Spinal antinociceptive action of an N-type voltage-dependent calcium channel blocker and the synergistic interaction with morphine

Abstract

Background: Four different voltage-dependent calcium channels (L-, N-, T- , and P-types) are distinguished in the central nervous system. Both L- and N-type calcium channels have been implicated in the release of neurotransmitters from sensory neurons in the spinal cord. It has been demonstrated that in trathecal L-type calcium channel blockers, which alone do not exhibit any antinociceptive effects, potentiate the antinociceptive effects of intrathecal morphine. The current study was designed to investigate the antinociceptive effects of the intrathecally administered N- type calcium channel blocker, ω-conotoxin GVlA (ω-CgTx). The interaction between morphine and ω-CgTx at the level of the spinal cord also was examined. Methods: In male Sprague-Dawley rats, lumbar intrathecal catheters were chronically implanted. Tail flick and mechanical paw pressure tests were used to assess thermal and mechanical nociceptive thresholds, respectively. Morphine, ω-CgTx, or a combination of morphine and ω-CgTx was administered intrathecally, and the nociceptive thresholds were determined. Isobolographic analyses were used to define the nature of the functional interactions between morphine and ω-CgTx. Results: Intrathecal ω-CgTx produced antinociception in a dose- and time-dependent manner. Isobolographic analyses revealed that intrathecal ω-CgTx and morphine interacted synergistically in both nociceptive tests. Conclusions: This study indicates the importance of the N-type calcium channel in the spinal cord on nociception and suggests the functional interaction between the N-type calcium channel blocker and opioid at the level of the spinal cord.