A5 cells are silenced when REM sleep-like signs are elicited by pontine carbachol

The role of orexinergic system in the regulation of cataplexy

Loss of orexin/hypocretin causes serious sleep disorder; narcolepsy. Cataplexy is the most striking symptom of narcolepsy, characterized by abrupt muscle paralysis induced by emotional stimuli, and has been considered pathological activation of REM sleep atonia system. Clinical treatments for cataplexy/narcolepsy and early pharmacological studies in narcoleptic dogs tell us about the involvement of monoaminergic and cholinergic systems in the control of cataplexy/narcolepsy. Muscle atonia may be induced by activation of REM sleep-atonia generating system in the brainstem. Emotional stimuli may be processed in the limbic systems including the amygdala, nucleus accumbens, and medial prefrontal cortex. It is now considered that orexin/hypocretin prevents cataplexy by modulating the activity of different points of cataplexy-inducing circuit, including monoaminergic/cholinergic systems, muscle atonia-generating systems, and emotion-related systems. This review will describe the recent advances in understanding the neural mechanisms controlling cataplexy, with a focus on the involvement of orexin/hypocretin system, and will discuss future experimental strategies that will lead to further understanding and treatment of this disease.

Activity of Pontine A7 Noradrenergic Neurons is suppressed during REM sleep

The activity of hypoglossal motoneurons plays a key role in the maintenance of upper airway patency. The withdrawal of noradrenergic excitatory drive and increase of cholinergic inhibition markedly decreases excitability of hypoglossal motoneurons during sleep and especially during rapid-eye-movement (REM) stage. This leads to increased collapsibility of upper airway during sleep, which is the major neurological factor of obstructive sleep apnea (OSA) pathophysiology. Anatomical and functional data suggests that noradrenergic A7 neurons are the main source of noradrenergic drive to hypoglossal motoneurons. However, it is unknown whether the behavior of A7 neurons during sleep-wake cycle is in accord with their proposed involvement in sleep-related depression of hypoglossal motoneuron activity. Therefore, we sought to assess the behavior of A7 neurons during sleep and wakefulness in naturally sleeping head-restrained rats. We have found that, similar to other pontine noradrenergic neurons, the putative A7 noradrenergic neurons fired with relatively long-lasting action potentials with a low frequency regular discharge. Importantly, all noradrenergic A7 neurons were predominantly silent during REM sleep. The REM-off activity of the A7 neurons supports our hypothesis that these neurons may significantly contribute to the withdrawal of excitatory noradrenergic drive from upper airway motoneurons during REM sleep and, consequently, play a critical role in maintaining upper airway patency and pathophysiology of OSA. Therefore, noradrenergic A7 neurons may serve as an additional target for designing pharmacological approaches to treat OSA.

Indexing brain state-dependent pupil dynamics with simultaneous fMRI and optical fiber calcium recording

Pupillometry, a noninvasive measure of arousal, complements human functional MRI (fMRI) to detect periods of variable cognitive processing and identify networks that relate to particular attentional states. Even under anesthesia, pupil dynamics correlate with brain-state fluctuations, and extended dilations mark the transition to more arousable states. However, cross-scale neuronal activation patterns are seldom linked to brain state-dependent pupil dynamics. Here, we complemented resting-state fMRI in rats with cortical calcium recording (GCaMP-mediated) and pupillometry to tackle the linkage between brain-state changes and neural dynamics across different scales. This multimodal platform allowed us to identify a global brain network that covaried with pupil size, which served to generate an index indicative of the brain-state fluctuation during anesthesia. Besides, a specific correlation pattern was detected in the brainstem, at a location consistent with noradrenergic cell group 5 (A5), which appeared to be dependent on the coupling between different frequencies of cortical activity, possibly further indicating particular brain-state dynamics. The multimodal fMRI combining concurrent calcium recordings and pupillometry enables tracking brain state-dependent pupil dynamics and identifying unique cross-scale neuronal dynamic patterns under anesthesia.

Neuroanatomical Basis of State-Dependent Activity of Upper Airway Muscles

Obstructive Sleep Apnea (OSA) is a common sleep-related respiratory disorder that is associated with cognitive, cardiovascular, and metabolic morbidities. The major cause of OSA is the sleep-related reduction of upper airway muscle tone that leads to airway obstructions in individuals with anatomically narrow upper airway. This reduction is mainly due to the suppressant effect of sleep on hypoglossal motoneurons that innervate upper airway muscles. The hypoglossal motoneurons have state-dependent activity, which is decreased during the transition from wakefulness to non-rapid eye movement sleep and is further suppressed during rapid eye movement sleep. Multiple neurotransmitters and their receptors have been implicated in the control of hypoglossal motoneuron activity across the sleep-wake states. However, to date, the results of the rigorous testing show that withdrawal of noradrenergic excitation and cholinergic inhibition essentially contribute to the depression of hypoglossal motoneuron activity during sleep. The present review will focus on origins of noradrenergic and cholinergic innervation of hypoglossal motoneurons and the functional role of these neurons in the state-dependent activity of hypoglossal motoneurons.

Catecholaminergic A1/C1 neurons contribute to the maintenance of upper airway muscle tone but may not participate in NREM sleep-related depression of these muscles

Neural mechanisms of obstructive sleep apnea, a common sleep-related breathing disorder, are incompletely understood. Hypoglossal motoneurons, which provide tonic and inspiratory activation of genioglossus (GG) muscle (a major upper airway dilator), receive catecholaminergic input from medullary A1/C1 neurons. We aimed to determine the contribution of A1/C1 neurons in control of GG muscle during sleep and wakefulness. To do so, we placed injections of a viral vector into DBH-cre mice to selectively express the hMD4i inhibitory chemoreceptors in A1/C1 neurons. Administration of the hM4Di ligand, clozapine-N-oxide (CNO), in these mice decreased GG muscle activity during NREM sleep (F_1,1,3=17.1, p<0.05); a similar non-significant decrease was observed during wakefulness. CNO administration had no effect on neck muscle activity, respiratory parameters or state durations. In addition, CNO-induced inhibition of A1/C1 neurons did not alter the magnitude of the naturally occurring depression of GG activity during transitions from wakefulness to NREM sleep. These findings suggest that A1/C1 neurons have a net excitatory effect on GG activity that is most likely mediated by hypoglossal motoneurons. However, the activity of A1/C1 neurons does not appear to contribute to NREM sleep-related inhibition of GG muscle activity, suggesting that A1/C1 neurons regulate upper airway patency in a state-independent manner.

2-Adrenergic blockade rescues hypoglossal motor defense against obstructive sleep apnea

Decreased noradrenergic excitation of hypoglossal motoneurons during sleep causing hypotonia of pharyngeal dilator muscles is a major contributor to the pathogenesis of obstructive sleep apnea (OSA), a widespread disease for which treatment options are limited. Previous OSA drug candidates targeting various excitatory/inhibitory receptors on hypoglossal motoneurons have proved unviable in reactivating these neurons, particularly during rapid-eye-movement (REM) sleep. To identify a viable drug target, we show that the repurposed α₂-adrenergic antagonist yohimbine potently reversed the depressant effect of REM sleep on baseline hypoglossal motoneuron activity (a first-line motor defense against OSA) in rats. Remarkably, yohimbine also restored the obstructive apnea-induced long-term facilitation of hypoglossal motoneuron activity (hLTF), a much-neglected form of noradrenergic-dependent neuroplasticity that could provide a second-line motor defense against OSA but was also depressed during REM sleep. Corroborating immunohistologic, optogenetic, and pharmacologic evidence confirmed that yohimbine's beneficial effects on baseline hypoglossal motoneuron activity and hLTF were mediated mainly through activation of pontine A7 and A5 noradrenergic neurons. Our results suggest a 2-tier (impaired first- and second-line motor defense) mechanism of noradrenergic-dependent pathogenesis of OSA and a promising pharmacotherapy for rescuing both these intrinsic defenses against OSA through disinhibition of A7 and A5 neurons by α₂-adrenergic blockade.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Locus Coeruleus and Tuberomammillary Nuclei Ablations Attenuate Hypocretin/Orexin Antagonist-Mediated REM Sleep

