Latent contextual inhibition of the cardiac component in the startle reaction to a sound stimulus in rats

Heart rate (HR), freezing score, and motor component were estimated during acoustic startle (ASR) habituation (two sessions with 24-hour interval, 10 trials in a session). It was shown that rats previously exposed to the experimental context (once for 5 min 24 h before training) demonstrated HR decrease in response to the first stimulus and tachycardia in response to the 4th-10th stimuli during the first session. The interstimulus HR declined from the 4th to the 6th trials. The same profile of cardiovascular response was observed in this group at the beginning of the second session with the following (to the 7th trial) habituation of tachycardia. These rats didn't demonstrate the intertrial HR decrease during the second session. Nonadapted rats responded by bradycardia to the 1st and 2nd trials of the first session. The response didn't change for tachycardia with continuation of stimuli presentation. Tachycardia appeared only in response to the 7th-10th trials of the second session and didn't habituate. The intertrial HR level decreased in this group only during the second session. The results are discussed in terms of contextual latent inhibition of the cardiac acoustic startle response.