The Journal of Neuroscience, January 1, 2002, 22(1):226-238
Interaction between Metabotropic and NMDA Subtypes of Glutamate
Receptors in Sprout Suppression at Young Synapses
Frank
Miskevich,
Wei
Lu,
Shuh-Yow
Lin, and
Martha
Constantine-Paton
Department of Biology, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Recently, NMDA receptors (NMDARs) have been implicated in a cell
contact-dependent suppression of sprouting in cultured
Xenopus tectal neurons during an early period when
neither AMPA/kainate (KA) receptors nor action potentials play a
prominent role in cell-cell communication. We asked how the NMDA
receptors function in the absence of the depolarizing effect of AMPA/KA
receptor activity. We show that type II metabotropic glutamate
receptors (mGluRs) can operate synergistically with NMDA receptors in
the absence of AMPA/KA receptor function to suppress an early
neurite sprouting response of the tectal neurons. Calcium
imaging with fluo-3 AM and morphological analyses after exposure to
glutamate receptor antagonists show that a combination of AMPA/KA
receptor and type II mGluR blockers mimics the decrease in
intracellular free calcium in response to glutamate and the structural
effects produced by NMDA receptor antagonists in these cultures.
Patch-clamp analyses confirmed a decrease in NMDA receptor-mediated
currents with type II mGluR blockade, and 8-bromo cAMP application
produced a decrease in NMDA receptor-mediated calcium influx. These
data suggest that type II mGluRs potentiate NMDA receptor function by
decreasing cAMP levels in tectal neurons. We also show that NMDARs
exhibit low magnesium sensitivity in tectal neurons during the first
few days in culture. Thus both metabotropic and ionotropic glutamate
receptors can play a role in the contact-mediated suppression of
ongoing sprouting at early neuron-neuron contacts before action potential activity.
Key words:
Xenopus; synaptogenesis; sprouting; AMPA; calcium; cAMP; electrophysiology
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