SPECIALIZED ELECTROPHYSIOLOGICAL PROPERTIES OF ANATOMICALLY IDENTIFIED NEURONS IN THE HILAR REGION OF THE RAT FASCIA-DENTATA

Because of their strategic position between the granule cell and pyram idal cell layers, neurons of the hilar region of the hippocampal forma tion are likely to play an important role in the information processin g between the entorhinal cortex and the hippocampus proper. Here we pr esent an electrophysiological characterization of anatomically identif ied neurons in the fascia dentata as studied using patch-pipette recor dings and subsequent biocytin-staining of neurons in slices. The resti ng potential, input resistance (R-N), membrane time constant (tau(m)), ''sag'' in hyperpolarizing responses, maximum firing rate during a 1- s current pulse, spike width. and fast and slow afterhyperpolarization s (AHPs) were determined for several different types of hilar neurons. Basket cells had a dense axonal plexus almost exclusively within the granule cell layer and were distinguishable by their low R-N, Short ta u(m), lack of sag, and rapid firing rates. Dentate granule cells also lacked sag and were identifiable by their higher R-N, longer tau(m), a nd lower firing rates than basket cells. Mossy cells had extensive axo n collaterals within the hilus and a few long-range collaterals to the inner molecular layer and CA3c and were characterized physiologically by small fast and slow AHPs. Spiny and aspiny hilar interneurons proj ected primarily either to the inner or outer segment of the molecular layer and had a dense intrahilar axonal plexus, terminating onto somat a within the hilus and CA3c. Physiologically, spiny hilar interneurons generally had higher R-N values than mossy cells and a smaller slow A HP than aspiny interneurons. The specialized physiological properties of different classes of hilar neurons are likely to be important deter minants of their functional operation within the hippocampal circuitry .