Rhythmic activity from transverse brainstem slice of neonatal rat is modulated by nitric oxide

A role for nitric oxide in the control of breathing in zebrafish (Danio rerio)

Nitric oxide (NO) is a gaseous neurotransmitter, which in adult mammals, modulates the acute hypoxic ventilatory response; its role in the control of breathing in fish during development is unknown. We addressed the interactive effects of developmental age and NO in the control of piscine breathing by measuring the ventilatory response of zebrafish (Danio rerio) adults and larvae to NO donors and by inhibiting endogenous production of NO. In adults, sodium nitroprusside (SNP), a NO donor, inhibited ventilation; the extent of the ventilatory inhibition was related to the pre-existing ventilatory drive, with the greatest inhibition exhibited during exposure to hypoxia (PO2=5.6 kPa). Inhibition of endogenous NO production using L-NAME supressed the hypoventilatory response to hyperoxia, supporting an inhibitory role of NO in adult zebrafish. Neuroepithelial cells, the putative oxygen chemoreceptors of fish, contain neuronal nitric oxide synthase (nNOS). In zebrafish larvae at 4 days post fertilization, SNP increased ventilation in a concentration-dependent manner. Inhibition of NOS activity with L-NAME or knockdown of nNOS inhibited the hypoxic (PO2=3.5 kPa) ventilatory response. Immunohistochemistry revealed the presence of nNOS in the NECs of larvae. Taken together, these data suggest that NO plays an inhibitory role in the control ventilation in adult zebrafish, but an excitatory role in larvae.

Nitric oxide synthase expression in the medullary respiratory related nuclei and its involvement in CO-mediated central respiratory effects in neonatal rats

The present study was conducted in order to observe the potential participation of the nitric oxide synthase-NO pathway in CO-mediated regulation of respiration of neonatal rats. An immunofluorescent histochemical technique was used to examine the existence of the neuronal nitric oxide synthase, a key enzyme of synthesizing NO, in medullary respiratory nuclei. The rhythmic respiratory-like discharges of hypoglossal rootlets of medullary slices were recorded to test the role of the nitric oxide synthase in CO-mediated respiratory effects. We observed neuronal nitric oxide synthase expressed in the medullary respiratory nuclei in conjunction with CO lengthened expiratory duration, decreased respiratory frequency, and increased inspiratory amplitude. These CO-mediated respiratory effects could be partially eliminated by prior treatment of the slices with Nω-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase. The results suggest that nitric oxide synthase-NO pathway might be involved in the CO-mediated central regulation of respiration at the level of medulla oblongata in neonatal rats.

Math1 is essential for the development of hindbrain neurons critical for perinatal breathing

Mice lacking the proneural transcription factor Math1 (Atoh1) lack multiple neurons of the proprioceptive and arousal systems and die shortly after birth from an apparent inability to initiate respiration. We sought to determine whether Math1 was necessary for the development of hindbrain nuclei involved in respiratory rhythm generation, such as the parafacial respiratory group/retrotrapezoid nucleus (pFRG/RTN), defects in which are associated with congenital central hypoventilation syndrome (CCHS). We generated a Math1-GFP fusion allele to trace the development of Math1-expressing pFRG/RTN and paratrigeminal neurons and found that loss of Math1 did indeed disrupt their migration and differentiation. We also identified Math1-dependent neurons and their projections near the pre-Bötzinger complex, a structure critical for respiratory rhythmogenesis, and found that glutamatergic modulation reestablished a rhythm in the absence of Math1. This study identifies Math1-dependent neurons that are critical for perinatal breathing that may link proprioception and arousal with respiration.

Nitric oxide and respiratory rhythm in mammals: a new modulator of phase transition?

NO (nitric oxide) modulates several central pattern generators, but its role in respiratory rhythmogenesis and its mode of action on medullary respiratory neurons during normoxia are unknown. We analysed the actions of NO on the mammalian respiratory network at the system and cellular levels. Given systemically, the NO donor diethylamine NONOate increased post-inspiratory duration in vagus, phrenic and hypoglossal nerves, whereas blockade of NO generation with L-NAME (N(G)-nitro-L-arginine methyl ester) produced the opposite response. At the cellular level, we pressure-ejected the NO donor on to respiratory neurons. NO had both inhibitory and excitatory effects on all types of respiratory neurons. Inhibitory effects involved soluble guanylate cyclase, as they were blocked with ODQ (1H-[1,2,4]oxadiazolo[4,3a]quinoxalin-1-one), whereas excitations were antagonized by uric acid and possibly mediated via peroxynitrite. Importantly, NO facilitated both GABA (gamma-aminobutyric acid)- and NMDA (N-methyl-D-aspartate)-induced neuronal responses, but this was restricted to post-inspiratory and pre-inspiratory neurons; other neuron types showed additive effects only. Our results support NO as modulator of centrally generated respiratory activity and specifically of ligand-mediated responses in respiratory neuron types involved in respiratory phase transition.

