title: AMPA and NMDA receptors expressed by differentiating Xenopus spinal neurons.
abstract: N-methyl--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, alpha-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 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 approximately 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 approximately 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.
authors.affiliation: Department of Biology and Center for Molecular Genetics, University of California, San Diego, La Jolla,California 92093, USA.
authors.forename: E L
authors.initials: EL
authors.lastname: Gleason
authors.forename: N C
authors.initials: NC
authors.lastname: Spitzer
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002454:Cell Differentiation / Q000502:physiology; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D004558:Electric Stimulation; D004594:Electrophysiology; D004625:Embryo, Nonmammalian; D015640:Ion Channel Gating / Q000187:drug effects; D008564:Membrane Potentials / Q000502:physiology; D009474:Neurons / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D018091:Receptors, AMPA / Q000096:biosynthesis*; D016194:Receptors, N-Methyl-D-Aspartate / Q000096:biosynthesis*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D013116:Spinal Cord / Q000166:cytology / Q000378:metabolism*; D014981:Xenopus
chemical_list: D018091:Receptors, AMPA; D016194:Receptors, N-Methyl-D-Aspartate; D002118:Calcium