The wake-promoting hypocretin-orexin neurons are in an intrinsic state of membrane depolarization

A circuit from lateral hypothalamic to dorsal hippocampal dentate gyrus modulates behavioral despair in mice

Behavioral despair is one of the clinical manifestations of major depressive disorder and an important cause of disability and death. However, the neural circuit mechanisms underlying behavioral despair are poorly understood. In a well-established chronic behavioral despair (CBD) mouse model, using a combination of viral tracing, in vivo fiber photometry, chemogenetic and optogenetic manipulations, in vitro electrophysiology, pharmacological profiling techniques, and behavioral tests, we investigated the neural circuit mechanisms in regulating behavioral despair. Here, we found that CBD enhanced CaMKIIα neuronal excitability in the dorsal dentate gyrus (dDG) and dDGCaMKIIα neurons involved in regulating behavioral despair in CBD mice. Besides, dDGCaMKIIα neurons received 5-HT inputs from median raphe nucleus (MRN) and were mediated by 5-HT1A receptors, whereas MRN5-HT neurons received CaMKIIα inputs from lateral hypothalamic (LH) and were mediated by AMPA receptors to regulate behavioral despair. Furthermore, fluvoxamine exerted its role in resisting behavioral despair through the LH-MRN-dDG circuit. These findings suggest that a previously unidentified circuit of LHCaMKIIα-MRN5-HT-dDGCaMKIIα mediates behavioral despair induced by CBD. Furthermore, these support the important role of AMPA receptors in MRN and 5-HT1A receptors in dDG that might be the potential targets for treatment of behavioral despair, and explain the neural circuit mechanism of fluvoxamine-resistant behavioral despair.

Neural flip-flops I: Short-term memory

The networks proposed here show how neurons can be connected to form flip-flops, the basic building blocks in sequential logic systems. The novel neural flip-flops (NFFs) are explicit, dynamic, and can generate known phenomena of short-term memory. For each network design, all neurons, connections, and types of synapses are shown explicitly. The neurons' operation depends only on explicitly stated, minimal properties of excitement and inhibition. This operation is dynamic in the sense that the level of neuron activity is the only cellular change, making the NFFs' operation consistent with the speed of most brain functions. Memory tests have shown that certain neurons fire continuously at a high frequency while information is held in short-term memory. These neurons exhibit seven characteristics associated with memory formation, retention, retrieval, termination, and errors. One of the neurons in each of the NFFs produces all of the characteristics. This neuron and a second neighboring neuron together predict eight unknown phenomena. These predictions can be tested by the same methods that led to the discovery of the first seven phenomena. NFFs, together with a decoder from a previous paper, suggest a resolution to the longstanding controversy of whether short-term memory depends on neurons firing persistently or in brief, coordinated bursts. Two novel NFFs are composed of two and four neurons. Their designs follow directly from a standard electronic flip-flop design by moving each negation symbol from one end of the connection to the other. This does not affect the logic of the network, but it changes the logic of each component to a logic function that can be implemented by a single neuron. This transformation is reversible and is apparently new to engineering as well as neuroscience.

Identification of a novel perifornical-hypothalamic-area-projecting serotonergic system that inhibits innate panic and conditioned fear responses

The serotonin (5-HT) system is heavily implicated in the regulation of anxiety and trauma-related disorders such as panic disorder and post-traumatic stress disorder, respectively. However, the neural mechanisms of how serotonergic neurotransmission regulates innate panic and fear brain networks are poorly understood. Our earlier studies have identified that orexin (OX)/glutamate neurons within the perifornical hypothalamic area (PFA) play a critical role in adaptive and pathological panic and fear. While site-specific and electrophysiological studies have shown that intracranial injection and bath application of 5-HT inhibits PFA neurons via 5-HT1a receptors, they largely ignore circuit-specific neurotransmission and its physiological properties that occur in vivo. Here, we investigate the role of raphe nuclei 5-HT inputs into the PFA in panic and fear behaviors. We initially confirmed that photostimulation of glutamatergic neurons in the PFA of rats produces robust cardioexcitation and flight/aversive behaviors resembling panic-like responses. Using the retrograde tracer cholera toxin B, we determined that the PFA receives discrete innervation of serotonergic neurons clustered in the lateral wings of the dorsal (lwDRN) and in the median (MRN) raphe nuclei. Selective lesions of these serotonergic projections with saporin toxin resulted in similar panic-like responses during the suffocation-related CO2 challenge and increased freezing to fear-conditioning paradigm. Conversely, selective stimulation of serotonergic fibers in the PFA attenuated both flight/escape behaviors and cardioexcitation responses elicited by the CO2 challenge and induced conditioned place preference. The data here support the hypothesis that PFA projecting 5-HT neurons in the lwDRN/MRN represents a panic/fear-off circuit and may also play a role in reward behavior.

Brain Circuits Underlying Narcolepsy

Narcolepsy is a sleep disorder manifesting symptoms such as excessive daytime sleepiness and often cataplexy, a sudden and involuntary loss of muscle activity during wakefulness. The underlying neuropathological basis of narcolepsy is the loss of orexin neurons from the lateral hypothalamus. To date numerous animal models of narcolepsy have been produced in the laboratory, being invaluable tools for delineating the brain circuits of narcolepsy. This review will examine the evidence regarding the function of the orexin system, and how loss of this wake-promoting system manifests in excessive daytime sleepiness. This review will also outline the brain circuits controlling cataplexy, focusing on the contribution of orexin signaling loss in narcolepsy. Although our understanding of the brain circuits of narcolepsy has made great progress in recent years, much remains to be understood.

Mild Traumatic Brain Injury Affects Orexin/Hypocretin Physiology Differently in Male and Female Mice

Traumatic brain injury (TBI) is known to affect the physiology of neural circuits in several brain regions, which can contribute to behavioral changes after injury. Disordered sleep is a behavior that is often seen after TBI, but there is little research into how injury affects the circuitry that contributes to disrupted sleep regulation. Orexin/hypocretin neurons (hereafter referred to as orexin neurons) located in the lateral hypothalamus normally stabilize wakefulness in healthy animals and have been suggested as a source of dysregulated sleep behavior. Despite this, few studies have examined how TBI affects orexin neuron circuitry. Further, almost no animal studies of orexin neurons after TBI have included female animals. Here, we address these gaps by studying changes to orexin physiology using ex vivo acute brain slices and whole-cell patch clamp recording. We hypothesized that orexin neurons would have reduced afferent excitatory activity after injury. Ultimately, this hypothesis was supported but there were additional physiological changes that occurred that we did not originally hypothesize. We studied physiological properties in orexin neurons approximately 1 week after mild traumatic brain injury (mTBI) in 6-8-week-old male and female mice. mTBI was performed with a lateral fluid percussion injury between 1.4 and 1.6 atmospheres. Mild TBI increased the size of action potential afterhyperpolarization in orexin neurons from female mice, but not male mice and reduced the action potential threshold in male mice, but not in female mice. Mild TBI reduced afferent excitatory activity and increased afferent inhibitory activity onto orexin neurons. Alterations in afferent excitatory activity occurred in different parameters in male and female animals. The increased afferent inhibitory activity after injury is more pronounced in recordings from female animals. Our results indicate that mTBI changes the physiology of orexin neuron circuitry and that these changes are not the same in male and female animals.

