Glutamatergic neurons in the Kölliker-Fuse nucleus project to the rostral ventral respiratory group and phrenic nucleus: a combined retrograde tracing and in situ hybridization study in the rat

Substantia Nigra Dopamine Neuronal Responses to Habenular Stimulation and Foot Shock Are Altered by Lesions of the Rostromedial Tegmental Nucleus

Dopamine (DA) neurons of the substantia nigra (SN) and ventral tegmental area generally respond to aversive stimuli or the absence of expected rewards with transient inhibition of firing rates, which can be recapitulated with activation of the lateral habenula (LHb) and eliminated by lesioning the intermediating rostromedial tegmental nucleus (RMTg). However, a minority of DA neurons respond to aversive stimuli, such as foot shock, with a transient increase in firing rate, an outcome that rarely occurs with LHb stimulation. The degree to which individual neurons respond to these two stimulation modalities with the same response phenotype and the role of the RMTg is not known. Here, we record responses from single SN DA neurons to alternating activation of the LHb and foot shock in male rats. Lesions of the RMTg resulted in a shift away from inhibition to no response during both foot shock and LHb stimulation. Furthermore, lesions unmasked an excitatory response during LHb stimulation. The response correspondence within the same neuron between the two activation sources was no different from chance in sham controls, suggesting that external inputs rather than intrinsic DA neuronal properties are more important to response outcome. These findings contribute to a literature that shows a complex neurocircuitry underlies the regulation of DA activity and, by extension, behaviors related to learning, anhedonia, and cognition.

Inhibitory synaptic transmission is impaired in the Kölliker-Fuse of male, but not female, Rett syndrome mice

Rett syndrome (RTT) is a severe neurodevelopmental disorder that mainly affects females due to silencing mutations in the X-linked MECP2 gene. One of the most troubling symptoms of RTT is breathing irregularity, including apneas, breath-holds, and hyperventilation. Mice with silencing mutations in Mecp2 exhibit breathing abnormalities similar to human patients and serve as useful models for studying mechanisms underlying breathing problems in RTT. Previous work implicated the pontine, respiratory-controlling Kölliker-Fuse (KF) in the breathing problems in RTT. The goal of this study was to test the hypothesis that inhibitory synaptic transmission is deficient in KF neurons from symptomatic male and female RTT mice. We performed whole cell voltage-clamp recordings from KF neurons in acute brain slices to examine spontaneous and electrically evoked inhibitory post-synaptic currents (IPSCs) in RTT mice and age- and sex-matched wild-type mice. The frequency of spontaneous IPSCs was reduced in KF neurons from male RTT mice but surprisingly not in female RTT mice. In addition, electrically evoked IPSCs were less reliable in KF neurons from male, but not female, RTT mice, which was positively correlated with paired-pulse facilitation, indicating decreased probability of release. KF neurons from male RTT mice were also more excitable and exhibited shorter-duration action potentials. Increased excitability of KF neurons from male mice was not explained by changes in axon initial segment length. These findings indicate impaired inhibitory neurotransmission and increased excitability of KF neurons in male but not female RTT mice and suggest that sex-dependent mechanisms contribute to breathing problems in RTT.NEW & NOTEWORTHY Kölliker-Fuse (KF) neurons in acute brain slices from male Rett syndrome (RTT) mice receive reduced inhibitory synaptic inputs compared with wild-type littermates. In female RTT mice, inhibitory transmission was not different in KF neurons compared with controls. The results from this study show that sex-specific alterations in synaptic transmission occur in the KF of RTT mice.

The integrated brain network that controls respiration

Respiration is a brain function on which our lives essentially depend. Control of respiration ensures that the frequency and depth of breathing adapt continuously to metabolic needs. In addition, the respiratory control network of the brain has to organize muscular synergies that integrate ventilation with posture and body movement. Finally, respiration is coupled to cardiovascular function and emotion. Here, we argue that the brain can handle this all by integrating a brainstem central pattern generator circuit in a larger network that also comprises the cerebellum. Although currently not generally recognized as a respiratory control center, the cerebellum is well known for its coordinating and modulating role in motor behavior, as well as for its role in the autonomic nervous system. In this review, we discuss the role of brain regions involved in the control of respiration, and their anatomical and functional interactions. We discuss how sensory feedback can result in adaptation of respiration, and how these mechanisms can be compromised by various neurological and psychological disorders. Finally, we demonstrate how the respiratory pattern generators are part of a larger and integrated network of respiratory brain regions.

Molecular and anatomical characterization of parabrachial neurons and their axonal projections

The parabrachial nucleus (PBN) is a major hub that receives sensory information from both internal and external environments. Specific populations of PBN neurons are involved in behaviors including food and water intake, nociceptive responses, breathing regulation, as well as learning and responding appropriately to threatening stimuli. However, it is unclear how many PBN neuron populations exist and how different behaviors may be encoded by unique signaling molecules or receptors. Here we provide a repository of data on the molecular identity, spatial location, and projection patterns of dozens of PBN neuron subclusters. Using single-cell RNA sequencing, we identified 21 subclusters of neurons in the PBN and neighboring regions. Multiplexed in situ hybridization showed many of these subclusters are enriched within specific PBN subregions with scattered cells in several other regions. We also provide detailed visualization of the axonal projections from 21 Cre-driver lines of mice. These results are all publicly available for download and provide a foundation for further interrogation of PBN functions and connections.

Intermittent hypoxia and respiratory recovery in pre-clinical rodent models of incomplete cervical spinal cord injury

Impaired respiratory function is a common and devastating consequence of cervical spinal cord injury. Accordingly, the development of safe and effective treatments to restore breathing function is critical. Acute intermittent hypoxia has emerged as a promising therapeutic strategy to treat respiratory insufficiency in individuals with spinal cord injury. Since the original report by Bach and Mitchell (1996) concerning long-term facilitation of phrenic motor output elicited by brief, episodic exposure to reduced oxygen, a series of studies in animal models have led to the realization that acute intermittent hypoxia may have tremendous potential for inducing neuroplasticity and functional recovery in the injured spinal cord. Advances in our understanding of the neurobiology of acute intermittent hypoxia have prompted us to begin to explore its effects in human clinical studies. Here, we review the basic neurobiology of the control of breathing and the pathophysiology and respiratory consequences of two common experimental models of incomplete cervical spinal cord injury (i.e., high cervical hemisection and mid-cervical contusion). We then discuss the impact of acute intermittent hypoxia on respiratory motor function in these models: work that has laid the foundation for translation of this promising therapeutic strategy to clinical populations. Lastly, we examine the limitations of these animal models and intermittent hypoxia and discuss how future work in animal models may further advance the translation and therapeutic efficacy of this treatment.

Medial Parabrachial Nucleus Is Essential in Controlling Wakefulness in Rats

Activation of the parabrachial nucleus (PB) in the brainstem induced wakefulness in rats, suggesting which is an important nucleus that controls arousal. However, the sub-regions of PB in regulating sleep-wake cycle is still unclear. Here, we employ chemogenetics and optogenetics strategies and find that activation of the medial part of PB (MPB), but not the lateral part, induces continuous wakefulness for 10 h without sleep rebound in neither sleep amount nor the power spectra. Optogenetic activation of glutamatergic MPB neurons in sleeping rats immediately wake rats mediated by the basal forebrain (BF) and lateral hypothalamus (LH), but not the ventral medial thalamus. Most importantly, chemogenetic inhibition of PB neurons decreases wakefulness for 10 h. Conclusively, these findings indicate that the glutamatergic MPB neurons are essential in controlling wakefulness, and that MPB-BF and MPB-LH pathways are the major neuronal circuits.

