The Journal of Neurophysiology Vol. 79 No. 6 June 1998, pp. 2986-2998
Copyright ©1998 The American Physiological Society

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, University of California, San Diego, La Jolla,California 92093

Gleason, Evanna L. and Nicholas C. Spitzer. AMPA and NMDA receptors expressed by differentiating Xenopus spinal neurons. J. Neurophysiol. 79: 2986-2998, 1998. N-methyl-D-aspartate (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, -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors also can 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 first are expressed. The P_Ca/P_monocation 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 before expression of NMDA receptors. The Ca²⁺ permeability of AMPA receptors, their ability to generate significant elevations of [Ca²⁺]_i, and their appearance before synapse formation position them to play roles in neural development. Spontaneous release of agonists 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.

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