Modulation of GABAergic synapses by brain-derived neurotrophic factor in cultured hippocampal neurons

Brain-derived neurotrophic factor, known for its critical role in neuron development, has also been shown to regulate glutamatergic synaptic transmission. The principal aim of this thesis was to determine how BDNF may also modulate GABAergic synapses, both acutely and long-term. Experiments were performed in neurons from the hippocampus, a brain region involved in learning and memory and that richly expresses BDNF and its receptor tyrosine kinase B (TrkB). Cultured hippocampal neurons were used to provide advantages of visualization of and accessibility to individual synapses.; First, the localization of BDNF and TrkB to GABAergic synapses was established using immunocytochemistry for presynaptic and postsynaptic markers. The development of GABAergic synapses in cultured hippocampal neurons was then characterized from 1--21 days in vitro (DIV) by performing immunocytochemistry to visualize GABAergic synapses and recording miniature inhibitory postsynaptic currents (mIPSCs) to determine functional kinetics of GABAergic synapses. Three major stages of GABAergic synaptogenesis were described: (1) the emergence of GABAergic synapses between 5--8 DIV, (2) proliferation of GABAergic synapses from 8--14 DIV, and (3) emergence of adult isoforms after 14 DIV, including transitions in the composition of GABA A receptor subunits and switches in the expression of GABA transporter subtypes. Therefore, an appropriate time to evaluate modifications induced by BDNF on relatively mature GABAergic synapses was determined to be 14--16 DIV.; The acute (0--30 minutes) and long-term (48 hours) effects of BDNF on GABAergic synapses were then determined using immunocytochemistry to visualize synapses and mIPSC recordings to evaluate the strength of GABAergic synapses. Endogenous activity of the TrkB receptor acutely reduced GABAergic synapse strength by promoting internalization of GABAA receptors. However, long-term BDNF enhanced GABAergic synapse strength in response to activity by increasing expression of the 65 kDa isoform of glutamic acid decarboxylase (GAD-65), the synthetic enzyme for GABA.; Overall, these results suggest that BDNF is an important modulator of GABAergic synapses. The acute effects of BDNF demonstrate rapid changes in synaptic plasticity due to phosphorylation which may promote short-term brain excitability. The long-term effects reflect synaptic scaling due to new protein synthesis, suggesting that BDNF is a mediator of homeostatic plasticity in the brain.