The Journal of Neurophysiology Vol. 88 No. 2 August 2002, pp. 1005-1015
Copyright ©2002 by the American Physiological Society

Excitatory Actions of GABA Increase BDNF Expression via a MAPK-CREB-Dependent MechanismA Positive Feedback Circuit in Developing Neurons

 ¹Department of Neuroscience, The Ohio State University, Columbus, Ohio 43210; and  ²Department of Neurosurgery, Yale University Medical School, New Haven, Connecticut 06520-8082

Obrietan, Karl, Xiao-Bing Gao, and Anthony N. van den Pol. Excitatory Actions of GABA Increase BDNF Expression via a MAPK-CREB-Dependent MechanismA Positive Feedback Circuit in Developing Neurons. J. Neurophysiol. 88: 1005-1015, 2002. During early neuronal development, GABA functions as an excitatory neurotransmitter, triggering membrane depolarization, action potentials, and the opening of plasma membrane Ca²⁺ channels. These excitatory actions of GABA lead to a number of changes in neuronal structure and function. Although the effects of GABA on membrane biophysics during early development have been well documented, little work has been done to examine the possible mechanisms underlying GABA-regulated plastic changes in the developing brain. This study focuses on GABA-regulated kinase activity and transcriptional control. We utilized a combination of Western blotting and immunocytochemical techniques to examine two potential downstream pathways regulated by GABA excitation: the p42/44 mitogen-activated protein kinase (MAPK) cascade and the transcription factor cyclic AMP response element binding protein (CREB). During early development of cultured hypothalamic neurons (5 days in vitro), stimulation with GABA triggered activation of the MAPK cascade and phosphorylation of CREB at Ser 133. These effects were mediated by the GABA_A receptor, since administration of the GABA_A receptor-specific agonist muscimol (50 µM) triggered pathway activation, and pretreatment with the GABA_A-receptor specific antagonist bicuculline (20 µM) blocked pathway activation. Immunocytochemistry revealed a spatial and temporal correlation between activation of the MAPK cascade and CREB phosphorylation. Pretreatment with the MAPK/ERK kinase (MEK) inhibitor U0126 (10 µM) attenuated CREB phosphorylation, indicating that the MAPK pathway regulates that activation state of CREB. In contrast to the excitatory effects observed during early development, in more mature neurons, GABA functions as an inhibitory transmitter. Consistent with this observation, GABA_A receptor activation did not stimulate MAPK cascade activation or CREB phosphorylation in mature cultures (18 days in vitro). To determine whether GABA_A receptor activation during early development stimulates gene expression, we examined the inducible expression of the neurotrophin brain-derived neurotrophic factor (BDNF). Both GABA and muscimol stimulated BDNF expression, and pretreatment with U0126 attenuated GABA-induced BDNF expression. Whole cell electrophysiological recording was used to assess the effects of BDNF on GABA release. BDNF (100 ng/ml) dramatically increased the frequency of excitatory GABAergic spontaneous postsynaptic currents. Together, these data suggest a positive excitatory feedback loop between GABA and BDNF expression during early development, where GABA facilitates BDNF expression, and BDNF facilitates the synaptic release of GABA. Signaling via the MAPK cascade and the transcription factor CREB appear to play a substantial role in this process.