Biomedical Engineering Reference
In-Depth Information
in vitro assays, cellular and alternative species models, and predictive compu-
tational methods that incorporate knowledge about toxicity pathways.
The current approaches to the characterization of neurotoxic chemical
hazards rely heavily on dose-effect characterization in whole animal models,
analyzing behavioral (e.g. motor activity, water maze, functional-observational
battery), neurophysiological (e.g. evoked potentials, EEG recordings, etc.)
and/or pathological or structural (morphometric) endpoints. Hundreds of
animals have to be tested using time-consuming behavioral and structural
assessments. Research now focuses on obtaining information regarding
cellular, molecular and submolecular actions of individual chemicals rather
than the identification of toxicity pathways common to many chemicals.
Neurophysiological assessments for compounds such as carbon disulfide,
toluene solvents, metals and pesticides have been explored.
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The blocking of
type A GABA (GABA
A
) receptors by a wide variety of insecticides increases
excitability at the cellular level and results in increased network activity due to
disinhibition of neuronal firing. This toxicity pathway ultimately underlies the
acute neurotoxicity observed and neurophysiological approaches are ideal for a
rapid detection of these types of toxicity.
Substrate-integrated microelectrode arrays provide important information
regarding the neuron network structure and function that is dicult or
impossible to obtain using other electrophysiological techniques. Some of the
manufacturers that currently provide hardware and software for MEA
systems include: Alpha MED Sciences, Osaka, Japan (MED systems formally
manufactured by Panasonic); Axion Biosystems, Atlanta, GA; Ayanda
Biosystems/SAS Biologics, Lausanne, Switzerland/Claix, France; 3-Brain,
Landquart, Switzerland; MultiChannel Systems, Reutlingen Germany; Plexon,
Inc., Dallas TX; and Tucker-Davis Technologies, Alachua, FL.
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MEA systems offer a great deal of flexibility regarding biological tissue and
experimental design. A wide variety of electrically excitable biological tissues may
be placed on the MEA. This includes cardiac tissue, primary cultures of nervous
system tissue from many different regions, i.e. mouse frontal cortex cells, tissue
slices (e.g. hippocampal slice) or retinas. MEA chips with characteristics
specialized to the tissue can be made in-house or purchased commercially. The
electrodes may be planar or they may have a 3D arrangement that allows
penetration of the electrodes through outer layers of damaged cells in tissue slices,
thereby enhancing the cell-electrode coupling. The spatial pattern of electrodes
can also be changed, for example, to place more electrodes in specific regions of a
hippocampal slice or retina. Following use, biological tissue is removed from the
MEA chips, and thus they can be reused multiple times and with special care are
quite economical ($10-30 per use). Figure 3.19 shows the assembling of an MEA
(A-D) and the liquid handling robot over the culture chambers (E).
Electrical activity can be recorded either immediately (slices, retina) or
following a growth phase (1-3 weeks) in culture. In neuronal cultures, activity
consists of extracellular potentials generated by action potentials in cell bodies,
axons and dendrites of neurons that are within the receptive field of an
electrode on the array. The recorded activity is spontaneous but can also be
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