Hypocretin 1 and 2 (Hcrts; also known as orexin A and B), excitatory neuropeptides synthesized in cells located in the tuberal hypothalamus, play a central role in the control of arousal. Hcrt inputs to the locus coeruleus norepinephrine (LC NE) system and the posterior hypothalamic histaminergic tuberomammillary nuclei (TMN HA) are important efferent pathways for Hcrt-induced wakefulness. The LC expresses Hcrt receptor 1 (HcrtR1), whereas HcrtR2 is found in the TMN. Although the dual Hcrt/orexin receptor antagonist almorexant (ALM) decreases wakefulness and increases NREM and REM sleep time, the neural circuitry that mediates these effects is currently unknown. To test the hypothesis that ALM induces sleep by selectively disfacilitating subcortical wake-promoting populations, we ablated LC NE neurons (LCx) or TMN HA neurons (TMNx) in rats using cell-type-specific saporin conjugates and evaluated sleep/wake following treatment with ALM and the GABA_A receptor modulator zolpidem (ZOL). Both LCx and TMNx attenuated the promotion of REM sleep by ALM without affecting ALM-mediated increases in NREM sleep. Thus, eliminating either HcrtR1 signaling in the LC or HcrtR2 signaling in the TMN yields similar effects on ALM-induced REM sleep without affecting NREM sleep time. In contrast, neither lesion altered ZOL efficacy on any measure of sleep–wake regulation. These results contrast with those of a previous study in which ablation of basal forebrain cholinergic neurons attenuated ALM-induced increases in NREM sleep time without affecting REM sleep, indicating that Hcrt neurotransmission influences distinct aspects of NREM and REM sleep at different locations in the sleep–wake regulatory network.

Revisiting Antagonist Effects in Hypoglossal Nucleus: Brainstem Circuit for the State-Dependent Control of Hypoglossal Motoneurons: A Hypothesis

We reassessed and provided new insights into the findings that were obtained in our previous experiments that employed the injections of combined adrenergic, serotonergic, GABAergic, and glycinergic antagonists into the hypoglossal nucleus in order to pharmacologically abolish the depression of hypoglossal nerve activity that occurred during carbachol-induced rapid-eye-movement (REM) sleep-like state in anesthetized rats. We concluded that noradrenergic disfacilitation is the major mechanism that is responsible for approximately 90% of the depression of hypoglossal motoneurons, whereas the remaining 10% can be explained by serotonergic mechanisms that have net inhibitory effect on hypoglossal nerve activity during REM sleep-like state. We hypothesized that both noradrenergic and serotonergic state-dependent mechanisms indirectly control hypoglossal motoneuron excitability during REM sleep; their activities are integrated and mediated to hypoglossal motoneurons by reticular formation neurons. In addition, we proposed a brainstem neural circuit that can explain the new findings.

The inhibition of the dorsal paragigantocellular reticular nucleus induces waking and the activation of all adrenergic and noradrenergic neurons: a combined pharmacological and functional neuroanatomical study

GABAergic neurons specifically active during paradoxical sleep (PS) localized in the dorsal paragigantocellular reticular nucleus (DPGi) are known to be responsible for the cessation of activity of the noradrenergic neurons of the locus coeruleus during PS. In the present study, we therefore sought to determine the role of the DPGi in PS onset and maintenance and in the inhibition of the LC noradrenergic neurons during this state. The effect of the inactivation of DPGi neurons on the sleep-waking cycle was examined in rats by microinjection of muscimol, a GABAA agonist, or clonidine, an alpha-2 adrenergic receptor agonist. Combining immunostaining of the different populations of wake-inducing neurons with that of c-FOS, we then determined whether muscimol inhibition of the DPGi specifically induces the activation of the noradrenergic neurons of the LC. Slow wave sleep and PS were abolished during 3 and 5 h after muscimol injection in the DPGi, respectively. The application of clonidine in the DPGi specifically induced a significant decrease in PS quantities and delayed PS appearance compared to NaCl. We further surprisingly found out that more than 75% of the noradrenergic and adrenergic neurons of all adrenergic and noradrenergic cell groups are activated after muscimol treatment in contrast to the other wake active systems significantly less activated. These results suggest that, in addition to its already know inhibition of LC noradrenergic neurons during PS, the DPGi might inhibit the activity of noradrenergic and adrenergic neurons from all groups during PS, but also to a minor extent during SWS and waking.

Sleep-wake control of the upper airway by noradrenergic neurons, with and without intermittent hypoxia

Hypoglossal (XII) motoneurons innervate muscles of the tongue whose tonic and inspiratory modulated activity protects the upper airway from collapse in patients affected by the obstructive sleep apnea (OSA) syndrome. Both norepinephrine and serotonin provide wakefulness-related excitatory drives that maintain activity in XII motoneurons, with the noradrenergic system playing a particularly prominent role in rats. When noradrenergic and serotonergic drives are antagonized, no further decline of XII nerve activity occurs during pharmacologically induced rapid eye movement (REM) sleep-like state. This is the best evidence to date that, at least in this model, the entire REM sleep-related decline of upper airway muscle tone results from withdrawal of these two excitatory inputs. A major component of noradrenergic input to XII motoneurons originates from pontine noradrenergic neurons that have state-dependent patterns of activity, maximal during wakefulness, and minimal, or absent during REM sleep. Our data suggest that not all ventrolateral medullary catecholaminergic neurons follow this pattern, with adrenergic C1 neurons probably increasing their activity during REM sleep. When rats are subjected to chronic-intermittent hypoxia, noradrenergic drive to XII motoneurons is increased by mechanisms that include sprouting of noradrenergic terminals in the XII nucleus, and increased expression of α1-adrenoceptors; an outcome that may underlie the elevated baseline activity of upper airway muscles during wakefulness in OSA patients.

Activation properties of trigeminal motoneurons in participants with and without bruxism

In animals, sodium- and calcium-mediated persistent inward currents (PICs), which produce long-lasting periods of depolarization under conditions of low synaptic drive, can be activated in trigeminal motoneurons following the application of the monoamine serotonin. Here we examined if PICs are activated in human trigeminal motoneurons during voluntary contractions and under physiological levels of monoaminergic drive (e.g., serotonin and norepinephrine) using a paired motor unit analysis technique. We also examined if PICs activated during voluntary contractions are larger in participants who demonstrate involuntary chewing during sleep (bruxism), which is accompanied by periods of high monoaminergic drive. In control participants, during a slowly increasing and then decreasing isometric contraction, the firing rate of an earlier-recruited masseter motor unit, which served as a measure of synaptic input to a later-recruited test unit, was consistently lower during derecruitment of the test unit compared with at recruitment (ΔF = 4.6 ± 1.5 imp/s). The ΔF, therefore, is a measure of the reduction in synaptic input needed to counteract the depolarization from the PIC to provide an indirect estimate of PIC amplitude. The range of ΔF values measured in the bruxer participants during similar voluntary contractions was the same as in controls, suggesting that abnormally high levels of monoaminergic drive are not continually present in the absence of involuntary motor activity. We also observed a consistent "onion skin effect" during the moderately sized contractions (<20% of maximal), whereby the firing rate of higher threshold motor units discharged at slower rates (by 4-7 imp/s) compared with motor units with relatively lower thresholds. The presence of lower firing rates in the more fatigue-prone, higher threshold trigeminal motoneurons, in addition to the activation of PICs, likely facilitates the activation of the masseter muscle during motor activities such as eating, nonnutritive chewing, clenching, and yawning.

