Search

Dysregulated excitability within the spinal dorsal horn is a critical mediator of chronic pain. Dedek et al. report that downregulation of tyrosine phosphatase STEP61 links disinhibition to NMDAR potentiation in human and rodent spinal pain processing, and develop an ex vivo human preclinical model to help bridge the translational divide., Dysregulated excitability within the spinal dorsal horn is a critical mediator of chronic pain. In the rodent nerve injury model of neuropathic pain, BDNF-mediated loss of inhibition (disinhibition) gates the potentiation of excitatory GluN2B N-methyl-d-aspartate receptor (NMDAR) responses at lamina I dorsal horn synapses. However, the centrality of this mechanism across pain states and species, as well as the molecular linker involved, remain unknown. Here, we show that KCC2-dependent disinhibition is coupled to increased GluN2B-mediated synaptic NMDAR responses in a rodent model of inflammatory pain, with an associated downregulation of the tyrosine phosphatase STEP61. The decreased activity of STEP61 is both necessary and sufficient to prime subsequent phosphorylation and potentiation of GluN2B NMDAR by BDNF at lamina I synapses. Blocking disinhibition reversed the downregulation of STEP61 as well as inflammation-mediated behavioural hypersensitivity. For the first time, we characterize GluN2B-mediated NMDAR responses at human lamina I synapses and show that a human ex vivo BDNF model of pathological pain processing downregulates KCC2 and STEP61 and upregulates phosphorylated GluN2B at dorsal horn synapses. Our results demonstrate that STEP61 is the molecular brake that is lost following KCC2-dependent disinhibition and that the decrease in STEP61 activity drives the potentiation of excitatory GluN2B NMDAR responses in rodent and human models of pathological pain. The ex vivo human BDNF model may thus form a translational bridge between rodents and humans for identification and validation of novel molecular pain targets.

The prevalence and severity of many chronic pain syndromes differ across sex, and recent studies have identified differences in immune signalling within spinal nociceptive circuits as a potential mediator. Although it has been proposed that sex-specific pain mechanisms converge once they reach neurons within the superficial dorsal horn (SDH), direct investigations using rodent and human preclinical pain models have been lacking. Here, we discovered that in the Freund’s Adjuvant in vivo model of inflammatory pain, where both male and female rats display tactile allodynia, a pathological coupling between KCC2-dependent disinhibition and NMDA receptor potentiation within SDH neurons was observed in male but not female rats. Unlike males, the neuroimmune mediator, BDNF, failed to downregulate inhibitory signalling elements (KCC2 and STEP61) and upregulate excitatory elements (pFyn, GluN2B, and pGluN2B) in female rats, resulting in no effect of ex vivo BDNF on synaptic NMDA receptor responses in female lamina I neurons. Importantly, this sex difference in spinal pain processing was conserved from rodents to humans. As in rodents, ex vivo spinal treatment with BDNF downregulated markers of disinhibition and upregulated markers of facilitated excitation in SDH neurons from male but not female human organ donors. Ovariectomy in female rats recapitulated the male pathological pain neuronal phenotype, with BDNF driving a coupling between disinhibition and NMDA receptor potentiation in adult lamina I neurons following the prepubescent elimination of sex hormones in females. This discovery of sexual dimorphism in a central neuronal mechanism of chronic pain across species provides a foundational step towards a better understanding and treatment for pain in both sexes.

While spinal microglia play a role in early stages of neuropathic pain etiology, whether they are useful targets to reverse chronic pain at late stages remains unknown. Here, we show that microglia activation in the spinal cord persists for >3 months following nerve injury in rodents, beyond involvement of proinflammatory cytokine and chemokine signalling. In this chronic phase, selective depletion of spinal microglia in male rats with the targeted immunotoxin Mac1-saporin and blockade of brain-derived neurotrophic factor-TrkB signalling with intrathecal TrkB Fc chimera, but not cytokine inhibition, almost completely reversed pain hypersensitivity. By contrast, local spinal administration of Mac1-saporin did not affect nociceptive withdrawal threshold in control animals nor did it affect the strength of afferent-evoked synaptic activity in the spinal dorsal horn in normal conditions. These findings show that the long-term, chronic phase of nerve injury-induced pain hypersensitivity is maintained by microglia-neuron interactions. The findings also effectively separate the central signalling pathways underlying the maintenance phase of the pathology from the early and peripheral inflammatory reactions to injury, pointing to different targets for the treatment of acute vs chronic injury-induced pain.

Spinal disinhibition has been hypothesized to underlie pain hypersensitivity in neuropathic pain. Apparently contradictory mechanisms have been reported, raising questions on the best target to produce analgesia. Here, we show that nerve injury is associated with a reduction in the number of inhibitory synapses in the spinal dorsal horn. Paradoxically, this is accompanied by a BDNF-TrkB-mediated upregulation of synaptic GABAARs and by an α1-to-α2GABAAR subunit switch, providing a mechanistic rationale for the analgesic action of the α2,3GABAAR benzodiazepine-site ligand L838,417 after nerve injury. Yet, we demonstrate that impaired Cl- extrusion underlies the failure of L838,417 to induce analgesia at high doses due to a resulting collapse in Cl- gradient, dramatically limiting the benzodiazepine therapeutic window. In turn, enhancing KCC2 activity not only potentiated L838,417-induced analgesia, it rescued its analgesic potential at high doses, revealing a novel strategy for analgesia in pathological pain, by combined targeting of the appropriate GABAAR-subtypes and restoring Cl- homeostasis. Disinhibition in the dorsal horn of the spinal cord may contribute to chronic pain. Here the authors show that, despite a paradoxical increase in α2/α3 subunits of the GABAA receptor in a neuropathic pain model, inhibition eventually fails due to KCC2 hypofunction.

