Astrocytes control breathing through pH-dependent release of ATP

Application of optogenetic glial cells to neuron-glial communication

Optogenetic techniques combine optics and genetics to enable cell-specific targeting and precise spatiotemporal control of excitable cells, and they are increasingly being employed. One of the most significant advantages of the optogenetic approach is that it allows for the modulation of nearby cells or circuits with millisecond precision, enabling researchers to gain a better understanding of the complex nervous system. Furthermore, optogenetic neuron activation permits the regulation of information processing in the brain, including synaptic activity and transmission, and also promotes nerve structure development. However, the optimal conditions remain unclear, and further research is required to identify the types of cells that can most effectively and precisely control nerve function. Recent studies have described optogenetic glial manipulation for coordinating the reciprocal communication between neurons and glia. Optogenetically stimulated glial cells can modulate information processing in the central nervous system and provide structural support for nerve fibers in the peripheral nervous system. These advances promote the effective use of optogenetics, although further experiments are needed. This review describes the critical role of glial cells in the nervous system and reviews the optogenetic applications of several types of glial cells, as well as their significance in neuron-glia interactions. Together, it briefly discusses the therapeutic potential and feasibility of optogenetics.

An autonomic mode of brain activity

The relevance of interactions between autonomic and central nervous systems remains unclear for human brain function and health, particularly when both systems are challenged under sleep deprivation (SD). We measured brain activity (with fMRI), pulse and respiratory signals, and baseline brain amyloid beta burden (with PET) in healthy participants. We found that SD relative to rested wakefulness (RW) resulted in a significant increase in synchronized low frequency (LF, < 0.1 Hz) activity in an autonomically-related network (AN), including dorsal attention, visual, and sensorimotor regions, which we previously found to have consistent temporal coupling with LF pulse signal changes (regulated by sympathetic tone). SD resulted in a significant phase coherence between the LF component of the pulse signal and a medial network with peak effects in the midbrain reticular formation, and between LF component of the respiratory variations (regulated by respiratory motor output) and a cerebellar network. The LF power of AN during SD was significantly and independently correlated with pulse-medial network and respiratory-cerebellar network phase coherences (total adjusted R2 = 0.78). Higher LF power of AN during SD (but not RW) was associated with lower amyloid beta burden (Cohen's d = 0.8). In sum, SD triggered an autonomic mode of synchronized brain activity that was associated with distinct autonomic-central interactions. Findings highlight the direct relevance of global cortical synchronization to brain clearance mechanisms.

Whole-brain analysis of CO2 chemosensitive regions and identification of the retrotrapezoid and medullary raphé nuclei in the common marmoset (Callithrix jacchus)

Respiratory chemosensitivity is an important mechanism by which the brain senses changes in blood partial pressure of CO2 (PCO2). It is proposed that special neurons (and astrocytes) in various brainstem regions play key roles as CO2 central respiratory chemosensors in rodents. Although common marmosets (Callithrix jacchus), New-World non-human primates, show similar respiratory responses to elevated inspired CO2 as rodents, the chemosensitive regions in marmoset brain have not been defined yet. Here, we used c-fos immunostainings to identify brain-wide CO2-activated brain regions in common marmosets. In addition, we mapped the location of the retrotrapezoid nucleus (RTN) and raphé nuclei in the marmoset brainstem based on colocalization of CO2-induced c-fos immunoreactivity with Phox2b, and TPH immunostaining, respectively. Our data also indicated that, similar to rodents, marmoset RTN astrocytes express Phox2b and have complex processes that create a meshwork structure at the ventral surface of medulla. Our data highlight some cellular and structural regional similarities in brainstem of the common marmosets and rodents.

Cell Responses of the Ventrolateral Medulla to PAR1 Activation and Changes in Respiratory Rhythm in Newborn Rat En Bloc Brainstem-Spinal Cord Preparations

Proteinase-activated receptor-1 (PAR1) is expressed in astrocytes of various brain regions, and its activation is involved in the modulation of neuronal activity. Here, we report effects of PAR1 selective agonist TFLLR on respiratory rhythm generation in brainstem-spinal cord preparations. Preparations were isolated from newborn rats (P0-P4) under deep isoflurane anesthesia and were transversely cut at the rostral medulla. Preparations were superfused with artificial cerebrospinal fluid (25-26 °C), and inspiratory C4 ventral root activity was monitored. The responses to TFLLR of cells close to the cut surface were detected by calcium imaging or membrane potential recordings. Application of 10 μM TFLLR (4 min) induced a rapid and transient increase of calcium signal in cells of the ventrolateral respiratory regions of the medulla. More than 88% of responding cells (223/254 cells from 13 preparations) were also activated by low (0.2 mM) K+ solution, suggesting that they were astrocytes. Immunohistochemical examination demonstrated that PAR1 was expressed on many astrocytes. Respiratory-related neurons in the medulla were transiently hyperpolarized (-1.8 mV) during 10 μM TFLLR application, followed by weak membrane depolarization after washout. C4 burst rate decreased transiently in response to application of TFLLR, followed by a slight increase. The inhibitory effect was partially blocked by 50 μM theophylline. In conclusion, activation of astrocytes via PAR1 resulted in a decrease of inspiratory C4 burst rate in association with transient hyperpolarization of respiratory-related neurons. After washout, slow and weak excitatory responses appeared. Adenosine may be partially involved in the inhibitory effect of PAR1 activation.

Molecular machines stimulate intercellular calcium waves and cause muscle contraction

Intercellular calcium waves (ICW) are complex signalling phenomena that control many essential biological activities, including smooth muscle contraction, vesicle secretion, gene expression and changes in neuronal excitability. Accordingly, the remote stimulation of ICW could result in versatile biomodulation and therapeutic strategies. Here we demonstrate that light-activated molecular machines (MM)-molecules that perform mechanical work on the molecular scale-can remotely stimulate ICW. MM consist of a polycyclic rotor and stator that rotate around a central alkene when activated with visible light. Live-cell calcium-tracking and pharmacological experiments reveal that MM-induced ICW are driven by the activation of inositol-triphosphate-mediated signalling pathways by unidirectional, fast-rotating MM. Our data suggest that MM-induced ICW can control muscle contraction in vitro in cardiomyocytes and animal behaviour in vivo in Hydra vulgaris. This work demonstrates a strategy for directly controlling cell signalling and downstream biological function using molecular-scale devices.

Criteria for central respiratory chemoreceptors: experimental evidence supporting current candidate cell groups

An interoceptive homeostatic system monitors levels of CO2/H+ and provides a proportionate drive to respiratory control networks that adjust lung ventilation to maintain physiologically appropriate levels of CO2 and rapidly regulate tissue acid-base balance. It has long been suspected that the sensory cells responsible for the major CNS contribution to this so-called respiratory CO2/H+ chemoreception are located in the brainstem-but there is still substantial debate in the field as to which specific cells subserve the sensory function. Indeed, at the present time, several cell types have been championed as potential respiratory chemoreceptors, including neurons and astrocytes. In this review, we advance a set of criteria that are necessary and sufficient for definitive acceptance of any cell type as a respiratory chemoreceptor. We examine the extant evidence supporting consideration of the different putative chemoreceptor candidate cell types in the context of these criteria and also note for each where the criteria have not yet been fulfilled. By enumerating these specific criteria we hope to provide a useful heuristic that can be employed both to evaluate the various existing respiratory chemoreceptor candidates, and also to focus effort on specific experimental tests that can satisfy the remaining requirements for definitive acceptance.