Eat, seek, rest? An orexin/hypocretin perspective

Seeking and ingesting nutrients is an essential cycle of life in all species. In classical neuropsychology these two behaviours are viewed as fundamentally distinct from each other, and known as appetitive and consummatory, respectively. Appetitive behaviour is highly flexible and diverse, but typically involves increased locomotion and spatial exploration. Consummatory behaviour, in contrast, typically requires reduced locomotion. Another long-standing concept is "rest and digest", a hypolocomotive response to calorie intake, thought to facilitate digestion and storage of energy after eating. Here, we note that the classical seek➔ingest➔rest behavioural sequence is not evolutionarily advantageous for all ingested nutrients. Our limited stomach capacity should be invested wisely, rather than spent on the first available nutrient. This is because nutrients are not simply calories: some nutrients are more essential for survival than others. Thus, a key choice that needs to be made soon after ingestion: to eat more and rest, or to terminate eating and search for better food. We offer a perspective on recent work suggesting how nutrient-specific neural responses shape this choice. Specifically, the hypothalamic hypocretin/orexin neurons (HONs) - cells that promote hyperlocomotive explorative behaviours - are rapidly and differentially modulated by different ingested macronutrients. Dietary non-essential (but not essential) amino acids activate HONs, while glucose depresses HONs. This nutrient-specific HON modulation engages distinct reflex arcs, seek➔ingest➔seek and seek➔ingest➔rest, respectively. We propose that these nutri-neural reflexes evolved to facilitate optimal nutrition despite the limitations of our body.

A cluster of mesopontine GABAergic neurons suppresses REM sleep and curbs cataplexy

Physiological rapid eye movement (REM) sleep termination is vital for initiating non-REM (NREM) sleep or arousal, whereas the suppression of excessive REM sleep is promising in treating narcolepsy. However, the neuronal mechanisms controlling REM sleep termination and keeping sleep continuation remain largely unknown. Here, we reveal a key brainstem region of GABAergic neurons in the control of both physiological REM sleep and cataplexy. Using fiber photometry and optic tetrode recording, we characterized the dorsal part of the deep mesencephalic nucleus (dDpMe) GABAergic neurons as REM relatively inactive and two different firing patterns under spontaneous sleep–wake cycles. Next, we investigated the roles of dDpMe GABAergic neuronal circuits in brain state regulation using optogenetics, RNA interference technology, and celltype-specific lesion. Physiologically, dDpMe GABAergic neurons causally suppressed REM sleep and promoted NREM sleep through the sublaterodorsal nucleus and lateral hypothalamus. In-depth studies of neural circuits revealed that sublaterodorsal nucleus glutamatergic neurons were essential for REM sleep termination by dDpMe GABAergic neurons. In addition, dDpMe GABAergic neurons efficiently suppressed cataplexy in a rodent model. Our results demonstrated that dDpMe GABAergic neurons controlled REM sleep termination along with REM/NREM transitions and represented a novel potential target to treat narcolepsy.

Do orexin/hypocretin neurons signal stress or reward?

Hypothalamic neurons that produce the peptide transmitters orexins/hypocretins (HONs) broadcast their predominantly neuroexcitatory outputs to the entire brain via their extremely wide axonal projections. HONs were originally reported to be activated by food deprivation, and to stimulate arousal, energy expenditure, and eating. This led to extensive studies of HONs in the context of nutrient-sensing and energy balance control. While activation of HONs by body energy depletion continues to be supported by experimental evidence, it has also become clear that HONs are robustly activated not only by nutrient depletion, but also by diverse sensory stimuli (both neutral and those associated with rewarding or aversive events), seemingly unrelated to each other or to energy balance. One theory that could unify these findings is that all these stimuli signal "stress" - defined either as a potentially harmful state, or an awareness of reward deficiency. If HON activity is conceptualized as a cumulative representation of stress, then many of the reported HONs outputs - including EEG arousal, sympathetic activation, place avoidance, and exploratory behaviours - could be viewed as logical stress-counteracting responses. We discuss evidence for and against this unifying theory of HON function, including the alterations in HON activity observed in anxiety and depression disorders. We propose that, in order to orchestrate stress-countering responses, HONs need to coactivate motivation and aversion brain systems, and the impact of HON stimulation on affective states may be perceived as rewarding or aversive depending on the baseline HON activity.

Sodium background currents in endocrine/neuroendocrine cells: Towards unraveling channel identity and contribution in hormone secretion

In endocrine/neuroendocrine tissues, excitability of secretory cells is patterned by the repertoire of ion channels and there is clear evidence that extracellular sodium (Na⁺) ions contribute to hormone secretion. While voltage-gated channels involved in action potential generation are well-described, the background 'leak' channels operating near the resting membrane potential are much less known, and in particular the channels supporting a background entry of Na⁺ ions. These background Na⁺ currents (called here 'I_Nab') have the ability to modulate the resting membrane potential and subsequently affect action potential firing. Here we compile and analyze the data collected from three endocrine/neuroendocrine tissues: the anterior pituitary gland, the adrenal medulla and the endocrine pancreas. We also model how I_Nab can be functionally involved in cellular excitability. Finally, towards deciphering the physiological role of I_Nab in endocrine/neuroendocrine cells, its implication in hormone release is also discussed.

