Nerve growth factor (NGF) monoclonal antibodies inhibit chronic pain, yet failed to gain approval due to worsened joint damage in osteoarthritis patients. We report that neuropilin-1 (NRP1) is a coreceptor for NGF and tropomyosin-related kinase A (TrkA) pain signaling. NRP1 was coexpressed with TrkA in human and mouse nociceptors. NRP1 inhibitors suppressed NGF-stimulated excitation of human and mouse nociceptors and NGF-evoked nociception in mice. NRP1 knockdown inhibited NGF/TrkA signaling, whereas NRP1 overexpression enhanced signaling. NGF bound NRP1 with high affinity and interacted with and chaperoned TrkA from the biosynthetic pathway to the plasma membrane and endosomes, enhancing TrkA signaling. Molecular modeling suggested that the C-terminal R/KXXR/K NGF motif interacts with the extracellular “b” NRP1 domain within a plasma membrane NGF/TrkA/NRP1 of 2:2:2 stoichiometry. G α interacting protein C-terminus 1 (GIPC1), which scaffolds NRP1 and TrkA to myosin VI, colocalized in nociceptors with NRP1/TrkA. GIPC1 knockdown abrogated NGF-evoked excitation of nociceptors and pain-like behavior. Thus, NRP1 is a nociceptor-enriched coreceptor that facilitates NGF/TrkA pain signaling. NRP binds NGF and chaperones TrkA to the plasma membrane and signaling endosomes via the GIPC1 adaptor. NRP1 and GIPC1 antagonism in nociceptors offers a long-awaited nonopioid alternative to systemic antibody NGF sequestration for the treatment of chronic pain.

Drugs enhancing the availability of noradrenaline are gaining prominence in the therapy of chronic neuropathic pain. However, underlying mechanisms are not well understood, and research has thus far focused on α2-adrenergic receptors and neuronal excitability. Adrenergic receptors are also expressed on glial cells, but their roles toward antinociception are not well deciphered. This study addresses the contribution of β2-adrenergic receptors (β2-ARs) to the therapeutic modulation of neuropathic pain in mice. We report that selective activation of β2-ARs with Formoterol inhibits pro-inflammatory signaling in microglia ex vivo and nerve injury-induced structural remodeling and functional activation of microglia in vivo. Systemic delivery of Formoterol inhibits behaviors related to neuropathic pain, such as mechanical hypersensitivity, cold allodynia as well as the aversive component of pain, and reverses chronically established neuropathic pain. Using conditional gene targeting for microglia-specific deletion of β2-ARs, we demonstrate that the anti-allodynic effects of Formoterol are primarily mediated by microglia. Although Formoterol also reduces astrogliosis at late stages of neuropathic pain, these functions are unrelated to β2-AR signaling in microglia. Our results underline the value of developing microglial β2-AR agonists for relief from neuropathic pain and clarify mechanistic underpinnings.

Chronic pain is a debilitating condition that influences the social, economic, and psychological aspects of patients’ lives. Hence, the need for better treatment is drawing extensive interest from the research community. Developmental molecules such as Wnt, ephrins, and semaphorins are acknowledged as central players in the proper growth of a biological system. Their receptors and ligands are expressed in a wide variety in both neurons and glial cells, which are implicated in pain development, maintenance, and resolution. Thereby, it is not surprising that the impairment of those pathways affects the activities and functions of the entire cell. Evidence indicates aberrant activation of their pathways in the nervous system in rodent models of chronic pain. In those conditions, Wnt, ephrin, and semaphorin signaling participate in enhancing neuronal excitability, peripheral sensitization, synaptic plasticity, and the production and release of inflammatory cytokines. This review summarizes the current knowledge on three main developmental pathways and their mechanisms linked with the pathogenesis and progression of pain, considering their impacts on neuronal and glial cells in experimental animal models. Elucidations of the downstream pathways may provide a new mechanism for the involvement of Wnt, ephrin, and semaphorin pathways in pain chronicity.

GABAergic signaling and GABA_B receptors play crucial roles in regulating the physiology of oligodendrocyte-lineage cells, including their proliferation, differentiation, and myelination. Therefore, they are promising targets for studying how spinal oligodendrocyte precursor cells (OPCs) respond to injuries and neurodegenerative diseases like multiple sclerosis. Taking advantage of the temporally controlled and cell-specific genetic downregulation of GABA_B receptors from OPCs, our investigation addresses their specific influence on OPC behavior in the gray and white matter of the mouse spinal cord. Our results show that, while GABA_B receptors do not significantly alter spinal cord myelination under physiological conditions, they distinctly regulate the OPC differentiation and Ca²⁺ signaling. In addition, we investigate the impact of OPC-GABA_B receptors in two models of toxic demyelination, namely, the cuprizone and the lysolecithin models. The genetic downregulation of OPC-GABA_B receptors protects against demyelination and oligodendrocyte loss. Additionally, we observe the enhanced resilience to cuprizone-induced pathological alterations in OPC Ca²⁺ signaling. Our results provide valuable insights into the potential therapeutic implications of manipulating GABA_B receptors in spinal cord OPCs and deepen our understanding of the interplay between GABAergic signaling and spinal cord OPCs, providing a basis for future research.

Several forms of chronic pain do not respond to the conventional analgesics, such as opioids, but can be treated with antidepressants, such as serotonin and noradrenalin reuptake inhibitors (SNRIs). Recent studies indicate that noradrenalin signalling is a key target for SNRI-induced analgesia in neuropathic pain. SNRIs inhibit chronic pain by blocking reuptake of noradrenalin and subsequent activation of adrenergic receptors on neurons in the dorsal horn of the spinal cord. However, in the nervous system, various subtypes of adrenergic receptors are highly expressed by astrocytes and microglial cells. Activation of these receptors on astrocytes engages complex intracellular signalling pathways and prevents inflammatory changes of microglia, which in turn can affect neuronal activity. Hence, SNRIs-induced modulations of the glial cell physiology can impact neural circuit functions and pain perception. In this review, we summarize our current knowledge on the impact of SNRIs on glial cells and in modulating chronic pain in experimental animal models.

GABAergic signaling and GABA _B receptors play crucial roles in regulating the physiology of oligodendrocyte-lineage cells, including their proliferation, differentiation, and myelination. Therefore, they are promising targets for studying how spinal oligodendrocyte precursor cells (OPCs) respond to injuries and neurodegenerative diseases like multiple sclerosis. Taking advantage of the temporally controlled and cell-specific genetic removal of GABA _B receptors from OPCs, our investigation addresses their specific influence on OPC behavior in the gray and white matter of the mouse spinal cord. Our results show that while GABA _B receptors do not significantly alter OPC cell proliferation and differentiation under physiological conditions, they distinctly regulate the Ca ²⁺ signaling of OPCs. In addition, we investigate the impact of OPC-GABA _B receptors in two models of toxic demyelination, namely the cuprizone and the lysolecithin models. The genetic removal of OPC-GABA _B receptors protects against demyelination and oligodendrocyte loss. Additionally, we observe enhanced resilience to cuprizone-induced pathological alterations in OPC Ca ²⁺ signaling. Our results provide valuable insights into the potential therapeutic implications of manipulating GABA _B receptors in spinal cord OPCs and deepen our understanding of the interplay between GABAergic signaling and spinal cord OPCs, providing a basis for future research.