Purinergic signaling mediates neuroglial interactions to modulate sighs

Sighs prevent the collapse of alveoli in the lungs, initiate arousal under hypoxic conditions, and are an expression of sadness and relief. Sighs are periodically superimposed on normal breaths, known as eupnea. Implicated in the generation of these rhythmic behaviors is the preBötzinger complex (preBötC). Our experimental evidence suggests that purinergic signaling is necessary to generate spontaneous and hypoxia-induced sighs in a mouse model. Our results demonstrate that driving calcium increases in astrocytes through pharmacological methods robustly increases sigh, but not eupnea, frequency. Calcium imaging of preBötC slices corroborates this finding with an increase in astrocytic calcium upon application of sigh modulators, increasing intracellular calcium through g-protein signaling. Moreover, photo-activation of preBötC astrocytes is sufficient to elicit sigh activity, and this response is blocked with purinergic antagonists. We conclude that sighs are modulated through neuron-glia coupling in the preBötC network, where the distinct modulatory responses of neurons and glia allow for both rhythms to be independently regulated.

Fiber optical imaging of astroglial calcium signaling in the respiratory network in the working heart brainstem preparation

The neuronal activity in the respiratory network strongly depends on a variety of different neuromodulators. Given the essential role of astrocytes in stabilizing respiratory network activity generated by neurons in the preBötzinger complex (preBötC), our aim was to investigate astrocytic calcium signaling in the working heart brainstem preparation using fiber-optical imaging. By using transgenic mice that express GCaMP6s specifically in astrocytes, we successfully recorded astrocytic calcium signals in response to norepinephrine from individual astrocytes.

The astrocytic Na+ -HCO3 - cotransporter, NBCe1, is dispensable for respiratory chemosensitivity

The interoceptive homeostatic mechanism that controls breathing, blood gases and acid-base balance in response to changes in CO2 /H+ is exquisitely sensitive, with convergent roles proposed for chemosensory brainstem neurons in the retrotrapezoid nucleus (RTN) and their supporting glial cells. For astrocytes, a central role for NBCe1, a Na+ -HCO3 - cotransporter encoded by Slc4a4, has been envisaged in multiple mechanistic models (i.e. underlying enhanced CO2 -induced local extracellular acidification or purinergic signalling). We tested these NBCe1-centric models by using conditional knockout mice in which Slc4a4 was deleted from astrocytes. In GFAP-Cre;Slc4a4fl/fl mice we found diminished expression of Slc4a4 in RTN astrocytes by comparison to control littermates, and a concomitant reduction in NBCe1-mediated current. Despite disrupted NBCe1 function in RTN-adjacent astrocytes from these conditional knockout mice, CO2 -induced activation of RTN neurons or astrocytes in vitro and in vivo, and CO2 -stimulated breathing, were indistinguishable from NBCe1-intact littermates; hypoxia-stimulated breathing and sighs were likewise unaffected. We obtained a more widespread deletion of NBCe1 in brainstem astrocytes by using tamoxifen-treated Aldh1l1-Cre/ERT2;Slc4a4fl/fl mice. Again, there was no difference in effects of CO2 or hypoxia on breathing or on neuron/astrocyte activation in NBCe1-deleted mice. These data indicate that astrocytic NBCe1 is not required for the respiratory responses to these chemoreceptor stimuli in mice, and that any physiologically relevant astrocytic contributions must involve NBCe1-independent mechanisms. KEY POINTS: The electrogenic NBCe1 transporter is proposed to mediate local astrocytic CO2 /H+ sensing that enables excitatory modulation of nearby retrotrapezoid nucleus (RTN) neurons to support chemosensory control of breathing. We used two different Cre mouse lines for cell-specific and/or temporally regulated deletion of the NBCe1 gene (Slc4a4) in astrocytes to test this hypothesis. In both mouse lines, Slc4a4 was depleted from RTN-associated astrocytes but CO2 -induced Fos expression (i.e. cell activation) in RTN neurons and local astrocytes was intact. Likewise, respiratory chemoreflexes evoked by changes in CO2 or O2 were unaffected by loss of astrocytic Slc4a4. These data do not support the previously proposed role for NBCe1 in respiratory chemosensitivity mediated by astrocytes.

Astrocytes of the eye and optic nerve: heterogeneous populations with unique functions mediate axonal resilience and vulnerability to glaucoma

The role of glia, particularly astrocytes, in mediating the central nervous system's response to injury and neurodegenerative disease is an increasingly well studied topic. These cells perform myriad support functions under physiological conditions but undergo behavioral changes - collectively referred to as 'reactivity' - in response to the disruption of neuronal homeostasis from insults, including glaucoma. However, much remains unknown about how reactivity alters disease progression - both beneficially and detrimentally - and whether these changes can be therapeutically modulated to improve outcomes. Historically, the heterogeneity of astrocyte behavior has been insufficiently addressed under both physiological and pathological conditions, resulting in a fragmented and often contradictory understanding of their contributions to health and disease. Thanks to increased focus in recent years, we now know this heterogeneity encompasses both intrinsic variation in physiological function and insult-specific changes that vary between pathologies. Although previous studies demonstrate astrocytic alterations in glaucoma, both in human disease and animal models, generally these findings do not conclusively link astrocytes to causative roles in neuroprotection or degeneration, rather than a subsequent response. Efforts to bolster our understanding by drawing on knowledge of brain astrocytes has been constrained by the primacy in the literature of findings from peri-synaptic 'gray matter' astrocytes, whereas much early degeneration in glaucoma occurs in axonal regions populated by fibrous 'white matter' astrocytes. However, by focusing on findings from astrocytes of the anterior visual pathway - those of the retina, unmyelinated optic nerve head, and myelinated optic nerve regions - we aim to highlight aspects of their behavior that may contribute to axonal vulnerability and glaucoma progression, including roles in mitochondrial turnover and energy provisioning. Furthermore, we posit that astrocytes of the retina, optic nerve head and myelinated optic nerve, although sharing developmental origins and linked by a network of gap junctions, may be best understood as distinct populations residing in markedly different niches with accompanying functional specializations. A closer investigation of their behavioral repertoires may elucidate not only their role in glaucoma, but also mechanisms to induce protective behaviors that can impede the progressive axonal damage and retinal ganglion cell death that drive vision loss in this devastating condition.

Astrocyte Endfeet in Brain Function and Pathology: Open Questions

Astrocyte endfeet enwrap the entire vascular tree within the central nervous system, where they perform important functions in regulating the blood-brain barrier (BBB), cerebral blood flow, nutrient uptake, and waste clearance. Accordingly, astrocyte endfeet contain specialized organelles and proteins, including local protein translation machinery and highly organized scaffold proteins, which anchor channels, transporters, receptors, and enzymes critical for astrocyte-vascular interactions. Many neurological diseases are characterized by the loss of polarization of specific endfoot proteins, vascular dysregulation, BBB disruption, altered waste clearance, or, in extreme cases, loss of endfoot coverage. A role for astrocyte endfeet has been demonstrated or postulated in many of these conditions. This review provides an overview of the development, composition, function, and pathological changes of astrocyte endfeet and highlights the gaps in our knowledge that future research should address.

Glial modulation of the parallel memory formation

Actions from glial cells could affect the readiness and efficacy of learning and memory. Using a mouse cerebellar-dependent horizontal optokinetic response motor learning paradigm, short-term memory (STM) formation during the online training period and long-term memory (LTM) formation during the offline rest period were studied. A large variability of online and offline learning efficacies was found. The early bloomers with booming STM often had a suppressed LTM formation and late bloomers with no apparent acute training effect often exhibited boosted offline learning performance. Anion channels containing LRRC8A are known to release glutamate. Conditional knockout of LRRC8A specifically in astrocytes including cerebellar Bergmann glia resulted in a complete loss of STM formation while the LTM formation during the rest period remained. Optogenetic manipulation of glial activity by channelrhodopsin-2 or archaerhodopsin-T (ArchT) during the online training resulted in enhancement or suppression of STM formation, respectively. STM and LTM are likely to be triggered simultaneously during online training, but LTM is expressed later during the offline period. STM appears to be volatile and the achievement during the online training is not handed over to LTM. In addition, we found that glial ArchT photoactivation during the rest period resulted in the augmentation of LTM formation. These data suggest that STM formation and LTM formation are parallel separate processes. Strategies to weigh more on the STM or the LTM could depend on the actions of the glial cells.