An overview of the orexinergic system in different animal species

Orexin (hypocretin), is a neuropeptide produced by a subset of neurons in the lateral hypothalamus. From the lateral hypothalamus, the orexin-containing neurons project their fibres extensively to other brain structures, and the spinal cord constituting the central orexinergic system. Generally, the term ''orexinergic system'' usually refers to the orexin peptides and their receptors, as well as to the orexin neurons and their projections to different parts of the central nervous system. The extensive networks of orexin axonal fibres and their terminals allow these neuropeptidergic neurons to exert great influence on their target regions. The hypothalamic neurons containing the orexin neuropeptides have been implicated in diverse functions, especially related to the control of a variety of homeostatic functions including feeding behaviour, arousal, wakefulness stability and energy expenditure. The broad range of functions regulated by the orexinergic system has led to its description as ''physiological integrator''. In the last two decades, the orexinergic system has been a topic of great interest to the scientific community with many reports in the public domain. From the documentations, variations exist in the neuroanatomical profile of the orexinergic neuron soma, fibres and their receptors from animal to animal. Hence, this review highlights the distinct variabilities in the morphophysiological aspects of the orexinergic system in the vertebrate animals, mammals and non-mammals, its presence in other brain-related structures, including its involvement in ageing and neurodegenerative diseases. The presence of the neuropeptide in the cerebrospinal fluid and peripheral tissues, as well as its alteration in different animal models and conditions are also reviewed.

Cellular, synaptic, and network effects of chemokines in the central nervous system and their implications to behavior

Accumulating evidence highlights chemokines as key mediators of the bidirectional crosstalk between neurons and glial cells aimed at preserving brain functioning. The multifaceted role of these immune proteins in the CNS is mirrored by the complexity of the mechanisms underlying its biological function, including biased signaling. Neurons, only in concert with glial cells, are essential players in the modulation of brain homeostatic functions. Yet, attempts to dissect these complex multilevel mechanisms underlying coordination are still lacking. Therefore, the purpose of this review is to summarize the current knowledge about mechanisms underlying chemokine regulation of neuron-glia crosstalk linking molecular, cellular, network, and behavioral levels. Following a brief description of molecular mechanisms by which chemokines interact with their receptors and then summarizing cellular patterns of chemokine expression in the CNS, we next delve into the sequence and mechanisms of chemokine-regulated neuron-glia communication in the context of neuroprotection. We then define the interactions with other neurotransmitters, neuromodulators, and gliotransmitters. Finally, we describe their fine-tuning on the network level and the behavioral relevance of their modulation. We believe that a better understanding of the sequence and nature of events that drive neuro-glial communication holds promise for the development of new treatment strategies that could, in a context- and time-dependent manner, modulate the action of specific chemokines to promote brain repair and reduce the neurological impairment.

Orexin/Hypocretin and MCH Neurons: Cognitive and Motor Roles Beyond Arousal

The lateral hypothalamus (LH) is classically implicated in sleep-wake control. It is the main source of orexin/hypocretin and melanin-concentrating hormone (MCH) neuropeptides in the brain, which have been both implicated in arousal state switching. These neuropeptides are produced by non-overlapping LH neurons, which both project widely throughout the brain, where release of orexin and MCH activates specific postsynaptic G-protein-coupled receptors. Optogenetic manipulations of orexin and MCH neurons during sleep indicate that they promote awakening and REM sleep, respectively. However, recordings from orexin and MCH neurons in awake, moving animals suggest that they also act outside sleep/wake switching. Here, we review recent studies showing that both orexin and MCH neurons can rapidly (sub-second-timescale) change their firing when awake animals experience external stimuli, or during self-paced exploration of objects and places. However, the sensory-behavioral correlates of orexin and MCH neural activation can be quite different. Orexin neurons are generally more dynamic, with about 2/3rds of them activated before and during self-initiated running, and most activated by sensory stimulation across sensory modalities. MCH neurons are activated in a more select manner, for example upon self-paced investigation of novel objects and by certain other novel stimuli. We discuss optogenetic and chemogenetic manipulations of orexin and MCH neurons, which combined with pharmacological blockade of orexin and MCH receptors, imply that these rapid LH dynamics shape fundamental cognitive and motor processes due to orexin and MCH neuropeptide actions in the awake brain. Finally, we contemplate whether the awake control of psychomotor brain functions by orexin and MCH are distinct from their “arousal” effects.

A sodium background conductance controls the spiking pattern of mouse adrenal chromaffin cells in situ

KEY POINTS

Mouse chromaffin cells in acute adrenal slices exhibit two distinct spiking patterns, a repetitive mode and a bursting mode. A sodium background conductance operates at rest as demonstrated by the membrane hyperpolarization evoked by a low Na⁺ -containing extracellular saline. This sodium background current is insensitive to TTX, is not blocked by Cs⁺ ions and displays a linear I-V relationship at potentials close to chromaffin cell resting potential. Its properties are reminiscent of those of the sodium leak channel NALCN. In the adrenal gland, Nalcn mRNA is selectively expressed in chromaffin cells. The study fosters our understanding of how the spiking pattern of chromaffin cells is regulated and adds a sodium background conductance to the list of players involved in the stimulus-secretion coupling of the adrenomedullary tissue.

ABSTRACT

Chromaffin cells (CCs) are the master neuroendocrine units for the secretory function of the adrenal medulla and a finely-tuned regulation of their electrical activity is required for appropriate catecholamine secretion in response to the organismal demand. Here, we aim at deciphering how the spiking pattern of mouse CCs is regulated by the ion conductances operating near the resting membrane potential (RMP). At RMP, mouse CCs display a composite firing pattern, alternating between active periods composed of action potentials spiking with a regular or a bursting mode, and silent periods. RMP is sensitive to changes in extracellular sodium concentration, and a low Na⁺ -containing saline hyperpolarizes the membrane, regardless of the discharge pattern. This RMP drive reflects the contribution of a depolarizing conductance, which is (i) not blocked by tetrodotoxin or caesium, (ii) displays a linear I-V relationship between -110 and -40 mV, and (iii) is carried by cations with a conductance sequence g_Na  > g_K  > g_Cs . These biophysical attributes, together with the expression of the sodium-leak channel Nalcn transcript in CCs, state credible the contribution of NALCN. This inaugural report opens new research routes in the field of CC stimulus-secretion coupling, and extends the inventory of tissues in which NALCN is expressed to neuroendocrine glands.

