Propagation of activity within the hippocampal dentate gyrus

Objectives: To investigate the spread of excitation in the dentate gyrus with stimulation of stratum granulosum at different sites.

Methods: Data were sampled from fluorescence changes of the voltage-sensitive dye RH 795 incorporated into cellular membranes after electric stimulation and recorded with a 464-photodiode array in a neuronal network in vitro. The decrease of fluorescence of the sensitive dye depends on the level of cell-excitation.

Results: The experiments showed that stimulation of the crest of the stratum granulosum evoked activity in this entire layer and in the hilus. The inner edge of the ectal limb adjacent to the presubiculum was omitted. Propagation of activity to the ectal limb and the hilus was twice (6.3ms) as fast as to the endal limb (12.6ms). The longer latency to the endal limb showed the involvement of multisynaptic processes indicating neural computation of activity.

If the point of stimulation was set in the stratum granulosum of the ectal limb the entire layer in the endal limb was activated whereas only the end of the stratum granulosum within the ectal limb and a small region adjacent to the prosubiculum of the entorhinal cortex were included.

Stimulation of the endal limb showed higher levels of activity within the dentate gyrus compared to other stimulation sites. The pattern of propagation, however, was similar to that evoked by stimulation of the stratum granulosum in the ectal limb. Activity from the endal limb reached the area around the crest within 3,78ms, i.e. that the time of information exchange and communication between these two regions was unusually fast. Propagation of activity to the stratum pyrimadale was nearly exclusively via the endal limb.

Throughout the different stimulation sites stratum granulosum of both limbs communicated with the hilus indicating the importance of this region.

Conclusion: In conclusion, the pattern of activity in the hilus, at both ends of the stratum granulosum and in the region adjacent to the prosubiculum was always the same independent of position and amplitude of the stimulus. The region adjacent to the presubiculum was always omitted. In spite of the macroscopic similar organization of both limbs the endal limb takes a more significant position regarding the propagation of activity.

According to the known course of the mossy fibres activity is primarily propagated to the stratum lucidum in the suprapyramidal zone and from there to the stratum pyramidale of the hippocampus proper.