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. 2000 Sep 15;20(18):7074-9.
doi: 10.1523/JNEUROSCI.20-18-07074.2000.

Dynorphin Promotes Abnormal Pain and Spinal Opioid Antinociceptive Tolerance

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Free PMC article

Dynorphin Promotes Abnormal Pain and Spinal Opioid Antinociceptive Tolerance

T W Vanderah et al. J Neurosci. .
Free PMC article

Abstract

The nonopioid actions of spinal dynorphin may promote aspects of abnormal pain after nerve injury. Mechanistic similarities have been suggested between opioid tolerance and neuropathic pain. Here, the hypothesis that spinal dynorphin might mediate effects of sustained spinal opioids was explored. Possible abnormal pain and spinal antinociceptive tolerance were evaluated after intrathecal administration of [D-Ala(2), N-Me-Phe(4), Gly-ol(5)]enkephalin (DAMGO), an opioid mu agonist. Rats infused with DAMGO, but not saline, demonstrated tactile allodynia and thermal hyperalgesia of the hindpaws (during the DAMGO infusion) and a decrease in antinociceptive potency and efficacy of spinal opioids (tolerance), signs also characteristic of nerve injury. Spinal DAMGO elicited an increase in lumbar dynorphin content and a decrease in the mu receptor immunoreactivity in the spinal dorsal horn, signs also seen in the postnerve-injury state. Intrathecal administration of dynorphin A(1-17) antiserum blocked tactile allodynia and reversed thermal hyperalgesia to above baseline levels (i.e., antinociception). Spinal dynorphin antiserum, but not control serum, also reestablished the antinociceptive potency and efficacy of spinal morphine. Neither dynorphin antiserum nor control serum administration altered baseline non-noxious or noxious thresholds or affected the intrathecal morphine antinociceptive response in saline-infused rats. These data suggest that spinal dynorphin promotes abnormal pain and acts to reduce the antinociceptive efficacy of spinal opioids (i.e., tolerance). The data also identify a possible mechanism for previously unexplained clinical observations and offer a novel approach for the development of strategies that could improve the long-term use of opioids for pain.

Figures

Fig. 1.
Fig. 1.
Left, Male Sprague Dawley rats received an intrathecal infusion of saline or of DAMGO (1 nmol · μl−1 · hr−1) for 7 d. The spinal cords were removed and assayed for dynorphin content with an enzyme immunoassay. The rats with DAMGO infusions showed a significant (*p ≤ 0.05; Student'st test) increase in spinal dynorphin content when compared with saline-infused rats. Right, Antinociceptive tolerance on day 8 to intrathecal DAMGO (1 μg/5 μl;n = 8; top) or cross-tolerance to intrathecal morphine (10 μg/5 μl; n = 8;bottom) in the 52°C water tail-flick test is demonstrated.
Fig. 2.
Fig. 2.
Immunohistochemical analysis of prodynorphin in lumbar spinal sections. Rats have been infused with either saline (control) or DAMGO for 7 d. Lumbar spinal cross sections (40 μm) were labeled with the antiserum for prodynorphin (1:40,000) and processed for DAB staining by the ABC method. Spinal cord halves are represented in close juxtaposition to allow visual comparison.Top, The lefthalfis obtained from the saline-infused control, and therighthalf is obtained from the DAMGO-infused rat. The micrographs were acquired via a Hamamatsu digital-imaging system with a Nikon microscope. Bottom, As seen under higher magnification, the superficial laminae (A, B), discrete cell bodies, and numerous fibers were labeled, whereas in laminae V and VI (C, D) and in lamina X (E, F), prodynorphin-IR was predominantly associated with fibers. Lumbar spinal sections from DAMGO-infused (B, D,F) rats exhibited a reduction of prodynorphin-IR in all the laminae stated above when compared with the saline-infused control (A, C, E). This reduction appears to be primarily caused by a loss of fiber staining in these laminae. In the superficial laminae, staining was still clearly present in cell bodies; the density of immunolabeled cell bodies was similar between control and DAMGO-infused rats.
Fig. 3.
Fig. 3.
Immunohistochemical analysis of the μ-opioid receptor in lumbar spinal sections. Rats have been infused with either saline (control) or DAMGO for 7 d. Lumbar spinal cross sections (40 μm) were labeled with the antiserum for rat MOR (1:20,000) and processed for DAB staining by the ABC method. Spinal cord halves are represented in close juxtaposition to allow visual comparison. Thelefthalf is obtained from the saline-infused control, and the righthalf is obtained from the DAMGO-infused rat. Substantially higher MOR immunoreactivity is observed in the superficial laminae (I and II) of the control cord, indicating a greater concentration of μ-opioid receptors, than in that of the DAMGO-treated group. The micrographs were acquired via a Hamamatsu digital-imaging system with a Nikon microscope.
Fig. 4.
Fig. 4.
Male Sprague Dawley rats received intrathecal infusions of saline or of DAMGO (1 nmol · μl−1 · hr−1) for 7 d. Antinociceptive tolerance after chronic infusion of DAMGO was accompanied by tactile allodynia, indicated by a significant (*p ≤ 0.05; Student's t test;n = 10) decrease in paw withdrawal thresholds to probing with von Frey filaments on the sixth day of infusion. The acute intrathecal injection of 200 μg of antiserum to dynorphin A(1–17) on day 7 blocked tactile allodynia when given 20 min before testing in DAMGO-tolerant rats (n = 10). DYN A/S, Dynorphin antiserum.
Fig. 5.
Fig. 5.
Male Sprague Dawley rats received intrathecal infusions of saline or of DAMGO (1 nmol · μl−1 · hr−1) for 7 d. Antinociceptive tolerance was accompanied by thermal hyperalgesia after chronic infusion of DAMGO, indicated by a significant (*p ≤ 0.05; Student'st test; n = 10) decrease in paw withdrawal latencies to radiant heat applied to the plantar aspect of the hindpaw on the sixth day of infusion. The acute intrathecal injection of 200 μg of antiserum to dynorphin A(1–17) on day 7 reversed thermal hyperalgesia when given 20 min before testing in DAMGO-tolerant rats (‡ p ≤ 0.05; Student'st test; n = 10).
Fig. 6.
Fig. 6.
Male Sprague Dawley rats received intrathecal infusions of saline or of DAMGO (1 nmol · μl−1 · hr−1) for 7 d. Antinociceptive dose–response functions for intrathecal morphine were generated in the 52°C water tail-flick test 10 min after treatment. The saline- or DAMGO-infused rats received either intrathecal morphine alone or morphine 10 min after intrathecal pretreatment with antiserum to dynorphin A(1–17) (i.e., dynorphin antiserum given 20 min before the test). The following groups were used: naive rats (○), rats infused with saline and challenged with morphine (■), rats infused with saline and challenged with morphine after pretreatment with dynorphin antiserum (▵), rats infused with DAMGO and challenged with morphine (●), and rats infused with DAMGO and challenged with morphine after pretreatment with dynorphin antiserum (▪). Five to 10 separate animals were used for eachpoint on the dose–response curve. i.th., Intrathecal.

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