Inhibitory interneurons can evoke vasodilation and vasoconstriction, making them potential cellular drivers of neurovascular coupling. However, the specific regulatory roles played by particular interneuron subpopulations remain unclear. Our purpose was therefore to adopt a cell-specific optogenetic approach to investigate how somatostatin (SST) and neuronal nitric oxide synthase (nNOS)-expressing interneurons might influence the neurovascular relationship. In mice, specific activation of SST- or nNOS-interneurons was sufficient to evoke hemodynamic changes. In the case of nNOS-interneurons, robust hemodynamic changes occurred with minimal changes in neural activity, suggesting that the ability of blood oxygen level dependent functional magnetic resonance imaging (BOLD fMRI) to reliably reflect changes in neuronal activity may be dependent on type of neuron recruited. Conversely, activation of SST-interneurons produced robust changes in evoked neural activity with shallow cortical excitation and pronounced deep layer cortical inhibition. Prolonged activation of SST-interneurons often resulted in an increase in blood volume in the centrally activated area with an accompanying decrease in blood volume in the surrounding brain regions, analogous to the negative BOLD signal. These results demonstrate the role of specific populations of cortical interneurons in the active control of neurovascular function.

20Inhibitory interneurons can evoke vasodilation and vasoconstriction, making them 21 potential cellular drivers of neurovascular coupling. However, the specific regulatory 22 roles played by particular interneuron subpopulations remain unclear. Our purpose 23 was therefore to adopt a cell-specific optogenetic approach to investigate how 24 somatostatin (SST) and neuronal nitric oxide synthase (NOS1)-expressing 25 interneurons might influence neurovascular relationships. In mice, specific activation 26 of SST-or NOS1-interneurons was sufficient to evoke haemodynamic changes similar 27 to those evoked by physiological whisker stimulation. In the case of NOS1-28 interneurons, robust haemodynamic changes occurred with minimal changes in neural 29 activity. Conversely, activation of SST-interneurons produced robust changes in 30 evoked neural activity with shallow cortical excitation and pronounced deep layer 31 cortical inhibition. This often resulted in a central increase in blood volume with 32 corresponding surround decrease, analogous to the negative BOLD signal. These 33 results demonstrate the role of specific populations of cortical interneurons in the 34 active control of neurovascular function.35 36 80 of cortical GABAergic interneurons have specific roles in NVC. Also, that the ability of 81 BOLD signals to act as a surrogate measure of local neural activation may in part be 82 dependent upon which subpopulation of neurons are being activated.83 84 4 Results 85 Short duration optogenetic stimulation of specific interneurons evokes a 86 localised haemodynamic response 87 Genetically modified mice expressing channelrhodopsin-2 (ChR2) in either SST-or 88 NOS1-expressing interneurons (referred to as SST-ChR2 or NOS1-ChR2 mice, 89 respectively) were used to investigate how light induced activity of these inhibitory 90 interneurons may alter cortical haemodynamics. Using an anaesthetised mouse 91 (Figure 1), we assessed whether short duration optogenetic stimulation of specific 92 subtypes of interneuron evoked a localised haemodynamic response, comparable to 93 that evoked by a mild physiological stimulus (mechanical whisker stimulation). 2-94 dimensional optical imaging spectroscopy (2D-OIS) was used to record high-95 resolution 2D maps of the changes in blood volume (Hbt), oxygenated haemoglobin 96 (HbO2) and reduced haemoglobin (Hbr) evoked by stimulation. Each animal initially 97 received a mechanical whisker stimulation (2s, 5Hz), evoking changes in Hbt, HbO2 98 and Hbr which were localised to the whisker barrel cortex (Figure 2A). These 99 haemodynamic changes allowed us to map the whisker barrel cortex and, in turn, 100 guide the placement of the optical fibre used for photostimulation (Figure 1). The time 101 series of the haemodynamic response to whisker stimulation shows an increase in Hbt 102and HbO2 during the stimulation with a corresponding washout of Hbr (Figure 2A). 103 5 104 A fibre-coupled blue (470nm) LED, placed directly above the whisker barrel cortex, 105 was used to apply photostimulat...

Neurovascular coupling is a critical brain mechanism whereby changes to blood flow accompany localised neural activity. The breakdown of neurovascular coupling is linked to the development and progression of several neurological conditions including dementia. In this study, we examined cortical haemodynamics in mouse preparations that modelled Alzheimer’s disease (J20-AD) and atherosclerosis (PCSK9-ATH) between 9 and 12 m of age. We report novel findings with atherosclerosis where neurovascular decline is characterised by significantly reduced blood volume, altered levels of oxyhaemoglobin and deoxyhaemoglobin, in addition to global neuroinflammation. In the comorbid mixed model (J20-PCSK9-MIX), we report a 3 x increase in hippocampal amyloid-beta plaques. A key finding was that cortical spreading depression (CSD) due to electrode insertion into the brain was worse in the diseased animals and led to a prolonged period of hypoxia. These findings suggest that systemic atherosclerosis can be detrimental to neurovascular health and that having cardiovascular comorbidities can exacerbate pre-existing Alzheimer’s-related amyloid-plaques.

Investigating neurovascular coupling in awake rodents is becoming ever more popular due, in part, to our increasing knowledge of the profound impacts that anaesthesia can have upon brain physiology. Although awake imaging brings with it many advantages, we still do not fully understand how voluntary locomotion during imaging affects sensory-evoked haemodynamic responses. In this study we investigated how evoked haemodynamic responses can be affected by the amount and timing of locomotion. Using an awake imaging set up, we used 2D-Optical Imaging Spectroscopy (2D-OIS) to measure changes in cerebral haemodynamics within the sensory cortex of the brain during either 2 s whisker stimulation or spontaneous (no whisker stimulation) experiments, whilst animals could walk on a spherical treadmill. We show that locomotion alters haemodynamic responses. The amount and timing of locomotion relative to whisker stimulation is important, and can significantly impact sensory-evoked haemodynamic responses. If locomotion occurred before or during whisker stimulation, the amplitude of the stimulus-evoked haemodynamic response was significantly altered. Therefore, monitoring of locomotion during awake imaging is necessary to ensure that conclusions based on comparisons of evoked haemodynamic responses (e.g., between control and disease groups) are not confounded by the effects of locomotion.