Klymko, N.; Sartori, A. M.; Leon, M.; Seifert, C. N.; Lee, R.; Mathai, J. C.; Verstegen, A. M. J.

Barrington\'s nucleus (Bar) is a brainstem hub essential for lower urinary tract (LUT) function, yet the molecular identity and functional specialization of its neuronal subtypes remain poorly defined. Here, we construct a single-nucleus transcriptional atlas of Bar and identify an excitatory Penk-expressing population of (BarPenk) critical for LUT regulation. BarPenk neurons are active selectively during voiding, when the external urethral sphincter (EUS) relaxes, and their optogenetic activation elicits time-locked suppression of EUS activity. Chemogenetic activation of these neurons induces a sex-specific, aberrant pattern of micturition behavior, while targeted ablation impairs voluntary marking behavior in response to female cues. We observe multiple regions involved in visceromotor regulation and behavioral state control projecting to BarPenk neurons, supporting their role as integrators of internal state and environmental context to drive urinary output. These findings provide new insights into the brainstem circuits that shape reflexive and voluntary micturition, and highlight how discrete neuronal subtypes contribute to sexually dimorphic regulation of LUT function.