Abstract
Purpose :
Thalamocortical (TC) relay neurons in the dorsal lateral geniculate nucleus (dLGN) of the thalamus receive convergent retinogeniculate (RG) synaptic input from retinal ganglion cells (RGCs) and convey signals to the primary visual cortex. The goal of this study was to determine how RGC synapses to TC neurons are altered in the DBA/2J (D2) mouse model of glaucoma.
Methods :
Experiments were performed with male and female D2 mice at 4-month (m), 6m, 9m, and 12m of age. Controls were the strain-match DBA/2J-gpnmb+ mice. We measured RG synaptic function using optic tract stimulation in parasagittal brain slices while performing whole-cell voltage clamp recordings from post-synaptic TC neurons. Anterograde RGC axon transport was measured as the fraction of dLGN area labeled by a unilateral intravitreal injection of cholera toxin-B-594 (CTB) tracer while microglia activation and RGC synaptic terminal loss was quantified using immunofluorescence staining for Iba1 and vGlut2, respectively. Data were analyzed with a one-way ANOVA and Dunnett’s post-hoc multiple comparison tests. Nested statistical tests were used to avoid pitfalls from pseudoreplication. Data are mean±SEM.
Results :
CTB transport was reduced in 9m D2 mice (29±10% dLGN labeled, n=7 mice) compared to controls (81±1%, n =14, p<0.0001). Microglia showed signs of activation at 9m and 12m including increased cell number and decreased total process length and process endpoints per cell. vGlut2 puncta were reduced at 9m and 12m (p=0.02 and p<0.0001, respectively). vGlut2 density weakly correlated with microglia activation (R2=0.3; p=0.0004 for microglia branch length). There was a statistically significant reduction in the maximum synaptic current amplitude (EPSCmax) detected in slices from 12m D2 mice (822±189 pA; n=31 cells, 9 mice) compared to controls (2269±226 pA; n=35 cells, 12 mice; p=0.0009). Reducing stimulus strength to isolate inputs from single RGC axons showed that single-fiber EPSC (EPSCsf) was similar between groups (p>0.05). EPSCsf/EPSCmax ratio was higher in slices from 12m D2 mice (0.42±0.07) compared to controls (0.15±0.03; p=0.004), indicative of reduced numbers of RGC inputs to each TC neuron.
Conclusions :
RGC synaptic loss is functionally detectable in older DBA/2J mice while microglial activation might contribute to pathological synaptic pruning.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.