Neurochemistry of the Kölliker-Fuse nucleus from a respiratory perspective

The Kölliker-Fuse nucleus (KF) is a functionally distinct component of the parabrachial complex, located in the dorsolateral pons of mammals. The KF has a major role in respiration and upper airway control. A comprehensive understanding of the KF and its contributions to respiratory function and dysfunction requires an appreciation for its neurochemical characteristics. The goal of this review is to summarize the diverse neurochemical composition of the KF, focusing on the neurotransmitters, neuromodulators, and neuropeptides present. We also include a description of the receptors expressed on KF neurons and transporters involved in each system, as well as their putative roles in respiratory physiology. Finally, we provide a short section reviewing the literature regarding neurochemical changes in the KF in the context of respiratory dysfunction observed in SIDS and Rett syndrome. By over-viewing the current literature on the neurochemical composition of the KF, this review will serve to aid a wide range of topics in the future research into the neural control of respiration in health and disease.

Inspiring song: The role of respiratory circuitry in the evolution of vertebrate vocal behavior

Vocalization is a common means of communication across vertebrates, but the evolutionary origins of the neural circuits controlling these behaviors are not clear. Peripheral mechanisms of sound production vary widely: fish produce sounds with a swimbladder or pectoral fins; amphibians, reptiles, and mammalians vocalize using a larynx; birds vocalize with a syrinx. Despite the diversity of vocal effectors across taxa, there are many similarities in the neural circuits underlying the control of these organs. Do similarities in vocal circuit structure and function indicate that vocal behaviors first arose in a single common ancestor, or have similar neural circuits arisen independently multiple times during evolution? In this review, we describe the hindbrain circuits that are involved in vocal production across vertebrates. Given that vocalization depends on respiration in most tetrapods, it is not surprising that vocal and respiratory hindbrain circuits across distantly related species are anatomically intermingled and functionally linked. Such vocal-respiratory circuit integration supports the hypothesis that vocal evolution involved the expansion and functional diversification of breathing circuits. Recent phylogenetic analyses, however, suggest vocal behaviors arose independently in all major tetrapod clades, indicating that similarities in vocal control circuits are the result of repeated co-options of respiratory circuits in each lineage. It is currently unknown whether vocal circuits across taxa are made up of homologous neurons, or whether vocal neurons in each lineage arose from developmentally and evolutionarily distinct progenitors. Integrative comparative studies of vocal neurons across brain regions and taxa will be required to distinguish between these two scenarios.

An Amygdalo-Parabrachial Pathway Regulates Pain Perception and Chronic Pain

The parabrachial (PB) complex mediates both ascending nociceptive signaling and descending pain modulatory information in the affective/emotional pain pathway. We have recently reported that chronic pain is associated with amplified activity of PB neurons in a rat model of neuropathic pain. Here we demonstrate that similar activity amplification occurs in mice, and that this is related to suppressed inhibition to lateral parabrachial (LPB) neurons from the CeA in animals of either sex. Animals with pain after chronic constriction injury of the infraorbital nerve (CCI-Pain) displayed higher spontaneous and evoked activity in PB neurons, and a dramatic increase in after-discharges, responses that far outlast the stimulus, compared with controls. LPB neurons in CCI-Pain animals showed a reduction in inhibitory, GABAergic inputs. We show that, in both rats and mice, LPB contains few GABAergic neurons, and that most of its GABAergic inputs arise from CeA. These CeA GABA neurons express dynorphin, somatostatin, and/or corticotropin releasing hormone. We find that the efficacy of this CeA-LPB pathway is suppressed in chronic pain. Further, optogenetically stimulating this pathway suppresses acute pain, and inhibiting it, in naive animals, evokes pain behaviors. These findings demonstrate that the CeA-LPB pathway is critically involved in pain regulation, and in the pathogenesis of chronic pain.SIGNIFICANCE STATEMENT We describe a novel pathway, consisting of inhibition by dynorphin, somatostatin, and corticotropin-releasing hormone-expressing neurons in the CeA that project to the parabrachial nucleus. We show that this pathway regulates the activity of pain-related neurons in parabrachial nucleus, and that, in chronic pain, this inhibitory pathway is suppressed, and that this suppression is causally related to pain perception. We propose that this amygdalo-parabrachial pathway is a key regulator of both chronic and acute pain, and a novel target for pain relief.

Kölliker-Fuse/Parabrachial complex mu opioid receptors contribute to fentanyl-induced apnea and respiratory rate depression

Overdoses caused by the opioid agonist fentanyl have increased exponentially in recent years. Identifying mechanisms to counter progression to fatal respiratory apnea during opioid overdose is desirable, but difficult to study in vivo. The pontine Kölliker-Fuse/Parabrachial complex (KF/PB) provides respiratory drive and contains opioid-sensitive neurons. The contribution of the KF/PB complex to fentanyl-induced apnea was investigated using the in situ arterially perfused preparation of rat. Systemic application of fentanyl resulted in concentration-dependent respiratory disturbances. At low concentrations, respiratory rate slowed and subsequently transitioned to an apneustic-like, 2-phase pattern. Higher concentrations caused prolonged apnea, interrupted by occasional apneustic-like bursts. Application of CTAP, a selective mu opioid receptor antagonist, directly into the KF/PB complex reversed and prevented fentanyl-induced apnea by increasing the frequency of apneustic-like bursting. These results demonstrate that countering opioid effects in the KF/PB complex is sufficient to restore phasic respiratory output at a rate similar to pre-fentanyl conditions, which could be beneficial in opioid overdose.

Activation of Brainstem Neurons During Mesencephalic Locomotor Region-Evoked Locomotion in the Cat

The distribution of locomotor-activated neurons in the brainstem of the cat was studied by c-Fos immunohistochemistry in combination with antibody-based cellular phenotyping following electrical stimulation of the mesencephalic locomotor region (MLR) – the anatomical constituents of which remain debated today, primarily between the cuneiform (CnF) and the pedunculopontine tegmental nuclei (PPT). Effective MLR sites were co-extensive with the CnF nucleus. Animals subject to the locomotor task showed abundant Fos labeling in the CnF, parabrachial nuclei of the subcuneiform region, periaqueductal gray, locus ceruleus (LC)/subceruleus (SubC), Kölliker–Fuse, magnocellular and lateral tegmental fields, raphe, and the parapyramidal region. Labeled neurons were more abundant on the side of stimulation. In some animals, Fos-labeled cells were also observed in the ventral tegmental area, medial and intermediate vestibular nuclei, dorsal motor nucleus of the vagus, n. tractus solitarii, and retrofacial nucleus in the ventrolateral medulla. Many neurons in the reticular formation were innervated by serotonergic fibers. Numerous locomotor-activated neurons in the parabrachial nuclei and LC/SubC/Kölliker–Fuse were noradrenergic. Few cholinergic neurons within the PPT stained for Fos. In the medulla, serotonergic neurons within the parapyramidal region and the nucleus raphe magnus were positive for Fos. Control animals, not subject to locomotion, showed few Fos-labeled neurons in these areas. The current study provides positive evidence for a role for the CnF in the initiation of locomotion while providing little evidence for the participation of the PPT. The results also show that MLR-evoked locomotion involves the parallel activation of reticular and monoaminergic neurons in the pons/medulla, and provides the anatomical and functional basis for spinal monoamine release during evoked locomotion. Lastly, the results indicate that vestibular, cardiovascular, and respiratory centers are centrally activated during MLR-evoked locomotion. Altogether, the results show a complex pattern of neuromodulatory influences of brainstem neurons by electrical activation of the MLR.

