Does glyceryl trinitrate cause central sympatholytic effects? Insights from a case of baroreflex failure.

Whether part of the blood pressure lowering effects of glyceryl trinitrate (GTN) is the result of centrally mediated reduction in sympathetic activity is debated. In humans, baroreflex activity potentially obscures the central sympatholytic effects of GTN. We examined this in a routine clinical tilt test in a patient with baroreflex failure secondary to previous neck radiotherapy. With reduced baroreflex function we observed an exaggerated fall in blood pressure and reduced sympathetic activity with GTN, supporting a peripheral vasodilation and central sympatholytic effect.

Sympathoexcitation following intermittent hypoxia in rat is mediated by circulating angiotensin II acting at the carotid body and subfornical organ.

KEY POINTS: In anaesthetized rats, acute intermittent hypoxia increases sympathetic nerve activity, sympathetic peripheral chemoreflex sensitivity and central sympathetic-respiratory coupling. Renin-angiotensin system inhibition prevents the sympathetic effects of intermittent hypoxia, with intermittent injections of angiotensin II into the systemic circulation replicating these effects. Bilateral carotid body denervation reduces the sympathetic effects of acute intermittent hypoxia and eliminates the increases in chemoreflex sensitivity and sympathetic-respiratory coupling. Pharmacological inhibition of the subfornical organ also reduces the sympathetic effects of acute intermittent hypoxia, although it has no effect on the increases in chemoreflex sensitivity and central sympathetic-respiratory coupling. Combining both interventions eliminates the sympathetic effects of both intermittent hypoxia and angiotensin II. ABSTRACT: Circulating angiotensin II (Ang II) is vital for arterial pressure elevation following intermittent hypoxia in rats, although its importance in the induction of sympathetic changes is unclear. We tested the contribution of the renin-angiotensin system to the effects of acute intermittent hypoxia (AIH) in anaesthetized and ventilated rats. There was a 33.7 ± 2.9% increase in sympathetic nerve activity (SNA), while sympathetic chemoreflex sensitivity and central sympathetic-respiratory coupling increased by one-fold following AIH. The sympathetic effects of AIH were prevented by blocking angiotensin type 1 receptors with systemic losartan. Intermittent systemic injections of Ang II (Int.Ang II) elicited similar sympathetic responses to AIH. To identify the neural pathways responsible for the effects of AIH and Int.Ang II, we performed bilateral carotid body denervation, which reduced the increase in SNA by 56% and 45%, respectively. Conversely, pharmacological inhibition of the subfornical organ (SFO), an established target of circulating Ang II, reduced the increase in SNA following AIH and Int.Ang II by 65% and 59%, respectively, although it did not prevent the sensitization of the sympathetic peripheral chemoreflex, nor the increase in central sympathetic-respiratory coupling. Combined carotid body denervation and inhibition of the SFO eliminated the enhancement of SNA following AIH and Int.Ang II. Repeated systemic injections of phenylephrine caused an elevation in SNA similar to AIH, and this effect was prevented by a renin inhibitor, aliskiren. Our findings show that the sympathetic effects of AIH are the result of RAS-mediated activations of the carotid bodies and the SFO.

Catecholaminergic C3 neurons are sympathoexcitatory and involved in glucose homeostasis.

Brainstem catecholaminergic neurons play key roles in the autonomic, neuroendocrine, and behavioral responses to glucoprivation, yet the functions of the individual groups are not fully understood. Adrenergic C3 neurons project widely throughout the brain, including densely to sympathetic preganglionic neurons in the spinal cord, yet their function is completely unknown. Here we demonstrate in rats that optogenetic stimulation of C3 neurons induces sympathoexcitatory, cardiovasomotor functions. These neurons are activated by glucoprivation, but unlike the C1 cell group, not by hypotension. The cardiovascular activation induced by C3 neurons is less than that induced by optogenetic stimulation of C1 neurons; however, combined stimulation produces additive sympathoexcitatory and cardiovascular effects. The varicose axons of C3 neurons largely overlap with those of C1 neurons in the region of sympathetic preganglionic neurons in the spinal cord; however, regional differences point to effects on different sympathetic outflows. These studies definitively demonstrate the first known function of C3 neurons as unique cardiovasomotor stimulatory cells, embedded in the brainstem networks regulating cardiorespiratory activity and the response to glucoprivation.

