Brain‐wide Network Restructuring after Chemogenetic Locus Coeruleus Activation: Implications of Tonic Noradrenergic Activity for Neurodegeneration (2022)

Loss of noradrenergic neurons in a small brainstem nucleus, the locus coeruleus (LC), is a common characteristic in the development of age‐related neurodegenerative diseases such as Alzheimer's disease (AD) and other dementias. The noradrenergic system (NS) projects from the locus coeruleus (LC) throughout the brain to modulate neurophysiology for memory, learning and cognition, for regulation of sleep and circadian rhythm, and in response to stress and reward. Thus dysregulation of NS activity, i.e ., blunted or tonic firing of LC neurons and its resultant effect on neural activity, is considered a significant factor in development and progression of neurodegenerative diseases as well as of psychiatric disorders. Our hypothesis is that tonic activation of LC neurons acutely discoordinates neuronal firing patterns across the brain, and ultimately provokes a prolonged abnormal brain state. We test this hypothesis via brain‐wide manganese‐enhanced MRI (MEMRI) in parallel with recordings of naturalistic behavior before, during, and long after chemogenetic (hM3Dq) stimulation of LC neurons. After systemic delivery, paramagnetic Mn(II) ions (0.3 mmol/kg, i.p.) accumulate in active neurons of awake‐behaving mice, highlighting neural activity by T 1 ‐weighted MRI. Adult mice (3‐6 months) were subjected either to longitudinal MEMRI (n = 18) or to open field behavior (n = 8), ± hM3Dq expression; and ± designer drug – clozapine‐N‐oxide (CNO). For behavior naïve C57BL/6 mice were video recorded and behavioral parameters tracked by Noldus Ethovision XT15, before and after administration of CNO (5 mg/kg, i.p.). Mice then underwent stereotactic injection of CAV2‐PRS‐hM3D(Gq)‐mCherry targeting the LC (AP = ‐5.4, ML = ±0.80, DV = ‐3.80). At 4‐8 weeks after transfection the effect of CNO ± hM3Dq on LC neurons was monitored in live animals by pupillary light response. Pre‐ and post‐CNO behavior was again recorded. For MEMRI two groups ± hM3Dq were imaged before and after CNO. MEMRI images were analyzed for Mn(II)‐dependent intensity increases by statistical parametric mapping (SPM), segment‐wise cross‐correlation and network analysis. Transgene expression was confirmed post‐mortem by microscopy of mCherry in LC brain sections, which showed transgene expression in bilateral LC. Comparison of MEMRI segment‐wise intensities revealed changes of strength and direction of correlations between time points that also diffed between groups. Hierarchical clustering of these correlations revealed 40% increase in modularity, i.e., the number of distinct clusters, after CNO only in the hM3Dq transfected group. Together our data suggest discoordination of brain regions after LC‐NS activation. In neurodegenerative disease the loss of LC input could result in the opposite effects, while none‐the‐less leading to discoordination of brain‐wide activity underlying sleep disorders and memory disturbance. This brain‐wide imaging strategy with chemogenetic LC manipulation provides critical insight into NS modulation of neural activity under both normal and pathological conditions.