The light switch-off response as a putative rodent test of innate fear

Recent discussions on the ethics in animal experimentation instigate the refinement of methods used in Behavioral Neuroscience, particularly regarding fear/anxiety paradigms. We propose the Light Switch-Off Test (LSOT), based on the innate motivation to cease an aversive stimulus (bright light), displayed naturally by rodents in their habitat. Forty-six male adult Wistar rats were allocated into independent groups: control, diazepam at 1 or 2mg/kg, and meta-Chlorophenylpiperazine (mCPP) at 0.5 or 1mg/kg. The experimental box has two square compartments, separated by an acrylic portal. In each side of the box, there is a 40-W incandescent light bulb. After a habituation period in the box, 40 light stimuli (trials lasting up to 20s each) are emitted at random intervals. By crossing compartments during the lighted period, the rat could switch-off the stimulus. Parameters observed are the number of switch-off responses (SORs), latency of SOR and intertrial locomotion. The SOR frequency was higher in rats treated with mCPP at 1mg/kg, an anxiogenic drug, while diazepam at the doses used in this study did not produce effects. Animals exposed solely to the box for the length of the test did not respond in a false positive way. Therefore, the SOR represents a good index to measure the innate rodent fear of bright-lighten areas, once they react quickly in order to turn off the stimulus. Among its many advantages, the LSOT is a simple, replicable, non-invasive and minimally stressful procedure, since it does not expose animals to excessively aversive stimulus.