Synaptic background noise controls the input/output characteristics of
single cells in an in vitro model of in-vivo activity.
Jean-Marc Fellous, Michael Rudolph, Alain Destexhe and Terrence J. Sejnowski
Neuroscience 122: 811-829, 2003.
In vivo, in vitro and computational studies were used to
investigate the impact of the synaptic background activity observed in
neocortical neurons in vivo. We simulated background activity in
vitro using two stochastic Ornstein-Uhlenbeck processes describing
glutamatergic and GABAergic synaptic conductances, which were injected into a
cell in real time using the dynamic clamp technique. With parameters chosen to
mimic in vivo conditions, layer 5 rat prefrontal cortex cells recorded in
vitro were depolarized by about 15 mV, their membrane fluctuated with a S.D. of
about 4 mV, their input resistances decreased five-fold, their spontaneous firing
had a high coefficient of variation and an average firing rate of about 5-10 Hz.
Brief changes in the variance of the
alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) synaptic
conductance fluctuations induced time-locked spiking without significantly
changing the average membrane potential of the cell. These transients mimicked
increases in the correlation of excitatory inputs. Background activity was highly
effective in modulating the firing-rate/current curve of the cell: the variance
of the simulated gamma-aminobutyric acid (GABA) and AMPA conductances
individually set the input/output gain, the mean excitatory and inhibitory
conductances set the working point, and the mean inhibitory conductance
controlled the input resistance. An average ratio of inhibitory to excitatory
mean conductances close to 4 was optimal in generating membrane potential
fluctuations with high coefficients of variation. We conclude that background
synaptic activity can dynamically modulate the input/output properties of
individual neocortical neurons in vivo.
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