Dynamics​ ​ of​ ​ synaptic​ ​ extracellular​ ​ field​ ​ potentials​ ​ in​ ​ the nucleus​ ​ laminaris​ ​ of​ ​ the​ ​ barn​ ​ owl.

Synaptic currents are frequently assumed to make a major contribution to the extracellular field potential (EFP). However, in any neuronal population, the explicit separation of synaptic sources from other contributions such as postsynaptic spikes remains a challenge. Here, we take advantage of the simple organization of the barn owl nucleus laminaris (NL) in the auditory brainstem to isolate synaptic currents through the iontophoretic application of the AMPA-receptor antagonist NBQX. Responses to auditory stimulation show that the temporal dynamics of the evoked synaptic contributions to the EFP are consistent with synaptic short-term depression (STD). The estimated time constants of an STD model fitted to the data are similar to the fast time constants reported from in vitro experiments in the chick. Overall, the putative synaptic EFPs in the barn owl NL are significant but small (< 1% change of the variance by NBQX). This result supports the hypothesis that the EFP in NL is generated mainly by axonal spikes, in contrast to most other neuronal systems.