Sung-Ho Lee

Blood-oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) has advanced brain mapping across species. This widely used technique, however, faces challenges such as acoustic noise, electromagnetic interference, motion artifacts, magnetic-field inhomogeneity and limitations in sensitivity and specificity. Here we introduce Steady-state On-the-Ramp Detection of INduction-decay with Oversampling (SORDINO), an improved zero echo time (ZTE)-based fMRI technique that maintains ultra-low slew rate and acquires data exclusively during gradient direction changes. At 9.4 T, SORDINO outperforms conventional gradient-recalled-echo-based echo-planar imaging and ZTE in metrics crucial for fMRI while being silent, sensitive, specific and resistant to motion and susceptibility artifacts. We showcase SORDINO’s superior compatibility with multimodal experiments and reveal T₁-based contrast mechanisms distinct from BOLD that are expected to generalize across ZTE-based fMRI techniques. We further demonstrate SORDINO brain-wide activity and functional connectivity mapping in awake, behaving mice, overcoming stress-related, motion-related and sensitivity-related limitations that remain common barriers in current animal fMRI studies.

The striatum, known as the input nucleus of the basal ganglia, is extensively studied for its diverse behavioral roles. However, the relationship between its neuronal and vascular activity, vital for interpreting functional magnetic resonance imaging (fMRI) signals, has not received comprehensive examination within the striatum. Here, we demonstrate that optogenetic stimulation of dorsal striatal neurons or their afferents from various cortical and subcortical regions induces negative striatal fMRI responses in rats, manifesting as vasoconstriction. These responses occur even with heightened striatal neuronal activity, confirmed by electrophysiology and fiber-photometry. In parallel, midbrain dopaminergic neuron optogenetic modulation, coupled with electrochemical measurements, establishes a link between striatal vasodilation and dopamine release. Intriguingly, in vivo intra-striatal pharmacological manipulations during optogenetic stimulation highlight a critical role of opioidergic signaling in generating striatal vasoconstriction. This observation is substantiated by detecting striatal vasoconstriction in brain slices after synthetic opioid application. In humans, manipulations aimed at increasing striatal neuronal activity likewise elicit negative striatal fMRI responses. Our results emphasize the necessity of considering vasoactive neurotransmission alongside neuronal activity when interpreting fMRI signal.

Task-free functional connectivity in animal models provides an experimental framework to examine connectivity phenomena under controlled conditions and allows for comparisons with data modalities collected under invasive or terminal procedures. Currently, animal acquisitions are performed with varying protocols and analyses that hamper result comparison and integration. Here we introduce StandardRat, a consensus rat functional magnetic resonance imaging acquisition protocol tested across 20 centers. To develop this protocol with optimized acquisition and processing parameters, we initially aggregated 65 functional imaging datasets acquired from rats across 46 centers. We developed a reproducible pipeline for analyzing rat data acquired with diverse protocols and determined experimental and processing parameters associated with the robust detection of functional connectivity across centers. We show that the standardized protocol enhances biologically plausible functional connectivity patterns relative to previous acquisitions. The protocol and processing pipeline described here is openly shared with the neuroimaging community to promote interoperability and cooperation toward tackling the most important challenges in neuroscience.

Summary: The striatum is the primary input nucleus of the basal ganglia, widely studied for its complex roles in health and disease. Functional magnetic resonance imaging (fMRI) studies are essential for discerning striatal function, however the relationship between neuronal and hemodynamic activity, critical for interpreting fMRI signals, has not been rigorously examined in striatum. We find that optogenetic stimulation of striatal neurons or afferents evokes negative striatal fMRI responses in rats that can occur despite broad increases in local neuronal activity. Intra-striatal pharmacological manipulations suggest that opioidergic, but not dopaminergic transmission contributes to negative striatal fMRI signals (the latter instead associated with positive signals). Striatal neuronal activity peaks are also associated with negative hemodynamic signals in behaving rats. Negative fMRI responses are observed in human striatum under conditions of anticipated neuronal activity increases. Our results prompt consideration of local cellular and neurochemical environments along with neuronal activity in fMRI signal interpretation.

The salience network (SN) and default mode network (DMN) play a crucial role in cognitive function. The SN, anchored in the anterior insular cortex (AI), has been hypothesized to modulate DMN activity during stimulus-driven cognition. However, the causal neural mechanisms underlying changes in DMN activity and its functional connectivity with the SN are poorly understood. Here we combine feedforward optogenetic stimulation with fMRI and computational modeling to dissect the causal role of AI neurons in dynamic functional interactions between SN and DMN nodes in the male rat brain. Optogenetic stimulation of Chronos-expressing AI neurons suppressed DMN activity, and decreased AI-DMN and intra-DMN functional connectivity. Our findings demonstrate that feedforward optogenetic stimulation of AI neurons induces dynamic suppression and decoupling of the DMN and elucidates previously unknown features of rodent brain network organization. Our study advances foundational knowledge of causal mechanisms underlying dynamic cross-network interactions and brain network switching.

The default mode network (DMN) of the brain is functionally associated with a wide range of behaviors. In this study, we used functional magnetic resonance imaging (fMRI), positron emission tomography (PET), and spectral fiber photometry to investigate the selective neuromodulatory effect of norepinephrine (NE)–releasing noradrenergic neurons in the locus coeruleus (LC) on the mouse DMN. Chemogenetic-induced tonic LC activity decreased cerebral blood volume (CBV) and glucose uptake and increased synchronous low-frequency fMRI activity within the frontal cortices of the DMN. Fiber photometry results corroborated these findings, showing that LC-NE activation induced NE release, enhanced calcium-weighted neuronal spiking, and reduced CBV in the anterior cingulate cortex. These data suggest that LC-NE alters conventional coupling between neuronal activity and CBV in the frontal DMN. We also demonstrated that chemogenetic activation of LC-NE neurons strengthened functional connectivity within the frontal DMN, and this effect was causally mediated by reduced modulatory inputs from retrosplenial and hippocampal regions to the association cortices of the DMN.