(

**A**,

**B**,

*left*) Raster plots of a depressing synapse, without (

**A**) and in presence of (

**B**) a single event of glutamate exocytosis from the astrocyte (onset at the

*black mark* at

*t* = 1 s, α = 0) for

*n* = 100 Poisson spike trains with the same average frequency

*f*_{in} (

*black line* in

**A**,

**B**,

*right*). A

*green dot* marks an input spike that released more resources than its preceding one, while a

*magenta dot* represents an input spike that released less resources than its previous one. (

**A**,

**B**,

*middle*) The increase of facilitated spike pairs by release-decreasing astrocytic glutamate on the depressing synapse is due to the decrease of synaptic basal release probability

*U*_{0} (

*black line*) beyond the switching threshold

*U*_{thr} (

*blue line*) while the limiting frequency (

*f*_{lim},

*dark red line*) increase above the average input frequency (

*f*_{in},

*black line*). In such situation in fact, both conditions needed for short-term facilitation are fulfilled (see “Mechanisms of short-term presynaptic plasticity” in “”). (

**C, D**) The opposite occurs for a facilitating synapse under the effect of a release-increasing astrocyte (α = 1). In this case in fact, astrocytic glutamate makes

*U*_{0} increase beyond

*U*_{thr} (

**D**,

*middle*) while

*f*_{lim} switches from above to below

*f*_{in}, thus marking onset of depression (

**D**,

*right*). The same results can alternatively be obtained analyzing the slope of the

curve (equation 3) for

*f*_{in}(

*t*) (). Parameters as in .

## PubMed Commons