Wise Young
10-02-2001, 12:12 AM
The Journal of Neuroscience, March 1, 2001, 21(5):1779-1786
Pronociceptive Actions of Dynorphin Maintain Chronic Neuropathic Pain
Zaijie Wang1, Luis R. Gardell1, Michael H. Ossipov1, Todd W. Vanderah1, Miles B. Brennan4, Ute Hochgeschwender5, Victor J. Hruby2, T. Phil Malan Jr1, 3, Josephine Lai1, and Frank Porreca1, 3
Departments of 1Â*Pharmacology, 2Â*Chemistry, and 3Â*Anesthesiology, University of Arizona Health Sciences Center, Tucson, Arizona 85724,Â*4Â*Eleanor Roosevelt Institute, Denver, Colorado 80206,Â*and 5Â*Developmental Biology Program, Oklahoma Medical Research Foundation, Oklahoma City, Oklahoma 73104
Whereas tissue injury increases spinal dynorphin expression, the functional relevance of this upregulation to persistent pain is unknown. Here, mice lacking the prodynorphin gene were studied for sensitivity to non-noxious and noxious stimuli, before and after induction of experimental neuropathic pain. Prodynorphin knock-out (KO) mice had normal responses to acute non-noxious stimuli and a mild increased sensitivity to some noxious stimuli. After spinal nerve ligation (SNL), both wild-type (WT) and KO mice demonstrated decreased thresholds to innocuous mechanical and to noxious thermal stimuli, indicating that dynorphin is not required for initiation of neuropathic pain. However, whereas neuropathic pain was sustained in WT mice, KO mice showed a return to baselines by post-SNL day 10.Â*In WT mice, SNL upregulated lumbar dynorphin content on day 10,Â*but not day 2,Â*after injury. Intrathecal dynorphin antiserum reversed neuropathic pain in WT mice at post-SNL day 10Â*(when dynorphin was upregulated) but not on post-SNL day 2; intrathecal MK-801 reversed SNL-pain at both times. Opioid (µ, , and ) receptor density and G-protein activation were not different between WT and KO mice and were unchanged by SNL injury. The observations suggest (1) an early, dynorphin-independent phase of neuropathic pain and a later dynorphin-dependent stage, (2) that upregulated spinal dynorphin is pronociceptive and required for the maintenance of persistent neuropathic pain, and (3) that processes required for the initiation and the maintenance of the neuropathic pain state are distinct. Identification of mechanisms that maintain neuropathic pain appears important for strategies to treat neuropathic pain.
Key words: prodynorphin; dynorphin; neuropathic pain; opioid receptors; spinal nerve injury; nociception; gene deletion; gene knockout; transgenic; mouse
Pronociceptive Actions of Dynorphin Maintain Chronic Neuropathic Pain
Zaijie Wang1, Luis R. Gardell1, Michael H. Ossipov1, Todd W. Vanderah1, Miles B. Brennan4, Ute Hochgeschwender5, Victor J. Hruby2, T. Phil Malan Jr1, 3, Josephine Lai1, and Frank Porreca1, 3
Departments of 1Â*Pharmacology, 2Â*Chemistry, and 3Â*Anesthesiology, University of Arizona Health Sciences Center, Tucson, Arizona 85724,Â*4Â*Eleanor Roosevelt Institute, Denver, Colorado 80206,Â*and 5Â*Developmental Biology Program, Oklahoma Medical Research Foundation, Oklahoma City, Oklahoma 73104
Whereas tissue injury increases spinal dynorphin expression, the functional relevance of this upregulation to persistent pain is unknown. Here, mice lacking the prodynorphin gene were studied for sensitivity to non-noxious and noxious stimuli, before and after induction of experimental neuropathic pain. Prodynorphin knock-out (KO) mice had normal responses to acute non-noxious stimuli and a mild increased sensitivity to some noxious stimuli. After spinal nerve ligation (SNL), both wild-type (WT) and KO mice demonstrated decreased thresholds to innocuous mechanical and to noxious thermal stimuli, indicating that dynorphin is not required for initiation of neuropathic pain. However, whereas neuropathic pain was sustained in WT mice, KO mice showed a return to baselines by post-SNL day 10.Â*In WT mice, SNL upregulated lumbar dynorphin content on day 10,Â*but not day 2,Â*after injury. Intrathecal dynorphin antiserum reversed neuropathic pain in WT mice at post-SNL day 10Â*(when dynorphin was upregulated) but not on post-SNL day 2; intrathecal MK-801 reversed SNL-pain at both times. Opioid (µ, , and ) receptor density and G-protein activation were not different between WT and KO mice and were unchanged by SNL injury. The observations suggest (1) an early, dynorphin-independent phase of neuropathic pain and a later dynorphin-dependent stage, (2) that upregulated spinal dynorphin is pronociceptive and required for the maintenance of persistent neuropathic pain, and (3) that processes required for the initiation and the maintenance of the neuropathic pain state are distinct. Identification of mechanisms that maintain neuropathic pain appears important for strategies to treat neuropathic pain.
Key words: prodynorphin; dynorphin; neuropathic pain; opioid receptors; spinal nerve injury; nociception; gene deletion; gene knockout; transgenic; mouse