Divergent sex-specific pannexin-1 mechanisms in microglia and T cells underlie neuropathic pain

A recent history of immune cell sex differences in the peripheral nervous system in persistent pain states

Pain is entwined with inflammation, and biological sex often influences mechanisms of the immune system. Due to possible differences in inflammatory mechanisms, women are predisposed to autoimmune diseases and chronic pain. Despite sex as a critical variable in clinical cases of autoimmune conditions and its pain comorbidities, fundamental investigations have long underrepresented female subjects in their studies. Fundamental research in the 2010s, however, identified a binary sex specific mechanism for pain in rodents: male pain is microglia-driven while female pain is T cell-driven. Since then, studies have expanded in neuro-immunology to indicate that the sex differences and immune cells involved in these processes take on more elaborate roles when expanded to other causal modalities and anatomical levels of neuropathic and inflammatory pain. In this mini-review, we highlight updated roles for macrophages, T cells, and B cells in the peripheral nervous system during persistent pain conditions: neuropathic pain and inflammatory pain. We discuss sex similarities and sex differences in these cell types. By parsing out the sex specific roles of immune cells in persistent pain states, we may be better positioned to find immune-based therapies that can effectively target chronic pain in sex-biased autoimmune conditions.

Fructose-1,6-Bisphosphate Reduces Chronic Constriction Injury Neuropathic Pain in Mice by Targeting Dorsal Root Ganglia Nociceptive Neuron Activation

Background/Objectives: Fructose-1,6-bisphosphate (FBP) is an intermediate product of the glycolytic pathway with analgesic effect in acute inflammatory pain model via the production of adenosine. However, whether FBP is active in neuropathic pain is unknown. Therefore, we reason that it would be suitable to investigate the analgesic effect and mechanism of action of FBP in a model of chronic constriction injury (CCI) of sciatic nerve-induced neuropathic pain in mice. Methods: After CCI induction, mice received FBP, adenosine, A1 and/or A2A receptor antagonists, and/or inhibitors of the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP)/protein kinase G (PKG)/ATP sensitive K channels (KATP) signaling pathway. Results: FBP (up to 85%) and adenosine (up to 84%) inhibited the mechanical hyperalgesia (electronic aesthesiometer) induced by CCI with similar profiles. FBP analgesia was dependent on adenosine because adenosine A1 and A2A receptors antagonists diminished FPB activity (100% and 79%, respectively). FBP analgesia was also dependent on activating the NO/cGMP/PKG/KATP signaling pathway. Furthermore, FBP treatment increased the production of NO in cultured dorsal root ganglia (DRG) neurons (100% increase), whereas neuronal nitric oxide synthase (nNOS) inhibition decreased (up to 70%) the analgesic effect of FBP. We also observed that FBP reduced the calcium levels of transient receptor potential ankyrin 1 (TRPA1)+ DRG neurons (85%) and paw-flinching triggered by TRPA1 activation (38%). Conclusions: FBP reduced neuropathic pain by reducing DRG neuron activation. The mechanisms involved the activation of adenosine A1 and A2A receptors to trigger the analgesic NO/cGMP/PKG/KATP signaling pathway and reducing TRPA1+ DRG neuron activity.

A look into the future: Your biological sex may guide chronic pain treatment

Neuropathic pain constitutes a clinical problem more prevalent among women. In this issue of Neuron, Fan et al.1 identify distinct sex-dependent pathways driving nociception. Importantly, T cells release the adipokine leptin in female but not male rodents, offering a novel approach for pain relief in women.