Distribution of glutamic acid decarboxylase mRNA-containing neurons in rat medulla projecting to thoracic spinal cord in relation to monoaminergic brainstem neurons

Chronic Pain-Associated Cardiovascular Disease: The Role of Sympathetic Nerve Activity

Chronic pain affects many people world-wide, and this number is continuously increasing. There is a clear link between chronic pain and the development of cardiovascular disease through activation of the sympathetic nervous system. The purpose of this review is to provide evidence from the literature that highlights the direct relationship between sympathetic nervous system dysfunction and chronic pain. We hypothesize that maladaptive changes within a common neural network regulating the sympathetic nervous system and pain perception contribute to sympathetic overactivation and cardiovascular disease in the setting of chronic pain. We review clinical evidence and highlight the basic neurocircuitry linking the sympathetic and nociceptive networks and the overlap between the neural networks controlling the two.

Subregional differences in GABAA receptor subunit expression in the rostral ventrolateral medulla of sedentary versus physically active rats

Neurons in the rostral ventrolateral medulla (RVLM) regulate blood pressure through direct projections to spinal sympathetic preganglionic neurons. Only some RVLM neurons are active under resting conditions due to significant, tonic inhibition by gamma-aminobutyric acid (GABA). Withdrawal of GABAA receptor-mediated inhibition of the RVLM increases sympathetic outflow and blood pressure substantially, providing a mechanism by which the RVLM could contribute chronically to cardiovascular disease (CVD). Here, we tested the hypothesis that sedentary conditions, a major risk factor for CVD, increase GABAA receptors in RVLM, including its rostral extension (RVLMRE ), both of which contain bulbospinal catecholamine (C1) and non-C1 neurons. We examined GABAA receptor subunits GABAAα1 and GABAAα2 in the RVLM/RVLMRE of sedentary or physically active (10-12 weeks of wheel running) rats. Western blot analyses indicated that sedentary rats had lower expression of GABAAα1 and GABAAα2 subunits in RVLM but only GABAAα2 was lower in the RVLMRE of sedentary rats. Sedentary rats had significantly reduced expression of the chloride transporter, KCC2, suggesting less effective GABA-mediated inhibition compared to active rats. Retrograde tracing plus triple-label immunofluorescence identified fewer bulbospinal non-C1 neurons immunoreactive for GABAAα1 but a higher percentage of bulbospinal C1 neurons immunoreactive for GABAAα1 in sedentary animals. Sedentary conditions did not significantly affect the number of bulbospinal C1 or non-C1 neurons immunoreactive for GABAAα2 . These results suggest a complex interplay between GABAA receptor expression by spinally projecting C1 and non-C1 neurons and sedentary versus physically active conditions. They also provide plausible mechanisms for both enhanced sympathoexcitatory and sympathoinhibitory responses following sedentary conditions.

Embracing diversity in the 5-HT neuronal system

Neurons that synthesize and release 5-hydroxytryptamine (5-HT; serotonin) express a core set of genes that establish and maintain this neurotransmitter phenotype and distinguish these neurons from other brain cells. Beyond a shared 5-HTergic phenotype, these neurons display divergent cellular properties in relation to anatomy, morphology, hodology, electrophysiology and gene expression, including differential expression of molecules supporting co-transmission of additional neurotransmitters. This diversity suggests that functionally heterogeneous subtypes of 5-HT neurons exist, but linking subsets of these neurons to particular functions has been technically challenging. We discuss recent data from molecular genetic, genomic and functional methods that, when coupled with classical findings, yield a reframing of the 5-HT neuronal system as a conglomeration of diverse subsystems with potential to inspire novel, more targeted therapies for clinically distinct 5-HT-related disorders.

Neurons with diverse phenotypes project from the caudal to the rostral nucleus of the solitary tract

The nucleus of the solitary tract is a potential site for taste-visceral interactions. Connections from the caudal, visceral area of the nucleus (cNST) to the rostral, gustatory zone (rNST) have been described, but the phenotype of cells giving rise to the projection(s) and their distribution among rNST subdivisions are unknown. To determine these characteristics of the intrasolitary pathway, we injected pan-neuronal and floxed AAV viruses into the cNST of mice expressing cre in glutamatergic, GABAergic, or catecholaminergic neurons. Particular attention was paid to the terminal field distribution in rNST subdivisions by simultaneously visualizing P2X2 localized to gustatory afferent terminals. All three phenotypically identified pathways terminated in rNST, with the density greatest for glutamatergic and sparsest for catecholaminergic projections, observations supported by retrograde tracing. Interestingly, cNST neurons had more prominent projections to rNST regions medial and ventral to P2X2 staining, i.e., the medial and ventral subdivisions. In addition, GABAergic neurons projected robustly to the lateral subdivision and adjacent parts of the reticular formation and spinal trigeminal nucleus. Although cNST neurons also projected to the P2X2-rich central subdivision, such projections were sparser. These findings suggest that cNST visceral signals exert stronger excitatory and inhibitory influences on local autonomic and reflex pathways associated with the medial and ventral subdivisions compared to weaker modulation of ascending pathways arising from the central subdivision and ultimately destined for the forebrain.

Ventral medullary control of rapid eye movement sleep and atonia

Discrete populations of neurons at multiple levels of the brainstem control rapid eye movement (REM) sleep and the accompanying loss of postural muscle tone, or atonia. The specific contributions of these brainstem cell populations to REM sleep control remains incompletely understood. Here we show in rats that viral vector-based lesions of the ventromedial medulla at a level rostral to the inferior olive (pSOM) produced violent myoclonic twitches and abnormal electromyographic spikes, but not complete loss of tonic atonia, during REM sleep. Motor tone during non-REM (NREM) sleep was unaffected in these same animals. Acute chemogenetic activation of pSOM neurons in rats robustly and selectively suppressed REM sleep but not NREM sleep. Similar lesions targeting the more rostral ventromedial medulla (RVM) did not affect sleep or atonia, while chemogenetic stimulation of the RVM produced wakefulness and reduced sleep. Finally, selective activation of vesicular GABA transporter (VGAT) pSOM neurons in mice produced complete suppression of REM sleep whereas their loss increased EMG spikes during REM sleep. These results reveal a key contribution of the pSOM and specifically the VGAT+ neurons in this region in REM sleep and motor control.

Interactions between respiratory oscillators in adult rats

Breathing in mammals is hypothesized to result from the interaction of two distinct oscillators: the preBötzinger Complex (preBötC) driving inspiration and the lateral parafacial region (pF_L) driving active expiration. To understand the interactions between these oscillators, we independently altered their excitability in spontaneously breathing vagotomized urethane-anesthetized adult rats. Hyperpolarizing preBötC neurons decreased inspiratory activity and initiated active expiration, ultimately progressing to apnea, i.e., cessation of both inspiration and active expiration. Depolarizing pF_L neurons produced active expiration at rest, but not when inspiratory activity was suppressed by hyperpolarizing preBötC neurons. We conclude that in anesthetized adult rats active expiration is driven by the pF_L but requires an additional form of network excitation, i.e., ongoing rhythmic preBötC activity sufficient to drive inspiratory motor output or increased chemosensory drive. The organization of this coupled oscillator system, which is essential for life, may have implications for other neural networks that contain multiple rhythm/pattern generators.

DOI:http://dx.doi.org/10.7554/eLife.14203.001

Mammals breathe air into and out of their lungs to absorb oxygen into the body and to remove carbon dioxide. The rhythm of breathing is most likely controlled by two groups of neurons in a part of the brain called the brain stem. One group called the preBötzinger Complex drives breathing in (inspiration), and normally, breathing out (expiration) occurs when the muscles responsible for inspiration relax. The other group of neurons – known as the lateral parafacial region – controls extra muscles that allow us to increase our breathing when we need to, such as during exercise.

Huckstepp et al. set out to determine how these two groups of neurons interact with one another in anesthetized rats to produce a reliable and efficient pattern of breathing. The experiments provide further evidence that inspiration is mainly driven by the preBötzinger Complex. Whilst activity from the lateral parafacial region is needed to cause the rats to breathe out more forcefully than normal, a second low level of activity from another source is also required. This source could either be the preBötzinger Complex, or some unknown neurons that change their activity in response to the levels of oxygen and carbon dioxide in the blood or fluid of the brain.