Loss of motoneurons in the ventral compartment of the rat hypoglossal nucleus following early postnatal exposure to alcohol

Perinatal alcohol exposure (AE) has multiple detrimental effects on cognitive and various behavioral outcomes, but little is known about its impact on the autonomic functions. In a rat model of fetal alcohol spectrum disorders (FASD), we investigated neurochemical and neuroanatomical alterations in two brainstem nuclei, the hypoglossal nucleus (XIIn) and the dorsal nucleus of the vagus nerve (Xdn). One group of male Sprague-Dawley rats (n=6) received 2.625 g/kg ethanol intragastrically twice daily on postnatal days (PD) 4-9, a period equivalent to the third trimester of human pregnancy, and another group (n=6) was sham-intubated. On PD 18-19, the rats were perfused and medullary sections were immunohistochemically processed for choline acetyltransferase (ChAT) or two aminergic receptors that mediate excitatory drive to motoneurons, α₁-adrenergic (α₁-R) and serotonin 2A (5-HT(2A)-R), and c-Fos. Based on ChAT labeling, AE rats had reduced numbers of motoneurons in the ventral XIIn (XIIn-v; 35.4±1.3 motoneurons per side and section vs. 40.0±1.2, p=0.022), but not in the dorsal XIIn or Xdn. Consistent with ChAT data, both the numbers of α₁-R-labeled motoneurons in the XIIn-v and the area of the XIIn-v measured using 5-HT(2A)-R staining were significantly smaller in AE rats (19.7±1.5 vs. 25.0±1.4, p=0.031 and 0.063 mm² ±0.002 vs. 0.074±0.002, p=0.002, respectively). Concurrently, both 5-HT(2A)-R and c-Fos staining tended to be higher in AE rats, suggesting an increased activation. Thus, postnatal AE causes motoneuronal loss in the XIIn-v. This may compromise upper airway control and contribute to increased risk of upper airway obstructions and sudden infant death in FASD victims.

Circadian dependence of receptors that mediate wake-related excitatory drive to hypoglossal motoneurons

Serotonin (5-HT), norepinephrine and orexins (ORX) are the three best established mediators of wake-related activation of hypoglossal (XII) motoneurons that innervate the muscles of the tongue. Since the tongue's use is temporarily closely aligned with the rest-activity cycle, we tested whether expression of mRNA for relevant 5-HT, norepinephrine and ORX receptors varies in the XII nucleus with the rest-activity cycle. Adult rats (n=7-9/group) were decapitated at 8-9 am (near rest period onset) or at 6-7 pm (near active period onset). Tissue micropunches were extracted from medullary slices containing the XII motor and sensory external cuneate (ECN) nuclei. 5-HT2A, α1-adrenergic and ORX type 2 receptor mRNAs were quantified using RT-PCR. Only 5-HT2A receptor mRNA levels differed between the two time points and were higher at the active period onset; no differences were detected in the ECN. Consistent with the mRNA results, 5-HT2A protein levels were also higher in the XII nucleus at the active period onset than at rest onset. Thus, the endogenous serotonergic excitatory drive to XII motoneurons may be enhanced through circadian- or activity-dependent mechanisms that increase the availability of 5-HT2A receptors prior to the active period. Conversely, reduced levels of 5-HT2A receptors during the rest-sleep period may exacerbate the propensity for sleep-disordered breathing in subjects with anatomically compromised upper airway.

Evidence that adrenergic ventrolateral medullary cells are activated whereas precerebellar lateral reticular nucleus neurons are suppressed during REM sleep

Rapid eye movement sleep (REMS) is generated in the brainstem by a distributed network of neurochemically distinct neurons. In the pons, the main subtypes are cholinergic and glutamatergic REMS-on cells and aminergic REMS-off cells. Pontine REMS-on cells send axons to the ventrolateral medulla (VLM), but little is known about REMS-related activity of VLM cells. In urethane-anesthetized rats, dorsomedial pontine injections of carbachol trigger REMS-like episodes that include cortical and hippocampal activation and suppression of motoneuronal activity; the episodes last 4–8 min and can be elicited repeatedly. We used this model to determine whether VLM catecholaminergic cells are silenced during REMS, as is typical of most aminergic neurons studied to date, and to investigate other REMS-related cells in this region. In 18 anesthetized, paralyzed and artificially ventilated rats, we obtained extracellular recordings from VLM cells when REMS-like episodes were elicited by pontine carbachol injections (10 mM, 10 nl). One major group were the cells that were activated during the episodes (n = 10). Their baseline firing rate of 3.7±2.1 (SD) Hz increased to 9.7±2.1 Hz. Most were found in the adrenergic C1 region and at sites located less than 50 µm from dopamine β-hydroxylase-positive (DBH⁺) neurons. Another major group were the silenced or suppressed cells (n = 35). Most were localized in the lateral reticular nucleus (LRN) and distantly from any DBH⁺ cells. Their baseline firing rates were 6.8±4.4 Hz and 15.8±7.1 Hz, respectively, with the activity of the latter reduced to 7.4±3.8 Hz. We conclude that, in contrast to the pontine noradrenergic cells that are silenced during REMS, medullary adrenergic C1 neurons, many of which drive the sympathetic output, are activated. Our data also show that afferent input transmitted to the cerebellum through the LRN is attenuated during REMS. This may distort the spatial representation of body position during REMS.

Optogenetic stimulation of c1 and retrotrapezoid nucleus neurons causes sleep state-dependent cardiorespiratory stimulation and arousal in rats

C1 catecholaminergic neurons and neurons of the retrotrapezoid nucleus are integrative nodes within the brain stem network regulating cardiorespiratory reflexes elicited by hypoxia and hypercapnia, stimuli that also produce arousal from sleep. In the present study, Channelrhodopsin-2 was selectively introduced into these neurons with a lentiviral vector to determine whether their selective activation also produces arousal in sleeping rats. Sleep stages were identified from electroencephalographic and neck muscle electromyographic recordings. Breathing was measured using unrestrained whole body plethysmography and blood pressure by telemetry. During nonrapid eye movement sleep, unilateral photostimulation of the C1 region caused arousal in 83.0±14.7% of trials and immediate and intense cardiorespiratory activation. Arousal during photostimulation was also observed during rapid eye movement sleep (41.9±5.6% of trials), but less reliably than during nonrapid eye movement sleep. The cardiorespiratory responses elicited by photostimulation were dramatically smaller during rapid eye movement sleep than nonrapid eye movement sleep or wakefulness. Systemic α1-adrenoreceptor blockade reduced the cardiorespiratory effects of photostimulation but had no effect on the arousal caused by photostimulation during nonrapid eye movement sleep. Postmortem histology showed that neurons expressing Channelrhodopsin 2-mCherry were predominantly catecholaminergic (81%). These results show that selective activation of C1 and retrotrapezoid nucleus neurons produces state-dependent arousal and cardiorespiratory stimulation. These neurons, which are powerfully activated by chemoreceptor stimulation, may contribute to the sleep disruption associated with obstructive sleep apnea.

