Oxytocin mediated excitation of hypoglossal motoneurons: implications for treating obstructive sleep apnea

Clinical studies have shown that oxytocin administered intranasally (IN) decreased the incidence and duration of obstructive events in patients with obstructive sleep apnea (OSA). Although the mechanisms by which oxytocin promotes these beneficial effects are unknown, one possible target of oxytocin could be the excitation of tongue-projecting hypoglossal motoneurons in the medulla, that exert central control of upper airway patency. This study tested the hypothesis that IN oxytocin enhances tongue muscle activity via the excitation of hypoglossal motoneurons projecting to tongue protrudor muscles (PMNs). To test this hypothesis we performed in vivo and in vitro electrophysiological studies in C57BL6/J mice as well as fluorescent imaging studies in transgenic mice in which neurons that express oxytocin receptors co-express fluorescent protein. IN oxytocin significantly increased the amplitude of inspiratory-related tongue muscle activity. This effect was abolished by severing the medial branch of hypoglossal nerve that innervates PMNs of the tongue. Oxytocin receptor-positive neurons were more prevalent in the population of PMNs than in retractor-projecting hypoglossal motoneurons (RMNs). Oxytocin administration increased action potential firing in PMNs, but had no significant effect on firing activity in RMNs. In conclusion, IN oxytocin stimulates respiratory-relating tongue muscle activity likely acting on central hypoglossal motoneurons that provide tongue protrusion and upper airway opening. This mechanism may play a role in oxytocin-induced reductions in upper airway obstructions in patients with OSA.

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.

GABA and glycine neurons from the ventral medullary region inhibit hypoglossal motoneurons

Obstructive sleep apnea (OSA) is a common disorder characterized by repetitive sleep-related losses of upper airway patency that occur most frequently during rapid eye movement (REM) sleep. Hypoglossal motoneurons play a key role in regulating upper airway muscle tone and patency during sleep. REM sleep activates GABA and glycine neurons in the ventral medulla (VM) to induce cortical desynchronization and skeletal muscle atonia during REM sleep; however, the role of this brain region in modulating hypoglossal motor activity is unknown. We combined optogenetic and chemogenetic approaches with in-vitro and in-vivo electrophysiology, respectfully, in GAD2-Cre mice of both sexes to test the hypothesis that VM GABA/glycine neurons control the activity of hypoglossal motoneurons and tongue muscles. Here, we show that there is a pathway originating from GABA/glycine neurons in the VM that monosynaptically inhibits brainstem hypoglossal motoneurons innervating both tongue protruder genioglossus (GMNs) and retractor (RMNs) muscles. Optogenetic activation of ChR2-expressing fibers induced a greater postsynaptic inhibition in RMNs than in GMNs. In-vivo chemogenetic activation of VM GABA/glycine neurons produced an inhibitory effect on tongue electromyographic (EMG) activity, decreasing both the amplitude and duration of inspiratory-related EMG bursts without any change in respiratory rate. These results indicate that activation of GABA/glycine neurons from the VM inhibits tongue muscles via a direct pathway to both GMNs and RMNs. This inhibition may play a role in REM sleep associated upper airway obstructions that occur in patients with OSA.

Designer Receptors Exclusively Activated by Designer Drugs Approach to Treatment of Sleep-disordered Breathing

Rationale: Obstructive sleep apnea is recurrent upper airway obstruction caused by a loss of upper airway muscle tone during sleep. The main goal of our study was to determine if designer receptors exclusively activated by designer drugs (DREADD) could be used to activate the genioglossus muscle as a potential novel treatment strategy for sleep apnea. We have previously shown that the prototypical DREADD ligand clozapine-N-oxide increased pharyngeal diameter in mice expressing DREADD in the hypoglossal nucleus. However, the need for direct brainstem viral injections and clozapine-N-oxide toxicity diminished translational potential of this approach, and breathing during sleep was not examined.Objectives: Here, we took advantage of our model of sleep-disordered breathing in diet-induced obese mice, retrograde properties of the adeno-associated virus serotype 9 (AAV9) viral vector, and the novel DREADD ligand J60.Methods: We administered AAV9-hSyn-hM3(Gq)-mCherry or control AAV9 into the genioglossus muscle of diet-induced obese mice and examined the effect of J60 on genioglossus activity, pharyngeal patency, and breathing during sleep.Measurements and Main Results: Compared with control, J60 increased genioglossus tonic activity by greater than sixfold and tongue uptake of 2-deoxy-2-[¹⁸F]fluoro-d-glucose by 1.5-fold. J60 increased pharyngeal patency and relieved upper airway obstruction during non-REM sleep.Conclusions: We conclude that following intralingual administration of AAV9-DREADD, J60 can activate the genioglossus muscle and improve pharyngeal patency and breathing during sleep.