Further investigation is required to identify how these interactions go awry in diseases that affect breathing, such as sleep apneas.

DOI:http://dx.doi.org/10.7554/eLife.14203.002

Inhibitory modulation of optogenetically identified neuron subtypes in the rostral solitary nucleus

Inhibition is presumed to play an important role in gustatory processing in the rostral nucleus of the solitary tract (rNST). One source of inhibition, GABA, is abundant within the nucleus and comes both from local, intrasolitary sources and from outside the nucleus. In addition to the receptor-mediated effects of GABA on rNST neurons, the hyperpolarization-sensitive currents, Ih and IA, have the potential to further modulate afferent signals. To elucidate the effects of GABAergic modulation on solitary tract (ST)-evoked responses in phenotypically defined rNST neurons and to define the presence of IA and Ih in the same cells, we combined in vitro recording and optogenetics in a transgenic mouse model. This mouse expresses channelrhodopsin 2 (ChR2) in GAD65-expressing GABAergic neurons throughout the rNST. GABA positive (GABA+) neurons differed from GABA negative (GABA-) neurons in their response to membrane depolarization and ST stimulation. GABA+ neurons had lower thresholds to direct membrane depolarization compared with GABA- neurons, but GABA- neurons responded more faithfully to ST stimulation. Both IA and Ih were present in subsets of GABA+ and GABA- neurons. Interestingly, GABA+ neurons with Ih were more responsive to afferent stimulation than inhibitory neurons devoid of these currents, whereas GABA- neurons with IA were more subject to inhibitory modulation. These results suggest that the voltage-gated channels underlying IA and Ih play an important role in modulating rNST output through a circuit of feedforward inhibition.

Sympathetic preganglionic neurons: properties and inputs

The sympathetic nervous system comprises one half of the autonomic nervous system and participates in maintaining homeostasis and enabling organisms to respond in an appropriate manner to perturbations in their environment, either internal or external. The sympathetic preganglionic neurons (SPNs) lie within the spinal cord and their axons traverse the ventral horn to exit in ventral roots where they form synapses onto postganglionic neurons. Thus, these neurons are the last point at which the central nervous system can exert an effect to enable changes in sympathetic outflow. This review considers the degree of complexity of sympathetic control occurring at the level of the spinal cord. The morphology and targets of SPNs illustrate the diversity within this group, as do their diverse intrinsic properties which reveal some functional significance of these properties. SPNs show high degrees of coupled activity, mediated through gap junctions, that enables rapid and coordinated responses; these gap junctions contribute to the rhythmic activity so critical to sympathetic outflow. The main inputs onto SPNs are considered; these comprise afferent, descending, and interneuronal influences that themselves enable functionally appropriate changes in SPN activity. The complexity of inputs is further demonstrated by the plethora of receptors that mediate the different responses in SPNs; their origins and effects are plentiful and diverse. Together these different inputs and the intrinsic and coupled activity of SPNs result in the rhythmic nature of sympathetic outflow from the spinal cord, which has a variety of frequencies that can be altered in different conditions.

How sympathetic are your spinal cord circuits?

NEW FINDINGS

What is the topic of this review? This review focuses on the role of gap junctions and interneurones in sympathetic control at the spinal cord level. What advances does it highlight? The review considers the importance of these local spinal circuits in contributing to rhythmic autonomic activity and enabling appropriate responses to homeostatic perturbations. Sympathetic control of end organs relies on the activity of sympathetic preganglionic neurones (SPNs) within the spinal cord. These SPNs exhibit heterogeneity with respect to function, neurochemistry, location, descending inputs and patterns of activity. Part of this heterogeneity is bestowed by local spinal circuitry. Our understanding of the role of these local circuits, including the significance of connections between the SPNs themselves through specialized gap junctions, is patchy. This report focuses on interneurones and gap junctions within these circuits. Gap junctions play a role in sympathetic control; they are located on SPNs in the intermediolateral cell column. Mefloquine, a chemical that blocks these gap junctions, reduces local rhythmic activity in the spinal cord slice and disrupts autonomic control in the working heart-brainstem preparation. The role that these gap junctions may play in health and disease in adult animals remains to be elucidated fully. Presympathetic interneurones are located in laminae V, VII and X and the intermediolateral cell column; those in lamina X are GABAergic and directly inhibit SPNs. The GABAergic inputs onto SPNs exert their effects through activation of synaptic and extrasynaptic receptors, which stabilize the membrane at negative potentials. The GABAergic interneurones contribute to rhythmic patterns of activity that can be generated in the spinal cord, because bicuculline reduces network oscillatory activity. These studies indicate that local spinal cord circuitry is critical in enabling appropriate levels and patterning of activity in sympathetic outflow. We need to understand how these circuits may be harnessed in the situation of spinal cord injury.

Distribution of raphespinal fibers in the mouse spinal cord

Background

Serotonergic raphespinal neurons and their fibers have been mapped in large mammals, but the non-serotonergic ones have not been studied, especially in the mouse. The present study aimed to investigate the termination pattern of fibers arising from the hindbrain raphe and reticular nuclei which also have serotonergic neurons by injecting the anterograde tracer BDA into them.

Results

We found that raphespinal fibers terminate in both the dorsal and ventral horns in addition to lamina 10. There is a shift of the fibers in the ventral horn towards the dorsal and lateral part of the gray matter. Considerable variation in the termination pattern also exists between raphe nuclei with raphe magnus having more fibers terminating in the dorsal horn. Fibers from the adjacent gigantocellular reticular nucleus show similar termination pattern as those from the raphe nuclei with slight difference. Immunofluorescence staining showed that raphespinal fibers were heterogeneous and serotoninergic fibers were present in all laminae but mainly in laminae 1, 2, medial lamina 8, laminae 9 and 10. Surprisingly, immunofluorescence staining on clarified spinal cord tissue revealed that serotoninergic fibers formed bundles regularly in a short distance along the rostrocaudal axis in the medial part of the ventral horn and they extended towards the lateral motor neuron column area.

Conclusion

Serotonergic and non-serotonergic fibers arising from the hindbrain raphe and reticular nuclei had similar termination pattern in the mouse spinal cord with subtle difference. The present study provides anatomical foundation for the multiple roles raphe and adjacent reticular nuclei play.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0046-x) contains supplementary material, which is available to authorized users.

CO2-inhibited neurons in the medullary raphé are GABAergic

Previous studies have reported subsets of medullary raphé neurons that are either stimulated or inhibited by CO2/pH in vitro, in situ, and in vivo. We tested the hypothesis that medullary raphé CO2-inhibited neurons are GABAergic. Extracellular recordings in unanesthetized juvenile in situ rat preparations showed reversible hypercapnia-induced suppression of 19% (63/323) of medullary raphé neurons, and this suppression persisted after antagonism of NMDA, AMPA/kainate, and GABAA receptors. We stained a subset of CO2-inhibited cells and found that most (11/12) had glutamic acid decarboxylase 67 immunoreactivity (GAD67-ir). These data indicate that the majority of acidosis-inhibited medullary raphé neurons are GABAergic, and that their chemosensitivity is independent of major fast synaptic inputs. Thus, CO2-sensitive GABAergic neurons may play a role in central CO2/pH chemoreception.

