ATP is released during hypoxia from the ventrolateral medulla (VLM) and activates purinergic P2 receptors (P2Rs) at unknown loci to offset the secondary hypoxic depression of breathing. In this study, we used rhythmically active medullary slices from neonatal rat to map, in relation to anatomical and molecular markers of the pre-Bötzinger complex (preBötC) (a proposed site of rhythm generation), the effects of ATP on respiratory rhythm and identify the P2R subtypes responsible for these actions. Unilateral microinjections of ATP in a three-dimensional grid within the VLM revealed a "hotspot" where ATP (

Just as in other neural systems, plasticity is a hallmark of the neural system controlling breathing. One spinal mechanism of respiratory plasticity is phrenic long-term facilitation (pLTF) following acute intermittent hypoxia. Although cellular mechanisms giving rise to pLTF occur within the phrenic motor nucleus, different signaling cascades elicit pLTF in different conditions. These cascades, referred to as “Q” and “S" pathways to phrenic motor facilitation (pMF), interact via cross-talk inhibition. Whereas the Q pathway dominates pLTF after mild to moderate hypoxic episodes, the S pathway dominates after severe hypoxic episodes. The biological significance of multiple pathways to pMF is not known. We discuss the possibility that interactions between pathways confer emergent properties to pLTF, including: 1) pattern sensitivity and 2) metaplasticity. Understanding these mechanisms and their interactions may enable us to optimize intermittent hypoxia induced plasticity as a treatment for patients that suffer from ventilatory impairment or other motor deficits.

Glia modulate neuronal activity by releasing transmitters in a process called gliotransmission. The role of this process in controlling the activity of neuronal networks underlying motor behavior is unknown. ATP features prominently in gliotransmission; it also contributes to the homeostatic ventilatory response evoked by low oxygen through mechanisms that likely include excitation of preBötzinger complex (preBötC) neural networks, brainstem centers critical for breathing. We therefore inhibited glial function in rhythmically active inspiratory networks in vitro to determine whether glia contribute to preBötC ATP sensitivity. Glial toxins markedly reduced preBötC responses to ATP, but not other modulators. Furthermore, since preBötC glia responded to ATP with increased intracellular Ca 2ϩ and glutamate release, we conclude that glia contribute to the ATP sensitivity of preBötC networks, and possibly the hypoxic ventilatory response. Data reveal a role for glia in signal processing within brainstem motor networks that may be relevant to similar networks throughout the neuraxis.

Although systemic inflammation occurs in most pathological conditions that challenge the neural control of breathing, little is known concerning the impact of inflammation on respiratory motor plasticity. Here, we tested the hypothesis that low-grade systemic inflammation induced by lipopolysaccharide (LPS, 100 μg/kg ip; 3 and 24 h postinjection) elicits spinal inflammatory gene expression and attenuates a form of spinal, respiratory motor plasticity: phrenic long-term facilitation (pLTF) induced by acute intermittent hypoxia (AIH; 3, 5 min hypoxic episodes, 5 min intervals). pLTF was abolished 3 h (vehicle control: 67.1 ± 27.9% baseline; LPS: 3.7 ± 4.2%) and 24 h post-LPS injection (vehicle: 58.3 ± 17.1% baseline; LPS: 3.5 ± 4.3%). Pretreatment with the nonsteroidal anti-inflammatory drug ketoprofen (12.5 mg/kg ip) restored pLTF 24 h post-LPS (55.1 ± 12.3%). LPS increased inflammatory gene expression in the spleen and cervical spinal cord (homogenates and isolated microglia) 3 h postinjection; however, all molecules assessed had returned to baseline by 24 h postinjection. At 3 h post-LPS, cervical spinal iNOS and COX-2 mRNA were differentially increased in microglia and homogenates, suggesting differential contributions from spinal cells. Thus LPS-induced systemic inflammation impairs AIH-induced pLTF, even after measured inflammatory genes returned to normal. Since ketoprofen restores pLTF even without detectable inflammatory gene expression, "downstream" inflammatory molecules most likely impair pLTF. These findings have important implications for many disease states where acute systemic inflammation may undermine the capacity for compensatory respiratory plasticity.