Subunit-Specific Photocontrol of Glycine Receptors by Azobenzene-Nitrazepam Photoswitcher

Photopharmacology is a unique approach that through a combination of photochemistry methods and advanced life science techniques allows the study and control of specific biological processes, ranging from intracellular pathways to brain circuits. Recently, a first photochromic channel blocker of anion-selective GABA_A receptors, the azobenzene-nitrazepam-based photochromic compound (Azo-NZ1), has been described. In the present study, using patch-clamp technique in heterologous system and in mice brain slices, site-directed mutagenesis and molecular modeling we provide evidence of the interaction of Azo-NZ1 with glycine receptors (GlyRs) and determine the molecular basis of this interaction. Glycinergic synaptic neurotransmission determines an important inhibitory drive in the vertebrate nervous system and plays a crucial role in the control of neuronal circuits in the spinal cord and brain stem. GlyRs are involved in locomotion, pain sensation, breathing, and auditory function, as well as in the development of such disorders as hyperekplexia, epilepsy, and autism. Here, we demonstrate that Azo-NZ1 blocks in a UV-dependent manner the activity of α2 GlyRs (GlyR2), while being barely active on α1 GlyRs (GlyR1). The site of Azo-NZ1 action is in the chloride-selective pore of GlyR at the 2’ position of transmembrane helix 2 and amino acids forming this site determine the difference in Azo-NZ1 blocking activity between GlyR2 and GlyR1. This subunit-specific modulation is also shown on motoneurons of brainstem slices from neonatal mice that switch during development from expressing “fetal” GlyR2 to “adult” GlyR1 receptors.

Alfaxalone Causes Reduction of Glycinergic IPSCs, but Not Glutamatergic EPSCs, and Activates a Depolarizing Current in Rat Hypoglossal Motor Neurons

We investigated effects of the neuroactive steroid anesthetic alfaxalone on intrinsic excitability, and on inhibitory and excitatory synaptic transmission to hypoglossal motor neurons (HMNs). Whole cell recordings were made from HMNs in brainstem slices from 7 to 14-day-old Wistar rats. Spontaneous, miniature, and evoked inhibitory post-synaptic currents (IPSCs), and spontaneous and evoked excitatory PSCs (EPSCs) were recorded at –60 mV. Alfaxalone did not alter spontaneous glycinergic IPSC peak amplitude, rise-time or half-width up to 10 μM, but reduced IPSC frequency from 3 μM. Evoked IPSC amplitude was reduced from 30 nM. Evoked IPSC rise-time was prolonged and evoked IPSC decay time was increased only by 10 μM alfaxalone. Alfaxalone also decreased evoked IPSC paired pulse ratio (PPR). Spontaneous glutamatergic EPSC amplitude and frequency were not altered by alfaxalone, and evoked EPSC amplitude and PPR was also unchanged. Alfaxalone did not alter HMN repetitive firing or action potential amplitude. Baseline holding current at −60 mV with a CsCl-based pipette solution was increased in an inward direction; this effect was not seen when tetrodotoxin (TTX) was present. These results suggest that alfaxalone modulates glycine receptors (GlyRs), causing a delayed and prolonged channel opening, as well as causing presynaptic reduction of glycine release, and activates a membrane current, which remains to be identified. Alfaxalone selectively reduces glycinergic inhibitory transmission to rat HMNs via a combination of pre- and post-synaptic mechanisms. The net effect of these responses to alfaxalone is to increase HMN excitability and may therefore underlie neuro-motor excitation during neurosteroid anesthesia.

Developmental Changes in the Inhibition of Glycinergic Synaptic Currents by Niflumic Acid in Hypoglossal Motoneurons

