Observations on field potentials at their point of generation

A technique for evoking then recording field potentials through one extracellular electrode was studied in the dentate gyrus of pentobarbital-anesthetized rats. In the molecular layer (the location of granule cell dendrites), a -5 microA pulse (0.4 ms, 0.2 Hz) consistently elicited a 'focal' response the major component of which was a negative-going wave of about 1 ms latency, 10 ms duration, and -0.8 to -1.5 mV amplitude. This wave resembled, and could partially occlude, field excitatory post-synaptic potentials (EPSPs) elicited electrically from the perforant path. It fatigued during high-frequency stimulation and is suggested to consist largely of granule-cell EPSPs produced by directly activated, perforant-path terminals. Focal and perforant-path tetanic stimulation led to stable potentiation of the focal negative phase. Stimulus-response curves for the negative phase were roughly linear over most or all of the stimulus range of -1 to -5 microA, but on a finer scale were serrated and irregular. After a tetanus, different stimulus-response curves showed parallel leftward shifts or slope changes along all or part of their range, implying multiple mechanisms of potentiation that might include both threshold and amplification changes. Several uses are suggested in the paper for focal recording of compound potentials in research and diagnosis.