Enhancement of neurotransmitter release induced by brain-derived neurotrophic factor in cultured hippocampal neurons - PubMed (original) (raw)

Enhancement of neurotransmitter release induced by brain-derived neurotrophic factor in cultured hippocampal neurons

Y X Li et al. J Neurosci. 1998.

Abstract

Brain-derived neurotrophic factor (BDNF), like other neurotrophins, has long-term effects on neuronal survival and differentiation; furthermore, recent work has shown that BDNF also can induce rapid changes in synaptic efficacy. We have investigated the mechanism(s) of these synaptic effects on cultured embryonic hippocampal neurons. In the presence of the GABAA receptor antagonist, picrotoxin, the application of BDNF (100 ng/ml) for 1-5 min increased the amplitude of evoked synaptic currents by 48 +/- 9% in 10 of 15 pairs of neurons and increased the frequency of EPSC bursts to 205 +/- 20% of the control levels. There was no detectable effect of BDNF on various measures of electrical excitability, including the resting membrane potential, input resistance, action potential threshold, and action potential amplitude. In addition, BDNF did not change the postsynaptic currents induced by the exogenous application of glutamate. BDNF did increase the frequency of miniature EPSCs (mEPSCs) (268.0 +/- 46.8% of control frequency), however, without affecting the mEPSC amplitude. The effect of BDNF on mEPSC frequency was blocked by the tyrosine kinase inhibitor K252a and also by the removal of extracellular calcium ([Ca2+]o). Fura-2 recordings showed that BDNF elicited an increase in intracellular calcium concentration ([Ca2+]c). This effect was dependent on [Ca2+]o; it was blocked by K252a and by thapsigargin, but not by caffeine. The results demonstrate that BDNF enhances glutamatergic synaptic transmission at a presynaptic locus and that this effect is accompanied by a rise in [Ca2+]c that requires the release of Ca2+ from IP3-gated stores.

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Figures

Fig. 1.

Fig. 1.

Bursts of synaptic activity in whole-cell recordings of E18 hippocampal neurons cultured after 2 weeks. The inward current deflections are produced by postsynaptic currents.A, Shown is an example of recordings. B, Shown is the time course of the BDNF effect. The effect of BDNF (100 ng/ml) on the burst frequency appeared ∼30 sec after application. The increased frequency and the increased magnitude of the integrated current per minute (denoted synaptic charge) recovered to baseline ∼5 min after washout of BDNF. C, D, Average values of bursting frequency and of synaptic charge in and after BDNF application.

Fig. 2.

Fig. 2.

The effect of BDNF on evoked synaptic currents between paired neurons. A, Micrograph taken during dual whole-cell recordings was made from a pair of nearby cultured hippocampal neurons; note the images of the micropipette electrodes.B, The bottom panel shows the current in the presynaptic cell during a voltage step to −10 mV from a holding potential of −70 mV. The top panel superimposes 12 continuous EPSCs recorded simultaneously from the postsynaptic cell before and during BDNF application. C, The average of EPSCs recorded during 5 min before (CONTROL) and 2–5 min after the beginning of BDNF application. D, The average of the amplitude of evoked EPSCs during and after the application of BDNF.

Fig. 3.

Fig. 3.

BDNF did not affect the electrical excitability of neurons. APV, CNQX, and picrotoxin were used to block the NMDA-, AMPA-, and GABA-induced responses from the input of other cells. Recordings were made in current-clamp mode. A, Shown are the I–V responses recorded from cells both before (A1) and during (A2) BDNF application. B, Shown is the relation between the amplitude of the injected current and the number of action potentials during the 600 msec current pulses.

Fig. 4.

Fig. 4.

BDNF increased the frequency, but not the amplitude, distribution of mEPSCs. A, Shown are examples of recordings. TTX (100 n

m

) was present to block the release of transmitter caused by spontaneous action potentials.B, Shown is the time course of the BDNF effect. At ∼30 sec after BDNF application the frequency of mEPSCs was increased. The increased frequency recovered to the baseline value 5 min after the washout of BDNF. C, The average values of frequency during and after BDNF application. D, Averages of cumulative amplitude distributions of mEPSCs obtained before and during BDNF application (n = 9). There is no obvious difference (p > 0.05) between the distributions of amplitudes before and during BDNF application.

Fig. 5.

Fig. 5.

The increased frequency of mEPSCs caused by BDNF was blocked by K252a and was dependent on the extracellular calcium.

Fig. 6.

Fig. 6.

BDNF did not affect the sensitivity of glutamate receptors. A, Shown are inward currents induced by glutamate (2–200 μ

m

) before and during BDNF application.B, Shown is the dose–response relation for the glutamate-induced current, measured during the plateau (900 msec after the start of application; n = 5).

Fig. 7.

Fig. 7.

BDNF induced an increase in intracellular Ca2+. [Ca2+]c was measured by image ratio fluorescence microscopy with fura-2.A, A hippocampal cell responded to BDNF with an increase in [Ca2+]c. The BDNF-induced increase in [Ca2+]c disappeared in Ca2+-free solution and was restored after a switch to normal saline. B, K252a blocked the BDNF-induced increase in [Ca2+]c. C, Exposure to 2 μ

m

thapsigargin blocked the [Ca2+]c response. D, The [Ca2+]c response was present during exposure to caffeine-treated cells. (+) and (−) in the graphs indicate the BDNF-induced responses in the presence and absence of the drug, respectively. The bar in each panel indicates the period of BDNF application.

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