Optogenetically-initiated low frequency dorsal hippocampal activity enhances resting-state functional MRI (rsfMRI) connectivity and long-term visual memory performance

Abstract

Introduction: Functional connectivity measured by resting-state functional MRI (rsfMRI) has been suggested to be an expression of brain-wide network behavior underlying cognitive functions[1]. However, there is a lack of direct evidence demonstrating their association. Here, we deploy a combined approach of functional MRI, rsfMRI, optogenetic neuromodulation and behavioral tests to examine the relationship between rsfMRI connectivity and cognition. Specifically, we interrogated the hippocampal-cortical network, pivotal for memory functions.

Methods: Animal preparation: 3μl of AAV5-CaMKIIa::ChR2(H134R)-mCherry (optogenetic) or saline (sham control) was injected to dorsal dentate gyrus (dDG) of adult male SD rats. Six weeks after injection, an opaque optical fiber cannula was implanted chronically at the injection site. rsfMRI and behavioral experiments: rsfMRI scans were performed before, during and after 1/40Hz optogenetic stimulation (n=18; 473nm; 40mW/mm; 10%/30% duty cycle; 10-mins) and analyzed based on our recently established protocols[2]. Novel-object recognition tests were utilized to assess short- and long-term memory performance[3]. In Group I, animals (optogenetic: n=15; sham control: n=11) received 1Hz stimulation twice before the assessment of short- and long-term memory. In Group II and III (optogenetic: n=7 and n=9), stimulation was only presented once prior to the assessment of short- and long-term memory, respectively.

Results: Low frequency, not high frequency, optogenetic stimulation of dDG excitatory neurons enhances brain-wide resting-state functional connectivity: We found that 1Hz, not 40Hz, stimulation increased interhemispheric rsfMRI connectivity in dorsal hippocampus and sensory cortices (i.e., visual, auditory and somatosensory). Remarkably, connectivities remained elevated after cessation of stimulation within the ~2.5-hrs rsfMRI experimental window. Low frequency dDG stimulation enhances long-term visual memory performance: As expected, all animals displayed good short-term memory performance[3]. However, for long-term memory performance, only optogenetic animals showed stronger preference for the novel object (Group I). Interestingly, we found that such enhanced long-term memory performance was only replicated in optogenetic animals that received stimulation prior to the assessment of long-, not short-term memory. This particular observation indicates that the stimulation enhanced the hippocampal memory retrieval, and not encoding processes.

Conclusion: Our findings reveal the presence of a strong association between brain connectivity (as measured by rsfMRI) and cognitive performance. Our optogenetic fMRI approach presents unique opportunities to not only understand rsfMRI connectivity, but also use them for examining and optimizing neuromodulation efficacies in the future.