Acute intermittent hypoxia induced neural plasticity in respiratory motor control.

Respiratory neural networks can adapt to rapid environmental change or be altered over the long term by various inputs. The mechanisms that underlie the plasticity necessary for adaptive changes in breathing remain unclear. Acute intermittent hypoxia (AIH)-induced respiratory long-term facilitation (LTF) is one of the most extensively studied types of respiratory plasticity. Acute intermittent hypoxia-induced LTF is present in several respiratory motor outputs, innervating both pump muscles (i.e. diaphragm) and valve muscles (i.e. tongue, pharynx and larynx). Long-term facilitation is present in various species, including humans, and the expression of LTF is influenced by gender, age and genetics. Serotonin plays a key role in initiating and modulating plasticity at the level of respiratory motor neurons. Recently, multiple intracellular pathways have been elucidated that are capable of giving rise to respiratory LTF. These mainly activate the metabolic receptors coupled to Gq ('Q' pathway) and Gs ('S' pathway) proteins. Herein, we discuss AIH-induced respiratory LTF in animals and humans, as well as recent advances in our understanding of the synaptic and intracellular pathways underlying this form of plasticity. We also discuss the potential to use intermittent hypoxia to induce functional recovery following cervical spinal injury.

Intrathecal melanin-concentrating hormone reduces sympathetic tone and blocks cardiovascular reflexes.

Melanin-concentrating hormone (MCH) is a neuropeptide that acts to increase feeding behavior and decrease energy expenditure. The role of MCH in central cardiorespiratory regulation is still poorly understood. Experiments were conducted on urethane-anesthetized, vagotomized, and artificially ventilated male Sprague-Dawley rats (n = 22) to ascertain whether MCH modulates sympathetic vasomotor tone, as well as barosympathetic, chemosympathetic, and somatosympathetic reflexes at the level of the spinal cord. Intrathecal injection of 10 µl of MCH produced a dose-dependent hypotension, bradycardia, and sympathoinhibition. Peak response was observed following administration of 1 mM MCH, causing a decrease in mean arterial pressure of 39 ± 2 mmHg (P < 0.001), splanchnic sympathetic nerve activity of 78 ± 11% (P < 0.001), and heart rate of 87 ± 11 beats per minute (bpm) (P < 0.01). The two peaks of the somatosympathetic reflex were decreased by intrathecal MCH, 7 ± 3% (P < 0.01) and 31 ± 6% (P < 0.01), respectively, and the spinal component of the reflex was accentuated 96 ± 23% (P < 0.05), with respect to the baseline for MCH, compared with the two peaks and spinal component of the somatosympathetic reflex elicited following saline injection with respect to the baseline for saline. MCH decreased the sympathetic gain to 120 s of hyperoxic hypercapnea (10% CO(2) in 90% O(2)) and to 10-12 s poikilocapneic anoxia (100% N(2)) from 0.74 ± 0.14%/s to 0.23 ± 0.04%/s (P < 0.05) and 16.47 ± 3.2% to 4.35 ± 1.56% (P < 0.05), respectively. There was a 34% decrease in gain and a 62% decrease in range of the sympathetic baroreflex with intrathecal MCH. These data demonstrate that spinal MCH blunts the central regulation of sympathetic tone and adaptive sympathetic reflexes.

Noxious somatic stimuli diminish respiratory-sympathetic coupling by selective resetting of the respiratory rhythm in anaesthetized rats.

