Cerebrospinal fluid and the Blood Brain Barrier (Lecture 16-Exam 2)

Hydrocephalus

An imbalance between formation and absorption can lead to _____

-Neurons must be protected from *neuroactive substances*, compounds that are capable of affecting membrane receptors, transporters, ion channels -Neurons in the CNS produce a large number of molecules which function as neurotransmitters. BBB helps to keep action of neurotransmitters localized

Describe how normal neuron function depends on a stable environment:

-CO2 diffuses across the BBB -More CO2 will increase hydrogen ion concentration in the brain interstitial fluid -This will activate central chemoreceptors -Consequently breathing is stimulated

Describe the carbon dioxides effects on the BBB

1. In the chorioid plexus 2. Flow of molecules across the blood-CSF-barrier 3. Micronutrients (vitamins) are actively transported into CSF 4. Na/K exchanger at apical surface secretes Na in to CSF 5. Cl- follows passively paracellular 6. H2O follows osmotic gradient 7. Pulsation creates small pressure fluctuations which helps CSF absorption

How is CSF formed? (7 steps)

Hydrocephalus

Increased CSF volume in skull associated with increased intracranial pressure

The mammalian BBB *MDR transporter* keeps the drug from entering the CNS

Macrocyclic lactones (ex: invermectin) bind with high affinity to GABA-gated chloride ion channels -In arthropods and nematodes, this results in an inhibition of nerve activity, flaccid paralysis, and death= Antiparasitic effect What keeps this drug from entering the CNS of the animal it is given to?

Constant oxygen supply, but blood contains: waste, antibodies, toxins, neurotransmitters, hormones which can affect the function of neurons -Those components need to be kept out

Normal neuron function depends also on constant:

1. *Selective entry mechanisms*: -Only *lipid-soluble substances diffuse* across the BBB (oxygen, carbon dioxide, ketones) -Ion channels- secondary active transport mechanisms (ex: NA-K-ATPase, Na-dependent amino acid transport) -Through *receptor mediated transport* specific water-soluble substances can enter the brain interstitial space (ex: GLUT-1; transport of glucose down a concentration gradient) 2. *Metabolic barriers and P-glycoprotein*: -enzymes in endothelial cells convert neuroactive substances and then can pass through the metabolic barriers -ex: monoamine oxidase (MAO) converts dopamine into 3,4-Dihydroxyphenylacetic acid (DOPAC), DOPAC cannot pass the membrane, but its receptor is located on the vascular side of the endothelial cells that returns it to the blood stream

The BBB allows only a selective passage of molecules. This is achieved by 1 of 2 mechanisms What are those mechanisms?

Pressure dependent -slightly higher pressure of subarachnoid space than in the venous system

The absorption of CSF is ______ dependent

1. Restricting drug penetration across the BBB 2. Promoting drug elimination into bile and urine

The intact MDR limits drug entry into the organism after oral administration by: (2 ways)

1. Cerebrospinal fluid (CSF) 2. Blood Brain Barrier (BBB): selective exchange of fluids and substances

What are 2 elements that help stabilize the environment of neurons in the CNS?

Cerebral Spinal Fluid (CSF) -An increase in protein might be associated with inflammation (ex: encephalitis, meningitis, neoplasia, spinal cord compression)

____ is collected to determine inflammatory processes in the CNS

Glial tumors (specifically *astrocytomas* and oligodendrogliomas) -Glial cells= cells that are surrounding neurons & supplying neurons, they are part of the BBB, these cells are predisposed to mutation and growth of a tumor

_____ are the second most common group of canine brain tumors causing injury to the BBB

Arterial pulsation

_____ in choroid plexus helps drive CSF from ventricular system

Glucose metabolism

_____ is the major energy source for the brain

Blood brain barrier

_____ separates blood and the neuronal environment

MDR, multidrug-resistance transporter (AKA *P-glucoprotein*)

______ facilitates the back transport of large lipophilic substances form the brain into the blood

Protein

CSF is collected to determine inflammatory processes in the CNS -An increase in ____ might be associated with inflammation (ex: encephalitis, meningitis, neoplasia, spinal cord compression)

The bulk flows down a pressure gradient: Lateral ventricles -> 3rd ventricle -> 4th ventricle -> subarachnoid space -> venous system -Cerebral aqueduct: connection between 3rd and 4th ventricle -The pressure gradient is highest in lateral ventricles and diminishes successively along subarachnoid space -Arterial pulsation in choroid plexus helps drive fluid from ventricular system

