  Recent evidence in rodent cerebral cortex and olfactory bulb suggests that
short-term dynamics of excitatory synaptic transmission is correlated to
stereotypical connectivity motifs. It was observed that neurons with short-term
facilitating synapses form predominantly reciprocal pairwise connections, while
neurons with short-term depressing synapses form unidirectional pairwise
connections. The cause of these structural differences in synaptic
microcircuits is unknown. We propose that these connectivity motifs emerge from
the interactions between short-term synaptic dynamics (SD) and long-term
spike-timing dependent plasticity (STDP). While the impact of STDP on SD was
shown in vitro, the mutual interactions between STDP and SD in large networks
are still the subject of intense research. We formulate a computational model
by combining SD and STDP, which captures faithfully short- and long-term
dependence on both spike times and frequency. As a proof of concept, we
simulate recurrent networks of spiking neurons with random initial connection
efficacies and where synapses are either all short-term facilitating or all
depressing. For identical background inputs, and as a direct consequence of
internally generated activity, we find that networks with depressing synapses
evolve unidirectional connectivity motifs, while networks with facilitating
synapses evolve reciprocal connectivity motifs. This holds for heterogeneous
networks including both facilitating and depressing synapses. Our study
highlights the conditions under which SD-STDP might the correlation between
facilitation and reciprocal connectivity motifs, as well as between depression
and unidirectional motifs. We further suggest experiments for the validation of
the proposed mechanism.