NBCe1-B/C-knockout mice exhibit an impaired respiratory response and an enhanced renal response to metabolic acidosis

The sodium-bicarbonate cotransporter (NBCe1) has three primary variants: NBCe1-A, -B and -C. NBCe1-A is expressed in renal proximal tubules in the cortical labyrinth, where it is essential for reclaiming filtered bicarbonate, such that NBCe1-A knockout mice are congenitally acidemic. NBCe1-B and -C variants are expressed in chemosensitive regions of the brainstem, while NBCe1-B is also expressed in renal proximal tubules located in the outer medulla. Although mice lacking NBCe1-B/C (KOb/c) exhibit a normal plasma pH at baseline, the distribution of NBCe1-B/C indicates that these variants could play a role in both the rapid respiratory and slower renal responses to metabolic acidosis (MAc). Therefore, in this study we used an integrative physiologic approach to investigate the response of KOb/c mice to MAc. By means of unanesthetized whole-body plethysmography and blood-gas analysis, we demonstrate that the respiratory response to MAc (increase in minute volume, decrease in pCO2) is impaired in KOb/c mice leading to a greater severity of acidemia after 1 day of MAc. Despite this respiratory impairment, the recovery of plasma pH after 3-days of MAc remained intact in KOb/c mice. Using data gathered from mice housed in metabolic cages we demonstrate a greater elevation of renal ammonium excretion and greater downregulation of the ammonia recycling enzyme glutamine synthetase in KOb/c mice on day 2 of MAc, consistent with greater renal acid-excretion. We conclude that KOb/c mice are ultimately able to defend plasma pH during MAc, but that the integrated response is disturbed such that the burden of work shifts from the respiratory system to the kidneys, delaying the recovery of pH.

pH-sensing supramolecular fluorescent probes discovered by library screening

A new design concept for pH-sensing supramolecular fluorescent probes is reported. Supramolecular fluorescent pH probes based on pro-guest are designed and prepared. Pro-guests are designed to degrade under acidic condition and convert to competitive guests to displace encapsulated dyes, which leads to a significant enhancement in fluorescence intensity. A library of potential fluorescent pH probes is generated and screened to discover workable probes. These probes are capable of detecting the acidic pH in solution phase. We confirm that these supramolecular probes could detect the acidic environment in endosomal compartments in live cells.

ATP-mediated signalling in the central synapses

ATP released from the synaptic terminals and astrocytes can activate neuronal P2 receptors at a variety of locations across the CNS. Although the postsynaptic ATP-mediated signalling does not bring a major contribution into the excitatory transmission, it is instrumental for slow and diffuse modulation of synaptic dynamics and neuronal firing in many CNS areas. Neuronal P2X and P2Y receptors can be activated by ATP released from the synaptic terminals, astrocytes and microglia and thereby can participate in the regulation of synaptic homeostasis and plasticity. There is growing evidence of importance of purinergic regulation of synaptic transmission in different physiological and pathological contexts. Here, we review the main mechanisms underlying the complexity and diversity of purinergic signalling and purinergic modulation in central neurons.

Respiration: The circuit for hypoxia-induced sighs

Sighs are a response to hypoxia, altered lung volume, and emotional state. A recent study employing in vivo physiology, optogenetics, chemoablation, and genetic silencing shows the importance of gastrin releasing peptide-expressing neurons in mediating sighs.

Fetal defenses against intrapartum head compression-implications for intrapartum decelerations and hypoxic-ischemic injury

Uterine contractions during labor and engagement of the fetus in the birth canal can compress the fetal head. Its impact on the fetus is unclear and still controversial. In this integrative physiological review, we highlight evidence that decelerations are uncommonly associated with fetal head compression. Next, the fetus has an impressive ability to adapt to increased intracranial pressure through activation of the intracranial baroreflex, such that fetal cerebral perfusion is well-maintained during labor, except in the setting of prolonged systemic hypoxemia leading to secondary cardiovascular compromise. Thus, when it occurs, fetal head compression is not necessarily benign but does not seem to be a common contributor to intrapartum decelerations. Finally, the intracranial baroreflex and the peripheral chemoreflex (the response to acute hypoxemia) have overlapping efferent effects. We propose the hypothesis that these reflexes may work synergistically to promote fetal adaptation to labor.

Astrocytic Kir4.1 channels regulate locomotion by orchestrating neuronal rhythmicity in the spinal network

Neuronal rhythmogenesis in the spinal cord is correlated with variations in extracellular K⁺ levels ([K⁺ ]_e ). Astrocytes play important role in [K⁺ ]_e homeostasis and compute neuronal information. Yet it is unclear how neuronal oscillations are regulated by astrocytic K⁺ homeostasis. Here we identify the astrocytic inward-rectifying K⁺ channel Kir4.1 (a.k.a. Kcnj10) as a key molecular player for neuronal rhythmicity in the spinal central pattern generator (CPG). By combining two-photon calcium imaging with electrophysiology, immunohistochemistry and genetic tools, we report that astrocytes display Ca²⁺ transients before and during oscillations of neighboring neurons. Inhibition of astrocytic Ca²⁺ transients with BAPTA decreases the barium-sensitive Kir4.1 current responsible of K⁺ clearance. Finally, we show in mice that Kir4.1 knockdown in astrocytes progressively prevents neuronal oscillations and alters the locomotor pattern resulting in lower motor performances in challenging tasks. These data identify astroglial Kir4.1 channels as key regulators of neuronal rhythmogenesis in the CPG driving locomotion.

Distinct astrocytic modulatory roles in sensory transmission during sleep, wakefulness, and arousal states in freely moving mice

Despite extensive research on astrocytic Ca²⁺ in synaptic transmission, its contribution to the modulation of sensory transmission during different brain states remains largely unknown. Here, by using two-photon microscopy and whole-cell recordings, we show two distinct astrocytic Ca²⁺ signals in the murine barrel cortex: a small, long-lasting Ca²⁺ increase during sleep and a large, widespread but short-lasting Ca²⁺ spike when aroused. The large Ca²⁺ wave in aroused mice was inositol trisphosphate (IP3)-dependent, evoked by the locus coeruleus-norepinephrine system, and enhanced sensory input, contributing to reliable sensory transmission. However, the small Ca²⁺ transient was IP3-independent and contributed to decreased extracellular K⁺, hyperpolarization of the neurons, and suppression of sensory transmission. These events respond to different pharmacological inputs and contribute to distinct sleep and arousal functions by modulating the efficacy of sensory transmission. Together, our data demonstrate an important function for astrocytes in sleep and arousal states via astrocytic Ca²⁺ waves.

The emerging science of Glioception: Contribution of glia in sensing, transduction, circuit integration of interoception

Interoception is the process by which the nervous system regulates internal functions to achieve homeostasis. The role of neurons in interoception has received considerable recent attention, but glial cells also contribute. Glial cells can sense and transduce signals including osmotic, chemical, and mechanical status of extracellular milieu. Their ability to dynamically communicate "listening" and "talking" to neurons is necessary to monitor and regulate homeostasis and information integration in the nervous system. This review introduces the concept of "Glioception" and focuses on the process by which glial cells sense, interpret and integrate information about the inner state of the organism. Glial cells are ideally positioned to act as sensors and integrators of diverse interoceptive signals and can trigger regulatory responses via modulation of the activity of neuronal networks, both in physiological and pathological conditions. We believe that understanding and manipulating glioceptive processes and underlying molecular mechanisms provide a key path to develop new therapies for the prevention and alleviation of devastating interoceptive dysfunctions, among which pain is emphasized here with more focused details.

