Intrathecal alpha 2-adrenergic agonists stimulate acetylcholine and norepinephrine release from the spinal cord dorsal horn in sheep. An in vivo microdialysis study.
BACKGROUND
Intrathecal injection of clonidine and dexmedetomidine produce behavioral analgesia by an alpha 2-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 alpha 2-adrenergic agonists in sheep.


METHODS
Twelve sheep with chronically implanted thoracic intrathecal catheters were anesthetized with halothane. Multiple 200-micron-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 micrograms), dexmedetomidine (100 micrograms), or saline were injected intrathecally. Microdialysis samples were analyzed by high-pressure liquid chromatography for acetylcholine and norepinephrine.


RESULTS
Both alpha 2-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 alpha 2-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.
Klimscha
W
W
Department of Anesthesia, Bowman Gray School of Medicine, Winston-Salem, North Carolina 27157-1009, USA.
Tong
C
C
Eisenach
J C
JC
D000109:Acetylcholine / Q000378:metabolism*; D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage*; D000818:Animals; D003000:Clonidine / Q000008:administration & dosage*; D007093:Imidazoles / Q000008:administration & dosage*; D007278:Injections, Spinal; D020926:Medetomidine; D017551:Microdialysis; D009638:Norepinephrine / Q000378:metabolism*; D012756:Sheep; D013116:Spinal Cord / Q000378:metabolism*
D000316:Adrenergic alpha-Agonists; D007093:Imidazoles; D003000:Clonidine; D020926:Medetomidine; D000109:Acetylcholine; D009638:Norepinephrine