Physiology of quantal norepinephrine release from somatodendritic sites of neurons in locus coeruleus

Hippocampal seizures differentially modulate locus coeruleus activity and result in consistent time-locked release of noradrenaline in rat hippocampus

The locus coeruleus (LC) is a small brainstem nucleus and is the sole source of noradrenaline in the neocortex, hippocampus and cerebellum. Noradrenaline is a powerful neuromodulator involved in the regulation of excitability and plasticity of large-scale brain networks. In this study, we performed a detailed assessment of the activity of locus coeruleus neurons and changes in noradrenergic transmission during acute hippocampal seizures evoked with perforant path stimulation, using state-of-the-art methodology. Action potentials of LC neurons, of which some were identified by means of optogenetics, were recorded in anesthetized rats using a multichannel high-density electrophysiology probe. The seizure-induced change in firing rate differed between LC neurons: 55% of neurons decreased in firing rate during seizures, while 28% increased their firing rate. Topographic analysis of multi-unit activity over the electrophysiology probe showed a topographic clustering of neurons that were inhibited or excited during seizures. Changes in hippocampal noradrenaline transmission during seizures were assessed using a fluorescent biosensor for noradrenaline, GRABNE2m, in combination with fiber photometry, in both anesthetized and awake rats. Although our neuronal recordings indicated both inhibition and excitation of LC neurons during seizures, a consistent release of noradrenaline was observed. Concentrations of noradrenaline increased at seizure onset and decreased during or shortly after the seizure. In conclusion, this study showed consistent but heterogeneous modulation of LC neurons and a consistent time-locked release of hippocampal noradrenaline during acute hippocampal seizures.

From pupil to the brain: New insights for studying cortical plasticity through pupillometry

Pupil size variations have been associated with changes in brain activity patterns related with specific cognitive factors, such as arousal, attention, and mental effort. The locus coeruleus (LC), a key hub in the noradrenergic system of the brain, is considered to be a key regulator of cognitive control on pupil size, with changes in pupil diameter corresponding to the release of norepinephrine (NE). Advances in eye-tracking technology and open-source software have facilitated accurate pupil size measurement in various experimental settings, leading to increased interest in using pupillometry to track the nervous system activation state and as a potential biomarker for brain disorders. This review explores pupillometry as a non-invasive and fully translational tool for studying cortical plasticity starting from recent literature suggesting that pupillometry could be a promising technique for estimating the degree of residual plasticity in human subjects. Given that NE is known to be a critical mediator of cortical plasticity and arousal, the review includes data revealing the importance of the LC-NE system in modulating brain plasticity and pupil size. Finally, we will review data suggesting that pupillometry could provide a quantitative and complementary measure of cortical plasticity also in pre-clinical studies.

Spike-Dependent Dynamic Partitioning of the Locus Coeruleus Network through Noradrenergic Volume Release in a Simulation of Nucleus Core

The Locus coeruleus (LC) modulates various neuronal circuits throughout the brain. Its unique architectural organization encompasses a net of axonal innervation that spans the entire brain, while its somatic core is highly compact. Recent research revealed an unexpected cellular input specificity within the nucleus that can give rise to various network states that either broadcast norepinephrine signals throughout the brain or pointedly modulate specific brain areas. Such adaptive input–output functions likely surpass our existing network models that build upon a given synaptic wiring configuration between neurons. As the distances between noradrenergic neurons in the core of the LC are unusually small, neighboring neurons could theoretically impact each other via volume transmission of NE. We therefore set out to investigate if such interaction could be mediated through noradrenergic alpha2-receptors in a spiking neuron model of the LC. We validated our model of LC neurons through comparison with experimental patch-clamp data and identified key variables that impact alpha2-mediated inhibition of neighboring LC neurons. Our simulation confirmed a reliable autoinhibition of LC neurons after episodes of high neuronal activity that continue even after neuronal activity subsided. Additionally, dendro-somatic synapses inhibited spontaneous spiking in the somatic compartment of connected neurons in our model. We determined the exact position of hundreds of LC neurons in the mouse brain stem via a tissue clearing approach and, based on this, further determined that 25 percent of noradrenergic neurons have a neighboring LC neuron within less than a 25-micrometer radius. By modeling NE diffusion, we estimated that more than 15 percent of the alpha2-adrenergic receptors fraction can bind NE within such a diffusion radius. Our spiking neuron model of LC neurons predicts that repeated or long-lasting episodes of high neuronal activity induce partitioning of the gross LC network and reduce the spike rate in neighboring neurons at distances smaller than 25 μm. As these volume-mediating neighboring effects are challenging to test with the current methodology, our findings can guide future experimental approaches to test this phenomenon and its physiological consequences.

