Acoustic startle response and habituation in freezing and nonfreezing rats

Rats can be divided according to their responses to startle-eliciting stimuli into 2 groups with different emotional states. About half of the 54 female Sprague-Dawley rats showed long-lasting freezing behavior after 1-8 stimuli (10 kHz, 110 dB spl). In freezing rats the startle amplitude was higher than in nonfreezing rats, even on the very first startle response. This finding demonstrates that the anxiety state of these animals before the first startle-eliciting stimulus, and not just the aversiveness of the stimulus, contributes to freezing behavior. In addition, in freezing rats there was no influence of spontaneous motor activity or of adaptation time on startle amplitude. Only in nonfreezing rats were high motor activities correlated with lowered startle amplitudes, and only in these rats did the course of startle habituation depend on adaptation time.