Collateral axonal projections from rostral ventromedial medullary nitric oxide synthase containing neurons to brainstem autonomic sites

The magnocellular reticular nucleus and adjacent lateral paragigantocellular nucleus have been shown to contain a large population of nitric oxide synthase (NOS) immunoreactive neurons. However, little is known about the projections of these neurons within the central nervous system. Retrograde tract-tracing techniques combined with immunohistochemistry were used in this study to investigate whether NOS neurons in this rostral ventromedial medullary (RVMM) region send collateral axonal projections to autonomic sites in the nucleus of the solitary tract (NTS) and in the nucleus ambiguus (Amb). Fluorogold and/or rhodamine labeled latex microspheres were microinjected into the NTS and Amb at sites that elicited bardycardia and/or depressor responses (l-glutamate; 0.25 M; 10 nl). After a survival period of 10-14 days, the rats were sacrificed and tissue sections of the brainstem were processed immunohistochemically for the identification of NOS containing neuronal perikarya. After unilateral injection of the tract-tracers into the NTS and Amb, retrogradely labeled neurons were observed bilaterally throughout the RVMM region. Of the number of RVMM neurons retrogradely labeled from the NTS (684+/-143), 9% were found to be immunoreactive to NOS. Similarly, of those RVMM neurons retrogradely labeled from the Amb (963+/-207), 7% also contained NOS immunoreactivity. Neurons with collateral axonal projections to NTS and Amb (14% and 10%, respectively) were observed predominantly within a region of RVMM that extended co-extensively with approximately the rostrocaudal extent of the facial nucleus. Of these double labeled neurons, 36.4+/-20 (39%) were also found to be immunoreactive to NOS. These data indicate that the RVMM contains at least three population of NOS neurons that send axons to innervate functionally similar cardiovascular responsive sites in the NTS and Amb. Although the function of these NOS containing medullary pathways in cardiovascular control is not known, it is likely that those with collateral axonal projections represent the anatomical substrate by which the RVMM may simultaneously coordinate cardiovascular responses during physiological changes associated with respiration and/or motor movements.

Autonomic response and Fos expression in the NTS following intermittent vagal stimulation: Importance of pulse frequency

Chronic intermittent stimulation of the vagus nerve (VNS) is an approved adjunctive therapy of refractory epilepsy. Nevertheless, the circuits triggered by VNS under the variable conditions used in patients are not well understood. We analyzed the effect of increasing pulse frequency on physiological variables (intragastric pressure, cardiac and respiratory frequencies) and neuronal activation in the solitary tract nucleus (NTS), the entry level of peripheral vagal afferents, in the rat. For this purpose, we compared the subnuclear distribution of Fos-like immunoreactivity within the NTS following VNS at frequencies selected for their low (1 Hz) or high (10 Hz) therapeutic efficacy. In addition, NADPH diaphorase histochemistry was conducted in double-labeling experiments to check whether activated neurons may express nitric oxide (NO). We demonstrated that increasing pulse frequency had a major influence on the cardiorespiratory response to VNS and on the amount of activated neurons within NTS subdivisions engaged in cardiorespiratory control. These data, in line with clinical observations, suggested that within the range of therapeutic frequency, VNS may favor the regulation by vagal inputs of cortical activities within limbic areas involved in both epileptogenesis and cardiorespiratory afferent control. Furthermore, we did not find any evidence that anticonvulsant VNS might trigger NOergic neurons in the NTS.