The Thermodynamically Expensive Contribution of Three Calcium Sources to Somatic Release of Serotonin

The soma, dendrites and axon of neurons may display calcium-dependent release of transmitters and peptides. Such release is named extrasynaptic for occurring in absence of synaptic structures. This review describes the cooperative actions of three calcium sources on somatic exocytosis. Emphasis is given to the somatic release of serotonin by the classical leech Retzius neuron, which has allowed detailed studies on the fine steps from excitation to exocytosis. Trains of action potentials induce transmembrane calcium entry through L-type channels. For action potential frequencies above 5 Hz, summation of calcium transients on individual action potentials activates the second calcium source: ryanodine receptors produce calcium-induced calcium release. The resulting calcium tsunami activates mitochondrial ATP synthesis to fuel transport of vesicles to the plasma membrane. Serotonin that is released maintains a large-scale exocytosis by activating the third calcium source: serotonin autoreceptors coupled to phospholipase C promote IP3 production. Activated IP3 receptors in peripheral endoplasmic reticulum release calcium that promotes vesicle fusion. The Swiss-clock workings of the machinery for somatic exocytosis has a striking disadvantage. The essential calcium-releasing endoplasmic reticulum near the plasma membrane hinders the vesicle transport, drastically reducing the thermodynamic efficiency of the ATP expenses and elevating the energy cost of release.

Intrinsic cardiac adrenergic cells contribute to LPS-induced myocardial dysfunction

Intrinsic cardiac adrenergic (ICA) cells regulate both developing and adult cardiac physiological and pathological processes. However, the role of ICA cells in septic cardiomyopathy is unknown. Here we show that norepinephrine (NE) secretion from ICA cells is increased through activation of Toll-like receptor 4 (TLR4) to aggravate myocardial TNF-α production and dysfunction by lipopolysaccharide (LPS). In ICA cells, LPS activated TLR4-MyD88/TRIF-AP-1 signaling that promoted NE biosynthesis through expression of tyrosine hydroxylase, but did not trigger TNF-α production due to impairment of p65 translocation. In a co-culture consisting of LPS-treated ICA cells and cardiomyocytes, the upregulation and secretion of NE from ICA cells activated cardiomyocyte β₁-adrenergic receptor driving Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) to crosstalk with NF-κB and mitogen-activated protein kinase pathways. Importantly, blockade of ICA cell-derived NE prevented LPS-induced myocardial dysfunction. Our findings suggest that ICA cells may be a potential therapeutic target for septic cardiomyopathy.

Extrasynaptic Communication

Streams of action potentials or long depolarizations evoke a massive exocytosis of transmitters and peptides from the surface of dendrites, axons and cell bodies of different neuron types. Such mode of exocytosis is known as extrasynaptic for occurring without utilization of synaptic structures. Most transmitters and all peptides can be released extrasynaptically. Neurons may discharge their contents with relative independence from the axon, soma and dendrites. Extrasynaptic exocytosis takes fractions of a second in varicosities or minutes in the soma or dendrites, but its effects last from seconds to hours. Unlike synaptic exocytosis, which is well localized, extrasynaptic exocytosis is diffuse and affects neuronal circuits, glia and blood vessels. Molecules that are liberated may reach extrasynaptic receptors microns away. The coupling between excitation and exocytosis follows a multistep mechanism, different from that at synapses, but similar to that for the release of hormones. The steps from excitation to exocytosis have been studied step by step for the vital transmitter serotonin in leech Retzius neurons. The events leading to serotonin exocytosis occur similarly for the release of other transmitters and peptides in central and peripheral neurons. Extrasynaptic exocytosis occurs commonly onto glial cells, which react by releasing the same or other transmitters. In the last section, we discuss how illumination of the retina evokes extrasynaptic release of dopamine and ATP. Dopamine contributes to light-adaptation; ATP activates glia, which mediates an increase in blood flow and oxygenation. A proper understanding of the workings of the nervous system requires the understanding of extrasynaptic communication.

