Introduction There is a recognised need for novel in vitro end- points in neurotoxicology (1). Electrophysiological techniques could provide such endpoints, because they have several advantages over methods currently in use. Electrophysiological effects can reveal damage at a very early stage, usually before morphological changes and overt cell damage occur (2, 3). Moreover, electrophysiological techniques are able to detect epileptogenic toxicity, a toxic property that is not detected by histological methods (4, 5). Extracellular electrophysiological recording in in vitro freshly-cut hippocampal slices is used as a standard and rela- tively simple method in neuroscience. For example, it has been used to investigate hypoxic neuronal dam- age (6–9) and the mechanisms of neurotoxicity by some compounds, including trimethyltin (TMT; 10–12), triethyltin (13) and kainic acid (KA; 14–16). Freshly-cut hippocampal slices have a life-span of a few hours, which makes them suitable for use in acute toxicity tests. Organotypic slices have a life- span of weeks, and could be used for chronic, as well as acute, toxicity tests, but their preparation and maintenance are technically much more cumbersome and time-consuming. Electrophysiology could be especially useful in screening tests, due to its expected high sensitivity (2). In fact, it is very unlikely that a toxic compound causes neuronal death or severe damage without suppressing or severely affecting neuronal electrical activity. Acute, as well as chronic, toxicity could be investigated by using an appropriate in vitro model, such as freshly-cut brain slices, for acute toxicity studies, and organotypic slices for chronic toxicity investigations. There is a risk that electrophysiol- ogy will give false-positive results; for example, bar- biturates can affect and even suppress electrical activity, without causing neuronal death. Never- theless, in screening tests, high sensitivity is mandatory, so electrophysiology could still be use- ful, even in the face of occasional false-positive results. Electrophysiological changes have rarely been suggested as possible endpoints for use in formal screening tests of neurotoxicity (17–19). By con- trast, a more systematic investigation of the use of electrophysiological techniques in neurotoxic- ity testing, and a comparison with other tests in use, could advance their use, and possibly pro- vide the rationale for their formal validation as tests. Changes in Extracellular Action Potential Detect Kainic Acid and Trimethyltin Toxicity in Hippocampal Slice Preparations Earlier than do MAP2 Density Measurements Raffaella Melani, 1 Renata Rebaudo, 1 Jens Noraberg, 2 Jens Zimmer 2 and Maurizio Balestrino 1 1 Department of Neuroscience, Ophthalmology and Genetics, University of Genova, Italy; 2 Department of Anatomy and Neurobiology, University of Southern Denmark, Odense, Denmark Summary — In vitro electrophysiological techniques for the assessment of neurotoxicity could have sev- eral advantages over other methods in current use, including the ability to detect damage at a very early stage, and could further assist in replacing animal experimentation in vivo. We investigated how an elec- trophysiological parameter, the extracellularly-recorded compound action potential (“population spike”, PS) could be used as a marker of in vitro neurotoxicity in the case of two well-known toxic compounds, kainic acid (KA) and trimethyltin (TMT). We compared the use of this electrophysiological endpoint with changes in immunoreactivity for microtubule-associated protein 2 (MAP2), a standard histological test for neurotoxicity. We found that both toxic compounds reliably caused disappearance of the PS, and that such disappearance occurred after only 1 hour of exposure to the drug. By contrast, densitometric measure- ments of MAP2 immunoreactivity were unaffected by both KA and TMT after such a short exposure time. We conclude that, in the case of KA and TMT, the extracellular PS was abolished at a very early time-point, when MAP2 immunoreactivity levels were still comparable to those of the untreated controls. Electrophysiology could be a reliable and early indicator of neurotoxicity, which could improve our ability to test for neurotoxicity in vitro, thus further replacing the need for in vivo experimentation. Key words: brain slices, electrophysiology, in vitro, kainic acid, MAP2, neurotoxicity, population spike, trimethyltin. Address for correspondence: M. Balestrino, Department of Neuroscience, Ophthalmology and Genetics, University of Genova, Via De Toni 5, 16132 Genova, Italy. E-mail: mbalestrino@neurologia.unige.it ATLA 33, 379–386, 2005 379