Optogenetic tool development and interrogation of frequency-dependent signaling in the hippocamposeptal pathway [electronic resource]
Keywords: Neurosciences.

Hippocampal oscillations are critical for information processing, and are strongly influenced by inputs from the medial septum. Hippocamposeptal neurons provide direct inhibitory feedback from the hippocampus onto septal cells, and are therefore likely to also play an important role in the circuit; these neurons fire at either low or high frequency, reflecting hippocampal network activity during theta oscillations or ripple events, respectively. Since the hippocamposeptal projection is sparse and long-range, the impact of high or low frequency hippocampal input on septal physiology has not been addressable with classical electrophysiological or pharmacological techniques. In order to understand the contribution of defined neuronal subtypes, such as hippocamposeptal neurons, to brain function, our laboratory has developed a technique termed optogenetics, which integrates genetic targeting and optical stimulation to achieve temporally precise manipulation of genetically and spatially defined cell types in intact tissue. Optogenetics employs light sensitive microbial proteins, including ion pumps and channels that can elicit or inhibit action potentials. Optogenetics has already proved invaluable to neuroscience, but several key limitations to its application have become apparent: First, increasingly diverse optogenetic tools allow more versatile control over neural activity, but since new tools have been developed in multiple laboratories and tested across different preparations it is difficult to draw direct comparisons between them. As a result, it has become increasingly challenging for end users to select the optimal reagents for their experimental needs. Second, as the power of genetically encoded interventional and observational tools for neuroscience expands, the boundary of experimental design is increasingly defined by limits in selectively expressing these tools in specific cell types. To date, cell-type has primarily referred to genetic specificity, achieved with promoter-driven expression either in transgenic animals or in viruses. This approach is limited in its ability to define a 'cell type': cells may be targeted based on only a single parameter, and genetic targeting does not take into account anatomic connectivity, in many cases the most salient feature of a target population. The aim of this thesis is thus three-fold: 1) To interrogate frequency-dependent signaling in the hippocamposeptal pathway, using optogenetics to gain cell-type specific, temporally-precise control over hippocamposeptal fibers, 2) To systematically compare microbial opsins under matched experimental conditions to extract essential principles and identify key parameters for the conduct, design and interpretation of experiments involving optogenetic techniques, and 3) To develop new viral and molecular strategies to target cells of interest based on both genetic and topological parameters. The investigation of the hippocamposeptal projection will increase our understanding of the larger circuit of which it is a part, and will also illustrate the importance of firing frequency in neuronal signaling. The tool development described will be useful for future work investigating the hippocamposeptal pathway in particular, and more generally for a broad variety of applications of optogenetics to neuroscience.