High-frequency ultrasonic vocalizations in rats in response to tickling: The effects of restraint stress

Abstract

We tested the hypotheses that (a) the propensity to emit high-frequency (∼50kHz) ultrasonic vocalizations in response to manual “tickling” by an experimenter, may serve as a behavioral marker of positive affect in rats and, (b) that tickling may reduce the severity of stress. Group-housed adult rats were subjected to the 15-s tickling procedure daily, and their ultrasonic vocalization response was measured over a period of two weeks, until it has stabilized. The animals were then subjected to the restraint stress lasting for one week. The experimental groups were exposed to stress 1h before or 1h after tickling and the controls were tickled without stressing. Rats that were stressed 1h before tickling demonstrated a decreased number of the high-frequency calls as compared with the non-stressed controls. Stressing 23h before tickling reduced the call response less effectively. The propensity to emit high-frequency calls has normalized 7 and 12 days following the end of stressing. In addition, stressed groups showed a diminution of sucrose preference, which in the case of rats stressed 23h before tickling persisted even for 12 days following the end of restraint. The present data suggest that repeated stress may decrease the propensity to produce high-frequency vocalizations, and that this measure may serve as a biomarker of the depressive state of animals.