Neuropathology of the Brainstem to Mechanistically Understand and to Treat Alzheimer's Disease

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder as yet without effective therapy. Symptoms of this disorder typically reflect cortical malfunction with local neurohistopathology, which biased investigators to search for focal triggers and molecular mechanisms. Cortex, however, receives massive afferents from caudal brain structures, which do not only convey specific information but powerfully tune ensemble activity. Moreover, there is evidence that the start of AD is subcortical. The brainstem harbors monoamine systems, which establish a dense innervation in both allo- and neocortex. Monoaminergic synapses can co-release neuropeptides either by precisely terminating on cortical neurons or, when being “en passant”, can instigate local volume transmission. Especially due to its early damage, malfunction of the ascending monoaminergic system emerges as an early sign and possible trigger of AD. This review summarizes the involvement and cascaded impairment of brainstem monoaminergic neurons in AD and discusses cellular mechanisms that lead to their dysfunction. We highlight the significance and therapeutic challenges of transmitter co-release in ascending activating system, describe the role and changes of local connections and distant afferents of brainstem nuclei in AD, and summon the rapidly increasing diagnostic window during the last few years.

The Locus Coeruleus- Norepinephrine System in Stress and Arousal: Unraveling Historical, Current, and Future Perspectives

Arousal may be understood on a spectrum, with excessive sleepiness, cognitive dysfunction, and inattention on one side, a wakeful state in the middle, and hypervigilance, panic, and psychosis on the other side. However, historically, the concepts of arousal and stress have been challenging to define as measurable experimental variables. Divergent efforts to study these subjects have given rise to several disciplines, including neurobiology, neuroendocrinology, and cognitive neuroscience. We discuss technological advancements that chronologically led to our current understanding of the arousal system, focusing on the multifaceted nucleus locus coeruleus. We share our contemporary perspective and the hypotheses of others in the context of our current technological capabilities and future developments that will be required to move forward in this area of research.

Acoustic startle stimuli inhibit pain but do not alter nociceptive flexion reflexes to sural nerve stimulation

Acoustic startle stimuli inhibit pain, but whether this is due to a cross-modal inhibitory process or some other mechanism is uncertain. To investigate this, electrical stimulation of the sural nerve either preceded or followed an acoustic startle stimulus (by 200 ms) or was presented alone in 30 healthy participants. Five electrical stimuli, five acoustic startle stimuli, 10 startle + electrical stimuli, and 10 electrical + startle stimuli were presented in mixed order at intervals of 30-60 s. Effects of the startle stimulus on pain ratings, pupillary dilatation and nociceptive flexion reflexes to the electric shock were assessed. The acoustic startle stimulus inhibited electrically evoked pain to the ensuing electric shock (p < .001), and the electrical stimulus inhibited the perceived loudness of a subsequent acoustic startle stimulus (p < .05). However, the startle stimulus did not affect electrically evoked pain when presented 200 ms after the electric shock, and electrically evoked pain did not influence the perceived loudness of a prior startle stimulus. Furthermore, stimulus order did not influence the pupillary responses or nociceptive flexion reflexes. These findings suggest that acoustic startle stimuli transiently inhibit nociceptive processing and, conversely, that electrical stimuli inhibit subsequent auditory processing. These inhibitory effects do not seem to involve spinal gating as nociceptive flexion reflexes to the electric shock were unaffected by stimulus order. Thus, cross-modal interactions at convergence points in the brainstem or higher centers may inhibit responses to the second stimulus in a two-stimulus train.

Impaired Phasic Discharge of Locus Coeruleus Neurons Based on Persistent High Tonic Discharge-A New Hypothesis With Potential Implications for Neurodegenerative Diseases

The locus coeruleus (LC) is a small brainstem nucleus with widely distributed noradrenergic projections to the whole brain, and loss of LC neurons is a prominent feature of age-related neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD). This article discusses the hypothesis that in early stages of neurodegenerative diseases, the discharge mode of LC neurons could be changed to a persistent high tonic discharge, which in turn might impair phasic discharge. Since phasic discharge of LC neurons is required for the release of high amounts of norepinephrine (NE) in the brain to promote anti-inflammatory and neuroprotective effects, persistent high tonic discharge of LC neurons could be a key factor in the progression of neurodegenerative diseases. Transcutaneous vagal stimulation (t-VNS), a non-invasive technique that potentially increases phasic discharge of LC neurons, could therefore provide a non-pharmacological treatment approach in specific disease stages. This article focuses on LC vulnerability in neurodegenerative diseases, discusses the hypothesis that a persistent high tonic discharge of LC neurons might affect neurodegenerative processes, and finally reflects on t-VNS as a potentially useful clinical tool in specific stages of AD and PD.