Blocking of sortilin ligand binding prevents neuropathic pain by inhibiting BDNF-induced spinal KCC2 down-regulation., Neuropathic pain is a major incurable clinical problem resulting from peripheral nerve trauma or disease. A central mechanism is the reduced expression of the potassium chloride cotransporter 2 (KCC2) in dorsal horn neurons induced by brain-derived neurotrophic factor (BDNF), causing neuronal disinhibition within spinal nociceptive pathways. Here, we demonstrate how neurotensin receptor 2 (NTSR2) signaling impairs BDNF-induced spinal KCC2 down-regulation, showing how these two pathways converge to control the abnormal sensory response following peripheral nerve injury. We establish how sortilin regulates this convergence by scavenging neurotensin from binding to NTSR2, thus modulating its inhibitory effect on BDNF-mediated mechanical allodynia. Using sortilin-deficient mice or receptor inhibition by antibodies or a small-molecule antagonist, we lastly demonstrate that we are able to fully block BDNF-induced pain and alleviate injury-induced neuropathic pain, validating sortilin as a clinically relevant target.

Neuronal inhibition mediated by GABAA receptors constrains nociceptive processing in the spinal cord, and loss of GABAergic inhibition can produce allodynia and hyperalgesia. Extrasynaptic α5 subunit-containing GABAA receptors (α5GABAA Rs) generate a tonic conductance that inhibits neuronal activity and constrains learning and memory; however, it is unclear whether α5GABAA Rs similarly generate a tonic conductance in the spinal cord dorsal horn to constrain nociception. We assessed the distribution of α5GABAA Rs in the spinal cord dorsal horn by immunohistochemical analysis, and the activity and function of α5GABAA Rs in neurons of the superficial dorsal horn using electrophysiological and behavioral approaches in male, null-mutant mice lacking the GABAA R α5 subunit (Gabra5-/-) and wild-type mice (WT). The expression of α5GABAA Rs in the superficial dorsal horn followed a laminar pattern of distribution, with a higher expression in lamina II than lamina I. Similarly, the tonic GABAA current in lamina II neurons had a larger contribution from α5GABAA Rs than in lamina I, with no significant contribution of these receptors to synaptic GABAA current. In behavioural tests, WT and Gabra5-/- mice exhibited similar acute thermal and mechanical nociception, and similar mechanical sensitization immediately following intraplantar capsaicin or Complete Freund's Adjuvant (CFA). However, Gabra5-/- mice showed prolonged recovery from sensitization in these models, and increased responses in the late phase of the formalin test. Overall, our data suggest that tonically-active α5GABAA Rs in the spinal cord dorsal horn accelerate the resolution of hyperalgesia and may therefore serve as a novel therapeutic target to promote recovery from pathological pain. © 2016 Wiley Periodicals, Inc.

Summary: The behavioral features of neuropathic pain are not sexually dimorphic despite sex differences in the underlying neuroimmune signaling. This raises questions about whether neural processing is comparably altered. Here, we test whether the K+-Cl− co-transporter KCC2, which regulates synaptic inhibition, plays an equally important role in development of neuropathic pain in male and female rodents. Past studies on KCC2 tested only males. We find that inhibiting KCC2 in uninjured animals reproduces behavioral and electrophysiological features of neuropathic pain in both sexes and, consistent with equivalent injury-induced downregulation of KCC2, that counteracting chloride dysregulation reverses injury-induced behavioral and electrophysiological changes in both sexes. These findings demonstrate that KCC2 downregulation contributes equally to pain hypersensitivity in males and females. Whereas diverse (and sexually dimorphic) mechanisms regulate KCC2, regulation of intracellular chloride relies almost exclusively on KCC2. Directly targeting KCC2 thus remains a promising strategy for treatment of neuropathic pain in both sexes. : Neuropathic pain arises in males and females through distinct neuroimmune signaling. Yet the behavioral (or clinical) features of neuropathic pain are not sexually dimorphic. Mapplebeck et al. demonstrate that KCC2—though regulated in multiple, sex-specific ways—is uniquely responsible for the synaptic disinhibition causing neuropathic pain in both sexes. Keywords: K+-Cl− co-transporter KCC2, sex differences, degeneracy, neuropathic pain, disinhibition, GABA, glycine, BDNF