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Structural and functional identification of two distinct inspiratory neuronal populations at the level of the phrenic nucleus in the rat cervical spinal cord

The diaphragm is driven by phrenic motoneurons that are located in the cervical spinal cord. Although the anatomical location of the phrenic nucleus and the function of phrenic motoneurons at a single cellular level have been extensively analyzed, the spatiotemporal dynamics of phrenic motoneuron group activity have not been fully elucidated. In the present study, we analyzed the functional and structural characteristics of respiratory neuron population in the cervical spinal cord at the level of the phrenic nucleus by voltage imaging, together with histological analysis of neuronal and astrocytic distribution in the cervical spinal cord. We found spatially distinct two cellular populations that exhibited synchronized inspiratory activity on the transversely cut plane at C4–C5 levels and on the ventral surface of the mid cervical spinal cord in the isolated brainstem–spinal cord preparation of the neonatal rat. Inspiratory activity of one group emerged in the central portion of the ventral horn that corresponded to the central motor column, and the other appeared in the medial portion of the ventral horn that corresponded to the medial motor column. We identified by retrogradely labeling study that the anatomical distributions of phrenic and scalene motoneurons coincided with optically detected central and medial motor regions, respectively. Furthermore, we anatomically demonstrated closely located features of putative motoneurons, interneurons and astrocytes in these regions. Collectively, we report that phrenic and scalene motoneuron populations show synchronized inspiratory activities with distinct anatomical locations in the mid cervical spinal cord.

Efferent projections of excitatory and inhibitory preBötzinger Complex neurons

The preBötzinger Complex (preBötC), a compact medullary region essential for generating normal breathing rhythm and pattern, is the kernel of the breathing central pattern generator (CPG). Excitatory preBötC neurons in rats project to major breathing-related brainstem regions. Here, we provide a brainstem connectivity map in mice for both excitatory and inhibitory preBötC neurons. Using a genetic strategy to label preBötC neurons, we confirmed extensive projections of preBötC excitatory neurons within the brainstem breathing CPG including the contralateral preBötC, Bötzinger Complex (BötC), ventral respiratory group, nucleus of the solitary tract, parahypoglossal nucleus, parafacial region (RTN/pFRG or alternatively, pF_L /pF_V ), parabrachial and Kölliker-Füse nuclei, as well as major projections to the midbrain periaqueductal gray. Interestingly, preBötC inhibitory projections paralleled the excitatory projections. Moreover, we examined overlapping projections in the pons in detail and found that they targeted the same neurons. We further explored the direct anatomical link between the preBötC and suprapontine brain regions that may govern emotion and other complex behaviors that can affect or be affected by breathing. Forebrain efferent projections were sparse and restricted to specific nuclei within the thalamus and hypothalamus, with processes rarely observed in cortex, basal ganglia, or other limbic regions, e.g., amygdala or hippocampus. We conclude that the preBötC sends direct, presumably inspiratory-modulated, excitatory and inhibitory projections in parallel to distinct targets throughout the brain that generate and modulate breathing pattern and/or coordinate breathing with other behaviors, physiology, cognition, or emotional state.

A Subregion of the Parabrachial Nucleus Partially Mediates Respiratory Rate Depression from Intravenous Remifentanil in Young and Adult Rabbits

BACKGROUND

The efficacy of opioid administration to reduce postoperative pain is limited by respiratory depression. We investigated whether clinically relevant opioid concentrations altered the respiratory pattern in the parabrachial nucleus, a pontine region contributing to respiratory pattern generation, and compared these effects with a medullary respiratory site, the pre-Bötzinger complex.

METHODS

Studies were performed in 40 young and 55 adult artificially ventilated, decerebrate rabbits. We identified an area in the parabrachial nucleus where α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid microinjections elicited tachypnea. Two protocols were performed in separate sets of animals. First, bilateral microinjections of the μ-opioid receptor agonist [D-Ala, N-MePhe, Gly-ol]-enkephalin (100 μM) into the "tachypneic area" determined the effect of maximal μ-opioid receptor activation. Second, respiratory rate was decreased with continuous IV infusions of remifentanil. The opioid antagonist naloxone (1 mM) was then microinjected bilaterally into the "tachypneic area" of the parabrachial nucleus to determine whether the respiratory rate depression could be locally reversed.

RESULTS

Average respiratory rate was 27 ± 10 breaths/min. First, [D-Ala, N-MePhe, Gly-ol]-enkephalin injections decreased respiratory rate by 62 ± 20% in young and 45 ± 26% in adult rabbits (both P < 0.001). Second, during IV remifentanil infusion, bilateral naloxone injections into the "tachypneic area" of the parabrachial nucleus reversed respiratory rate depression from 55 ± 9% to 20 ± 14% in young and from 46 ± 20% to 18 ± 27% in adult rabbits (both P < 0.001). The effects of bilateral [D-Ala, N-MePhe, Gly-ol]-enkephalin injection and IV remifentanil on respiratory phase duration in the "tachypneic area" of the parabrachial nucleus was significantly different from the pre-Bötzinger complex.

CONCLUSIONS

The "tachypneic area" of the parabrachial nucleus is highly sensitive to μ-opioid receptor activation and mediates part of the respiratory rate depression by clinically relevant administration of opioids.

Kölliker-Fuse GABAergic and glutamatergic neurons project to distinct targets

The Kölliker-Fuse nucleus (KF) is known primarily for its respiratory function as the "pneumotaxic center" or "pontine respiratory group." Considered part of the parabrachial (PB) complex, KF contains glutamatergic neurons that project to respiratory-related targets in the medulla and spinal cord (Yokota, Oka, Tsumori, Nakamura, & Yasui, 2007). Here we describe an unexpected population of neurons in the caudal KF and adjacent lateral crescent subnucleus (PBlc), which are γ-aminobutyric acid (GABA)ergic and have an entirely different pattern of projections than glutamatergic KF neurons. First, immunofluorescence, in situ hybridization, and Cre-reporter labeling revealed that many of these GABAergic neurons express FoxP2 in both rats and mice. Next, using Cre-dependent axonal tracing in Vgat-IRES-Cre and Vglut2-IRES-Cre mice, we identified different projection patterns from GABAergic and glutamatergic neurons in this region. GABAergic neurons in KF and PBlc project heavily and almost exclusively to trigeminal sensory nuclei, with minimal projections to cardiorespiratory nuclei in the brainstem, and none to the spinal cord. In contrast, glutamatergic KF neurons project heavily to the autonomic, respiratory, and motor regions of the medulla and spinal cord previously identified as efferent targets mediating KF cardiorespiratory effects. These findings identify a novel, GABAergic subpopulation of KF/PB neurons with a distinct efferent projection pattern targeting the brainstem trigeminal sensory system. Rather than regulating breathing, we propose that these neurons influence vibrissal sensorimotor function.

Seizure-associated central apnea in a rat model: Evidence for resetting the respiratory rhythm and activation of the diving reflex

Respiratory derangements, including irregular, tachypnic breathing and central or obstructive apnea can be consequences of seizure activity in epilepsy patients and animal models. Periods of seizure-associated central apnea, defined as periods >1s with rapid onset and offset of no airflow during plethysmography, suggest that seizures spread to brainstem respiratory regions to disrupt breathing. We sought to characterize seizure-associated central apneic episodes as an indicator of seizure impact on the respiratory rhythm in rats anesthetized with urethane and given parenteral kainic acid to induce recurring seizures. We measured central apneic period onsets and offsets to determine if onset-offset relations were a consequence of 1) a reset of the respiratory rhythm, 2) a transient pausing of the respiratory rhythm, resuming from the pause point at the end of the apneic period, 3) a transient suppression of respiratory behavior with apnea offset predicted by a continuation of the breathing pattern preceding apnea, or 4) a random re-entry into the respiratory cycle. Animals were monitored with continuous ECG, EEG, and plethysmography. One hundred ninety central apnea episodes (1.04 to 36.18s, mean: 3.2±3.7s) were recorded during seizure activity from 7 rats with multiple apneic episodes. The majority of apneic period onsets occurred during expiration (125/161 apneic episodes, 78%). In either expiration or inspiration, apneic onsets tended to occur late in the cycle, i.e. between the time of the peak and end of expiration (82/125, 66%) or inspiration (34/36, 94%). Apneic period offsets were more uniformly distributed between early and late expiration (27%, 34%) and inspiration (16%, 23%). Differences between the respiratory phase at the onset of apnea and the corresponding offset phase varied widely, even within individual animals. Each central apneic episode was associated with a high frequency event in EEG or ECG records at onset. High frequency events that were not associated with flatline plethysmographs revealed a constant plethysmograph pattern within each animal, suggesting a clear reset of the respiratory rhythm. The respiratory rhythm became highly variable after about 1s, however, accounting for the unpredictability of the offset phase. The dissociation of respiratory rhythm reset from the cessation of airflow also suggested that central apneic periods involved activation of brainstem regions serving the diving reflex to eliminate the expression of respiratory movements. This conclusion was supported by the decreased heart rate as a function of apnea duration. We conclude that seizure-associated central apnea episodes are associated with 1) a reset of the respiratory rhythm, and 2) activation of brainstem regions serving the diving reflex to suppress respiratory behavior. The significance of these conclusions is that these details of seizure impact on brainstem circuitry represent metrics for assessing seizure spread and potentially subclassifying seizure patterns.

