The prominence of blood-oxygen-level dependent functional magnetic resonance imaging (BOLD fMRI) in human imaging results from its noninvasiveness in mapping of brain activation and functional connectivity. However, the extent to which the BOLD fMRI signal actually reports local neuronal activity remains a matter of debate, which has led to the intense development of multiscale and multimodal recording approaches of brain function, in humans and small animals. In rodents, functional ultrasound (fUS) imaging has recently emerged as an efficient alternative to BOLD fMRI for functional brain mapping in anesthetized and awake animals, with millisecond time and mesoscopic spatial resolutions.¹ fUS detects changes of cerebral blood volume (CBV) of given axial speed with high imaging sensitivity (for reviews, see Refs. 2 3.–4). It can be combined (more easily than BOLD fMRI) with single or multiunit electrodes to correlate the fUS signal and neuronal activity.⁵ However, one drawback of electrodes recording is the impossibility to distinguish the firing of all neuron subtypes. In addition, the chronic placement of metal electrodes interferes with glial physiology and brain elasticity.⁶ This favors the single use of imaging modalities for long-term investigation of brain function, i.e., the combination of fUS or BOLD fMRI with two-photon laser scanning microscopy (TPLSM). TPLSM allows imaging of specific cell types, which can be genetically targeted to express fluorescence reporters of cell activity. As a result, it becomes possible to bridge cellular responses from given neuron subtypes or other cells of the neurovascular unit to mesoscopic signals collected with BOLD fMRI or fUS. This multimodal imaging approach imposes a preparation that complies with all technique limitations: (1) BOLD fMRI acquisition cannot be performed during normal behavioral experiments, but it does not impose any surgery on the mouse head; (2) the fUS signal-to-noise ratio (SNR) is theoretically best without a skull, but is still very good with a thinned-skull or a cranial polymethylpentene (PMP) window⁷ and remains workable through the normal skull and skin;⁸ (3) imaging with TPLSM is best through a cranial glass window but can be achieved through a thinned-skull; (4) both fUS and TPLSM can be performed during behavior in freely moving animals, but glass coverslips prevent fUS and BOLD fMRI. With all these limitations, TPLSM and BOLD fMRI have rarely been combined yet,^9,10 the three techniques only once.¹¹

Neurovascular coupling (NVC), which mediates rapid increases in cerebral blood flow in response to neuronal activation, is commonly used to map brain activation or dysfunction. Here we tested the reemerging hypothesis that CO₂ generated by neuronal metabolism contributes to NVC. We combined functional ultrasound and two-photon imaging in the mouse barrel cortex to specifically examine the onsets of local changes in vessel diameter, blood flow dynamics, vascular/perivascular/intracellular pH, and intracellular calcium signals along the vascular arbor in response to a short and strong CO₂ challenge (10 s, 20%) and whisker stimulation. We report that the brief hypercapnia reversibly acidifies all cells of the arteriole wall and the periarteriolar space 3–4 s prior to the arteriole dilation. During this prolonged lag period, NVC triggered by whisker stimulation is not affected by the acidification of the entire neurovascular unit. As it also persists under condition of continuous inflow of CO₂, we conclude that CO₂ is not involved in NVC.

INTRODUCTION: Metabolic disorders are risk factors for Alzheimer's disease (AD), although underlying mechanisms remain unclear. We investigated the relationship between peripheral metabolic markers – adiponectin, fibroblast growth factor 21 (FGF‐21), and insulin‐like growth factor binding protein 2 (IGFBP‐2) – and AD. METHODS: Participants with subjective cognitive decline (SCD), mild cognitive impairment (MCI), AD, and cognitively healthy (CH) controls were from the Consortium for the Early Identification of Alzheimer's Disease–Quebec cohort ( n = 287). Serum adiponectin, FGF‐21, and IGFBP‐2 concentrations were measured, compared between groups, and assessed for associations with clinical, cognitive, biochemical, and magnetic resonance imaging (MRI) data. RESULTS: Metabolic dysfunction was linked to lower adiponectin and IGFBP‐2, but higher FGF‐21. Both FGF‐21 and IGFBP‐2 increased with age and were inversely associated with cognitive performance. IGFBP‐2 was elevated at SCD stage and correlated with plasma tau phosphorylated at threonine 181 and amygdala atrophy. Adiponectin was unrelated to cognition. DISCUSSION: These findings suggest that IGFBP‐2 and, to a lesser extent, FGF‐21 may serve as early biomarkers of cognitive impairment, reflecting intricate links between peripheral dysmetabolism and AD.

Neurovascular coupling, linking neuronal activity to cerebral blood flow, is essential for brain function and underpins functional brain imaging. Whereas mechanisms involved in vasodilation are well-documented, those controlling vasoconstriction remain overlooked. This study unravels the mechanisms by which pyramidal cells elicit arteriole vasoconstriction. Using patch-clamp recording, vascular and Ca²⁺ imaging in mouse cortical slices, we show that strong optogenetic activation of layer II/III pyramidal cells induces vasoconstriction, correlating with firing frequency and somatic Ca²⁺ increase. Ex vivo and in vivo pharmacological investigations indicate that this vasoconstriction predominantly recruits prostaglandin E2 through the cyclooxygenase-2 pathway, and activation of EP1 and EP3 receptors. We also present evidence that specific interneurons releasing neuropeptide Y, and astrocytes, through 20-hydroxyeicosatetraenoic acid, contribute to this process. By revealing the mechanisms by which pyramidal cells lead to vasoconstriction, our findings shed light on the complex regulation of neurovascular coupling.

Old age, the most important risk factor for Alzheimer’s disease (AD), is associated with thermoregulatory deficits. Brown adipose tissue (BAT) is the main thermogenic driver in mammals and its stimulation, through β3-adrenergic receptor (β3AR) agonists or cold acclimation, counteracts metabolic deficits in rodents and humans. Studies in animal models show that AD neuropathology leads to thermoregulatory deficits and cold-induced tau hyperphosphorylation is prevented by BAT stimulation through cold acclimation. Since metabolic disorders and AD share strong pathogenic links, we hypothesized that BAT stimulation through a β3AR agonist could exert benefits in AD as well. Here, we show that β3AR agonist administration (CL-316,243, 1 mg/kg/day i.p., from 15 to 16 months of age) decreased body weight and improved peripheral glucose metabolism and BAT thermogenesis in both non-transgenic and 3xTg-AD mice. One-month treatment with a β3AR agonist increased recognition index by 19% in 16-month-old 3xTg-AD mice compared to pre-treatment (14-month-old). Locomotion, anxiety and tau pathology were not modified. Finally, insoluble Aβ42/Aβ40 ratio was decreased by 27% in the hippocampus of CL-316,243-injected 3xTg-AD mice. Overall, our results indicate that β3AR stimulation reverses memory deficits and shifts downward the insoluble Aβ42/Aβ40 ratio in 16-month-old 3xTg-AD mice. As β3AR agonists are being clinically developed for metabolic disorders, repurposing them in AD could be a valuable therapeutic strategy.