The Journal of physiology
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The Journal of physiology · Aug 2019
Competing mechanisms of plasticity impair compensatory responses to repetitive apnoea.
Intermittent reductions in respiratory neural activity, a characteristic of many ventilatory disorders, leads to inadequate ventilation and arterial hypoxia. Both intermittent reductions in respiratory neural activity and intermittent hypoxia trigger compensatory enhancements in inspiratory output when experienced separately, forms of plasticity called inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. Reductions in respiratory neural activity that lead to moderate, but not mild, arterial hypoxia occludes plasticity expression, indicating that concurrent induction of iMF and LTF impairs plasticity through cross-talk inhibition of their respective signalling pathways. Moderate hypoxia undermines iMF by enhancing NR2B-containing NMDA receptor signalling, which can be rescued by exogenous retinoic acid, a molecule necessary for iMF. These data suggest that in ventilatory disorders characterized by reduced inspiratory motor output, such as sleep apnoea, endogenous mechanisms of compensatory plasticity may be impaired, and that exogenously activating respiratory plasticity may be a novel strategy to improve breathing. ⋯ Many forms of sleep apnoea are characterized by recurrent reductions in respiratory neural activity, which leads to inadequate ventilation and arterial hypoxia. Both recurrent reductions in respiratory neural activity and hypoxia activate mechanisms of compensatory plasticity that augment inspiratory output and lower the threshold for apnoea, inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. However, despite frequent concurrence of reduced respiratory neural activity and hypoxia, mechanisms that induce and regulate iMF and LTF have only been studied separately. Here, we demonstrate that recurrent reductions in respiratory neural activity ('neural apnoea') accompanied by cessations in ventilation that result in moderate (but not mild) hypoxaemia do not elicit increased inspiratory output, suggesting that concurrent induction of iMF and LTF occludes plasticity. A key role for NMDA receptor activation in impairing plasticity following concurrent neural apnoea and hypoxia is indicated since recurrent hypoxic neural apnoeas triggered increased phrenic inspiratory output in rats in which spinal NR2B-containing NMDA receptors were inhibited. Spinal application of retinoic acid, a key molecule necessary for iMF, bypasses NMDA receptor-mediated constraints, thereby rescuing plasticity following hypoxic neural apnoeas. These studies raise the intriguing possibility that endogenous mechanisms of compensatory plasticity may be impaired in some individuals with sleep apnoea, and that exogenously activating pathways giving rise to respiratory plasticity may be a novel pharmacological strategy to improve breathing.
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The Journal of physiology · Jul 2019
Simultaneous assessment of central and peripheral chemoreflex regulation of muscle sympathetic nerve activity and ventilation in healthy young men.
Central chemoreceptor stimulation, by hypercapnia (acidosis), and peripheral, by hypoxia plus hypercapnia, evoke reflex increases in ventilation and sympathetic outflow. The assumption that central or peripheral chemoreceptor-mediated sympathetic activation elicited when P C O 2 increases parallels concurrent ventilatory responses is unproven. Applying a modified rebreathing protocol that equilibrates central and peripheral chemoreceptor P C O 2 whilst clamping O2 tension at either hypoxic or hyperoxic concentrations, the independent ventilatory and muscle sympathetic stimulus-response properties of the central and peripheral chemoreflexes were quantified and compared in young men. The novel findings were that ventilatory and sympathetic responses to central and peripheral chemoreflex stimulation are initiated at similar P C O 2 recruitment thresholds but individual specific sympathetic responsiveness cannot be predicted from the ventilatory sensitivities of either chemoreceptor reflex. Such findings in young men, if replicated in heart failure or hypertension, should temper present enthusiasm for trials targeting the peripheral chemoreflex based solely on ventilatory responsiveness to non-specific chemoreceptor stimulation. ⋯ In humans, stimulation of peripheral or central chemoreceptor reflexes is assumed to evoke equivalent ventilatory and sympathetic responses. We evaluated whether central or peripheral chemoreceptor-mediated sympathetic activation elicited by increases in CO2 tension ( P C O 2 ) parallels concurrent ventilatory responses. Twelve healthy young men performed a modified rebreathing protocol designed to equilibrate central and peripheral chemoreceptor P C O 2 tensions with end-tidal P C O 2 ( P ETC O 2 ) at two isoxic end-tidal P O 2 ( P ET O 2 ) such that central responses can be segregated, by hyperoxia, from the net response (hypoxia minus hyperoxia). Ventilation and muscle sympathetic nerve activity (MSNA) were recorded continuously during rebreathing at isoxic P ET O 2 of 150 and 50 mmHg. During rebreathing, the P ETC O 2 values at which ventilation (L min-1 ) and total MSNA (units) began to rise were identified ( P ETC O 2 recruitment thresholds) and their slopes above the recruitment threshold were determined (sensitivity). The central chemoreflex recruitment threshold for ventilation (46 ± 3 mmHg) and MSNA (45 ± 4 mmHg) did not differ (P = 0.55) and slopes were 2.3 ± 0.9 L min-1 mmHg-1 and 2.1 ± 1.5 units mmHg-1 , respectively. The peripheral chemoreflex recruitment thresholds, at 41 ± 3 mmHg for both ventilation and MSNA were lower (P < 0.05) compared to the central chemoreflex recruitment thresholds. Peripheral chemoreflex sensitivity was 1.7 ± 0.1 L min-1 mmHg-1 for ventilation and 2.9 ± 2.6 units mmHg-1 for MSNA. There was no relationship between the ventilatory and MSNA sensitivity for either the central (r2 = 0.01, P = 0.76) or peripheral (r2 = 0.01, P = 0.73) chemoreflex. In healthy young men, ventilatory and sympathetic responses to central and peripheral chemoreceptor reflex stimulation are initiated at similar P ETC O 2 recruitment thresholds but individual ventilatory responsiveness does not predict sympathetic sensitivities of either chemoreflex.
