INTERNEURONS IN THE HIPPOCAMPAL DENTATE GYRUS - AN IN-VIVO INTRACELLULAR STUDY

Interneurons in the dentate area were characterized physiologically an d filled with biocytin in urethane-anaesthetized rats. On the basis of axonal targets the following groups could be distinguished. (i) Large multipolar interneurons with spiny dendrites in the deep hilar region densely innervated the outer molecular layer and contacted both granu le cells and parvalbumin-positive neurons (hilar interneuron with perf orant pathway-associated axon terminals; HIPP cells). (ii) A pyramidal -shaped neuron with a cell body located in the subgranular layer inner vated mostly the inner molecular layer and the granule cell layer (hil ar interneuron with commissural-associational pathway-associated axon terminals; HICAP cell). It contacted both granule cells and interneuro ns, Axon collaterals of HIPP and HICAP neurons covered virtually the e ntire septo-temporal extent of the dorsal dentate gyrus. (iii) Calbind in-immunoreactive neurons with horizontal dendrites in stratum oriens of the CA3c region gave rise to a rich axon arbor in strata oriens, py ramidale and radiatum and innervated almost the entire extent of the d orsal hippocampus, with some collaterals entering the subicular area ( putative trilaminar cell). (iv) Hilar basket cells innervated mostly t he granule cell layer and to some extent the inner molecular layer and the CA3c pyramidal layer. HIPP and trilaminar interneurons could be a ntidromically activated by stimulation of the fimbria. Only the HICAP cells could be monosynaptically discharged by the perforant path input . All interneurons examined showed phase-locked activity to the extrac ellularly recorded theta/gamma oscillations or to irregular dentate el ectroencephalogram spikes. These observations indicate that the interc onnected interneuronal system plays a critical role in coordinating po pulation activity of the dentate gyrus and Ammon's horn.