Locus Coeruleus Optogenetic Light Activation Induces Long-Term Potentiation of Perforant Path Population Spike Amplitude in Rat Dentate Gyrus

Norepinephrine (NE) in dentate gyrus (DG) produces NE-dependent long-term potentiation (NE-LTP) of the perforant path-evoked potential population spike both in vitro and in vivo. Chemical activators infused near locus coeruleus (LC), the source of DG NE, produce a NE-LTP that is associative, i.e., requires concurrent pairing with perforant path (PP) input. Here, we ask if LC optogenetic stimulation that allows us to activate only LC neurons can induce NE-LTP in DG. We use an adeno-associated viral vector containing a depolarizing channel (AAV8-Ef1a-DIO-eChR2(h134r)-EYFP-WPRE) infused stereotaxically into the LC of TH:Cre rats to produce light-sensitive LC neurons. A co-localization of ~62% in LC neurons was observed for these channels. Under urethane anesthesia, we demonstrated that 5-10 s 10 Hz trains of 30 ms light pulses in LC reliably activated neurons near an LC optoprobe. Ten minutes of the same train paired with 0.1 Hz PP electrical stimulation produced a delayed NE-LTP of population spike amplitude, but not EPSP slope. A leftward shift in the population spike input/output curve at the end of the experiment was also consistent with long-term population spike potentiation. LC neuron activity during the 10 min light train was unexpectedly transient. Increased LC neuronal firing was seen only for the first 2 min of the light train. NE-LTP was more delayed and less robust than reported with LC chemo-activation. Previous estimates of LC axonal conduction times suggest acute release of NE occurs 40-70 ms after an LC neuron action potential. We used single LC light pulses to examine acute effects of NE release and found potentiated population spike amplitude when a light pulse in LC occurred 40-50 ms, but not 20-30 ms, prior to a PP pulse, consistent with conduction estimates. These effects of LC optogenetic activation reinforce evidence for a continuum of NE potentiation effects in DG. The single pulse effects mirror an earlier report using LC electrical stimulation. These acute effects support an attentional role of LC activation. The LTP of PP responses induced by optogenetic LC activation is consistent with the role of LC in long-term learning and memory.

Functional Organization of the Sympathetic Pathways Controlling the Pupil: Light-Inhibited and Light-Stimulated Pathways

Pupil dilation is mediated by a sympathetic output acting in opposition to parasympathetically mediated pupil constriction. While light stimulates the parasympathetic output, giving rise to the light reflex, it can both inhibit and stimulate the sympathetic output. Light-inhibited sympathetic pathways originate in retina-receptive neurones of the pretectum and the suprachiasmatic nucleus (SCN): by attenuating sympathetic activity, they allow unimpeded operation of the light reflex. Light stimulates the noradrenergic and serotonergic pathways. The hub of the noradrenergic pathway is the locus coeruleus (LC) containing both excitatory sympathetic premotor neurones (SympPN) projecting to preganglionic neurones in the spinal cord, and inhibitory parasympathetic premotor neurones (ParaPN) projecting to preganglionic neurones in the Edinger-Westphal nucleus (EWN). SympPN receive inputs from the SCN via the dorsomedial hypothalamus, orexinergic neurones of the latero-posterior hypothalamus, wake- and sleep-promoting neurones of the hypothalamus and brain stem, nociceptive collaterals of the spinothalamic tract, whereas ParaPN receive inputs from the amygdala, sleep/arousal network, nociceptive spinothalamic collaterals. The activity of LC neurones is regulated by inhibitory α₂-adrenoceptors. There is a species difference in the function of the preautonomic LC. In diurnal animals, the α₂-adrenoceptor agonist clonidine stimulates mainly autoreceptors on SymPN, causing miosis, whereas in nocturnal animals it stimulates postsynaptic α₂-arenoceptors in the EWN, causing mydriasis. Noxious stimulation activates SympPN in diurnal animals and ParaPN in nocturnal animals, leading to pupil dilation via sympathoexcitation and parasympathetic inhibition, respectively. These differences may be attributed to increased activity of excitatory LC neurones due to stimulation by light in diurnal animals. This may also underlie the wake-promoting effect of light in diurnal animals, in contrast to its sleep-promoting effect in nocturnal species. The hub of the serotonergic pathway is the dorsal raphe nucleus that is light-sensitive, both directly and indirectly (via an orexinergic input). The light-stimulated pathways mediate a latent mydriatic effect of light on the pupil that can be unmasked by drugs that either inhibit or stimulate SympPN in these pathways. The noradrenergic pathway has widespread connections to neural networks controlling a variety of functions, such as sleep/arousal, pain, and fear/anxiety. Many physiological and psychological variables modulate pupil function via this pathway.