C1 neurons: the body's EMTs

The C1 neurons reside in the rostral and intermediate portions of the ventrolateral medulla (RVLM, IVLM). They use glutamate as a fast transmitter and synthesize catecholamines plus various neuropeptides. These neurons regulate the hypothalamic pituitary axis via direct projections to the paraventricular nucleus and regulate the autonomic nervous system via projections to sympathetic and parasympathetic preganglionic neurons. The presympathetic C1 cells, located in the RVLM, are probably organized in a roughly viscerotopic manner and most of them regulate the circulation. C1 cells are variously activated by hypoglycemia, infection or inflammation, hypoxia, nociception, and hypotension and contribute to most glucoprivic responses. C1 cells also stimulate breathing and activate brain stem noradrenergic neurons including the locus coeruleus. Based on the various effects attributed to the C1 cells, their axonal projections and what is currently known of their synaptic inputs, subsets of C1 cells appear to be differentially recruited by pain, hypoxia, infection/inflammation, hemorrhage, and hypoglycemia to produce a repertoire of stereotyped autonomic, metabolic, and neuroendocrine responses that help the organism survive physical injury and its associated cohort of acute infection, hypoxia, hypotension, and blood loss. C1 cells may also contribute to glucose and cardiovascular homeostasis in the absence of such physical stresses, and C1 cell hyperactivity may contribute to the increase in sympathetic nerve activity associated with diseases such as hypertension.

Multiple roles of serotonin in pain control mechanisms--implications of 5-HT₇ and other 5-HT receptor types

Among monoamine neurotransmitters, serotonin (5-HT) is known to play complex modulatory roles in pain signaling mechanisms since the first reports, about forty years ago, on its essentially pro-nociceptive effects at the periphery and anti-nociceptive effects when injected directly at the spinal cord level. The discovery of multiple 5-HT receptor subtypes allowed possible explanations to this dual action at the periphery versus the central nervous system (CNS) since both excitatory and inhibitory effects can be exerted through 5-HT activation of different 5-HT receptors. However, it also appeared that activation of the same receptor subtype at CNS level might induce variable effects depending on the physiological or pathophysiological status of the animal administered with agonists. In particular, the marked neuroplastic changes induced by nerve lesion, which account for sensitization of pain signaling mechanisms, can contribute to dramatic changes in the effects of a given 5-HT receptor agonist in neuropathic rats versus intact healthy rats. This has notably been observed with 5-HT₇ receptor agonists which exert a pronociceptive action in healthy rats but alleviate hyperalgesia consecutive to nerve lesion in neuropathic animals. Analysis of cellular mechanisms underlying such dual 5-HT actions mediated by a single receptor subtype indicates that the neuronal phenotype which expresses this receptor also plays a key role in determining which modulatory action 5-HT would finally exert on pain signaling mechanisms.

Brain sources of inhibitory input to the rat rostral ventrolateral medulla

The rostral ventrolateral medulla (RVLM) contains neurons critical for cardiovascular, respiratory, metabolic, and motor control. The activity of these neurons is controlled by inputs from multiple identified brain regions; however, the neurochemistry of these inputs is largely unknown. Gamma-aminobutyric acid (GABA) and enkephalin tonically inhibit neurons within the RVLM. The aim of this study was to identify all brain regions that provide GABAergic or enkephalinergic input to the rat RVLM. Neurons immunoreactive for cholera toxin B (CTB-ir), retrogradely transported from the RVLM, were assessed for expression of glutamic acid decarboxylase (GAD67) or preproenkephalin (PPE) mRNA using in situ hybridization. GAD67 mRNA was expressed in CTB-ir neurons in the following regions: the nucleus of the solitary tract (NTS, 6% of CTB-ir neurons), area postrema (AP, 8%), caudal ventrolateral medulla (17%), midline raphe (40%), ventrolateral periaqueductal gray (VLPAG, 15%), lateral hypothalamic area (LHA, 25%), central nucleus of the amygdala (CeA, 77%), sublenticular extended amygdala (SLEA, 86%), interstitial nucleus of the posterior limb of the anterior commissure (IPAC, 56%), bed nucleus of the stria terminals (BNST, 59%), and medial preoptic area (MPA, 53%). PPE mRNA was expressed in CTB-ir neurons in the following regions: the NTS (14% of CTB-ir neurons), midline raphe (26%), LHA (22%), zona incerta (ZI, 15%), CeA (5%), paraventricular nucleus (PVN, 13%), SLEA (66%), and MPA (26%). Thus, limited brain regions contribute GABAergic and/or enkephalinergic input to the RVLM. Multiple neurochemically distinct pathways originate from these brain regions projecting to the RVLM.

Arterial chemoreceptor activation reduces the activity of parapyramidal serotonergic neurons in rats

The parapyramidal (ppy) region targets primarily the intermediolateral cell column and is probably involved in breathing and thermoregulation. In the present study, we tested whether ppy serotonergic neurons respond to activation of central and peripheral chemoreceptors. Bulbospinal ppy neurons (n=30) were recorded extracellularly along with the phrenic nerve activity in urethane/α-chloralose-anesthetized, paralyzed, intact (n=7) or carotid body denervated (n=6) male Wistar rats. In intact animals, most of the ppy neurons were inhibited by hypoxia (n=14 of 19) (8% O2, 30s) (1.5 ± 0.03 vs. control: 2.4 ± 0.2 Hz) or hypercapnia (n=15 of 19) (10% CO2) (1.7 ± 0.1 vs. control: 2.2 ± 0.2 Hz), although some neurons were insensitive to hypoxia (n=3 of 19) or hypercapnia (n=4 of 19). Very few neurons (n=2 of 19) were activated after hypoxia, but not after hypercapnia. In carotid body denervated rats, all the 5HT-ppy neurons (n=11) were insensitive to hypercapnia (2.1 ± 0.1 vs. control: 2.3 ± 0.09 Hz). Biotinamide-labeled cells that were recovered after histochemistry were located in the ppy region. Most labeled cells (90%) showed strong tryptophan hydroxylase immunocytochemical reactivity, indicating that they were serotonergic. The present data reveal that peripheral chemoreceptors reduce the activity of the serotonergic premotor neurons located in the ppy region. It is plausible that the serotonergic neurons of the ppy region could conceivably regulate breathing automaticity and be involved in autonomic regulation.

Medullary GABAergic mechanisms contribute to electroacupuncture modulation of cardiovascular depressor responses during gastric distention in rats

Electroacupuncture (EA) at P5-P6 acupoints overlying the median nerves typically reduces sympathoexcitatory blood pressure (BP) reflex responses in eucapnic rats. Gastric distention in hypercapnic acidotic rats, by activating both vagal and sympathetic afferents, decreases heart rate (HR) and BP through actions in the rostral ventrolateral medulla (rVLM) and nucleus ambiguus (NAmb), leading to sympathetic withdrawal and parasympathetic activation, respectively. A GABAA mechanism in the rVLM mediates the decreased sympathetic outflow. The present study investigated the hypothesis that EA modulates gastric distention-induced hemodynamic depressor and bradycardia responses through nuclei that process parasympathetic and sympathetic outflow. Anesthetized hypercapnic acidotic rats manifested repeatable decreases in BP and HR with gastric distention every 10 min. Bilateral EA at P5-P6 for 30 min reversed the hypotensive response from -26 ± 3 to -6 ± 1 mmHg and the bradycardia from -35 ± 11 to -10 ± 3 beats/min for a period that lasted more than 70 min. Immunohistochemistry and in situ hybridization to detect c-Fos protein and GAD 67 mRNA expression showed that GABAergic caudal ventral lateral medulla (cVLM) neurons were activated by EA. Glutamatergic antagonism of cVLM neurons with kynurenic acid reversed the actions of EA. Gabazine used to block GABAA receptors microinjected into the rVLM or cVLM reversed EA's action on both the reflex depressor and bradycardia responses. EA modulation of the decreased HR was inhibited by microinjection of gabazine into the NAmb. Thus, EA through GABAA receptor mechanisms in the rVLM, cVLM, and NAmb modulates gastric distention-induced reflex sympathoinhibition and vagal excitation.

