Parabrachial Tac1 Neurons Are Conditionally Activated During Dynamic Breathing Patterns

Homeostatic breathing is continuously regulated by reflexive neural circuits that provide physiological feedback. However, in the awake state, breathing is also conditionally altered by behaviors and emotions. Relatively little is known about the circuits that mediate this dynamic “non-homeostatic'' breathing control. We previously found that optogenetic activation of a Tac1, but not Calca, expressing subpopulation of lateral parabrachial (PBN) neurons drives breathing frequencies that far exceed those relevant for the homeostatic control of breathing. Here, we combine cell-type-specific silencing and multi-unit recording approaches to further test the hypothesis that this circuit plays an important role in state-dependent, but not automatic, respiratory control. To render Tac1 PBN neurons non-functional, the PBN of adult Tac1 Cre mice was injected bilaterally with an AAV that drives Cre-dependent expression of tetanus light chain. After allowing 4 weeks for viral expression, plethysmography did not reveal any differences in breathing frequency or pattern compared to mice injected with control AAV. Changes in breathing were also absent during acute optogenetic inhibition of Tac1+; Calca- neurons following injection of AAV that expresses iC++ in neurons that contain Cre but not FlpO in the PBN of Tac1 Cre ; Calca FlpO mice. To assess the spiking activity of Tac1 PBN neurons relative to ongoing breathing patterns, the PBN of Tac1 Cre ; Calca FlpO mice was injected with an AAV that expresses ChRmine in neurons containing Cre but not FlpO, a cranial window was created dorsal to the PBN and a head bar secured to the skull. After 2 weeks, mice were head-fixed and a high-density neuropixels probe was inserted into the PBN via the cranial window. Multi-unit activity was recorded from awake, freely breathing mice during simultaneous recording of respiratory pattern. Illumination of the cranial window with red light was sufficient for ChRmine mediated activation of Tac1+;Calca- PBN neurons, producing a robust increase in breathing frequency. Recorded PBN units were then optically tagged by assessing their response latency and fidelity to repeated 10 ms light pulses. “Optotagging” during Neuropixels multi-unit recordings in awake mice suggested that this PBN subpopulation is mostly quiescent during resting homeostatic breathing, but switches to tonic spiking during periods of non-homeostatic breathing including rapid sniffing bouts induced by odorants. Isoflurane anesthesia abolished light-evoked activity of optotagged neurons, underlying their state-dependent function. We conclude that this PBN subpopulation functions as a switch-like mechanism that is inactive at rest and conditionally activated during non-homeostatic breathing patters that are unique to the awake state. R01HL166317 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.