Brain-derived neurotrophic factor acutely depresses excitatory synaptic transmission to GABAergic neurons in visual cortical slices

Brain-derived neurotrophic factor (BDNF) acutely modulates synaptic transmission to excitatory neurons in hippocampus and neocortex. The question of whether BDNF acts similarly on excitatory synaptic transmission to GABAergic neurons was eluded in previous studies using cortical slices. To address this question, we used transgenic mice in which expression of green fluorescence protein (GFP) is regulated by glutamic acid decarboxylase 67 (GAD67) promoter. In cortical slices prepared from these GAD67-GFP knock-in mice, we could detect GABAergic neurons under a fluorescent microscope. An application of BDNF rapidly depressed excitatory postsynaptic currents (EPSCs) evoked by layer IV stimulation in most GFP-positive neurons in layer II/III of the cortex. This effect was seen at synapses activated during the BDNF application and blocked by anti-TrkB IgG, indicating that the acute inhibitory action of BDNF is activity-dependent and mediated through TrkB. Paired-pulse ratios of the amplitude of EPSCs to paired stimulation at intervals of 10-100 ms were not significantly changed after BDNF application, suggesting that the site of depression may be postsynaptic. Responses to directly applied glutamate were also depressed by BDNF in most of neurons, being consistent with the interpretation of postsynaptic action of BDNF. The depressive action of BDNF was blocked by an intracellular injection of a Ca(2+) chelator, suggesting that a rise in Ca(2+) is involved in the acute depression of EPSCs. This action of BDNF was seen in 67% of parvalbumin (PV)-positive neurons, but in only 19% of PV-negative neurons, indicating that the depressive action is biased to PV-positive GABAergic neurons.