Geneeskundige Stichting Koningin Elisabeth ... - GSKE - FMRE

PART 2: The use of Endothelin 1 in a model for focal transient ischemia :
a microdialysis study in rat brain.
Stroke is the third major cause of death, and the main disabling neurologic disease in the
industrialized countries. In an ischaemic situation, the brain is deprived of oxygen and glucose,
and consequently, the ATP-driven ion pumps fail to work. Hence, energy insufficiency leads
to membrane depolarization, which in its turn results in a large release of various neurotransmitters,
such as glutamate, dopamine and GABA. In various experimental studies however, it
was observed that neuronal death is not occurring immediately after the ischaemic injury. This
observation has encouraged the research and development of neuroprotective agents, which
would be administered upon stroke, but still be able to prevent further neuronal damage.
In general, this study is situated in the framework of the research for neuroprotective drugs
in cerebral ischaemia. Ultimately, its goal is to find a pharmacological treatment, which can
mitigate, or even prevent, any neuronal damage following an ischaemic insult.
For this purpose, first, an animal model for studying focal, transient cerebral ischaemia was
developed. Since in humans an ischaemic insult is seldom permanent, it was aimed to develop
an animal model, incorporating transient occlusion, i.e. with reperfusion. Hence, the clinical
situation is mimicked as closely as conceivable with an animal model.
Studying the different animal models described in literature, the Et-1 model was selected.
Et-1, an endogenous peptide, is one of the most potent vasoconstrictors known to date. Using
this peptide as a tool to constrict one of the main cerebral arteries (i.e. the MCA), creates an
animal model for focal transient cerebral ischaemia.
The characterization of this model was done using microdialysis, laser Doppler flowmetry
and histology. With microdialysis, the release pattern of extracellular striatal dopamine, glutamate
and GABA was monitored. After a dose-response study, a 120 pmol Et-1 dose was selected
to conduct further work with. Injecting 120 pmol Et-1 provoked a reproducible ischaemic
infarct, with reperfusion incorporated into the design of the model. Laser Doppler studies confirmed
the drop in striatal blood flow after the micro-application of Et-1 in the proximity of the
MCA, as well as the return to basal blood flow levels, with a certain time delay. Administration
of a non selective Et-1 antagonist, locally in the striatum, before Et-1 administration in the proximity
of the MCA, resulted in a similar release pattern of the monitored transmitters, compared
with the Et-1 only treated animals. On the other hand, when administering the Et-1 antagonist
at the same injection site as Et-1, but beforehand, an inhibition of the Et-1 effects on extracellular
striatal transmitter levels was observed. Hence, this confirmed the hypothesis that Et-1 was
active via its receptor sites on the MCA and/or its receptor sites on the MCA’s lenticulostriatal
branches, resulting in a vasoconstriction, and not by a direct action of Et-1 upon striatal dopaminergic
neurons through diffusion.
Histological studies finally showed the existence of a reproducible ischaemic infarct, located
in the striatum and expanding to the cortex.
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