Antagonism of orexin receptors in the posterior hypothalamus reduces hypoglossal and cardiorespiratory excitation from the perifornical hypothalamus

The perifornical (PF) region of the posterior hypothalamus promotes wakefulness and facilitates motor activity. In anesthetized rats, local disinhibition of PF neurons by GABA(A) receptor antagonists activates orexin (OX) neurons and elicits a systemic response, including increases of hypoglossal nerve activity (XIIa), respiratory rate, heart rate, and blood pressure. The increase of XIIa is mediated to hypoglossal (XII) motoneurons by pathways that do not require noradrenergic or serotonergic projections. We hypothesized that the pathway might include OX-dependent activation locally within the PF region or direct projections of OX neurons to the XII nucleus. Adult, male Sprague-Dawley rats were urethane anesthetized, vagotomized, paralyzed, and ventilated. Gabazine (GABA(A) receptor antagonist, 0.18 mM, 20 nl) was injected into the PF region, and ~2 h later, a second gabazine injection was performed preceded by injection of a dual OX1/2 receptor antagonist (almorexant; 90 mM) either into the XII nucleus (40-60 nl at 2-3 rostrocaudal levels; n = 6 rats), or into the PF region (40-60 nl; n = 6 rats). XIIa, respiratory rate, heart rate, and arterial blood pressure were analyzed for 70 min after each gabazine injection. The excitatory effects of PF gabazine on XIIa, respiratory, and heart rates were significantly reduced by up to 44-82% when gabazine injections were preceded by PF almorexant injections, but not when almorexant was injected into the XII nucleus. These data suggest that a significant portion of XII motoneuronal and cardiorespiratory activation evoked by disinhibition of PF neurons is mediated by local OX-dependent mechanisms within the posterior hypothalamus.

Inhibition of A5 Neurons Facilitates the Occurrence of REM Sleep-Like Episodes in Urethane-Anesthetized Rats: A New Role for Noradrenergic A5 Neurons?

When rapid eye movement (REM) sleep occurs, noradrenergic cells become silent, with the abolition of activity in locus coeruleus (LC) neurons seen as a key event permissive for the occurrence of REM sleep. However, it is not known whether silencing of other than LC noradrenergic neurons contributes to the generation of REM sleep. In urethane-anesthetized rats, stereotyped REM sleep-like episodes can be repeatedly elicited by injections of the cholinergic agonist, carbachol, into a discrete region of the dorsomedial pons. We used this preparation to test whether inhibition of ventrolateral pontine noradrenergic A5 neurons only, or together with LC neurons, also can elicit REM sleep-like effects. To silence noradrenergic cells, we sequentially injected the α₂-adrenergic agonist clonidine (20–40 nl, 0.75 mM) into both A5 regions and then the LC. In two rats, successful bilateral clonidine injections into the A5 region elicited the characteristic REM sleep-like episodes (hippocampal theta rhythm, suppression of hypoglossal nerve activity, reduced respiratory rate). In five rats, bilateral clonidine injections into the A5 region and then into one LC triggered REM sleep-like episodes, and in two rats injections into both A5 and then both LC were needed to elicit the effect. In contrast, in three rats, uni- or bilateral clonidine injections only into the LC had no effect, and clonidine injections placed in another six rats outside of the A5 and/or LC regions were without effect. The REM sleep-like episodes elicited by clonidine had similar magnitude of suppression of hypoglossal nerve activity (by 75%), similar pattern of hippocampal changes, and similar durations (2.5–5.3 min) to the episodes triggered in the same preparation by carbachol injections into the dorsomedial pontine reticular formation. Thus, silencing of A5 cells may importantly enable the occurrence of REM sleep-like episodes, at least under anesthesia. This is a new role for noradrenergic A5 neurons.

Effect of chronic intermittent hypoxia on noradrenergic activation of hypoglossal motoneurons

In obstructive sleep apnea patients, elevated activity of the lingual muscles during wakefulness protects the upper airway against occlusions. A possibly related form of respiratory neuroplasticity is present in rats exposed to acute and chronic intermittent hypoxia (CIH). Since rats exposed to CIH have increased density of noradrenergic terminals and increased α(1)-adrenoceptor immunoreactivity in the hypoglossal (XII) nucleus, we investigated whether these anatomic indexes of increased noradrenergic innervation translate to increased sensitivity of XII motoneurons to noradrenergic activation. Adult male Sprague-Dawley rats were subjected to CIH for 35 days, with O(2) level varying between 24% and 7% with 180-s period for 10 h/day. They were then anesthetized, vagotomized, paralyzed, and artificially ventilated. The dorsal medulla was exposed, and phenylephrine (2 mM, 10 nl) and then the α(1)-adrenoceptor antagonist prazosin (0.2 mM, 3 × 40 nl) were microinjected into the XII nucleus while XII nerve activity (XIIa) was recorded. The area under integrated XIIa was measured before and at different times after microinjections. The excitatory effect of phenylephrine on XII motoneurons was similar in sham- and CIH-treated rats. In contrast, spontaneous XIIa was more profoundly reduced following prazosin injections in CIH- than sham-treated rats [to 21 ± 7% (SE) vs. 40 ± 8% of baseline, P < 0.05] without significant changes in central respiratory rate, arterial blood pressure, or heart rate. Thus, consistent with increased neuroanatomic measures of noradrenergic innervation of XII motoneurons following exposure to CIH, prazosin injections revealed a stronger endogenous noradrenergic excitatory drive to XII motoneurons in CIH- than sham-treated anesthetized rats.

The involvement of noradrenaline in rapid eye movement sleep mentation

Noradrenaline, one of the main brain monoamines, has powerful central influences on forebrain neurobiological processes which support the mental activities occurring during the sleep-waking cycle. Noradrenergic neurons are activated during waking, decrease their firing rate during slow wave sleep, and become silent during rapid eye movement (REM) sleep. Although a low level of noradrenaline is still maintained during REM sleep because of diffuse extrasynaptic release without rapid withdrawal, the decrease observed during REM sleep contributes to the mentation disturbances that occur during dreaming, which principally resemble symptoms of schizophrenia but seemingly also of attention deficit hyperactivity disorder.

Theta synchronization between the hippocampus and the nucleus incertus in urethane-anesthetized rats

Oscillatory coupling between distributed areas can constitute a mechanism for neuronal integration. Theta oscillations provide temporal windows for hippocampal processing and only appear during certain active states of animals. Since previous studies have demonstrated that nucleus incertus (NI) contributes to the generation of hippocampal theta activity, in this paper, we evaluated the oscillatory coupling between both structures. We compared hippocampal and NI field potentials that were simultaneously recorded in urethane-anesthetized rats. Electrical and cholinergic stimulations of the reticularis pontis oralis nucleus have been used as hippocampal theta generation models. The spectral analyses reveal that electrical stimulation induced an increase in theta oscillations in both channels, whose frequencies depended on the intensity of stimulation. The intensity range used simultaneously increased the normalized spectral energy in the fast theta band (6-12 Hz) in HPC and NI. Frequencies within the theta range were found to be very similar in both channels. In order to validate coupling, spectral coherence was inspected. The data reveal that coherence in the high theta band also increased while stimuli were applied. Cholinergic activation progressively increased the main frequency in both structures to reach an asymptotic period with stable peak frequency in the low theta range (3-6 Hz), which could be first observed in NI and lasted about 1,500 s. Coherence in this band reached values close to 1. Taken together, these results support an electrophysiological and functional coupling between the hippocampus and the reticular formation, suggesting NI to be part of a distributed network working at theta frequencies.

State-dependent control of lumbar motoneurons by the hypocretinergic system

Neurons in the lateral hypothalamus (LH) that synthesize hypocretins (Hcrt-1 and Hcrt-2) are active during wakefulness and excite lumbar motoneurons. Because hypocretinergic cells also discharge during phasic periods of rapid eye movement (REM) sleep, we sought to examine their action on the activity of motoneurons during this state. Accordingly, cat lumbar motoneurons were intracellularly recorded, under alpha-chloralose anesthesia, prior to (control) and during the carbachol-induced REM sleep-like atonia (REMc). During control conditions, LH stimulation induced excitatory postsynaptic potentials (composite EPSP) in motoneurons. In contrast, during REMc, identical LH stimulation induced inhibitory PSPs in motoneurons. We then tested the effects of LH stimulation on motoneuron responses following the stimulation of the nucleus reticularis gigantocellularis (NRGc) which is part of a brainstem-spinal cord system that controls motoneuron excitability in a state-dependent manner. LH stimulation facilitated NRGc stimulation-induced composite EPSP during control conditions whereas it enhanced NRGc stimulation-induced IPSPs during REMc. These intriguing data indicate that the LH exerts a state-dependent control of motor activity. As a first step to understand these results, we examined whether hypocretinergic synaptic mechanisms in the spinal cord were state dependent. We found that the juxtacellular application of Hcrt-1 induced motoneuron excitation during control conditions whereas motoneuron inhibition was enhanced during REMc. These data indicate that the hypocretinergic system acts on motoneurons in a state-dependent manner via spinal synaptic mechanisms. Thus, deficits in Hcrt-1 may cause the coexistence of incongruous motor signs in cataplectic patients, such as motor suppression during wakefulness and movement disorders during REM sleep.

