Characterization of GABAergic and Cholinergic Medial Septal Neurons for Non-Selective Optogenetic Control of the Septohippocampal Network

Science Center, Bent Corridor

The hippocampal theta rhythm is a robust oscillation ranging from 5-12 Hz and is most prevalent during voluntary locomotive, “exploratory” behavior and paradoxical sleep (Vanderwolf, 1969). The medial septum (MS) has been implicated as being the primary pacemaker of hippocampal theta rhythm, supported by research documenting robust anatomical and neurophysiological connections between the structures in the form of GABAergic, cholinergic, and glutamateric neuronal subpopulations. As such, the MS may represent a target for influencing hippocampal oscillations, particularly in the theta band. Recent work in the ECD group has sought to utilize optogenetic control of the MS to study the effects of septal stimulation on hippocampal rhythmic activity in a variety of behavioral contexts. Recent work demonstrated that rats performed better on training day 1 of a T-maze training task when receiving 6 or 30 Hz septal stimulation compared to yellow light control stimulation (Blumberg et al., 2016). However, it has not been determined if the viral vector used to introduce channelrhodopsin-2 (ChR2) into the septum preferentially transfects specific neuronal subpopulations, or transfects in a non-specific manner. Here, we undertook a histological investigation in order to determine the precise breakdown of cell types transfected with the virus. Three rats were injected in the medial septum with an AAV viral vector and tissue was collected and stained with antibodies for glutamate decarboxylase (GAD) and choline acetyltransferase (ChAT) for quantitative cell counts of GABAergic and cholinergic neurons, respectively. Preliminary data of quantitative cell counts revealed a trend of more GABAergic neurons than cholinergic neurons, though this did not approach significance. However, the proportion of cells transfected with the virus was roughly equal across GABAergic and cholinergic cells. Taken together, these preliminary findings suggest a trend of non-selective optogenetic control of the medial septum.