Gs. Liu et al., Variability of neurotransmitter concentration and nonsaturation of postsynaptic AMPA receptors at synapses in hippocampal cultures and slices, NEURON, 22(2), 1999, pp. 395-409
To understand the elementary unit of synaptic communication between CNS neu
rons, one must know what causes the variability of quantal postsynaptic cur
rents and whether unitary packets of transmitter saturate postsynaptic rece
ptors. We studied single excitatory synapses between hippocampal neurons in
culture. Focal glutamate application at individual postsynaptic sites evok
ed currents (I-glu) with little variability compared with quantal excitator
y postsynaptic currents (EPSCs). The maximal I-glu was >2-fold larger than
the median EPSC. Thus, variations in [glu](cleft) are the main source of va
riability in EPSC size, and glutamate receptors are generally far from satu
ration during quantal transmission. This conclusion was verified by molecul
ar antagonism experiments in hippocampal cultures and slices. The general l
ack of glutamate recepor saturation leaves room for increases in [glu](clef
t) as a mechanism for synaptic plasticity.