Neuromodulation of STDP through short-term changes in firing causality

Area of Science:

• Neuroscience
• Computational Neuroscience
• Synaptic Plasticity

Background:

• Spike-timing dependent plasticity (STDP) is a key mechanism for synaptic modification based on the causal relationship between pre- and postsynaptic neuron firing.
• Neuromodulators like dopamine are thought to influence learning by acting as a third factor in STDP, but this doesn't explain rapid changes in neuronal excitability.
• Existing models fail to account for the instantaneous effects of neuromodulators on neuronal activity, particularly in regions like the striatum.

Purpose of the Study:

• To propose a novel model of synaptic transmission where neuromodulators modulate neuronal firing causality rather than directly altering synaptic plasticity.
• To explain the rapid, direct effects of neuromodulators on postsynaptic neuron excitability.
• To provide a mechanism that can indirectly influence learning outcomes within both two-factor and three-factor STDP frameworks.

Main Methods:

• Development of a computational model for synaptic transmission incorporating neuromodulator-induced changes in relative firing causality.
• Analysis of how altered firing causality affects synaptic modification under STDP rules.
• Comparison of the model's predictions with experimental observations of neuromodulator effects on neuronal activity.

Main Results:

• The proposed model demonstrates that neuromodulators can indirectly influence synaptic plasticity by altering the causal relationship between pre- and postsynaptic spiking.
• This mechanism explains the observed instantaneous facilitation of neuronal excitability upon neuromodulator application.
• The model supports learning through standard two-factor STDP while also being compatible with three-factor STDP rules.

Conclusions:

• Neuromodulators may primarily act by shifting the balance of neuronal firing causality, which then indirectly shapes synaptic plasticity.
• This perspective reconciles the slow, indirect effects of neuromodulated STDP with the rapid, direct effects on neuronal excitability.
• The proposed model offers a more comprehensive understanding of neuromodulation in synaptic learning and brain function.