Figure 3: Presynaptic pathway of gliotransmission. Gliotransmitter released from the astrocyte (a) binds extrasynaptically located presynaptic receptors (b) thereby decreasing or increasing synaptic release depending on the type of gliotransmitter and receptor. In the release-decreasing case, synaptic release probability could approach zero by gliotransmission ((c), red trace, ), although, in practice, less dramatic reductions are more likely to be measured with respect to the original value in the absence of gliotransmission (black dashed line). The reduction in synaptic release probability changes pair pulse plasticity increasing the pair pulse ratio (d). In the case of release-increasing gliotransmission, synaptic release probability could instead increase up to one ((e), green trace, ). In turn, pair pulse plasticity changes towards a decrease of the ensuing pair pulse ratio (f). Parameters as in Table 1 except for , ,  s, , mV, and  M.