Altered sleep intensity upon DBS to hypothalamic sleep-wake centers in rats

Deep brain stimulation (DBS) has been scarcely investigated in the field of sleep research. We hypothesize that DBS onto hypothalamic sleep- and wake-promoting centers will produce significant neuromodulatory effects and potentially become a therapeutic strategy for patients suffering severe, drug-refractory sleep–wake disturbances. We aimed to investigate whether continuous electrical high-frequency DBS, such as that often implemented in clinical practice, in the ventrolateral preoptic nucleus (VLPO) or the perifornical area of the posterior lateral hypothalamus (PeFLH), significantly modulates sleep–wake characteristics and behavior. We implanted healthy rats with electroencephalographic/electromyographic electrodes and recorded vigilance states in parallel to bilateral bipolar stimulation of VLPO and PeFLH at 125 Hz and 90 µA over 24 h to test the modulating effects of DBS on sleep–wake proportions, stability and spectral power in relation to the baseline. We unexpectedly found that VLPO DBS at 125 Hz deepens slow-wave sleep (SWS) as measured by increased delta power, while sleep proportions and fragmentation remain unaffected. Thus, the intensity, but not the amount of sleep or its stability, is modulated. Similarly, the proportion and stability of vigilance states remained altogether unaltered upon PeFLH DBS but, in contrast to VLPO, 125 Hz stimulation unexpectedly weakened SWS, as evidenced by reduced delta power. This study provides novel insights into non-acute functional outputs of major sleep–wake centers in the rat brain in response to electrical high-frequency stimulation, a paradigm frequently used in human DBS. In the conditions assayed, while exerting no major effects on the sleep–wake architecture, hypothalamic high-frequency stimulation arises as a provocative sleep intensity-modulating approach.

The dynamic regulation of appetitive behavior through lateral hypothalamic orexin and melanin concentrating hormone expressing cells

The lateral hypothalamic area (LHA) is a heterogeneous brain structure extensively studied for its potent role in regulating energy balance. The anatomical and molecular diversity of the LHA permits the orchestration of responses to energy sensing cues from the brain and periphery. Two of the primary cell populations within the LHA associated with integration of this information are Orexin (ORX) and Melanin Concentrating Hormone (MCH). While both of these non-overlapping populations exhibit orexigenic properties, the activities of these two systems support feeding behavior through contrasting mechanisms. We describe the anatomical and functional properties as well as interaction with other neuropeptides and brain reward and hedonic systems. Specific outputs relating to arousal, food seeking, feeding, and metabolism are coordinated through these mechanisms. We then discuss how both the ORX and MCH systems harmonize in a divergent yet overall cooperative manner to orchestrate feeding behavior through transitions between various appetitive states, and thus offer novel insights into LHA allostatic control of appetite.

Orexin neurons and inhibitory Agrp→orexin circuits guide spatial exploration in mice

KEY POINTS

Photoinhibition of endogenous activity of lateral hypothalamic orexin neurons causes place preference and reduces innate avoidance Endogenous activity of orexin neurons correlates with place preference Mediobasal hypothalamic Agrp neurons inhibit orexin neurons via GABA, and chemogenetic suppression of Agrp neurons increases avoidance in an orexin receptor-dependent manner.

ABSTRACT

Hypothalamic orexin/hypocretin neurons integrate multiple sensory cues and project brain-wide to orchestrate diverse innate behaviours. Their loss impairs many context-appropriate actions, but the motivational characteristics of orexin cell activity remain unclear. We and others previously approached this question by artificial orexin stimulation, which could induce either rewarding (positive valence) or aversive (negative valence) brain activity. It is unknown to what extent such approaches replicate natural/endogenous orexin signals, which rapidly fluctuate during wakefulness. Here we took an alternative approach, focusing on observing and silencing natural orexin cell signals associated with a fundamental innate behaviour, self-paced spatial exploration. We found that mice are more likely to stay in places paired with orexin cell optosilencing. The orexin cell optosilencing also reduced avoidance of places that mice find innately aversive. Correspondingly, calcium recordings revealed that orexin cell activity rapidly reduced upon exiting the innately aversive places. Furthermore, we provide optogenetic evidence for an inhibitory GABAergic Agrp→orexin hypothalamic neurocircuit, and find that Agrp cell suppression increases innate avoidance behaviour, consistent with orexin disinhibition. These results imply that exploration may be motivated and oriented by a need to reduce aversive orexin cell activity, and suggest a hypothalamic circuit for fine-tuning orexin signals to changing ethological priorities.

Intellectual disability-associated UNC80 mutations reveal inter-subunit interaction and dendritic function of the NALCN channel complex

The sodium-leak channel NALCN forms a subthreshold sodium conductance that controls the resting membrane potentials of neurons. The auxiliary subunits of the channel and their functions in mammals are largely unknown. In this study, we demonstrate that two large proteins UNC80 and UNC79 are subunits of the NALCN complex. UNC80 knockout mice are neonatal lethal. The C-terminus of UNC80 contains a domain that interacts with UNC79 and overcomes a soma-retention signal to achieve dendritic localization. UNC80 lacking this domain, as found in human patients, still supports whole-cell NALCN currents but lacks dendritic localization. Our results establish the subunit composition of the NALCN complex, uncover the inter-subunit interaction domains, reveal the functional significance of regulation of dendritic membrane potential by the sodium-leak channel complex, and provide evidence supporting that genetic variations found in individuals with intellectual disability are the causes for the phenotype observed in patients.

N-benzhydryl quinuclidine compounds are a potent and Src kinase-independent inhibitor of NALCN channels

BACKGROUND AND PURPOSE

NALCN is a Na⁺ leak, GPCR-activated channel that regulates the resting membrane potential and neuronal excitability. Despite numerous possible roles for NALCN in both normal physiology and disease processes, lack of specific blockers hampers further investigation.

EXPERIMENTAL APPROACH

The effect of N-benzhydryl quinuclidine compounds on NALCN channels was demonstrated using whole-cell patch-clamp recordings in HEK293T cells overexpressing NALCN and acutely isolated nigral dopaminergic neurons that express NALCN endogenously. Src kinase activity was measured using a Src kinase assay kit, and voltage and current-clamp recordings from nigral dopaminergic neurons were used to measure NALCN currents and membrane potentials.

KEY RESULTS

N-benzhydryl quinuclidine compounds inhibited NALCN channels without affecting TRPC channels, another important route for Na⁺ leak. In HEK293T cells overexpressing NALCN, N-benzhydryl quinuclidine compounds potently suppressed muscarinic M₃ receptor-activated NALCN currents. Structure-function relationship studies suggest that the quinuclidine ring with a benzhydryl group imparts the ability to inhibit NALCN currents regardless of Src family kinases. Moreover, N-benzhydryl quinuclidine compounds inhibited not only GPCR-activated NALCN currents but also background Na⁺ leak currents and hyperpolarized the membrane potential in native midbrain dopaminergic neurons that express NALCN endogenously.

CONCLUSION AND IMPLICATIONS

These findings suggest that N-benzhydryl quinuclidine compounds have a pharmacological potential to directly inhibit NALCN channels and could be a useful tool to investigate functions of NALCN channels.

