Assessment of Blood–Brain Barrier Breakdown - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 382

limit and regulate molecular exchange at the interfaces between
the blood and the neural tissue or its fl uid spaces ( 2 ), the BBB
exerts the greatest control over the immediate microenvironment
of brain cells, maintaining a stable and unique extracellular envi-
ronment within the neuropil. It is formed by brain endothelial cells
(ECs) lining the brain vasculature and interconnected by tight
junctions (TJs). The ECs are surrounded by pericytes and astro-
glial foot processes that severely restrict the paracellular (tight
junctional) pathway for diffusion of hydrophilic solutes. Thus,
penetration across the brain endothelium is effectively confi ned to
transcellular mechanisms ( 3, 4 ).
Recent studies in animals confi rm that breakdown of the BBB
is a critical event in the development and progression of impaired
neurovascular unit activities and has a direct underlying role in the
dysfunction of astrocytes, in the brain's infl ammatory response (for
reviews, see refs. ( 5, 6 )). In some cases, a primary damage to the
vascular bed like that observed in ischemic stroke ( 7 ) or traumatic
brain injury ( 8 )—leads immediately and directly to leakage of
serum components into the neuropil. Separately, an increase in
BBB permeability is often found to occur several days after the
primary insult, representing part of the brain's response to the
injury. While for most patients, clinical data indicate that BBB
permeability returns to normal within days to weeks following the
acute event ( 9-11 ), in some patients BBB disruption has been doc-
umented months or even years after the insult ( 12, 13 ). There is,
however, no fi rm evidence indicating whether these long-lasting
disruptions are primary or secondary to the ongoing pathological
processes that perpetuate them. The mechanisms underlying
delayed increase in BBB permeability are not completely under-
stood. Following intracerebral bleeding, hemolysis products in the
subarachnoid space have been hypothesized to underlie delayed
ischemic neurological defi cits ( 14, 15 ). During the course of hemo-
lysis, Hb and K + are released and their concentrations in the extra-
cellular space are increased ( 16, 17 ). Such changes may result in a
direct effect on vascular ECs, smooth muscles, and pericytes induc-
ing vasodilation and increased BBB permeability (see below and
Fig. 1 ). Additionally (or alternatively), the altered extracellular
environment may lead to the spreading of abnormal neuronal
activity (spreading depolarizations) ( 18-20 ), which may be associ-
ated with vasodilation (but also with vasoconstriction, see refs.
( 21-23 )), and possibly delayed BBB opening due to the upregula-
tion of matrix metaloproteases (e.g., MMP9, see refs. ( 24, 25 )).
While accumulated data from experimental animals and human
patients point to the complexity of BBB breakdown in extent, spa-
tial and temporal domains, and to the key role of BBB breakdown
to the pathophysiology of brain damage and repair mechanisms,
quantitative and reliable methods for measuring BBB permeability
in the living brain still do not exist. Here, we describe a new