Final Report Summary - NILSBO (Investigating the role of adult neurogenesis in spatial memory through optogenetic monitoring of neural activity)
The extracellular unit recording technique with tetrodes (a bundle of four electrodes) is a widely used method for recording neuronal activity in behaving animals. Simultaneously recorded spikes from different neurons are separated by spike sorting based on the amplitudes and shapes of the spikes as they appear differently on each of the four channels. While the technique can identify neuronal types based on their firing properties, its limitation is the inability to determine which anatomically defined cell is the source of the spikes. In the dentate gyrus (DG), unit recording technique detected a few electrophysiologically-defined cell types which are proposed by different studies to correspond to different anatomically-defined cell types (Neunuebel, 2012 (J Neuro); Leutgeb et al, 2007 (Science); Jung and McNaughton, 1993 (Hippocampus)). Thus, it is unknown how granule cells (GCs), the major anatomically-defined type of output neurons in the dentate gyrus, are activated even though we have information on distinct firing patterns recorded in the dentate gyrus. In this study, we aim to implement a technical approach overcoming this limitation (Lima, 2009; Zhang et al., 2013) and to record from identified GCs in the DG of behaving mice. We expressed the Channelrhodopsin-2 (Chr2) gene encoding a light-sensitive cation channel in GCs. In this way, these neurons are tagged so that they respond to light stimulation by generating spikes. Light-induced firing can then be used to identify the activity of targeted neurons in behaving animals. We have been working with transgenic animals that sparsely express Chr2 in DG GCs (POMC-Cre x B6;129S-Gt(ROSA)26Sortm32(CAG-COP4*H134R/EYFP)Hze/J). We used two types of chronically implanted micro drives, one with four independently moveable tetrodes and a separately implanted optic fiber and one with eight independently moveable tetrodes and an integrated moveable optic fiber (VersaDrive8 optical, Neuralynx). When using the drive with the separately implanted tetrode light pulses (472nm) of 3ms induced spikes with short latency (5-10ms) in the transgenic animals but not in Chr2 negative controls. This short latency suggests that light-induced activation is not synaptic but direct. Variability in spiking latency is substantial in most cases and spiking probability low. With increasing light intensity the variability in spiking latency is reduced and the probability increases. Taken together these results suggest we can indeed directly activate GCs that express Chr2 and record the light-induced activity. This result represents a major step towards our goal by showing that this is a promising approach to identify GCs in behaving mice. We were however not able to identify single-units. When we started using the newly available VersaDrive8 optical we were able to identify single-units. Moreover, we have been able to identify a mature granule cell in the dentate gyrus and subsequently record its spatially modulated activity during exploration. This represents proof of principle.