CCR2⁺ cells infiltrate the ventral horn of the spinal cord after nerve injury, and CCR2 activation is necessary for VGLUT1 losses from dendrites. A–C, 2D projections of confocal image stacks (50-μm-thick section) showing CCR2⁺ cells (white cells) infiltrated in the spinal cord 3 or 21 d after either tibial or sciatic nerve transections. These cells are located predominantly in the ventral horn, close to retrogradely labeled LG MNs (blue). The area highlighted in the rectangle is shown at higher magnification below. D, Time course of CCR2⁺ cell infiltration after tibial (red line) or sciatic (blue line) nerve injury. Data are represented as average number of total RFP⁺ cells in the ventral horn of 50-μm-thick spinal cord sections. Each data point represents the average from all L4-L5 sections containing Fast Blue MNs in each mouse (average of 23.2 sections per animal). Lines indicate the average of 3 or 4 mice per time point. Error bars indicate SE. Significant increases in CCR2⁺ cells compared with control are found from 3 d to 8 weeks after injury for both injuries (black asterisks), whereas significant differences between STL and TTL are found at 14 d, 21 d, and 8 weeks after injury (gray asterisks): *p < 0.05; ***p < 0.001. E, Time course of CX3CR1-GFP⁺ cell activation (green lines), infiltration of CCR2-RFP⁺ cells (red lines), and overall VGLUT1 loss after sciatic or tibial nerve injury (blue lines) normalized to maximum change for each condition and measurement. The time course of VGLUT1 loss overlaps with CCR2⁺ cell entry; both occur at a delay after the onset of microglia activation. F, VGLUT1 synapses on LG reconstructed MNs after injury in dual-heterozygote Cx3cr1^GFP/+::Ccr2^RFP/+ animals compared with CCR2 KOs. G, Quantification of VGLUT1 synapses on LG MN dendrites. Dual-heterozygote animals, but not CCR2 KOs, show a significant decrease in dendritic synapses (***p < 0.001), each compared with their respective sham controls (n = 6 animals per group; each animal estimate is the average of 6 MNs per animal; error bars indicate SE). H, VGLUT1 densities on cell bodies. There was only partial preservation in CCR2 KO animals: *p < 0.05; ***p < 0.001. Scale bars: A1, A2, 100 μm; B1, C1, B2, C2, respectively, at the same, F, 50 μm.