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| Home > Publications database > Optical stimulation evokes sustained activity in the isolated medial septum |
| Abstract | FZJ-2019-04534 |
;
2019
Abstract: The processing of spatially related input during locomotion involves oscillatory hippocampal (HPC) activity in the theta band. It is known that the medial septum (MS) plays a central role in the generation of HPC theta activity, but the underlying mechanisms have not yet been described. Fuhrmann et al. [1] have shown that a brief stimulation of glutamatergic (VGluT2) neurons in the mouse MS in vivo evokes sustained theta activity in the HPC local-field potential (LFP), lasting for at least 10 seconds and preceding the onset of locomotion. Blocking of glutamatergic synapses in the MS suppresses sustained theta activity.Here, we investigate to what extent the MS alone can generate sustained activity. To this end, we study responses of individual MS neurons to optical stimulation in acute mouse MS slices recorded by microelectrode arrays (MEAs). MS slices exhibit spontaneous activity, with a fraction of neurons being active at rates of 5-15 spikes/s. Brief 1-second optical stimulation of VGluT2 neurons consistently leads to a sustained increase in the activity in some of the MS neurons, lasting for several, sometimes more than 10 seconds. The same effect is observed in slices with blocked glutamatergic and/or GABAergic connections (see Figure 1). Irrespective of the blocking condition, we do not detect any signs of spike-train synchronization or spatial clustering of stimulus evoked sustained activity. Stimulation of parvalbumin-expressing (PV) neurons does not lead to any significant firing rate modulation after stimulus offset.We conclude that the isolated MS is capable of generating sustained activity at time scales comparable to those found in the HPC [1]. The generation of this sustained activity seems to be the result of a bistable dynamics of individual VGluT2 neurons, and does not rely on synaptic interactions within the MS network. Single neurons exhibiting bistable dynamics have been described in earlier studies [2,3].It remains to be shown how coherent HPC theta activity can emerge from asynchronous sustained activation of MS neurons, and to what extent the stimulus-evoked generation of sustained HPC theta activity relies on direct projections from VGluT2 neurons to the HPC. Future work is further dedicated to a systematic comparison between the characteristics (duration, stimulus efficiency) of sustained spiking activity in the MS, sustained theta activity in HPC LFPs, and behavioral responses.
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