Figure 3C shows calcium responses of all five responsive axon terminals in a single retina, and four of them exhibited enhanced responses after TBS. The results from all experiments performed in different retinae showed that TBS induced persistent enhancement of calcium responses of BC axon terminals for more than 30 min (Figure 3D). The mean amplitude of calcium responses during
10–30 min after TBS was 169% ± 16% (n = 20; p = 0.0001) of the mean control value observed before TBS. However, TBS could not induce significant changes in calcium responses of BC axon terminals in zebrafish at 15–20 dpf (Figure S2B), consistent with previous electrophysiological results (Figure S2A). Because presynaptic calcium changes can efficiently lead to changes in neurotransmitter release (Neher and Sakaba, 2008), our results suggest that presynaptic changes in OSI-744 datasheet neurotransmitter release may be involved in the expression of LTP at BC-RGC synapses. To further examine the presynaptic involvement in the LTP expression, we first examined changes in mEPSCs of RGCs before and after LTP induction INCB018424 order by TBS when TTX (1 μM) was bath applied. As shown by the example in Figure 4A,
an increase in the frequency but not the amplitude of mEPSCs was observed after the TBS, which enhanced the e-EPSCs of the same RGC (compare the top and bottom traces in Figure 4A, right). In total the frequencies of mEPSCs were 5.3 ± 0.7 and 7.3 ± 0.9 Hz before and 10–40 min after LTP induction (n = 10; p = 0.01; Figure 4B), respectively. Meanwhile, the amplitudes of mEPSCs were heptaminol 4.8 ± 0.7 and 5.1 ± 0.6 pA before and 10–40 min after LTP induction (n = 10; p = 0.1; Figure 4C), respectively. Consistently, similar observations were found for spontaneous EPSCs (sEPSCs) (Figure S5). We then measured the PPR and CV of RGC e-EPSCs, two parameters mainly
reflecting properties of presynaptic neurotransmitter release (Faber and Korn, 1991; Singer and Diamond, 2006; Zucker, 1989), before and after LTP induction. As shown by the example in Figure 4D, we found that the development of LTP was accompanied by a significant decrease of the PPR, which was measured at an interpulse interval of 1 s (von Gersdorff et al., 1998). In total the PPRs of RGC e-EPSCs were 0.85 ± 0.12 and 0.62 ± 0.07 before and 10–40 min after LTP induction by TBS (n = 7; p = 0.007; Figures 4E and S6), respectively. Furthermore, the CV of RGC e-EPSCs also showed significant decrease after LTP induction (Before TBS, 0.22 ± 0.02; After TBS, 0.17 ± 0.01; n = 18; p = 0.005; Figure 4F). Taken together, these findings imply that presynaptic change in the probability of neurotransmitter release is involved in the expression of LTP at BC-RGC synapses. The induction of LTP at BC-RGC synapses requires postsynaptic NMDAR activation, whereas its expression involves presynaptic changes.