Figure 4. mGluR2 blockade causes an enhanced but delayed preferred-direction inhibition onto DSGCs.

A. A model of mGluR2-dependent dendritic compartmentalization in SACs. Left: During endogenous mGluR2 signaling, SAC dendritic branches are more isolated from each other. A bar moving in the preferred direction of the DSGC (leftward) triggers minimal GABA release (red dots) from the centripetally activated sector A of the SAC. Subsequent activation of sector B in the centrifugal direction does not propagate efficiently to sector A to trigger GABA release. Right: During mGluR2 blockade, electrotonic isolation between SAC branches is reduced. Therefore, the strong centrifugal response of sector B propagates more efficiently to sector A. This leads to enhanced dendritic activation of sector A and more GABA release onto the DSGC during motion in the DSGC’s preferred direction.

B – D. DSGC IPSCs before and after adding LY341495 at the bar speed of 440 μm/s.

B. Example On components of IPSC traces of DSGCs in a wild type mouse evoked by a dark bar moving in the preferred and null directions at a bar speed of 440 μm/s before (black) and after (red) adding LY341495. Dark traces represent mean values and light traces represent individual trials. Also see Figures S5B–S5D for IPSC data evoked by a bright bar.

C. Summary of charge transfer of IPSCs in the preferred and null directions at the speed of 440 μm/s in wild type mice. Preferred: baseline, 121.8 ± 21.6 pA·s; LY341495, 227.5 ± 36.0 pA·s; **p = 0.002. Null: baseline, 275.9 ± 48.9 pA·s; LY341495, 325.7 ± 60.8 pA·s; p = 0.23. n = 12 cells from 5 mice. Also see Figures S5B–S5D.

D. As in C, summary of DSI values for IPSCs in wild type mice at the bar speed of 440 μm/s. Baseline, 0.37 ± 0.03; LY341495, 0.17 ± 0.04; *** p = 0.0001.

E – G. DSGC IPSCs before and after adding LY341495 at the bar speed of 1100 μm/s.

E. As in B, example IPSC traces of DSGCs at the bar speed of 1100 μm/s.

F. As in C, summary of charge transfer of IPSCs at the bar speed of 1100 μm/s. Preferred: baseline, 118.0 ± 15.8 pA·s; LY341495, 204.6 ± 29.9 pA·s; *p = 0.01. Null: baseline, 236.9 ± 26.8 pA·s; LY341495, 274.1 ± 41.1 pA·s; p = 0.13. n = 8 cells from 5 mice.

G. As in D, summary of DSI values for IPSCs at the bar speed of 1100 μm/s. Baseline, 0.33 ± 0.05; LY341495, 0.13 ± 0.04; ** p = 0.008.

H. Boxplot of the temporal delay of IPSC peak due to LY341495 application in wild type mice at the bar speed of 440 um/s and 1100 um/s. Mean ± SEM: 440 um/s, 289 ± 39 ms; 1100 um/s, 147 ± 20 ms; **p = 0.005. n = 12 cells from 7 mice. Also see Figure S5A.

Also see Figure S6 for the effect of LY341495 on DSGCs from mGluR2 KO mice and alpha RGCs.