Mammalian brainstem hypoglossal motoneurones (HMs) receive powerful synaptic glycinergic inputs and are involved in a variety of motor functions, including respiration, chewing, sucking, swallowing, and phonation. During the early postnatal development, subunit composition of chloride-permeable glycine receptors (GlyRs) changes leading to a decrease of "fetal" alpha2 and elevation of "adult" alpha1 GlyR subunits. It has been recently demonstrated that niflumic acid (NFA), a member of the fenamate class of non-steroidal anti-inflammatory drugs, is an efficient subunits-specific blocker of GlyRs. At a heterologous expression of different GlyR subunits it has been shown that blocking potency of NFA is more than one order higher for alpha2 GlyRs than for receptors formed by alpha1 subunit. To reveal the action of NFA on the synaptic activity we analyzed here the effects of NFA on the glycinergic inhibitory post-synaptic currents in the HMs from mouse brainstem slices. In the whole-cell patch clamp configuration, the amplitude and the frequency of glycinergic synaptic currents from two age groups have been analyzed: "neonate" (P2-P4) and "juvenile" (P7-P12). Addition of NFA in the presence of antagonists of glutamate and GABA receptors caused a decrease in the mean amplitude and frequency of synaptic events. The degree of the inhibition induced by NFA decreased with the postnatal development, being higher on the motoneurons from "neonate" brainstem slices in comparison with the "juvenile" age group. Analysis of the pair-pulse facilitation suggests the post-synaptic origin of NFA action. These observations provide evidence on the developmental changes in the inhibition by NFA of glycinergic synaptic transmission, which reflects increase in the alpha1 and decrease in the alpha2 GlyR subunits expression in synapses to hypoglossal motoneurons during the early stages of postnatal life.

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.

Modulation of Motoneuronal Activity With Sleep-Wake States and Motoneuronal Gene Expression Vary With Circadian Rest-Activity Cycle

In both nocturnal and diurnal mammals, sleep and wake states differentially aggregate during the rest and active phases of circadian cycle. Closely associated with this rhythm are prominent changes in motor activity. Here, we quantified the magnitudes of electromyographic activity (EMG) measured separately during different sleep-wake states across the rest-activity cycle, thereby separating amplitude measurements from the known dependance of the timing of wake and sleep on the phase of circadian rest-activity cycle. In seven rats chronically instrumented for electroencephalogram and EMG monitoring, nuchal and lingual muscle EMGs were measured as a commonly used postural output in behavioral sleep studies and as a cranial motor output with potential clinical relevance in obstructive sleep apnea (OSA) syndrome, respectively. We found that, for both motor outputs, EMG measured during wake episodes was significantly higher during the active phase, than during the rest phase, of circadian cycle. The corresponding patterns observed during slow-wave sleep (SWS) and rapid eye movement sleep (REMS) were different. During SWS, lingual EMG was very low and did not differ between the rest and active phase, whereas nuchal EMG had pattern similar to that during wakefulness. During REMS, lingual EMG was, paradoxically, higher during the rest phase due to increased twitching activity, whereas nuchal EMG was very low throughout the rest and active periods (postural atonia). In the follow-up comparison of differences in transcript levels in tissue samples obtained from the medullary hypoglossal motor nucleus and inferior olive (IO) at rest onset and active period onset conducted using microarrays, we identified significant differences for multiple transcripts representing the core members of the molecular circadian clock and other genes important for the regulation of cell metabolism and activity (up to n = 130 at p < 0.001). Collectively, our data indicate that activity of motoneurons is regulated to optimally align it with the rest-activity cycle, with the process possibly involving transcriptional mechanisms at the motoneuronal level. Our data also suggest that OSA patients may be relatively better protected against sleep-related upper airway obstructions during REMS episodes generated during the rest phase, than during active phase, of the circadian cycle.

Contribution of Neurochemical Inputs to the Decrease of Motoneuron Excitability During Non-REM and REM Sleep: A Systematic Review

The sleep-related depression of excitability of upper airway motoneurons is a major neurological cause of obstructive sleep apnea whereas a disruption in the inhibition of spinal motoneurons during rapid eye movement (REM) sleep causes the REM sleep behavioral disorder. The large amount of experimental data has been obtained that deal with neurochemical mechanisms that are responsible for sleep-related depression of various motoneuron groups. However, there is a disagreement regarding the outcome of these studies primarily due to the use of different animal models and approaches, as well as due to differences in quantification and interpretation of obtained results. In this study, we sought to apply the same calculation methodology in order to uniformly quantify and compare the relative contribution of excitatory or inhibitory inputs to the decrease of excitability of different motoneuronal pools during REM and/or non-REM sleep. We analyzed only published quantitative data that were obtained by using receptor antagonists or chemogenetic approach to block receptors or silence neuronal populations. The outcomes of this analysis highlight the differences in the neurotransmitter mechanisms of sleep-related motoneuron depression between different motoneuronal pools and demonstrate the consistency of these mechanisms for hypoglossal motoneurons among various animal models.

