Somatostatin-expressing parafacial neurons are CO2/H+ sensitive and regulate baseline breathing

Glutamatergic neurons in the retrotrapezoid nucleus (RTN) function as respiratory chemoreceptors by regulating breathing in response to tissue CO2/H+. The RTN and greater parafacial region may also function as a chemosensing network composed of CO2/H+-sensitive excitatory and inhibitory synaptic interactions. In the context of disease, we showed that loss of inhibitory neural activity in a mouse model of Dravet syndrome disinhibited RTN chemoreceptors and destabilized breathing (Kuo et al., 2019). Despite this, contributions of parafacial inhibitory neurons to control of breathing are unknown, and synaptic properties of RTN neurons have not been characterized. Here, we show the parafacial region contains a limited diversity of inhibitory neurons including somatostatin (Sst)-, parvalbumin (Pvalb)-, and cholecystokinin (Cck)-expressing neurons. Of these, Sst-expressing interneurons appear uniquely inhibited by CO2/H+. We also show RTN chemoreceptors receive inhibitory input that is withdrawn in a CO2/H+-dependent manner, and chemogenetic suppression of Sst+ parafacial neurons, but not Pvalb+ or Cck+ neurons, increases baseline breathing. These results suggest Sst-expressing parafacial neurons contribute to RTN chemoreception and respiratory activity.

Isoflurane inhibits a Kir4.1/5.1-like conductance in neonatal rat brainstem astrocytes and recombinant Kir4.1/5.1 channels in a heterologous expression system

All inhalation anesthetics used clinically including isoflurane can suppress breathing; since this unwanted side effect can persist during the postoperative period and complicate patient recovery, there is a need to better understand how isoflurane affects cellular and molecular elements of respiratory control. Considering that astrocytes in a brainstem region known as the retrotrapezoid nucleus (RTN) contribute to the regulation of breathing in response to changes in CO2/H+ (i.e., function as respiratory chemoreceptors), and astrocytes in other brain regions are highly sensitive to isoflurane, we wanted to determine whether and how RTN astrocytes respond to isoflurane. We found that RTN astrocytes in slices from neonatal rat pups (7-12 days postnatal) respond to clinically relevant levels of isoflurane by inhibition of a CO2/H+-sensitive Kir4.1/5.1-like conductance [50% effective concentration (EC50) = 0.8 mM or ~1.7%]. We went on to confirm that similar levels of isoflurane (EC50 = 0.53 mM or 1.1%) inhibit recombinant Kir4.1/5.1 channels but not homomeric Kir4.1 channels expressed in HEK293 cells. We also found that exposure to CO2/H+ occluded subsequent effects of isoflurane on both native and recombinant Kir4.1/5.1 currents. These results identify Kir4.1/5.1 channels in astrocytes as novel targets of isoflurane. These results suggest astrocyte Kir4.1/5.1 channels contribute to certain aspects of general anesthesia including altered respiratory control.NEW & NOTEWORTHY An unwanted side effect of isoflurane anesthesia is suppression of breathing. Despite this clinical significance, effects of isoflurane on cellular and molecular elements of respiratory control are not well understood. Here, we show that isoflurane inhibits heteromeric Kir4.1/5.1 channels in a mammalian expression system and a Kir4.1/5.1-like conductance in astrocytes in a brainstem respiratory center. These results identify astrocyte Kir4.1/5.1 channels as novel targets of isoflurane and potential substrates for altered respiratory control during isoflurane anesthesia.

Disordered breathing in a mouse model of Dravet syndrome

Dravet syndrome (DS) is a form of epilepsy with a high incidence of sudden unexpected death in epilepsy (SUDEP). Respiratory failure is a leading cause of SUDEP, and DS patients' frequently exhibit disordered breathing. Despite this, mechanisms underlying respiratory dysfunction in DS are unknown. We found that mice expressing a DS-associated Scn1a missense mutation (A1783V) conditionally in inhibitory neurons (Slc32a1cre/+::Scn1aA1783V fl/+; defined as Scn1aΔE26) exhibit spontaneous seizures, die prematurely and present a respiratory phenotype including hypoventilation, apnea, and a diminished ventilatory response to CO2. At the cellular level in the retrotrapezoid nucleus (RTN), we found inhibitory neurons expressing the Scn1a A1783V variant are less excitable, whereas glutamatergic chemosensitive RTN neurons, which are a key source of the CO2/H+-dependent drive to breathe, are hyper-excitable in slices from Scn1aΔE26 mice. These results show loss of Scn1a function can disrupt respiratory control at the cellular and whole animal levels.

