Understanding pup affective state through ethologically significant ultrasonic vocalization frequency

Abstract

Throughout life, rats emit ultrasonic vocalizations (USV) when confronted with an aversive situation. However, the conditions classically used to elicit USV vary greatly with the animal’s age (isolation from the dam in infancy, versus nociceptive stimulation in adults). The present study is the first to characterize USV responses to the same aversive event throughout development. Specifically, infant, juvenile and adult rats were presented with mild foot-shocks and their USV frequency, duration, and relationship with respiration and behavior were compared. In juvenile and adult rats, a single class of USV is observed with an age-dependent main frequency and duration (30 kHz/400 ms in juveniles, 22 kHz/900 ms in adults). In contrast, infant rat USV were split into two classes with specific relationships with respiration and behavior: 40 kHz/300 ms and 66 kHz/21 ms. Next, we questioned if these infant USV were also emitted in a more naturalistic context by exposing pups to interactions with the mother treating them roughly. This treatment enhanced 40-kHz USV while leaving 66-kHz USV unchanged suggesting that the use of USV goes far beyond a signal studied in terms of amount of emission, and can inform us about some aspects of the infant’s affective state.

Isolation‐induced ultrasonic vocalizations in pups: A comparison between Long‐Evans, Sprague–Dawley, and Wistar rats

“…These features are very similar to our present type 1 and type 2 calls, respectively, which supports the validity of our results. In our study, type 1 and type 2 calls accounted for about 84% of all calls detected and for most of the remaining 16%, we identified a third class, which was not specifically addressed by Boulanger‐Bertolus et al (), although they studied LE rats, which according to our findings also display this type of calls in a considerable percentage. Boulanger‐Bertolus et al () pointed out that their two call types (40‐kHz, 66‐kHz, i.e., our types 1 and 2 calls, respectively) are emitted during isolation from mother and nest, and that especially the 66‐kHz calls were related to behavioral activity, that is, movement (see also Wöhr & Schwarting, ), whereas 40‐, but not 66‐kHz calls, affect respiration since they seem to increase expiration.…”

“…In our study, type 1 and type 2 calls accounted for about 84% of all calls detected and for most of the remaining 16%, we identified a third class, which was not specifically addressed by Boulanger‐Bertolus et al (), although they studied LE rats, which according to our findings also display this type of calls in a considerable percentage. Boulanger‐Bertolus et al () pointed out that their two call types (40‐kHz, 66‐kHz, i.e., our types 1 and 2 calls, respectively) are emitted during isolation from mother and nest, and that especially the 66‐kHz calls were related to behavioral activity, that is, movement (see also Wöhr & Schwarting, ), whereas 40‐, but not 66‐kHz calls, affect respiration since they seem to increase expiration. Re‐exposure of the dam seemed to specifically increase pup 40‐kHz calls, especially when this dam showed “rough maternal behavior.” Furthermore, Boulanger‐Bertolus et al () hypothesized that pup 40‐kHz calls have similarities with adult aversive calls, which are emitted at frequencies around 20 and 30 kHz, whereas 66‐kHz calls might resemble a certain class of adult 50‐kHz calls (Taylor, Urbano, & Cooper, ), which unlike most adult 50‐kHz calls are not related to appetitive but aversive situations.…”

“…Boulanger‐Bertolus et al () pointed out that their two call types (40‐kHz, 66‐kHz, i.e., our types 1 and 2 calls, respectively) are emitted during isolation from mother and nest, and that especially the 66‐kHz calls were related to behavioral activity, that is, movement (see also Wöhr & Schwarting, ), whereas 40‐, but not 66‐kHz calls, affect respiration since they seem to increase expiration. Re‐exposure of the dam seemed to specifically increase pup 40‐kHz calls, especially when this dam showed “rough maternal behavior.” Furthermore, Boulanger‐Bertolus et al () hypothesized that pup 40‐kHz calls have similarities with adult aversive calls, which are emitted at frequencies around 20 and 30 kHz, whereas 66‐kHz calls might resemble a certain class of adult 50‐kHz calls (Taylor, Urbano, & Cooper, ), which unlike most adult 50‐kHz calls are not related to appetitive but aversive situations. Of note, similar call types differing in peak frequency and frequency modulation were detected in mouse pups (Wöhr et al, ).…”

“…The former might be equivalent to our type 2 class, which is also characterized by short duration and higher peak frequency, together with rather small bandwidth, a measure which was not addressed by Harmon et al (). Recently, Boulanger‐Bertolus, Rincón‐Cortés, Sullivan, and Mouly (), who used a different behavioral testing procedure, reported two types of USV (see also Ise & Ohta, for SD pups) in isolated male and female LE pups around PND 12–15: One type, termed “40‐kHz calls,” had mean frequencies around 40 kHz and long call durations (around 140 ms), whereas the other, “66‐kHz calls,” was higher (around 66 kHz) and much shorter (around 21 ms). These features are very similar to our present type 1 and type 2 calls, respectively, which supports the validity of our results.…”

“…Furthermore, Boulanger-Bertolus et al (2017) hypothesized that pup 40-kHz calls have similarities with adult aversive calls, which are emitted at frequencies around 20 and 30 kHz, whereas 66-kHz calls might resemble a certain class of adult 50-kHz calls (Taylor, Urbano, & Cooper, 2017), which unlike most adult 50-kHz calls are not related to appetitive but aversive situations. Of note, similar call types differing in peak frequency and frequency modulation were detected in mouse pups .…”