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

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.