Amyloid beta peptides, locus coeruleus-norepinephrine system and dense core vesicles

The evolution of peptidergic signaling systems in the central nervous system serves a distinct and crucial role in brain processes and function. The diversity of physiological peptides and the complexity of their regulation and secretion from the dense core vesicles (DCV) throughout the brain is a topic greatly in need of investigation, though recent years have shed light on cellular and molecular mechanisms that are summarized in this review. Here, we focus on the convergence of peptidergic systems onto the Locus Coeruleus (LC), the sole provider of norepinephrine (NE) to the cortex and hippocampus, via large DCV. As the LC-NE system is one of the first regions of the brain to undergo degeneration in Alzheimer's Disease (AD), and markers of DCV have consistently been demonstrated to have biomarker potential for AD progression, here we summarize the current literature linking the LC-NE system with DCV dysregulation and Aβ peptides. We also include neuroanatomical data suggesting that the building blocks of senile plaques, Aβ monomers, may be localized to DCV of the LC and noradrenergic axon terminals of the prefrontal cortex. Finally, we explore the putative consequences of chronic stress on Aβ production and the role that DCV may play in LC degeneration. Clinical data of immunological markers of DCV in AD patients are discussed.

Morphological correlates of sex differences in acoustic startle response and prepulse inhibition through projections from locus coeruleus to cochlear root neurons

The noradrenergic locus coeruleus (LC) plays an important role in the promotion and maintenance of arousal and alertness. Our group recently described coerulean projections to cochlear root neurons (CRNs), the first relay of the primary acoustic startle reflex (ASR) circuit. However, the role of the LC in the ASR and its modulation, prepulse inhibition (PPI), is not clear. In this study, we damaged LC neurons and fibers using a highly selective neurotoxin, DSP-4, and then assessed ASR and PPI in male and female rats. Our results showed that ASR amplitude was higher in males at 14 days after DSP-4 injection when compared to pre-administration values and those in the male control group. Such modifications in ASR amplitude did not occur in DSP-4-injected females, which exhibited ASR amplitude within the range of control values. PPI differences between males and females seen in controls were not observed in DSP-4-injected rats for any interstimulus interval tested. DSP-4 injection did not affect ASR and PPI latencies in either the male or the female groups, showing values that were consistent with the sex-related variability observed in control rats. Furthermore, we studied the noradrenergic receptor system in the cochlear nerve root using gene expression analysis. When compared to controls, DSP-4-injected males showed higher levels of expression in all adrenoceptor subtypes; however, DSP-4-injected females showed varied effects depending on the receptor type, with either up-, downregulations, or maintenance of expression levels. Lastly, we determined noradrenaline levels in CRNs and other LC-targeted areas using HPLC assays, and these results correlated with behavioral and adrenoceptor expression changes post DSP-4 injection. Our study supports the participation of LC in ASR and PPI, and contributes toward a better understanding of sex-related differences observed in somatosensory gating paradigms.

Regulation of neurological and neuropsychiatric phenotypes by locus coeruleus-derived galanin

Decades of research confirm that noradrenergic locus coeruleus (LC) neurons are essential for arousal, attention, motivation, and stress responses. While most studies on LC transmission focused unsurprisingly on norepinephrine (NE), adrenergic signaling cannot account for all the consequences of LC activation. Galanin coexists with NE in the vast majority of LC neurons, yet the precise function of this neuropeptide has proved to be surprisingly elusive given our solid understanding of the LC system. To elucidate the contribution of galanin to LC physiology, here we briefly summarize the nature of stimuli that drive LC activity from a neuroanatomical perspective. We go on to describe the LC pathways in which galanin most likely exerts its effects on behavior, with a focus on addiction, depression, epilepsy, stress, and Alzheimer׳s disease. We propose a model in which LC-derived galanin has two distinct functions: as a neuromodulator, primarily acting via the galanin 1 receptor (GAL1), and as a trophic factor, primarily acting via galanin receptor 2 (GAL2). Finally, we discuss how the recent advances in neuropeptide detection, optogenetics and chemical genetics, and galanin receptor pharmacology can be harnessed to identify the roles of LC-derived galanin definitively. This article is part of a Special Issue entitled SI: Noradrenergic System.

