Playback of 22-kHz and 50-kHz ultrasonic vocalizations induces differential c-fos expression in rat brain

Rodent ultrasonic vocalizations, which serve as sensitive measures in a number of relevant individual and social behaviours, have become increasingly interesting for biopsychological studies on emotion and motivation. Of these, high frequency (50-kHz) ultrasonic vocalizations can index a positive emotional state, and induce approach, whereas low frequency (22-kHz) ultrasonic vocalizations can induce avoidance and may index anxiety, since they are emitted during various unconditioned and conditioned aversive situations. While cholinergic and dopaminergic systems have been implicated, specific neural substrates that sub-serve these vocalization-dependent states remain to be elucidated. Using c-fos immunocytochemistry, we revealed neural activity in brain areas of naïve male Wistar rats in response to playback of 22-kHz and flat and frequency-modulated 50-kHz ultrasonic vocalizations. Presentation of background noise or no acoustic stimulus at all constituted the controls. Playback of 50-kHz ultrasonic vocalizations led to approach behaviour. Acoustically stimulated animals demonstrated differential activation in auditory areas, with a frequency-dependent activation in the auditory cortex. Specific forebrain, thalamic, hypothalamic and brainstem areas were also activated differentially. While 50-kHz playback induced sparse fos-like immunoreactivity in frontal association cortex, nucleus accumbens, thalamic parafascicular and paraventricular nuclei, 22-kHz playback elicited c-fos expression in the perirhinal cortex, amygdalar nuclei and the periaqueductal gray. This study unveils neural substrates that are activated during ultrasonic playback perception, which could sub-serve the affective states elicited by these vocalizations.