Intrathecal α2-adrenergic agonists stimulate acetylcholine and norepinephrine release from the spinal cord dorsal horn in sheep: An in vivo microdialysis study

Abstract

Background: Intrathecal injection of clonidine and dexmedetomidine produce behavioral analgesia by an α2-adrenergic mechanism. Functional and anatomic studies suggest that this analgesia is mediated by cholinergic activation. This hypothesis was directly tested by measuring extracellular acetylcholine concentrations in spinal cord interstitial fluid by means of microdialysis after intrathecal injection of these α2-adrenergic agonists in sheep. Methods: Twelve sheep with chronically implanted thoracic intrathecal catheters were anesthetized with halothane. Multiple 200-μm- diameter dialysis fibers were inserted surgically at a mid-thoracic level through the dorsal horn and perfused with artificial cerebrospinal fluid. After baseline sampling, either clonidine (100 μg), dexmedetomidine (100 μg), or saline were injected intrathecally. Microdialysis samples were analyzed by high-pressure liquid chromatography for acetylcholine and norepinephrine. Results: Both α2-adrenergic agonists increased acetylcholine in microdialysate, whereas intrathecal saline had no effect. Analysis of the raw data showed that all groups differed significantly, with greater levels of acetylcholine following administration of dexmedetomidine than clonidine or saline. Unexpectedly, intrathecal clonidine also increased microdialysate norepinephrine levels. Conclusions: These data are consistent with previous experiments measuring acetylcholine concentrations in cerebrospinal fluid and support analgesia from α2-adrenergic agonists mediated in part by cholinergic activation. In addition, the increase in norepinephrine concentrations after intrathecal administration of clonidine suggest stimulation of norepinephrine release by this agent.