We want to understand how the brain balances its energy supply with demand – and how variations in this balance affect brain function. Specifically, we want to learn which vascular cells control the brain’s energy supply, and how much neurovascular coupling between energy supply (blood flow) and demand (neuronal activity) varies during normal and pathological brain function. For example, we are investigating whether neurovascular function changes during different arousal states, or in conditions such as Alzheimer’s disease and obesity. To do this, we image blood vessels in brain slices and in vivo, in behaving mice, while manipulating neuronal activity pharmacologically or using physiological stimuli, and imaging neuronal activity using genetically encoded calcium indicators. By concurrently measuring neuronal activity and vascular responses during different conditions, we will be able to determine which factors modulate the neurovascular coupling relationship. Our results will be important for understanding how brain activity is fuelled and how the brain’s energy supply is affected by disease. Because human functional magnetic resonance imaging (fMRI) measures blood flow as a surrogate of neuronal activity, our results will also be important for understanding what fMRI signals actually tell us about neuronal activity.
Brain capillaries can be imaged in behaving mice, by intravenously injecting a fluorescent dye and imaging the brain with a 2-photon microscope. Either the whole capillary can be imaged (A), so changes in area can be measured, or a line can be scanned very fast (C), allowing blood cell velocity to be determined. When mice run on a ball, capillaries in somatosensory cortex dilate (B), and blood cells move faster (D). Data collected by CH in collaboration with Patrick Drew, at Pennsylvania State University.
Investigating neurovascular coupling in hippocampus compared to visual cortex
Kira Shaw, Postdoc
The brain needs energy to fuel its activity. Active neurons produce “neurovascular coupling” signals, which diffuse to blood vessels to increase blood flow, and supply oxygen and glucose. The undersupply of oxygen and glucose to the brain is associated with neuronal damage (e.g. Alzheimer’s disease). Neurovascular coupling has been widely studied in primary sensory cortices in vivo, but less so in hippocampus. Evidence has suggested that neuronal activity can occur without a positive BOLD signal in hippocampus 1,2, and that this region is uniquely more vulnerable to hypoxia 3. Why and when such “neurovascular uncoupling” occurs in the hippocampus is unclear.
I use two-photon imaging to visualise blood vessels and excitatory neurons (A) in the dorsal hippocampus in vivo from awake mice traversing a virtual reality environment (B). Image A shows the pyramidal cell layer (stratum pyramidale) of CA1, with active excitatory neurons labelled in green (GCaMP6f) and blood vessels in fluorescent red (Texas red dextran). Using these techniques I am able to measure neuronal firing rates and response profiles, relative to blood flow and vessel diameter changes.
1 – Mishra et al. J. Neurosci. 31, 15053-64, (2011). 2 – Angenstein et al. J. Neurosci. 29, 2428-39, (2009). 3 - Cervos-Navarro & Diemer. Crit Re Neurobiol. 6, 149-82, (1991).
Hippocampal encoding of a virtual environment
Dorieke Grijseels, PhD student
The hippocampus is a brain area that is important for spatial navigation and spatial memory. It contains excitatory pyramidal cells that fire at a specific place in the environment, aptly named ‘place cells’. Although place cells are relatively stable within the same environment, they remap when part of the environment is changed. This might be a chance in local cues, e.g. objects added or removed, or to the context, e.g. a different scent is introduced. Not all place cells remap in the same way, some cells might start firing in a different location, others might stop firing alltogether.
By combining two-photon imaging and presentation of a virtual environment, I am able to very precisely determine the individual response of cells to small changes to the environment. I will try to use this to determine the encoding of individual aspects of the environment and look at the functional heterogeneity of the place cells population.
Do monoamines affect neurovascular coupling?
Katie Boyd, PhD student
My project will test whether transmitters such as noradrenaline and dopamine alter the relationship between neuronal activity and vascular responses. To explore this, I image blood vessels in brain slices, and apply drugs to change blood vessel diameters. I also measure neuronal and blood vessel responses in vivo, in conditions where monoamine release varies.
Â In this image, a capillary is constricting in response to applied noradrenaline and dilating in response to glutamate.
Investigating the detrimental effects of a short-term high fat diet
Devin Clarke, PhD student
Obesity is rising rapidly in the western world thanks in part to the abundance of foods that are high in energy and fats. A remarkable aspect of these high energy, high fat diets, is that in animal models they induce not only inflammation in the brain, but also changes in neuronal activity and animal behavior as little as 48-72 hours after beginning the diet. I am interested in detailing, and eventually manipulating, the neuroimmune and neurovascular effects of short term high fat diet feeding. The techniques I use to do this are immunostaining of brain tissue and 2-photon microscopy in awake mice, that have fluorescently labeled blood vessels, pericytes, and microglia, among others.