Evolution of vocal patterns: tuning hindbrain circuits during species divergence

The neural circuits underlying divergent courtship behaviors of closely related species provide a framework for insight into the evolution of motor patterns. In frogs, male advertisement calls serve as unique species identifiers and females prefer conspecific to heterospecific calls. Advertisement calls of three relatively recently (∼8.5 Mya) diverged species - Xenopus laevis, X. petersii and X. victorianus - include rapid trains of sound pulses (fast trills). We show that while fast trills are similar in pulse rate (∼60 pulses s^-1) across the three species, they differ in call duration and period (time from the onset of one call to the onset of the following call). Previous studies of call production in X. laevis used an isolated brain preparation in which the laryngeal nerve produces compound action potentials that correspond to the advertisement call pattern (fictive calling). Here, we show that serotonin evokes fictive calling in X. petersii and X. victorianus as it does in X. laevis As in X. laevis, fictive fast trill in X. petersii and X. victorianus is accompanied by an N-methyl-d-aspartate receptor-dependent local field potential wave in a rostral hindbrain nucleus, DTAM. Across the three species, wave duration and period are strongly correlated with species-specific fast trill duration and period, respectively. When DTAM is isolated from the more rostral forebrain and midbrain and/or more caudal laryngeal motor nucleus, the wave persists at species-typical durations and periods. Thus, intrinsic differences within DTAM could be responsible for the evolutionary divergence of call patterns across these related species.

Inhibition of the pontine Kölliker-Fuse nucleus reduces genioglossal activity elicited by stimulation of the retrotrapezoid chemoreceptor neurons

The Kölliker-Fuse (KF) region, located in the dorsolateral pons, projects to several brainstem areas involved in respiratory regulation, including the chemoreceptor neurons within the retrotrapezoid nucleus (RTN). Several lines of evidence indicate that the pontine KF region plays an important role in the control of the upper airways for the maintenance of appropriate airflow to and from the lungs. Specifically, we hypothesized that the KF region is involved in mediating the response of the hypoglossal motor activity to central respiratory chemoreflex activation and to stimulation of the chemoreceptor neurons within the RTN region. To test this hypothesis, we combined immunohistochemistry and physiological experiments. We found that in the KF, the majority of biotinylated dextran amine (BDA)-labeled axonal varicosities contained detectable levels of vesicular glutamate transporter-2 (VGLUT2), but few contained glutamic acid decarboxylase-67 (GAD67). The majority of the RTN neurons that were FluorGold (FG)-immunoreactive (i.e., projected to the KF) contained hypercapnia-induced Fos, but did not express tyrosine hydroxylase. In urethane-anesthetized sino-aortic denervated and vagotomized male Wistar rats, hypercapnia (10% CO2) or N-methyl-d-aspartate (NMDA) injection (0.1mM) in the RTN increased diaphragm (DiaEMG) and genioglossus muscle (GGEMG) activities and elicited abdominal (AbdEMG) activity. Bilateral injection of muscimol (GABA-A agonist; 2mM) into the KF region reduced the increase in DiaEMG and GGEMG produced by hypercapnia or NMDA into the RTN. Our data suggest that activation of chemoreceptor neurons in the RTN produces a significant increase in the genioglossus muscle activity and the excitatory pathway is dependent on the neurons located in the dorsolateral pontine KF region.

Commentary on: Efferent connections of the parabrachial nucleus in the rat. C.B. Saper and A.D. Loewy, Brain Research 197:291-317, 1980

By the late 1970׳s, the pathways had been identified from neurons in the nucleus of the solitary tract that control visceral sensory inflow and from the paraventricular nucleus and lateral hypothalamus that directly innervate the autonomic preganglionic neurons, thereby controlling autonomic outflow. However, the connections between the two were not yet clear. This paper identified the parabrachial nucleus as a key intermediary, receiving the bulk of outflow from the nucleus of the solitary tract and distributing it to a set of brainstem and forebrain sites that constituted a central autonomic control network. This work also identified the insular cortex as a key visceral sensory cortical area. This article is part of a Special Issue entitled SI:50th Anniversary Issue.

Morphological and physiological evidence of a synaptic connection between the lateral parabrachial nucleus and neurons in the A7 catecholamine cell group in rats

BACKGROUND

The descending noradrenergic (NAergic) system is one of the important endogenous analgesia systems. It has been suggested that noxious stimuli could activate descending NAergic system; nevertheless, the underlying neuronal circuit remains unclear. As NAergic neurons in the A7 catecholamine cell group (A7) are a part of the descending NAergic system and the lateral parabrachial nucleus (LPB) is an important brainstem structure that relays ascending nociceptive signal, we aimed to test whether LPB neurons have direct synaptic contact with NAergic A7 neurons.

RESULTS

Stereotaxic injections of an anterograde tracer, biotinylated dextran-amine (BDA), were administered to LPB in rats. The BDA-labeled axonal terminals that have physical contacts with tyrosine hydroxylase-positive (presumed noadrenergic) neurons were identified in A7. Consistent with these morphological observations, the excitatory synaptic currents (EPSCs) were readily evoked in NAergic A7 neurons by extracellular stimulation of LPB. The EPSCs evoked by LPB stimulation were blocked by CNQX, a non-NMDA receptor blocker, and AP5, a selective NMDA receptor blocker, showing that LPB-A7 synaptic transmission is glutamatergic. Moreover, the amplitude of LPB-A7 EPSCs was significantly attenuated by DAMGO, a selective μ-opioid receptor agonist, which was associated with an increase in paired-pulse ratio.

CONCLUSIONS

Taken together, the above results showed direct synaptic connections between LPB and A7 catecholamine cell group, the function of which is subject to presynaptic modulation by μ-opioid receptors.

Neural Control of Breathing and CO2 Homeostasis

Recent advances have clarified how the brain detects CO2 to regulate breathing (central respiratory chemoreception). These mechanisms are reviewed and their significance is presented in the general context of CO2/pH homeostasis through breathing. At rest, respiratory chemoreflexes initiated at peripheral and central sites mediate rapid stabilization of arterial PCO2 and pH. Specific brainstem neurons (e.g., retrotrapezoid nucleus, RTN; serotonergic) are activated by PCO2 and stimulate breathing. RTN neurons detect CO2 via intrinsic proton receptors (TASK-2, GPR4), synaptic input from peripheral chemoreceptors and signals from astrocytes. Respiratory chemoreflexes are arousal state dependent whereas chemoreceptor stimulation produces arousal. When abnormal, these interactions lead to sleep-disordered breathing. During exercise, central command and reflexes from exercising muscles produce the breathing stimulation required to maintain arterial PCO2 and pH despite elevated metabolic activity. The neural circuits underlying central command and muscle afferent control of breathing remain elusive and represent a fertile area for future investigation.