The Lateral Hypothalamus: An Uncharted Territory for Processing Peripheral Neurogenic Inflammation

The roles of the hypothalamus and particularly the lateral hypothalamus (LH) in the regulation of inflammation and pain have been widely studied. The LH consists of a parasympathetic area that has connections with all the major parts of the brain. It controls the autonomic nervous system (ANS), regulates feeding behavior and wakeful cycles, and is a part of the reward system. In addition, it contains different types of neurons, most importantly the orexin neurons. These neurons, though few in number, perform critical functions such as inhibiting pain transmission and interfering with the reward system, feeding behavior and the hypothalamic pituitary axis (HPA). Recent evidence has identified a new role for orexin neurons in the modulation of pain transmission associated with several inflammatory diseases, including rheumatoid arthritis and ulcerative colitis. Here, we review recent findings on the various physiological functions of the LH with special emphasis on the orexin/receptor system and its role in mediating inflammatory pain.

Sleep deprivation-induced pre- and postsynaptic modulation of orexin neurons

Sleep/wake states are controlled by sleep- and wake-promoting systems, and transitions between states are thought to be regulated by their reciprocal inhibition and homeostatic sleep need. Orexin neurons are known to promote wake maintenance and stabilize the sleep/wake switch. Thus, we asked whether orexin neurons are modulated by homeostatic sleep need. Rats were sleep deprived or left undisturbed to rest for 6 h, then acute brain slices were generated for patch clamp recordings. We found that sleep deprivation increased firing and reduced spike frequency adaptation in response to excitatory drive in orexin neurons. These changes were specific to D-type orexin neurons which, unlike H-type orexin neurons, lack A-type current. In D-type orexin neurons, sleep deprivation decreased afterhyperpolarizing potential, which was associated with increased gain, measured as the slope of the input-output relationship. These effects were mimicked by inhibition of SK channels. Furthermore, sleep deprivation resulted in presynaptic inhibition of excitatory inputs to both D-type and H-type orexin neurons, which preferentially affected sparse synaptic inputs while sparing high frequency synaptic activities. Taken together, our results indicate that sleep deprivation modulates the gain control and synaptic gating in orexin neurons. These pre-and postsynaptic changes would tune orexin neurons to strong wake-promoting excitatory signals, while dampening weak synaptic inputs to allow transition to sleep in the absence of such strong signals. These mechanisms are consistent with a role of orexin neurons not only as a key state stabilizer, but also as a homeostatic wake integrator in the sleep/wake switch. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

TRPCing around the hypothalamus

All of the canonical transient receptor potential channels (TRPC) with the exception of TRPC 2 are expressed in hypothalamic neurons and are involved in multiple homeostatic functions. Although the metabotropic glutamate receptors have been shown to be coupled to TRPC channel activation in cortical and sub-cortical brain regions, in the hypothalamus multiple amine and peptidergic G protein-coupled receptors (GPCRs) and growth factor/cytokine receptors are linked to activation of TRPC channels that are vital for reproduction, temperature regulation, arousal and energy homeostasis. In addition to the neurotransmitters, circulating hormones like insulin and leptin through their cognate receptors activate TRPC channels in POMC neurons. Many of the post-synaptic effects of the neurotransmitters and hormones are regulated in different physiological states by expression of TRPC channels in the post-synaptic neurons. Therefore, TRPC channels are key targets not only for neurotransmitters but circulating hormones in their vital role to control multiple hypothalamic functions, which is the focus of this review.

Control of Energy Expenditure by AgRP Neurons of the Arcuate Nucleus: Neurocircuitry, Signaling Pathways, and Angiotensin

PURPOSE OF REVIEW

Here, we review the current understanding of the functional neuroanatomy of neurons expressing Agouti-related peptide (AgRP) and the angiotensin 1A receptor (AT_1A) within the arcuate nucleus (ARC) in the control of energy balance.

RECENT FINDINGS

The development and maintenance of obesity involves suppression of resting metabolic rate (RMR). RMR control is integrated via AgRP and proopiomelanocortin neurons within the ARC. Their projections to other hypothalamic and extrahypothalamic nuclei contribute to RMR control, though relatively little is known about the contributions of individual projections and the neurotransmitters involved. Recent studies highlight a role for AT_1A, localized to AgRP neurons, but the specific function of AT_1A within these cells remains unclear. AT_1A functions within AgRP neurons to control RMR, but additional work is required to clarify its role within subpopulations of AgRP neurons projecting to distinct second-order nuclei, and the molecular mediators of its signaling within these cells.

Electrophysiological Properties of Melanin-Concentrating Hormone and Orexin Neurons in Adolescent Rats

Orexin and melanin-concentrating hormone (MCH) neurons have complementary roles in various physiological functions including energy balance and the sleep/wake cycle. in vitro electrophysiological studies investigating these cells typically use post-weaning rodents, corresponding to adolescence. However, it is unclear whether these neurons are functionally mature at this period and whether these studies can be generalized to adult cells. Therefore, we examined the electrophysiological properties of orexin and MCH neurons in brain slices from post-weaning rats and found that MCH neurons undergo an age-dependent reduction in excitability, but not orexin neurons. Specifically, MCH neurons displayed an age-dependent hyperpolarization of the resting membrane potential (RMP), depolarizing shift of the threshold, and decrease in excitatory transmission, which reach the adult level by 7 weeks of age. In contrast, basic properties of orexin neurons were stable from 4 weeks to 14 weeks of age. Furthermore, a robust short-term facilitation of excitatory synapses was found in MCH neurons, which showed age-dependent changes during the post-weaning period. On the other hand, a strong short-term depression was observed in orexin neurons, which was similar throughout the same period. These differences in synaptic responses and age dependence likely differentially affect the network activity within the lateral hypothalamus where these cells co-exist. In summary, our study suggests that orexin neurons are electrophysiologically mature before adolescence whereas MCH neurons continue to develop until late adolescence. These changes in MCH neurons may contribute to growth spurts or consolidation of adult sleep patterns associated with adolescence. Furthermore, these results highlight the importance of considering the age of animals in studies involving MCH neurons.

Orexin and Alzheimer's Disease

Alzheimer's disease (AD) is the most frequent age-related dementia. It prevalently causes cognitive decline, although it is frequently associated with secondary behavioral disturbances. AD neurodegeneration characteristically produces a remarkable destruction of the sleep-wake cycle, with diurnal napping, nighttime arousals, sleep fragmentation, and REM sleep impairment. It was recently hypothesized that the orexinergic system was involved in AD pathology. Accordingly, recent papers showed the association between orexinergic neurotransmission dysfunction, sleep impairment, and cognitive decline in AD. Orexin is a hypothalamic neurotransmitter which physiologically produces wakefulness and reduces REM sleep and may alter the sleep-wake cycle in AD patients. Furthermore, the orexinergic system seems to interact with CSF AD biomarkers, such as beta-amyloid and tau proteins. Beta-amyloid accumulation is the main hallmark of AD pathology, while tau proteins mark brain neuronal injury due to AD pathology. Investigations so far suggest that orexinergic signaling overexpression alters the sleep-wake cycle and secondarily induces beta-amyloid accumulation and tau-mediated neurodegeneration. Therefore, considering that orexinergic system dysregulation impairs sleep-wake rhythms and may influence AD pathology, it is hypothesized that orexin receptor antagonists are likely potential preventive/therapeutic options in AD patients.