Exploring the role of neuropeptide S in the regulation of arousal: a functional anatomical study

Neuropeptide S (NPS) is a regulatory peptide expressed by limited number of neurons in the brainstem. The simultaneous anxiolytic and arousal-promoting effect of NPS suggests an involvement in mood control and vigilance, making the NPS-NPS receptor system an interesting potential drug target. Here we examined, in detail, the distribution of NPS-immunoreactive (IR) fiber arborizations in brain regions of rat known to be involved in the regulation of sleep and arousal. Such nerve terminals were frequently apposed to GABAergic/galaninergic neurons in the ventro-lateral preoptic area (VLPO) and to tyrosine hydroxylase-IR neurons in all hypothalamic/thalamic dopamine cell groups. Then we applied the single platform-on-water (mainly REM) sleep deprivation method to study the functional role of NPS in the regulation of arousal. Of the three pontine NPS cell clusters, the NPS transcript levels were increased only in the peri-coerulear group in sleep-deprived animals, but not in stress controls. The density of NPS-IR fibers was significantly decreased in the median preoptic nucleus-VLPO region after the sleep deprivation, while radioimmunoassay and mass spectrometry measurements showed a parallel increase of NPS in the anterior hypothalamus. The expression of the NPS receptor was, however, not altered in the VLPO-region. The present results suggest a selective activation of one of the three NPS-expressing neuron clusters as well as release of NPS in distinct forebrain regions after sleep deprivation. Taken together, our results emphasize a role of the peri-coerulear cluster in the modulation of arousal, and the importance of preoptic area for the action of NPS on arousal and sleep.

Brain Glycogen Decreases During Intense Exercise Without Hypoglycemia: The Possible Involvement of Serotonin

Brain glycogen stored in astrocytes, a source of lactate as a neuronal energy source, decreases during prolonged exercise with hypoglycemia. However, brain glycogen dynamics during exercise without hypoglycemia remain unknown. Since intense exercise increases brain noradrenaline and serotonin as known inducers for brain glycogenolysis, we hypothesized that brain glycogen decreases with intense exercise not accompanied by hypoglycemia. To test this hypothesis, we employed a well-established acute intense exercise model of swimming in rats. Rats swam for fourteen 20 s bouts with a weight equal to 8 % of their body mass and were sacrificed using high-power (10 kW) microwave irradiation to inactivate brain enzymes for accurate detection of brain glycogen and monoamines. Intense exercise did not alter blood glucose, but did increase blood lactate levels. Immediately after exercise, brain glycogen decreased and brain lactate increased in the hippocampus, cerebellum, cortex, and brainstem. Simultaneously, serotonin turnover in the hippocampus and brainstem mutually increased and were associated with decreased brain glycogen. Intense swimming exercise that does not induce hypoglycemia decreases brain glycogen associated with increased brain lactate, implying an importance of glycogen in brain energetics during intense exercise even without hypoglycemia. Activated serotonergic regulation is a possible underlying mechanism for intense exercise-induced glycogenolysis at least in the hippocampus and brainstem.

GHB for cataplexy: Possible mode of action

The sleep disorder narcolepsy is caused by the loss of orexinergic neurones in the lateral hypothalamus. A troublesome symptom of narcolepsy is cataplexy, the sudden loss of muscle tone in response to strong emotions. It can be alleviated by antidepressants and sodium oxybate (γ-hydroxybutyric acid (GHB)). It is likely that the noradrenergic nucleus locus coeruleus (LC) is involved since it is essential for the maintenance of muscle tone, and ceases to fire during cataplectic attacks. Furthermore, alpha-2 adrenoceptors proliferate in the LC in cataplexy, probably due to 'heterologous denervation supersensitivity' resulting from the loss/weakening of the orexinergic input to the LC. This would lead to the sensitization of the autoinhibition mechanism of LC neurones mediated by inhibitory alpha-2 adrenoceptors ('autoreceptors'). Thus the excitatory input from the amygdala to the LC, activated by an emotional stimulus, would lead to the 'switching off' of LC activity via the supersensitive auto-inhibition mechanism. GHB is an agonist at both γ-aminobutyric acid (GABA) GABA (B) and GHB receptors that may be a subtype of an extrasynaptic GABA(A) receptor. GHB may prevent a cataplectic attack by dampening the tone of LC neurones via the stimulation of inhibitory extrasynaptic GABA receptors in the LC, and thus increasing the threshold for autoinhibition.