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The Journal of physiology · Jul 2019
NaV 1.6 regulates excitability of mechanosensitive sensory neurons.
Voltage-gated sodium channels are critical for peripheral sensory neuron transduction and have been implicated in a number of painful and painless disorders. The β-scorpion toxin, Cn2, is selective for NaV 1.6 in dorsal root ganglion neurons. NaV 1.6 plays an essential role in peripheral sensory neurons, specifically at the distal terminals of mechanosensing fibres innervating the skin and colon. NaV 1.6 activation also leads to enhanced response to mechanical stimulus in vivo. This works highlights the use of toxins in elucidating pain pathways moreover the importance of non-peripherally restricted NaV isoforms in pain generation. ⋯ Peripheral sensory neurons express multiple voltage-gated sodium channels (NaV ) critical for the initiation and propagation of action potentials and transmission of sensory input. Three pore-forming sodium channel isoforms are primarily expressed in the peripheral nervous system (PNS): NaV 1.7, NaV 1.8 and NaV 1.9. These sodium channels have been implicated in painful and painless channelopathies and there has been intense interest in them as potential therapeutic targets in human pain. Emerging evidence suggests NaV 1.6 channels are an important isoform in pain sensing. This study aimed to assess, using pharmacological approaches, the function of NaV 1.6 channels in peripheral sensory neurons. The potent and NaV 1.6 selective β-scorpion toxin Cn2 was used to assess the effect of NaV 1.6 channel activation in the PNS. The multidisciplinary approach included Ca2+ imaging, whole-cell patch-clamp recordings, skin-nerve and gut-nerve preparations and in vivo behavioural assessment of pain. Cn2 facilitates NaV 1.6 early channel opening, and increased persistent and resurgent currents in large-diameter dorsal root ganglion (DRG) neurons. This promotes enhanced excitatory drive and tonic action potential firing in these neurons. In addition, NaV 1.6 channel activation in the skin and gut leads to increased response to mechanical stimuli. Finally, intra-plantar injection of Cn2 causes mechanical but not thermal allodynia. This study confirms selectivity of Cn2 on NaV 1.6 channels in sensory neurons. Activation of NaV 1.6 channels, in terminals of the skin and viscera, leads to profound changes in neuronal responses to mechanical stimuli. In conclusion, sensory neurons expressing NaV 1.6 are important for the transduction of mechanical information in sensory afferents innervating the skin and viscera.
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The Journal of physiology · Jun 2019
Studies into the determinants of skeletal muscle oxygen consumption: novel insight from near-infrared diffuse correlation spectroscopy.
