INVIVO AND INVITRO STUDIES ON PUTATIVE INTERNEURONES IN THE RAT HIPPOCAMPUS - POSSIBLE MEDIATORS OF FEEDFORWARD INHIBITION

Extracellular and intracellular recordings were made from non-pyramidal neurons in the rat hippocampus in vivo and in vitro. These cells were situated in the stratum pyramidale but were orthodromically activated with a lower threshold than pyramidal neurons in response to stimulation of the Schaffer collateral/commissural afferents. Cells fired earlier than pyramidal neurons in response to suprathreshold stimulation and, in contrast to pyramidal cells, often fired a burst of action potentials. The non-pyramidal neurons also appeared to be orthodromically activated on stimulating the alveus and fired later than antidromically activated pyramidal cells. A very short action potential duration and the ability to fire at high frequencies in response to long depolarizing current pulses also distinguished these neurons from pyramidal cells. These non-pyramidal cells are interneurons which could mediate an early feed-forward activity onto pyramidal cells such as feed-forward inhibition. They may also be recurrently activated and hence could conceivably mediate a recurrent inhibition.