The Journal of Neurophysiology Vol. 82 No. 1 July 1999, pp. 94-102
Copyright ©1999 by the American Physiological Society

GABA_B Receptor-Mediated Regulation of Glutamate-Activated Calcium Transients in Hypothalamic and Cortical Neuron Development

Department of Biological Science, Stanford University, Stanford, California 94305; and Department of Neurosurgery, Yale University School of Medicine, New Haven, Connecticut 06520

Obrietan, Karl and Anthony van den Pol. GABA_B Receptor-Mediated Regulation of Glutamate-Activated Calcium Transients in Hypothalamic and Cortical Neuron Development. J. Neurophysiol. 82: 94-102, 1999.In the mature nervous system excitatory neurotransmission mediated by glutamate is balanced by the inhibitory actions of GABA. However, during early development, GABA acting at the ligand-gated GABA_A Cl channel also exerts excitatory actions. This raises a question as to whether GABA can exert inhibitory activity during early development, possibly by a mechanism that involves activation of the G protein-coupled GABA_B receptor. To address this question we used Ca²⁺ digital imaging to assess the modulatory role of GABA_B receptor signaling in relation to the excitatory effects of glutamate during hypothalamic and cortical neuron development. Ca²⁺ transients mediated by synaptic glutamate release in neurons cultured from embryonic rat were dramatically depressed by the administration of the GABA_B receptor agonist baclofen in a dose-dependent manner. The inhibitory effects of GABA_B receptor activation persisted for the duration of baclofen administration (>10 min). Preincubation with the Gi protein inhibitor pertussis toxin resulted in a substantial decrease in the inhibitory actions of baclofen, confirming that a Gi-dependent mechanism mediated the effects of the GABA_B receptor. Co-administration of the GABA_B receptor antagonist 2-hydroxy-saclofen eliminated the inhibitory action of baclofen. Alone, GABA_B antagonist application elicited a marked potentiation of Ca²⁺ transients mediated by glutamatergic neurotransmission, suggesting that tonic synaptic GABA release exerts an inhibitory tone on glutamate receptor-mediated Ca²⁺ transients via GABA_B receptor activation. In the presence of TTX to block action potential-mediated neurotransmitter release, stimulation with exogenously applied glutamate triggered a robust postsynaptic Ca²⁺ rise that was dramatically depressed (>70% in cortical neurons, >40% in hypothalamic neurons) by baclofen. Together these data suggest both a pre- and postsynaptic component for the modulatory actions of the GABA_B receptor. These results indicate a potentially important role for the GABA_B receptor as a modulator of the excitatory actions of glutamate in developing neurons.