Emerging principles and neural substrates underlying tonic sleep-state-dependent influences on respiratory motor activity

Respiratory muscles with dual respiratory and non-respiratory functions (e.g. the pharyngeal and intercostal muscles) show greater suppression of activity in sleep than the diaphragm, a muscle almost entirely devoted to respiratory function. This sleep-related suppression of activity is most apparent in the tonic component of motor activity, which has functional implications of a more collapsible upper airspace in the case of pharyngeal muscles, and decreased functional residual capacity in the case of intercostal muscles. A major source of tonic drive to respiratory motoneurons originates from neurons intimately involved in states of brain arousal, i.e. neurons not classically involved in generating respiratory rhythm and pattern per se. The tonic drive to hypoglossal motoneurons, a respiratory motor pool with both respiratory and non-respiratory functions, is mediated principally by noradrenergic and glutamatergic inputs, these constituting the essential components of the wakefulness stimulus. These tonic excitatory drives are opposed by tonic inhibitory glycinergic and gamma-amino butyric acid (GABA) inputs that constrain the level of respiratory-related motor activity, with the balance determining net motor tone. In sleep, the excitatory inputs are withdrawn and GABA release into the brainstem is increased, thus decreasing respiratory motor tone and predisposing susceptible individuals to hypoventilation and obstructive sleep apnoea.

Central nervous system plus autonomic nervous system disorders responsible for gastrointestinal and pancreatobiliary diseases

Clinical digestive disorders depend on the non-adequate coupling of functioning of the gastrointestinal tract with that of its affluent systems, namely, the pancreatic exocrine and the hepato-biliary secretions. The secretion of gastrointestinal hormones is monitored by the peripheral autonomic nervous system. However, the latter is regulated by the central nervous system (CNS) circuitry localized at the medullary pontine segment of the CNS. In turn, both parasympathetic and adrenergic medullary circuitries are regulated by the pontine A5 noradrenergic (NA) and the dorsal raphe serotonergic nuclei, respectively. DR-5HT is positively correlated with the C1-Ad medullary nuclei (responsible for adrenal gland secretion), whereas the MR-5HT nucleus is positively correlated with the A5-NA pontomedullary nucleus. The latter is responsible for neural sympathetic activity (sympathetic nerves). Both types of sympathetic activities maintain an alternation with the peripheral parasympathetic branch, which is positively correlated with the enterochromaffin cells that secrete serotonin. Serotonin displays hormonal antagonism to the circulating catecholamines.

Noradrenergic neurons expressing Fos during waking and paradoxical sleep deprivation in the rat

Noradrenaline is known to induce waking (W) and to inhibit paradoxical sleep (PS or REM). Both roles have been exclusively attributed to the noradrenergic neurons of the locus coeruleus (LC, A6), shown to be active during W and inactive during PS. However, the A1, A2, A5 and A7 noradrenergic neurons could also be responsible. Therefore, to determine the contribution of each of the noradrenergic groups in W and in PS inhibition, rats were maintained in continuous W for 3h in a novel environment or specifically deprived of PS for 3 days, with some of them allowed to recover from this deprivation. A double immunohistochemical labeling with Fos and tyrosine hydroxylase was then performed. Thirty percent of the LC noradrenergic cells were found to be Fos-positive after exposure to the novel environment and less than 2% after PS deprivation. In contrast, a significant number of double-labeled neurons (up to 40% of the noradrenergic neurons) were observed in the A1/C1, A2 and A5 groups, after both novel environment and PS deprivation. After PS recovery and in control condition, less than 1% of the noradrenergic neurons were Fos-immunoreactive, regardless of the noradrenergic group. These results indicate that the brainstem noradrenergic cell groups are activated during W and silent during PS. They further suggest that the inhibitory effect of noradrenaline on PS may be due to the A1/C1, A2 and to a lesser degree to A5 neurons but not from those of the LC as previously hypothesized.

Differential localization of carbachol- and bicuculline-sensitive pontine sites for eliciting REM sleep-like effects in anesthetized rats

Carbachol, a cholinergic agonist, and GABA(A) receptor antagonists injected into the pontine dorsomedial reticular formation can trigger rapid eye movement (REM) sleep-like state. Data suggest that GABAergic and cholinergic effects interact to produce this effect but the sites where this occurs have not been delineated. In urethane-anesthetized rats, in which carbachol effectively elicits REM sleep-like episodes (REMSLE), we tested the ability of 10 nL microinjections of carbachol (10 mm) and bicuculline (0.5 or 2 mm) to elicit REMSLE at 47 sites located within the dorsal pontine reticular formation at the levels -8.00 to -10.80 from bregma (B) (Paxinos and Watson, The Rat Brain in Stereotaxic Coordinates, Academic Press, San Diego, 1997). At rostral levels, most carbachol and some bicuculline injections elicited REMSLE with latencies that gradually decreased from 242 to 12 s for carbachol and from 908 to 38 s for bicuculline for more caudal injection sites. As the latencies decreased, the durations of bicuculline-elicited REMSLE increased from 104 s to over 38 min, and the effect was dose dependent, whereas the duration of carbachol-elicited REMSLE changed little (104-354 s). Plots of REMSLE latency versus the antero-posterior coordinates revealed that both drugs were maximally effective near B-8.80. At levels caudal to B-8.80, carbachol was effective at few sites, whereas bicuculline-elicited REMSLE to at least B-9.30 level. Thus, the bicuculline-sensitive sites extended further caudally than those for carbachol and antagonism of GABA(A) receptors both triggered REMSLE and controlled their duration, whereas carbachol effects on REMSLE duration were small or limited by its concurrent REMSLE-opposing actions.

Inhibition of pontine noradrenergic A7 cells reduces hypoglossal nerve activity in rats

Noradrenergic (NE) excitatory drive maintains activity of hypoglossal (XII) motoneurons during wakefulness. In predisposed persons, sleep-related decrements of NE cell activity may contribute to hypotonia of upper airway muscles innervated by XII motoneurons. The goal of this study was to determine whether NE neurons of the pontine A7 group, an anatomically identified source of NE projections to the XII nucleus, provide significant, endogenous NE excitatory drive to XII motoneurons. In anesthetized rats, we microinjected clonidine (0.75 mM, 20-40 nl), an alpha(2)-adrenergic receptor agonist that inhibits pontine NE cells, aiming at the A7 region. Nine injections were placed within 0.4 mm from the A7 group identified using tyrosine hydroxylase immunohistochemistry: they reduced XII nerve activity by 31.3+/-2.8% (standard error) and decreased the central respiratory rate by 6%. Another 21 injections, including eight placed near NE cells of the sub-coeruleus region, were made at distances over 0.5 mm from the A7 group and they did not alter either XII nerve activity or respiratory rate. In control experiments, clonidine injections into the A7 group preceded by injections of an alpha(2)-receptor antagonist, RS-79948, did not change XII nerve activity. Four experiments with unilateral clonidine injections into the A7 region and with Fos immunohistochemistry used as a marker of cell activity revealed that the percentage of Fos-positive A7 cells was significantly reduced on the injected side. There was also a significant positive correlation between Fos expression in A7 cells and XII nerve activity. Thus, decrements of NE excitatory drive from the A7 group may significantly reduce XII nerve activity. In obstructive sleep apnea patients, in whom the muscles innervated by XII motoneurons act as upper airway dilators, this may contribute to sleep-related respiratory disorders.

