GABAB RECEPTOR MEDIATED INHIBITION OF GABAA RECEPTOR CALCIUM ELEVATIONS IN
DEVELOPING HYPOTHALAMIC NEURONS.
Karl Obrietan and Anthony N. van den Pol.
Department of Biological Science, Stanford University, Stanford, CA 94305.
Department of Neurosurgery, Yale University School of Medicine, New Haven, CT
06520.
APStracts 4:307N, 1997.
ABSTRACT
In the CNS, GABA affects neuronal activity through both the ligand-gated GABAA
receptor channel and the G protein-coupled GABAB receptor. In the mature
nervous system, both receptor subtypes decrease neural excitability, whereas
in most neurons during development, the GABAA receptor increases neural
excitability and raises cytosolic Ca2+ levels. We used Ca2+ digital imaging to
test the hypothesis that GABAA receptor-mediated Ca2+ rises were regulated by
GABAB receptor activation. In young, embryonic day 18, hypothalamic neurons
cultured for 6+1 days in vitro, we found that cytosolic Ca2+ rises triggered
by synaptically-activated GABAA receptors were dramatically depressed (> 80%)
in a dose-dependent manner by application of the GABAB receptor agonist
baclofen (100 nM-100 æM). Coadministration of the GABAB receptor antagonist 2-
hydroxy-saclofen or CGP 35348 reduced the inhibitory action of baclofen.
Administration of the GABAB 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 GABAA receptor-mediated Ca2+ rises
via GABAB receptor activation. In the presence of tetrodotoxin the GABAA
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 3 days,
suggesting that GABAB 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 GABAB receptor activation depresses GABAA receptor-mediated Ca2+
rises by both reducing the synaptic release of GABA and decreasing the
postsynaptic Ca2+ responsiveness. Collectively, these data suggest that GABAB
receptors play an important inhibitory role regulating Ca2+ rises elicited by
GABAA 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.
Received 23 July 1997; accepted in final form 31 October 1997.
APS Manuscript Number J604-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 14 November 1997