Influence of gender and estrous cycle in the acoustic startle reflex in Wistar rats

Abstract

The acoustic startle reflex (ASR) is a fast and intense motor reaction that results in contraction of skeletal muscles of the face and body in response to an unexpected and high intensity sound stimulus. The ASR is an acoustic-motor reflex triggered by structures of the brainstem. It is present in several species of mammals including man. The ASR presents many modulations such as prepulse inhibition (PPI) that is characterized by reduced amplitude of ASR when the triggering sound stimulus is preceded by a stimulus of lower intensity. The responses of ASR and PPI can be assessed in man and in experimental models noninvasively and they have a high degree of homology between rodents and humans. ASR and PPI have been used for basic and clinical research since they are altered in many neurological and psychiatric conditions that have deficiencies in processing of sensorimotor information. In rats, ASR manifests as a ballistic movement with the shortening of the total length of the animal, and increased blood pressure and heart rate, mediated by the autonomic nervous system. In man, it is known that there are differences in gender-related PPI reflecting hormonal changes that occur during the menstrual cycle. However studies in rodents are conflicting regarding the differences between males and females, rarely mentioning the possible variations during the estrous cycle. Partially it is due to the frequent exclusion of females in physiological, pharmacological and behavioral studies to eliminate the effect of hormonal fluctuations. This makes difficult the use of previous data in the literature from females to support the experimental results and their comparisons with results obtained in humans. To evaluate the influence of gender and the estrous cycle phases in ASR and PPI in Wistar rats, we will study 10 male and 20 female adult rats, between 100 and 150 days of age. Initially, both groups will undergo a period of habituation with the researcher and the manipulations necessary for evaluation of ASR, PPI and collection of vaginal lavage in females. In the experimental phase, the estrous cycle of females will be accompanied by daily vaginal lavage and animals will be subjected to repeated sessions of evaluation of ASR and PPI. In females, a sample of blood will be collected for determination of estradiol and progesterone by ELISA assay, after each behavioral measurement. After an interval of 15 days since the end of the behavioral assessment, 3 males and 3 females in each phase of estrous cycle will be stimulated for an ASR and PPI session, and after 1 hour will be perfused in order to evaluate the neuronal activation of the neural circuitry of ASR and PPI by Fos immunohistochemistry. Therefore, we will intend to relate the gender, the stage of the estrous cycle of females, the behavioral evaluation of ASR and PPI, the levels of estradiol and progesterone and the neuronal activation of the brainstem nuclei involved in the ASR and PPI. The results of this work will be important for studies using RAS and PPI as tools for assessing deficits in sensorimotor integration and to improve the design of future translational studies.