Brain-derived neurotrophic factor inhibits calcium channel activation, exocytosis, and endocytosis at a central nerve terminal

Abstract

Brain-derived neurotrophic factor (BDNF) is a neurotrophin that regulates synaptic function and plasticity and plays important roles in neuronal development, survival, and brain disorders. Despite such diverse and important roles, how BDNF, or more generally speaking, neurotrophins affect synapses, particularly nerve terminals, remains unclear. By measuring calcium currents and membrane capacitance during depolarization at a large mammalian central nerve terminal, the rat calyx of Held, we report for the first time that BDNF slows down calcium channel activation, including P/Q-type channels, and inhibits exocytosis induced by brief depolarization or single action potentials, inhibits slow and rapid endocytosis, and inhibits vesicle mobilization to the readily releasable pool. These presynaptic mechanisms may contribute to the important roles of BDNF in regulating synapses and neuronal circuits and suggest that regulation of presynaptic calcium channels, exocytosis, and endocytosis are potential mechanisms by which neurotrophins achieve diverse neuronal functions.

Keywords: BDNF; calcium channels; calyx of Held; endocytosis; exocytosis.

Figure 1.

BDNF slows down I_Ca activation kinetics. A, B, Antibody staining of vGlut1 (red) and β-galactosidase (green) to label the calyx and BDNF, respectively, in Bdnf^LacZ/+ (A) and TrkB in TrkB^LacZ/+ (B) knock-in mice at P14. Scale bar, 10 μm. C, Top, Averaged traces of I_Ca induced by depol_{20 ms} in control (n = 12; black) and in the presence of BDNF (100 ng/ml, n = 11; red) in P8–P10 rat calyces (mean ± SEM; SEM is plotted every 2 ms). Bottom, Same traces as in the top but showing the rise phase on a larger timescale. D, I_Ca 20–80% rise time, I_Ca amplitude, and QI_Ca (mean ± SEM) induced by depol_{20 ms} in control (Ctrl; n = 12) and in the presence of BDNF (100 ng/ml, n = 11) in P8–P10 calyces. **p < 0.01. E, F, Similar plots as in C and D, respectively, but from P13–P14 calyces (control, n = 12; BDNF, n = 10). G, Sampled I_Ca traces in response to 200 ms depolarization from −80 to −40 (gray), −10 (blue), 0 (pink), 10 (green), and 40 (black) mV in control and BDNF-treated calyces at P9. H, I–V relationship in control and BDNF-treated P8–P10 calyces (n = 3 for each data point).

Figure 2.

BDNF reduces the release probability of vesicles in the RRP by reducing QI_Ca. A, B, Sampled I_Ca and capacitance (C_m) changes induced by 1 (pink), 2 (green), 5 (blue), and 30 (black) ms depolarization from a P9 calyx in the absence (A, Control) or presence (B) of BDNF. Capacitance within 100 ms after depolarization was not shown to avoid artifacts (Yamashita et al., 2005). C, D, ΔC_m (C) and QI_Ca (D) plotted versus the depolarization duration in the absence (control, n = 9; black) or presence (n = 6, mean ± SEM; red) of BDNF. *p < 0.05, **p < 0.01. The dotted box on the left is enlarged on the right. E–H, Similar to A–D, respectively, but from P13–P14 calyces (control, n = 5; BDNF, n = 4). I, Sampled EPSCs (top) and EPSC amplitude (mean ± SEM; bottom) taken 5–10 min before and 30–40 min after applying a control solution (control; black) or BDNF (red).

Figure 3.

BDNF inhibits slow and rapid endocytosis in P8–P10 calyces. A, Sampled and averaged (mean ± SEM) C_m changes induced by 20 APe at 100 Hz (arrow) in control (Ctrl; n = 6; black) and in the presence of BDNF (100 ng/ml, n = 4; red) in P8–P10 calyces (P8–P10 applies to all panels). The averaged traces are superimposed (right), and SEM is plotted every 1 s. B, Similar arrangement as in A, except the stimulus was 200 APe at 100 Hz. Control, n = 6; BDNF, n = 4. C, Rate_decay, QI_Ca, and ΔC_m induced by 20 APe at 100 Hz in control (Ctrl; n = 5) and in the presence of BDNF (n = 4). *p < 0.05; **p < 0.01. D, Similar to C, except that the stimulus was 200 APe at 100 Hz. E, F, Sampled (left 3 panels) and averaged C_m (right) changes induced by depol_{20 ms} (E) or depol_{20 ms × 10} (F) in control (n = 6; black), in the presence of BDNF (100 ng/ml, n = 4; red), or in the presence of K252a (200 nm) and BDNF (n = 4; blue). The averaged traces are superimposed (right). G, H, Rate_decay, QI_Ca, and ΔC_m induced by depol_{20 ms} (G) or depol_{20 ms × 10} (H) in control (Ctrl; n = 6), in the presence of BDNF (n = 4), or both BDNF and K252a (200 nm, n = 4).

Figure 4.

BDNF inhibits slow and rapid endocytosis in P13–P14 calyces. A–H, Similar arrangements as Figure 3A–H, respectively, but from P13–P14 calyces. A, B, Control (Ctrl), n = 4; BDNF, n = 4. C, D, Control, n = 6; BDNF, n = 4. E, F, Control, n = 6; BDNF, n = 4. G, H, Control, n = 5; BDNF, n = 4. *p < 0.05; **p < 0.01.