Connexin 43-Mediated Astroglial Metabolic Networks Contribute to the Regulation of the Sleep-Wake Cycle

Astrocytes produce and supply metabolic substrates to neurons through gap junction-mediated astroglial networks. However, the role of astroglial metabolic networks in behavior is unclear. Here, we demonstrate that perturbation of astroglial networks impairs the sleep-wake cycle. Using a conditional Cre-Lox system in mice, we show that knockout of the gap junction subunit connexin 43 in astrocytes throughout the brain causes excessive sleepiness and fragmented wakefulness during the nocturnal active phase. This astrocyte-specific genetic manipulation silenced the wake-promoting orexin neurons located in the lateral hypothalamic area (LHA) by impairing glucose and lactate trafficking through astrocytic networks. This global wakefulness instability was mimicked with viral delivery of Cre recombinase to astrocytes in the LHA and rescued by in vivo injections of lactate. Our findings propose a novel regulatory mechanism critical for maintaining normal daily cycle of wakefulness and involving astrocyte-neuron metabolic interactions.

Toll-Like Receptor 2 Is a Regulator of Circadian Active and Inactive State Consolidation in C57BL/6 Mice

Regulatory systems required to maintain behavioral arousal remain incompletely understood. We describe a previously unappreciated role that toll-like receptor 2 (Tlr2, a membrane bound pattern recognition receptor that recognizes specific bacterial, viral, and fungal peptides), contributes toward regulation of behavioral arousal. In 4–4.5 month old mice with constitutive loss of Tlr2 function (Tlr2^−/− mice), we note a marked consolidation in the circadian pattern of both active and inactive states. Specifically, Tlr2^−/− mice demonstrated significantly fewer but longer duration active states during the circadian dark cycle, and significantly fewer but longer duration inactive states during the circadian light cycle. Tlr2^−/− mice also consumed less food and water, and moved less during the circadian light cycle. Analysis of circadian rhythms further suggested that Tlr2^−/− mice demonstrated less day-to-day variability in feeding, drinking, and movement behaviors. Reevaluation of this same mouse cohort at age 8–8.5 months revealed a clear blunting of these differences. However, Tlr2^−/− mice were still noted to have fewer short-duration active states during the circadian dark cycle, and continued to demonstrate significantly less day-to-day variability in feeding, drinking, and movement behaviors. These results suggest that Tlr2 function may have a role in promoting transitions between active and inactive states. Prior studies have demonstrated that Tlr2 regulates sickness behaviors including hypophagia, hyperthermia, and decreased activity. Our work suggests that Tlr2 function also evokes behavioral fragmentation, another aspect of sickness behavior and a clinically significant problem of older adults.

Endocannabinoid-dependent disinhibition of orexinergic neurons: Electrophysiological evidence in leptin-knockout obese mice

OBJECTIVES

In the ob/ob mouse model of obesity, chronic absence of leptin causes a significant increase of orexin (OX) production by hypothalamic neurons and excessive food intake. The altered OX level is linked to a dramatic increase of the inhibitory innervation of OX producing neurons (OX neurons) and the over expression of the endocannabinoid 2-arachidonoylglycerol (2-AG) by OX neurons of ob/ob mice. Little is known about the function of the excitatory synapses of OX neurons in ob/ob mice, and their modulation by 2-AG. In the present study, we fill this gap and provide the first evidence of the overall level of activation of OX neurons in the ob/ob mice.

METHODS

We performed in vitro whole-cell patch-clamp recordings on OX neurons located in the perifornical area of the lateral hypothalamus in acute brain slices of wt and ob/ob mice. We identified OX neurons on the basis of their electrophysiological membrane properties, with 96% of concordance with immunohistochemisty.

RESULTS

We found that OX neurons of ob/ob mice are innervated by less efficient and fewer excitatory synapses than wt mice. Consequently, ob/ob OX neurons show more negative resting membrane potential and lower action potential firing frequency than wt. The bath application of the cannabinoid type-1 receptor agonist WIN55,212-2, depresses both the excitatory and the inhibitory synapses in ob/ob animals, but only the excitatory synapses in wt animals. Finally, the physiologic release of 2-AG induces a prevalent depression of inhibition (disinhibition) of OX neurons in ob/ob animals but not in wt.

CONCLUSIONS

In ob/ob mice, chronic absence of leptin induces a 2-AG mediated functional disinhibition of OX neurons. This helps explain the increase of OX production and, consequently, the excessive food intake of ob/ob mice.

High sensitivity of spontaneous spike frequency to sodium leak current in a Lymnaea pacemaker neuron

The spontaneous rhythmic firing of action potentials in pacemaker neurons depends on the biophysical properties of voltage-gated ion channels and background leak currents. The background leak current includes a large K⁺ and a small Na⁺ component. We previously reported that a Na⁺ -leak current via U-type channels is required to generate spontaneous action potential firing in the identified respiratory pacemaker neuron, RPeD1, in the freshwater pond snail Lymnaea stagnalis. We further investigated the functional significance of the background Na⁺ current in rhythmic spiking of RPeD1 neurons. Whole-cell patch-clamp recording and computational modeling approaches were carried out in isolated RPeD1 neurons. The whole-cell current of the major ion channel components in RPeD1 neurons were characterized, and a conductance-based computational model of the rhythmic pacemaker activity was simulated with the experimental measurements. We found that the spiking rate is more sensitive to changes in the Na⁺ leak current as compared to the K⁺ leak current, suggesting a robust function of Na⁺ leak current in regulating spontaneous neuronal firing activity. Our study provides new insight into our current understanding of the role of Na⁺ leak current in intrinsic properties of pacemaker neurons.

