AMPA and NMDA Receptors Expressed by Differentiating Xenopus Spinal
Neurons.
Evanna L. Gleason and Nicholas C. Spitzer.
Department of Biology and Center for Molecular Genetics, UCSD, La Jolla, CA
92093.
APStracts 5:123N, 1998.
ABSTRACT
NMDA receptors are often the first ionotropic glutamate receptors expressed at
early stages of development and appear to influence neuronal differentiation
by mediating Ca2+ influx. Although less well studied, AMPA receptors can also
generate Ca2+ elevations and may have developmental roles. We document the
presence of AMPA and NMDA class receptors and the absence of kainate class
receptors with whole cell voltage clamp recordings from Xenopus embryonic
spinal neurons differentiated in vitro. Reversal potential measurements
indicate that AMPA receptors are permeable to Ca2+ both in differentiated
neurons and at the time they are first expressed. The PCa/Pmonocation of 1.9
is close to that of cloned Ca2+-permeable AMPA receptors expressed in
heterologous systems. Ca2+ imaging reveals that Ca2+ elevations are elicited
by AMPA, or NMDA in the absence of Mg2+. The amplitudes and durations of these
agonist-induced Ca2+ elevations are similar to those of spontaneous Ca2+
transients known to act as differentiation signals in these cells. Two sources
of Ca2+ amplify AMPA- and NMDA-induced Ca2+ elevations. Activation of voltage-
gated Ca2+ channels by AMPA- or NMDA-mediated depolarization contributes 15%
or 30% of cytosolic Ca2+ elevations, respectively. Activation of either class
of receptor produces elevations of Ca2+ that elicit further release of Ca2+
from thapsigargin-sensitive but ryanodine-insensitive stores, contributing an
additional 30% of Ca2+ elevations. Voltage clamp recordings and Ca2+ imaging
both show that these spinal neurons express functional AMPA receptors soon
after neurite initiation and prior to expression of NMDA receptors. The Ca2+
permeability of AMPA receptors, their ability to generate significant
elevations of [Ca2+]i, and their appearance prior to synapse formation
position them to play roles in neural development. Spontaneous release of
agonist from growth cones is detected with glutamate receptors in outside-out
patches, suggesting that spinal neurons are early, nonsynaptic sources of
glutamate that can influence neuronal differentiation in vivo.
Received 25 August 1997; accepted in final form 9 March 1998.
APS Manuscript Number J703-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1998 The American Physiological Society.
Published in APStracts on 6 April 1998