Nanoparticles-mediated ion channels manipulation: From their membrane interactions to bioapplications

Ion channels are transmembrane proteins ubiquitously expressed in all cells that control various ions (e.g. Na⁺, K⁺, Ca²⁺ and Cl^- etc) crossing cellular plasma membrane, which play critical roles in physiological processes including regulating signal transduction, cell proliferation as well as excitatory cell excitation and conduction. Abnormal ion channel function is usually associated with dysfunctions and many diseases, such as neurodegenerative disorders, ophthalmic diseases, pulmonary diseases and even cancers. The precise regulation of ion channels not only helps to decipher physiological and pathological processes, but also is expected to become cutting-edge means for disease treatment. Recently, nanoparticles-mediated ion channel manipulation emerges as a highly promising way to meet the increasing requirements with respect to their simple, efficient, precise, spatiotemporally controllable and non-invasive regulation in biomedicine and other research frontiers. Thanks the advantages of their unique properties, nanoparticles can not only directly block the pore sites or kinetics of ion channels through their tiny size effect, and perturb active voltage-gated ion channel by their charged surface, but they can also act as antennas to conduct or enhance external physical stimuli to achieve spatiotemporal, precise and efficient regulation of various ion channel activities (e.g. light-, mechanical-, and temperature-gated ion channels etc). So far, nanoparticles-mediated ion channel regulation has shown potential prospects in many biomedical fields at the interfaces of neuro- and cardiovascular modulation, physiological function regeneration and tumor therapy et al. Towards such important fields, in this typical review, we specifically outline the latest studies of different types of ion channels and their activities relevant to the diseases. In addition, the different types of stimulation responsive nanoparticles, their interaction modes and targeting strategies towards the plasma membrane ion channels will be systematically summarized. More importantly, the ion channel regulatory methods mediated by functional nanoparticles and their bioapplications associated with physiological modulation and therapeutic development will be discussed. Last but not least, current challenges and future perspectives in this field will be covered as well.

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.

Astrocyte heterogeneity and interactions with local neural circuits

Astrocytes are ubiquitous within the central nervous system (CNS). These cells possess many individual processes which extend out into the neuropil, where they interact with a variety of other cell types, including neurons at synapses. Astrocytes are now known to be active players in all aspects of the synaptic life cycle, including synapse formation and elimination, synapse maturation, maintenance of synaptic homeostasis and modulation of synaptic transmission. Traditionally, astrocytes have been studied as a homogeneous group of cells. However, recent studies have uncovered a surprising degree of heterogeneity in their development and function, suggesting that astrocytes may be matched to neurons to support local circuits. Hence, a better understanding of astrocyte heterogeneity and its implications are needed to understand brain function.

Astrocytes Transplanted during Early Postnatal Development Integrate, Mature, and Survive Long Term in Mouse Cortex

Astrocytes have complex structural, molecular, and physiological properties and form specialized microenvironments that support circuit-specific functions in the CNS. To better understand how astrocytes acquire their unique features, we transplanted immature mouse cortical astrocytes into the developing cortex of male and female mice and assessed their integration, maturation, and survival. Within days, transplanted astrocytes developed morphologies and acquired territories and tiling behavior typical of cortical astrocytes. At 35-47 d post-transplantation, astrocytes appeared morphologically mature and expressed levels of EAAT2/GLT1 similar to nontransplanted astrocytes. Transplanted astrocytes also supported excitatory/inhibitory (E/I) presynaptic terminals within their territories, and displayed normal Ca2+ events. Transplanted astrocytes showed initially reduced expression of aquaporin 4 (AQP4) at endfeet and elevated expression of EAAT1/GLAST, with both proteins showing normalized expression by 110 d and one year post-transplantation, respectively. To understand how specific brain regions support astrocytic integration and maturation, we transplanted cortical astrocytes into the developing cerebellum. Cortical astrocytes interlaced with Bergmann glia (BG) in the cerebellar molecular layer to establish discrete territories. However, transplanted astrocytes retained many cortical astrocytic features including higher levels of EAAT2/GLT1, lower levels of EAAT1/GLAST, and the absence of expression of the AMPAR subunit GluA1. Collectively, our findings demonstrate that immature cortical astrocytes integrate, mature, and survive (more than one year) following transplantation and retain cortical astrocytic properties. Astrocytic transplantation can be useful for investigating cell-autonomous (intrinsic) and non-cell-autonomous (environmental) mechanisms contributing to astrocytic development/diversity, and for determining the optimal timing for transplanting astrocytes for cellular delivery or replacement in regenerative medicine.SIGNIFICANCE STATEMENT The mechanisms that enable astrocytes to acquire diverse molecular and structural properties remain to be better understood. In this study, we systematically analyzed the properties of cortical astrocytes following their transplantation to the early postnatal brain. We found that immature cortical astrocytes transplanted into cerebral cortex during early postnatal mouse development integrate and establish normal astrocytic properties, and show long-term survival in vivo (more than one year). In contrast, transplanted cortical astrocytes display reduced or altered ability to integrate into the more mature cerebral cortex or developing cerebellum, respectively. This study demonstrates the developmental potential of transplanted cortical astrocytes and provides an approach to tease apart cell-autonomous (intrinsic) and non-cell-autonomous (environmental) mechanisms that determine the structural, molecular, and physiological phenotype of astrocytes.

Local brain environment changes associated with epileptogenesis

Plastic change of the neuronal system has traditionally been assumed to be governed primarily by the long-term potentiation/depression mechanisms of synaptic transmission. However, a rather simple shift in the ambient ion, transmitter and metabolite concentrations could have a pivotal role in generating plasticity upon the physiological process of learning and memory. Local brain environment and metabolic changes could also be the cause and consequences of the pathogenesis leading to epilepsy. Governing of the local brain environment is the primal function of astrocytes. The metabolic state of the entire brain is strongly linked to the activity of the lateral hypothalamus. In this study, plastic change of astrocyte reactions in the lateral hypothalamus was examined using epileptogenesis as an extreme form of plasticity. Fluorescent sensors for calcium or pH expressed in astrocytes were examined for up to one week by in vivo fibre photometry in freely moving transgenic male mice. Optical fluctuations on a timescale of seconds is difficult to assess because these signals are heavily influenced by local brain blood volume changes and pH changes. Using a newly devised method for the analysis of the optical signals, changes in Ca2+ and pH in astrocytes and changes in local brain blood volume associated with hippocampal-stimulated epileptic seizures were extracted. Following a transient alkaline shift in the astrocyte triggered by neuronal hyperactivity, a prominent acidic shift appeared in response to intensified seizure which developed with kindling. The acidic shift was unexpected as transient increase in local brain blood volume was observed in response to intensified seizures, which should lead to efficient extrusion of the acidic CO2. The acidic shift could be a result of glutamate transporter activity and/or due to the increased metabolic load of astrocytes leading to increased CO2 and lactate production. This acidic shift may trigger additional gliotransmitter release from astrocytes leading to the exacerbation of epilepsy. As all cellular enzymic reactions are influenced by Ca2+ and pH, changes in these parameters could also have an impact on the neuronal circuit activity. Thus, controlling the astrocyte pH and/or Ca2+ could be a new therapeutic target for treatment of epilepsy or prevention of undesired plasticity associated with epileptogenesis.

Neuronal and astrocytic protein connections and associated adhesion molecules

Astrocytes are morphologically complex, with a myriad of processes which allow contact with other astrocytes, blood vessels, and neurons. Adhesion molecules expressed by these cells regulate this connectivity. Adhesion molecules are required to form and maintain functional neural circuits, but their importance and mechanisms of action, particularly in astrocyte-neuron contact, remain unresolved. Several studies of neuron-astrocyte connections have demonstrated the vital functions of adhesion molecules, including neuron-glia cell adhesion molecules, astrotactins, and protocadherins. In this review, we provide an overview and perspective of astrocyte-neuron contacts mediated by adhesion molecules in developing neural circuits and synapse formation, especially in the cerebellum. We also outline a novel mechanism of interaction between neurons and astrocytes in the tripartite synapses that has been recently found by our group.

