Figure 4: Gliotransmitter-mediated modulation of synaptic frequency response. Decrease (a) or increase (d) of synaptic release probability by gliotransmission modulates the average per-spike synaptic release, resulting in a change of the synapse frequency response. Monotonically decreasing frequency responses that are typical of depressing synapses could be flattened by release-decreasing gliotransmission ((b), black versus red points), and vice versa, almost nonmonotonic ones, characteristic of facilitating synapses, could turn into monotonically decreasing responses by release-increasing gliotransmission ((e), black versus green points). Changes in frequency response depend on whether gliotransmission impinges on the very synapse that is triggered by (homosynaptic/closed-loop scenario) or not (heterosynaptic/open-loop scenario). In the homosynaptic scenario, the synaptic response is expected to change only for presynaptic firing rates that are sufficiently high to trigger gliotransmitter release from the astrocyte ((b), (e), cyan points). Data points and error bars: meanSTD for (no gliot. and heterosyn. gliot.) or simulations (homosyn. gliot.) with 60 s long Poisson-distributed presynaptic spike trains. ((c), (f)) The change of synaptic frequency response mediated by gliotransmission (three consecutive gliotransmitter releases at the time instants marked by triangles) leads to changes in how presynaptic firing rates (top panels) are transmitted by the synapse (bottom panels). Simulated postsynaptic currents (PSCs) are shown as average traces of simulations for gliotransmitter release at 1 Hz. Release-decreasing gliotransmission was achieved for , whereas was used for release-increasing gliotransmission. Depressing synapse in ((a), (b)):  s,  s, and ; facilitating synapse in ((d), (e)):  s,  s, and . Other model parameters as in Figure 3 except for  M.