Specific expansion of motor cortical projections in a singing mouse

Key Points:

• Researchers investigated whether differences in vocal behavior between laboratory and singing mice are due to variations in projection patterns of orofacial motor cortex (OMC) neurons, testing three models: novel projections, differences in innervation strength, or differences in projection probability.
• Bulk axonal tracing and whole-brain imaging showed that OMC neurons in both species project to the same brain regions with similar overall morphology and projection patterns, ruling out novel projections as a source of behavioral differences.
• Single-cell resolution mapping using MAPseq revealed no significant differences in innervation strength per neuron but identified a selective increase in the probability of OMC neurons projecting to the auditory cortex region (AudR) and the midbrain periaqueductal gray (PAG) in singing mice compared to laboratory mice.
• The expanded OMC projections to AudR and PAG, brain regions implicated in vocal communication, suggest enhanced cortical control over vocalizations in singing mice, providing a potential neural circuit mechanism underlying their complex song behavior.
• Findings support the concept that quantitative modifications of existing neural circuits, rather than drastic rewiring, can underlie rapid behavioral evolution; this approach and model system offer a framework for studying neural circuit evolution related to vocal communication and may have parallels in human language evolution.