International Journal of Psychophysiology, 14 (1993) 5-19 0 1993 Elsevier Science Publishers B.V. All rights reserved 0167-8760/93/$06.00 5 INTPSY 00430 Neurotoxicity screening methods are sensitive to experimental history P.J. Spencer, J.L. Mattsson, K.A. Johnson and R.R. Albee The Toxicology Research Laboratory, The Dow Chemical Company, Midland, MI (USA) (Accepted 22 September 1992) Key words: Environmental enrichment; Evoked potential; Guideline test; Flash evoked potential; Somatosensory evoked potential; Auditory brainstem response; Motor activity; Grip strength Toxicity studies commonly include unavoidable environmental differences (experimental history) among test groups, such as chemical taste, odor and irritation. The influence of environmental variables on USEPA guideline neurotoxicity tests was evaluated using an environmental enrichment model. h-week-old male Fischer 344 rats were housed for 13 weeks in pairs with access to an exercise wheel, trained to run on a rotating rod and handled frequently. Control animals were housed singly. lacked the exercise wheel and rotating rod training, and had only routine interaction with caretakers. At the end of 13 weeks, flash evoked potentials (FEPs), somatosensory evoked potentials (SEPs), auditory brainstem responses (ABRs), grip performance, motor activity (MA), elements of the functional observational battery (activity and reactivity to handling/restraint) and brain histopathology with glial fibrillary acidic protein immunohistochemistry (GFAP IHC) were evaluated. Animals from the enriched group demonstrated changes (P < 0.05) in FEPs, SEPs and grip performance. Enriched animals were more active and reactive to their surroundings, and were highly reactive to physical restraint. Control (unenriched) animals showed little to no exploratory behavior and were more tolerant of restraint. Differences in experimental history can be detected using elements of standard guideline tests and may confound interpretation of such data if not taken into consideration. INTRODUCTION The United States Environmental Protection Agency has published guidelines for the neuro- toxicity screening of industrial and agricultural chemicals (USEPA, 1985, 1991). These guidelines require rats be at least 42 days of age and be tested 4-5 times during studies that can last from 2 weeks to 1 year. The screening battery includes a functional observational battery (FOB), motor activity (MA) and neuropathology. The FOB in- cludes a clinical evaluation, qualitative assess- ment of spontaneous activity, reactivity to hand- Correspondence to: P.J. Spencer, Neurotoxicology Depart- ment, Health and Environmental Sciences, The Dow Chemi- cal Company, 1803 Building, Midland, MI 48674, USA. ling, coordination, sensory evaluation and quanti- tative measurements of grip strength and landing foot splay. The guideline recommends a proce- dure for assaying glial fibrillary acidic protein as a marker for astrocyte hypertrophy, and guidelines for visual, auditory and somatosensory evoked potentials are being developed. The concept of an adequate control group has been inherited from classical toxicology, where controls are simply those animals without expo- sure to the test substance. Obviously, many fea- tures of the test environment remain uncon- trolled, e.g., taste of food is altered by the test substance, odor of food and air can be different, the material may be irritating to eyes or respira- tory passages, and there may be systemic toxicity that causes malaise and other indirect effects on behavior.