In vivo and in vitro studies on putative interneurones in the rat hippocampus: possible mediators of feed-forward inhibition.

Extracellular and intracellular recordings have been made from non-pyramidal neurones 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 neurones in response to stimulation of the Schaffer collateral/commissural afferents. Cells fired earlier than pyramidal neurones in response to suprathreshold stimulation and, in contrast to pyramidal cells, often fired a burst of action potentials. The non-pyramidal neurones 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. It is suggested that these non-pyramidal cells are interneurones 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.