Spinal dopaminergic projections control the transition to pathological pain plasticity via a D1/D5-mediated mechanism.

The mechanisms that lead to the maintenance of chronic pain states are poorly understood, but their elucidation could lead to new insights into how pain becomes chronic and how it can potentially be reversed. We investigated the role of spinal dorsal horn neurons and descending circuitry in plasticity mediating a transition to pathological pain plasticity suggesting the presence of a chronic pain state using hyperalgesic priming. We found that when dorsal horn neurokinin 1 receptor-positive neurons or descending serotonergic neurons were ablated before hyperalgesic priming, IL-6- and carrageenan-induced mechanical hypersensitivity was impaired, and subsequent prostaglandin E2 (PGE2) response was blunted. However, when these neurons were lesioned after the induction of priming, they had no effect on the PGE2 response, reflecting differential mechanisms driving plasticity in a primed state. In stark contrast, animals with a spinally applied dopaminergic lesion showed intact IL-6- and carrageenan-induced mechanical hypersensitivity, but the subsequent PGE2 injection failed to cause mechanical hypersensitivity. Moreover, ablating spinally projecting dopaminergic neurons after the resolution of the IL-6- or carrageenan-induced response also reversed the maintenance of priming as assessed through mechanical hypersensitivity and the mouse grimace scale. Pharmacological antagonism of spinal dopamine D1/D5 receptors reversed priming, whereas D1/D5 agonists induced mechanical hypersensitivity exclusively in primed mice. Strikingly, engagement of D1/D5 coupled with anisomycin in primed animals reversed a chronic pain state, consistent with reconsolidation-like effects in the spinal dorsal horn. These findings demonstrate a novel role for descending dopaminergic neurons in the maintenance of pathological pain plasticity.

Proteomic and functional annotation analysis of injured peripheral nerves reveals ApoE as a protein upregulated by injury that is modulated by metformin treatment.

BACKGROUND: Peripheral nerve injury (PNI) results in a fundamental reorganization of the translational machinery in the injured peripheral nerve such that protein synthesis is increased in a manner linked to enhanced mTOR and ERK activity. We have shown that metformin treatment, which activates adenosine monophosphate-activated protein kinase (AMPK), reverses tactile allodynia and enhanced translation following PNI. To gain a better understanding of how PNI changes the proteome of the sciatic nerve and ascertain how metformin treatment may cause further change, we conducted a range of unbiased proteomic studies followed by biochemical experiments to confirm key results. RESULTS: We used multidimensional protein identification technology (MUDPIT) on sciatic nerve samples taken from rats with sham surgery, spinal nerve ligation (SNL) surgery or SNL + 200 mg/kg metformin treatment. MUDPIT analysis on these complex samples yielded a wide variety of proteins that were sorted according to their peptide counts in SNL and SNL + metformin compared to sham. These proteins were then submitted to functional annotation analysis to identify potential functional networks altered by SNL and SNL + metformin treatment. Additionally, we used click-chemistry-based labeling and purification of nascently synthesized proteins followed by MUDPIT to further identify peptides that were synthesized within the injured nerve. With these methods, we identified apolipoprotein E (ApoE) as a protein profoundly increased by PNI and further increased by PNI and metformin. This result was confirmed by Western Blot of samples from SNL rats and spared nerve injury (SNI) mice. Furthermore, we show that 7-day treatment with metformin in naïve mice leads to an increase in ApoE expression in the sciatic nerve. CONCLUSIONS: These proteomic findings support the hypothesis that PNI leads to a fundamental reorganization of gene expression within the injured nerve. Our data identify a key association of ApoE with PNI that is regulated by metformin treatment. We conclude from the known functions of ApoE in the nervous system that ApoE may be an intrinsic factor linked to nerve regeneration after PNI, an effect that is further enhanced by metformin treatment.

Spinal protein kinase M ζ underlies the maintenance mechanism of persistent nociceptive sensitization.

Sensitization of the pain pathway is believed to promote clinical pain disorders. We hypothesized that the persistence of a sensitized state in the spinal dorsal horn might depend on the activity of protein kinase M ζ (PKMζ), an essential mechanism of late long-term potentiation (LTP). To test this hypothesis, we used intraplantar injections of interleukin-6 (IL-6) in mice to elicit a transient allodynic state that endured ∼3 d. After the resolution of IL-6-induced allodynia, a subsequent intraplantar injection of prostaglandin E(2) (PGE(2)) or intrathecal injection of the metabotropic glutamate receptor 1/5 (mGluR1/5) agonist DHPG (dihydroxyphenylglycol) precipitated allodynia and/or nocifensive responses. Intraplantar injection of IL-6 followed immediately by intrathecal injection of a PKMζ inhibitor prevented the expression of subsequent PGE(2)-induced allodynia. Inhibitors of protein translation were effective in preventing PGE(2)-induced allodynia when given immediately after IL-6, but not after the initial allodynia had resolved. In contrast, spinal PKMζ inhibition completely abolished both prolonged allodynia to hindpaw PGE(2) and enhanced nocifensive behaviors evoked by intrathecal mGluR1/5 agonist injection after the resolution of IL-6-induced allodynia. Moreover, spinal PKMζ inhibition prevented the enhanced response to subsequent stimuli following resolution of hypersensitivity induced by plantar incision. The present findings demonstrate that the spinal cord encodes an engram for persistent nociceptive sensitization that is analogous to molecular mechanisms of late LTP and suggest that spinally directed PKMζ inhibitors may offer therapeutic benefit for injury-induced pain states.