The Journal of Neurophysiology Vol. 79 No. 3 March 1998, pp. 1360-1370
Copyright ©1998 The American Physiological Society

GABA_B Receptor-Mediated Inhibition of GABA_A Receptor Calcium Elevations in Developing Hypothalamic Neurons

Karl Obrietan and Anthony N. van den Pol

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

Obrietan, K. and Anthony N. van den Pol. GABA_B receptor-mediated inhibition of GABA_A receptor calcium elevations in developing hypothalamic neurons. J. Neurophysiol. 79: 1360-1370, 1998. In the CNS, -aminobutyric acid (GABA) affects neuronal activity through both the ligand-gated GABA_A receptor channel and the G protein-coupled GABA_B receptor. In the mature nervous system, both receptor subtypes decrease neural excitability, whereas in most neurons during development, the GABA_A receptor increases neural excitability and raises cytosolic Ca²⁺ levels. We used Ca²⁺ digital imaging to test the hypothesis that GABA_A receptor-mediated Ca²⁺ rises were regulated by GABA_B receptor activation. In young, embryonic day 18, hypothalamic neurons cultured for 5 ± 2 days in vitro, we found that cytosolic Ca²⁺ rises triggered by synaptically activated GABA_A receptors were dramatically depressed (>80%) in a dose-dependent manner by application of the GABA_B receptor agonist baclofen (100 nM-100 µM). Coadministration of the GABA_B receptor antagonist 2-hydroxy-saclofen or CGP 35348 reduced the inhibitory action of baclofen. Administration of the GABA_B antagonist alone elicited a reproducible Ca²⁺ rise in >25% of all synaptically active neurons, suggesting that synaptic GABA release exerts a tonic inhibitory tone on GABA_A receptor-mediated Ca²⁺ rises via GABA_B receptor activation. In the presence of tetrodotoxin the GABA_A receptor agonist muscimol elicited robust postsynaptic Ca²⁺ rises that were depressed by baclofen coadministration. Baclofen-mediated depression of muscimol-evoked Ca²⁺ rises were observed in both the cell bodies and neurites of hypothalamic neurons taken at embryonic day 15 and cultured for three days, suggesting that GABA_B receptors are functionally active at an early stage of neuronal development. Ca²⁺ rises elicited by electrically induced synaptic release of GABA were largely inhibited (>86%) by baclofen. These results indicate that GABA_B receptor activation depresses GABA_A receptor-mediated Ca²⁺ rises by both reducing the synaptic release of GABA and decreasing the postsynaptic Ca²⁺ responsiveness. Collectively, these data suggest that GABA_B receptors play an important inhibitory role regulating Ca²⁺ rises elicited by GABA_A receptor activation. Changes in cytosolic Ca²⁺ during early neural development would, in turn, profoundly affect a wide array of physiological processes, such as gene expression, neurite outgrowth, transmitter release, and synaptogenesis.

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