Field potential analysis of synaptic transmission in spiking neurons in a sparse and irregular neuronal structure in vitro

Extracellular field potential (FP) recording of dendritic depolarizations evoked by afferent stimulation is widely used as a quantitative measure of excitatory synaptic transmission in brain slices in-vitro for structures with high neuronal density and regularity such as hippocampus, neocortex and cerebellum. On the other hand, FP recordings of somatic depolarizations induced by orthodromic or antidromic stimulation of afferent or efferent nerves have been used in some in-vivo preparations for mapping the central projections of these nerves. In this study, we examined the applicability of somatic FPs as a measure of excitatory synaptic transmission in a sparse and irregular brain structure. Excitatory FPs were induced in nucleus tractus solitarius (NTS) in the dorsal medulla by electrical impulse stimulation of primary afferent fibers in the tractus solitarius (TS) in rat brainstem slices in vitro. The evoked FP was rapid and biphasic, and was stimulus-intensity dependent and saturable. The morphology of these somatic FPs resembled the dendritic FPs found in hippocampal and neocortical slices, with an excitatory postsynaptic component that exhibited similar pharmacological and stimulus frequency-dependent properties as found in NTS cells with intracellular or whole-cell recordings. Simultaneous FP and whole-cell recordings revealed that the postsynaptic component of FP was associated with neuronal firing rather than subthreshold membrane depolarizations. We conclude that somatic FP recording provides a simple and reliable measure of excitatory neurotransmission in the TS-NTS pathway and is a useful alternative or adjunct to intracellular or whole-cell recordings especially for studies of long-term synaptic plasticity in spiking neurons. This technique may also be applicable to other brain regions that lack the regular and dense organization of hippocampal and neocortical structures.