Abstract
Breathing is generated by a respiratory network in the brainstem. At its core, a population of neurons expressing neurokinin-1 receptors (NK1R) and the peptide somatostatin (SST) form the preBötzinger Complex (preBötC), a site essential for the generation of breathing. PreBötC interneurons generate rhythm and follower neurons shape motor outputs by activating upper airway respiratory muscles. Since NK1R-expressing preBötC neurons are preferentially inhibited by μ-opioid receptors via activation of GIRK channels, NK1R stimulation may also involve GIRK channels. Hence, we identify the contribution of GIRK channels to rhythm, motor output and respiratory modulation by NK1Rs and SST. In adult rats, GIRK channels were identified in NK1R-expressing preBötC cells. Their activation decreased breathing rate and genioglossus muscle activity, an important upper airway muscle. NK1R activation increased rhythmic breathing and genioglossus muscle activity in wild-type mice, but not in mice lacking GIRK2 subunits (GIRK2−/−). Conversely, SST decreased rhythmic breathing via SST2 receptors, reduced genioglossus muscle activity likely through SST4 receptors, but did not involve GIRK channels. In summary, NK1R stimulation of rhythm and motor output involved GIRK channels, whereas SST inhibited rhythm and motor output via two SST receptor subtypes, therefore revealing separate circuits mediating rhythm and motor output.
Highlights
Breathing is an autonomic behaviour generated by a complex respiratory network in the brainstem
In the preBötzinger Complex (preBötC), MORs inhibit rhythmic breathing through potassium channels[16], and we recently identified that GIRK channels contribute to MOR respiratory slowing[17]
To identify the types of preBötC cells expressing GIRK channels, we visualized the expression of GIRK2 subunits and neurokinin-1 receptors (NK-1R) using immunohistochemistry of brainstem sections from adult rats (n = 3)
Summary
Breathing is an autonomic behaviour generated by a complex respiratory network in the brainstem. SST-expressing preBötC neurons project to hypoglossal premotor areas[5] and may regulate upper airway muscle activity. The roles of MOR have been characterized[14,16], the functional roles of NK-1R, SST, and SST receptors in regulating rhythmic breathing and upper airway muscle activity, especially in vivo, are unclear. We propose that GIRK channels contribute to excitation of rhythmic breathing by NK-1Rs. MORs, SST, and SST2A receptors are co-expressed in NK-1R-expressing preBötC cells[9], activation of SST receptors inhibits glutamate release by activating potassium leak current or by inhibiting voltage-dependent calcium current[19]. To better understand the circuitry generating rhythmic breathing and motor outputs, we aimed to establish the functional roles of GIRK-expressing preBötC neurons in mediating rhythmic breathing and hypoglossal motor activity, as well as excitation by NK-1Rs and inhibition by SST in adult rodents in vivo. We propose a functional framework identifying the roles of GIRK channels, NK-1Rs, and SST receptors in rhythmic breathing and motor output
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