Distinct functional roles determined by synaptic plasticity expression locus
Abstract: Long-term plasticity can be expressed pre- or postsynaptically depending on a range of factors such as developmental stage, synapse type, and activity patterns. However, the functional consequences of this diversity are unknown. Whereas postsynaptic expression of plasticity predominantly affects synaptic response amplitude, presynaptic expression alters both synaptic response amplitude and short-term dynamics. Theoretical outcomes are usually based on fixed changes in the postsynaptic response amplitude, which can constitute a postsynaptic bias as presynaptic plasticity may also affect the stochasticity of vesicle release and the redistribution of presynaptic resources. To explore the functional impact of different expressions, we simulated two different scenarios in which a postsynaptic neuron learns to respond to excitatory inputs, either through its latency to spike or through correlated activities. We implemented long-term plasticity presynaptically, postsynaptically, or both, across different STDP models. We found that presynaptic changes adjusted the speed of learning, while postsynaptic expression was better at regulating spike timing and frequency. When combining both expression loci, postsynaptic changes amplified the response range, while presynaptic plasticity maintained control over postsynaptic firing rates, potentially providing a possible form of activity homeostasis. With this work we aim to show that different plasticity expressions may not be interchangeable, but instead enable complimentary functions.