Rat Group I Metabotropic Glutamate Receptors Inhibit Neuronal Ca2+ Channels
Via Multiple Signal Transduction Pathways in HEK 293 Cells.
Brian A. McCool, Jean-Phillipe Pin, Michael M. Harpold, Paul F. Brust, Kenneth
A. Stauderman, and David M. Lovinger.
Department of Molecular Physiology and Biophysics, Vanderbilt University
School of Medicine, Nashville, Tennessee; M‚canismes Mol‚culares des
Communications Cellulaires, Centre National de la Recherche Scientifique-
Institut National de la Sant‚ et de la Recherche M‚dicale de Pharmacolgie-
Endocrinologie, Monpelier Cedex 5, France; and SIBIA Neurosciences, Inc., La
Jolla, California.
APStracts 4:229N, 1997.
ABSTRACT
We have previously shown that metabotropic glutamate receptors with Group I-
like pharmacology couple to N-type and P/Q-type calcium channels in acutely
isolated cortical neurons using G proteins most likely belonging to the Gi/Go
subclass (Choi and Lovinger 1996). To better understand the potential
mechanisms forming the basis for Group I mGluR modulation of voltage-gated
calcium channels in the central nervous system, we have examined the ability
of specific mGluRs to couple to neuronal N-type (ŕ1B-1/ŕ2d/á1b) and P/Q-type
(ŕ1A-2/ŕ2d/á1b) voltage-gated calcium channels in an HEK 293 heterologous
expression system. Using the whole-cell patch clamp technique where
intracellular calcium is buffered to low levels, we have shown that Group I
receptors inhibit both N-type and P/Q-type calcium channels in a voltage-
dependent fashion. Similar to our observations in cortical neurons, this
voltage-dependent inhibition is mediated almost entirely by N-ethylmaleimide
(NEM)-sensitive heterotrimeric G proteins, strongly suggesting that these
receptors can utilize Gi/Go-like G proteins to couple to N-type and P/Q-type
calcium channels. However, inconsistent with the apparent NEM-sensitivity of
Group I modulation of calcium channels, modulation of N-type channels in Group
I mGluR-expressing cells was only partially sensitive to pertussis toxin
(PTX), indicating the potential involvement of both PTX-sensitive and -
resistant G proteins. The PTX-resistant modulation was voltage-dependent and
entirely resistant to NEM and cholera toxin. A time course of treatment with
PTX revealed that this toxin caused Group I receptors to slowly shift from
using a primarily NEM-sensitive G protein to using an NEM-resistant form. The
PTX-induced switch from NEM-sensitive to NEM-resistant modulation was also
dependent upon protein synthesis, indicating some reliance on active cellular
processes. In addition to these voltage-dependent pathways, perforated patch
recordings on Group I mGluR-expressing cells indicate that another slowly-
developing, calcium-dependent form of modulation for N-type channels may be
seen when intracellular calcium is not highly buffered. We conclude that Group
I mGluRs can modulate neuronal Ca2+ channels using a variety of signal
transduction pathways and propose that the relative contributions of different
pathways may exemplify the diversity of responses mediated by these receptors
in the central nervous system.
Received 25 April 1997; accepted in final form 4 September 1997.
APS Manuscript Number J330-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 October 1997