Harmonics and frequency-modulated calls indicate different motivational states in rats.

Rats emit 50-kHz ultrasonic vocalizations (USVs) via the accumbal dopaminergic system in response to rhythmic stroking (RS). However, it is unknown whether RS can lead to approach behavior, which is a reliable marker of positive affective states. To assess the effects of RS on the behavioral response and its correlation with call subtypes, we measured the total number of 50-kHz USVs (overall and within call subtypes) and approach behavior in response to RS. Rats were randomly divided into two equal groups. One group received RS and the control group received light touch (LT). RS stimulation was administered for 30 s once a day for 7 consecutive days, and USVs were recorded immediately before, during, and after stimulation. RS induced 50-kHz USVs from day 1 and led to quick approach on day 2 and after. However, those responses were absent in the LT group. Specific frequency-modulated (FM) calls observed after RS were significantly correlated with approach latency. These results suggest that RS has rewarding effects, and the specific FM calls observed immediately after RS can be used as indices of innate positive reinforcement.