Conditional activity of parabrachial Tac1 neurons turns on rapid breathing
Authors: Arthurs Joe , Bush Nicholas , Reilly Elora , Guan Steven , Baertsch Nathan 
Published: May 2024

Breathing is regulated automatically to preserve homeostasis. This reflex regulation of breathing is achieved through neural circuits that function as negative feedback loops. Similar to how a thermostat regulates temperature in a room, the activity of these circuits can be adjusted up or down to fine tune breathing and restore a homeostatic set-point. Yet, breathing is also conditionally altered by behaviors and emotions, requiring dissociation of breathing frequency and pattern from its underlying automatic control. Relatively little is known about the circuits that mediate this “state-dependent'' 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 Tac1Cre 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 Tac1Cre; CalcaFlpO mice. To assess the spiking activity of Tac1 PBN neurons relative to ongoing breathing patterns, the PBN of Tac1Cre; CalcaFlpO 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 suffcient for ChRmine mediated deep brain 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. Based on our preliminary observations, “optotagged” Tac1+; Calca- neurons were mostly quiescent during resting breathing but switched to tonic spiking during bouts of sniffng/rapid breathing. Thus, we predict that Tac1 PBN neurons do not regulate breathing via thermostat-like changes in activity similar to breathing reflexes. Instead, Tac1 PBN neurons represent a switch-like mechanism where these neurons are primarily inactive with minimal respiratory role under normal conditions, but when activated they turn on rapid breathing. R01HL166317. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.