Probing activation-induced neurochemical changes using optogenetics combined with functional magnetic resonance...

Knowledge about biochemical processes is a prerequisite for a better understanding of mechanisms underlying brain activity. ¹ H functional magnetic resonance spectroscopy (¹ H-fMRS) at high magnetic fields allows for the non-invasive measurement of metabolic changes during brain activation. Optogenetics, on the other hand, has revolutionized the field of neuroscience. It was previously coupled with functional magnetic resonance imaging (fMRI) techniques in rodents enabling population-specific targeting of cells, investigating brain networks with unprecedented in vivo precision. The coupling of optogenetics and ¹ H-fMRS is expected to enhance the specificity of metabolic readouts to validate neuro-energetic theories underlying brain activity. To date, the feasibility of combining optogenetic stimulation with fMRS has not been explored. We used green laser stimulation delivered through an optical fiber implanted superficially above the primary somatosensory forelimb cortex (S1FL) of rats expressing the C1V1 opsin in excitatory neurons. A protocol for the acquisition of functional ¹ H MR spectra upon optogenetic stimulation without craniotomy-induced artefacts was established. Quantification of metabolite concentrations in S1FL upon optogenetic and electrical forepaw stimulation demonstrated significantly increased glutamate levels (+8.8% and +9.9%, p < 0.05 respectively), which were compensated by decreased glutamine levels (-17% and -18%, p < 0.05 respectively). Our results demonstrate for the first time the feasibility of combining optogenetic control and functional MR spectroscopy (o-fMRS) in the rat somatosensory cortex, opening new possibilities for monitoring the energetic demands of specific cell populations and for exploring the underpinnings of energy metabolism during brain activity. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.