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Vol. 298, Issue 1, 257-263, July 2001
Physical Therapy Graduate Program (A.K., K.A.S.), Neuroscience
Graduate Program (K.A.S.), and Department of Pharmacology (M.O.U.),
College of Medicine, University of Iowa, Iowa City, Iowa
Although transcutaneous electrical nerve stimulation (TENS) is used
extensively in inflammatory joint conditions such as arthritis, the
underlying mechanisms are unclear. This study aims to demonstrate an
opiate-mediated activation of descending inhibitory pathways from the
rostral ventral medulla (RVM) in the antihyperalgesia produced by low-
(4 Hz) or high-frequency (100 Hz) TENS. Paw withdrawal latency
to radiant heat, as an index of secondary hyperalgesia, was recorded
before and after knee joint inflammation (induced by intra-articular
injection of 3% kaolin and carrageenan) and after TENS/no TENS
coadministered with naloxone (20 µg/1 µl), naltrindole (5 µg/1
µl), or vehicle (1 µl) microinjected into the RVM. The
selectivity of naloxone and naltrindole doses was tested against
the µ-opioid receptor agonist
[D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin
(DAMGO) (20 ng, 1 µl) and the
2-opioid receptor agonist deltorphin (5 µg, 1 µl) in the RVM. Naloxone microinjection into the RVM blocks the antihyperalgesia produced by low frequency (p < 0.001), but not that produced by
high-frequency TENS (p > 0.05). In contrast,
naltrindole injection into the RVM blocks the antihyperalgesia produced
by high-frequency (p < 0.05), but not
low-frequency (p > 0.05) TENS. The analgesia
produced by DAMGO and deltorphin is selectively blocked by naloxone
(p < 0.05) and naltrindole (p < 0.05), respectively. Thus, the dose of naloxone and naltrindole used
in the current study blocks µ- and
-opioid receptors,
respectively. Hence, low-frequency and high-frequency TENS produces
antihyperalgesia by activation of µ- and
-opioid receptors,
respectively, in the RVM.
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