Noxious somatic stimulation evokes respiratory and autonomic responses. The mechanisms underlying the responses and the manner in which they are co-ordinated are still unclear. The effects of activation of somatic nociceptive fibres on lumbar sympathetic nerve activity at slow (2-10 Hz) and fast frequency bands (100-1000 Hz) and the effects on respiratory-sympathetic coupling are unknown. In anaesthetized, artificially ventilated Sprague-Dawley rats under neuromuscular blockade, ensemble averaging of sympathetic activity following high-intensity single-pulse stimulation of the sciatic nerve revealed two peaks (~140 and ~250 ms) that were present at similar latencies whether or not slow or fast band filtering was used. Additionally, in the slow band of both lumbar and splanchnic sympathetic nerve activity, a third peak with a very slow latency (~650 ms) was apparent. In the respiratory system, activation of the sciatic nerve decreased the expiratory period when the stimulus occurred during the first half of expiration, but increased the expiratory period if the stimulus was delivered in the second half of the expiratory phase. The phase shifting of the respiratory cycle also impaired the respiratory-sympathetic coupling in both splanchnic and lumbar sympathetic nerve activity in the subsequent respiratory cycle. The findings suggest that noxious somatosympathetic responses reduce the co-ordination between respiration and perfusion by resetting the respiratory pattern generator.

The ventilatory response to hypoxia and hypercapnia is absent in the neonatal fat-tailed dunnart.

At birth, the newborn fat-tailed dunnart relies on cutaneous gas exchange to meet metabolic demands, with continuous lung ventilation emerging several days later. We hypothesised that the delayed expression of lung ventilation (V(E)) in these animals is in part due to a low responsiveness of the respiratory control system to blood gas perturbations. To address this hypothesis, we assessed the ventilatory and metabolic response to hypoxia (10% O(2)) and hypercapnia (5% CO(2)) using closed-system respirometry from birth to 23 days postpartum (P). Neonatal fat-tailed dunnarts displayed no significant hypoxic or hypercapnic ventilatory responses at any age. Regardless, significant hyperventilation through a suppression of metabolic rate (V(O(2))) was observed at birth in response to hypercapnia and in response to hypoxia at all ages, except P12. Therefore, reliance on cutaneous gas exchange during early life may be partially attributed to reduced chemosensitivity or a lack of central integration of chemosensitive afferent information. This may be in part due to the relative immaturity of this species at birth, compared with other mammals.

Effect of temperature, age and the pons on respiratory rhythm in the rat brainstem-spinal cord.

Neonatal animals are extremely tolerant of hypothermia. However, cooling will ultimately lead to ventilatory arrest, or cessation of respiratory movements. Upon rewarming, ventilation can recover spontaneously (autoresuscitation). This study examined the effect of age (P0-P5) and the pons on respiratory-related output during hypothermic ventilatory arrest and recovery using a brainstem-spinal cord preparation of neonatal rats. As temperature fell, burst frequency slowed, burst duration increased, burst shape became fragmented and eventually respiratory arrest occurred in all preparations. Removing the pons had little effect on younger preparations (P0-P2). Older preparations (P4-P5) with the pons removed continued to burst at cooler temperatures compared to pons-intact preparations and burst durations were significantly longer. Episodic breathing patterns were observed in all preparations (all ages, pons on or off) at lower temperatures. At 27 °C, however, episodic breathing was only observed in younger preparations with the pons on. These data suggest that developmental changes occurring at the level of the pons underlie the loss of hypothermic tolerance and episodic breathing.

PACAP-PAC1 Receptor Activation Is Necessary for the Sympathetic Response to Acute Intermittent Hypoxia.

