Breathing pace

Each breath is two phases that alternate like the push and pull of riders on a seesaw; inspiration when air is drawn into the lungs and expiration when it is expelled. Breathing is when these are linked one-after-another. Several thousand neurons in the brainstem comprise the breathing rhythm generator and two key neural clusters control the inspiration and expiration. These two groups are reciprocally connected by inhibitory neurons, providing a scaffold for their activities to alternate. Additionally, the inspiratory neurons have specialized properties that bestow them with the ability to form an autonomous pacemaker. For example, these neurons produce a rhythm when explanted in a thin brain slice. So, it is possible for the cycles of breaths to rhythmically occur in vivo by inspiration terminating expiration and vice versa, or for the autonomous pacemaker to determine when each breath occurs and then expiration simply follows. We use sophisticated approaches to identify key neuron types in each of these neural clusters to dissect which and when these two mechanisms are used to pace breathing.

 

Volitional control

Take a deep breath and hold it. How is it that you learned to stop your most vital behavior? At birth, we possess innate protective reflexes that halt breathing, like if we fall into water. Eventually, we learn to volitionally stop breathing so we hold our breath before diving into the water. This capacity is extremely powerful, exemplified by the world-record for a breath hold being ~24 minutes! A big picture question our lab studies is if the mechanism to volitionally hold our breath is to “turn-on” the innate reflexes that evolved to stop breathing or, instead, do we develop a novel approach? Also, when we choose to stop breathing, do we stop the breathing pacemaker or instead the downstream motor pathways? We have created a first-of-its kind training paradigm to teach mice to control breathing in order to answer these types of questions.

 

Sensory control

One of our most powerful urges is to breathe. Breathlessness overrides all thoughts, desires, and actions and can manifest in many emotions, a prominent being anxiety. Sensory signals that originate in the lung signal to the brain to provide feedback about breathing, but we lack an understanding for how these lung sensory signals enter the brain and then how these signals are transformed to control the breathing pacemaker during basal respiration and in clinically relevant circumstances, like asthma. We have established a framework for characterizing the neural circuit by which a lung sensory signal tunes, and even stops breathing, and plan to study how it can manifest in the powerful emotion and feeling of breathlessness and chest-tightness.

 

Vocalization

Stereotyped vocalizations are innately produced in distantly related species, like our first cries after birth. This alone suggests that phonation is created by a hardwired ‘central pattern generating’ (CPG) system. Since vocalizations are coordinated with breathing, the putative brainstem vocalization CPG might be reciprocally connected with the breathing pacemaker. How breathing is co-opted by or integrated with vocalization circuits, or even what the vocalization circuits are in the brainstem, was unknown until our recent work. We discovered the first brainstem CPG dedicated to, at least in part, forming neonatal murine cries, dubbed the iRO (1). iRO produces an autonomous rhythm that patterns the rhythmic closure of the larynx and breath airflow to form cries that contain one, two, three, or more stereotyped syllables. To coordinate this, the iRO and the breathing pacemaker are reciprocally connected. Now, we have shown that the iRO is capable of coordinating the larynx and breathing muscles to produce the ten complex adult mouse vocalizations (2). We are extending upon this work by characterizing if and how this system is aberrantly patterned in disorders with atypical speech, like autism, and how other orofacial behaviors, such as swallowing, are coordinated with breathing.

(1) Wei et al. A novel reticular node in the brainstem synchronizes neonatal crying with breathing. Neuron, 2022

(2) MacDonald et al. The breath shape controls intonation of mouse vocalizations. Elife, 2024.

 

Opioid induced respiratory depression

Opioids are our most powerful analgesics and a primary medicine in pain management. However, opioid misuse has caused hundreds of thousands of deaths since 1999, a number that continues grow. The lethal side-effect of opioids is respiratory depression, where breathing becomes very slow and shallow, often leading to death. Recently, we demonstrated that most of murine opioid depressed breathing is due to µ-opioid receptor (MOR) signaling in the breathing pacemaker (1, 2). The identification of both the cellular and molecular mechanism(s) of OIRD will establish a platform to determine how it can be blocked or overridden, leading to transformative treatments for a devastating societal challenge that fails to slow. ~8% of the pacemaker neurons express MOR, and now we seek to determine how they control breathing and slow it upon opioid overdose.

(1) Bachmutsky, Wei et al. ß-arrestin 2 germline knockout does not attenuate opioid respiratory depression. Elife, 2021

(2) Bachmutsky et al. Opioids depress breathing through two small brainstem sites. Elife, 2020