Slow Bursting Neurons of Mouse Cortical Layer 6b Are Depolarized by Hypocretin/Orexin and Major Transmitters of Arousal

Neurons firing spontaneously in bursts in the absence of synaptic transmission have been previously recorded in different layers of cortical brain slices. It has been suggested that such neurons could contribute to the generation of alternating UP and DOWN states, a pattern of activity seen during slow-wave sleep. Here, we show that in layer 6b (L6b), known from our previous studies to contain neurons highly responsive to the wake-promoting transmitter hypocretin/orexin (hcrt/orx), there is a set of neurons, endowed with distinct intrinsic properties, which displayed a strong propensity to fire spontaneously in rhythmic bursts. In response to small depolarizing steps, they responded with a delayed firing of action potentials which, upon higher depolarizing steps, invariably inactivated and were followed by a depolarized plateau potential and a depolarizing afterpotential. These cells also displayed a strong hyperpolarization-activated rectification compatible with the presence of an I_h current. Most L6b neurons with such properties were able to fire spontaneously in bursts. Their bursting activity was of intrinsic origin as it persisted not only in presence of blockers of ionotropic glutamatergic and GABAergic receptors but also in a condition of complete synaptic blockade. However, a small number of these neurons displayed a mix of intrinsic bursting and synaptically driven recurrent UP and DOWN states. Most of the bursting L6b neurons were depolarized and excited by hcrt/orx through a direct postsynaptic mechanism that led to tonic firing and eventually inactivation. Similarly, they were directly excited by noradrenaline, histamine, dopamine, and neurotensin. Finally, the intracellular injection of these cells with dye and their subsequent Neurolucida reconstruction indicated that they were spiny non-pyramidal neurons. These results lead us to suggest that the propensity for slow rhythmic bursting of this set of L6b neurons could be directly impeded by hcrt/orx and other wake-promoting transmitters.

Effects of Hypocretin/Orexin and Major Transmitters of Arousal on Fast Spiking Neurons in Mouse Cortical Layer 6B

Fast spiking (FS) GABAergic neurons are thought to be involved in the generation of high-frequency cortical rhythms during the waking state. We previously showed that cortical layer 6b (L6b) was a specific target for the wake-promoting transmitter, hypocretin/orexin (hcrt/orx). Here, we have investigated whether L6b FS cells were sensitive to hcrt/orx and other transmitters associated with cortical activation. Recordings were thus made from L6b FS cells in either wild-type mice or in transgenic mice in which GFP-positive GABAergic cells are parvalbumin positive. Whereas in a control condition hcrt/orx induced a strong increase in the frequency, but not amplitude, of spontaneous synaptic currents, in the presence of TTX, it had no effect at all on miniature synaptic currents. Hcrt/orx effect was thus presynaptic although not by an action on glutamatergic terminals but rather on neighboring cells. In contrast, noradrenaline and acetylcholine depolarized and excited these cells through a direct postsynaptic action. Neurotensin, which is colocalized in hcrt/orx neurons, also depolarized and excited these cells but the effect was indirect. Morphologically, these cells exhibited basket-like features. These results suggest that hcrt/orx, noradrenaline, acetylcholine, and neurotensin could contribute to high-frequency cortical activity through an action on L6b GABAergic FS cells.

Understanding how discrete populations of hypothalamic neurons orchestrate complicated behavioral states

A major question in systems neuroscience is how a single population of neurons can interact with the rest of the brain to orchestrate complex behavioral states. The hypothalamus contains many such discrete neuronal populations that individually regulate arousal, feeding, and drinking. For example, hypothalamic neurons that express hypocretin (Hcrt) neuropeptides can sense homeostatic and metabolic factors affecting wakefulness and orchestrate organismal arousal. Neurons that express agouti-related protein (AgRP) can sense the metabolic needs of the body and orchestrate a state of hunger. The organum vasculosum of the lamina terminalis (OVLT) can detect the hypertonicity of blood and orchestrate a state of thirst. Each hypothalamic population is sufficient to generate complicated behavioral states through the combined efforts of distinct efferent projections. The principal challenge to understanding these brain systems is therefore to determine the individual roles of each downstream projection for each behavioral state. In recent years, the development and application of temporally precise, genetically encoded tools has greatly improved our understanding of the structure and function of these neural systems. This review will survey recent advances in our understanding of how these individual hypothalamic populations can orchestrate complicated behavioral states due to the combined efforts of individual downstream projections.

Concentration-dependent activation of dopamine receptors differentially modulates GABA release onto orexin neurons

Dopamine (DA) and orexin neurons play important roles in reward and food intake. There are anatomical and functional connections between these two cell groups: orexin peptides stimulate DA neurons in the ventral tegmental area and DA inhibits orexin neurons in the hypothalamus. However, the cellular mechanisms underlying the action of DA on orexin neurons remain incompletely understood. Therefore, the effect of DA on inhibitory transmission to orexin neurons was investigated in rat brain slices using the whole-cell patch-clamp technique. We found that DA modulated the frequency of spontaneous and miniature IPSCs (mIPSCs) in a concentration-dependent bidirectional manner. Low (1 μM) and high (100 μM) concentrations of DA decreased and increased IPSC frequency, respectively. These effects did not accompany a change in mIPSC amplitude and persisted in the presence of G-protein signaling inhibitor GDPβS in the pipette, suggesting that DA acts presynaptically. The decrease in mIPSC frequency was mediated by D2 receptors whereas the increase required co-activation of D1 and D2 receptors and subsequent activation of phospholipase C. In summary, our results suggest that DA has complex effects on GABAergic transmission to orexin neurons, involving cooperation of multiple receptor subtypes. The direction of dopaminergic influence on orexin neurons is dependent on the level of DA in the hypothalamus. At low levels DA disinhibits orexin neurons whereas at high levels it facilitates GABA release, which may act as negative feedback to curb the excitatory orexinergic output to DA neurons. These mechanisms may have implications for consummatory and motivated behaviours.

TRPV4 and K(Ca) ion channels functionally couple as osmosensors in the paraventricular nucleus

BACKGROUND AND PURPOSE

Transient receptor potential vanilloid type 4 (TRPV4) and calcium-activated potassium channels (KCa ) mediate osmosensing in many tissues. Both TRPV4 and KCa channels are found in the paraventricular nucleus (PVN) of the hypothalamus, an area critical for sympathetic control of cardiovascular and renal function. Here, we have investigated whether TRPV4 channels functionally couple to KCa channels to mediate osmosensing in PVN parvocellular neurones and have characterized, pharmacologically, the subtype of KCa channel involved.

EXPERIMENTAL APPROACH

We investigated osmosensing roles for TRPV4 and KCa channels in parvocellular PVN neurones using cell-attached and whole-cell electrophysiology in mouse brain slices and rat isolated PVN neurons. Intracellular Ca(2+) was recorded using Fura-2AM. The system was modelled in the NEURON simulation environment.

