Resident macrophages from dorsal root ganglia are important for the development of traumatic-induced neuropathic pain. In the first 5–7 days after a traumatic sciatic nerve injury (i.e., spinal nerve ligation (SNL), spared nerve injury (SNI), sciatic nerve transection or sciatic nerve ligation and transection), Ionized binding adapter protein 1 (Iba1) (+) resident macrophages cluster around dorsal root ganglia neurons, possibly contributing to nerve injury-induced hypersensitivity. Since infiltrating macrophages gradually recruited to the lesion site peak at about 7 days, the first few days post-lesion offer a window of opportunity when the contribution of Iba1 (+) resident macrophages to neuropathic pain pathogenesis could be investigated. Iba1 is an actin cross-linking cytoskeleton protein, specifically located only in macrophages and microglia. In this study, we explored the contribution of rat Iba1 (+) macrophages in SNL-induced neuropathic pain by using intra-ganglionic injections of naked Iba1-siRNA, delivered at the time the lesion occurred. The results show that 5 days after Iba1 silencing, Iba1 (+) resident macrophages are switched from an M1 (pro-inflammatory) phenotype to an M2 (anti-inflammatory) phenotype, which was confirmed by a significant decrease of M1 markers (CD32 and CD86), a significant increase of M2 markers (CD163 and Arginase-1), a reduced secretion of pro-inflammatory cytokines (IL-6, TNF-α and IL-1β) and an increased release of pro-regenerative factors (BDNF, NGF and NT-3) which initiated the regrowth of adult DRG neurites and reduced SNL-induced neuropathic pain. Our data show for the first time, that it is possible to induce macrophages towards an anti-inflammatory phenotype by interacting with their cytoskeleton.

(1) Background: As membrane channels contribute to different cell functions, understanding the underlying mechanisms becomes extremely important. A large number of neuronal channels have been investigated, however, less studied are the channels expressed in the glia population, particularly in microglia. In the present study, we focused on the function of the Kv1.3, Kv1.5 and Kir2.1 potassium channels expressed in both BV2 cells and primary microglia cultures, which may impact the cellular migration process. (2) Methods: Using an immunocytochemical approach, we were able to show the presence of the investigated channels in BV2 microglial cells, record their currents using a patch clamp and their role in cell migration using the scratch assay. The migration of the primary microglial cells in culture was assessed using cell culture inserts. (3) Results: By blocking each potassium channel, we showed that Kv1.3 and Kir2.1 but not Kv1.5 are essential for BV2 cell migration. Further, primary microglial cultures were obtained from a line of transgenic CX3CR1-eGFP mice that express fluorescent labeled microglia. The mice were subjected to a spared nerve injury model of pain and we found that microglia motility in an 8 µm insert was reduced 2 days after spared nerve injury (SNI) compared with sham conditions. Additional investigations showed a further impact on cell motility by specifically blocking Kv1.3 and Kir2.1 but not Kv1.5; (4) Conclusions: Our study highlights the importance of the Kv1.3 and Kir2.1 but not Kv1.5 potassium channels on microglia migration both in BV2 and primary cell cultures.

Traumatic peripheral neuropathic pain is a complex syndrome caused by a primary lesion or dysfunction of the peripheral nervous system. Secondary to the lesion, resident or infiltrating macrophages proliferate and initiate a cross-talk with the sensory neurons, at the level of peripheral nerves and sensory ganglia. The neuron–macrophage interaction, which starts very early after the lesion, is very important for promoting pain development and for initiating changes that will facilitate the chronicization of pain, but it also has the potential to facilitate the resolution of injury-induced changes and, consequently, promote the reduction of pain. This review is an overview of the unique characteristics of nerve-associated macrophages in the peripheral nerves and sensory ganglia and of the molecules and signaling pathways involved in the neuro-immune cross-talk after a traumatic lesion, with the final aim of better understanding how the balance between pro- and anti-nociceptive dialogue between neurons and macrophages may be modulated for new therapeutic approaches.