The role of astrocytes in behaviors related to emotion and motivation

Astrocytes are present throughout the brain and intimately interact with neurons and blood vessels. Three decades of research have shown that astrocytes reciprocally communicate with neurons and other non-neuronal cells in the brain and dynamically regulate cell function. Astrocytes express numerous receptors for neurotransmitters, neuromodulators, and cytokines and receive information from neurons, other astrocytes, and other non-neuronal cells. Among those receptors, the main focus has been G-protein coupled receptors. Activation of G-protein coupled receptors leads to dramatic changes in intracellular signaling (Ca²⁺ and cAMP), which is considered a form of astrocyte activity. Methodological improvements in measurement and manipulation of astrocytes have advanced our understanding of the role of astrocytes in circuits and have begun to reveal unexpected functions of astrocytes in behavior. Recent studies have suggested that astrocytic activity regulates behavior flexibility, such as coping strategies for stress exposure, and plays an important role in behaviors related to emotion and motivation. Preclinical evidence suggests that impairment of astrocytic function contributes to psychiatric diseases, especially major depression. Here, we review recent progress on the role of astrocytes in behaviors related to emotion and motivation.

Could respiration-driven blood oxygen changes modulate neural activity?

Oxygen is critical for neural metabolism, but under most physiological conditions, oxygen levels in the brain are far more than are required. Oxygen levels can be dynamically increased by increases in respiration rate that are tied to the arousal state of the brain and cognition, and not necessarily linked to exertion by the body. Why these changes in respiration occur when oxygen is already adequate has been a long-standing puzzle. In humans, performance on cognitive tasks can be affected by very high or very low oxygen levels, but whether the physiological changes in blood oxygenation produced by respiration have an appreciable effect is an open question. Oxygen has direct effects on potassium channels, increases the degradation rate of nitric oxide, and is rate limiting for the synthesis of some neuromodulators. We discuss whether oxygenation changes due to respiration contribute to neural dynamics associated with attention and arousal.

Lactate-Mediated Signaling in the Brain-An Update

Lactate is a universal metabolite produced and released by all cells in the body. Traditionally it was viewed as energy currency that is generated from pyruvate at the end of the glycolytic pathway and sent into the extracellular space for other cells to take up and consume. In the brain, such a mechanism was postulated to operate between astrocytes and neurons many years ago. Later, the discovery of lactate receptors opened yet another chapter in the quest to understand lactate actions. Other ideas, such as modulation of NMDA receptors were also proposed. Up to this day, we still do not have a consensus view on the relevance of any of these mechanisms to brain functions or their contribution to human or animal physiology. While the field develops new ideas, in this brief review we analyze some recently published studies in order to focus on some unresolved controversies and highlight the limitations that need to be addressed in future work. Clearly, only by using similar and overlapping methods, cross-referencing experiments, and perhaps collaborative efforts, we can finally understand what the role of lactate in the brain is and why this ubiquitous molecule is so important.

Role for Astrocytes in mGluR-Dependent LTD in the Neocortex and Hippocampus

Astroglia are an active element of brain plasticity, capable to release small molecule gliotransmitters by various mechanisms and regulate synaptic strength. While importance of glia-neuron communications for long-term potentiation has been rather widely reported, research into role for astrocytes in long-depression (LTD) is just gaining momentum. Here, we explored the role for astrocytes in the prominent form of synaptic plasticity-mGluR-dependent LTD. We found out the substantial contribution of the Group I receptors, especially mGluR1 subtype, into Ca²⁺-signaling in hippocampal and neocortical astrocytes, which can be activated during synaptic stimulation used for LTD induction. Our data demonstrate that mGluR receptors can activate SNARE-dependent release of ATP from astrocytes which in turn can directly activate postsynaptic P2X receptors in the hippocampal and neocortical neurons. The latter mechanism has recently been shown to cause the synaptic depression via triggering the internalisation of AMPA receptors. Using mouse model of impaired glial exocytosis (dnSNARE mice), we demonstrated that mGluR-activated release of ATP from astrocytes is essential for regulation of mGluR-dependent LTD in CA3-CA1 and layer 2/3 synapses. Our data also suggest that astrocyte-related pathway relies mainly on mGluR1 receptors and act synergistically with neuronal mechanisms dependent mainly on mGluR5.

Deconstruction of a hypothalamic astrocyte-white adipocyte sympathetic axis that regulates lipolysis in mice

The role of non-neuronal glial cells in the regulation of adipose sympathetic nerve activity and adipocyte functions such as white adipose tissue lipid lipolysis is poorly understood. Here, we combine chemo/optogenetic manipulations of medio-basal hypothalamic astrocytes, real-time fiber photometry monitoring of white adipose tissue norepinephrine (NE) contents and nerve activities, electrophysiological recordings of local sympathetic inputs to inguinal white adipose tissue (iWAT), and adipose tissue lipid lipolytic assays to define the functional roles of hypothalamic astrocytes in the regulation of iWAT sympathetic outflow and lipolysis. Our results show that astrocyte stimulation elevates iWAT NE contents, excites sympathetic neural inputs and promotes lipolysis. Mechanistically, we find that sympathetic paravertebral ganglia (PG) partake in those astrocyte effects. We also find that astrocyte stimulation excites pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARH), and chemogenetic inhibition of POMC neurons blunts the effects induced by astrocyte stimulation. While we cannot exclude potential roles played by other cell populations such as microglia, our findings in this study reveal a central astrocyte-peripheral adipocyte axis modulating sympathetic drive to adipose tissues and adipocyte functions, one that might serve as a target for therapeutic intervention in the treatment of obesity.

Hippocampal astrocytes modulate anxiety-like behavior

Astrocytes can affect animal behavior by regulating tripartite synaptic transmission, yet their influence on affective behavior remains largely unclear. Here we showed that hippocampal astrocyte calcium activity reflects mouse affective state during virtual elevated plus maze test using two-photon calcium imaging in vivo. Furthermore, optogenetic hippocampal astrocyte activation elevating intracellular calcium induced anxiolytic behaviors in astrocyte-specific channelrhodopsin 2 (ChR2) transgenic mice (hGFAP-ChR2 mice). As underlying mechanisms, we found ATP released from the activated hippocampal astrocytes increased excitatory synaptic transmission in dentate gyrus (DG) granule cells, which exerted anxiolytic effects. Our data uncover a role of hippocampal astrocytes in modulating mice anxiety-like behaviors by regulating ATP-mediated synaptic homeostasis in hippocampal DG granule cells. Thus, manipulating hippocampal astrocytes activity can be a therapeutic strategy to treat anxiety.

Co-culture platform for neuron-astrocyte interaction using optogenetic modulation

For decades, the role of glial cells has attracted attention in the neuroscience field. Particularly, although the astrocyte is the most abundant glial cell type, it was believed to function as a passive support cell. However, recent evidence suggests that astrocytes actively release various gliotransmitters and signaling entities that regulate the excitability of pre-and post-synaptic neurons in the brain. In this study, we optimized the ratio of astrocytes and neurons to investigate the interaction between astrocytes and neurons. To this end, postnatal day 0-1 rodent hippocampi were dissociated and cultured. The neuron-astrocyte ratio was monitored for up to 3 weeks after treating the cultures with 0, 1, and 5 µM of cytosine arabinoside (Ara-C) at DIV 2. Subsequently, from postnatal transgenic (TG) mouse expressing ChR2 on astrocytes, hippocampi were cultured on the microelectrode array (MEA) with the desired neuron-astrocyte ratio. The astrocyte was irradiated using a 473 nm blue laser for 30 s in a cycle of 10 Hz and electrophysiological recording was performed to verify the activities of neurons induced by the stimulated astrocytes. Astrocytes and neurons in both co-cultures increased at an identical ratio when treated with 1 µM Ara-C, whereas they decreased significantly when treated with 5 µM Ara-C. Particularly, the laser-stimulated astrocytes induced an increase in the frequency of neuronal activities and lasted after illumination. The proposed co-culture platform is expected to be used in experiments to investigate the network between astrocytes and neurons in vitro.

