Abstracts

Rationale: Wistar Audiogenic Rat (WAR) strain is a genetic model of sound-induced reflex epilepsy that, in the acute situation, mimics tonic-clonic seizures (audiogenic seizures; AS) and, in the chronic protocol, mimics temporal lobe epilepsy. AS have been shown to evoke neuroendocrine responses, however, hypothalamic-pituitary-adrenal activity in WAR is not established. The aim of this study was to phenotypically characterize the WAR strain, and to evaluate the HPA axis responses to ACTH stimulation, restraint stress and circadian variation in these animals. Methods: In order to determine the body weight curve, different litters were weekly weighted from birthday to 63rd day of life. Heart, liver, adrenals, spleen and kidneys were collected, dissected and weighted under basal conditions (fresh weight). Organs were placed in the oven at 50°C and weighted daily until a stable weight was obtained for 3 days. WAR and Wistar (controls) groups were decapitated under basal condition (08 AM or 08 PM) or after restraint for 15 minutes for plasma corticosterone (B) and ACTH measurements by specific radioimmunoassays, and to verify the adrenal weight. To verify the adrenal responsiveness to ACTH, rats were anesthetized with 2.5% tribromoethanol and had a silastic cannula placed at right jugular vein. Next day, rats were pre-treated with dexamethasone (100 ug/100g) and 2 hours after they received an intravenous injection of ACTH (8 ng/rat) or vehicle. Trunk blood samples were collected for plasma B determination by decapitation 15 minutes after the injection. All procedures are in agreement with Brazilian College of Animal Experimentation, and were approved by FMRP-USP Ethical Commission of Ethics in Animal Research (protocol n°203/2005). The results were analyzed using GraphPad Prism software choosing the appropriate test for each experiment and significance were adopted when p<0.05. Results: Plasma B values after ACTH stimulus were significantly higher in WAR (19±8.7 μg/dl), compared to Wistar (9.2±2.5 μg/dl). The results of restrain stress and circadian rhythm variation are shown in Table 1 and indicate that there was an increase of plasma B and ACTH levels after restraint, compared with basal conditions. Although there was no difference in B responses to stress between both groups of rats, plasma ACTH levels after stress were higher (p<0.05) in WAR compared to Wistar. The body weight of WAR was higher (p<0.05) than Wistar from the first week to the 63rd day. In general, organs weight is higher in WAR (see Table 2). Conclusions: Our data indicate that, despite the lower body weight, WAR have increased organ weights. In the adrenal, its increased weight could account for the accentuated pituitary and adrenal responsiveness. The causal association of the increased HPA axis responsiveness and the susceptibility to seizure in WAR remains to be determined. Finacial support: FAPESP, Cinapce-FAPESP, PROEX-CAPES, CNPq and FAEPA.