Developmental differences in broad versus selective approach responses to ultrasonic vocalizations in rats:...

Rats emit ultrasonic vocalizations (USVs) that function as affective and social acoustic cues. In adults, 50-kHz USVs are associated with appetitive states, whereas 22-kHz USVs are associated with aversive or defensive contexts. However, developmental changes in receiver responses to these calls remain unclear. Here, we compared sound-source approach behavior and c-Fos expression in juvenile and adult male rats exposed to playback of 50-kHz USVs, 22-kHz USVs, or control sounds. Each rat was assigned to one playback condition and perfused after the behavioral test for c-Fos immunohistochemistry. Playback condition significantly affected approach time. Adult rats showed selective approach to 50-kHz USVs, with longer approach time to 50-kHz USVs than to 22-kHz USVs or control sounds. In contrast, juvenile rats showed a broader, less differentiated pattern of approach times across the two USV conditions. Direct age-group comparisons showed that adults approached 22-kHz USVs less than juveniles. USV playback increased c-Fos expression in several affect- and reward-related regions, including the agranular insular cortex, anterior cingulate cortex, nucleus accumbens core, and basolateral amygdala. Following playback of 50-kHz USVs, adults showed higher c-Fos expression than juveniles in the nucleus accumbens core and basolateral amygdala. Moreover, c-Fos expression in the nucleus accumbens core and central amygdala was positively correlated with approach behavior. These findings suggest that responses to USVs involve distributed sensory-affective processing and adult-biased recruitment of reward- and affect-related regions following playback of 50-kHz USVs.