Fos expression in pontomedullary catecholaminergic cells following rapid eye movement sleep-like episodes elicited by pontine carbachol in urethane-anesthetized rats

Pontine noradrenergic neurons of the locus coeruleus (LC) and sub-coeruleus (SubC) region cease firing during rapid eye movement sleep (REMS). This plays a permissive role in the generation of REMS and may contribute to state-dependent modulation of transmission in the CNS. Whether all pontomedullary catecholaminergic neurons, including those in the A1/C1, A2/C2 and A7 groups, have REMS-related suppression of activity has not been tested. We used Fos protein expression as an indirect marker of the level of neuronal activity and linear regression analysis to determine whether pontomedullary cells identified by tyrosine hydroxylase (TH) immunohistochemistry have reduced Fos expression following REMS-like state induced by pontine microinjections of a cholinergic agonist, carbachol in urethane-anesthetized rats. The percentage of Fos-positive TH cells was negatively correlated with the cumulative duration of REMS-like episodes induced during 140 min prior to brain harvesting in the A7 and rostral A5 groups bilaterally (P < 0.01 for both), and in SubC neurons on the side opposite to carbachol injection (P < 0.05). Dorsal medullary A2/C2 neurons did not exhibit such correlation, but their Fos expression (and that in A7, rostral A5 and SubC neurons) was positively correlated with the duration of the interval between the last REMS-like episode and the time of perfusion (P < 0.05). In contrast, neither of these correlations was significant for A1 /C1 or caudal A5 neurons. These findings suggest that, similar to the prototypic LC neurons, neurons of the A7, rostral A5 and A2/C2 groups have reduced or abolished activity during REMS, whereas A1 /IC1 and caudal A5 neurons do not have this feature. The reduced activity of A2/C2, A5 and A7 neurons during REMS, and the associated decrements in norepinephrine release, may cause state-dependent modulation of.transmission in brain somato- and viscerosensory, somatomotor, and cardiorespiratory pathways.

Disinhibition of perifornical hypothalamic neurones activates noradrenergic neurones and blocks pontine carbachol-induced REM sleep-like episodes in rats

Studies in behaving animals suggest that neurones located in the perifornical (PF) region of the posterior hypothalamus promote wakefulness and suppress sleep. Among such cells are those that synthesize the excitatory peptides, orexins (ORX). Lack of ORX, or their receptors, is associated with narcolepsy/cataplexy, a disorder characterized by an increased pressure for rapid eye movement (REM) sleep. We used anaesthetized rats in which pontine microinjections of a cholinergic agonist, carbachol, can repeatedly elicit REM sleep-like episodes to test whether activation of PF cells induced by antagonism of endogenous, GABA(A) receptor-mediated, inhibition suppresses the ability of the brainstem to generate REM sleep-like state. Microinjections of the GABA(A) receptor antagonist, bicuculline (20 nl, 1 mm), into the PF region elicited cortical and hippocampal activation, increased the respiratory rate and hypoglossal nerve activity, induced c-fos expression in ORX and other PF neurones, and increased c-fos expression in pontine A7 and other noradrenergic neurones. The ability of pontine carbachol to elicit any cortical, hippocampal or brainstem component of the REM sleep-like response was abolished during the period of bicuculline-induced activation. The activating and REM sleep-suppressing effect of PF bicuculline was not attenuated by systemic administration of the ORX type 1 receptor antagonist, SB334867. Thus, activation of PF neurones that are endogenously inhibited by GABA(A) receptors is sufficient to turn off the brainstem REM sleep-generating network; the effect is, at least in part, due to activation of pontine noradrenergic neurones, but is not mediated by ORX type 1 receptors. A malfunction of the pathway that originates in GABA(A) receptor-expressing PF neurones may cause narcolepsy/cataplexy.

Respiratory motor activity: influence of neuromodulators and implications for sleep disordered breathing

Sleep, especially rapid-eye-movement sleep, causes fundamental modifications of respiratory muscle activity and control mechanisms, modifications that can predispose individuals to sleep-related breathing disorders. One of the most common of these disorders is obstructive sleep apnea (OSA) that affects approximately 4% of adults. OSA is caused by repeated episodes of pharyngeal airway obstruction that can occur hundreds of times per night, leading to recurrent asphyxia, arousals from sleep, daytime sleepiness, and adverse cardiovascular and cerebrovascular consequences. OSA is caused by the effects of sleep on pharyngeal muscle tone in individuals with already narrow upper airways. Moreover, since OSA occurs only in sleep, this disorder by definition is a state-dependent process ultimately caused by the influence of sleep neural mechanisms on the activity of pharyngeal motoneurons. This review synthesizes recent findings relating to control of pharyngeal muscle activity across sleep-wake states, with special emphasis on the influence of neuromodulators acting at the hypoglossal motor nucleus that inervates the genioglossus muscle of the tongue. The results of such basic physiological studies may be relevant to identifying and developing new pharmacological strategies to augment pharyngeal muscle activity in sleep, especially rapid-eye-movement sleep, as potential treatments for OSA.

GABAergic processes in the mesencephalic tegmentum modulate the occurrence of active (rapid eye movement) sleep in guinea pigs

The ventrolateral subdivision of the periaqueductal gray (vlPAG) and the adjacent dorsal mesencephalic reticular formation (dMRF) are involved in the modulation of active (rapid eye movement) sleep (AS). In order to determine the effects on AS of the suppression of neuronal activity in these regions, muscimol, a GABA receptor A (GABA(A)) receptor agonist, and bicuculline, a GABA(A) receptor antagonist, were microinjected bilaterally in guinea pigs and the states of sleep and wakefulness were examined. The main effect of muscimol was an increase in AS; this increase occurred in conjunction with a reduction in the time spent in wakefulness. The powerful effect of muscimol was striking especially when considering the small amount of naturally-occurring AS that is present in this species. Additional observable effects that were induced by muscimol were: 1) long lasting episodes of hypotonia/atonia during wakefulness and quiet sleep that included a lack of extensor tone in the hind limbs, and 2) frequently occurring cortical spindles, similar to those observed during naturally-occurring quiet sleep (sleep spindles), that were present during wakefulness. Conversely, bilateral microinjections of bicuculline induced a prolonged state of wakefulness and blocked the effect of subsequent injections of muscimol. These data suggest that endogenous GABA acts on GABA(A) receptors within the vlPAG and dMRF to promote AS in the guinea pig.

Muscarinic receptors and alpha2-adrenoceptors interact to modulate the respiratory rhythm in mouse neonates

The respiratory rhythm generator (RRG) is modulated by several endogenous substances, including acetylcholine (ACh) and noradrenaline (NA) that interact in several modulatory processes. To know whether ACh and NA interacted to modulate the RRG activity, we used medullary "en bloc" and slice preparations from neonatal mice where the RRG has been shown to receive a facilitatory modulation from A1/C1 neurons, via a continuous release of endogenous NA and activation of alpha2 adrenoceptors. Applying ACh at 25 microM activated the RRG but ACh had no effects at 50 microM. Applying the ACh receptor agonists nicotine and muscarine facilitated and depressed the RRG, respectively. After yohimbine pre-treatment that blocked the alpha2 facilitation, the nicotinic facilitation was not altered, the muscarinic depression was reversed and ACh 50 microM significantly facilitated the RRG. After L-tyrosine pre-treatment that potentiated the alpha2 facilitation, the muscarinic depression was enhanced. Thus, ACh regulates the RRG activity via nicotinic and muscarinic receptors, the muscarinic receptors interacting with alpha2 adrenoceptors.

