Fig. 3
Hebbian LTP depends on post-synaptic AP bursts, and can be induced only in BS. A Left, Representative voltage responses in RS (black) and BS (red) to HFSH-2 which elicited 3 APs. HFSH-2 is defined as high intensity HFS that was applied following HFSL. Right upper, The boxed traces are superimposed for comparison at expanded time scale. Right lower, Initial AP frequency (Finit) of each group (RS/BS, n = 18/21; **p < 0.01). B Relationship between Finit of APs evoked by somatic current injection and that by synaptic stimulation. Two parameters were significantly correlated in BS (r = 0.55, **p < 0.01) but not in RS (r = 0.11). Black bold line, linear regression in BS. r, Pearson’s correlation coefficient. C Time course of normalized EPSP changes induced by applying two sequential HFS (HFSL and HFSH) in RS (black, n = 5) and BS (red, n = 5). Note that LTPAP was induced on top of LTPsub in BS, not in RS. D Left, Time course before and after HFSH-2. Right, Magnitude of LTPAP in RS- and BS-GCs (RS: − 4.2 ± 7.0%, n = 12; BS, 44.0 ± 4.8%, n = 7, ***p < 0.001). E LTP magnitude as a function of synaptically evoked Finit. Finit was correlated to LTP magnitude (r = 0.50, *p < 0.05). Open circles, individual data; closed circles, averaged value for each group. Black line, linear regression line. F Pairing protocol for LTPAP induction. It consists of subthreshold HFS (HFSL-2) and post-synaptic 3 APs evoked by somatic pulses (2 ms, 3 nA at 100 Hz). G Left, Time course of normalized EPSP before and after a pairing protocol. Right, Pairing protocol-induced LTPAP in RS- and BS-GCs (RS/BS, n = 8/6). H LTP was not induced in the presence of intracellular solution containing BAPTA (10 mM, pink, n = 7). The pairing protocol-induced LTP time courses in RS and BS under control conditions shown in G were merged, and superimposed in gray (n = 14). I Left, Representative voltage traces evoked by HFSH-2 in the presence of APV (brown, 50 μM). No AP burst was elicited in the presence of APV. Right, Time course of normalized EPSP before and after HFSH-2. J Similar as in I, but a pairing protocol was applied instead of HFSH-2. The control trace (gray) was reproduced from panel H for comparison. K Early (LTP5, open circle) and late phase (LTP30, closed circle) LTP induced by a pairing protocol with and without APV (Control, n = 14; APV, n = 11). Shades and error bars, S.E.M. *p < 0.05. **p < 0.01. ***p < 0.001. n.s., not significant (p > 0.05)