Optogenetic functional magnetic resonance imaging investigation of brain-wide long-range projections

Abstract

One challenge in contemporary neuroscience is to achieve an integrated understanding of the large-scale brain-wide interactions, particularly the spatiotemporal patterns of neural activity that give rise to functions and behavior. At present, little is known about the spatiotemporal properties of long-range neuronal networks. Blood-oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is a noninvasive technique that can measure the hemodynamic responses as neural correlates throughout a nucleus with high spatial and temporal resolution. The objectives of this doctoral work were to develop state-of-the-art optogenetic and large-scale fMRI methods, for in vivo investigation of the patterns of large-scale brain-wide interactions in rodent models.

Firstly, brain-wide neural activity patterns elicited by stimulating ventral posteromedial (VPM) thalamocortical excitatory neurons and their functional relevance were examined through combined optogenetic stimulation and functional MRI (fMRI). Robust optogenetically-evoked fMRI activation was detected bilaterally in primary visual, somatosensory and auditory cortices at low (1 Hz) but not high frequencies (5-40 Hz). Subsequent electrophysiological recordings indicated interactions over long temporal windows across thalamo-cortical, cortico-cortical, and interhemispheric callosal projections at low frequencies. Visually-evoked fMRI activation during and after VPM stimulation in superior colliculus were enhanced, indicating that visual processing was subcortically modulated by low frequency activity originating from VPM. Secondly, a comparison of a parallel yet distinct somatosensory thalamo-cortical pathway was made. Stimulating posteromedial complex thalamocortical excitatory neurons also evoked brain-wide BOLD activation, although with a distinct spatiotemporal profile. The results directly demonstrate that low frequency activity governs large-scale, brain-wide connectivity and interactions through long-range excitatory projections to coordinate the functional integration of remote brain regions. These findings provide a fresh impetus to study the mysteries of the brain.

Lastly, optogenetics and pharmacological inactivation techniques were used to uncover the neural bases of resting-state fMRI (rsfMRI). Low frequency (1 Hz) optogenetic activation of VPM thalamocortical excitatory neurons enhanced interhemispheric/bilateral primary somatosensory barrel field (S1BF), secondary somatosensory (S2), primary visual (V1) and primary auditory (A1) cortical functional connectivities. Moreover, the increase in functional connectivities persisted even after the cessation of stimulation. Comparatively, pharmacological inactivation of VPM thalamocortical neurons using Tetrodotoxin (TTX) decreased interhemispheric S1BF, S2, V1 and A1 functional connectivities. These results demonstrated the power of combining neuromodulatory techniques with rsfMRI for investigation of the neural bases underlying brain-wide functional connectivity. Future studies will apply this method in combination with electrophysiological recordings to examine the changes in neural activity that underpin the changes observed in rsfMRI connectivity.