Module 7 - Brain Circulation, Barriers and Cerebrospinal Fluid Flashcards
L7.1 - The vasculature of the brain, the anatomy and principles
(describe all and the circle of Willis, arterial input, posterior/anterior supply and areas)
The main input is the vertebral arteries, which form the basilar artery. These supply mainly the brainstem and make up the posterior blood supply. The anterior blood supply come from the carotid arteries, which supplies the cerebral cortex and the diencephalon. The circle of Willis gives some level of flexibility, because a small occlusion can be compensated for, given the system is circular.
In the arteries, the vessels are surrounded by a lot of smooth muscle, which will protect from the pressure of the blood. These are less present in venous system, which has formed the theory that function shapes phenotype, which also explains why different types of arteries look different.
Once the blood vessels leave the circle of Willis, they will penetrate the brain from the surface in. Vessels will differ in looks depending where we see them or how deep – a penetrating arteriole (penetrating the brain) will have astrocytic end feet and a perivascular space, where the pial arteriole have neither, but more muscle to resist pressure.
The smallest vessels are capillaries, which are 4x more prevalent that arterioles (arteries arterioles capillaries) and are where the exchange happens with the brain. To ensure blood reaches this far down, there are pre-capillary sphincters that will constrict the vessel slightly to ensure that there is enough blood pressure to keep going. Capillaries dilate before arterioles, as they’re closer to the target that requires more energy. It’s therefore believed that the capillaries signal this need retrogradely.
L7.1 - Explain regulation of cerebral blood flow by blood pressure and blood gasses
Generally, the cerebral blood flow is constant, and we only see slight decreases in deep sleep. The blood flow does not change based on the pressure due to auto regulation, which will keep the blood flow constant in the brain by modulating the blood vessel dilation.
Different gasses will modulate the cerebral blood-flow (and the level that autoregulation regulates to) to a small extend. When there is an increase in arterial CO2, there will be an increase of CFB to make up for this. Similarly, as high level of arterial O2 will lead to a decrease in CFB. As CO2 will be associated with lower PH levels of the blood, it has also been found that injection alkaline solutions (high PH) lead to vasoconstriction, showing that the vessels are PH sensitive.
Nitric oxide is produced by endothelia cells and is a gass-neurotransmitter that can freely diffuse. NO will cause smooth muscle relaxation around the arteries, through downstream affects of the cGMP and protein kinase G. This leads to vasodilation.
L7.2 - Describe the neurovascular unit and the mechanisms that couple neurotransmission to brain energy supply.
The neurovascular unit controls the cerebral bloodflow, the blood-brain-barier permeability and the brain’s microenvironment. It consists of brain endothelia cells, which form tight junctions, pericytes or smooth muscle, which can constrict/dialate to control bloodflow, glial cells (astrocytes, microglia and oligodendrocytes), which can signal about the activity in the neurons and is therefore important in neurovascular coupling. Additionally, we have neurons and the basement membrane.
We care because: problems with the NVU can give brain dysfunction (BBB controlled by the NVU), not delivering enough O2 can lead to neurodegeneration (the NVU controls brain blood flow) and neurovascular coupling underly fMRI
The coupling of neurotransmission and brain energy supply is known as the neurovascular coupling. This concept describes that areas of the brain that are very active will require more energy. The global bloodflow to the brain remains more or less constant, but when a certain area is more active than normal, increased levels of CBF will flow there, to provide oxygen and glucose and therefore avoid running out of ATP. The relationship between activation and increase in CBF is thought to be linear, but so is the relationship between O2 decreasing and synaptic activity. The first mechanism is seen when we e.g. have glutamate signaling, leading to Ca2+ influx and an increase in NO from interneurons, which causes vasodilation. This change is supported by astrocyte signaling. As the majority of the energy for the brain is used in synaptic transmission and information processing, this network is logical.
