A Novel Simple Method for Assessing Rodent Innate Fear Using Round Elevated Platform

Abstract

Open field (OF) and elevated plus maze (EPM) have been widely used for assessing rodent innate fear. In OF task, animals with more fear will spend less time in the center zone (unsafe zone). Regarding EPM, animals with more fear will spent less time in the open arms (unsafe zone). Thus less activity in unsafe zone will be observed in animals with more fear. Based on this assumption, we speculated that our previously designed opaque round elevated platform (O-REP) with open space can be used for assessing rodent innate fear by analyzing animal activity. In O-REP, height and the open space make the animals feel fearful in O-REP and decrease its activity. To verify this novel method, the difference between female and male mice were examined by OF, EPM, and O-REP respectively. The results showed that EPM task could not find the significant fear difference between female and male mice using ICR and C57/BL6 strain. However, C57/BL6 female mice displayed less fear both in OF and O-REP task, which indicates that OF and O-REP task displayed more sensitivity than EPM task in assessing rodent innate fear. Furthermore, O-REP task rather than OF task found that female ICR mice displayed significantly more innate fear in O-REP, which indicated that O-REP might be more sensitive than OF and EPM in assessing innate fear. Further investigations showed that the animals displayed similar activities in O-REP, opaque square elevated platform (O-SEP) and transparent round elevated platform (T-REP), indicating that shape and transparency of EP did not affect the sensitivity of EP. Thus our data demonstrated that the REP is novel alternative task for evaluating rodent innate fear, which sensitivity was not affected by the shape and transparency. The present studies will greatly facilitate fear related research.