Repetitive hypoxia is a key feature of obstructive sleep apnoea (OSA), a condition characterized by intermittent airways obstruction. Patients with OSA present with persistent increases in sympathetic activity and commonly develop hypertension. The objectives of this study were to determine if the persistent increases in sympathetic nerve activity, known to be induced by acute intermittent hypoxia (AIH), are mediated through activation of the pituitary adenylate cyclase activating polypeptide (PACAP) signaling system. Here, we show that the excitatory neuropeptide PACAP, acting in the spinal cord, is important for generating the sympathetic response seen following AIH. Using PACAP receptor knockout mice, and pharmacological agents in Sprague Dawley rats, we measured blood pressure, heart rate, pH, PaCO2, and splanchnic sympathetic nerve activity, under anaesthesia, to demonstrate that the sympathetic response to AIH is mediated via the PAC1 receptor, in a cAMP-dependent manner. We also report that both intermittent microinjection of glutamate into the rostroventrolateral medulla (RVLM) and intermittent infusion of a sub-threshold dose of PACAP into the subarachnoid space can mimic the sympathetic response to AIH. All the sympathetic responses are independent of blood pressure, pH or PaCO2 changes. Our results show that in AIH, PACAP signaling in the spinal cord helps drive persistent increases in sympathetic nerve activity. This mechanism may be a precursor to the development of hypertension in conditions of chronic intermittent hypoxia, such as OSA.

Respiratory rhythm of brainstem-spinal cord preparations: Effects of maturation, age, mass and oxygenation.

We examined the effect of age, mass and the presence of the pons on the longevity (length of time spontaneous respiratory-related activity is produced) of brainstem-spinal cord preparations of neonatal rodents (rats and hamsters) and the level of oxygenation in the medulla respiratory network in these preparations. We found the longevity of the preparations from both species decreased with increasing postnatal age. Physical removal of the pons increased respiratory frequency and the longevity of the preparation. However, tissue oxygenation at the level of the medullary respiratory network was not affected by removal of the pons or increasing postnatal age (up to postnatal day 4). Taken together, these data suggest that the effect of removing the pons on respiratory frequency and the longevity of brainstem-spinal cord preparations with increasing postnatal age are primarily due to postnatal development and appear to be unrelated to mass or changes in levels of tissue oxygenation.

Neurokinin-1 receptor activation in the Bötzinger complex evokes bradypnea and is involved in mediating the Hering-Breuer reflex.

The role of substance P (SP) and its receptor, the neurokinin-1 (NK1R), in the generation of respiratory rhythm has received considerable attention, particularly at the Pre-Bötzinger Complex of the ventral respiratory group (VRG). However, the functional role of SP and NK1R in other VRG regions has not been explored in detail. We review the current literature and describe recent data demonstrating that selective activation of NK1R in the Bötzinger Complex (BötC) of the VRG evoked bradypnea by lengthening expiratory period. In addition, endogenous activation of NK1R in the BötC participates in the expiratory lengthening effect of the Hering-Breuer reflex. These data suggest that NK1R expressing neurons in different subregions of the VRG have functionally diverse roles and provide new insight on the modulatory role of SP on respiratory reflexes.

The Expression of Galanin in the Parafacial Respiratory Group and its Effects on Respiration in Neonatal Rats.

The inhibitory peptide galanin is expressed within the retrotrapezoidal nucleus (RTN) - a key central chemoreceptor site that also contains the active expiratory oscillator. It was previously reported that microinjection of galanin into pre-Bötzinger complex - containing the inspiratory oscillator - exerts inhibitory effects on inspiratory motor output and respiratory rhythm. In neonatal rats, the present study aimed to investigate: (1) expression of galanin within the parafacial respiratory group (pFRG), which overlaps anatomically and functionally with the adult RTN, and; (2) effects of galanin on respiratory rhythm using the in vitro brainstem-spinal cord preparation. We showed that 14 ±â€¯2% of Phox2b-immunoreactive (ir) neurons in the parafacial region were also galanin-ir. Galanin peptide expression was confirmed within 3/9 CO2-sensitive, Phox2b-ir Pre-Inspiratory neurons (Pre-I) recorded in parafacial region. Bath application of galanin (0.1-0.2 µM): (1) decreased the duration of membrane depolarization in both Pre-I and inspiratory pFRG neurons, and; (2) decreased the number of C4 bursts that were associated with each burst in Pre-I neurons within the pFRG. In preparations showing episodic breathing at baseline, the respiratory patterning reverted to the 'normal' pattern of single, uniformly rhythmic C4 bursts (n = 10). In preparations with normal respiratory patterning at baseline, slowing of C4 rhythm (n = 7) resulted although rhythmic bursting in recorded Pre-I neurons remained unperturbed (n = 6). This study therefore demonstrates that galanin is expressed within the pFRG of neonatal rats, including neurons that are intrinsically chemosensitive. Overall the peptide has an inhibitory effect on inspiratory motor output, as previously shown in adults.