Overall, we see a decrease in oxygen when an area first becomes active. The bloodflow will increase in response to this, giving a surplus of O2 (hemodynamic response function)
While vasodilation is mediated by smooth muscle in the arterioles, it’s mainly done by pericytes in the capillaries. Together with the precapillary sphincters, these cells form the “microvascular inflow tract” (MIT) cells - when they relax, blood flow increases.
L7.1 - EXTRA (Describe the neurovascular unit and the mechanisms that couple neurotransmission to brain energy supply.)
Pial arteries will dive into the tissue as a penetrating arteries.
Pial arteries have smooth muscle cell to withstand blood pressure and can cause changes in the CBF. The penetrating ones have thinner layers of smooth muscle. Smooth muscle helps determine neurovascular coupling and is not present in veins. Both types are lines with endothelia cells and penetrating are lines with astrocytes too. Penetrating also have a perivascular space.
Is the mechanism feed forward or back? Intuitive is feedback (we need more nutrients and less waste when we’re active), however, it seems that most is determined by feed forward, based on downstream effects that are present when we e.g. have glutamate signaling, leading to Ca2+ influx and an increase in NO from interneurons, which causes vasodilation. We known an increases in excitatory signaling can lead to vasodilation, as most energy for the brain is used in synaptic transmission and therefore information processing.
Astrocytes are important in neurovascular coupling. Glutamate can lead to ca2+ in astrocytes, which can give downstream effects of vasodilation. Adenosine can also be a vasodialator
Pericyte line cappilaries and can dialate and constrict based on NO responses.
Endothelia cells contain NO and other vasodilaters and can therefore lead to changes in arterial diamaters. Cappilaries are the closest to the active neurons, so they receive the signal of when we need more O2, so the cappilaries will send signals retrogradely to signal to pial arteries
Neurovascular coupling = rise in local cerebral blood flow that accompanies rises in nerve cell activity. When there is an increase in blood usage, oxygenated blood rushes in.
The increase in blood flow is necessary to provide the brain with oxygen and glucose, which the brain can use in producing ATP.
It’s been found that synaptic activity dilates the brain capillaries and that capillary dilation through the pericyte relaxation is associated with drop in ca2+
L7.3 - Describe cellular and non-cellular constituents of the BBB
Cellular:
Endothelia cells (thin layer), principal component (0.2 microns thick) - blocks most stuff – forms physical and transcellular barrier. Blocks influx of substances by having efflux pumps
Pericytes: cover the capillaries (30% of circumference), stabilize the proliferation and create the basement membrane.
Astrocytic end feet: wrap around the blood vessel (support the endothelia cell), support tight junction formation and influence transcellular transport.
Neurons: influence BBB permeability
non-cellular:
basement membrane (collagen layer), Stabilizes the BBB complex
glycocalyx: hairy structure on the lumen side and are made of proteins/sugars and cover half of the surface and is negatively charged, creates an enzymatic barrier, regulate cell adheasion
L7.3 - Explain the difference between active and passive transport across the BBB
Diffusion is a passive transport that works for the gradient – the paracellular is for small hydrophilic molecules and transcellular is small and lipophilic (most drugs work like). The active is the transport take ATP (the carrier mediated is also called augmented diffusion: transporter across the membrane but diffusion in the cell)
L7.3 - Describe the routes across the BBB (paracellular, transcellular)
Paracellular: small hydrophilic molecules that moved between the endothelia cells – diffusion, restricted in the brain (tight junction and adherance junction proteins), not saturable/competitive
Transcellular: is small and lipophilic (most drugs work like) – goes in the cell. Sensitive to efflux pumps (PGP is the strongest) – block 50 % of all drug). Transport caffeine, nicotine, alcohol & sterioids
Carrier mediated: uses a carrier to get in and out of the cell and diffuse across the cytosol (Carrier on either side - glucose is transported like this by GLUT1, but also ions, NTs, vitamins and amino acids) – bi-directional active transport
Absorptive mediated transcytosis (endocytosis + exocytosis across a membrane): non-specific and is suppressed in the brain (caveolin-mediated) – works mainly for positively changed molecules
Example: transports albumin
Like with RMT, the transported substance can be degenerated inside the epithelia cell if PH is too low
Receptor-mediated transport: transcytosis (endocytosis + exocytosis across a membrane): very specific and clathrin mediated. Transports the substance and the receptor across (either in-out or out-in depending on substance).