Inhibitory interneurons are an important component of dorsal horn circuitry where they serve to modulate spinal nociception. There is now considerable evidence indicating that reduced inhibition in the spinal dorsal horn contributes to neuropathic pain. A loss of these inhibitory neurons after nerve injury is one of the mechanisms being proposed to account for reduced inhibition; however, this remains controversial. This is in part because previous studies have focused on global measurements of inhibitory neurons without assessing the number of inhibitory synapses. To address this, we conducted a quantitative analysis of the spatial and temporal changes in the number of inhibitory terminals, as detected by glutamic acid decarboxylase 65 (GAD65) immunoreactivity, in the superficial dorsal horn of the spinal cord following a chronic constriction injury (CCI) to the sciatic nerve in rats. Isolectin B4 (IB4) labelling was used to define the location within the dorsal horn directly affected by the injury to the peripheral nerve. The density of GAD65 inhibitory terminals was reduced in lamina I (LI) and lamina II (LII) of the spinal cord after injury. The loss of GAD65 terminals was greatest in LII with the highest drop occurring around 3–4 weeks and a partial recovery by 56 days. The time course of changes in the number of GAD65 terminals correlated well with both the loss of IB4 labeling and with the altered thresholds to mechanical and thermal stimuli. Our detailed analysis of GAD65+ inhibitory terminals clearly revealed that nerve injury induced a transient loss of GAD65 immunoreactive terminals and suggests a potential involvement for these alterations in the development and amelioration of pain behaviour.

Morphine-induced hyperalgesia (MIH) is a severe adverse effect accompanying repeated morphine treatment, causing a paradoxical decrease in nociceptive threshold. Previous reports associated MIH with a decreased expression of the Cl− extruder KCC2 in the superficial dorsal horn (SDH) of the spinal cord, weakening spinal GABAA/glycine-mediated postsynaptic inhibition. Here, we tested whether the administration of small molecules enhancing KCC2, CLP257 and its pro-drug CLP290, may counteract MIH. MIH was typically expressed within 6–8 days of morphine treatment. Morphine-treated rats exhibited decreased withdrawal threshold to mechanical stimulation and increased vocalizing behavior to subcutaneous injections. Chloride extrusion was impaired in SDH neurons measured as a depolarizing shift in EGABA under Cl− load. Delivering CLP257 to spinal cord slices obtained from morphine-treated rats was sufficient to restore Cl− extrusion capacity in SDH neurons. In vivo co-treatment with morphine and oral CLP290 prevented membrane KCC2 downregulation in SDH neurons. Concurrently, co-treatment with CLP290 significantly mitigated MIH and acute administration of CLP257 in established MIH restored normal nociceptive behavior. Our data indicate that enhancing KCC2 activity is a viable therapeutic approach for counteracting MIH. Chloride extrusion enhancers may represent an effective co-adjuvant therapy to improve morphine analgesia by preventing and reversing MIH.

A major unresolved issue in treating pain is the paradoxical hyperalgesia produced by the gold-standard analgesic morphine and other opiates. We found that hyperalgesia-inducing treatment with morphine resulted in downregulation of the K(+)-Cl(-) co-transporter KCC2, impairing Cl(-) homeostasis in rat spinal lamina l neurons. Restoring the anion equilibrium potential reversed the morphine-induced hyperalgesia without affecting tolerance. The hyperalgesia was also reversed by ablating spinal microglia. Morphine hyperalgesia, but not tolerance, required μ opioid receptor-dependent expression of P2X4 receptors (P2X4Rs) in microglia and μ-independent gating of the release of brain-derived neurotrophic factor (BDNF) by P2X4Rs. Blocking BDNF-TrkB signaling preserved Cl(-) homeostasis and reversed the hyperalgesia. Gene-targeted mice in which Bdnf was deleted from microglia did not develop hyperalgesia to morphine. However, neither morphine antinociception nor tolerance was affected in these mice. Our findings dissociate morphine-induced hyperalgesia from tolerance and suggest the microglia-to-neuron P2X4-BDNF-KCC2 pathway as a therapeutic target for preventing hyperalgesia without affecting morphine analgesia.

Central neurons, communicating via excitatory and inhibitory synapses, constitute specific neural circuits that generate and modulate movements. As the main inhibitory transmitter, GABA is a crucial factor in spinal motor networks, which execute movements, and at all supraspinal levels where motor programs are selected and regulated: indeed, in two major control structures, cerebellum and basal ganglia, GABAergic neurons predominate in the internal organization and the output signals sent to upper and lower motor centers. Hence, defective GABAergic inhibition causes a variety of motor disorders, most often characterized by uncontrolled movements (such as chorea) in Huntington’s disease – the result of progressive loss of striatal GABA neurons – or by tremor and ataxia with inaccurate and poorly coordinated movements in cerebellar GABAergic dysfunction. In Parkinson’s disease, the primary defect is a loss of dopaminergic neurons of the substantial nigra, but as these cells normally excite or inhibit different populations of GABA neurons, abnormal striatal output to motor cortex via thalamus results in motor dysfunction, manifested by resting tremor, muscle rigidity, and akinesia. GABAergic deficiency can sometimes be alleviated by drugs that enhance GABA function, but the best hope for the treatment of severe GABA neuronal loss is by intrastriatal implants that release growth factors or by replacement with neural stem cells.