Interaction of opioid and local anesthetics on spinal dorsal horn neuron

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Male Sprague-Dawley rats were used in this study. The animals were divided into two experiments ; 1) electrophysiological study or 2) behavioral study.1) Under halothane anesthesia, lumbar laminectomy and intrathecal catheterization were peformed. The extracellular activity was recorded from a single dorsal horn neurons. When wide dynamic range neurons responsive to both noxious and non-noxious stimuli were isolated, a few neuron simultaneously responded to the visceral stimulus (colorectal distension). Intrathecal administration of morphine inhibited the response of those neurons to noxious stimuli. However, it has not yet determined the interaction of intrathecally coadminitered morphine and lidocaine due to small sample, to date.2) Tail flick latency and colorectal distension threshold were measured before and for 180 min after the epidural injection or continuous intrathecal infusion of morphine, lidocaine, or combination of those drugs. Isobolographic analysis, potency ratio analysis and fractional analysis revealed that morphine and lidocaine interact synergistically in both somatic and visceral nociception tests.