Optogenetic approaches to study complex neuronal circuits during cognitive aging

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DESCRIPTION (provided by applicant): Cognitive impairment during aging is a primary factor contributing to morbidity and mortality in the elderly and is one of the major challenges facing medicine today. It is thought that disruption of the excitatory/inhibitory synaptic ratio may represent the final common pathway contributing to cognitive decline during aging, and that new drug development is needed to resurrect altered synaptic function. The goal of this project is to develop new experimental models for aging research utilizing channelrhodopsin-2-yellow fluorescent protein ChR2(H134R)-EYFP BAC transgenic mice specific for the vesicular gamma-aminobutyric acid (GABA) transporter (VGAT) to study selective GABAergic synaptic terminal activation, and transgenic mice specific for the vesicular glutamate transporter 2 (Vglut2) to study selective glutamatergic (GLU) synaptic terminal activation. We will establish a colony for use at 4-6, 10-12 and 20-22 mo of age. We will employ optogenetic stimulation, patch-clamp recording, Ca2+ sensitive fluorescent imaging, confocal microscopy, water maze behavioral characterization and single cell RT-PCR (scRT-PCR) for cell identification. Caloric restriction wil be used to reverse or prevent age-related synaptic and cognitive deficits in order to support the relevance of the synaptic change to cognition and to establish the practicality for therapeutic intervention. We have a new "reduced synaptic preparation" amenable to optogenetic manipulation that will allow us to identify critical synaptic parameters contributing to detrimenta aging. The optogenetic method to control neuronal excitability is based on the cell-type specific expression of the light-activated channelrhodospin-2 (ChR2) which is a cation-permeable channel that enables cell depolarization in response to blue light. We hypothesize that GABAergic and GLU synaptic transmission is altered in basal forebrain cholinergic neurons during aging such that an increased ratio of synaptic excitation to inhibition (E/I ratio) contribues to cognitive impairment. It would be exceedingly difficult to support this hypothesis without the optogenetic resources present in these transgenic mouse lines. Our ultimate objective is to improve the quality of life in cognitively-impaired aged individuals by restoring "youthful synapses" through the use of better research tools and rational drug design. These studies are important because optogenetic techniques are emerging as a powerful tool to selectively study neurotransmitter systems in the brain; however, little is known about the functional expression of ChR2- EYFP in neurons across aging. This project is innovative because we will selectively stimulate and quantitate isolated GABAergic and GLU synapses at an unprecedented level for aging research in our reduced synaptic preparation. We will be the first to quantitate ChR2 functional expression at the cellular level across late aging in rodents, and we will establish an inexpensive animal model for future aging research.