Locus Coeruleus (LC) is a small brainstem nucleus that synthesizes nearly all the norepinephrine (NE) in the brain¹. Aberrant firing of LC neurons has been linked to changes in attention and memory². In neurodegenerative disorders, senile plaques and neurofibrillary tangles first appear in the LC and neuronal loss in LC is reported greater than any other nuclei in the brain³. Despite the importance of probing LC functions on a systems level, traditional pharmacology and brain stimulation techniques fail to unveil the LC signaling cascades due to the lack of specificity. In this study, we employed a novel approach that expresses Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) solely on the noradrenergic cells of the LC and performed a parallel fMRI and PET studies in these transgenic mice. DREADDs are mutated G-protein coupled muscarinic receptors that have lost their binding affinity towards acetylcholine, yet gained a binding affinity to the otherwise pharmacologically inert ligand clozapine-N-oxide (CNO)⁴. Upon CNO binding, neurons become depolarized generating an prolonged changes in neuronal firing that lasts several minutes to hours. To our knowledge, this is the first reported chemogenetic study that maps neurocircuits in vivo using MRI and PET. Methods: Male C57BL6;129 En1:cre; Dbh;FLPo; RC::FL-hM3Dq mice genetically expressing hM3Dq and mCherry (Red) in NE neurons of the LC were used (Figure 1A). The genetic profile of the mice was confirmed by PCR genotyping for expression of both Cre and FLPo on both alleles, guaranteeing complete accuracy of hM3Dq expression on noradrenergic neurons of the LC. CNO modulation of these DREADD expressing neurons was verified by electrophysiology (Figure 1B). For PET experiments, mice were initially anesthetized under 1-3% isoflurane and injected with saline or CNO (10 mg/kg, i.p.) in separate sessions. 18FDG was injected after 5 min, followed by a recovery period were the mice were allowed to resume their normal activity in their cage. After 45 minutes, the mice underwent a 30 min static mode PET scan. All PET data were processed using PMOD software. For fMRI studies, mice were initially anesthetized using 1-3% isoflurane and maintained under light anesthetized (0.75-1% isoflurane) while maintain physiological homeostasis. CBV-weighted fMRI responses were measured by injecting a bolus dose of an in-house iron oxide nanoformulation (30 mg Fe/kg) via a tail vein catheter. Single shot, single sampled GE-EPI sequences (BW= 250 kHz, TR= 3000 ms, TE= 8 ms, matrix=64X64x64, FOV= 1.92x1.92x1.92 cm3) were acquired using a Bruker 9.4T MR scanner with a 72 mm quad-transmit only volume coil and a quad-receive only mouse brain coil. fMRI data was continuously acquired for 40 min, with CNO administered 10 min into the scan (10 mg/kg, ip). Isotropic 3D fMRI data were motion corrected, bandpass filtered (0.01-0.1Hz), and aligned to a baseline EPI population atlas using AFNI-based pipeline⁵ and manually skull stripped on ITK-SNAP. Independent Component Analysis (ICA) was performed on fMRI data using 60 IC maps with FSL MELODIC and rendered in 3D using AMIRA. Functional microangiography of the cerebrovasculature was also assessed during pre- and post-CNO acquisitions using a 3D FLASH sequence similar to that described elsewhere6 and analyzed using AMIRA. Three dimensional ΔR2* maps were calculated using ΔR2* = ln(S_pre/S_post)/TE, where S_pre and S_post are the pre-contrast background and post-contrast signal intensities of T2-weighted images. Changes in vascular tone were visualized in 3D using AMIRA and changes in vessel diameter were quantified through the Skeltonization Option. LC neuronal firing was stimulated and maintained by CNO administration, producing significant decreases in glucose metabolism in the cortex, hippocampus and cerebellum and increases in the thalamus when compared to the baseline glucose uptake on the same saline treated mice (Figure 2A; p<0.5). In support of the PET data, ICA maps generate with a statistical threshold of (p<0.5) showed decreased CBV across cortical nuclei, cerebellum and hippocampus after CNO-induced LC firing and statistical increases in perfusion at the caudate putamen, thalamus and hypothalamus (Figure 2B). Functional MRA confirmed that the vessels feeding the parenchymal tissues of the brain showed vasodilation with ~20% reduction in vessel diameters after CNO administration (Figure 2C), which was supported by findings that the LC neurons directly innervate parenchymal vessels and not pial vessels (Figure 1C). These data suggest that increased release of NE from LC neurons selectively modulate glucose metabolism and vascular tone throughout the brain. Interestingly, the stimulating LC firing induces systemic vasoconstriction in most brain regions except the thalamus, hypothalamus and cortical regions near pial vessels (Figure 1C). This study suggests that NE released by the LC may modulate neurocircuits through limiting perfusion and glucose uptake.