State-dependent control of breathing by the retrotrapezoid nucleus

KEY POINTS

This study explores the state dependence of the hypercapnic ventilatory reflex (HCVR). We simulated an instantaneous increase or decrease of central chemoreceptor activity by activating or inhibiting the retrotrapezoid nucleus (RTN) by optogenetics in conscious rats. During quiet wake or non-REM sleep, hypercapnia increased both breathing frequency (fR ) and tidal volume (VT ) whereas, in REM sleep, hypercapnia increased VT exclusively. Optogenetic inhibition of RTN reduced VT in all sleep-wake states, but reduced fR only during quiet wake and non-REM sleep. RTN stimulation always increased VT but raised fR only in quiet wake and non-REM sleep. Phasic RTN stimulation produced active expiration and reduced early expiratory airflow (i.e. increased upper airway resistance) only during wake. We conclude that the HCVR is highly state-dependent. The HCVR is reduced during REM sleep because fR is no longer under chemoreceptor control and thus could explain why central sleep apnoea is less frequent in REM sleep.

ABSTRACT

Breathing has different characteristics during quiet wake, non-REM or REM sleep, including variable dependence on PCO2. We investigated whether the retrotrapezoid nucleus (RTN), a proton-sensitive structure that mediates a large portion of the hypercapnic ventilatory reflex, regulates breathing differently during sleep vs. wake. Electroencephalogram, neck electromyogram, blood pressure, respiratory frequency (fR ) and tidal volume (VT ) were recorded in 28 conscious adult male Sprague-Dawley rats. Optogenetic stimulation of RTN with channelrhodopsin-2, or inhibition with archaerhodopsin, simulated an instantaneous increase or decrease of central chemoreceptor activity. Both opsins were delivered with PRSX8-promoter-containing lentiviral vectors. RTN and catecholaminergic neurons were transduced. During quiet wake or non-REM sleep, hypercapnia (3 or 6% FI,CO2 ) increased both fR and VT whereas, in REM sleep, hypercapnia increased VT exclusively. RTN inhibition always reduced VT but reduced fR only during quiet wake and non-REM sleep. RTN stimulation always increased VT but raised fR only in quiet wake and non-REM sleep. Blood pressure was unaffected by either stimulation or inhibition. Except in REM sleep, phasic RTN stimulation entrained and shortened the breathing cycle by selectively shortening the post-inspiratory phase. Phasic stimulation also produced active expiration and reduced early expiratory airflow but only during wake. VT is always regulated by RTN and CO2 but fR is regulated by CO2 and RTN only when the brainstem pattern generator is in autorhythmic mode (anaesthesia, non-REM sleep, quiet wake). The reduced contribution of RTN to breathing during REM sleep could explain why certain central apnoeas are less frequent during this sleep stage.

Respiratory-related outputs of glutamatergic, hypercapnia-responsive parabrachial neurons in mice

In patients with obstructive sleep apnea, airway obstruction during sleep produces hypercapnia, which in turn activates respiratory muscles that pump air into the lungs (e.g., the diaphragm) and that dilate and stabilize the upper airway (e.g., the genioglossus). We hypothesized that these responses are facilitated by glutamatergic neurons in the parabrachial complex (PB) that respond to hypercapnia and project to premotor and motor neurons that innervate the diaphragm and genioglossus muscles. To test this hypothesis, we combined c-Fos immunohistochemistry with in situ hybridization for vGluT2 or GAD67 or with retrograde tracing from the ventrolateral medullary region that contains phrenic premotor neurons, the phrenic motor nucleus in the C3-C5 spinal ventral horn, or the hypoglossal motor nucleus. We found that hypercapnia (10% CO2 for 2 hours) activated c-Fos expression in neurons in the external lateral, lateral crescent (PBcr), and Kölliker-Fuse (KF) PB subnuclei and that most of these neurons were glutamatergic and virtually none γ-aminobutyric acidergic. Numerous CO2 -responsive neurons in the KF and PBcr were labeled after retrograde tracer injection into the ventrolateral medulla or hypoglossal motor nuclei, and in the KF after injections into the spinal cord, making them candidates for mediating respiratory-facilitatory and upper-airway-stabilizing effects of hypercapnia.

Multiscale fingerprinting of neuronal functional connectivity

Current cellular-based connectomics approaches aim to delineate the functional or structural organizations of mammalian brain circuits through neuronal activity mapping and/or axonal tracing. To discern possible connectivity between functionally identified neurons in widely distributed brain circuits, reliable and efficient network-based approaches of cross-registering or cross-correlating such functional-structural data are essential. Here, a novel cross-correlation approach that exploits multiple timing-specific, response-specific, and cell-specific neuronal characteristics as coincident fingerprint markers at the systems, network, and cellular levels is proposed. Application of this multiscale temporal-cellular coincident fingerprinting assay to the respiratory central pattern generator network in rats revealed a descending excitatory pathway with characteristic activity pattern and projecting from a distinct neuronal population in pons to its counterparts in medulla that control the post-inspiratory phase of the respiratory rhythm important for normal breathing, airway protection, and respiratory-vocalization coordination. This enabling neurotracing approach may prove valuable for functional connectivity mapping of other brain circuits.

MLLT11/AF1q is differentially expressed in maturing neurons during development

Myeloid/lymphoid or mixed-lineage leukemia; translocated to chromosome 11 or ALL1 fused from chromosome 1q (MLLT11/AF1q) is a highly conserved 90 amino acid protein that functions in hematopoietic differentiation. Its translocation to the Trithorax locus has been implicated in malignancies of the hematopoietic system. However, the spatio-temporal profile of MLLT11 expression during embryonic development has not been characterized. Here we show that MLLT11 has a remarkably specific expression pattern in the developing central and peripheral nervous system. We find high levels of MLLT11 transcript and protein expression in the developing marginal zone of the cortex and spinal cord. MLLT11 co-localized with Tbr2 in the developing subplate region of the cortex and expanded to encompass the cortical plate at late fetal stages. Expression in the peripheral nervous system initiated at E9.5 in the facio-acoustic cranial ganglia and elaborated to identify all the cranio-facial and dorsal root ganglia by E10.5. We also observed expression in the eye and gastrointestinal tract, where MLLT11 transcripts localized to Tuj1-positive inner retinal layer and autonomic neurons, respectively. Altogether these results show that MLLT11 is a pan-neuronal marker, suggesting a role in neural differentiation in the central nervous system and neural crest-cell derived peripheral ganglia.

Regulation of the chemosensory control of breathing by Kölliker-Fuse neurons

The Kölliker-Fuse region (KF) and the lateral parabrachial nucleus (LPBN) have been implicated in the maintenance of cardiorespiratory control. Here, we evaluated the involvement of the KF region and the LPBN in cardiorespiratory responses elicited by chemoreceptor activation in unanesthetized rats. Male Wistar rats (280-330 g; n = 5-9/group) with bilateral stainless-steel guide cannulas implanted in the KF region or the LPBN were used. Injection of muscimol (100 and 200 pmol/100 nl) in the KF region decreased resting ventilation (1,140 ± 68 and 978 ± 100 vs. saline: 1,436 ± 155 ml·kg(-1)·min(-1)), without changing mean arterial pressure (MAP) and heart rate (HR). Bilateral injection of the GABA-A antagonist bicuculline (1 nmol/100 nl) in the KF blocked the inhibitory effect on ventilation (1,418 ± 138 vs. muscimol: 978 ± 100 ml·kg(-1)·min(-1)) elicited by muscimol. Muscimol injection in the KF reduced the increase in ventilation produced by hypoxia (8% O2) (1,827 ± 61 vs. saline: 3,179 ± 325 ml·kg(-1)·min(-1)) or hypercapnia (7% CO2) (1,488 ± 277 vs. saline: 3,539 ± 374 ml·kg(-1)·min(-1)) in unanesthetized rats. Bilateral injection of bicuculline in the KF blocked the decrease in ventilation produced by muscimol in the KF during peripheral or central chemoreflex activation. Bilateral injection of muscimol in the LPBN did not change resting ventilation or the increase in ventilation elicited by hypoxia or hypercapnia. The results of the present study suggest that the KF region, but not the LPBN, has mechanisms to control ventilation in resting, hypoxic, or hypercapnic conditions in unanesthetized rats.