Describe CSF circulation:

Normally: The mammalian BBB *MDR transporter* doesn't allow the ivermectin to enter the CNS -MDR is encoded by the *MDR1 (ABCB1) gene* In collies: -*MDR1 (-/-) dogs do not express a functional MDR (aka: P-glycoprotein)* -Macrocyclic lactones, such as ivermectin bind with high affinity to GABA-gated chloride ion channels of the dog -Ivermectin therefore causes neurological symptoms (same effects it would have on the parasite) -Some symptoms: mydriasis, ataxia, and depression

Describe Ivermectin neurotoxicity in collies:

If blood supply is interrupted, neurons begin to die and after a while the separation between blood environment and brain neurons begins to break down with detrimental effects on neuron function *Blood brain barrier* separates blood and neuronal environment

A large oxygen demand of neurons is facilitated by rich perfusion -If blood supply is interrupted:

Arachnoid villi

CSF is absorbed across the _____ into the venous circulation and through drainage into lymphatic vessels

Impairment of absorption -Function of arachnoid villi is not working correctly, the passage form the lateral ventricles to the subarachnoid space is open and flowing, its just the last step that isn't working right -Secondary to meningitis hemorrhage -Cellular debris obstruct transfer of CSF at arachnoid villi -Possibly volume increase in subarachnoid space and pressure increase on outside of the brain

Describe *communicating hydrocephalus*

-Obstruction at cerebral aqueduct or at exit from 4th ventricle (2 lateral apertures) -Build-up of fluid before the blockage -Ventricular regions in brain expand -ex: secondary to FIP; congenital

Describe *non-communicating* hydrocephalus

-Absorption occurs predominantly in the dorsal Sagittal sinus (between left and right hemisphere) and transverse sinuses (between cerebrum and cerebellum) -CSF is absorbed across the arachnoid villi into the venous circulation and through drainage into lymphatic vessels -The absorption of CSF is pressure dependent (slightly higher pressure of subarachnoid space than into venous system -Normally pressure of CSF is larger than that of venous system -> "one-way bulk flow" (all constituents of CSF move)

Describe CSF absorption

-MDR (multidrug-resistance transporter), AKA *P-glycoprotein*: facilitates the back transport of large lipohilic substances form the brain into the blood -MDR is an *ATP driven* efflux transporter which pumps its substrates out of the cell. -The intact MDR limits drug entry into the organism after oral administration by *(a) restricting drug penetration across the BBB* and *(b) promoting drug elimination into bile and urine* -MDR is not specific

Describe P-Glycoprotein of the BBB

GABA receptors are channel receptors 1. When GABA binds to them, they change shape and allow *Chloride ions8 to pass through the central channel 2. This causes *hyperpolarization* and thus *reduces the neuron's excitability* (has the effect of inhibition) -GABA receptors are very common in the mammalian nervous system. It is estimated that close to 40% of the synapses in the brain work with GABA and therefore have GABA receptors

Describe how GABA receptors work: (2 steps)

-Balance between production at choroid plexus and absorption at arachnoid villi -Formation is (+/-) constant -Absorption responds to pressure changes -Obstruction of the absorption causes pressure changes -An imbalance between formation and absorption can lead to *hydrocephalus*

Describe pressure regulation in ventricles and subarachnoid space:

-Glucose, the major fuel in the brain, is transported across the cell membranes by facilitated diffusion mediated by glucose transporter proteins (GLUT-1) -Location of the different GLUT-1 glucose transporter isoforms (55-kDa, 45-kDa) in the brain -GLUT-3 is neuronal glucose transporter

Describe receptor mediated transport and a method of the selective entry mechanism of the BBB:

1. Only *lipid-soluble substances diffuse* across the BBB (oxygen, carbon dioxide, ketone bodies, etc.) 2. Ion channels; secondary active transport mechanism (ex: Na-K-ATPase; Na-dependent amino acid transport) 3. Through *receptor mediated transport* specific water-soluble substances can enter the brain interstitial space (ex: GLUT-1: transport of glucose down a concentration gradient)

Describe the *Selective entry mechanism* of the BBB that allows only a selective passage of molecules (3 ways of transport to this mechanism)