Nitric oxide changes its role as a modulator of respiratory motor activity during development in the bullfrog (Rana catesbeiana)

Nitric oxide (NO) is a unique chemical messenger that has been shown to play a role in the modulation of breathing in amphibians and other vertebrates. In the post-metamorphic tadpole and adult amphibian brainstem, NO, acting via the neuronal isoform of nitric oxide synthase (nNOS), is excitatory to the generation of lung burst activity. In this study, we examine the modulation of breathing by NO during development of the amphibian brainstem. Isolated brainstem preparations from pre-metamorphic and late-stage post-metamorphic tadpoles (Rana catesbeiana) were used to determine the role of NO in modulating central respiratory neural activity. Respiratory neural activity was monitored with suction electrodes recording extracellular activity of cranial nerve rootlets that innervate respiratory musculature. Brainstems were superfused with an artificial cerebrospinal fluid (aCSF) at 20-22 degrees C containing l-nitroarginine (l-NA; 1-10 mM), a non-selective NOS inhibitor. In pre-metamorphic tadpoles, l-NA increased fictive gill ventilation frequency and amplitude, and increased lung burst frequency. By contrast, l-NA applied to the post-metamorphic tadpole brainstem had little effect on fictive buccal activity, but significantly decreased lung burst frequency and the frequency of lung burst episodes. These data indicate that early in development, NO provides a tonic inhibitory input to gill and lung burst activity, but as development progresses, NO provides an excitatory input to lung ventilation. This changing role for NO coincides with the shift in importance in the different respiratory modes during development in amphibians; that is, pre-metamorphic tadpoles rely predominantly on gill ventilation whereas post-metamorphic tadpoles have lost the gills and are obligate air-breathers primarily using lungs for gas exchange. We hypothesize that NO provides a tonic input to the respiratory CPG during development and this changing role reflects the modulatory influence of NO on inhibitory or excitatory modulators or neurotransmitters involved in the generation of respiratory rhythm.

Maintenance of eupnea and gasping following alterations in potassium ion concentration of perfusates of in situ rat preparation

Levels of extracellular potassium ion, above those in vivo, are required for the generation of rhythmic activities of in vitro preparations. Our purpose was to define whether hyperkalemia in the perfusate of an in situ preparation was likewise necessary for the expression of eupnea and gasping. Studies were performed using the in situ preparation of the juvenile rat, in which activity of the phrenic nerve was recorded as an index of the respiratory rhythm. Eupnea and gasping were impervious to modifications of potassium ion of the perfusate. Eupnea was maintained uninterrupted for more than 60 min whether the total concentration of potassium was hypokalemic (2.75 mM), normokalemic (4.0 mM), or hyperkalemic (6.25 and 7.75 mM). Gasping, with identical characteristics, was elicited at any concentration of potassium ion. We conclude that both eupnea and gasping are unaffected by modest changes in the concentration of potassium ion in the perfusate with which the in situ rat preparation is maintained. These results add further support to the conclusion that the in situ preparation represents a model that accurately reproduces mechanisms of rhythm generation of both eupnea and gasping in vivo.

Endogenous rhythm generation in the pre-Bötzinger complex and ionic currents: modelling and in vitro studies

The pre-Bötzinger complex is a small region in the mammalian brainstem involved in generation of the respiratory rhythm. As shown in vitro, this region, under certain conditions, can generate endogenous rhythmic bursting activity. Our investigation focused on the conditions that may induce this bursting behaviour. A computational model of a population of pacemaker neurons in the pre-Bötzinger complex was developed and analysed. Each neuron was modelled in the Hodgkin-Huxley style and included persistent sodium and delayed-rectifier potassium currents. We found that the firing behaviour of the model strongly depended on the expression of these currents. Specifically, bursting in the model could be induced by a suppression of delayed-rectifier potassium current (either directly or via an increase in extracellular potassium concentration, [K+]o) or by an augmentation of persistent sodium current. To test our modelling predictions, we recorded endogenous population activity of the pre-Bötzinger complex and activity of the hypoglossal (XII) nerve from in vitro transverse brainstem slices (700 micro m) of neonatal rats (P0-P4). Rhythmic activity was absent at 3 mm[K+]o but could be triggered by either the elevation of [K+]o to 5-7 mm or application of potassium current blockers (4-AP, 50-200 micro m, or TEA, 2 or 4 mm), or by blocking aerobic metabolism with NaCN (2 mm). This rhythmic activity could be abolished by the persistent sodium current blocker riluzole (25 or 50 micro m). These findings are discussed in the context of the role of endogenous bursting activity in the respiratory rhythm generation in vivo vs. in vitro and during normal breathing in vivo vs. gasping.