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Vol. 62, Issue 5, 1084-1093, November 2002
Laboratory of Bioorganic Chemistry, National Institute of
Diabetes Digestive and Kidney Diseases, Bethesda, Maryland (A.D., J.G.,
J.-W.G., N.S., A.S.B., J.W.); Department of Chemistry, University of
Virginia, Charlottesville, Virginia (W.D.H., P.L.S.); Lilly Research
Laboratories, Eli Lilly & Company, Indianapolis, Indiana (C.C.F.); and
Department of Neurology, Emory University School of Medicine, Atlanta,
Georgia (A.I.L.)
Centrally active muscarinic agonists display pronounced analgesic
effects. Identification of the specific muscarinic acetylcholine receptor (mAChR) subtype(s) mediating this activity is of considerable therapeutic interest. To examine the roles of the M2 and
M4 receptor subtypes, the two
Gi/Go-coupled mAChRs, in mediating
agonist-dependent antinociception, we generated a mutant mouse line
deficient in both M2 and M4 mAChRs
[M2/M4 double-knockout (KO) mice]. In
wild-type mice, systemic, intrathecal, or intracerebroventricular
administration of centrally active muscarinic agonists resulted in
robust analgesic effects, indicating that muscarinic analgesia can be
mediated by both spinal and supraspinal mechanisms. Strikingly,
muscarinic agonist-induced antinociception was totally abolished in
M2/M4 double-KO mice, independent of the route
of application. The nonselective muscarinic agonist oxotremorine showed
reduced analgesic potency in M2 receptor single-KO mice,
but retained full analgesic activity in M4 receptor
single-KO mice. In contrast, two novel muscarinic agonists chemically
derived from epibatidine, CMI-936 and CMI-1145, displayed reduced
analgesic activity in both M2 and M4 receptor single-KO mice, independent of the route of application. Radioligand binding studies indicated that the two CMI compounds, in contrast to
oxotremorine, showed >6-fold higher affinity for M4 than
for M2 receptors, providing a molecular basis for the
observed differences in agonist activity profiles. These data provide
unambiguous evidence that muscarinic analgesia is exclusively mediated
by a combination of M2 and M4 mAChRs at both
spinal and supraspinal sites. These findings should be of considerable
relevance for the development of receptor subtype-selective muscarinic
agonists as novel analgesic drugs.
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