Cytoarchitecture and CO(2) sensitivity of Phox2b-positive Parafacial neurons in the newborn rat medulla

Preinspiratory (Pre-I) neurons in the parafacial respiratory group (pFRG) compose one of the respiratory rhythm generators in the medulla of the newborn rat. It has been shown that a subgroup of pFRG/Pre-I neurons could also work as central chemoreceptor neurons, because the CO(2) sensitivity of these Pre-I neurons was preserved even after blockade of Na(+) channels and Ca(2+) channels, and the membrane depolarization induced by hypercapnic stimulation was mainly because of the closing of K(+) channels. These neurons, some of which were identified to be glutamatergic, express the transcription factor Phox2b. Phox2b expression was one of the most noticeable characteristics of pFRG/Pre-I neurons. We also found that Phox2b-expressing neurons in the parafacial region of the rostral ventral medulla tended to assemble around capillary blood vessels. In contrast, another subclass of the pFRG/Pre-I neurons was Phox2b-negative and CO(2)-insensitive. Some of these neurons were identified to be glycinergic or GABAergic. Thus, Phox2b expression is a key genetic marker that can be used to more clearly establish the cell architecture of the pFRG, which consists of heterogeneous neuronal subtypes. In this chapter, we elaborate on the CO(2) sensitivity of Phox2b-positive/negative parafacial neurons and the cytoarchitecture in the newborn rat medulla, and discuss ionic mechanisms of CO(2) responsiveness.

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

Expression of 5-HT2A receptors in prefrontal cortex pyramidal neurons projecting to nucleus accumbens. Potential relevance for atypical antipsychotic action

The prefrontal cortex (PFC) is involved in higher brain functions altered in schizophrenia. Classical antipsychotic drugs modulate information processing in cortico-limbic circuits via dopamine D2 receptor blockade in nucleus accumbens (NAc) whereas atypical antipsychotic drugs preferentially target cortical serotonin (5-HT) receptors. The brain networks involved in the therapeutic action of atypical drugs are not fully understood. Previous work indicated that medial PFC (mPFC) pyramidal neurons projecting to ventral tegmental area express 5-HT2A receptors suggesting that atypical antipsychotic drugs modulate dopaminergic activity distally, via 5-HT2A receptor (5-HT2A-R) blockade in PFC. Since the mPFC also projects heavily to NAc, we examined whether NAc-projecting pyramidal neurons also express 5-HT2A-R. Using a combination of retrograde tracing experiments and in situ hybridization we report that a substantial proportion of mPFC-NAc pyramidal neurons in rat brain express 5-HT2A-R mRNA in a layer- and area-specific manner (up to 68% in layer V of contralateral cingulate). The functional relevance of 5-HT2A-R to modulate mPFC-NAc projections was examined in dual-probe microdialysis experiments. The application of the preferential 5-HT2A-R agonist DOI into mPFC enhanced glutamate release locally (+66 ± 18%) and in NAc (+74 ± 12%) indicating that cortical 5-HT2A-R activation augments glutamatergic transmission in NAc. Since NAc integrates glutamatergic and dopaminergic inputs, blockade of 5-HT2A-R by atypical drugs may reduce cortical excitatory inputs onto GABAergic neurons of NAc, adding to dopamine D2 receptor blockade. Together with previous observations, the present results suggest that atypical antipsychotic drugs may control the activity of the mesolimbic pathway at cell body and terminal level.

Chemoreception and asphyxia-induced arousal

Arousal protects against the adverse and potentially fatal effects of asphyxia during sleep. Asphyxia stimulates the carotid bodies and central chemoreceptors but the sequence of events leading to arousal is uncertain. In this review, the theoretical mechanisms leading to arousal from sleep are briefly summarized and the issue of whether central respiratory chemoreceptors (CRCs) or other types of CO2-responsive CNS neurons contribute to asphyxia-induced arousal is discussed. We focus on the role of the retrotrapezoid nucleus, the raphe and the locus coeruleus and emphasize the anatomical and neurophysiological evidence which suggests that these putative central chemoreceptors could contribute to arousal independently of their effects on breathing. Finally, we describe recent attempts to test the contribution of specific brainstem pathways to asphyxia-induced arousal using optogenetic and other tools and the possible contribution of a group of hypoxia-sensitive brainstem neurons (the C1 cells) to breathing and arousal.

Glutamatergic signaling from the parabrachial nucleus plays a critical role in hypercapnic arousal

The mechanisms of arousal from apneas during sleep in patients suffering from obstructive sleep apnea are not well understood. However, we know that respiratory chemosensory pathways converge on the parabrachial nucleus (PB), which sends glutamatergic projections to a variety of forebrain structures critical to arousal, including the basal forebrain, lateral hypothalamus, midline thalamus, and cerebral cortex. We tested the role of glutamatergic signaling in this pathway by developing an animal model for repetitive CO2 arousals (RCAs) and investigating the effect of deleting the gene for the vesicular glutamate transporter 2 (Vglut2) from neurons in the PB. We used mice with lox P sequences flanking exon2 of the Vglut2 gene, in which adeno-associated viral vectors containing genes encoding Cre recombinase and green fluorescent protein were microinjected into the PB to permanently and selectively disrupt Vglut2 expression while labeling the affected neurons. We recorded sleep in these mice and then investigated the arousals during RCA. Vglut2 deletions that included the external lateral and lateral crescent subdivisions of the lateral PB more than doubled the latency to arousal and resulted in failure to arouse by 30 s in >30% of trials. By contrast, deletions that involved the medial PB subdivision had minimal effects on arousal during hypercapnia but instead increased non-rapid eye movement (NREM) sleep by ∼43% during the dark period, and increased delta power in the EEG during NREM sleep by ∼50%. Our results suggest that glutamatergic neurons in the lateral PB are necessary for arousals from sleep in response to CO2, while medial PB glutamatergic neurons play an important role in promoting spontaneous waking.

The emerging role of the parabrachial complex in the generation of wakefulness drive and its implication for respiratory control

The parabrachial complex is classically seen as a major neural knot that transmits viscero- and somatosensory information toward the limbic and thalamic forebrain. In the present review we summarize recent findings that imply an emerging role of the parabrachial complex as an integral part of the ascending reticular arousal system, which promotes wakefulness and cortical activation. The ascending parabrachial projections that target wake-promoting hypothalamic areas and the basal forebrain are largely glutamatergic. Such fast synaptic transmission could be even more significant in promoting wakefulness and its characteristic pattern of cortical activation than the cholinergic or mono-aminergic ascending pathways that have been emphasized extensively in the past. A similar role of the parabrachial complex could also apply for its more established function in control of breathing. Here the parabrachial respiratory neurons may modulate and adapt breathing via the control of respiratory phase transition and upper airway patency, particularly during respiratory and non-respiratory behavior associated with wakefulness.

Parabrachial nucleus involvement in multiple system atrophy

Multiple system atrophy (MSA) is associated with respiratory dysfunction, including sleep apnea, respiratory dysrhythmia, and laryngeal stridor. Neurons of the parabrachial nucleus (PBN) control respiratory rhythmogenesis and airway resistance.

OBJECTIVES

The objective of this study is to determine whether there was involvement of putative respiratory regions of the PBN in MSA.

