Figure 3. ArcKR expression prevents GI-mGluR-mediated priming of long-term potentiation (LTP)
A, Graph plotting mean normalised (to the baseline) fEPSP slope against time for WT (n = 10 slices, 5 mice) and ArcKR mice (n = 12 slices, 6 mice). After a 20-minute baseline, LTP was induced with high frequency stimulation (1s, 100 Hz). Inset, example fEPSP waveforms before and after LTP induction from WT and ArcKR mice. B, Bar chart plotting the mean potentiation of fEPSP slope (between 55-60 minutes post LTP induction). There was no significant difference (p = 0.3791, Z =-0.89, U = 46, Mann Whitney) in the amplitude of long-term potentiation between WT (mean potentiation 60.2 ± 17.9 %) and ArcKR slices (74.0 ± 14.9 %). C, Graph plotting mean normalised (to the baseline) fEPSP slope against time for WT (n = 7 slices, 4 mice) and ArcKR mice (n = 7 slices, 5 mice). Following a 20-minute baseline, a low concentration of DHPG (20 µM) was applied for 10 minutes to activate GI-mGluRs (mean peak inhibition in WT 41.9 ± 4.8 %; ArcKR 42.8 ± 4.7 %). Following a 20-minute wash, LTP was induced by high frequency stimulation (1s, 100 Hz). Inset, example fEPSP waveforms before and after LTP induction (average of waveforms at 55-60 minutes) from WT and ArcKR mice. D, Bar chart plotting mean potentiation (55-60 minutes post LTP induction). There was a significant difference (p = 0.022, U =37, Z = 2.2) in the amplitude of LTD following GI-mGluR priming in WT (mean potentiation 76.6 ± 8.6 %) compared to ArcKR slices (mean potentiation 36.6 ± 8.3 %). Data points are the mean potentiation from individual experiments. E, Graph plotting mean normalised (to the baseline) fEPSP slope against time for WT (n = 7 slices, 4mice) and ArcKR mice (n = 7 slices, mice) illustrating the actions of 20 µM DHPG. F, Bar chart plotting the mean inhibition after 20 minutes following DHPG wash. There was no significant difference in the amount of inhibition in WT compared to ArcKR mice (P = 0.069, Z = -1.78, U =14.5, Mann Whitney). The points (B, D and F) are the means from individual experiments. G, Western blot showing Arc protein expression in hippocampal lysates obtained from WT (n = 4) and ArcKR (n = 4) mice. Slices were incubated in control (vehicle), 20 µM DHPG and 100 µM DHPG for 10 min. H, Bar chart analysis show a significant increase in Arc expression after DHPG (100 µM) exposure compared to control in WT (*p=0.012) and ArcKR (**p=0.008), but not after 20 µM DHPG exposure (WT: p=0.26 and ArcKR: p=0.75). GAPDH was used as loading control. The data points are the Arc/GAPDH ratios from individual experiments. Error bars indicate ± S.E.M. Statistical comparisons were performed with one-way analysis of variance (ANOVA) followed by Tukey’s multiple comparisons.