ATP released during hypoxia from the ventrolateral medulla activates purinergic receptors (P2Rs) to attenuate the secondary hypoxic depression of breathing by a mechanism that likely involves a P2Y 1 R-mediated excitation of preBötzinger complex (preBötC) inspiratory rhythm-generating networks. In this study, we used rhythmically active in vitro preparations from embryonic and postnatal rats and ATP microinjection into the rostral ventral respiratory group (rVRG)/preBötC to reveal that these networks are sensitive to ATP when rhythm emerges at embryonic day 17 (E17). The peak frequency elicited by ATP at E19 and postnatally was the same (ϳ45 bursts/min), but relative sensitivity was threefold greater at E19, reflecting a lower baseline frequency (5.6 Ϯ 0.9 vs 19.0 Ϯ 1.3 bursts/min). Combining microinjection techniques with ATP biosensors revealed that ATP concentration in the rVRG/preBötC falls rapidly as a result of active processes and closely correlates with inspiratory frequency. A phosphate assay established that preBötC-containing tissue punches degrade ATP at rates that increase perinatally. Thus, the agonist profile [ATP/ADP/adenosine (ADO)] produced after ATP release in the rVRG/preBötC will change perinatally. Electrophysiology further established that the ATP metabolite ADP is excitatory and that, in fetal but not postnatal animals, ADO at A 1 receptors exerts a tonic depressive action on rhythm, whereas A 1 antagonists extend the excitatory action of ATP on inspiratory rhythm. These data demonstrate that ATP is a potent excitatory modulator of the rVRG/preBötC inspiratory network from the time it becomes active and that ATP actions are determined by a dynamic interaction between the actions of ATP at P2 receptors, ectonucleotidases that degrade ATP, and ATP metabolites on P2Y and P1 receptors.

Inflammation is characteristic of most clinical disorders that challenge the neural control of breathing. Since inflammation modulates neuroplasticity, we studied the impact of inflammation caused by prolonged intermittent hypoxia on an important form of respiratory plasticity, acute intermittent hypoxia (three, 5 min hypoxic episodes, 5 min normoxic intervals) induced phrenic long-term facilitation (pLTF). Because chronic intermittent hypoxia elicits neuroinflammation and pLTF is undermined by lipopolysaccharide-induced systemic inflammation, we hypothesized that one night of intermittent hypoxia (IH-1) elicits spinal inflammation, thereby impairing pLTF by a p38 MAP kinase-dependent mechanism. pLTF and spinal inflammation were assessed in anesthetized rats pretreated with IH-1 (2 min hypoxia, 2 min normoxia; 8 h) or sham normoxia and allowed 16 h for recovery. IH-1 (1) transiently increased IL-6 (1.5 Ϯ 0.2-fold; p ϭ 0.02) and inducible nitric oxide synthase (iNOS) (2.4 Ϯ 0.4-fold; p ϭ 0.01) mRNA in cervical spinal homogenates, (2) elicited a sustained increase in IL-1␤ mRNA (2.4 Ϯ 0.2-fold; p Ͻ 0.001) in isolated cervical spinal microglia, and (3) abolished pLTF (Ϫ1 Ϯ 5% vs 56 Ϯ 10% in controls; p Ͻ 0.001). pLTF was restored after IH-1 by systemic NSAID administration (ketoprofen; 55 Ϯ 9%; p Ͻ 0.001) or spinal p38 MAP kinase inhibition (58 Ϯ 2%; p Ͻ 0.001). IH-1 increased phosphorylated (activated) p38 MAP kinase immunofluorescence in identified phrenic motoneurons and adjacent microglia. In conclusion, IH-1 elicits spinal inflammation and impairs pLTF by a spinal p38 MAP kinase-dependent mechanism. By targeting inflammation, we may develop strategies to manipulate respiratory motor plasticity for therapeutic advantage when the respiratory control system is compromised (e.g., sleep apnea, apnea of prematurity, spinal injury, or motor neuron disease).

Acute intermittent hypoxia (AIH) induces a form of spinal motor plasticity known as phrenic long-term facilitation (pLTF); pLTF is a prolonged increase in phrenic motor output after AIH has ended. In anesthetized rats, we demonstrate that pLTF requires activity of the novel PKC isoform, PKC, and that the relevant PKC is within phrenic motor neurons. Whereas spinal PKC inhibitors block pLTF, inhibitors targeting other PKC isoforms do not. PKC is highly expressed in phrenic motor neurons, and PKC knockdown with intrapleural siRNAs abolishes pLTF. Intrapleural siRNAs targeting PKC, an atypical PKC isoform expressed in phrenic motor neurons that underlies a distinct form of phrenic motor plasticity, does not affect pLTF. Thus, PKC plays a critical role in spinal AIH-induced respiratory motor plasticity, and the relevant PKC is localized within phrenic motor neurons. Intrapleural siRNA delivery has considerable potential as a therapeutic tool to selectively manipulate plasticity in vital respiratory motor neurons.

Many lung and central nervous system disorders require robust and appropriate physiological responses to assure adequate breathing. Factors undermining the efficacy of ventilatory control will diminish the ability to compensate for pathology, threatening life itself. Although most of these same disorders are associated with systemic and/or neuroinflammation, and inflammation affects neural function, we are only beginning to understand interactions between inflammation and any aspect of ventilatory control (e.g. sensory receptors, rhythm generation, chemoreflexes, plasticity). Here we review available evidence, and present limited new data suggesting that systemic (or neural) inflammation impairs two key elements of ventilatory control: chemoreflexes and respiratory motor (vs. sensory) plasticity. Achieving an understanding of mechanisms whereby inflammation undermines ventilatory control is fundamental since inflammation may diminish the capacity for natural, compensatory responses during pathological states, and the ability to harness respiratory plasticity as a therapeutic strategy in the treatment of devastating breathing disorders, such as during cervical spinal injury or motor neuron disease.