Figure 5. DG-restricted YFP-tomosyn1-expression impairs basal synaptic transmission and paired-pulse facilitation but not frequency facilitation or LTP at MF-CA3 synapses.
Basal synaptic transmission and plasticity in the MF-CA3 synapses were measured in acute hippocampal slices derived from YFP-tomosyn1- and YFP-overexpressing mice. (a) Representative hippocampal slice expressing lentivirus with schematic representation of the stimulating and recording electrodes locations. (b) fEPSP slopes in an input-output curve versus the fiber volley of the MF-CA3 synapses in response to increasing stimulation intensities as was measured by extracellular field recordings. The curve reveals that basal synaptic transmission was significantly lower (P<0.0003) in slices derived from YFP-tomosyn1-overexpressing (n=6 slices from five different mice) vs. YFP-overexpressing mice (n=6 slices from five different mice). Paired-pulse facilitation (PPF) is impaired by YFP-tomosyn1-overexpression in (c) acute hippocampal slices derived from YFP-tomosyn1-overexpressing mice (n=8 slices for YFP and n=6 for YFP-tomosyn1, both from four different mice), and in (d) autapses from primary hippocampal neurons growing on microislands (YFP-tomosyn1, n=11 cells; YFP, n=9 cells). Bar graph shows the paired-pulse response ratio (2nd EPSP/1st EPSP) at 40 ms paired-pulse intervals. (e) No difference in frequency facilitation response was measured by extracellular field recordings between YFP-tomosyn1- and YFP-overexpressing mice (n=8 slices for YFP and n=6 for YFP-tomosyn1, both from four different mice). EPSP amplitude was plotted against stimulation frequencies. (f) No difference in long-term potentiation was measured by extracellular field recordings (n=8 slices for YFP and n=6 for YFP-tomosyn1, both from four different mice) between YFP-tomosyn1- and YFP-overexpressing mice. To confirm that the measured responses resulted from release from MF terminals, we blocked release from the MF terminals by application of DCG-IV (a selective agonist of presynaptic mGluR receptors on the MF terminals) at the end of each experiment. DCG-IV caused gradual and complete blockage of the responses. (g) Sample traces of PPF (left column) and LTP (right column) from YFP-overexpressing mice (upper row) and YFP-tomosyn1-overexpressing mice (lower row). PPF; sample traces showing average of 15 traces; black, 0.05Hz; blue, 0.2Hz; red, 0.5Hz. LTP; EPSP amplitude showing the time course of potentiation after tetanic stimulation; red, pre-LTP; blue, post-LTP, black, after DCG-IV application. GrDG; granular cells of the dentate gyrus, SLu; stratum lucidum. Values are means±s.e. **P=0.02, ***P<0.0003