HIPPOCAMPAL CA1 INTERNEURONS - AN IN-VIVO INTRACELLULAR LABELING STUDY

Fast spiking interneurons in the CA1 area of the dorsal hippocampus we re recorded from and filled with biocytin in anesthetized rats. The fu ll extent of their dendrites and axonal arborizations as well as their calcium binding protein content were examined. Based on the spatial e xtent of axon collaterals, local circuit cells (basket and O-LM neuron s) and long-range cells (bistratified, trilaminar, and backprojection neurons) could be distinguished. Basket cells were immunoreactive for parvalbumin and their axon collaterals were confined to the pyramidal layer. A single basket cell contacted more than 1500 pyramidal neurons and 60 other parvalbumin-positive interneurons. Commissural stimulati on directly discharged basket cells, followed by an early and late IPS Ps, indicating interneuronal inhibition of basket cells. The dendrites of another local circuit neuron (O-LM) were confined to stratum orien s and it had a small but high-density axonal terminal field in stratum lacunosum-moleculare. The fastest firing cell of all interneurons was a calbindin-immunoreactive bistratified neuron with axonal targets in stratum oriens and radiatum. Two neurons with their cell bodies in th e alveus innervated the CA3 region (backprojection cells), in addition to rich axon collaterals in the CA1 region. The trilaminar interneuro n had axon collaterals in strata radiatum, oriens and pyramidale with its dendrites confined to stratum oriens. Commissural stimulation evok ed an early EPSP-IPSP-late depolarizing potential sequence in this cel l. All interneurons formed symmetric synapses with their targets at th e electron microscopic level. These findings indicate that interneuron s with distinct axonal targets have differential functions in-shaping the physiological patterns of the CA1 network.