Excessive Respiratory Modulation of Blood Pressure Triggers Hypertension.

The etiology of hypertension, the world's biggest killer, remains poorly understood, with treatments targeting the established symptom, not the cause. The development of hypertension involves increased sympathetic nerve activity that, in experimental hypertension, may be driven by excessive respiratory modulation. Using selective viral and cell lesion techniques, we identify adrenergic C1 neurons in the medulla oblongata as critical for respiratory-sympathetic entrainment and the development of experimental hypertension. We also show that a cohort of young, normotensive humans, selected for an exaggerated blood pressure response to exercise and thus increased hypertension risk, has enhanced respiratory-related blood pressure fluctuations. These studies pinpoint a specific neuronal target for ameliorating excessive sympathetic activity during the developmental phase of hypertension and identify a group of pre-hypertensive subjects that would benefit from targeting these cells.

Immunohistochemical localization of GAD67-expressing neurons and processes in the rat brainstem: subregional distribution in the nucleus tractus solitarius.

The role of gamma-aminobutyric acid (GABA) in homeostatic control in the brainstem, in particular, in the nucleus tractus solitarius (NTS), is well established. However, to date, there is no detailed description of the distribution of GABAergic neurons within the NTS. The goal of the current study was to reexamine the efficacy of immunohistochemical localization of glutamic acid decarboxylase (GAD) protein, specifically the 67-kDa isoform (GAD67), as a marker for GABAergic neurons in the medulla and to provide a detailed map of GAD67-immunoreactive (-ir) cells within rat NTS by using a recently developed mouse monoclonal antibody. We describe a distribution of GAD67-ir cells in the medulla similar to that reported previously from in situ hybridization study. GAD67-ir cells were localized in regions known to contain high GABA content, including the ventrolateral medulla, raphe nuclei, and area postrema, but were absent from all motor nuclei, although dense terminal labeling was discerned in these regions. In the NTS, GAD67-ir was localized in all subregions. Semiquantitative analysis of the GAD67-ir distribution in the NTS revealed greater numbers of GAD67-ir cells medial to the solitary tract. Finally, dense GAD67 terminal labeling was found in the medial, central, intermediate, commissural, and subpostremal subregions, whereas sparse labeling was observed in the ventral subregion. Our findings support the use of immunohistochemistry for GAD67 as a marker for the localization of GABAergic cells and terminal processes in the rat brainstem. Furthermore, the reported heterogeneous distribution of GAD67-ir in the NTS suggests differential inhibitory modulation of sensory processing.

Distribution and innervation of putative peripheral arterial chemoreceptors in the red-eared slider (Trachemys scripta elegans).

Peripheral arterial chemoreceptors have been isolated to the common carotid artery, aorta, and pulmonary artery of turtles. However, the putative neurotransmitters associated with these chemoreceptors have not yet been described. The goal of the present study was to determine the neurochemical content, innervations, and distribution of putative oxygen-sensing cells in the central vasculature of turtles and to derive homologies with peripheral arterial chemoreceptors of other vertebrates. We used tract tracing together with immunohistochemical markers for cholinergic cells (vesicular acetylcholine transporter [VAChT]), tyrosine hydroxylase (TH; the rate-limiting enzyme in catecholamine synthesis), and serotonin (5HT) to identify putative oxygen-sensing cells and to determine their anatomical relation to branches of the vagus nerve (Xth cranial nerve). We found potential oxygen-sensing cells in all three chemosensory areas innervated by branches of the Xth cranial nerve. Cells containing either 5HT or VAChT were found in all three sites. The morphology and size of these cells resemble glomus cells found in amphibians, mammals, tortoises, and lizards. Furthermore, we found populations of cholinergic cells located at the base of the aorta and pulmonary artery that are likely involved in efferent regulation of vessel resistance. Catecholamine-containing cells were not found in any of the putative chemosensitive areas. The presence of 5HT- and VAChT-immunoreactive cells in segments of the common carotid artery, aorta, and pulmonary artery appears to reflect a transition between cells containing the major neurotransmitters seen in fish (5HT) and mammals (ACh and adenosine).