Low-fidelity GABA transmission within a dense excitatory network of the solitary tract nucleus

Visceral primary afferents enter the CNS at the caudal solitary tract nucleus (NTS), and activate central pathways key to autonomic and homeostatic regulation. Excitatory transmission from primary solitary tract (ST)-afferents consists of multiple contacts originating from single axons that offer a remarkably high probability of glutamate release and high safety factor for ST afferent excitation. ST afferent activation sometimes triggers polysynaptic GABAergic circuits, which feedback onto second-order NTS neurons. Although inhibitory transmission is observed at second-order neurons, much less is known about the organization and mechanisms regulating GABA transmission. Here, we used a focal pipette to deliver minimal stimulus shocks near second-order NTS neurons in rat brainstem slices and directly activated single GABAergic axons. Most minimal focal shocks activated low jitter EPSCs from single axons with characteristics resembling ST afferents. Much less commonly (9% of sites), minimal focal shocks activated monosynaptic IPSCs at fixed latency (low jitter) that often failed (30%) and had no frequency-dependent facilitation or depression. These GABA release characteristics contrasted markedly to the unfailing, large amplitudes for glutamate released during ST-EPCSs recorded from the same neurons. Surprisingly, unitary GABAergic IPSCs were only weakly calcium dependent. In some neurons, strong focal shocks evoked compound IPSCs indicating convergent summation of multiple inhibitory axons. Our studies demonstrate that second-order NTS neurons receive GABAergic transmission from a diffuse network of inhibitory axons that rely on an intrinsically less reliable and substantially weaker release apparatus than ST excitation. Effective inhibition depends on co-activation of convergent inputs to blunt excitatory drive.

The soma and proximal dendrites of sympathetic preganglionic neurons innervating the major pelvic ganglion in female rats receive predominantly inhibitory inputs

Sympathetic preganglionic neurons (SPNs) in the intermediolateral (IML) and dorsal commissural nucleus (DCN) of the thoracolumbar segments of the spinal cord contribute to the autonomic control of the pelvic visceral organs. We examined the morphology of these neurons at the light and electron microscopic level and quantified the boutons apposing the soma and proximal dendrites of the SPNs innervating the major pelvic ganglion (MPG) in female rats. The majority of these cells resided in the DCN (61.6±6.2%) and IML (33.2±4.4%) nuclei. Measurements of cell volume and shape revealed no differences between SPNs sampled from the DCN and IML populations. Ultrastructural studies of DCN and IML SPNs revealed that coverage of SPNs by synaptic inputs is sparse, with an average of 11.60±2.41% of the soma membrane and 16.33±6.18% of proximal dendrites apposed by boutons, though some somata exhibited no synaptic coverage. Three distinct types of boutons were found to appose the SPN somata and dendrites. The putatively inhibitory F-type bouton covered a significantly greater percentage of membrane on the soma (8.48±2.12%) and dendrites (12.65±4.34%), than the S-type bouton, a putatively excitatory bouton, which only covered 2.94±0.70% of the somatic and 3.68±2.98% of the dendritic membranes. Boutons with dense-core vesicles were rare. Our results demonstrate that SPNs of the DCN and IML of female rats are similar morphologically, and that synaptic input on these cells, though sparse, is predominantly inhibitory.

Synaptic and intrinsic activation of GABAergic neurons in the cardiorespiratory brainstem network

GABAergic pathways in the brainstem play an essential role in respiratory rhythmogenesis and interactions between the respiratory and cardiovascular neuronal control networks. However, little is known about the identity and function of these GABAergic inhibitory neurons and what determines their activity. In this study we have identified a population of GABAergic neurons in the ventrolateral medulla that receive increased excitatory post-synaptic potentials during inspiration, but also have spontaneous firing in the absence of synaptic input. Using transgenic mice that express GFP under the control of the Gad1 (GAD67) gene promoter, we determined that this population of GABAergic neurons is in close apposition to cardioinhibitory parasympathetic cardiac neurons in the nucleus ambiguus (NA). These neurons fire in synchronization with inspiratory activity. Although they receive excitatory glutamatergic synaptic inputs during inspiration, this excitatory neurotransmission was not altered by blocking nicotinic receptors, and many of these GABAergic neurons continue to fire after synaptic blockade. The spontaneous firing in these GABAergic neurons was not altered by the voltage-gated calcium channel blocker cadmium chloride that blocks both neurotransmission to these neurons and voltage-gated Ca²⁺ currents, but spontaneous firing was diminished by riluzole, demonstrating a role of persistent sodium channels in the spontaneous firing in these cardiorespiratory GABAergic neurons that possess a pacemaker phenotype. The spontaneously firing GABAergic neurons identified in this study that increase their activity during inspiration would support respiratory rhythm generation if they acted primarily to inhibit post-inspiratory neurons and thereby release inspiration neurons to increase their activity. This population of inspiratory-modulated GABAergic neurons could also play a role in inhibiting neurons that are most active during expiration and provide a framework for respiratory sinus arrhythmia as there is an increase in heart rate during inspiration that occurs via inhibition of premotor parasympathetic cardioinhibitory neurons in the NA during inspiration.

Distribution of glycine/GABA neurons in the ventromedial medulla with descending spinal projections and evidence for an ascending glycine/GABA projection

The ventromedial medulla (VM), subdivided in a rostral (RVM) and a caudal (CVM) part, has a powerful influence on the spinal cord. In this study, we have identified the distribution of glycine and GABA containing neurons in the VM with projections to the cervical spinal cord, the lumbar dorsal horn, and the lumbar ventral horn. For this purpose, we have combined retrograde tracing using fluorescent microspheres with fluorescent in situ hybridization (FISH) for glycine transporter 2 (GlyT2) and GAD67 mRNAs to identify glycinergic and/or GABAergic (Gly/GABA) neurons. Since the results obtained with FISH for GlyT2, GAD67, or GlyT2 + GAD67 mRNAs were not significantly different, we concluded that glycine and GABA coexisted in the various projection neurons. After injections in the cervical cord, we found that 29% ± 1 (SEM) of the retrogradely labeled neurons in the VM were Gly/GABA (RVM: 43%; CVM: 21%). After lumbar dorsal horn injections 31% ± 3 of the VM neurons were Gly/GABA (RVM: 45%; CVM: 12%), and after lumbar ventral horn injections 25% ± 2 were Gly/GABA (RVM: 35%; CVM: 17%). In addition, we have identified a novel ascending Gly/GABA pathway originating from neurons in the area around the central canal (CC) throughout the spinal cord and projecting to the RVM, emphasizing the interaction between the ventromedial medulla and the spinal cord. The present study has now firmly established that GABA and glycine are present in many VM neurons that project to the spinal cord. These neurons strongly influence spinal processing, most notably the inhibition of nociceptive transmission.

Hypoxia activates nucleus tractus solitarii neurons projecting to the paraventricular nucleus of the hypothalamus

Peripheral chemoreceptor afferent information is sent to the nucleus tractus solitarii (nTS), integrated, and relayed to other brain regions to alter cardiorespiratory function. The nTS projects to the hypothalamic paraventricular nucleus (PVN), but activation and phenotype of these projections during chemoreflex stimulation is unknown. We hypothesized that activation of PVN-projecting nTS neurons occurs primarily at high intensities of hypoxia. We assessed ventilation and cardiovascular parameters in response to increasing severities of hypoxia. Retrograde tracers were used to label nTS PVN-projecting neurons and, in some rats, rostral ventrolateral medulla (RVLM)-projecting neurons. Immunohistochemistry was performed to identify nTS cells that were activated (Fos-immunoreactive, Fos-IR), catecholaminergic, and GABAergic following hypoxia. Conscious rats underwent 3 h normoxia (n = 4, 21% O(2)) or acute hypoxia (12, 10, or 8% O(2); n = 5 each). Hypoxia increased ventilation and the number of Fos-IR nTS cells (21%, 13 ± 2; 12%, 58 ± 4; 10%, 166 ± 22; 8%, 186 ± 6). Fos expression after 10% O(2) was similar whether arterial pressure was allowed to decrease (-13 ± 1 mmHg) or was held constant. The percentage of PVN-projecting cells activated was intensity dependent, but contrary to our hypothesis, PVN-projecting nTS cells exhibiting Fos-IR were found at all hypoxic intensities. Notably, at all intensities of hypoxia, ∼75% of the activated PVN-projecting nTS neurons were catecholaminergic. Compared with RVLM-projecting cells, a greater percentage of PVN-projecting nTS cells was activated by 10% O(2). Data suggest that increasing hypoxic intensity activates nTS PVN-projecting cells, especially catecholaminergic, PVN-projecting neurons. The nTS to PVN catecholaminergic pathway may be critical even at lower levels of chemoreflex activation and more important to cardiorespiratory responses than previously considered.

