ANATOMICAL PROPERTIES OF FAST SPIKING CELLS THAT INITIATE SYNCHRONIZED POPULATION DISCHARGES IN IN IMMATURE HIPPOCAMPUS

Minislices of the CA3 hippocampal subfield were prepared from 10- to 1 5-day-old rats and exposed to penicillin, a GABA(A) receptor antagonis t. Synchronized population discharges occurred spontaneously but could also be entrained by action potentials in single, fast spiking cells. This was unexpected, since fast spiking cells in the hippocampus are normally thought to be inhibitory interneurons. Experiments were thus undertaken to determine the anatomical identity of these cells. Biocyt in injections showed that these cells had the anatomical feature of in hibitory interneurons. Two classes of cells were identified: basket ce lls (including cells with pyramidal or multipolar dendritic arbors) an d bistratified cells. Basket cells had characteristic dense axonal arb ors in the stratum pyramidale. They also possessed wide ranging axons in strata radiatum and oriens. The axons of bistratified cells avoided the cell body layer and produced a web-like plexus of axons in strata radiatum and oriens. In the majority of minislices, dye coupling was also observed. Interneurons were preferentially dye-coupled to other i nterneurons. We speculate that, in early life, hippocampal interneuron s may have dualistic synaptic properties. Normally, they inhibit nearb y pyramidal cells; however, when GABA, receptors are suppressed a seco ndary excitatory properly of these cells is uncovered. Copyright (C) 1 996 IBRO.