Spontaneous swallowing occurs during autoresuscitation in the in situ brainstem preparation of rat.

Previous studies report that upper airway reflexes are operational during autoresuscitation from respiratory arrest. We investigated swallowing/breathing interactions, measured by recording of vagal (VNA) and phrenic nerve activities (PNA), during autoresuscitation in the in situ perfused brainstem preparation of juvenile rats. During the initial surgery, respiratory arrest was induced by exsanguination and cooling. Reperfusion (i.e. re-oxygenation and re-warming) of the brainstem circuits was associated with frequent spontaneous swallowing before resumption of respiration (n=6, 'stage 1 autoresuscitation'). When recovered, the respiratory pattern was transiently apneustic-like ('stage 2 autoresuscitation'). Spontaneous swallowing often occurred at the end of the prolonged PNA (n=9/12). Successful autoresuscitation was characterised by re-establishment of the 3 phase respiratory motor pattern and no spontaneous swallowing. Pharmacological inhibition (isoguvacine, 10 mM, 50-75 nl; n=10) of the Kölliker-Fuse nucleus (KF) mimicked stage 2 autoresuscitation. However, the frequency of spontaneous swallowing after KF inhibition did not correlate with subsequent recovery of the eupneic respiratory motor pattern.

Mechanism of sympathetic activation and blood pressure elevation in humans and animals following acute intermittent hypoxia.

Sleep apnea is associated with repeated episodes of hypoxemia, causing marked increase in sympathetic nerve activity and blood pressure. Considerable evidence suggests that intermittent hypoxia (IH) resulting from apnea is the primary stimulus for sympathetic overactivity in sleep apnea patients. Several IH protocols have been developed either in animals or in humans to investigate mechanisms underlying the altered autonomic regulation of the circulation. Most of these protocols involve several days (10-40 days) of IH exposure, that is, chronic intermittent hypoxia (CIH). Recent data suggest that a single session of IH exposure, that is, acute intermittent hypoxia (AIH), is already capable of increasing tonic sympathetic nerve output (sympathetic long-term facilitation, LTF) and altering chemo- and baroreflexes with or without elevation of blood pressure. This indicates that IH alters the autonomic neurocirculatory at a very early time point, although the mechanisms underlying this neuroplasticity have not been explored in detail. The purpose of this chapter is to briefly review the effects of AIH on sympathetic LTF and alteration of autonomic reflexes in comparison with the studies from CIH studies. We will also discuss the potential central and peripheral mechanism underlying sympathetic LTF.

Distribution and innervation of putative arterial chemoreceptors in the bullfrog (Rana catesbeiana).

Peripheral arterial chemoreceptors have been located previously in the carotid labyrinth, the aortic arch, and the pulmocutaneous artery of frogs. In the present study we used cholera toxin B neuronal tract tracing and immunohistochemical markers for cholinergic cells (vesicular acetylcholine transporter [VAChT]), tyrosine hydroxylase (TH), and serotonin (5HT) to identify putative O2-sensing cells in Rana catesbeiana. We found potential O2-sensing cells in all three vascular areas innervated by branches of the vagus nerve, whereas only cells in the carotid labyrinth were innervated by the glossopharyngeal nerve. Cells containing either 5HT or TH were found in all three sites, whereas cells containing both neurotransmitters were found only in the carotid labyrinth. Cell bodies containing VAChT were not found at any site. The morphology and innervation of putative O2-sensing cells were similar to those of glomus cells found in other vertebrates. The presence of 5HT- and TH-immunoreactive cells in the aorta, pulmocutaneous artery, and carotid labyrinth appears to reflect a phylogenetic transition between the major neurotransmitter seen in the putative O2-sensing cells of fish (5HT) and those found in the glomus cells of mammals (acetylcholine, adenosine, and catecholamines).