Glia determine the course of brain-derived neurotrophic factor-mediated dendritogenesis and provide a soluble inhibitory cue to dendritic growth in the brainstem

Cardiorespiratory control neurons in the brainstem nucleus tractus solitarius (NTS) undergo dramatic expansion of dendritic arbors during the early postnatal period, when functional remodeling takes place within the NTS circuitry. However, the underlying molecular mechanisms of morphological maturation of NTS neurons are largely unknown. Our previous studies point to the neurotrophin brain-derived neurotrophic factor (BDNF), which is abundantly expressed by NTS-projecting primary sensory neurons, as a candidate mediator of NTS dendritogenesis. In the current study, we used neonatal rat NTS neurons in vitro to examine the role of BDNF in the dendritic development of neurochemically identified subpopulations of NTS neurons. In the presence of abundant glia, BDNF promoted NTS dendritic outgrowth and complexity, with the magnitude of the BDNF effect dependent on neuronal phenotype. Surprisingly, BDNF switched from promoting to inhibiting NTS dendritogenesis upon glia depletion. Moreover, glia depletion alone led to a significant increase in NTS dendritic outgrowth. Consistent with this result, astrocyte-conditioned medium (ACM), which promoted hippocampal dendritogenesis, inhibited dendritic growth of NTS neurons. The latter effect was abolished by heat-inactivation of ACM, pointing to a diffusible astrocyte-derived negative regulator of NTS dendritic growth. Together, these data demonstrate a role for BDNF in the postnatal development of NTS neurons, and reveal novel effects of glia on this process. Moreover, previously documented dramatic increases in NTS glial proliferation in victims of sudden infant death syndrome (SIDS) underscore the importance of our findings and the need to better understand the role of glia and their interactions with BDNF during NTS circuit maturation. Furthermore, while it has previously been demonstrated that the specific effects of BDNF on dendritic growth are context-dependent, the role of glia in this process is unknown. Thus, our data carry important implications for mechanisms of dendritogenesis likely beyond the NTS.

Control of the central chemoreflex by A5 noradrenergic neurons in rats

Central chemoreflex stimulation produces an increase in phrenic nerve activity (PNA) and sympathetic nerve activity (SNA). The A5 noradrenergic region projects to several brainstem areas involved in autonomic regulation and contributes to the increase in SNA elicited by peripheral chemoreflex activation. The aim of the present study was to further test the hypothesis that the A5 noradrenergic region could contribute to central chemoreflex activation. In urethane-anesthetized, sino-aortic denervated, and vagotomized male Wistar rats (n=6-8/group), hypercapnia (end-expiratory CO₂ from 5% to 10%) increased mean arterial pressure (MAP; Δ=+33±4 mmHg, P<0.05), splanchnic SNA (sSNA; Δ=+97±13%, P<0.05), and PNA frequency and amplitude. Bilateral injection of muscimol (GABA-A agonist; 2 mM) into the A5 noradrenergic region reduced the rise in MAP (Δ=+19±3 mmHg, P<0.05), sSNA (Δ=+63±5%, P<0.05), and PNA frequency and amplitude produced by hypercapnia. Injections of the immunotoxin anti-dopamine β-hydroxylase-saporin (anti-DβH-SAP) into the A5 region destroyed TH⁺ neurons but spared facial motoneurons and the chemosensitive neurons in the retrotrapezoid nucleus that express the transcription factor Phox2b and that are non-catecholaminergic (TH⁻Phox2b⁺). Two weeks after selective destruction of the A5 region with the anti-DβH-SAP toxin, the increase in MAP (Δ=+22±5 mmHg, P<0.05), sSNA (Δ=+68±9%, P<0.05), and PNA amplitude was reduced after central chemoreflex activation. These results suggest that A5 noradrenergic neurons contribute to the increase in MAP, sSNA, and PNA activation during central chemoreflex stimulation.

Molecular and neuroanatomical characterization of single neurons in the mouse medullary gigantocellular reticular nucleus

Medullary gigantocellular reticular nucleus (mGi) neurons have been ascribed a variety of behaviors, many of which may fall under the concepts of either arousal or motivation. Despite this, many details of the connectivity of mGi neurons, particularly in reference to those neurons with ascending axons, remain unknown. To provide a neuroanatomical and molecular characterization of these cells, with reference to arousal and level-setting systems, large medullary reticular neurons were characterized with retrograde dye techniques and with real-time reverse transcriptase PCR (RT-PCR) analyses of single-neuron mRNA expression in the mouse. We have shown that receptors consistent with participation in generalized arousal are expressed by single mGi neurons and that receptors from different families of arousal-related neurotransmitters are rarely coexpressed. Through retrograde labeling, we have shown that neurons with ascending axons and neurons with descending axons tend to form like-with-like clusters, a finding that is consistent across age and gender. In comparing the two groups of retrogradely labeled neurons in neonatal animals, those neurons with axons that ascend to the midbrain show markers for GABAergic or coincident GABAergic and glutamatergic function; in contrast, approximately 60% of the neurons with axons that descend to the spinal cord are glutamatergic. We discuss the mGi's relationship to the voluntary and emotional motor systems and speculate that neurons in the mGi may represent a mammalian analogue to Mauthner cells, with a separation of function for neurons with ascending and descending axons.

Opioid receptor modulation of GABAergic and serotonergic spinally projecting neurons of the rostral ventromedial medulla in mice

The rostral ventromedial medulla (RVM) is an important site of opioid actions and forms part of an analgesic pathway that projects to the spinal cord. The neuronal mechanisms by which opioids act within this brain region remain unclear, particularly in relation to the neurotransmitters GABA and serotonin. In the present study, we examined serotonergic and GABAergic immunoreactivity, identified using immunohistochemistry for tryptophan hydroxylase (TPH) and glutamate decarboxylase (GAD), in combination with in vitro whole cell patch clamping to investigate the role of opioids on the mouse RVM with identified projections to the spinal cord. Tyr-d-Ala-Gly-N-Me-Phe-Gly-ol enkephalin (DAMGO) produced μ-opioid receptor-mediated outward currents in virtually all TPH-immunoreactive projecting neurons and GAD-immunoreactive nonprojecting neurons (87% and 86%). The other groups of RVM neurons displayed mixed responsiveness to DAMGO (40-68%). Deltorphin II and U-69593 produced δ- and κ-opioid receptor-mediated outward currents in smaller subpopulations of RVM neurons, with many of the δ-opioid responders forming a subpopulation of μ-opioid-sensitive GABAergic nonprojecting neurons. These findings are consistent with prior electrophysiological and anatomic studies in the rat RVM and indicate that both serotonergic and GABAergic RVM neurons mediate the actions of μ-opioids. Specifically, μ-opioids have a direct postsynaptic inhibitory influence over both GABAergic and serotonergic neurons, including those that project to the dorsal spinal cord.

Asymmetrical changes in lumbar sympathetic nerve activity following stimulation of the sciatic nerve in rat

Noxious stimulation of the leg increases hind limb blood flow (HBF) to the ipsilateral side and decreases to the contralateral in rat. Whether or not this asymmetrical response is due to direct control by sympathetic terminals or mediated by other factors such as local metabolism and hormones remains unclear. The aim of this study was to compare responses in lumbar sympathetic nerve activity, evoked by stimulation of the ipsilateral and contralateral sciatic nerve (SN). We also sought to determine the supraspinal mechanisms involved in the observed responses. In anesthetized and paralyzed rats, intermittent electrical stimulation (1 mA, 0.5 Hz) of the contralateral SN evoked a biphasic sympathoexcitation. Following ipsilateral SN stimulation, the response is preceded by an inhibitory potential with a latency of 50 ms (N=26). Both excitatory and inhibitory potentials are abolished following cervical C1 spinal transection (N=6) or bilateral microinjections of muscimol (N=6) in the rostral ventrolateral medulla (RVLM). This evidence is suggestive that both sympathetic potentials are supraspinally mediated in this nucleus. Blockade of RVLM glutamate receptors by microinjection of kynurenic acid (N=4) selectively abolished the excitatory potential elicited by ipsilateral SN stimulation. This study supports the physiological model that activation of hind limb nociceptors evokes a generalized sympathoexcitation, with the exception of the ipsilateral side where there is a withdrawal of sympathetic tone resulting in an increase in HBF.

