperative pain. We sought to determine if this drug affects nociception when administered epidurally by testing its effects on responses to heat in normal rats. The effects of epidural tezampanel on pain-related behaviors in rats that underwent plantar incision were also studied.
Methods: Rats were anesthetized and epidural catheters were placed. One day after epidural catheterization, the baseline heat withdrawal latency was measured. Epidural tezampanel or morphine was tested for analgesia by examining their effects against heat withdrawal latency. Motor function was also tested. Comparisons to subcutaneous drug administration were made. Other rats underwent plantar incision after epidural catheterization to assess pain behavior caused by incision. The effects of epidural tezampanel on the cumulative pain scoring, based on guarding, the withdrawal threshold to von Frey filament application, and the withdrawal latency to heat, were measured. The effects of epidural tezampanel on arterial blood pressure and heart rate were also tested.
Results: Both epidural morphine and epidural tezampanel increased withdrawal latency to heat. Only subcutaneous morphine affected heat withdrawal latency. After plantar incision, epidural tezampanel decreased the median guarding pain score, increased the heat withdrawal latency and increased the mechanical withdrawal threshold indicating analgesic effects. Arterial blood pressure and heart rate did not change after epidural drug administration.
Conclusion: These experiments demonstrate that epidural administration of tezampanel produces analgesia to heat, motor side effects in some rats, and reduces pain behaviors caused by incision. No systemic analgesia was apparent using the largest dose.