Analyzing the brainstem circuits for respiratory chemosensitivity in freely moving mice

Regulation of systemic PCO2 is a life-preserving homeostatic mechanism. In the medulla oblongata, the retrotrapezoid nucleus (RTN) and rostral medullary Raphe are proposed as CO2 chemosensory nuclei mediating adaptive respiratory changes. Hypercapnia also induces active expiration, an adaptive change thought to be controlled by the lateral parafacial region (pFL). Here, we use GCaMP6 expression and head-mounted mini-microscopes to image Ca2+ activity in these nuclei in awake adult mice during hypercapnia. Activity in the pFL supports its role as a homogenous neuronal population that drives active expiration. Our data show that chemosensory responses in the RTN and Raphe differ in their temporal characteristics and sensitivity to CO2, raising the possibility these nuclei act in a coordinated way to generate adaptive ventilatory responses to hypercapnia. Our analysis revises the understanding of chemosensory control in awake adult mouse and paves the way to understanding how breathing is coordinated with complex non-ventilatory behaviours.

Astrocyte Heterogeneity in Regulation of Synaptic Activity

Our awareness of the number of synapse regulatory functions performed by astroglia is rapidly expanding, raising interesting questions regarding astrocyte heterogeneity and specialization across brain regions. Whether all astrocytes are poised to signal in a multitude of ways, or are instead tuned to surrounding synapses and how astroglial signaling is altered in psychiatric and cognitive disorders are fundamental questions for the field. In recent years, molecular and morphological characterization of astroglial types has broadened our ability to design studies to better analyze and manipulate specific functions of astroglia. Recent data emerging from these studies will be discussed in depth in this review. I also highlight remaining questions emerging from new techniques recently applied toward understanding the roles of astrocytes in synapse regulation in the adult brain.

Kir5.1 channels: potential role in epilepsy and seizure disorders

Inwardly rectifying potassium (Kir) channels are broadly expressed in many mammalian organ systems, where they contribute to critical physiological functions. However, the importance and function of the Kir5.1 channel (encoded by the KCNJ16 gene) have not been fully recognized. This review focuses on the recent advances in understanding the expression patterns and functional roles of Kir5.1 channels in fundamental physiological systems vital to potassium homeostasis and neurological disorders. Recent studies have described the role of Kir5.1-forming Kir channels in mouse and rat lines with mutations in the Kcnj16 gene. The animal research reveals distinct renal and neurological phenotypes, including pH and electrolyte imbalances, blunted ventilatory responses to hypercapnia/hypoxia, and seizure disorders. Furthermore, it was confirmed that these phenotypes are reminiscent of those in patient cohorts in which mutations in the KCNJ16 gene have also been identified, further suggesting a critical role for Kir5.1 channels in homeostatic/neural systems health and disease. Future studies that focus on the many functional roles of these channels, expanded genetic screening in human patients, and the development of selective small-molecule inhibitors for Kir5.1 channels, will continue to increase our understanding of this unique Kir channel family member.

Effects of ANP and BNP on the generation of respiratory rhythms in brainstem-spinal cord preparation isolated from newborn rats

Natriuretic peptides (NPs) are a family of peptide hormones produced in cardiac muscle cells and consist mainly of three types: atrial NP (ANP), B-type (or brain) NP (BNP), and C-type NP. We herein report the effects of ANP and BNP on central respiratory activity in brainstem-spinal cord preparation isolated from newborn rats. Bath application of these peptides (100 nM) induced a weak transient depression of the respiratory rhythm followed by recovery. Respiratory-related neurons in the rostral ventrolateral medulla showed a tendency for transient hyperpolarization followed by recovery during the application of ANP or BNP. The application of a membrane-permeable cGMP, 8-Br-cGMP (10 or 20 μM), did not induce significant effects on respiratory rhythm, suggesting no involvement of guanylyl cyclase in effects of ANP or BNP. We also examined effects of BNP on respiratory depression induced by the sedative dexmedetomidine, which exerts an inhibitory influence on respiratory rhythm. When pretreated with 50 nM BNP, the inhibitory effect of 100 nM dexmedetomidine was significantly reduced. Our findings suggest that ANP and BNP act as mild excitatory agents with sustained effects on respiratory rhythm after an initial transient depression.

Chemogenetic and Optogenetic Manipulations of Microglia in Chronic Pain

Chronic pain relief remains an unmet medical need. Current research points to a substantial contribution of glia-neuron interaction in its pathogenesis. Particularly, microglia play a crucial role in the development of chronic pain. To better understand the microglial contribution to chronic pain, specific regional and temporal manipulations of microglia are necessary. Recently, two new approaches have emerged that meet these demands. Chemogenetic tools allow the expression of designer receptors exclusively activated by designer drugs (DREADDs) specifically in microglia. Similarly, optogenetic tools allow for microglial manipulation via the activation of artificially expressed, light-sensitive proteins. Chemo- and optogenetic manipulations of microglia in vivo are powerful in interrogating microglial function in chronic pain. This review summarizes these emerging tools in studying the role of microglia in chronic pain and highlights their potential applications in microglia-related neurological disorders.

Brain H+ /CO2 sensing and control by glial cells

Maintenance of constant brain pH is critically important to support the activity of individual neurons, effective communication within the neuronal circuits, and, thus, efficient processing of information by the brain. This review article focuses on how glial cells detect and respond to changes in brain tissue pH and concentration of CO2 , and then trigger systemic and local adaptive mechanisms that ensure a stable milieu for the operation of brain circuits. We give a detailed account of the cellular and molecular mechanisms underlying sensitivity of glial cells to H+ and CO2 and discuss the role of glial chemosensitivity and signaling in operation of three key mechanisms that work in concert to keep the brain pH constant. We discuss evidence suggesting that astrocytes and marginal glial cells of the brainstem are critically important for central respiratory CO2 chemoreception-a fundamental physiological mechanism that regulates breathing in accord with changes in blood and brain pH and partial pressure of CO2 in order to maintain systemic pH homeostasis. We review evidence suggesting that astrocytes are also responsible for the maintenance of local brain tissue extracellular pH in conditions of variable acid loads associated with changes in the neuronal activity and metabolism, and discuss potential role of these glial cells in mediating the effects of CO2 on cerebral vasculature.

Patch-to-Seq and Transcriptomic Analyses Yield Molecular Markers of Functionally Distinct Brainstem Serotonin Neurons

Acute regulation of CO₂ and pH homeostasis requires sensory feedback from peripheral (carotid body) and central (central) CO₂/pH sensitive cells - so called respiratory chemoreceptors. Subsets of brainstem serotonin (5-HT) neurons in the medullary raphe are CO₂ sensitive or insensitive based on differences in embryonic origin, suggesting these functionally distinct subpopulations may have unique transcriptional profiles. Here, we used Patch-to-Seq to determine if the CO₂ responses in brainstem 5-HT neurons could be correlated to unique transcriptional profiles and/or unique molecular markers and pathways. First, firing rate changes with hypercapnic acidosis were measured in fluorescently labeled 5-HT neurons in acute brainstem slices from transgenic, Dahl SS (SSMcwi) rats expressing T2/ePet-eGFP transgene in Pet-1 expressing (serotonin) neurons (SS ^ePet1-eGFP rats). Subsequently, the transcriptomic and pathway profiles of CO₂ sensitive and insensitive 5-HT neurons were determined and compared by single cell RNA (scRNAseq) and bioinformatic analyses. Low baseline firing rates were a distinguishing feature of CO₂ sensitive 5-HT neurons. scRNAseq of these recorded neurons revealed 166 differentially expressed genes among CO₂ sensitive and insensitive 5-HT neurons. Pathway analyses yielded novel predicted upstream regulators, including the transcription factor Egr2 and Leptin. Additional bioinformatic analyses identified 6 candidate gene markers of CO₂ sensitive 5-HT neurons, and 2 selected candidate genes (CD46 and Iba57) were both expressed in 5-HT neurons determined via in situ mRNA hybridization. Together, these data provide novel insights into the transcriptional control of cellular chemoreception and provide unbiased candidate gene markers of CO₂ sensitive 5-HT neurons. Methodologically, these data highlight the utility of the patch-to-seq technique in enabling the linkage of gene expression to specific functions, like CO₂ chemoreception, in a single cell to identify potential mechanisms underlying functional differences in otherwise similar cell types.