Paraventricular hypothalamic nucleus: axonal projections to the brainstem

The paraventricular hypothalamic nucleus (PVH) contains many neurons that innervate the brainstem, but information regarding their target sites remains incomplete. Here we labeled neurons in the rat PVH with an anterograde axonal tracer, Phaseolus vulgaris leucoagglutinin (PHAL), and studied their descending projections in reference to specific neuronal subpopulations throughout the brainstem. While many of their target sites were identified previously, numerous new observations were made. Major findings include: 1) In the midbrain, the PVH projects lightly to the ventral tegmental area, Edinger-Westphal nucleus, ventrolateral periaqueductal gray matter, reticular formation, pedunculopontine tegmental nucleus, and dorsal raphe nucleus. 2) In the dorsal pons, the PVH projects heavily to the pre-locus coeruleus, yet very little to the catecholamine neurons in the locus coeruleus, and selectively targets the viscerosensory subregions of the parabrachial nucleus. 3) In the ventral medulla, the superior salivatory nucleus, retrotrapezoid nucleus, compact and external formations of the nucleus ambiguous, A1 and caudal C1 catecholamine neurons, and caudal pressor area receive dense axonal projections, generally exceeding the PVH projection to the rostral C1 region. 4) The medial nucleus of the solitary tract (including A2 noradrenergic and aldosterone-sensitive neurons) receives the most extensive projections of the PVH, substantially more than the dorsal vagal nucleus or area postrema. Our findings suggest that the PVH may modulate a range of homeostatic functions, including cerebral and ocular blood flow, corneal and nasal hydration, ingestive behavior, sodium intake, and glucose metabolism, as well as cardiovascular, gastrointestinal, and respiratory activities.

Inhibition of brown adipose tissue thermogenesis by neurons in the ventrolateral medulla and in the nucleus tractus solitarius

Neurons in the ventrolateral medulla (VLM) and in the nucleus tractus solitarius (NTS) play important roles in the regulation of cardiovascular and other autonomic functions. In the present study, we demonstrate an inhibition of brown adipose tissue (BAT) thermogenesis evoked by activation of neurons in the VLM, as well as by neurons in the intermediate NTS, of chloralose/urethane-anesthetized, artificially ventilated rats. Activation of neurons in either rostral VLM or caudal VLM with N-methyl-d-aspartate (12 nmol) reversed the cold-evoked increase in BAT sympathetic nerve activity (SNA), BAT temperature, and end-expired CO(2). Disinhibition of neurons in either VLM or NTS with the GABA(A) receptor antagonist, bicuculline (30 pmol), reversed the increases in BAT SNA, BAT temperature, and end-expired CO(2) that were elicited 1) by cold defense; 2) during the febrile model of nanoinjection of prostaglandin E(2) into the medial preoptic area; 3) by activation of neurons in the dorsomedial hypothalamus or in the rostral raphe pallidus (rRPa); or 4) by the micro-opioid receptor agonist fentanyl. Combined, but not separate, inhibitions of neurons in the VLM and in the NTS, with the GABA(A) receptor agonist, muscimol (120 pmol/site), produced increases in BAT SNA, BAT temperature, and expired CO(2), which were reversed by nanoinjection of glycine (30 nmol) into the rRPa. These findings suggest that VLM and NTS contain neurons whose activation inhibits BAT thermogenesis, that these neurons receive GABAergic inputs that are active under these experimental conditions, and that neurons in both sites contribute to the tonic inhibition of sympathetic premotor neuronal activity in the rRPa that maintains a low level of BAT thermogenesis in normothermic conditions.

Properties of GABAergic neurons in the rostral solitary tract nucleus in mice

The rostral nucleus of the solitary tract (rNST) plays a pivotal role in taste processing. The rNST contains projection neurons and interneurons that differ in morphology and intrinsic membrane properties. Although characteristics of the projection neurons have been detailed, similar information is lacking on the interneurons. We determined the intrinsic properties of the rNST GABAergic interneurons using a transgenic mouse model that expresses enhanced green fluorescent protein under the control of a GAD67 promoter. Glutamic acid decarboxylase-green fluorescent protein (GAD67-GFP) neurons were distributed throughout the rNST but were concentrated in the ventral subdivision with minimal interaction with the terminal field of the afferent input. Furthermore, the density of the GAD67-GFP neurons decreased in more rostral areas of rNST. In whole cell recordings, GAD67-GFP neurons responded with either an initial burst (73%), tonic (18%), or irregular (9%) discharge pattern of action potentials (APs) in response to membrane depolarization. These three groups also differed in passive and AP characteristics. Initial burst neurons had small ovoid or fusiform cell bodies, whereas tonic firing neurons had large multipolar or fusiform cell bodies. Irregular firing neurons had larger spherical soma. Some of the initial burst and tonic firing neurons were also spontaneously active. The GAD67-GFP neurons could also be categorized in subgroups based on colocalization with somatostatin and parvalbumin immunolabeling. Initial burst neurons would transmit the early dynamic portion of the encoded sensory stimuli, whereas tonic firing neurons could respond to both dynamic and static components of the sensory input, suggesting different roles for GAD67-GFP neurons in taste processing.

Cardiovascular and behavioural responses to conditioned fear and restraint are not affected by retrograde lesions of A5 and C1 bulbospinal neurons

The aim of this study was to test a possible role of A5 neurons in the expression of the pressor and tachycardic responses to conditioned fear and restraint, two forms of psychological stress. Previous Fos studies have shown that the C1 adrenergic neurons and spinally projecting neurons in the vasopressor region of the rostral ventrolateral medulla are not activated by these two stressors, suggesting that these cardiovascular changes may be mediated by other premotor sympathetic (presympathetic) cell groups. The same studies also revealed that the A5 noradrenergic group was one of the main presympathetic cell groups to be activated in response to these two stressors. Thus, we hypothesized that the A5 group could mediate these cardiovascular responses. Conditioned fear and restraint were tested in rats implanted with radiotelemetric probes before and after retrograde lesion with the selective toxin anti-dopamine-beta-hydroxylase-saporin bilaterally injected in the spinal cord at T2-T3. Six animals were selected that had the most extensive loss of spinally projecting catecholaminergic neurons: A5 (81%-95%) and rostral C1 (59%-86%, which would include most C1 bulbospinal neurons). However, despite this major loss of noradrenergic and adrenergic presympathetic neurons, the magnitude of the cardiovascular response to conditioned fear and restraint was the same before and after the lesion. Associated behavioural changes were not affected either. The results indicate that A5 presympathetic neurons are not essential for the expression of the tachycardic and pressor responses to conditioned fear and restraint. They also confirm that C1 bulbospinal neurons are not involved in these responses. The presympathetic neurons driving the tachycardic and pressor responses to conditioned fear and restraint must be elsewhere.

Acid sensitivity and ultrastructure of the retrotrapezoid nucleus in Phox2b-EGFP transgenic mice

The retrotrapezoid nucleus (RTN) contains noncholinergic noncatecholaminergic glutamatergic neurons that express the transcription factor Phox2b (chemically coded or "cc" RTN neurons). These cells regulate breathing and may be central chemoreceptors. Here we explore their ultrastructure and their acid sensitivity by using two novel BAC eGFP-Phox2b transgenic mice (B/G, GENSAT JX99) in which, respectively, 36% and 100% of the cc RTN neurons express the transgene in complete or partial anatomical isolation from other populations of eGFP neurons. All but one of the eGFP-labeled RTN neurons recorded in these mice were acid activated in slices. These cells contained VGLUT2 mRNA, and 50% contained preprogalanin mRNA (determined by single-cell PCR in the B/G mouse). Two neuronal subgroups were revealed, which differed in discharge rate at pH 7.3 (type I approximately 2; type II approximately 4 Hz) and the degree of alkalization that silenced the cells (type I 7.4-7.6, type II 7.8-8.0). Medial to the RTN, C1 neurons recorded in a tyrosine hydroxylase-GFP mouse were pH insensitive between pH 6.9 and pH 7.5. Ultrastructural studies demonstrated that eGFP-labeled RTN neurons were surrounded by numerous capillaries and were often in direct contact with glial cells, pericytes, and the basement membrane of capillaries. Terminals contacting large proximal eGFP dendrites formed mainly symmetric, likely inhibitory, synapses. Terminals on more distal eGFP dendrites formed preferentially asymmetric, presumably excitatory, synapses. In sum, C1 cells are pH insensitive, whereas cc RTN neurons are uniformly acid sensitive. The RTN neurons receive inhibitory and excitatory synaptic inputs and may have unfettered biochemical interactions with glial cells and the local microvasculature.