Diffuse correlation spectroscopy (DCS) is emerging as a powerful tool to assess skeletal muscle perfusion. Near-infrared spectroscopy (NIRS) is an established technique for characterizing the transport and utilization of oxygen through the microcirculation. Here we compared a combined NIRS-DCS system with conventional measures of oxygen delivery and utilization during handgrip exercise. The data show good concurrent validity between convective oxygen delivery and DCS-derived blood flow index, as well as between oxygen extraction at the conduit and microvascular level. We then manipulated forearm arterial perfusion pressure by adjusting the position of the exercising arm relative to the position of the heart. The data show that microvascular perfusion can be uncoupled from convective oxygen delivery, and that tissue saturation seemingly compensates to maintain skeletal muscle oxygen consumption. Taken together, these data support a novel role for NIRS-DCS in understanding the determinants of muscle oxygen consumption at the microvascular level. ⋯ Diffuse correlation spectroscopy (DCS) is emerging as a powerful tool to assess skeletal muscle perfusion. Combining DCS with near-infrared spectroscopy (NIRS) introduces exciting possibilities for understanding the determinants of muscle oxygen consumption; however, no investigation has directly compared NIRS-DCS to conventional measures of oxygen delivery and utilization in an exercising limb. To address this knowledge gap, nine healthy males performed rhythmic handgrip exercise with simultaneous measurements by NIRS-DCS, Doppler blood flow and venous oxygen content. The two approaches showed good concurrent validity, with directionally similar responses between: (a) Doppler-derived forearm blood flow and DCS-derived blood flow index (BFI), and (b) venous oxygen saturation and NIRS-derived tissue saturation. To explore the utility of combined NIRS-DCS across the physiological spectrum, we manipulated forearm arterial perfusion pressure by altering the arm position above or below the level of the heart. As expected, Doppler-derived skeletal muscle blood flow increased with exercise in both arm positions, but with markedly different magnitudes (below: +424.3 ± 41.4 ml/min, above: +306 ± 12.0 ml/min, P = 0.002). In contrast, DCS-derived microvascular BFI increased to a similar extent with exercise, regardless of arm position (P = 0.65). Importantly, however, the time to reach BFI steady state was markedly slower with the arm above the heart, supporting the experimental design. Notably, we observed faster tissue desaturation at the onset of exercise with the arm above the heart, resulting in similar muscle oxygen consumption profiles throughout exercise. Taken together, these data support a novel role for NIRS-DCS in understanding the determinants of skeletal muscle oxygen utilization non-invasively and throughout exercise.
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The Journal of physiology · May 2019
5-HT2A receptor activation enhances NMDA receptor-mediated glutamate responses through Src kinase in the dendrites of rat jaw-closing motoneurons.
5-HT increases the excitability of brainstem and spinal motoneurons, including the jaw-closing motoneurons, by depolarizing the membrane potential and decreasing the medium-duration afterhyperpolarization. In this study, we focused on how 5-HT enhances postsynaptic glutamatergic responses in the dendrites of the jaw-closing motoneurons. We demonstrate that 5-HT augments glutamatergic signalling by enhancing the function of the GluN2A-containing NMDA receptor (NMDAR) through the activation of 5-HT2A receptors (5-HT2A Rs) and Src kinase. To enhance glutamatergic responses, activation of the 5-HT2A Rs must occur within ∼60 μm of the location of the glutamate responses. 5-HT inputs to the jaw-closing motoneurons can significantly vary their input-output relationship, which may contribute to wide-range regulation of contractile forces of the jaw-closing muscles. ⋯ Various motor behaviours are modulated by 5-HT. Although the masseter (jaw-closing) motoneurons receive both glutamatergic and serotonergic inputs, it remains unclear how 5-HT affects the glutamatergic inputs to the motoneuronal dendrites. We examined the effects of 5-HT on postsynaptic responses evoked by single- or two-photon uncaging of caged glutamate (glutamate responses) to the dendrites of masseter motoneurons in postnatal day 2-5 rats of either sex. Application of 5-HT induced membrane depolarization and enhanced the glutamate-response amplitude. This enhancement was mimicked by the 5-HT2A receptor (5-HT2A R) agonist and was blocked by the 5-HT2A/2C R antagonist. However, neither the 5-HT2B R nor the 5-HT2C R agonists altered glutamate responses. Blockade of the NMDA receptors (NMDARs), but not AMPA receptors, abolished the 5-HT-induced enhancement. Furthermore, the selective antagonist for the GluN2A subunit abolished the 5-HT-induced enhancement. 5-HT increased GluN2A phosphorylation, while the Src kinase inhibitor reduced the 5-HT-induced enhancement and GluN2A phosphorylation. When exposure to the 5-HT2A R agonist was targeted to the dendrites, the enhancement of glutamate responses was restricted to the loci of the dendrites near the puff loci. Electron microscopic immunohistochemistry revealed that both the NMDARs and the 5-HT2A Rs were close to each other in the same dendrite. These results suggest that activation of dendritic 5-HT2A Rs enhances the function of local GluN2A-containing NMDARs through Src kinase. Such enhancement of the glutamate responses by 5-HT may contribute to wide-range regulation of contractile forces of the jaw-closing muscles.