α1- and α2-adrenergic receptors in the retrotrapezoid nucleus differentially regulate breathing in anesthetized adult rats

Norepinephrine (NE) is a potent modulator of breathing that can increase/decrease respiratory activity by α1-/α2-adrenergic receptor (AR) activation, respectively. The retrotrapezoid nucleus (RTN) is known to contribute to central chemoreception, inspiration, and active expiration. Here we investigate the sources of catecholaminergic inputs to the RTN and identify respiratory effects produced by activation of ARs in this region. By injecting the retrograde tracer Fluoro-Gold into the RTN, we identified back-labeled catecholaminergic neurons in the A7 region. In urethane-anesthetized, vagotomized, and artificially ventilated male Wistar rats unilateral injection of NE or moxonidine (α2-AR agonist) blunted diaphragm muscle activity (DiaEMG) frequency and amplitude, without changing abdominal muscle activity. Those inhibitory effects were reduced by preapplication of yohimbine (α2-AR antagonist) into the RTN. Conversely, unilateral RTN injection of phenylephrine (α1-AR agonist) increased DiaEMG amplitude and frequency and facilitated active expiration. This response was blocked by prior RTN injection of prazosin (α1-AR antagonist). Interestingly, RTN injection of propranolol (β-AR antagonist) had no effect on respiratory inhibition elicited by applications of NE into the RTN; however, the combined blockade of α2- and β-ARs (coapplication of propranolol and yohimbine) revealed an α1-AR-dependent excitatory response to NE that resulted in increase in DiaEMG frequency and facilitation of active expiration. However, blockade of α1-, α2-, or β-ARs in the RTN had minimal effect on baseline respiratory activity, on central or peripheral chemoreflexes. These results suggest that NE signaling can modulate RTN chemoreceptor function; however, endogenous NE signaling does not contribute to baseline breathing or the ventilatory response to central or peripheral chemoreceptor activity in urethane-anesthetized rats.

In vitro characterization of noradrenergic modulation of chemosensitive neurons in the retrotrapezoid nucleus

Chemosensitive neurons in the retrotrapezoid nucleus (RTN) regulate breathing in response to CO2/H(+) changes and serve as an integration center for other autonomic centers, including brain stem noradrenergic neurons. Norepinephrine (NE) contributes to respiratory control and chemoreception, and, since disruption of NE signaling may contribute to several breathing disorders, we sought to characterize effects of NE on RTN chemoreception. All neurons included in this study responded similarly to CO2/H(+) but showed differential sensitivity to NE; we found that NE activated (79%), inhibited (7%), or had no effect on activity (14%) of RTN chemoreceptors. The excitatory effect of NE on RTN chemoreceptors was dose dependent, retained in the presence of neurotransmitter receptor blockers, and could be mimicked and blocked by pharmacological manipulation of α1-adrenergic receptors (ARs). In addition, NE-activation was blunted by XE991 (KCNQ channel blocker), and partially occluded the firing response to serotonin, suggesting involvement of KCNQ channels. However, in whole cell voltage clamp, activation of α1-ARs decreased outward current and conductance by what appears to be a mixed effect on multiple channels. The inhibitory effect of NE on RTN chemoreceptors was blunted by an α2-AR antagonist. A third group of RTN chemoreceptors was insensitive to NE. We also found that chemosensitive RTN astrocytes do not respond to NE with a change in voltage or by releasing ATP to enhance activity of chemosensitive neurons. These results indicate NE modulates subsets of RTN chemoreceptors by mechanisms involving α1- and α2-ARs.

Cholinergic control of ventral surface chemoreceptors involves Gq/inositol 1,4,5-trisphosphate-mediated inhibition of KCNQ channels

KEY POINTS

ACh is an important modulator of breathing, including at the level of the retrotrapezoid nucleus (RTN), where evidence suggests that ACh is essential for the maintenance of breathing. Despite this potentially important physiological role, little is known about the mechanisms responsible for the cholinergic control of RTN function. In the present study, we show at the cellular level that ACh increases RTN chemoreceptor activity by a CO2/H(+) independent mechanism involving M1/M3 receptor-mediated inositol 1,4,5-trisphosphate/Ca(+2) signalling and downstream inhibition of KCNQ channels. These results dispel the theory that ACh is required for RTN chemoreception by showing that ACh, similar to serotonin and other modulators, controls the activity of RTN chemoreceptors without interfering with the mechanisms by which these cells sense H(+). By identifying the mechanisms by which wake-on neurotransmitters such as ACh modulate RTN chemoreception, the results of the present study provide a framework for understanding the molecular basis of the sleep-wake state-dependent control of breathing.

