Phenotyping the chronobiology of microglia

Abstract

Chronic neuropathic pain affects 7-10% of the population and is caused by damage to the somatosensory nervous system that persist after the initial injury has healed. The development of neuropathic pain is influenced by interactions between neurons and immune cells. Microglia are a component of the innate immune system and help in maintaining homeostasis in the central nervous system (CNS). Following injury, microglia alter their morphology, receptor expression and proliferate near the site of the damaged axons. Previous work has identified a rhythmic pattern to certain microglia functions indicating the presence of an intrinsic clock in these cells. Rhythmicity in neuropathic pain has also been observed by our group in the human population and in the mechanical sensitivity of male mice after spared nerve injury (SNI). We therefore sought to characterize the rhythmic expression of microglia receptors in neuropathic pain and determine if microglia present sex-specific rhythms. Rhythmic expression of microglia surface receptors in the brain and spinal cord (SC) of naïve male and female mice were characterized using flow cytometry. Naïve mice showed sex-dependent rhythmicity in the canonical receptors CD11b and CX3CR1 and the state dependent receptors CD86 and CD206. Using SNI as a model of neuropathic pain, microglia from males and females lost rhythmicity of their canonical receptors but showed sex-dependent rhythms in the receptors CD86 and CD206, unique from naïve mice. Lastly, the effects of disrupting the circadian clock by removing the gene Bmal1 from microglia were analyzed using the von Frey behavioural assay to observe changes in the oscillations of mechanical allodynia in male mice post-SNI. Male mice showed no rhythmic sensitivity after SNI following the excision of the Bmal1 gene from microglia. While rhythmic expression of microglia receptors was evident in both the naïve and neuropathic pain model, more work is needed to elucidate the underlying functions these rhythms regulate and determine how they relate to the modulation of neuropathic pain. This investigation enhances the knowledge surrounding sex-dependent microglia receptor rhythms and provides new avenues for therapeutic development in neuropathic pain.