ALTERED INHIBITION OF DENTATE GRANULE CELLS DURING SPATIAL-LEARNING IN AN EXPLORATION TASK

To investigate the extent to which inhibitory interneurons control imp ulse flow through the dentate gyrus during spatial learning in an expl oration task, dentate field potentials were recorded in response to pa ired stimulation of the perforant path while rats rested or explored. Recurrent inhibition of the granule cells was measured as the reductio n of the second waveform when a population spike was present in the fi rst. Both the population spike and the field EPSP (fEPSP) were suppres sed at interstimulus intervals shorter than similar to 40 msec. Consis tent differences were observed between potentials recorded at equivale nt brain temperature in the exploration and resting (reference) condit ions, During exploration, the fEPSP of the second (test) waveform was reduced further compared with reference potentials with a similar resp onse to the first (conditioning) stimulus. This reduction was observed only when the first pulse elicited a population spike. The population spike of the second waveform was facilitated compared with reference potentials with similar fEPSP slopes. These observations suggest that exploration is coupled to increased inhibition on the perforant-path t erminals or the dendrites of the granule cells, whereas the inhibition on the somata is decreased. The two phenomena were not correlated and followed different time courses. The suppression of the fEPSP decayed gradually, although it was still present at 15 min, whereas the facil itation of the population spike was stable. Together, these changes, w hich likely involve different populations of interneurons, may focus a nd amplify incoming signals from the entorhinal cortex.