VALIDATION OF IN-VITRO TESTS IN NEUROTOXICOLOGY

The major challenges in neurotoxicity testing relate to the complexity of the nervous system, the diversity of cell types involved, and the level of integration in the mammalian nervous system. In addition, com pounds which have selective pharmacological effects as receptor agonis ts/antagonists (for example, strychnine) may be neurotoxic at non-cyto toxic concentrations. Tests should answer the following questions: a) when is an effect toxic?, b) when is a substance to be considered toxi c? and c) is the long-term risk assessment valid? The two major strate gies used in the development of in vitro neurotoxicity tests are: mech anistic, in which an attempt is made to elucidate the biochemical proc esses involved in neurotoxicity, and disease based, which could ultima tely be the most useful strategy but which is currently constrained by lack of knowledge of the aetiology of most neurological illnesses. Po tential in vitro test systems which are being developed include: a) si mple tests which measure the activity of an enzyme (for example, acety lcholine esterase); b) studies involving single cell type culture (for example, neuroblastomas or dorsal root ganglion cells); c) complex pr imary co-culture systems (for example, reaggregate culture; 1, 2); and d) combinations of these, including tiered testing and battery testin g (3). Unfortunately, the more complex a system is, the more extensive is the characterisation needed, and, arguably, such systems will neve r fully mimic the intact central nervous system/peripheral nervous sys tem (CNS/PNS).To demonstrate some of the problems inherent in neurotox icity test development, the use of a system which involves the inhibit ion of outgrowth is described. The ideas are developed to include the shift toward proliferation and/or apoptosis of non-terminally differen tiated neurons. The most realistic objective for the optimised, integr ated and validated in vitro reductionist approach for neurotoxicologic al assessment is for the screening of new compounds in parallel with: a) the in vivo holistic approach (for example, to obtain pharmacokinet ics and absorption/receptor-binding data); and b) quantitative structu re activity relationships (QSARs). This is necessarily a selective rev iew, and more details of methodologies and strategies are presented in other publications (3, 4).