INTERNEURONS AND INHIBITION IN THE DENTATE GYRUS OF THE RAT IN-VIVO

Inhibitory cells are critically involved in shaping normal hippocampal function and are thought to be important elements in the development of hippocampal pathologies. However, there is relatively little inform ation about the extent and pattern of axonal arborization of hippocamp al interneurons and, therefore, about the sphere of influence of these cells. What we do know about these cells is based largely on in vitro slice studies, in which interneuronal interactions may be severely at tenuated. The present study was carried out to provide a more realisti c picture of interneuron influence. Intracellular recordings were obta ined from dentate interneurons in the intact brain of anesthetized rat s, and cells were intracellularly labeled with biocytin. The axonal ar bors of two classes of dentate interneurons were traced through the hi ppocampus; each was found to extend long distances (up to half of the total septotemporal length of the hippocampus) perpendicular to the hi ppocampal lamellae and to target preferential strata. These results su ggest that dentate interneurons have far-reaching effects on target ce lls in distant hippocampal lamellae. One implication of this finding i s that dentate neurons should receive more inhibitory synaptic drive i n vivo than in slice preparations, in which many inhibitory axon colla terals are amputated. Synaptic responses to perforant path stimulation were examined in granule cells, messy cells, and CA3 pyramidal cells in vivo, for comparison with previously published results from hippoca mpal slice studies. In vivo, all cell types showed excitatory synaptic responses that were brief and limited by robust IPSPs that were large r in amplitude and conductance than responses to comparable stimuli re corded in vitro. This difference could not be explained by a change in the intrinsic physiological properties of the cells in the slice prep aration, because those parameters were similar in vivo and in vitro. W e conclude that dentate gyrus inhibitory interneurons can affect the e xcitability of neurons in distant areas of the hippocampus, and that t hese distant influences cannot be appreciated in conventional in vitro preparations.