Regulation of blood vessels by ATP in the ventral medullary surface in a rat model of Parkinson's disease

Parkinson's disease (PD) patients often experience impairment of autonomic and respiratory functions. These include conditions such as orthostatic hypotension and sleep apnea, which are highly correlated with dysfunctional central chemoreception. Blood flow is a fundamental determinant of tissue CO₂/H⁺, yet the extent to which blood flow regulation within chemoreceptor regions contributes to respiratory behavior during neurological disease remains unknown. Here, we tested the hypothesis that 6-hydroxydopamine injection to inducing a known model of PD results in dysfunctional vascular homeostasis, biochemical dysregulation, and glial morphology of the ventral medullary surface (VMS). We show that hypercapnia (FiCO₂ = 10%) induced elevated VMS pial vessel constriction in PD animals through a P2-receptor dependent mechanism. Similarly, we found a greater CO₂-induced vascular constriction after ARL67156 (an ectonucleotidase inhibitor) in control and PD-induced animals. In addition, we also report that weighted gene correlational network analysis of the proteomic data showed a protein expression module differentially represented between both groups. This module showed that gene ontology enrichment for components of the ATP machinery were reduced in our PD-model compared to control animals. Altogether, our data indicate that dysfunction in purinergic signaling, potentially through altered ATP bioavailability in the VMS region, may compromise the RTN neuroglial vascular unit in a PD animal model.

Crosstalk of Astrocytes and Other Cells during Ischemic Stroke

Stroke is a leading cause of death and long-term disability worldwide. Astrocytes structurally compose tripartite synapses, blood–brain barrier, and the neurovascular unit and perform multiple functions through cell-to-cell signaling of neurons, glial cells, and vasculature. The crosstalk of astrocytes and other cells is complicated and incompletely understood. Here we review the role of astrocytes in response to ischemic stroke, both beneficial and detrimental, from a cell–cell interaction perspective. Reactive astrocytes provide neuroprotection through antioxidation and antiexcitatory effects and metabolic support; they also contribute to neurorestoration involving neurogenesis, synaptogenesis, angiogenesis, and oligodendrogenesis by crosstalk with stem cells and cell lineage. In the meantime, reactive astrocytes also play a vital role in neuroinflammation and brain edema. Glial scar formation in the chronic phase hinders functional recovery. We further discuss astrocyte enriched microRNAs and exosomes in the regulation of ischemic stroke. In addition, the latest notion of reactive astrocyte subsets and astrocytic activity revealed by optogenetics is mentioned. This review discusses the current understanding of the intimate molecular conversation between astrocytes and other cells and outlines its potential implications after ischemic stroke. “Neurocentric” strategies may not be sufficient for neurological protection and recovery; future therapeutic strategies could target reactive astrocytes.

Medullary astrocytes mediate irregular breathing patterns generation in chronic heart failure through purinergic P2X7 receptor signalling

Background

Breathing disorders (BD) (apnoeas/hypopneas, periodic breathing) are highly prevalent in chronic heart failure (CHF) and are associated with altered central respiratory control. Ample evidence identifies the retrotrapezoid nucleus (RTN) as an important chemosensitivity region for ventilatory control and generation of BD in CHF, however little is known about the cellular mechanisms underlying the RTN/BD relationship. Within the RTN, astrocyte‐mediated purinergic signalling modulates respiration, but the potential contribution of RTN astrocytes to BD in CHF has not been explored.

Methods

Selective neuron and/or astrocyte-targeted interventions using either optogenetic and chemogenetic manipulations in the RTN of CHF rats were used to unveil the contribution of the RTN on the development/maintenance of BD, the role played by astrocytes in BD and the molecular mechanism underpinning these alterations.

Findings

We showed that episodic photo-stimulation of RTN neurons triggered BD in healthy rats, and that RTN neurons ablation in CHF animals eliminates BD. Also, we found a reduction in astrocytes activity and ATP bioavailability within the RTN of CHF rats, and that chemogenetic restoration of normal RTN astrocyte activity and ATP levels improved breathing regularity in CHF. Importantly, P"X/ P2X7 receptor (P2X7r) expression was reduced in RTN astrocytes from CHF rats and viral vector-mediated delivery of human P2X7 P2X7r into astrocytes increases ATP bioavailability and abolished BD.

Interpretation

Our results support that RTN astrocytes play a pivotal role on BD generation and maintenance in the setting CHF by a mechanism encompassing P2X7r signalling.

Funding

This study was funded by the National Research and Development Agency of Chile (ANID).

Astrocyte-derived adenosine excites sleep-promoting neurons in the ventrolateral preoptic nucleus: Astrocyte-neuron interactions in the regulation of sleep

Although ATP and/or adenosine derived from astrocytes are known to regulate sleep, the precise mechanisms underlying the somnogenic effects of ATP and adenosine remain unclear. We selectively expressed channelrhodopsin-2 (ChR2), a light-sensitive ion channel, in astrocytes within the ventrolateral preoptic nucleus (VLPO), which is an essential brain nucleus involved in sleep promotion. We then examined the effects of photostimulation of astrocytic ChR2 on neuronal excitability using whole-cell patch-clamp recordings in two functionally distinct types of VLPO neurons: sleep-promoting GABAergic projection neurons and non-sleep-promoting local GABAergic neurons. Optogenetic stimulation of VLPO astrocytes demonstrated opposite outcomes in the two types of VLPO neurons. It led to the inhibition of non-sleep-promoting neurons and excitation of sleep-promoting neurons. These responses were attenuated by blocking of either adenosine A₁ receptors or tissue-nonspecific alkaline phosphatase (TNAP). In contrast, exogenous adenosine decreased the excitability of both VLPO neuron populations. Moreover, TNAP was expressed in galanin-negative VLPO neurons, but not in galanin-positive sleep-promoting projection neurons. Taken together, these results suggest that astrocyte-derived ATP is converted into adenosine by TNAP in non-sleep-promoting neurons. In turn, adenosine decreases the excitability of local GABAergic neurons, thereby increasing the excitability of sleep-promoting GABAergic projection neurons. We propose a novel mechanism involving astrocyte-neuron interactions in sleep regulation, wherein endogenous adenosine derived from astrocytes excites sleep-promoting VLPO neurons, and thus decreases neuronal excitability in arousal-related areas of the brain.

Intercellular Communication in the Central Nervous System as Deduced by Chemical Neuroanatomy and Quantitative Analysis of Images: Impact on Neuropharmacology

In the last decades, new evidence on brain structure and function has been acquired by morphological investigations based on synergic interactions between biochemical anatomy approaches, new techniques in microscopy and brain imaging, and quantitative analysis of the obtained images. This effort produced an expanded view on brain architecture, illustrating the central nervous system as a huge network of cells and regions in which intercellular communication processes, involving not only neurons but also other cell populations, virtually determine all aspects of the integrative function performed by the system. The main features of these processes are described. They include the two basic modes of intercellular communication identified (i.e., wiring and volume transmission) and mechanisms modulating the intercellular signaling, such as cotransmission and allosteric receptor-receptor interactions. These features may also open new possibilities for the development of novel pharmacological approaches to address central nervous system diseases. This aspect, with a potential major impact on molecular medicine, will be also briefly discussed.