Neuroanatomical tracing combined with in situ hybridization: analysis of gene expression patterns within brain circuits of interest

Most of our current understanding of brain circuits is based on hodological studies carried out using neuroanatomical tract-tracing. Our aim is to advance one step further by visualizing the functional correlate in a given circuit. In this regard, we believe it is feasible to combine retrograde tracing with fluorescence, non-radioactive in situ hybridization (ISH) protocols. The subsequent detection at the single-cell level of the expression of a given mRNA within retrograde-labeled neurons provides information regarding cellular function. This may be of particular interest when trying to elucidate the performance of brain circuits of interest in animal models of brain diseases. Several combinations of retrograde tracing with either single- and double-ISH are presented here, together with some criteria that influence the selection of the tracer to be used in conjunction with the strong demands of the ISH.

Activation of the retrotrapezoid nucleus by posterior hypothalamic stimulation

The retrotrapezoid nucleus (RTN) contains chemically defined neurons (ccRTN neurons) that provide a pH-regulated excitatory drive to the central respiratory pattern generator. Here we test whether ccRTN neurons respond to stimulation of the perifornical hypothalamus (PeF), a region that regulates breathing during sleep, stress and exercise. PeF stimulation with gabazine increased blood pressure, phrenic nerve discharge (PND) and the firing rate of ccRTN neurons in isoflurane-anaesthetized rats. Gabazine produced an approximately parallel upward shift of the steady-state relationship between ccRTN neuron firing rate and end-tidal CO(2), and a similar shift of the relationship between PND and end-tidal CO(2). The central respiratory modulation of ccRTN neurons persisted after gabazine without a change in pattern. Morphine administration typically abolished PND and reduced the discharge rate of most ccRTN neurons (by 25% on average). After morphine administration, PeF stimulation activated the ccRTN neurons normally but PND activation and the central respiratory modulation of the ccRTN neurons were severely attenuated. In the same rat preparation, most (58%) ccRTN neurons expressed c-Fos after exposure to hypercapnic hyperoxia (6-7% end-tidal CO(2); 3.5 h; no hypothalamic stimulation) and 62% expressed c-Fos under hypocapnia (approximately 3% end-tidal CO(2)) after PeF stimulation. Under baseline conditions (approximately 3% end-tidal CO(2), hyperoxia, no PeF stimulation) few (11%) ccRTN neurons expressed c-Fos. In summary, most ccRTN neurons are excited by posterior hypothalamic stimulation while retaining their normal response to CNS acidification. ccRTN neurons probably contribute both to the chemical drive of breathing and to the feed-forward control of breathing associated with emotions and or locomotion.

Photostimulation of channelrhodopsin-2 expressing ventrolateral medullary neurons increases sympathetic nerve activity and blood pressure in rats

To explore the specific contribution of the C1 neurons to blood pressure (BP) control, we used an optogenetic approach to activate these cells in vivo. A lentivirus that expresses channelrhodopsin-2 (ChR2) under the control of the catecholaminergic neuron-preferring promoter PRSx8 was introduced into the rostral ventrolateral medulla (RVLM). After 2-3 weeks, ChR2 was largely confined to Phox2b-expressing neurons (89%). The ChR2-expressing neurons were non-GABAergic, non-glycinergic and predominantly catecholaminergic (54%). Photostimulation of ChR2-transfected RVLM neurons (473 nm, 20 Hz, 10 ms, 9 mW) increased BP (15 mmHg) and sympathetic nerve discharge (SND; 64%). Light pulses at 0.2-0.5 Hz evoked a large sympathetic nerve response (16 x baseline) followed by a silent period (1-2 s) during which another stimulus evoked a reduced response. Photostimulation activated most (75%) RVLM baroinhibited neurons sampled with 1/1 action potential entrainment to the light pulses and without accommodation during 20 Hz trains. RVLM neurons unaffected by either CO(2) or BP were light-insensitive. Bötzinger respiratory neurons were activated but their action potentials were not synchronized to the light pulses. Juxtacellular labelling of recorded neurons revealed that, of these three cell types, only the cardiovascular neurons expressed the transgene. In conclusion, ChR2 expression had no discernable effect on the putative vasomotor neurons at rest and was high enough to allow precise temporal control of their action potentials with light pulses. Photostimulation of RVLM neurons caused a sizable sympathoactivation and rise in blood pressure. These results provide the most direct evidence yet that the C1 neurons have a sympathoexcitatory function.

Angiotensin-(1-7) antagonist, A-779, microinjection into the caudal ventrolateral medulla of renovascular hypertensive rats restores baroreflex bradycardia

In the present study we evaluated the effect of caudal ventrolateral medulla (CVLM) microinjection of the main angiotensin (Ang) peptides, Ang II and Ang-(1-7), and their selective antagonists on baseline arterial pressure (AP) and on baroreceptor-mediated bradycardia in renovascular hypertensive rats (2K1C). Microinjection of Ang II and Ang-(1-7) into the CVLM of 2K1C rats produced similar decrease in AP as observed in Sham rats. In both Sham and 2K1C, the hypotensive effect of Ang II and Ang-(1-7) at the CVLM was blocked, for up to 30 min, by previous CVLM microinjection of the Ang II AT1 receptor antagonist, Losartan, and Ang-(1-7) Mas antagonist, A-779, respectively. As expected, the baroreflex bradycardia was lower in 2K1C in comparison to Sham rats. CVLM microinjection of A-779 improved the sensitivity of baroreflex bradycardia in 2K1C hypertensive rats. In contrast, Losartan had no effect on the baroreflex bradycardia in either 2K1C or Sham rats. These results suggest that Ang-(1-7) at the CVLM may contribute to the low sensitivity of the baroreflex control of heart rate in renovascular hypertensive rats.

Neurochemistry of bulbospinal presympathetic neurons of the medulla oblongata

This review focuses on presympathetic neurons in the medulla oblongata including the adrenergic cell groups C1-C3 in the rostral ventrolateral medulla and the serotonergic, GABAergic and glycinergic neurons in the ventromedial medulla. The phenotypes of these neurons including colocalized neuropeptides (e.g., neuropeptide Y, enkephalin, thyrotropin-releasing hormone, substance P) as well as their relative anatomical location are considered in relation to predicting their function in control of sympathetic outflow, in particular the sympathetic outflows controlling blood pressure and thermoregulation. Several explanations are considered for how the neuroeffectors coexisting in these neurons might be functioning, although their exact purpose remains unknown. Although there is abundant data on potential neurotransmitters and neuropeptides contained in the presympathetic neurons, we are still unable to predict function and physiology based solely on the phenotype of these neurons.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

Galanin is a selective marker of the retrotrapezoid nucleus in rats

The rat retrotrapezoid nucleus (RTN) contains CO(2)-activated neurons that contribute to the central chemoreflex and to breathing automaticity. These neurons have two known markers, the transcription factor Phox2b and vesicular glutamate transporter 2 (VGLUT2). Noncatecholaminergic galanin-immunoreactive (ir) neurons within a region of the lower brainstem that seems identical to what is currently defined as the RTN have been previously described. Here we ask whether these galanin-expressing neurons are the same cells as the recently characterized CO(2)-sensitive neurons of the RTN. By using in situ hybridization, we found that pre-pro-galanin (PPGal) mRNA is expressed by an isolated cluster of neurons that is co-extensive with the RTN as defined by a population of strongly Phox2b-ir neurons devoid of tyrosine hydroxylase (Phox2b(+)/TH(-) neurons). This bilateral structure contains about 1,000 PPGal mRNA-positive neurons in the rat. The PPGal mRNA-positive neurons were Phox2b(+)/TH(-) and as susceptible to destruction by the toxin [Sar(9), Met (O(2))(11)]-substance P as the rest of the RTN Phox2b(+)/TH(-) cells of the RTN. CO(2)-activated neurons were recorded in the RTN of anesthetized rats and were labeled with biotinamide. Many of those cells (7/17, 41%, five rats) contained PPGal-mRNA. In conclusion, galanin mRNA is a very specific marker of the glutamatergic Phox2b(+)/TH(-) neurons of the RTN, but galanin mRNA identifies only half of these putative central respiratory chemoreceptors.

