The rat gracile nucleus in vitro: II. Field potentials and their conditioned depression.

The field potentials which follow the compound tract action potential recorded from the rat gracile nucleus in vitro were studied. The field potentials were evoked by stimulating the ipsilateral dorsal column. Depth profile experiments revealed that the negative (N) wave recorded near the dorsal surface of the nucleus inverted to a positive (P) wave deeper in the nucleus. The negative wave consisted of an initial spiky component (NA) which peaked at a latency of 2-5 ms and temporally corresponded with the firing of post-synaptic units. This wave was usually fused with a long slow wave which peaked at 8-10 ms and often lasted for over a second. This slow wave was composed of a relatively brief bicuculline-sensitive wave (NB) superimposed on a longer bicuculline-resistant wave (NC). All of these components could be distinguished in the P wave recorded deeper in the nucleus. The amplitude of the field potential was depressed following an identical preceding stimulus delivered to the dorsal columns. This conditioned depression of the field potential had a similar time-course to the field potential itself and, likewise, had both a bicuculline-sensitive and a bicuculline-resistant component. The depression of field potentials outlasted the depolarization of the gracile afferents indicating that post-synaptic mechanisms may be involved in this long-lasting phenomenon.