Global and local fMRI signals driven by neurons defined optogenetically by type and wiring

Top Cited Papers
Publisher Website
Google Scholar
Abstract
Functional magnetic resonance imaging (fMRI) studies are widely used to determine which brain regions are active during particular tasks or behaviours. There is much controversy over the source and interpretation of the BOLD (blood oxygenation level-dependent) signals that this imaging technique detects. Now, using fMRI in combination with optogenetics, a group of excitatory neurons in the rat brain has been identified as a source of the BOLD signals. Blood oxygenation level-dependent (BOLD) signals are the basis for much of the work on which regions of the human brain are active during particular tasks or behaviours, but there is controversy over their source and interpretation. Here a combination of optogenetics and BOLD signal monitoring shows that specific excitatory neurons within a mixed population are sufficient to produce positive BOLD signals, and could be used to map connections. Despite a rapidly-growing scientific and clinical brain imaging literature based on functional magnetic resonance imaging (fMRI) using blood oxygenation level-dependent (BOLD)1 signals, it remains controversial whether BOLD signals in a particular region can be caused by activation of local excitatory neurons2. This difficult question is central to the interpretation and utility of BOLD, with major significance for fMRI studies in basic research and clinical applications3. Using a novel integrated technology unifying optogenetic4,5,6,7,8,9,10,11,12,13 control of inputs with high-field fMRI signal readouts, we show here that specific stimulation of local CaMKIIα-expressing excitatory neurons, either in the neocortex or thalamus, elicits positive BOLD signals at the stimulus location with classical kinetics. We also show that optogenetic fMRI (ofMRI) allows visualization of the causal effects of specific cell types defined not only by genetic identity and cell body location, but also by axonal projection target. Finally, we show that ofMRI within the living and intact mammalian brain reveals BOLD signals in downstream targets distant from the stimulus, indicating that this approach can be used to map the global effects of controlling a local cell population. In this respect, unlike both conventional fMRI studies based on correlations14 and fMRI with electrical stimulation that will also directly drive afferent and nearby axons, this ofMRI approach provides causal information about the global circuits recruited by defined local neuronal activity patterns. Together these findings provide an empirical foundation for the widely-used fMRI BOLD signal, and the features of ofMRI define a potent tool that may be suitable for functional circuit analysis as well as global phenotyping of dysfunctional circuitry.