Casein Kinase II RegulatesN-Methyl-d-Aspartate Receptor Activity in Spinal Cords and Pain Hypersensitivity Induced by Nerve Injury

Abstract

Increased N-methyl-D-aspartate receptor (NMDAR) activity and phosphorylation in the spinal cord are critically involved in the synaptic plasticity and central sensitization associated with neuropathic pain. However, the mechanisms underlying increased NMDAR activity in neuropathic pain conditions remain poorly understood. Here we show that peripheral nerve injury induces a large GluN2A-mediated increase in NMDAR activity in spinal lamina II, but not lamina I, neurons. However, NMDAR currents in spinal dorsal horn neurons are not significantly altered in rat models of diabetic neuropathic pain and resiniferatoxin-induced painful neuropathy (postherpedic neuralgia). Inhibition of protein tyrosine kinases or protein kinase C has little effect on NMDAR currents potentiated by nerve injury. Strikingly, casein kinase II (CK2) inhibitors normalize increased NMDAR currents of dorsal horn neurons in nerve-injured rats. In addition, inhibition of calcineurin, but not protein phosphatase 1/2A, augments NMDAR currents only in control rats. CK2 inhibition blocks the increase in spinal NMDAR activity by the calcineurin inhibitor in control rats. Furthermore, nerve injury significantly increases CK2a and CK2b protein levels in the spinal cord. In addition, inhibition of CK2 or CK2b knockdown at the spinal level substantially reverses pain hypersensitivity induced by nerve injury. Our study indicates that neuropathic pain conditions with different etiologies do not share the same mechanisms, and increased spinal NMDAR activity is distinctly associated with traumatic nerve injury. CK2 plays a prominent role in the potentiation of NMDAR activity in the spinal dorsal horn and may represent a new target for treatments of chronic pain caused by nerve injury.

Nerve Injury Diminishes Opioid Analgesia through Lysine Methyltransferase-mediated Transcriptional Repression of μ-Opioid Receptors in Primary Sensory Neurons

“…Nerve Injury Diminishes MOR Expression in the DRG-SNL is a commonly used animal model of neuropathic pain (22,24). Using quantitative PCR, we determined the mRNA level of Oprm1 in the L5 and L6 DRGs at 5, 10, and 21 days after SNL.…”

“…Briefly, we made a small incision at the back of the neck of the animal. Next, we made a small opening in the atlantooccipital membrane of the cisterna magna and inserted a PE-10 catheter (ϳ8.0 cm) so that the caudal tip reached the lumbar spinal cord (8,24). Rats displaying motor or neurological dysfunction after catheter insertion were immediately euthanized.…”

“…To activate the device, a foot pedal was pressed, triggering a motor that applied a constantly increasing force on a linear scale. When the animal displayed pain by either withdrawing its paw or vocalizing, the pedal was immediately released, and the nociceptive threshold of the animal was read on the scale (24,28,29). Each trial was repeated two or three times at ϳ2-min intervals, and the mean value was used as the force to produce withdrawal responses.…”

“…Behavioral Assessment of Tactile Allodynia and Hyperalgesia-Behavioral tests were conducted as described previously (24,28). In brief, to detect tactile allodynia, we applied von Frey filaments to the left hind paw of the animals (ipsilateral to SNL or SNI).…”

“…If there was no response, the filament of the next greater force was applied. We calculated the tactile stimulus force that produced a 50% likelihood of a withdrawal response using the "up-down" method (22,24).…”