British Journal of Anaesthesia, Vol 81, Issue 2 208-215, Copyright © 1998 by The Board of Management and Trustees of the British Journal of Anaesthesia
H. Buerkle and T. L. Yaksh
Alpha2-adrenergic agonists given intrathecally result in antinociception
and intracerebroventricularly (ICV) in sedation. To examine whether
different alpha2-adrenergic receptor subtypes differentially mediate
antinociception and sedation, we measured the relative potency of three
alpha2-adrenergic agonists, dexmedetomidine (DMET), clonidine (CLON) and
UK-14.304 (UK), after spinal and ICV administration. Each agonist was given
either alone or in the presence of systemically administered yohimbine,
which acts as a competitive alpha2-antagonist in unanaesthetized rats.
Intrathecal delivery of the agonists alone resulted in a dose-dependent
antinociceptive effect (ED50 (nmol): DMET = 1.2, UK = 1.7, CLON = 5.6) with
little sedative effect at the lower doses. Yohimbine pretreatment resulted
in a rightward shift of the dose-response curves (DMET > CLON > UK).
ICV alpha2-adrenergic agonists produced a dose-dependent sedation (ED50
(nmol): DMET = 10.5; UK = 28.7; CLON = 126), with little antinociceptive
action. Again, yohimbine pretreatment produced a right shift of the ICV
sedation dose-response curves (UK > DMET > CLON). Thus, we conclude
that the spinal analgesic effects of DMET, CLON and UK appear to be
mediated by two sites. After ICV delivery, DMET, CLON and UK appear to act
at a common supra-spinal site to produce sedation and this site resembles
that acted upon by UK in the spinal cord.
LABORATORY INVESTIGATIONS
Pharmacological evidence for different alpha 2-adrenergic receptor sites mediating analgesia and sedation in the rat
Department of Anesthesiology, University of California, San Diego, California, USA
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