Endogenous Excitatory Drive Modulating Respiratory Muscle Activity across Sleep–Wake States

RATIONALE

The concept of a tonic drive activating respiratory muscle in wakefulness but not sleep has been an important and enduring notion in respiratory medicine, not least because it is useful in modeling sleep effects on breathing and understanding the pathogenesis of sleep-related breathing disorders such as obstructive sleep apnea. However, a neurotransmitter substrate mediating respiratory muscle activation across sleep-wake states has not been identified.

OBJECTIVES

We determined if alpha1 receptor antagonism at the hypoglossal motor nucleus (HMN) decreases genioglossus (GG) activity consistent with a role for an endogenous noradrenergic drive contributing to GG activation across sleep-wake states. We also determined if alpha1 receptor stimulation could counteract reduced endogenous noradrenergic drive and increase sleeping GG activity.

METHODS

Thirty-five rats were implanted with electroencephalogram and neck electrodes to record sleep-wake states and GG and diaphragm electrodes for respiratory muscle recordings. Microdialysis probes were inserted into the HMN.

MEASUREMENTS AND MAIN RESULTS

Microdialysis perfusion of the alpha1 receptor antagonist terazosin into the HMN significantly decreased GG activity in wakefulness and nonrapid eye movement (non-REM) sleep but not REM sleep. The alpha1 receptor agonist phenylephrine increased GG activity in wakefulness and sleep, but periods of motor inactivity persisted in REM sleep; there was no potentiating effect of combined alpha1 and 5-HT2 receptor stimulation.

CONCLUSIONS

Identification of an endogenous noradrenergic drive contributing to GG activation in wakefulness and non-REM sleep, but not REM sleep, is important given the prevalence and clinical significance of sleep-induced hypoventilation and obstructive sleep apnea in humans and the potential for pharmacologic treatment.

Differential pontomedullary catecholaminergic projections to hypoglossal motor nucleus and viscerosensory nucleus of the solitary tract

In individuals with a narrow or collapsible upper airway, sleep-related hypotonia of upper airway muscles leads to recurrent airway obstructions. Brainstem noradrenergic neurons reduce their activity during slow-wave sleep and become silent during rapid eye movement sleep; this may cause state-dependent changes in the motor output and reflexes. The loss of noradrenergic excitation is a major cause of sleep-related depression of activity in upper airway muscles innervated by the hypoglossal nerve. Our goal was to identify and compare the pontomedullary sources of catecholaminergic (CA) projections to the hypoglossal motor nucleus (Mo12) and the adjacent viscerosensory nucleus of the solitary tract (NTS). In 10 Sprague-Dawley rats, retrograde tracers, Fluoro-Gold or B sub-unit of cholera toxin, were microinjected (5-20nl) into the Mo12, NTS, or both nuclei. Tyrosine hydroxylase (TH) was used as a marker for CA neurons. Following tracer injections into the Mo12, retrogradely labeled and TH-positive neurons were found in the A1/C1 (18.5%), A5 (43.5%), A7 (15.0%), and sub-coeruleus (21.0%) regions, and locus coeruleus (1.7%). In contrast, following injections into the NTS, these proportions were: 48.0, 46.5, 0.2, 0.9, and 4.3%, respectively. The projections to both nuclei were bilateral, with a 3:2 ipsilateral predominance. In four animals with one tracer injected into the Mo12 and the other in NTS, TH-positive cells containing both tracers were found only in the A5 region. Thus, the pontomedullary sources of CA projections to the Mo12 and NTS differ, with only A1/C1 and A5 groups having significant projections to these two functionally distinct targets.

Central pathways of pulmonary and lower airway vagal afferents

Lung sensory receptors with afferent fibers coursing in the vagus nerves are broadly divided into three groups: slowly (SAR) and rapidly (RAR) adapting stretch receptors and bronchopulmonary C fibers. Central terminations of each group are found in largely nonoverlapping regions of the caudal half of the nucleus of the solitary tract (NTS). Second order neurons in the pathways from these receptors innervate neurons located in respiratory-related regions of the medulla, pons, and spinal cord. The relative ease of selective activation of SARs, and to a lesser extent RARs, has allowed for more complete physiological and morphological characterization of the second and higher order neurons in these pathways than for C fibers. A subset of NTS neurons receiving afferent input from SARs (termed pump or P-cells) mediates the Breuer-Hering reflex and inhibits neurons receiving afferent input from RARs. P-cells and second order neurons in the RAR pathway also provide inputs to regions of the ventrolateral medulla involved in control of respiratory motor pattern, i.e., regions containing a predominance of bulbospinal premotor neurons, as well as regions containing respiratory rhythm-generating neurons. Axon collaterals from both P-cells and RAR interneurons, and likely from NTS interneurons in the C-fiber pathway, project to the parabrachial pontine region where they may contribute to plasticity in respiratory control and integration of respiratory control with other systems, including those that provide for voluntary control of breathing, sleep-wake behavior, and emotions.

Central nervous system circuitry involved in the hyperinsulinism syndrome

Raised plasma levels of insulin, glucose and glucagon are found in patients affected by 'hyperinsulinism'. Obesity, hypertension, mammary plus ovary cysts and rheumatic symptoms are frequently observed in these patients. Sleep disorders and depression are also present in most subjects affected by this polysymptomatic disorder. The simultaneous increases of glucose, insulin and glucagon plasma levels seen in these patients indicate that the normal crosstalk between A cells, B cells and D cells is disrupted. With respect to this, it is well known that glucose excites B cells (which secrete insulin) and inhibits A cells (which secrete glucagon), which in turn excites D cells (which secrete somatostatin). Gastrointestinal hormones (incretins) modulate this crosstalk both directly and indirectly throughout pancreatic and hepatobiliary mechanisms. The above factors depend on autonomic nervous system mediation. For instance, acetylcholine released from parasympathetic nerves excites both B and A cells. Noradrenaline released from sympathetic nerves and adrenaline secreted from the adrenal glands inhibit B cells and excite A cells, which are crowded with beta(2)- and alpha(2)-receptors, respectively. Noradrenaline released from sympathetic nerves also excites A cells by acting at alpha(1)-receptors located at this level. According to this, the excessive release of noradrenaline from these nerves should provoke an enhancement of glucagon secretion which will result in overexcitation of insulin secretion from B cells. That is the disorder seen in the so-called 'hyperinsulinism', in which raised plasma levels of glucose, insulin and glucagon coexist. Taking into account that neural sympathetic activity is positively correlated to the A5 noradrenergic nucleus and median raphe serotonergic neurons, and negatively correlated to the A6 noradrenergic, the dorsal raphe serotonergic and the C1 adrenergic neurons, we postulate that this unbalanced central nervous system circuitry is responsible for the hyperinsulinism syndrome.

Noradrenergic, serotonergic and GABAergic antagonists injected together into the XII nucleus abolish the REM sleep-like depression of hypoglossal motoneuronal activity

Recently, we reported that the suppression of hypoglossal (XII) motoneuronal activity that occurs during the carbachol-induced, rapid eye movement (REM) sleep-like state is abolished by the microinjection into the XII nucleus of a drug mix that antagonizes aminergic excitation and amino acid-mediated inhibition (prazosin, methysergide, bicuculline and strychnine). We now assess the role of glycinergic inhibition in the depression of XII motoneuronal activity and estimate the distribution of the antagonists around the XII nucleus at the time when they are effective. Towards the first goal, REM sleep-like episodes were elicited in urethane-anesthetized rats by 10 nl carbachol microinjections into the dorsomedial pons prior to, and at different times after, combined microinjections into the XII nucleus of only three antagonists (strychnine omitted). As in our previous study, the carbachol-induced depression of XII activity was abolished during tests performed 42-88 min after the antagonists, whereas other characteristic effects of carbachol (appearance of hippocampal theta, cortical activation, decreased respiratory rate) remained intact. The depressant effect of carbachol on XII motoneurons partially recovered after 2.5 h. Towards the second goal, using a drug diffusion model, we determined that the tissue concentrations of the antagonists at the time when they were effective were within the range of their selective actions, and the drugs acted within 0.9-1.4 mm from the injection sites, thus within a space containing XII motoneurons and their dendrites. We conclude that antagonism of alpha-adrenergic, serotonergic, and GABA(A) receptors are sufficient to abolish the REM sleep-like atonia of XII motoneurons.

