Light sheet microscopy for 3-D description of networks in hippocampal output stages
Abstract
Light sheet microscopy, combined with tissue clearing methods, allows for extremely detailed 3-D imaging of large portions of the brain. This imaging method uses a thin sheet of light to excite a fluorescent marker in a focal region of tissue, thus allowing for relatively fast sampling speed and high optical resolution. Sampling though the entire extent of a brain region allows for very detailed 3-D imaging of labeled structures. The technique is readily used in conjunction with immunofluorescence methods for labeling axon projections from small populations of neurons. Combined these techniques make it possible to visualize the entirety of the connections between discrete brain subfields across very large distances. The power of the system becomes evident when considering interdigitated brain structures that rotate along three axes, a long-standing problem for more traditional neuroanatomical methods. We propose to use light sheet microscopy to address a critical roadblock for the development of a theory ofhippocampus. The hippocampus plays an essential role in the acquisition and recall of the episodic (narrative) memories used by people to organize the flow of everyday experience. These operations are critical for much of cognition including planning of future activities and inferential thinking. A recent study provided striking evidence that discrete damage to hippocampus entirely eliminates the ability of people to recall the order in which events had occurred during a ~20 walk across a university campus. These individuals had lesser problems remembering the identity of the events or their spatial locations. We then discovered a unique network within the rodent hippocampus that maintains traces of inputs for several minutes, a time period unparalleled for brain circuits. Treatments that block this self-sustained, reverberating activity eliminated the capacity of the animals to acquire temporal order for a sequence of cues without interfering with learning the identity and location of the cues. We have constructed realistic simulations that help elucidate the neuronal events that enable these network level functions [Cox et al, Nature, Comm Biology, 2019].The multiple stages of hippocampus form a giant loop with the multiple association regions of the neocortex. The latter, which are enormous in human brain, are generally held to perform the most complex types of processing to be found in cortex. They provide the primary input to hippocampus, the multiple stages of which then funnel into a set of output nodes that project back to the association regions. Based on results noted above, we propose that a primary function of this remarkable cortico-hippocampal-cortical loop is to add time and sequencing to the highest levels of cortical operations. The anatomy and physiology of the inputs to hippocampus and the first three nodes of the intra-hippocampal network have been described in great detail. Much less is known about the networks and functional organization of the three output stages. The proposed light sheet microscope will be used in conjunction with discrete tracer injections to develop a 3-D picture of the connections within and between the subdivisions of these missing stages. The emerging map of output stage circuitry will be used to guide physiological experiments and simulations. We believe that the project, though ambitious, will lead to breakthroughs in our understanding of hippocampus and enable the construction of algorithms for brain based autonomous devices with the capability of dealing with dynamic real world situations.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 05, 2021
- Source ID
- N000142112436
Entities
People
- Gary Lynch
Organizations
- Office of Naval Research
- United States Navy
- University of California, Irvine