Monosynaptic glutamatergic activation of locus coeruleus and other lower brainstem noradrenergic neurons by the C1 cells in mice

The C1 neurons, located in the rostral ventrolateral medulla (VLM), are activated by pain, hypotension, hypoglycemia, hypoxia, and infection, as well as by psychological stress. Prior work has highlighted the ability of these neurons to increase sympathetic tone, hence peripheral catecholamine release, probably via their direct excitatory projections to sympathetic preganglionic neurons. In this study, we use channelrhodopsin-2 (ChR2) optogenetics to test whether the C1 cells are also capable of broadly activating the brain's noradrenergic system. We selectively expressed ChR2(H134R) in rostral VLM catecholaminergic neurons by injecting Cre-dependent adeno-associated viral vectors into the brain of adult dopamine-β-hydroxylase (DβH)(Cre/0) mice. Most ChR2-expressing VLM neurons (75%) were immunoreactive for phenylethanolamine N-methyl transferease, thus were C1 cells, and most of the ChR2-positive axonal varicosities were immunoreactive for vesicular glutamate transporter-2 (78%). We produced light microscopic evidence that the axons of rostral VLM (RVLM) catecholaminergic neurons contact locus coeruleus, A1, and A2 noradrenergic neurons, and ultrastructural evidence that these contacts represent asymmetric synapses. Using optogenetics in tissue slices, we show that RVLM catecholaminergic neurons activate the locus coeruleus as well as A1 and A2 noradrenergic neurons monosynaptically by releasing glutamate. In conclusion, activation of RVLM catecholaminergic neurons, predominantly C1 cells, by somatic or psychological stresses has the potential to increase the firing of both peripheral and central noradrenergic neurons.

Chromogranin A and vesicular monoamine transporter 2 immunolocalization in protein bodies of human locus coeruleus neurons

Our previous histochemical and ultrastructural studies have identified, in human catecholamine neurons, abundant spherical acidophilic protein bodies (pb), which originate from regular mitochondria, retaining their double membrane. In locus coeruleus (LC) neurons, pb have somatodendritic distribution and are unequivocal storage vesicles for noradrenaline, as demonstrated by immunolocalization of Dopamine-β-Hydroxylase. In the present study, in order to reinforce the identity of pb as monoamine storage sites in human LC, and to assess their potential of somatodendritic release, we studied the subcellular immunolocalization of chromogranin A (CgA) and vesicular monoamine transporter 2 (VMAT2), given the fact that their localization defines the vesicles capacity of filling with monoamine and hence exocytotic release. The data provided in the present study, demonstrate the novel ultrastructural immunolocalization of both CgA and VMAT2 in protein bodies, supporting their involvement in somatodendritic storage and release of noradrenaline in human LC. Since the molecular mechanism of LC somatodendritic exocytosis remains largely elusive, the present study may shed light to a better understanding of this mechanism.

Functional neuroanatomy of the central noradrenergic system

The central noradrenergic neurone, like the peripheral sympathetic neurone, is characterized by a diffusely arborizing terminal axonal network. The central neurones aggregate in distinct brainstem nuclei, of which the locus coeruleus (LC) is the most prominent. LC neurones project widely to most areas of the neuraxis, where they mediate dual effects: neuronal excitation by α₁-adrenoceptors and inhibition by α₂-adrenoceptors. The LC plays an important role in physiological regulatory networks. In the sleep/arousal network the LC promotes wakefulness, via excitatory projections to the cerebral cortex and other wakefulness-promoting nuclei, and inhibitory projections to sleep-promoting nuclei. The LC, together with other pontine noradrenergic nuclei, modulates autonomic functions by excitatory projections to preganglionic sympathetic, and inhibitory projections to preganglionic parasympathetic neurones. The LC also modulates the acute effects of light on physiological functions ('photomodulation'): stimulation of arousal and sympathetic activity by light via the LC opposes the inhibitory effects of light mediated by the ventrolateral preoptic nucleus on arousal and by the paraventricular nucleus on sympathetic activity. Photostimulation of arousal by light via the LC may enable diurnal animals to function during daytime. LC neurones degenerate early and progressively in Parkinson's disease and Alzheimer's disease, leading to cognitive impairment, depression and sleep disturbance.

Differences in the H2S-induced quantal release of catecholamine in adrenal chromaffin cells of neonatal and adult rats

Both catecholamine (CA) released from adrenal chromaffin cells and hydrogen sulfide (H2S) have been shown to play critical roles in the regulation of hypoxic stress response. Our previous study has demonstrated that exogenous H2S directly induced quantal CA released from adult rat adrenal chromaffin cells (ARACCs) by inhibiting Ca(2+)-activated K(+) current [IK(Ca) current]. However, it is not clear now whether H2S can also directly induce quantal CA released from neonatal rat adrenal chromaffin cells (NRACCs). In the present study, we investigated whether exogenous H2S can stimulate quantal CA released from NRACCs, and whether there were differences in the kinetics of H2S-induced quantal CA released between ARACCs and NRACCs. Using carbon-fiber amperometry and whole-cell patch clamping techniques, our experimental results showed: (1) H2S can directly induce quantal CA released from NRACCs; (2) H2S induced the depolarization of membrane potential and inhibited IK(Ca) current; (3) compared with ARACCs, much smaller quantal size and faster quantal release were showed in NRACCs through the kinetic analysis of the single-vesicle secretion induced by H2S. Our results may not only help to further understand the H2S-induced CA released from adrenal chromaffin cells in the aspect of development, but also provide the insights for the clinical prevention and therapy for hypoxic stress-induced injury in neonates at birth.

