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Vol. 289, Issue 2, 993-999, May 1999
1 and
2 Opioid Receptors in the Production of Antinociception
in the Rat1
Committee on Neurobiology (R.W.H., D.L.H.) and
Department of
Anesthesia and Critical Care (R.W.H., D.L.H., T.S.G.), University of
Chicago, Chicago, Illinois; and
Department of Pharmacology (R.J.T.),
Temple University, Philadelphia, Pennsylvania
Previous work supports the existence of two types of
opioid
receptor (
1 and
2) and a role of both
subtypes in the spinal cord and the ventromedial medulla (VMM) in the
production of antinociception. Although it is well established that
spinal and supraspinal µ opioid receptors interact in a synergistic
manner to produce antinociception, little is known about the
interaction of
opioid receptors. This study used isobolographic
analysis to determine how
1 and
2 opioid
receptors in the VMM interact with their respective receptors in the
spinal cord to produce antinociception. Concurrent administration of
the
1 opioid receptor agonist
[D-Pen2,D-Pen5]enkephalin
at spinal and supraspinal sites in a fixed-dose ratio produced
antinociception in an additive manner in the tail-flick test. In
contrast, concurrent administration of very low doses of the
2 opioid receptor agonist
[D-Ala2,Glu4]deltorphin at spinal
and medullary sites produced antinociception in a synergistic manner.
However, as the total dose of
[D-Ala2,Glu4]deltorphin
increased, this interaction converted to additivity. These observations
suggest that different mechanisms mediate the antinociceptive effects
of different doses of
2 opioid receptor agonists. The
difference in the nature of the interaction produced by
1 and
2 opioid receptor agonists
provides additional evidence for the existence of different subtypes of
the
opioid receptor. These results also suggest that
2 opioid receptor agonists capable of crossing the
blood-brain barrier will be more potent or efficacious analgesics than
1 opioid receptor agonists after systemic administration.
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