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The blood-brain barrier (BBB) provides a physical separation between the blood tissue and the brain tissue (central nervous system). It is found throughout the vertebrate species o f the animal kingdom^. It functions to separate the environment o f the central nervous system (CNS) from the cocktail o f substances that circulate through the body via the bloodstream. As such it enables the delicate balance that exists in the CNS and its interstitial fluid to be homeostatically maintained. Not only are toxic substances prevented from unhindered entry to the brain, but also a large variety o f substances including nutrients essential for brain function, as well as hormones, amino acids, potassium ions, drug compounds and cells.^ These substances, whether useful or not, could disturb brain function if they enter the brain in uncontrolled quantities. Even small fluctuations that occur in blood levels o f these substances such as after eating or

exercise could be enough to disrupt the balance. Instead o f free entry into the brain, the BBB allows for controlled, carrier mediated or active transport processes that ensure that vital substances reach the brain, to maintain the crucial brain function. Severe CNS depression or uncontrolled nervous activity are examples o f the effect o f an imbalance in the brains chemical environment. The important rôle o f the BBB is highlighted by conditions causing BBB dysfunction. Grave conditions such as brain tumours, haemorrhage, stroke, hypertension and trauma are all associated with damage to the BBB".

4.2.1 Historical Background

Until relatively recently the structure and form o f the BBB was not understood. The first indication o f the existence o f the BBB was as early as the 19th century when Ehrlich^ in 1885 noted that certain dyes injected into the blood, used to stain animals for light microscopy, stained all organs except the brain. He proposed that this was because the dyes did not have an affinity for brain tissue. Later, in 1913, Goldmann^ conducted experiments in which he used dyes injected into both the blood and cerebrospinal fluid* (CSF). He showed that the brain tissue did in-fact stain readily when the dye was injected into the CSF, but this dye did not diffuse into the blood, leaving the other organs unstained. The same dye, trypan blue, however when injected into the blood stained all the organs except the brain. Therefore he concluded that the brain and blood where separated by an undefined barrier, which trapped the dye in either the CNS or the blood.

Other research conducted around that time was also explained by this concept. Biedl and Kraus^ discovered that bile acids that are not neurotoxic when injected into the blood, caused seizures and coma when injected into the brain. A similar effect was observed by Lewandow^ in 1900 with sodium ferrocyanate as the sample. Goldmann and these workers were foresighted and proposed that the barrier was due to some special

properties of the blood capillaries supplying the brain. The German pathologist Spatz (1933y, recognised that actual capillaries remained unstained in the brain, after intravenous injection of various dyes. He was the first to propose that the BBB was a facet o f the capillary endothelium.^ Although there was some scepticism as to the existence o f the BBB through the intervening time, Spatz’s proposal was confirmed conclusively with the development of electron microscopy by Reese and Kamovsky^ in

1967.

4.2.2 The Structure o f the BBB

Most organs in the body receive blood borne substances by diffusion between the blood and organ membranes, allowing solutes of up to 30,000^ daltons to pass freely. The blood capillary walls, composed of endothelial cells, at the interface between blood and organ are fenestrated, having small gaps to allow a free interchange. The ultrastrucure

Fig 4.1

Cross section of the brain. The brain cells are surrounded by interstitial fluid which is maintained in a homeostatic milieu by the (mainly) impervious BBB.

(Taken from Tuomanen, E., Sci. Amer. 1993, 2, 56)

of some capillaries however is different to those of other organs, especially the vascular system o f the brain and placenta. The endothelial cells that form these capillaries are joined by what are known as continuous tight junctions, so that the gaps of a fenestrated structure sltq filled by cell overlap. Thus free movement o f substances is prevented and

instead substances must have the right physicochemical properties to diffuse through the luminal capillary wall or there has to be an active transport mechanism which will allow

the process to be controlled. Since the drug must diffuse through the cell wall it is worth examining the lipid bi-layer structure that constitutes the cell wall.

NUCLEUS

c a s e m e n t V«WBRANE

Fig. 4.2

Brain Capillary displaying tight junctions and astrocytes. The figure illustrates the cross section of a brain capillary. The perivascular astrocyctes are believed to modulate the structure and function o f the

, • 1 0 11

barrier. ’

(T aken from G o ld stein , G .W ., B etz, A .L. Sci.Amer. 1986,255,10)

4.2.3 The Fluid Mosaic Model o f the Cell Membrane

The so called flu id mosaic model of the cell membrane describes a bi-layer of phospholipids. The hydrocarbon tails of the phospholipid chain form the central core, with hydrophilic phosphate ends on either side, forming the outer layer. Interspersed in the structure are proteins which are often either active or passive transport mechanisms to transport specific solutes across the membrane. These form the mosaic of the structure. The conflicting physicochemical properties (hydrophilic head, hydrophobic tail) make passive diffusion across the membrane difficult.

Figure 4.3

Fluid mosaic model o f the cell membrane.

A schematic diagram depicting phospholipid bi-layer, with protein attached in either side o f the membrane, across the membrane or on the membrane

surface.

m