  Two elements of neural information processing have primarily been proposed:
firing rate and spike timing of neurons. In the case of synaptic plasticity,
although spike-timing-dependent plasticity (STDP) depending on presynaptic and
postsynaptic spike times had been considered the most common rule, recent
studies have shown the inhibitory nature of the brain in vivo for precise spike
timing, which is key to the STDP. Thus, the importance of the firing frequency
in synaptic plasticity in vivo has been recognized again. However, little is
understood about how the frequency-dependent synaptic plasticity (FDP) is
regulated in vivo. Here, we focused on the presynaptic input pattern, the
intracellular calcium decay time constants, and the background synaptic
activity, which vary depending on neuron types and the anatomical and
physiological environment in the brain. By analyzing a calcium-based model, we
found that the synaptic weight differs depending on these factors
characteristic in vivo, even if neurons receive the same input rate. This
finding suggests the involvement of multifaceted factors other than input
frequency in FDP and even neural coding in vivo.
