Frequency-modulated 50 kHz ultrasonic vocalizations: a tool for uncovering the molecular substrates of positive affect

The evidence that frequency modulated (FM) 50 kHz ultrasonic vocalizations (USVs) reflect a positive emotional state in rats is reviewed. Positive emotional states in humans are measured by facial-vocal displays (e.g., Duchenne smiling and laughter), approach behavior, and subjective self-report of feeling states. In laboratory animals, only facial-vocal displays, along with approach behavior can be measured. FM 50 kHz USVs are uniquely elevated by hedonic stimuli and suppressed by aversive stimuli. Rates of FM 50 kHz USVs are positively correlated to the rewarding value of the eliciting stimulus. Additionally, playbacks of these vocalizations are rewarding. The neural and pharmacological substrates of 50 kHz USVs are consistent with those of human positive affective states. By experimentally eliciting FM 50 kHz USVs, the novel molecular underpinning of positive affect can be elucidated and may be similar to those in humans. In humans, positive emotional states confer resilience to depression and anxiety, as well as promote overall health. Using rough-and-tumble play induced hedonic USVs, we have identified insulin like growth factor I and the NR2B subunit of the NMDA receptor as playing a functional role in positive affective states. From this research, we have developed a promising new class of antidepressants that is entering phase II clinical trials for the treatment of depression.