Computational model of brain-stem circuit for state-dependent control of hypoglossal motoneurons

In patients with obstructive sleep apnea (OSA), the pharyngeal muscles become relaxed during sleep, which leads to a partial or complete closure of upper airway. Experimental studies suggest that withdrawal of noradrenergic and serotonergic drives importantly contributes to depression of hypoglossal motoneurons and, therefore, may contribute to OSA pathophysiology; however, specific cellular and synaptic mechanisms remain unknown. In this new study, we developed a biophysical network model to test the hypothesis that, to explain experimental observations, the neuronal network for monoaminergic control of excitability of hypoglossal motoneurons needs to include excitatory and inhibitory perihypoglossal interneurons that mediate noradrenergic and serotonergic drives to hypoglossal motoneurons. In the model, the state-dependent activation of the hypoglossal motoneurons was in qualitative agreement with in vivo data during simulated rapid eye movement (REM) and non-REM sleep. The model was applied to test the mechanisms of action of noradrenergic and serotonergic drugs during REM sleep as observed in vivo. We conclude that the proposed minimal neuronal circuit is sufficient to explain in vivo data and supports the hypothesis that perihypoglossal interneurons may mediate state-dependent monoaminergic drive to hypoglossal motoneurons. The population of the hypothesized perihypoglossal interneurons may serve as novel targets for pharmacological treatment of OSA. NEW & NOTEWORTHY In vivo studies suggest that during rapid eye movement sleep, withdrawal of noradrenergic and serotonergic drives critically contributes to depression of hypoglossal motoneurons (HMs), which innervate the tongue muscles. By means of a biophysical model, which is consistent with a broad range of empirical data, we demonstrate that the neuronal network controlling the excitability of HMs needs to include excitatory and inhibitory interneurons that mediate noradrenergic and serotonergic drives to HMs.

Developmental nicotine exposure alters potassium currents in hypoglossal motoneurons of neonatal rat

We previously showed that nicotine exposure in utero and after birth via breast milk [developmental nicotine exposure (DNE)] is associated with many changes in the structure and function of hypoglossal motoneurons (XIIMNs), including a reduction in the size of the dendritic arbor and an increase in cell excitability. Interestingly, the elevated excitability was associated with a reduction in the expression of glutamate receptors on the cell body. Together, these observations are consistent with a homeostatic compensation aimed at restoring cell excitability. Compensation for increased cell excitability could also occur by changing potassium conductance, which plays a critical role in regulating resting potential, spike threshold, and repetitive spiking behavior. Here we test the hypothesis that the previously observed increase in the excitability of XIIMNs from DNE animals is associated with an increase in whole cell potassium currents. Potassium currents were measured in XIIMNs in brain stem slices derived from DNE and control rat pups ranging in age from 0 to 4 days by whole cell patch-clamp electrophysiology. All currents were measured after blockade of action potential-dependent synaptic transmission with tetrodotoxin. Compared with control cells, XIIMNs from DNE animals showed significantly larger transient and sustained potassium currents, but this was observed only under conditions of increased cell and network excitability, which we evoked by raising extracellular potassium from 3 to 9 mM. These observations suggest that the larger potassium currents in nicotine-exposed neurons are an important homeostatic compensation that prevents "runaway" excitability under stressful conditions, when neurons are receiving elevated excitatory synaptic input.NEW & NOTEWORTHY Developmental nicotine exposure is associated with increased cell excitability, which is often accompanied by compensatory changes aimed at normalizing excitability. Here we show that whole cell potassium currents are also increased in hypoglossal motoneurons from nicotine-exposed neonatal rats under conditions of increased cell and network excitability. This is consistent with a compensatory response aimed at preventing instability under conditions in which excitatory synaptic input is high and is compatible with the concept of homeostatic plasticity.

Presynaptic facilitation of glycinergic mIPSC is reduced in mice lacking α3 glycine receptor subunits

Glycinergic neurons provide an important mechanism to control excitation of motoneurons in the brainstem and a reduction or loss of glycinergic inhibition can be deleterious by leading to hyperexcitation such as in hyperekplexia or neurodegeneration and neuronal death as in amyotrophic lateral sclerosis (ALS). Second messenger systems that change cyclic AMP and lead to phosphorylation of the α3 subunit of the glycine receptor (GlyR α3) have been shown to be potent modulators of synaptic inhibition in the spinal cord and brain stem. In this study we analyzed the role of GlyR α3 in synaptic inhibition to the hypoglossal nucleus using Glra3 (the gene encoding the glycine receptor α3 subunit) knockout mice. We observed that baseline glycinergic synaptic transmission to nucleus of hypoglossal motoneurons is rather normal in Glra3 knockout mice. Interestingly, we found that the modulation of synaptic transmission by cAMP-mediated pathways appeared to be reduced in Glra3 knockout mice. In the second postnatal week the forskolin-induced increase of miniature inhibitory postsynaptic potential (mIPSC) frequency was significantly larger in control as compared to Glra3 knockout mice suggesting that presynaptic glycine release in the hypoglossal nucleus is partially depending on GlyR α3.