ABSTRACT

ACh has long been considered important for the CO2/H(+)-dependent drive to breathe produced by chemosensitive neurons in the retrotrapezoid nucleus (RTN). However, despite this potentially important physiological role, almost nothing is known about the mechanisms responsible for the cholinergic control of RTN function. In the present study, we used slice-patch electrophysiology and pharmacological tools to characterize the effects of ACh on baseline activity and CO2/H(+)-sensitivity of RTN chemoreceptors, as well as to dissect the signalling pathway by which ACh activates these neurons. We found that ACh activates RTN chemoreceptors in a dose-dependent manner (EC50 = 1.2 μm). The firing response of RTN chemoreceptors to ACh was mimicked by a muscarinic receptor agonist (oxotremorine; 1 μm), and blunted by M1- (pirezenpine; 2 μm) and M3- (diphenyl-acetoxy-N-methyl-piperidine; 100 nm) receptor blockers, but not by a nicotinic-receptor blocker (mecamylamine; 10 μm). Furthermore, pirenzepine, diphenyl-acetoxy-N-methyl-piperidine and mecamylamine had no measurable effect on the CO2/H(+)-sensitivity of RTN chemoreceptors. The effects of ACh on RTN chemoreceptor activity were also blunted by inhibition of inositol 1,4,5-trisphosphate receptors with 2-aminoethoxydiphenyl borate (100 μm), depletion of intracellular Ca(2+) stores with thapsigargin (10 μm), inhibition of casein kinase 2 (4,5,6,7-tetrabromobenzotriazole; 10 μm) and blockade of KCNQ channels (XE991; 10 μm). These results show that ACh activates RTN chemoreceptors by a CO2/H(+) independent mechanism involving M1/M3 receptor-mediated inositol 1,4,5-trisphosphate/Ca(+2) signalling and downstream inhibition of KCNQ channels. Identifying the components of the signalling pathway coupling muscarinic receptor activation to changes in chemoreceptor activity may provide new potential therapeutic targets for the treatment of respiratory control disorders.

Enhanced expression of glycine N-methyltransferase by adenovirus-mediated gene transfer in CNS culture is neuroprotective

Glycine N-methyltransferase (GNMT) is the most abundant hepatic methyltransferase and plays important roles in regulating methyl group metabolism. In the central nervous system, GNMT expression is low and its function has not been revealed. The present study examines the effect of GNMT overexpression by adenovirus-mediated transfer in cortical mixed neuron-glial cultures. Infection of adenovirus encoding green fluorescence protein to cultures demonstrates high preference for non-neuronal cells. Optimal GNMT overexpression in cultures by adenoviral GNMT (Ad-GNMT) infection not only induces protein kinase C phosphorylation, but also increases neuronal/oligodendroglial survival. Furthermore, these Ad-GNMT-infected cultures are significantly resistant to H(2)O(2) toxicity and lipopolysaccharide stimulation. Conditioned media from Ad-GNMT-infected microglia also significantly enhance neuronal survival. Taken together, enhanced GNMT expression in mixed neuronal-glial cultures is neuroprotective, most likely mediated through non-neuronal cells.

The effect of metalloporphyrins and heme liposomes on delta-aminolevulinate synthase activity in rat liver

Heme administration causes inhibition of delta-aminolevulinate synthase (ALAS), best tested in the allylisopropylacetamide (AIA)-treated rat, a model for hepatic porphyrias. Because heme suspended in aqueous media (for injection) is unstable and has adverse effects on coagulation, alternate therapeutic modalities are being explored. The present study tries to answer two questions: 1) are any heme analogs as effective inhibitors of ALAS as heme is; and 2) does heme administration in the form of liposomes increase its effectiveness? None of the liposome compositions tested, even if containing lactosylceramide for preferential hepatocyte uptake, was more effective in inhibiting AIA-induced ALAS activity than heme in buffer. As for the function of the heme analogs, although deuteroheme and heme dimethyl ester proved ineffective, mesoheme and cobalt protoporphyrin were nearly as effective as heme itself, indicating that both hydrophobic side chains in positions 2 and 4 and free propionate groups at 6 and 7 are essential for ALAS inhibition, as is the presence of a central cobalt or iron atom.

Kinetics of the interaction of hemin liposomes with heme binding proteins

As a model for the transport of hemin across biological membranes, sonicated phosphatidylcholine liposomes with incorporated hemin were characterized. The interaction of the hemin liposomes with the heme binding proteins albumin, apomyoglobin, and hemopexin was examined as a function of liposome charge and cholesterol content. In all cases, there was an almost complete transfer of hemin from liposome to protein; a rapid phase and a slow phase were observed for the transfer. For negatively charged liposomes (with 11% dicetyl phosphate), the rapid and slow phases showed observed rates of transfer of ca. 2 and 0.01 s-1, respectively, for all three proteins. The presence of cholesterol in the liposomes decreased the observed rates by a factor of 2, and positively charged liposomes (with 11% stearylamine) showed about one-fifth the observed rates of negatively charged liposomes. The observed rates were independent of protein concentration, indicating that the rate-determining step is hemin efflux from the lipid bilayer. The hemin interaction with the phospholipid bilayer is suggested to be primarily hydrophobic with some electrostatic character. The two phases are suggested to arise from two different populations of hemin within the liposomes and are interpreted as arising from two different orientations of hemin within the bilayer.