Distribution of acetylcholine and catecholamines in fish gills and their potential roles in the hypoxic ventilatory response.

Carotid body glomus cells in mammals contain a plethora of different neurochemicals. Several hypotheses exist to explain their roles in oxygen-chemosensing. In the present study we assessed the distribution of serotonin, acetylcholine and catecholamines in the gills of trout (Oncorhynchus mykiss) and goldfish (Carassius auratus) using immunohistochemistry, and an activity-dependent dye, Texas Red hydrazide (TXR). In fish the putative oxygen sensing cells are neuroepithelial cells (NECs) and the focus in recent studies has been on the role of serotonin in oxygen chemoreception. The NECs of trout and goldfish contain serotonin, but, in contrast to the glomus cells of mammals, not acetylcholine or catecholamines. Acetylcholine was expressed in chain and proximal neurons and in extrinsic nerve bundles in the filaments. The serotonergic NECs did not label with the HNK-1 antibody suggesting that if they are derived from the neural crest, they are no longer proliferative or migrating. Furthermore, we predicted that if serotonergic NECs were chemosensory, they would increase their activity during hypoxia (endocytose TXR), but following 30 min of hypoxic exposure (45 Torr), serotonergic NECs did not take up TXR. Based on these and previous findings we propose several possible models outlining the ways in which serotonin and acetylcholine could participate in oxygen chemoreception in completing the afferent sensory pathway.

Recurrent laryngeal nerve activity exhibits a 5-HT-mediated long-term facilitation and enhanced response to hypoxia following acute intermittent hypoxia in rat.

A progressive and sustained increase in inspiratory-related motor output ("long-term facilitation") and an augmented ventilatory response to hypoxia occur following acute intermittent hypoxia (AIH). To date, acute plasticity in respiratory motor outputs active in the postinspiratory and expiratory phases has not been studied. The recurrent laryngeal nerve (RLN) innervates laryngeal abductor muscles that widen the glottic aperture during inspiration. Other efferent fibers in the RLN innervate adductor muscles that partially narrow the glottic aperture during postinspiration. The aim of this study was to investigate whether or not AIH elicits a serotonin-mediated long-term facilitation of laryngeal abductor muscles, and if recruitment of adductor muscle activity occurs following AIH. Urethane anesthetized, paralyzed, unilaterally vagotomized, and artificially ventilated adult male Sprague-Dawley rats were subjected to 10 exposures of hypoxia (10% O(2) in N(2), 45 s, separated by 5 min, n = 7). At 60 min post-AIH, phrenic nerve activity and inspiratory RLN activity were elevated (39 ± 11 and 23 ± 6% above baseline, respectively). These responses were abolished by pretreatment with the serotonin-receptor antagonist, methysergide (n = 4). No increase occurred in time control animals (n = 7). Animals that did not exhibit postinspiratory RLN activity at baseline did not show recruitment of this activity post-AIH (n = 6). A repeat hypoxia 60 min after AIH produced a significantly greater peak response in both phrenic and RLN activity, accompanied by a prolonged recovery time that was also prevented by pretreatment with methysergide. We conclude that AIH induces neural plasticity in laryngeal motoneurons, via serotonin-mediated mechanisms similar to that observed in phrenic motoneurons: the so-called "Q-pathway". We also provide evidence that the augmented responsiveness to repeat hypoxia following AIH also involves a serotonergic mechanism.