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- O. Beck, M. Chistiakova,
K. Obermayer, and M. Volgushev. Adaptation of Synaptic Connections to
Layer 2/3 Pyramidal Cells in Rat Visual Cortex.
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J. Neurophysiol., 94:363-376, 2005.
(PDF)
Neocortical synapses express differential dynamic properties. When
activated at high frequencies, the amplitudes of the subsequent postsynaptic
responses may increase or decrease, depending on the stimulation frequency
and on the properties of that particular synapse. Changes in the synaptic
dynamics can dramatically affect the communication between nerve cells.
Motivated by this question, we studied dynamic properties at synapses to
layer 2/3 pyramidal cells with intracellular recordings in slices of rat
visual cortex. Synaptic responses were evoked by trains of test stimuli,
which consisted of 10 pulses at different frequencies (5-40 Hz). Test
stimulation was applied either without any adaptation (control) or 2 s after
an adaptation stimulus, which consisted of 4 s stimulation of these same
synapses at 10, 25 or 40 Hz. The synaptic parameters were then assessed from
fitting the data with a model of synaptic dynamics. Our estimates of the
synaptic parameters in control, without adaptation are broadly consistent
with previous studies. Adaptation led to pronounced changes of synaptic
transmission. After adaptation, the amplitude of the response to the first
pulse in the test train decreased for several seconds and then recovered back
to the control level with a time constant of 2-18 seconds. Analysis of the
data with extended models, which include interaction between different pools
of synaptic vesicles, suggests that the decrease of the response amplitude
was due to a synergistic action of two factors, decrease of the release
probability and depletion of the available transmitter. After a weak (10 Hz)
adaptation, the decrease of the response amplitude was accompanied by and
correlated with the decrease of the release probability. After a strong
adaptation (25 or 40 Hz), the depletion of synaptic resources was the main
cause for the reduced response amplitude. Adaptation also led to pronounced
changes of the time constants of facilitation and recovery, however, these
changes were not uniform in all synapses, and on the population level the
only consistent and significant effect was an acceleration of the recovery
after a strong adaptation. Taken together, our results suggest, that apart
from decreasing the amplitude of postsynaptic responses, adaptation may
produce synapse-specific effects, which could result in a kind of
re-distribution of activity within neural networks.
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