METHODS

We examined the pons at autopsy in 10 cases with neuropathologically confirmed MSA and 8 age-matched controls. Sections obtained throughout the pons were processed for calcitonin-gene related peptide (CGRP) and Nissl staining to identify the lateral crescent of the lateral PBN (LPB) and the Kölliker-Fuse nucleus (K-F), which are involved in respiratory control. Cell counts were performed using stereology.

RESULTS

There was loss of CGRP neurons in the PBN in MSA (total estimated cell counts for the external LPB cluster was 12,584 ± 1146 in controls and 5917 ± 389 in MSA, p<0.0001); for the external medial PBN (MPB) cluster it was 15,081 ± 1758 in controls and 7842 ± 466 in MSA, p<0.001. There was also neuronal loss in putative respiratory regions of the PBN, including the lateral crescent of the LPB (13,039 ± 1326 in controls and 4164 ± 872 in MSA, p<0.0001); and K-F (5120 ± 495 in controls and 999 ± 308 in MSA, p<0.0001).

CONCLUSIONS

There is involvement of both CGRP and putative respiratory cell groups in the PBN in MSA. Whereas the clinical implications of CGRP cell loss are still undetermined, involvement of the LPB and K-F may contribute to respiratory dysfunction in this disorder.

Projections from the anterior basomedial and anterior cortical amygdaloid nuclei to melanin-concentrating hormone-containing neurons in the lateral hypothalamus of the rat

Melanin-concentrating hormone (MCH) is involved in the regulation of feeding behavior as well as in goal oriented behaviors, and MCH-containing neurons are distributed mainly in the lateral hypothalamic area (LHA). The anterior basomedial nucleus (BMA) and anterior cortical nucleus (CoA) of the amygdala form part of a circuit involved in processing olfactory, gustatory and visceral information, and the BMA-LHA and CoA-LHA pathways are suggested to be implicated in the control of feeding behavior. However, it is still unknown whether or not MCH-containing LHA neurons are under the direct influence of the BMA and CoA. Here the organization of projections from the BMA and CoA to MCH-containing LHA neurons was examined. Using a combined anterograde tracing with biotinylated dextranamine and immunohistochemistry for MCH, we first demonstrated that the distribution pattern of BMA fibers was almost similar to that of CoA fibers in the LHA, and a prominent overlapping distribution of these fibers and MCH-immunoreactive neurons existed in the ventral peripeduncular region of the LHA. We further revealed that asymmetrical synapses were made between these fibers and neurons. Using a combination of retrograde tract-tracing with cholera toxin B subunit and in situ hybridization for vesicular glutamate transporter (VGLUT) 2 mRNA, we finally showed that most of the LHA-projecting BMA and CoA neurons expressed VGLUT2 mRNA. These data suggest that the BMA and CoA of the amygdala may exert excitatory influence upon the MCH-containing LHA neurons for the regulation of feeding behavior.

Glutamatergic Kölliker-Fuse nucleus neurons innervate hypoglossal motoneurons whose axons form the medial (protruder) branch of the hypoglossal nerve in the rat

This study was performed to understand the anatomical substrates for Kölliker-Fuse nucleus (KFN) modulation of respiratory-related tongue movement. After application of cholera toxin B subunit (CTb) to the medial branch of the hypoglossal nerve (HGn) and injection of biotinylated dextran amine (BDA) into the KFN ipsilaterally, an overlapping distribution of BDA-labeled axon terminals and CTb-labeled neurons was found in the ventral compartment of the hypoglossal nucleus (HGN) ipsilateral to the application and injection sites. At the electron microscopic level, the BDA-labeled terminals made asymmetrical synaptic contacts predominantly with dendrites of the HGN neurons, some of which were labeled with CTb. Using retrograde tracing combined with in situ hybridization, we demonstrated that almost all the KFN neurons sending their axons to the HGN were positive for vesicular glutamate transporter (VGLUT) 2 mRNA but not glutamic acid decarboxylase 67 mRNA. Using a combination of anterograde and retrograde tracing techniques and immunohistochemistry for VGLUT2, we further demonstrated that the KFN axon terminals with VGLUT2 immunoreactivity established close contact with the HGN motoneurons whose axons constitute the medial branch of the HGn. The present results suggest that glutamatergic KFN fibers may exert excitatory influence upon the HGN motoneurons sending their axons to the medial branch of the HGn for the control of protruder tongue muscles contraction to maintain airway patency during respiration.

Reassessment of the structural basis of the ascending arousal system

The "ascending reticular activating system" theory proposed that neurons in the upper brainstem reticular formation projected to forebrain targets that promoted wakefulness. More recent formulations have emphasized that most neurons at the pontomesencephalic junction that participate in these pathways are actually in monoaminergic and cholinergic cell groups. However, cell-specific lesions of these cell groups have never been able to reproduce the deep coma seen after acute paramedian midbrain lesions that transect ascending axons at the caudal midbrain level. To determine whether the cortical afferents from the thalamus or the basal forebrain were more important in maintaining arousal, we first placed large cell-body-specific lesions in these targets. Surprisingly, extensive thalamic lesions had little effect on electroencephalographic (EEG) or behavioral measures of wakefulness or on c-Fos expression by cortical neurons during wakefulness. In contrast, animals with large basal forebrain lesions were behaviorally unresponsive and had a monotonous sub-1-Hz EEG, and little cortical c-Fos expression during continuous gentle handling. We then retrogradely labeled inputs to the basal forebrain from the upper brainstem, and found a substantial input from glutamatergic neurons in the parabrachial nucleus and adjacent precoeruleus area. Cell-specific lesions of the parabrachial-precoeruleus complex produced behavioral unresponsiveness, a monotonous sub-1-Hz cortical EEG, and loss of cortical c-Fos expression during gentle handling. These experiments indicate that in rats the reticulo-thalamo-cortical pathway may play a very limited role in behavioral or electrocortical arousal, whereas the projection from the parabrachial nucleus and precoeruleus region, relayed by the basal forebrain to the cerebral cortex, may be critical for this process.

Identification of central projections from amylin-activated neurons to the lateral hypothalamus

The ability of the pancreatic hormone amylin to inhibit food intake relies on a direct activation of the area postrema (AP). This activation is synaptically transmitted to the nucleus of the solitary tract (NTS), the lateral parabrachial nucleus (LPB), the central amygdaloid nucleus (Ce) and the lateral bed nucleus of stria terminalis (BSTL). Interestingly, neurons of the rostro-dorsal lateral hypothalamic area (dLHA), which are activated during fasting, are inhibited by peripheral amylin, although they lack amylin receptors. Using the retrograde tracer cholera toxin-B (Ctb) we analyzed whether the dLHA receives neuronal projections from amylin-activated brain areas. The anterograde tracer biotinylated dextran-amine (BDA) was used to confirm the projections and to identify further neuronal pathways potentially involved in amylin signaling. We identified dense projections from the amylin activated neurons in the LPB and sparse projections from the NTS to the dLHA. LPB fiber efferents were found in close proximity to dLHA nuclei activated by 24h of fasting. The AP and the Ce showed no projections to the dLHA. Dense efferents were also observed from the LPB to other hypothalamic areas, namely to the ventromedial, dorsomedial, paraventricular and arcuate nuclei. This study provides neuroanatomical evidence that among the amylin activated areas, the LPB provides the strongest input to the dLHA, thus it may mediate the amylin-induced inhibition of the dLHA.

