Noxious mechanical stimulation evokes the segmental release of opioid peptides that induce mu-opioid receptor internalization in the presence of peptidase inhibitors.
The internalization of mu-opioid receptors (MORs) provides an ideal way to locate areas of opioid peptide release. We used this method to study opioid release in the spinal cord evoked by noxious stimuli in anesthetized rats. Previous studies have shown that opioids released in the spinal cord produce MOR internalization only when they are protected from peptidase degradation. Accordingly, rats were implanted with chronic intrathecal catheters that were used to inject a mixture of peptidase inhibitors (amastatin, captopril and phosphoramidon) onto the lumbar spinal cord. Five minutes later, a noxious stimulus was delivered to the paw. Lumbar spinal segments were double-stained with antibodies against MORs and neurokinin 1 receptors (NK1Rs) using immunofluorescence. Mechanical stimulation of the hindpaw consisted of repeated 10 s clamps with a hemostat for 10 min. In the ipsilateral dorsal horn, the stimulus produced abundant NK1R internalization in segments L3-L6, and a more modest but significant MOR internalization in segments L5 and L6. In the contralateral dorsal horn, NK1R was substantially lower and MOR internalization was negligible. The same mechanical stimulus applied to a forepaw did not produce NK1R or MOR internalization in the lumbar spinal cord. Thermal stimulation consisted of immersing a hindpaw in water at 52 degrees C for 2 min. It produced substantial NK1R internalization ipsilaterally in segment L6, but no MOR internalization. These results show that mechanical stimulation induces segmental opioid release, i.e., in the dorsal horn receiving the noxious signals and not in other spinal segments.
Lao
Lijun
L
Center for Neurobiology of Stress and CURE: Digestive Diseases Research Center, Division of Digestive Diseases, Department of Medicine, David Geffen School of Medicine at UCLA, Los Angeles, California 90095, USA.
Song
Bingbing
B
Chen
Wenling
W
Marvizón
Juan Carlos G
JC
D000818:Animals; D005455:Fluorescent Antibody Technique; D007278:Injections, Spinal; D008297:Male; D018613:Microscopy, Confocal; D018847:Opioid Peptides / Q000187:drug effects / Q000378:metabolism*; D010812:Physical Stimulation; D020671:Posterior Horn Cells / Q000187:drug effects / Q000378:metabolism*; D011480:Protease Inhibitors / Q000008:administration & dosage*; D051381:Rats; D017207:Rats, Sprague-Dawley; D018040:Receptors, Neurokinin-1 / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000378:metabolism*; D013116:Spinal Cord / Q000187:drug effects / Q000378:metabolism
D018847:Opioid Peptides; D011480:Protease Inhibitors; D018040:Receptors, Neurokinin-1; D017450:Receptors, Opioid, mu