The breathing brainstem : development of inspiration

Abstract

Breathing is essential for life, and yet we do not fully understand the mechanisms that control it. The main central pattern generators for respiration include the inspiratory generating region called the preBötzinger Complex (preBötC), and the chemosensitive region called the parafacial respiratory group/retrotrapezoid nucleus (pFRG/RTN). These are located in the ventrolateral medulla oblongata of the brainstem.

To study these centers in detail, organotypic cultures of the respiratory brainstem were developed to keep the structural integrity of the neural tissue essentially intact while still allowing careful control of the microenvironment. The cultures generated synchronized respiratory network activity and motor output for up to three weeks. Characterization revealed a network organization of the respiratory regions with a so called small-world structure. These respiratory networks consists of both neurons and astrocytes. Detailed examination identified two subgroups of astrocytes. Most appeared dormant, but a subset of the astrocytes displayed persistent, rhythmic oscillating calcium activity. These active astrocytes formed an individual network, interacting with a distinct neuronal network.

Furthermore, stimulation of the astrocytes increased their calcium oscillation frequency in both the preBötC and pFRG/RTN. However, while neuronal calcium oscillations in the preBötC were unaffected, they were significantly increased in frequency in the pFRG/RTN. The organotypic culture also preserved the respiratory center reactivity towards exogenous stimulus, such as opioids and hypercapnia. Hypercapnic challenge elicited a gapjunction dependent release of the inflammatory molecule prostaglandin E2 (PGE2) in the pFRG/RTN, increasing the overall network activity of this region. A release of PGE2 was also triggered by stimulation of astrocytes, which blunted a subsequent hypercapnic challenge. Thus, a new respiratory signaling pathway where PGE2 release from astrocytes following hypercapnic challenge modifies respiratory behavior to meet physiological demands was identified. In a clinical setting this might be beneficial at birth, where high PGE2 levels set the respiratory system to perform deep breaths. However, increased levels of PGE2 would also blunt the hypercapnic response, inducing a vulnerable period for infants. The first week after birth, cardiorespiratory dysfunction may lead to sudden unexpected postnatal collapse (SUPC), where seemingly healthy infants collapse and require resuscitation.

In a retrospective study, we identified 115 SUPC cases among 313 351 live births during a 15.5-year period. Thus, the incidence of 36.7 SUPC events per 100 000 live births makes SUPC events more common than sepsis caused by group B streptococci. Seven percent of the affected children died, and about one fourth developed hypoxic ischemic encephalopathy. The majority of SUPCs occurred during the first hours after birth and where related to co-bedding, emphasizing the importance of these risk factors. Urinary PGE2 metabolite levels were high during the first days after birth when most (97 %) of SUPC events occur.

These findings are important for understanding how respiratory behavior is affected during inflammatory states, such as immediately after birth and during infections, and have an impact on our ability to detect and protect against respiratory dysfunction.