Thyrotropin-Releasing Hormone Increases Dorsal Premammillary Nucleus Neuron Activity and Darting Behavior

Thyrotropin-releasing hormone (TRH) is classically recognized for its role in regulating the hypothalamic-pituitary-thyroid (HPT) axis; however, it also functions as a central neuromodulator influencing arousal, locomotion, and autonomic processes. Despite these established roles, the circuit-level mechanisms through which TRH modulates behavior remain poorly defined. The dorsal premammillary nucleus (PMd) is a hypothalamic structure implicated in high-imminence defensive states, yet whether TRH directly engages PMd circuits is unknown. Here, we assessed the role of TRH as a modulator of PMd neuronal activity and behavioral output using in situ hybridization, electrophysiology, calcium imaging, and behavioral assays in male and female mice. TRHR mRNA was co-localized with a subset of PMd CCK-positive neurons. TRH increased firing rate in a subset of PMd neurons in brain slices, revealing functional activation of a subset of the population. This finding was corroborated by calcium imaging that showed increased activity following TRH exposure. In vivo, intrahypothalamic administration of TRH produced darting and behavioral effects. Together, these findings identify a subpopulation of PMd neurons that serve as a functional hypothalamic target of TRH signaling associated with rapid locomotor behavioral output.