Synergistic effects of presynaptic and postsynaptic neurons give rise to three-stage short-term plasticity in vivo

Short-term plasticity (STP) is the transient fluctuation of connection strength between two neurons depending on the recent history of neuronal activity. STP shapes neurotransmission over time and plays important roles in circuit computations. It is classically quantified ex vivo either at the synaptic level or at the level of spike transmission from the presynaptic neuron to the postsynaptic neuron. However, the exact relationship between the postsynaptic dendritic responses and spike transmission during STP still remains unclear in vivo. Here, we characterized together the STP of both postsynaptic dendritic responses, measured by the postsynaptic field potential (PFP), and spike transmission at the retinocollicular pathway of mice. We found mostly facilitating STP, where the second presynaptic spike occurring within 25 ms induces a larger PFP, and consequently a higher postsynaptic firing rate compared to responses from the first presynaptic spike. Both PFP and spike transmission exhibit short-term facilitation, but to a different degree, where the facilitation in the spike transmission is larger than the PFP. The PFP and spike transmission also exhibit facilitation of different decay time constants, indicating a nonlinear relationship between the two. Interestingly, a preceding postsynaptic spike can induce a similarly large but longer lasting facilitation on the spike transmission upon receiving a subsequent presynaptic input. However, STP of the PFP does not depend on the preceding postsynaptic spike, suggesting that this longer postsynaptic facilitation has a nonsynaptic origin. Overall, our results indicate that STP of the retinocollicular pathway exhibits three different stages: 1) a weak synaptic facilitation of postsynaptic dendritic responses, 2) a strong synaptic facilitation of spike transmission, and 3) a longer lasting nonsynaptic facilitation of spike transmission. Using a computational model, we show that the second STP stage is a direct inheritance from the first STP stage, whereas two opposing nonsynaptic mechanisms with different time constants are needed for the emergence of the third STP stage. These findings provide direct evidence that synaptic and nonsynaptic STPs coexist in vivo, paving the way for large-scale measurement of these STPs and offering a means to monitor the transmission of information in neural circuits of behaving animals.