In Transgenic Erythropoietin Deficient Mice, an Increase in Respiratory Response to Hypercapnia Parallels Abnormal Distribution of CO2/H+-Activated Cells in the Medulla Oblongata

Erythropoietin (Epo) and its receptor are expressed in central respiratory areas. We hypothesized that chronic Epo deficiency alters functioning of central respiratory areas and thus the respiratory adaptation to hypercapnia. The hypercapnic ventilatory response (HcVR) was evaluated by whole body plethysmography in wild type (WT) and Epo deficient (Epo-TAg^h) adult male mice under 4%CO₂. Epo-TAg^h mice showed a larger HcVR than WT mice because of an increase in both respiratory frequency and tidal volume, whereas WT mice only increased their tidal volume. A functional histological approach revealed changes in CO₂/H⁺-activated cells between Epo-TAg^h and WT mice. First, Epo-TAg^h mice showed a smaller increase under hypercapnia in c-FOS-positive number of cells in the retrotrapezoid nucleus/parafacial respiratory group than WT, and this, independently of changes in the number of PHOX2B-expressing cells. Second, we did not observe in Epo-TAg^h mice the hypercapnic increase in c-FOS-positive number of cells in the nucleus of the solitary tract present in WT mice. Finally, whereas hypercapnia did not induce an increase in the c-FOS-positive number of cells in medullary raphe nuclei in WT mice, chronic Epo deficiency leads to raphe pallidus and magnus nuclei activation by hyperacpnia, with a significant part of c-FOS positive cells displaying an immunoreactivity for serotonin in the raphe pallidus nucleus. All of these results suggest that chronic Epo-deficiency affects both the pattern of ventilatory response to hypercapnia and associated medullary respiratory network at adult stage with an increase in the sensitivity of 5-HT and non-5-HT neurons of the raphe medullary nuclei leading to stimulation of f_R for moderate level of CO₂.

Astrocyte regulation of neural circuit activity and network states

Astrocytes are known to influence neuronal activity through different mechanisms, including the homeostatic control of extracellular levels of ions and neurotransmitters and the exchange of signaling molecules that regulate synaptic formation, structure, and function. While a great effort done in the past has defined many molecular mechanisms and cellular processes involved in astrocyte-neuron interactions at the cellular level, the consequences of these interactions at the network level in vivo have only relatively recently been identified. This review describes and discusses recent findings on the regulatory effects of astrocytes on the activity of neuronal networks in vivo. Accumulating but still limited, evidence indicates that astrocytes regulate neuronal network rhythmic activity and synchronization as well as brain states. These studies demonstrate a critical contribution of astrocytes to brain activity and are paving the way for a more thorough understanding of the cellular bases of brain function.

CO2 signaling mediates neurovascular coupling in the cerebral cortex

Neurovascular coupling is a fundamental brain mechanism that regulates local cerebral blood flow (CBF) in response to changes in neuronal activity. Functional imaging techniques are commonly used to record these changes in CBF as a proxy of neuronal activity to study the human brain. However, the mechanisms of neurovascular coupling remain incompletely understood. Here we show in experimental animal models (laboratory rats and mice) that the neuronal activity-dependent increases in local CBF in the somatosensory cortex are prevented by saturation of the CO2-sensitive vasodilatory brain mechanism with surplus of exogenous CO2 or disruption of brain CO2/HCO3- transport by genetic knockdown of electrogenic sodium-bicarbonate cotransporter 1 (NBCe1) expression in astrocytes. A systematic review of the literature data shows that CO2 and increased neuronal activity recruit the same vasodilatory signaling pathways. These results and analysis suggest that CO2 mediates signaling between neurons and the cerebral vasculature to regulate brain blood flow in accord with changes in the neuronal activity.

Potential Role of the Retrotrapezoid Nucleus in Mediating Cardio-Respiratory Dysfunction in Heart Failure With Preserved Ejection Fraction

A strong association between chemoreflex hypersensitivity, disordered breathing, and elevated sympathetic activity has been shown in experimental and human heart failure (HF). The contribution of chemoreflex hypersensitivity in HF pathophysiology is incompletely understood. There is ample evidence that increased peripheral chemoreflex drive in HF with reduced ejection fraction (HFrEF; EF<40%) leads to pathophysiological changes in autonomic and cardio-respiratory control, but less is known about the neural mechanisms mediating cardio-respiratory disturbances in HF with preserved EF (HFpEF; EF>50%). Importantly, it has been shown that activation of the central chemoreflex worsens autonomic dysfunction in experimental HFpEF, an effect mediated in part by the activation of C1 catecholaminergic neurons neighboring the retrotrapezoid nucleus (RTN), an important region for central chemoreflex control of respiratory and autonomic function. Accordingly, the main purpose of this brief review is to discuss the possible role played by activation of central chemoreflex pathways on autonomic function and its potential role in precipitating disordered breathing in HFpEF. Improving understanding of the contribution of the central chemoreflex to the pathophysiology of HFpEF may help in development of novel interventions intended to improve cardio-respiratory outcomes in HFpEF.

5-HT7 receptors expressed in the mouse parafacial region are not required for respiratory chemosensitivity

Abstract

A brainstem homeostatic system senses CO₂/H⁺ to regulate ventilation, blood gases and acid–base balance. Neurons of the retrotrapezoid nucleus (RTN) and medullary raphe are both implicated in this mechanism as respiratory chemosensors, but recent pharmacological work suggested that the CO₂/H⁺ sensitivity of RTN neurons is mediated indirectly, by raphe‐derived serotonin acting on 5‐HT7 receptors. To investigate this further, we characterized Htr7 transcript expression in phenotypically identified RTN neurons using multiplex single cell qRT‐PCR and RNAscope. Although present in multiple neurons in the parafacial region of the ventrolateral medulla, Htr7 expression was undetectable in most RTN neurons (Nmb⁺/Phox2b⁺) concentrated in the densely packed cell group ventrolateral to the facial nucleus. Where detected, Htr7 expression was modest and often associated with RTN neurons that extend dorsolaterally to partially encircle the facial nucleus. These dorsolateral Nmb⁺/Htr7⁺ neurons tended to express Nmb at high levels and the intrinsic RTN proton detectors Gpr4 and Kcnk5 at low levels. In mouse brainstem slices, CO₂‐stimulated firing in RTN neurons was mostly unaffected by a 5‐HT7 receptor antagonist, SB269970 (n = 11/13). At the whole animal level, microinjection of SB269970 into the RTN of conscious mice blocked respiratory stimulation by co‐injected LP‐44, a 5‐HT7 receptor agonist, but had no effect on CO₂‐stimulated breathing in those same mice. We conclude that Htr7 is expressed by a minor subset of RTN neurons with a molecular profile distinct from the established chemoreceptors and that 5‐HT7 receptors have negligible effects on CO₂‐evoked firing activity in RTN neurons or on CO₂‐stimulated breathing in mice.

Key points

Neurons of the retrotrapezoid nucleus (RTN) are intrinsic CO₂/H⁺ chemosensors and serve as an integrative excitatory hub for control of breathing.

Serotonin can activate RTN neurons, in part via 5‐HT7 receptors, and those effects have been implicated in conferring an indirect CO₂ sensitivity.

Multiple single cell molecular approaches revealed low levels of 5‐HT7 receptor transcript expression restricted to a limited population of RTN neurons.

Pharmacological experiments showed that 5‐HT7 receptors in RTN are not required for CO₂/H⁺‐stimulation of RTN neuronal activity or CO₂‐stimulated breathing.

These data do not support a role for 5‐HT7 receptors in respiratory chemosensitivity mediated by RTN neurons.