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PAIN MEDICINE

The Effects of Intrathecal Gabapentin on Spinal Morphine Tolerance in the Rat Tail-Flick and Paw Pressure Tests

C. Hansen^*, I. Gilron^*,, and M. Hong^*,

Departments of Anesthesiology and *Pharmacology & Toxicology, Kingston General Hospital, Queen’s University, Ontario, Canada

Address correspondence and reprint requests to Ian Gilron, MD, MSc, FRCPC, Department of Anesthesiology, Queen’s University, Victory 2 Pavilion, 76 Stuart St., Kingston, ON K7L 2V7, Canada. Address e-mail to gilroni{at}post.queensu.ca

Analgesic tolerance to opioids has been described in both experimental and clinical conditions and may limit the clinical utility of these drugs. We have previously shown that systemic gabapentin (GBP), a non-opioid drug, prevents and reverses tolerance to systemic morphine in the rat. In this study, we investigated the effect of intrathecal GBP on spinal morphine tolerance. Studied rats were given 7 days of intrathecal injections with saline (10 µL), GBP (300 µg), morphine (15 µg), or a GBP-morphine combination, and analgesic testing using tail-flick and paw-pressure tests was conducted before and 30 min after the drug injection. On Day 8, an antinociceptive dose-response curve was constructed and the 50% effective dose (ED₅₀) values for morphine (given alone) were calculated for each study group. Coinjection of GBP with morphine blocked the development of tolerance, as shown by the preservation of morphine analgesia over 7 days as well as by a concomitant decrease in ED₅₀ values on Day 8, as compared with the morphine-alone group. Although additive analgesia over Days 1–7 cannot be ruled out, ED₅₀ reductions in the GBP-morphine combination group indeed suggest some suppression of tolerance. These data support previous evidence that GBP prevents opioid tolerance and, more specifically, indicate that intrathecal GBP prevents the development of spinal opioid tolerance. Future studies are required to examine the respective roles of supraspinal and peripheral sites of GBP-morphine interaction and to investigate the mechanisms underlying the action of GBP on opioid tolerance.

IMPLICATIONS: Analgesic opioid tolerance may limit the efficacy of opioids, such as morphine, and has underlying mechanisms that are also important in the development of neuropathic pain. This study supports previous evidence that gabapentin prevents opioid tolerance and more specifically indicates that intrathecal gabapentin prevents the development of spinal opioid tolerance.

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