Example: transports transferrin in and beta-amyloid out
Like with AMT, the transported substance can be degenerated inside the epithelia cell if PH is too low
L7.3 - Provide examples of the BBB dysfunction in brain pathologies
Main features are always increase in AMT and a failure of the tight junction. In some cases, we also see decreased RMT.
Aging: AMT is increased, tight junctions and RMT are decreased
Stoke: first, AMT is increased, then tight junctions are lost
Alzheimer’s: AMT is increased, tight junctions are lost leads to cognitive decline and decreased RMT of beta amyloid out
Neuroinflammation: AMT is increased, tight junctions are lost leads to leukocyte infiltration
L7.4 - Describe the ventricular system
We drain through the foramen of Luschka and Magendie to the sub-arachnoid space
L7.4 - Describe the structure and function of the choroid plexus
It’s present in the inner part of the ventricles in the medial part of the lateral ventricles, in the 3rd and 4th ventricle. Here the 4th seems to be the most important.
The choroid plexus are epithelia cells and reside in pia mater close to the ventricles. They help form the blood-CSF barrier through tight junction (no paracellular passage) and can maintain an ionic homeostasis (as it controls the movement of ions and other substances).
Choroid plexus produces CSF by transporting water from the vessels into the ventricles. The water follows the ions through co-transporters such as the Na+/K+/2Cl- cotransporter in depended of the osmotic gradient
The produced CSF is pushed into the ventricles through the choroid epithelium, which have microvilli to create a larger surface.
Vessel choroid epithelia CSF/ventricles ependymal cells brain
L7.4 - Describe the circulation of the cerebrospinal fluid
CSF is pushed around, as there is a large production pressure and the cilia from the ependymal cells help it move. Arterial and respiratory cycles helps too. CSF is drained through the arachnoid granulations.
L7.5 - Describe brain barrier function in drug delivery and drug delivery routes
The BBB is important in maintaining homeostasis for ions and protect the brain from outside sources (like xenobiotic substances) and other drugs not designed for the brain.
In drug delivery, the BBB can be a problem, because it keeps many types of drugs out. This can either be because the drugs can’t diffuse, don’t have a receptor or will bind to efflux pumps.
Drugs can pass through passive diffusion (mainly the transcellular subtype – paracellular is almost impossible and requires hydrophilic drugs). Alternatively, we need drug to be transported through active transport, through e.g. carrier mediated, AMT or RMT
L7.5 - Explain relationship between drug physicochemical properties and barrier permeation
Generally, certain criteria have to be fulfilled for drugs to pass the BBB. For passive diffusion (transcellular) we want small, lipophilic and not super charged drugs. However, some might not be able to stay past the BBB if they attach to efflux pumps. They might be present to a large extend in the CNS (like L-DOPA) if they can bind to a receptor and therefore e transported into the CNS (receptor mediated transport).
L7.5 - Describe ex vivo and in vivo experimental approaches to study brain drug delivery
Ex-vivo: make sure it’s not a substrate for the efflux pumps and check if could be a substrate for the solute carriers (like an active transporter).
In-vivo: breaching the BBB
*Cause global BBB disruption via intra-carotid artery infusion of mannitol (hyperosmolar mannitol infusion). This causes an osmotic shock that makes the endothelial cells shrink and the tight junctions open
* Local BBB disruption via pulsed focused ultrasound + microbubbles. The focused ultrasound makes the microbubbles oscillate which will open the junctions between the endothelial cells at specific areas
L7.6 - Explain how the brain eliminates waste protein.
