Microglia in the Rostral Ventromedial Medulla Mediate Synaptic Pruning via the C1q/C3-CR3 Signaling Pathway-A Mechanism for the Chronic Orofacial Pain.

Abstract

The mechanism of chronic orofacial pain was investigated by examining the interaction between activated microglia, C1q, and neurons in the rostral ventromedial medulla (RVM) of rats with orofacial pain caused by temporomandibular joint injection of complete Freund's adjuvant (CFA). The results demonstrated that the pain threshold in the CFA group exhibited a continuous decline, reaching its lowest point on the third day. During the modeling process, administered daily stereotactic injections of ANX-005 and minocycline into the RVM, which resulted in a notable recovery in the rats' pain threshold and a significant increase in C1q/C3 and microglia in the RVM of CFA rats. The application of ANX-005 or minocycline resulted in a reduction in the expression of C1q/C3 and microglia. Notably, the expression of excitatory presynaptic membrane markers reduced, and the length and density of dendritic spines decreased on neurons in the RVM. Additionally, C1q was abundantly localized on excitatory presynaptic membranes and expressed in microglial lysosomes. Treatment with ANX-005 or minocycline resulted in a reduced number of immunofluorescence colocalizations and an elevated dendritic spine density. These findings indicate that initial orofacial pain induced by CFA and microglia in the RVM are involved in the pruning of excitatory presynaptic membranes through the complement C1q/C3-CR3 signaling pathway. This process results in a reduction in the proportion of excitatory synapses and a disruption in the physiological balance between RVM descending facilitation and descending inhibition. This leads to the predominance of descending facilitation in pain transmission in the RVM, which in turn facilitates the chronification of orofacial pain.