Direct afferent excitation and long-term potentiation of hippocampal interneurons.

Abstract

Physiological properties of interneurons in the CA1 region and dentate gyrus were investigated in urethane-anesthetized rats using extracellular recording techniques. Commissural and associational pathways to CA1 and commissural and perforant path inputs to the dentate gyrus were stimulated with single pulses of low intensity. Interneurons (17), were recorded from the CA1 region and 16 in the dentate gyrus. Of these, 22 were activated by > 1 input. All of them could be fired repetitively to single-pulse stimulation with frequencies .ltoreq. 700 Hz. Nearly all of them fired on the rising part of the field potential preceding the population spike. Of the interneurons, > 70% had lower thresholds than the threshold of the population spike to the respective pathways. Interneurons of the hippocampal formation seem directly excited by various inputs, consequently principal cells may be inhibited in a feedforward manner. Following a single 40-Hz train of 5 s, 8 of 11 interneurons tested showed increased probability of cell discharge to subsequent single pulses for up to 2 h. The low stimulus intensities used made it unlikely that this long-term potentiation was conveyed disynaptically via a recurrent circuit.