Neuroprotection against oxidative stress by serum from heat acclimated rats Elie Beit-Yannai a , Victoria Trembovler a , Michal Horowitz b , Philip Lazarovici a , Ron Kohen c , Esther Shohami a, * a Department of Pharmacology and Experimental Therapeutics, The Hebrew University of Jerusalem, Jerusalem, 91120 Israel b Pharmacy School of Pharmacy, The Hebrew University of Jerusalem, Jerusalem, 91120 Israel c Department of Physiology, Faculty of Dental Medicine, The Hebrew University of Jerusalem, Jerusalem, 91120 Israel Received 1 July 1998; received in revised form 1 August 1998; accepted 12 August 1998 Abstract Exposure of PC12 cells, to 1% serum derived from normothermic (CON) rats resulted in 79% cell death. Sister cultures treated with 1% serum derived from heat acclimated (ACC) rats, were neuroprotected and expressed a significant reduction in cell death. In PC12 cells exposed to a free radical generator causing an oxidative stress, 90% cell death was measured in CON serum treated cultures, while ACC serum treated cultures were neuroprotected. Xanthine oxidase activity and uric acid (UA) levels were lower in ACC serum compared to CON. Addition of UA to both sera abolished the difference in cell viability, and toxicity of ACC serum reached that of CON. These findings suggest a causal relationship between the lower levels of UA in ACC and the neuroprotective effect observed. The present study proposes heat acclimation as an experimental and/or clinical tool for the achievement of neuroprotection.  1998 Elsevier Science Ireland Ltd. All rights reserved Keywords: Heat-acclimation; Uric acid; Xanthine-oxidase; Cell viability; PC12 Chronic exposure to high ambient temperature results in adaptive metabolic changes which lead to a decrease in metabolic rate [8]. Consequently, heat acclimated (ACC) rats show faster and better recovery following closed head injury, as compared with normothermic, control rats (CON), subjected to a similar severity of injury [15]. We have demonstrated that although the brain and the heart of ACC rats have a lower basal level of antioxidants compared to those of normothermic rats, both organs have a greater capacity to raise the levels of these compounds upon the insult. Furthermore, the temporal profile of the elevated antioxidants in the hearts of ACC rats after brain injury parallels that found in the brain, and suggests the involve- ment of a systemic, humoral response [3]. Uric acid (UA), considered a waste product of purine metabolism, is one of the major antioxidants found in high concentrations in plasma and brain tissue [2]. Urate contributes up to 60% of the total plasma antioxidant power in healthy subjects [16], and acts as a preventive antioxidant by interacting with 10–15% of the daily produced hydroxyl radicals. In the present study we tested the effect of sera taken from ACC and CON rats as well as UA, on the survival of a sympathetic pheochromocytoma (PC12) cell line, a widely used neuronal cell model [6]. PC12 cell line viability served as a convenient tool for investigating the effects of the serum under both normal conditions and during oxida- tive stress [1,13], and under oxidative stress, produced by 2,2′-Azobis(2-amidinopropane dihydrochloride) (AAPH), a water soluble peroxyl radical initiator [17]. This substance generates radicals at a known and constant rate [17]. The activity of the enzyme xanthine-oxidase, and the levels of its product, UA, in the brains of ACC and CON rats, were compared in order to assess whether any observed differ- ences in the brain are also reflected in the serum. Male Sabra rats (Hebrew University strain), weighing 180–220 g were used in this study. The animals were Neuroscience Letters 254 (1998) 89–92 0304-3940/98/$19.00  1998 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940(98)00670-3 * Corresponding author. Tel.: +972 2 6757513; fax: +972 2 6431094/5714923; e-mail: esty@cc.huji.ac.il