The Journal of Neuroscience, December 15, 1998, 18(24):10409-10419
Postsynaptic Ca2+ Influx Mediated by Three Different
Pathways during Synaptic Transmission at a Calyx-Type Synapse
Johann H.
Bollmann,
Fritjof
Helmchen,
J. Gerard G.
Borst, and
Bert
Sakmann
Abteilung Zellphysiologie, Max-Planck-Institut für
medizinische Forschung, D-69120 Heidelberg, Germany
Whole-cell recordings and Ca2+ flux measurements
were made at a giant calyx-type synapse in rat brainstem slices to
determine the contribution of glutamate receptor (GluR) channels and
voltage-dependent Ca2+ channels (VDCCs) to
postsynaptic Ca2+ influx during synaptic
transmission. A single presynaptic action potential (AP) evoked an
EPSP, followed by a single AP. The EPSP-AP sequence caused a
postsynaptic Ca2+ influx of ~3.0 pC, primarily
through VDCCs (~70%) and NMDA-type (up to 30%) channels but also
through AMPA-type (<5%) GluR channels. At 
80 mV, the fractional
Ca2+ current (Pf)
mediated by AMPA receptor (AMPAR) and NMDA receptor (NMDAR)
channels was 1.3 and 11-12%, respectively. Simulations of the time
course of Ca2+ influx through GluR channels
suggested that the small contribution of AMPAR channels occurred only
during the first few milliseconds of an EPSP, whereas influx through
NMDAR channels dominated later. The NMDAR-mediated
Ca2+ influx was localized in regions covered by the
presynaptic terminal, whereas the Ca2+ influx
mediated by VDCCs was more homogeneously distributed. Because of the
temporal and spatial differences, calcium ions entering through the
three different pathways are likely to activate different intracellular
targets in the postsynaptic cell.
Key words:
action potential; fura-2; fractional Ca2+
current; postsynaptic Ca2+ influx; medial nucleus of the
trapezoid body; calyx of Held; glutamate receptors; Ca2+
channels; Ca2+ imaging
Copyright © 1998 Society for Neuroscience 0270-6474/98/182410409-11$05.00/0
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