The parabrachial nucleus is a critical link in the transmission of short latency nociceptive information to midbrain dopaminergic neurons

Many dopaminergic neurons exhibit a short-latency response to noxious stimuli, the source of which is unknown. Here we report that the nociceptive-recipient parabrachial nucleus appears to be a critical link in the transmission of pain related information to dopaminergic neurons. Injections of retrograde tracer into the substantia nigra pars compacta of the rat labelled neurons in both the lateral and medial parts of the parabrachial nucleus, and intra-parabrachial injections of anterograde tracers revealed robust projections to the pars compacta and ventral tegmental area. Axonal boutons were seen in close association with tyrosine hydroxylase-positive (presumed dopaminergic) and negative elements in these regions. Simultaneous extracellular recordings were made from parabrachial and dopaminergic neurons in the anaesthetized rat, during the application of noxious footshock. Parabrachial neurons exhibited a short-latency, short duration excitation to footshock while dopaminergic neurons exhibited a short-latency inhibition. Response latencies of dopaminergic neurons were reliably longer than those of parabrachial neurons. Intra-parabrachial injections of the local anasethetic lidocaine or the GABA_A receptor antagonist muscimol reduced tonic parabrachial activity and the amplitude (and in the case of lidocaine, duration) of the phasic response to footshock. Suppression of parabrachial activity with lidocaine reduced the baseline firing rate of dopaminergic neurons, while both lidocaine and muscimol reduced the amplitude of the phasic inhibitory response to footshock, in the case of lidocaine sometimes abolishing it altogether. Considered together, these results suggest that the parabrachial nucleus is an important source of short-latency nociceptive input to the dopaminergic neurons.

Anatomical changes of phrenic motoneurons during development

Although the phrenic motoneurons are relatively well-developed at the time of birth as compared to non-respiratory motoneurons, they show distinct anatomical changes during postnatal development. In the present review we summarize anatomical changes of phrenic motoneurons during pre- and postnatal development. Cell bodies of phrenic motoneurons migrate into the ventromedial region of the ventral horn of C3-C6 by E13-E14 in the rat. During development the sizes and surface areas of phrenic motoneurons are increased with changes in dendritic morphology.

Vesicular glutamate transporter 2-immunoreactive synapses onto phrenic motoneurons in the neonatal rat

The synaptic organization between vesicular glutamate transporter 2 (VGLUT2)-immunoreactive (ir) axon terminals and phrenic motoneurons in the neonatal rat was examined using a combined retrograde tracing and immunohistochemistry for VGLUT2. The phrenic nucleus (PhN) contained large numbers of VGLUT2-ir axon terminals, some of which made axosomatic and axodendritic synapses with PhN motoneurons. These terminals were of asymmetrical type and contained spherical clear synaptic vesicles. The results suggest that in the neonatal rat glutamatergic synapses onto PhN motoneurons exist and mediate excitatory transmission to drive PhN motoneurons.

Pyramidal neurons in rat prefrontal cortex projecting to ventral tegmental area and dorsal raphe nucleus express 5-HT2A receptors

The prefrontal cortex (PFC) is involved in higher brain functions altered in schizophrenia. Classical antipsychotics modulate cortico-limbic circuits mainly through subcortical D2 receptor blockade, whereas second generation (atypical) antipsychotics preferentially target cortical 5-HT receptors. Anatomical and functional evidence supports a PFC-based control of the brainstem monoaminergic nuclei. Using a combination of retrograde tracing experiments and in situ hybridization we report that a substantial proportion of PFC pyramidal neurons projecting to the dorsal raphe (DR) and/or ventral tegmental area (VTA) express 5-HT(2A) receptors. Cholera-toxin B application into the DR and the VTA retrogradely labeled projection neurons in the medial PFC (mPFC) and in orbitofrontal cortex (OFC). In situ hybridization of 5-HT(2A) receptor mRNA in the same tissue sections labeled a large neuronal population in mPFC and OFC. The percentage of DR-projecting neurons expressing 5-HT(2A) receptor mRNA was approximately 60% in mPFC and approximately 75% in OFC (n = 3). Equivalent values for VTA-projecting neurons were approximately 55% in both mPFC and ventral OFC. Thus, 5-HT(2A) receptor activation/blockade in PFC may have downstream effects on dopaminergic and serotonergic systems via direct descending pathways. Atypical antipsychotics may distally modulate monoaminergic cells through PFC 5-HT(2A) receptor blockade, presumably decreasing the activity of neurons receiving direct cortical inputs.

The chemical neuroanatomy of breathing

The chemical neuroanatomy of breathing must ultimately encompass all the various neuronal elements physiologically identified in brainstem respiratory circuits and their apparent aggregation into "compartments" within the medulla and pons. These functionally defined respiratory compartments in the brainstem provide the major source of input to cranial motoneurons controlling the airways, and to spinal motoneurons activating inspiratory and expiratory pump muscles. This review provides an overview of the neuroanatomy of the major compartments comprising brainstem respiratory circuits, and a synopsis of the transmitters used by their constituent respiratory neurons.

Identification of neurotransmitters and co-localization of transmitters in brainstem respiratory neurons

Identifying the major ionotropic neurotransmitter in a respiratory neuron is of critical importance in determining how the neuron fits into the respiratory system, whether in producing or modifying respiratory drive and rhythm. There are now several groups of respiratory neurons whose major neurotransmitters have been identified and in some of these cases, more than one transmitter has been identified in particular neurons. This review will describe the physiologically identified neurons in major respiratory areas that have been phenotyped for major ionotropic transmitters as well as those where more than one transmitter has been identified. Although the purpose of the additional transmitter has not been elucidated for any of the respiratory neurons, some examples from other systems will be discussed.

CO2-sensitive preinspiratory neurons of the parafacial respiratory group express Phox2b in the neonatal rat

Phox2b protein is a specific marker for neurons in the parafacial region of the ventral medulla, which are proposed to play a role in central chemoreception and postnatal survival. Mutations of PHOX2B cause congenital central hypoventilation syndrome. However, there have been no reports concerning electrophysiological characteristics of these Phox2b-expressing neurons in the parafacial region of the neonate immediately after birth. This region overlaps with the parafacial respiratory group (pFRG) composed predominantly of preinspiratory (Pre-I) neurons that are involved in respiratory rhythm generation. We studied (1) whether pFRG neurons are Phox2b immunoreactive or not and (2) whether they show intrinsic CO(2) chemosensitivity. We found that most pFRG/Pre-I neurons were Phox2b immunoreactive and depolarized upon increase in CO(2) concentration under condition of action potential-dependent synaptic transmission blockade by tetrodotoxin. We also confirmed that these pFRG neurons expressed neurokinin-1 receptor. They were tyrosine hydroxylase negative and presumed to be glutamatergic. Our findings suggest that Phox2b-expressing parafacial neurons play a role in respiratory rhythm generation as well as central chemoreception and thus are essential for postnatal survival.

Anatomical architecture and responses to acidosis of a novel respiratory neuron group in the high cervical spinal cord (HCRG) of the neonatal rat

It has been postulated that there exists a neuronal mechanism that generates respiratory rhythm and modulates respiratory output pattern in the high cervical spinal cord. Recently, we have found a novel respiratory neuron group in the ventral portion of the high cervical spinal cord, and named it the high cervical spinal cord respiratory group (HCRG). In the present study, we analyzed the detailed anatomical architecture of the HCRG region by double immunostaining of the region using a neuron-specific marker (NeuN) and a marker for motoneurons (ChAT) in the neonatal rat. We found a large number of small NeuN-positive cells without ChAT-immunoreactivity, which were considered interneurons. We also found two and three clusters of motoneurons in the ventral portion of the ventral horn at C1 and C2 levels, respectively. Next, we examined responses of HCRG neurons to respiratory and metabolic acidosis in vitro by voltage-imaging together with cross correlation techniques, i.e., by correlation coefficient imaging, in order to understand the functional role of HCRG neurons. Both respiratory and metabolic acidosis caused the same pattern of changes in their spatiotemporal activation profiles, and the respiratory-related area was enlarged in the HCRG region. After acidosis was introduced, preinspiratory phase-dominant activity was recruited in a number of pixels, and more remarkably inspiratory phase-dominant activity was recruited in a large number of pixels. We suggest that the HCRG composes a local respiratory neuronal network consisting of interneurons and motoneurons and plays an important role in respiratory augmentation in response to acidosis.