Through the glymphatic system, which consists of 3 major parts: Astrocyte mediated influx of CSF, CSF-ISF exchange of waste and CSF drainage into the perivenous space.
The CSF is transported into the brain tissue through e.g aquaporine 4 channels the astrocytes (astrocytes decide how much goes through), the CSF mixes with the ISF and removes waste and aggregated proteins, as it is absorbed into the perivenous space and will drain to the venous blood through the arachnoid granulations or to the meningeal lymphatic vessels.
This clearance is increased during sleep and will decrease with sleep deprivation.
L7.6 - Describe brain’s lymphatic system.
The ISF/CSF found in the perivenular space (due to the glymphatic system) seem to be able to drain into newly re-discovered meningeal lymphatic vessels, which will drain to the cervical lympnodes. How the drainage works is still not well understood, but it might be an alternative to the arachnoid granulations draining to the venous blood and opens a discussion about neuroimmunology and the function of the glymphatic system.
The brains lympatic system seems t be decreased in aging,
L7.7 - Identify what are the main fuels and what drives energy consumption in the brain
The brain runs on ATP, which mainly is derived from glucose, but can also come from amino
acids, ketone bodies and fatty acids. The faster neurons fire, the more energy they use.
Action potential consumes around 47 % of ATP
-> most energy is used in areas where synaptic transmission happens
L7.7 - Describe how is metabolic energy produced in the brain. What are some of the main energy producing pathways?
Metabolism in the brain is compartmentalized (different areas have different metabolic functions), and neuron are uniquely interdependent. However, they have pathways of energy metabolism to other places in the body:
Glycolysis generates pyruvate from glucose (in the cytosol), which can be turned into acetyl CoA, that is the input to the citric acid cycle (in the mitochondria). These processes in total creates 38 ATP.
Glucose is taken up from the blood, and glycolysis is seen both in the presynaptic neuron, postsynaptic neuron and in astrocytes. In the postsynaptic neuron, pyruvate goes through the citric acid cycle in the mitochondria, to have enough ATP to sustain the Na+/K+ pump, which takes part in maintaining the membrane potential. Similarly in the presynapse, ATP is also generated by the citric acid cycle to maintain synaptic functions. The astrocyte can convert the pyruvate into lactate, which is sent to the presynapse, turned back into pyruvate and converted to ATP. Alternatively, the astrocyte converts the pyruvate into ATP by the citric acid cycle to fuel the conversion of glutamate to glutamine.
L7.7 - Identify shared metabolites in neurons and astrocytes during brain activation.
Astrocytes and neurons share the glutamate/glutamine cycle, where glutamate is taken up by astrocytes to be converted into glutamine. This metabolite is exported to neurons, which converts it back to glutamine and releases it when needed. It’s important that it’s not just send back as glutamate, as this could lead to hyperexcitation.
In the idea of the lactate shuttle, lactate is created in the astrocyte from pyruvate and then exported to the neuron, which converts it back into pyruvate. This is debated, as neurons largly rely on glucose
L7.10 - Describe age-related changes in brain energy metabolism.
The decrease in brain metabolism makes the brain more fragile, as there is less glycose and oxygen to cope with any challenges to homeostasis (e.g. through trauma - the homeostatic reserve is decreased). It’s believed that caloric deficits and intermittent fasting might help improve the decline in metabolic functions in aging.
Brain energy goes down (neurons don’t die without lack of energy)
Oxidative damage leads to mitochondrial dysfunction
It can’t dispose of bad proteins (glymphatic system declines)
It can’t repair the DNA as well - DNA is repaired 100K times a day (heart of aging research - some believe we accumulate DNA mutation with age)
We can’t signal properly and our stem cells (that are though the be linked to aging) are exhausted.
The glial cells become inflammatory and secrete cytokines, which will interact with microglia
