Pro-social ultrasonic communication in rats: insights from playback studies

Rodent ultrasonic vocalizations (USV) serve as situation-dependent affective signals and convey important communicative functions. In the rat, three major USV types exist: (I) 40-kHz USV, which are emitted by pups during social isolation; (II) 22-kHz USV, which are produced by juvenile and adult rats in aversive situations, including social defeat; and (III) 50-kHz USV, which are uttered by juvenile and adult rats in appetitive situations, including rough-and-tumble play. Here, evidence for a communicative function of 50-kHz USV is reviewed, focusing on findings obtained in the recently developed 50-kHz USV radial maze playback paradigm. Up to now, the following five acoustic stimuli were tested in this paradigm: (A) natural 50-kHz USV, (B) natural 22-kHz USV, (C) artificial 50-kHz sine wave tones, (D) artificial time- and amplitude-matched white noise, and (E) background noise. All studies using the 50-kHz USV radial maze playback paradigm indicate that 50-kHz USV serve a pro-social affiliative function as social contact calls. While playback of the different kinds of acoustic stimuli used so far elicited distinct behavioral response patterns, 50-kHz USV consistently led to social approach behavior in the recipient, indicating that pro-social ultrasonic communication can be studied in a reliable and highly standardized manner by means of the 50-kHz USV radial maze playback paradigm. This appears to be particularly relevant for rodent models of neurodevelopmental disorders, as there is a tremendous need for reliable behavioral assays with face validity to social communication deficits seen in autism and schizophrenia in order to study underlying genetic and neurobiological alterations.