Mapping and identification of GABAergic neurons in transgenic mice projecting to cardiac vagal neurons in the nucleus ambiguus using photo-uncaging

The neural control of heart rate is determined primarily by the activity of preganglionic parasympathetic cardiac vagal neurons (CVNs) originating in the nucleus ambiguus (NA) in the brain stem. GABAergic inputs to CVNs play an essential role in determining the activity of these neurons including a robust inhibition during each inspiratory burst. The origin of GABAergic innervation has yet to be determined however. A transgenic mouse line expressing green florescent protein (GFP) in GABAergic cells was used in conjunction with caged glutamate to identify both clusters and individual GABAergic neurons that evoke inhibitory GABAergic synaptic responses in CVNs. Transverse slices were taken with CVNs patch-clamped in the whole cell configuration. Sections containing both the pre-Botzinger complex as well as the calamus scriptorius were divided into approximately 90 quadrants, each 200 x 200 microm and were sequentially photostimulated. Inhibitory post synaptic currents (IPSCs) were recorded in CVNs after a 5-ms photostimulation of 50 microM caged glutamate. The four areas that contained GABAergic cells projecting to CVNs were 200 microm medial, 400 microm medial, 200 microm ventral, and 1,200 microm dorsal and 1,000 microm medial to patched CVNs. Once foci of GABAergic cells projecting to CVNs were determined, photostimulation of individual GABAergic neurons was conducted. The results from this study suggest that GABAergic cells located in four specific areas project to CVNs, and that these cells can be individually identified and stimulated using photouncaging to recruit GABAergic neurotransmission to CVNs.

Expression of Group I metabotropic glutamate receptors on phenotypically different cells within the nucleus of the solitary tract in the rat

Group I metabotropic glutamate receptors (mGluRs) are G-coupled receptors that modulate synaptic activity. Previous studies have shown that Group I mGluRs are present in the nucleus of the solitary tract (NTS), in which many visceral afferents terminate. Microinjection of selective Group I mGluR agonists into the NTS results in a depressor response and decrease in sympathetic nerve activity. There is, however, little evidence detailing which phenotypes of neurons within the NTS express Group I mGluRs. In brainstem slices, we performed immunohistochemical localization of Group I mGluRs and either glutamic acid decarboxylase 67 kDa isoform (GAD67), neuronal nitric oxide synthase (nNOS) or tyrosine hydroxylase (TH). Fluoro-Gold (FG, 2%; 15 nl) was microinjected in the caudal ventrolateral medulla (CVLM) of the rat to retrogradely label NTS neurons that project to CVLM. Group I mGluRs were distributed throughout the rostral-caudal extent of the NTS and were found within most NTS subregions. The relative percentages of Group I mGluR expressing neurons colabeled with the different markers were FG (6.9+/-0.7) nNOS (5.6+/-0.9), TH (3.9+/-1.0), and GAD67 (3.1+/-1.4). The percentage of FG containing cells colabeled with Group I mGluR (13.6+/-2.0) was greater than the percent colabeled with GAD67 (3.1+/-0.5), nNOS (4.7+/-0.5), and TH (0.1+/-0.08). Cells triple labeled for FG, nNOS, and Group I mGluRs were identified in the NTS. Thus, these data provide an anatomical substrate by which Group I mGluRs could modulate activity of CVLM projecting neurons in the NTS.

Generation of diverse neuronal subtypes in cloned populations of stem-like cells

Background

The central nervous tissue contains diverse subtypes of neurons with characteristic morphological and physiological features and different neurotransmitter phenotypes. The generation of neurons with defined neurotransmitter phenotypes seems to be governed by factors differently expressed along the anterior-posterior and dorsal-ventral body axes. The mechanisms of the cell-type determination, however, are poorly understood. Selected neuronal phenotypes had been generated from embryonic stem (ES) cells, but similar results were not obtained on more restricted neural stem cells, presumably due to the lack of homogeneous neural stem cell populations as a starting material.

Results

In the presented work, the establishment of different neurotransmitter phenotypes was investigated in the course of in vitro induced neural differentiation of a one-cell derived neuroectodermal cell line, in conjunction with the activation of various region-specific genes. For comparison, similar studies were carried out on the R1 embryonic stem (ES) and P19 multipotent embryonic carcinoma (EC) cells. In response to a short treatment with all-trans retinoic acid, all cell lines gave rise to neurons and astrocytes. Non-induced neural stem cells and self-renewing cells persisting in differentiated cultures, expressed "stemness genes" along with early embryonic anterior-dorsal positional genes, but did not express the investigated CNS region-specific genes. In differentiating stem-like cell populations, on the other hand, different region-specific genes, those expressed in non-overlapping regions along the body axes were activated. The potential for diverse regional specifications was induced in parallel with the initiation of neural tissue-type differentiation. In accordance with the wide regional specification potential, neurons with different neurotransmitter phenotypes developed. Mechanisms inherent to one-cell derived neural stem cell populations were sufficient to establish glutamatergic and GABAergic neuronal phenotypes but failed to manifest cathecolaminergic neurons.

Conclusion

The data indicate that genes involved in positional determination are activated along with pro-neuronal genes in conditions excluding any outside influences. Interactions among progenies of one cell derived neural stem cells are sufficient for the activation of diverse region specific genes and initiate different routes of neuronal specification.

Selective lesion of retrotrapezoid Phox2b-expressing neurons raises the apnoeic threshold in rats

Injection of the neurotoxin saporin-substance P (SSP-SAP) into the retrotrapezoid nucleus (RTN) attenuates the central chemoreflex in rats. Here we ask whether these deficits are caused by the destruction of a specific type of interneuron that expresses the transcription factor Phox2b and is non-catecholaminergic (Phox2b(+)TH(-)). We show that RTN contains around 2100 Phox2b(+)TH(-) cells. Injections of SSP-SAP into RTN destroyed Phox2b(+)TH(-) neurons but spared facial motoneurons, catecholaminergic and serotonergic neurons and the ventral respiratory column caudal to the facial motor nucleus. Two weeks after SSP-SAP, the apnoeic threshold measured under anaesthesia was unchanged when fewer than 57% of the Phox2b(+)TH(-) neurons were destroyed. However, destruction of 70 +/- 3.5% of these cells was associated with a dramatic rise of the apnoeic threshold (from 5.6 to 7.9% end-expiratory P(CO(2))). In anaesthetized rats with unilateral lesions of around 70% of the Phox2b(+)TH(-) neurons, acute inhibition of the contralateral intact RTN with muscimol instantly eliminated phrenic nerve discharge (PND) but normal PND could usually be elicited by strong peripheral chemoreceptor stimulation (8/12 rats). Muscimol had no effect in rats with an intact contralateral RTN. In conclusion, the destruction of the Phox2b(+)TH(-) neurons is a plausible cause of the respiratory deficits caused by injection of SSP-SAP into RTN. Two weeks after toxin injection, 70% of these cells must be killed to cause a severe attenuation of the central chemoreflex under anaesthesia. The loss of an even greater percentage of these cells would presumably be required to produce significant breathing deficits in the awake state.