REM sleep-like atonia of hypoglossal (XII) motoneurons is caused by loss of noradrenergic and serotonergic inputs

RATIONALE

Studies of hypoglossal (XII) motoneurons that innervate the genioglossus muscle, an upper airway dilator, suggested that the suppression of upper airway motor tone during REM sleep is caused by withdrawal of excitation mediated by norepinephrine and serotonin.

OBJECTIVES

Our objectives were to determine whether antagonism of aminergic receptors located in the XII nucleus region can abolish the REM sleep-like atonia of XII motoneurons, and whether both serotonergic and noradrenergic antagonists are required to achieve this effect.

METHODS

REM sleep-like episodes were elicited in anesthetized rats by pontine carbachol injections before and at various times after microinjection of prazosin and methysergide combined, or of only one of the drugs, into the XII nucleus.

MEASUREMENTS AND MAIN RESULTS

Spontaneous XII nerve activity was significantly reduced, by 35 to 81%, by each antagonist alone and in combination, indicating that XII motoneurons were under both noradrenergic and serotonergic endogenous excitatory drives. During the 32 to 81 min after microinjections of both antagonists, pontine carbachol caused no depression of XII nerve activity, whereas other characteristic effects (activation of the hippocampal and cortical EEG, and slowing of the respiratory rate) remained intact. A partial recovery of the depressant effect of carbachol then occurred parallel to the recovery of spontaneous XII nerve activity from the depressant effect of the antagonists. Microinjections of either antagonist alone did not eliminate the depressant effect of carbachol.

CONCLUSIONS

The REM sleep-like depression of XII motoneuronal activity induced by pontine carbachol can be fully accounted for by the combined withdrawal of noradrenergic and serotonergic effects on XII motoneurons.

Modulation of the respiratory rhythm generator by the pontine noradrenergic A5 and A6 groups in rodents

The aim of the present review is to summarise available studies dealing with the respiratory control exerted by pontine noradrenergic neurones in neonatal and adult mammals. During the perinatal period, in vitro studies on neonatal rodents have shown that A5 and A6 neurones exert opposite modulations onto the respiratory rhythm generator, inhibitory and facilitatory respectively, that the anatomical support for these modulations already exists at birth, and that genetically induced alterations in the formation of A5 and A6 neurones affect the maturation of the respiratory rhythm generator, leading to lethal respiratory deficits at birth. The A5-A6 modulation of the respiratory rhythm generator is not transient, occurring solely during the perinatal period but it persists throughout life: A5 and A6 neurones display a respiratory-related activity, receive inputs from and send information to the medullary respiratory centres and contribute to the adaptation of adult breathing to physiological needs.

Pontine cholinergic mechanisms and their impact on respiratory regulation

Activation of pontomedullary cholinergic neurons may directly and indirectly cause depression of respiratory motoneuronal activity, activation of respiratory premotor neurons and acceleration of the respiratory rate during REM sleep, as well as activation of breathing during active wakefulness. These effects may be mediated by distinct subpopulations of cholinergic neurons. The relative inactivity of cholinergic neurons during slow-wave sleep also may contribute to the depressant effects of this state on breathing. Cholinergic muscarinic and nicotinic receptors are expressed in central respiratory neurons and motoneurons, thus allowing cholinergic neurons to act on the respiratory system directly. Additional effects of cholinergic activation are mediated indirectly by noradrenergic, serotonergic and other neurons of the reticular formation. Excitatory and suppressant respiratory effects with features of natural states of REM sleep or active wakefulness can be elicited in urethane-anesthetized rats by pontine microinjections of the cholinergic agonist, carbachol. Carbachol models help elucidate the neural basis of respiratory disorders associated with central cholinergic activation.

The modulation by 5-HT of glutamatergic inputs from the raphe pallidus to rat hypoglossal motoneurones, in vitro

Decreases in the activity of 5-HT-containing caudal raphe neurones during sleep are thought to be partially responsible for the resultant disfacilitation of hypoglossal motoneurones. Whilst 5-HT has a direct excitatory action on hypoglossal motoneurones as a result of activation of 5-HT2 receptors, microinjection of 5-HT2 antagonists into the hypoglossal nucleus reduces motor activity to a much lesser extent compared to the suppression observed during sleep suggesting other transmitters co-localised in caudal raphe neurones may also be involved. The aim of the present study was therefore to characterise raphe pallidus inputs to hypoglossal motoneurones. Whole cell recordings were made from hypoglossal motoneurones in vitro. 5-HT evoked a direct membrane depolarisation (8.45 +/- 3.8 mV, P < 0.001) and increase in cell input resistance (53 +/- 40 %, P < 0.001) which was blocked by the 5-HT2 antagonist, ritanserin (2.40 +/- 2.7 vs. 7.04 +/- 4.6 mV). Stimulation within the raphe pallidus evoked a monosynaptic EPSC that was significantly reduced by the AMPA/kainate antagonist, NBQX (22.8 +/- 16 % of control, P < 0.001). In contrast, the 5-HT2 antagonist, ritanserin, had no effect on the amplitude of these EPSCs (106 +/- 31 % of control, P = n.s.). 5-HT reduced these EPSCs to 50.0 +/- 13 % of control (P < 0.001), as did the 5-HT1A agonist, 8-OH-DPAT (52.5 +/- 17 %, P < 0.001) and the 5-HT1B agonist, CP 93129 (40.6 +/- 29 %, P < 0.01). 8-OH-DPAT and CP 93129 increased the paired pulse ratio (1.38 +/- 0.27 to 1.91 +/- 0.54, P < 0.05 & 1.27 +/- 0.08 to 1.44 +/- 0.13, P < 0.01 respectively) but had no effect on the postsynaptic glutamate response (99 +/- 4.4 % and 100 +/- 2.5 %, P = n.s.). They also increased the frequency (P < 0.001), but not the amplitude, of miniature glutamatergic EPSCs in hypoglossal motoneurones. These data demonstrate that raphe pallidus inputs to hypoglossal motoneurones are predominantly glutamatergic in nature, with 5-HT decreasing the release of glutamate from these projections as a result of activation of 5-HT1A and/or 5-HT1B receptors located on presynaptic terminals.

Elimination of the post-hypoxic frequency decline in conscious rats lesioned in pontine A5 region

A decrease in the frequency of breathing following a hypoxic exposure that is below baseline values is called the post-hypoxic frequency decline (phfd) and is due to an elongation of expiratory time (TE). We hypothesized that lesioning the pontine A5 region would eliminate the phfd in conscious rats. Fourteen conscious male rats that demonstrated a phfd received lesions either within the A5 region (n=9) or outside this region (controls, n=5). Compared with pre-lesion values, body temperature decreased and frequency of breathing was lower during exposure to air, hypoxia, and hypercapnia in A5-lesioned, but not in the control-lesioned rats. No effect of A5 lesions was noted on tidal volume. Rats with A5 lesions no longer exhibited a phfd, and TE values following hypoxia were comparable to baseline TE values. These data suggest that the A5 region of the ventrolateral pons modulates the phfd in conscious rats and affects frequency of breathing in response to both hypoxia and hypercapnia.