Synaptic heterogeneity and stimulus-induced modulation of depression in central synapses

Short-term plasticity is a pervasive feature of synapses. Synapses exhibit many forms of plasticity operating over a range of time scales. We develop an optimization method that allows rapid characterization of synapses with multiple time scales of facilitation and depression. Investigation of paire d neurons that are postsynaptic to the same identified interneuron in the b uccal ganglion of Aplysia reveals that the responses of the two neurons dif fer in the magnitude of synaptic depression. Also, for single neurons, prol onged stimulation of the presynaptic neuron causes stimulus-induced increas es in the early phase of synaptic depression. These observations can be des cribed by a model that incorporates two availability factors, e.g., depleta ble vesicle pools or desensitizing receptor populations, with different tim e courses of recovery, and a single facilitation component. This model accu rately predicts the responses to novel stimuli. The source of synaptic hete rogeneity is identified with variations in the relative sizes of the two av ailability factors, and the stimulus-induced decrement in the early synapti c response is explained by a slowing of the recovery rate of one of the ava ilability factors. The synaptic heterogeneity and stimulus-induced modifica tions in synaptic depression observed here emphasize that synaptic efficacy depends on both the individual properties of synapses and their past histo ry.