Distinct classes of low frequency ultrasonic vocalizations in rats during sexual interactions relate to different emotional states

This study examined low-frequency ultrasonic vocalizations (IUSVs) in rats during two types of sexual interactions: postejaculatory interval (PEI) and barrier - noncontact (NC) test. We report distinct classes of IUSVs that can be assigned to different emotional states; relaxation vs. frustration. Totally flat, 22-kHz calls (Class A), were observed during the relaxation state following ejaculation; characterized by immobilization or grooming during the PEI. On the other hand, two-three component IUSVs (Class B) that start at a higher frequency (45-kHz: flat, upward or short signal) and then shift to 35-23-kHz (mostly to 28-23-kHz), correspond as we assume, to arousal and frustration - active states associated with sniffing a hole or exploration during the NC test. We suggest that momentary, abrupt decreases of arousal during the frustration state correspond to Class B IUSVs. The detailed spectral analysis of the high-frequency component of two-component IUSVs is crucial for establishing the relationship between such IUSVs and the corresponding behavior and emotional states. Our studies indicate that while the two-component Class B 22-kHz IUSVs may relate to the frustration state, a single component, flat, Class A IUSV relates to the relaxation state. The results of these studies support a notion that rats emit distinct vocalization patterns, reflecting their emotional states.