1. *Blood CSF barrier (BCSFB)*: -CSF (blue arrows) is produced by the choroid plexus (CP) or -CSF flows through the ventricle system into the subarachnoid space (SAS) -CSF returns to the venous blood to superior sagittal sinus (SSS) and lymphatic system 2. *Blood-brain barrier (BBB)* -BBB separates blood from the majority of the brain

Describe the 2 barriers:

1. *Subfornical organ (SFO)*: responsive to hormones; allows the passage of certain hormones (ex: vasopressin) 2. *Pituitary*: releases hormones from the brain (the passage of hormones from the brain into the bloodstream) 3. *Area postrema*: responsive to certain toxins that are present in the blood stream, it is connected to the vomiting center allowing vomiting to occur

Describe the brain regions without/with a less efficient BBB: (3)

1. Enzymes in endothelial cells convert neuroactive substances into metabolites 2. Metabolite cells can't pass the endothelial cells. There are transporters on the vascular side of the endothelial cells that take the metabolites and return them to the blood stream -ex: monoamine oxidase (MAO) enzyme converts dopamine into 3,4-Dihydroxyphenylacetic acid (DOPAC) -DOPAC cannot pass the membrane, so the receptor is located on the vascular side of the endothelial cell and allows it to pass

Describe the metabolic barrier of the BBB: (2 steps)

1. Many brain tumors contain vessels with poorly developed BBB 2. Ischemic stroke 3. Traumatic brain injury 4. Neuroinflammatory disease

What are some things that can injure/damage the BBB? (4)

*3 cavities/compartments*: 1. CSF 2. Vascular system 3. Interstitial space between neurons (brain tissue) *2 spaces/barriers*: 1. Between blood and interstitial space between neurons 2. Between blood and CSF

What are the 3 cavities/compartments of the BBB? What are the 2 spaces/barriers of the BBB?

1. Endothelial cells 2. 3 types of cell-cell connections: tight junctions, adherence junctions, desmosomes 3. Basement membrane 4. Glia (Astrocytes end feet)- surround the whole barrier -These structures being to break down after only a few hours of ischemia, Endothelial cells and basal membrane become "leakier"

What are the components of the BBB (4)

1. Contains no blood cells, little protein, ions 2. Is located in ventricles of the brain and central canal of spinal cord (in subarachnoid space) 3. Is continuously produced (replaced several times daily) 4. Circulates through ventricles and subarachnoid space 5. If formed in the choroid plexus 6. Is absorbed in sagittal sinus via arachnoid villi

What is CSF? (6 characteristics)

A catheter located in the ventricle and is passed through a valve that is going to remove the fluid to regulate the amount of fluid. -The amount that is removed is regulated or determined be the absolute pressure in the ventricles. You want to maintain a certain pressure about zero, this valve responds if that certain pressure is exceeded and it is closed when it hasn't met that certain pressure

What is a ventriculoperitoneal shunt?

During starvation, the brain can use ketone bodies in place of glucose (acetoacetate and beta-hydroxybutyrate; formed from catabolism of fatty acids)

What is ketone bodies role in the BBB?

1. Separates the blood and the neuronal environment 2. Stable environment for neurons and glia despite dramatic changes in blood due to diet, exercise, metabolic activity, illness, age, environmental toxins 3. Selective exchange of fluids and substances

What is the function of the Blood brain barrier? (3)

1. Maintain homeostasis of environment of neurons/maintain consistent extracellular microenvironment (ion concentration, nutrients) 2. Remove toxins 3. Distribution of polypeptide hormones and growth factors 4. Cushion the brain

What is the function of the Cerebral Spinal Fluid (CSF)? (4)

1. *Cerebellomedullary cistern*: subarachnoid space around cerebellum of fluids and substance accessed cranially from the atlas. -Easy access but danger of cerebellar herniation (from removing the fluid too fast 2. *Subarachnoid space in lumbar spinal column* -less risk but technically challenging

Where are the 2 places to do a CSF spinal tap

Highest in Lateral ventricles -the pressure gradient diminishes successively along subarachnoid space

Where is the pressure highest in CSF circulation

Macrocyclic lactones (ex: invermectin)

______ bind with high affinity to GABA-gated chloride ion channels -In Arthropods and nematodes this results in an inhibition of nerve activity, flaccid paralysis and death= antiparasitic effect -The *mammalian BBB (MDR transporter)* keeps the drug from entering the CNS