CONTEXTO GEOGRÁFICO:

Many animal models have been successfully used to study cerebral ischaemia in a variety of species. Animal models have provided us with considerable insight into the pathophysiology of cerebral ischaemia, and will continue to do so. However, for the study of cerebral ischaemia there is no such thing as an "ideal" animal model.

As far as possible the animal model should:

closely mimic the development of the clinical insult in humans,

cause a reproducible injury, i.e. experiments should elicit similar responses in each individual animal tested,

be closely physiologically monitored, i.e. core temperature, respiration rate, blood glucose and blood pressure,

show similar pathology as a result of the experimental insult to that pathology seen in humans.

Nevertheless, even if the above principles for animal models can be closely adhered to, there are a number of limitations which must be borne in mind. Firstly, reproducible results are often very difficult to achieve because of anatomical and physiological variability both within and across species. It is also often arduous to tightly regulate physiological parameters, but a lack of strict control can lead to spurious, misleading results. This has now been realised to have been the case with some of the in vivo testing of the NMDA receptor antagonist MK-801. In some of the studies a lowered animal body temperature, caused by MK-801, accounted for a number of the cerebroprotective effects which were incorrectly attributed to the direct action of MK-801 on the NMDA receptor (Choi, 1992). Another important point to consider with animal models is that the animals are usually anaesthetised and have undergone surgery, which may profoundly alter the tissue response to an ischaemic episode.

2.1.1.b "Reduced preparations"

Intact animal models are not always ideal for basic cellular research. A large number of researchers study brain function by working on what may be termed "reduced preparations". These include cultured neurons and glia, brain slices and synaptosomes. In some cases such preparations are used simply because they are easier to manipulate than the intact physiological system. However, there are also a number of scientifically valid reasons for using these reduced systems.

chapter 2 The Model

The use of reduced systems can be justified when the investigation necessitates altering the extracellular milieu of the tissue. This ability to manipulate extracellular composition is lacking in in vivo studies of the adult brain. In vivo the intact, semi- permeable blood-brain barrier limits the ability to alter ion and metabolite concentrations surrounding brain cells, and makes quantifying any changes difficult. Another inherent problem with in vivo work is that the study of the cellular response and the separate quantification of metabolic products generated by both normal and injured cells is relatively difficult. Thus the reduced preparation becomes preferable when a study involves altering and measuring the composition of the extracellular environment.

When using intact systems it becomes hard to dissect out the mechanisms that are occurring in individual cell types or organelles. The use of culture systems and synaptosomes can be very important in enabling the investigator to assign functions to either a specific cell type, or to localise a function to synaptic endings. Another approach which is commonly used by investigators to enable such dissection and assigning of functions to specific cell types or organelles, is the isolation of cells and organelles from animal models that have undergone an ischaemic insult. However, this approach more often than not uses well established methods that have been optimised for healthy, non-pathological animals. It is important to realise that in using such an approach, researchers could be isolating the damaged cells and organelles incorrectly. This is particularly important when swelling of cells or organelles is a possibility following an ischaemic insult. Any changes in the density of the pathological cells and organelles (such as mitochondria) will have implications for the effectiveness of isolation methods using density gradients, which achieve separation by exploiting the different densities of cells and organelles. A further limitation of using the approach of post-ischaemic isolation is that only the post-ischaemic changes can effectively be studied, and the isolation procedure itself may allow a short time window for some post-ischaemic recovery. The manipulation of in vitro reduced systems is advantageous in this case, because they enable the investigator to study responses as they occur, rather than being limited to some time-period after isolation.

Any decision to use a reduced preparation is made at a cost because the ideal situation is always to study the intact physiological system. However, in vitro models are useful tools for studying cellular and molecular mechanisms of ischaemic damage. Scientific models, whether involving in vivo animal work or in vitro cell work, have their own unique advantages and limitations. It is imperative that such limitations are borne in mind and that extrapolation back to the human situation proceeds with extreme caution. However, if the researcher is aware of the applications and limitations of a particular model, much useful information can be gleaned from carefully designed experiments using in vitro scientific models.