Modulation of physiological reflexes by pain: role of the locus coeruleus

The locus coeruleus (LC) is activated by noxious stimuli, and this activation leads to inhibition of perceived pain. As two physiological reflexes, the acoustic startle reflex and the pupillary light reflex, are sensitive to noxious stimuli, this review considers evidence that this sensitivity, at least to some extent, is mediated by the LC. The acoustic startle reflex, contraction of a large body of skeletal muscles in response to a sudden loud acoustic stimulus, can be enhanced by both directly ("sensitization") and indirectly ("fear conditioning") applied noxious stimuli. Fear-conditioning involves the association of a noxious (unconditioned) stimulus with a neutral (conditioned) stimulus (e.g., light), leading to the ability of the conditioned stimulus to evoke the "pain response". The enhancement of the startle response by conditioned fear ("fear-potentiated startle") involves the activation of the amygdala. The LC may also be involved in both sensitization and fear potentiation: pain signals activate the LC both directly and indirectly via the amygdala, which results in enhanced motoneurone activity, leading to an enhanced muscular response. Pupil diameter is under dual sympathetic/parasympathetic control, the sympathetic (noradrenergic) output dilating, and the parasympathetic (cholinergic) output constricting the pupil. The light reflex (constriction of the pupil in response to a light stimulus) operates via the parasympathetic output. The LC exerts a dual influence on pupillary control: it contributes to the sympathetic outflow and attenuates the parasympathetic output by inhibiting the Edinger-Westphal nucleus, the preganglionic cholinergic nucleus in the light reflex pathway. Noxious stimulation results in pupil dilation ("reflex dilation"), without any change in the light reflex response, consistent with sympathetic activation via the LC. Conditioned fear, on the other hand, results in the attenuation of the light reflex response ("fear-inhibited light reflex"), consistent with the inhibition of the parasympathetic light reflex via the LC. It is suggested that directly applied pain and fear-conditioning may affect different populations of autonomic neurones in the LC, directly applied pain activating sympathetic and fear-conditioning parasympathetic premotor neurones.

Extrasynaptic exocytosis and its mechanisms: a source of molecules mediating volume transmission in the nervous system

We review the evidence of exocytosis from extrasynaptic sites in the soma, dendrites, and axonal varicosities of central and peripheral neurons of vertebrates and invertebrates, with emphasis on somatic exocytosis, and how it contributes to signaling in the nervous system. The finding of secretory vesicles in extrasynaptic sites of neurons, the presence of signaling molecules (namely transmitters or peptides) in the extracellular space outside synaptic clefts, and the mismatch between exocytosis sites and the location of receptors for these molecules in neurons and glial cells, have long suggested that in addition to synaptic communication, transmitters are released, and act extrasynaptically. The catalog of these molecules includes low molecular weight transmitters such as monoamines, acetylcholine, glutamate, gama-aminobutiric acid (GABA), adenosine-5-triphosphate (ATP), and a list of peptides including substance P, brain-derived neurotrophic factor (BDNF), and oxytocin. By comparing the mechanisms of extrasynaptic exocytosis of different signaling molecules by various neuron types we show that it is a widespread mechanism for communication in the nervous system that uses certain common mechanisms, which are different from those of synaptic exocytosis but similar to those of exocytosis from excitable endocrine cells. Somatic exocytosis has been measured directly in different neuron types. It starts after high-frequency electrical activity or long experimental depolarizations and may continue for several minutes after the end of stimulation. Activation of L-type calcium channels, calcium release from intracellular stores and vesicle transport towards the plasma membrane couple excitation and exocytosis from small clear or large dense core vesicles in release sites lacking postsynaptic counterparts. The presence of synaptic and extrasynaptic exocytosis endows individual neurons with a wide variety of time- and space-dependent communication possibilities. Extrasynaptic exocytosis may be the major source of signaling molecules producing volume transmission and by doing so may be part of a long duration signaling mode in the nervous system.