Identification of midbrain neurons essential for vocal communication

Abstract

Vocalizations are an essential medium for communication and courtship in numerous mammalian species ranging from mice to humans. In mammals, the midbrain PAG serves as an obligatory node in a vocalization-related network that spans the forebrain and brainstem1-3, as bilateral lesions of the PAG result in mutism2-5. Despite the PAGs importance for vocal production, the identity, function, and connectivity of PAG neurons involved in vocalization has remained elusive, in part because the PAG is a functionally and anatomically heterogeneous structure that serves myriad roles including nociception, defensive behaviors, and autonomic regulation6-9. Here we used a viral genetic \"tagging\" method10,11 to identify a distinct subset of PAG neurons in the male mouse that are selectively activated during the production of ultrasonic vocalizations (USVs) elicited by female cues. Silencing these PAG-USV neurons rendered males mute without affecting their other courtship behaviors and also impaired their ability to attract female mice in a social choice assay. Activating these neurons using chemogenetic or optogenetic methods strongly elevated USV production, even in the absence of female cues. Notably, the timing of individual USVs was entrained to the expiratory phase of breathing but not to the pattern of optogenetic stimulation, suggesting that PAG-USV neural activity initiates and sets the duration of vocal bouts and recruits downstream premotor circuits that precisely pattern vocal output. Consistent with this idea, we found that PAG-USV neurons extend axons into pontine and medullary regions that are speculated to contain premotor central pattern generators important for vocalization3,12,13. These experiments establish the identity of the PAG neurons selectively required for USV production in mice, map their efferent connections, and demonstrate the communicative salience of male USVs in promoting female social affiliation.