Role of brain-derived neurotrophic factor in the excitatory-inhibitory imbalance during the critical period of postnatal respiratory development in the rat

The critical period of respiratory development in rats is a narrow window toward the end of the second postnatal week (P12–13), when abrupt neurochemical, electrophysiological, and ventilatory changes occur, when inhibition dominates over excitation, and when the animals’ response to hypoxia is the weakest. The goal of this study was to further test our hypothesis that a major mechanism underlying the synaptic imbalance during the critical period is a reduced expression of brain-derived neurotrophic factor (BDNF) and its TrkB receptors. Our aims were to determine (1) that the inhibitory dominance observed in hypoglossal motoneurons during the critical period was also demonstrable in a key respiratory chemosensor, NTS_VL; (2) if in vivo application of a TrkB agonist, 7,8-DHF, would prevent, but a TrkB antagonist, ANA-12, would accentuate the synaptic imbalance; and (3) if hypoxia would also heighten the imbalance. Our results indicate that (1) the synaptic imbalance was evident in the NTS_VL during the critical period; (2) intraperitoneal injections of 7,8-DHF prevented the synaptic imbalance during the critical period, whereas ANA-12 in vivo accentuated such an imbalance; and (3) acute hypoxia induced the weakest response in both the amplitude and frequency of sEPSCs during the critical period, but it increased the frequency of sIPSCs during the critical period. Thus, our findings are consistent with and strengthen our hypothesis that BDNF and TrkB play a significant role in inducing a synaptic imbalance during the critical period of respiratory development in the rat.

Developmental nicotine exposure enhances inhibitory synaptic transmission in motor neurons and interneurons critical for normal breathing

Nicotine exposure in utero negatively affects neuronal growth, differentiation, and synaptogenesis. We used rhythmic brainstems slices and immunohistochemistry to determine how developmental nicotine exposure (DNE) alters inhibitory neurotransmission in two regions essential to normal breathing, the hypoglossal motor nucleus (XIIn), and preBötzinger complex (preBötC). We microinjected glycine or muscimol (GABAA agonist) into the XIIn or preBötC of rhythmic brainstem slices from neonatal rats while recording from XII nerve roots to obtain XII motoneuron population activity. Injection of glycine or muscimol into the XIIn reduced XII nerve burst amplitude, while injection into the preBötC altered nerve burst frequency. These responses were exaggerated in preparations from DNE animals. Quantitative immunohistochemistry revealed a significantly higher GABAA receptor density on XII motoneurons from DNE pups. There were no differences in GABAA receptor density in the preBötC, and there were no differences in glycine receptor expression in either region. Nicotine, in the absence of other chemicals in tobacco smoke, alters normal development of brainstem circuits that are critical for normal breathing.

Using a computational model to analyze the effects of firing frequency on synchrony of a network of gap junction-coupled hypoglossal motoneurons

Hypoglossal motoneurons (HMs) are located in the brainstem and play an important role in the maintenance of upper airway patency. HMs are known to be coupled to one another via gap junctions and exhibit synchronous firing behavior when driven by premotor inputs. In the current study, we used a computational model to analyze the influence of firing frequency on synchronous firing behavior of a network of gap junction-coupled HMs. As there are many factors that can influence excitability and firing frequency of HMs, our simulations were focused on the effects of modulation of SK channel conductance and the magnitude of the input currents as a means of modifying firing frequency. Our simulations revealed that regardless of the mechanism by which firing frequency was modulated, increasing the firing frequency disrupted the synchrony in gap junction-coupled HM networks with low, medium, and high levels of gap junction coupling.

Effects of calcium (Ca(2+)) extrusion mechanisms on electrophysiological properties in a hypoglossal motoneuron: insight from a mathematical model

Spike-frequency dynamics and spike shape can provide insight into the types of ion channels present in any given neuron and give a sense for the precise response any neuron may have to a given input stimulus. Motoneuron firing frequency over time is especially important due to its direct effect on motor output. Of particular interest is intracellular Ca(2+), which exerts a powerful influence on both firing properties over time and spike shape. In order to better understand the cellular mechanisms for the regulation of intracellular Ca(2+) and their effect on spiking behavior, we have modified a computational model of an HM to include a variety of Ca(2+) handling processes. For the current study, a series of HM models that include Ca(2+) pumps, Na(+)/Ca(2+) exchangers, and a generic exponential decay of excess Ca(2+) were generated. Simulations from these models indicate that although each extrusion mechanism exerts a similar effect on voltage, the firing properties change distinctly with the inclusion of additional Ca(2+)-related mechanisms: BK channels, Ca(2+) buffering, and diffusion of [Ca(2+)]i modeled via a linear diffusion partial differential equation. While an exponential decay of Ca(2+) seems to adequately capture short-term changes in firing frequency seen in biological data, internal diffusion of Ca(2+) appears to be necessary for capturing longer term frequency changes.

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.

Mesopontine cholinergic projections to the hypoglossal motor nucleus

Mesopontine cholinergic (ACh) neurons have increased discharge during wakefulness, rapid eye movement (REM) sleep, or both. Hypoglossal (12) motoneurons, which play an important role in the control of upper airway patency, are postsynaptically excited by stimulation of nicotinic receptors, whereas muscarinic receptors presynaptically inhibit inputs to 12 motoneurons. These data suggest that ACh contributes to sleep/wake-related changes in the activity of 12 motoneurons by acting within the hypoglossal motor nucleus (Mo12), but the origins of ACh projections to Mo12 are not well established. We used retrograde tracers to assess the projections of ACh neurons of the mesopontine pedinculopontine tegmental (PPT) and laterodorsal tegmental (LDT) nuclei to the Mo12. In six Sprague-Dawley rats, Fluorogold or B subunit of cholera toxin, were pressure injected (5-20nl) into the Mo12. Retrogradely labeled neurons, identified as ACh using nitric oxide synthase (NOS) immunohistochemistry, were found bilaterally in discrete subregions of both PPT and LDT nuclei. Most retrogradely labeled PPT cells (96%) were located in the PPT pars compacta region adjacent to the ventrolateral tip of the superior cerebellar peduncle. In the LDT, retrogradely labeled neurons were located exclusively in its pars alpha region. Over twice as many ACh neurons projecting to the Mo12 were located in the PPT than LDT. The results demonstrate direct mesopontine ACh projections to the Mo12. These projections may contribute to the characteristic of wakefulness and REM sleep increases, as well as REM sleep-related decrements, of 12 motoneuronal activity.

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.

The nonuniform distribution of the GABA(A) receptor alpha 1 subunit influences inhibitory synaptic transmission to motoneurons within a motor nucleus

Using immunohistochemistry we studied the distribution of GABA(A) and glycine receptor alpha1 subunits in the rat hypoglossal nucleus during postnatal development. In the neonate [postnatal day (P) 1-3] and adult nucleus (P28-30), GABA(A) receptor alpha1 subunit labeling was relatively modest. However, in the juvenile nucleus (P9-13), labeling was strong in the ventrolateral region and moderate in the dorsal region. Glycine receptor alpha1 subunit labeling was strong and uniform in the juvenile and adult nucleus and absent in the neonate nucleus. GABA and glycine neurotransmitter labeling was uniform throughout the neonatal and juvenile nucleus. To study the functional consequences of this regional differential GABA(A) receptor alpha1 subunit distribution, we voltage clamped juvenile hypoglossal motoneurons (HMs) from the ventrolateral and dorsal regions and recorded spontaneous miniature IPSCs (mIPSCs). Pure GABAergic events had slower decay times than glycinergic events. Although pure GABAergic and glycinergic decay times did not differ depending on HM location, the decays of mixed mIPSCs from ventrolateral HMs, recorded without GABA(A) and glycine receptor antagonists, had significantly slower decays than mIPSCs from dorsal HMs. Focally applied GABA and glycine onto outside-out patches revealed that the GABAergic to glycinergic peak current amplitude ratio was larger for patches from ventrolateral HMs compared with dorsal HMs. Dual component mIPSCs, presumably caused by co-release of GABA and glycine, were recorded more frequently in the ventrolateral nucleus. These data suggest that the number of synapses using GABA(A) receptor-mediated transmission is greater on ventrolateral HMs than dorsal HMs, demonstrating a nonuniformity of synaptic function within a defined motor nucleus.