KEY RESULTS

Hypotonic saline reduced action current frequency in hypothalamic slices; a response mimicked by TRPV4 channel agonists 4αPDD (1 μM) and GSK1016790A (100 nM), and blocked by inhibitors of either TRPV4 channels (RN1734 (5 μM) and HC067047 (300 nM) or the low-conductance calcium-activated potassium (SK) channel (UCL-1684 30 nM); iberiotoxin and TRAM-34 had no effect. Our model was compatible with coupling between TRPV4 and KCa channels, predicting the presence of positive and negative feedback loops. These predictions were verified using isolated PVN neurons. Both hypotonic challenge and 4αPDD increased intracellular Ca(2+) and UCL-1684 reduced the action of hypotonic challenge.

CONCLUSIONS AND IMPLICATIONS

There was functional coupling between TRPV4 and SK channels in parvocellular neurones. This mechanism contributes to osmosensing in the PVN and may provide a novel pharmacological target for the cardiovascular or renal systems.

To ingest or rest? Specialized roles of lateral hypothalamic area neurons in coordinating energy balance

Survival depends on an organism’s ability to sense nutrient status and accordingly regulate intake and energy expenditure behaviors. Uncoupling of energy sensing and behavior, however, underlies energy balance disorders such as anorexia or obesity. The hypothalamus regulates energy balance, and in particular the lateral hypothalamic area (LHA) is poised to coordinate peripheral cues of energy status and behaviors that impact weight, such as drinking, locomotor behavior, arousal/sleep and autonomic output. There are several populations of LHA neurons that are defined by their neuropeptide content and contribute to energy balance. LHA neurons that express the neuropeptides melanin-concentrating hormone (MCH) or orexins/hypocretins (OX) are best characterized and these neurons play important roles in regulating ingestion, arousal, locomotor behavior and autonomic function via distinct neuronal circuits. Recently, another population of LHA neurons containing the neuropeptide Neurotensin (Nts) has been implicated in coordinating anorectic stimuli and behavior to regulate hydration and energy balance. Understanding the specific roles of MCH, OX and Nts neurons in harmonizing energy sensing and behavior thus has the potential to inform pharmacological strategies to modify behaviors and treat energy balance disorders.

Different distributions of preproMCH and hypocretin/orexin in the forebrain of the pig (Sus scrofa domesticus)

Neurons producing melanin-concentrating hormone (MCH) or hypocretin/orexin (Hcrt) have been implicated in the sleep/wake cycle and feeding behavior. Sleep and feeding habits vary greatly among mammalian species, depending in part of the prey/predatory status of animals. However, the distribution of both peptides has been described in only a limited number of species. In this work, we describe the distribution of MCH neurons in the brain of the domestic pig. Using in situ hybridization and immunohistochemistry, their cell bodies are shown to be located in the posterior lateral hypothalamic area (LHA), as expected. They form a dense cluster ventro-lateral to the fornix while only scattered cells are present dorsal to this tract. By comparison, Hcrt cell bodies are located mainly dorsal to the fornix. Therefore, the two populations of neurons display complementary distributions in the posterior LHA. MCH projections are, as indicated by MCH-positive axons, very abundant in all cortical fields ventral to the rhinal sulcus, as well as in the lateral, basolateral and basomedial amygdala. In contrast, most of the isocortex is sparsely innervated. To conclude, the distribution of MCH cell bodies and projections shows some very specific features in the pig brain, that are clearly different of that described in the rat, mouse or human. In contrast, the Hcrt pattern seems more similar to that in these species, i.e. more conserved. These results suggest that the LHA anatomic organization shows some very significant interspecies differences, which may be related to the different behavioral repertoires of animals with regard to feeding and sleep/wake cycles.

CXCL12 chemokine and its receptors as major players in the interactions between immune and nervous systems

The chemokine CXCL12/stromal cell-derived factor 1 alpha has first been described in the immune system where it functions include chemotaxis for lymphocytes and macrophages, migration of hematopoietic cells from fetal liver to bone marrow and the formation of large blood vessels. Among other chemokines, CXCL12 has recently attracted much attention in the brain as it has been shown that it can be produced not only by glial cells but also by neurons. In addition, its receptors CXCR4 and CXCR7, which are belonging to the G protein-coupled receptors family, are abundantly expressed in diverse brain area, CXCR4 being a major co-receptor for human immunodeficiency virus 1 entry. This chemokine system has been shown to play important roles in brain plasticity processes occurring during development but also in the physiology of the brain in normal and pathological conditions. For example, in neurons, CXCR4 stimulation has been shown regulate the synaptic release of glutamate and γ-aminobutyric acid (GABA). It can also act post-synaptically by activating a G protein activated inward rectifier K⁺ (GIRK), a voltage-gated K channel Kv2.1 associated to neuronal survival, and by increasing high voltage activated Ca²⁺ currents. In addition, it has been recently evidenced that there are several cross-talks between the CXCL12/CXCR4–7 system and other neurotransmitter systems in the brain (such as GABA, glutamate, opioids, and cannabinoids). Overall, this chemokine system could be one of the key players of the neuro-immune interface that participates in shaping the brain in response to changes in the environment.

Role for T-type Ca2+ channels in sleep waves

Since their discovery more than 30 years ago, low-threshold T-type Ca(2+) channels (T channels) have been suggested to play a key role in many EEG waves of non-REM sleep, which has remained exclusively linked to the ability of these channels to generate low-threshold Ca(2+) potentials and associated high-frequency bursts of action potentials. Our present understanding of the biophysics and physiology of T channels, however, highlights a much more diverse and complex picture of the pivotal contributions that they make to different sleep rhythms. In particular, recent experimental evidence has conclusively demonstrated the essential contribution of thalamic T channels to the expression of slow waves of natural sleep and the key role played by Ca(2+) entry through these channels in the activation or modulation of other voltage-dependent channels that are important for the generation of both slow waves and sleep spindles. However, the precise contribution to sleep rhythms of T channels in cortical neurons and other sleep-controlling neuronal networks remains unknown, and a full understanding of the cellular and network mechanisms of sleep delta waves is still lacking.

Control of arousal by the orexin neurons

The orexin-producing neurons in the lateral hypothalamus play an essential role in promoting arousal and maintaining wakefulness. These neurons receive a broad variety of signals related to environmental, physiological and emotional stimuli; they project to almost every brain region involved in the regulation of wakefulness; and they fire most strongly during active wakefulness, high motor activation, and sustained attention. This review focuses on the specific neuronal pathways through which the orexin neurons promote wakefulness and maintain high level of arousal, and how recent studies using optogenetic and pharmacogenetic methods have demonstrated that the locus coeruleus, the tuberomammillary nucleus, and the basal forebrain are some of the key sites mediating the arousing actions of orexins.

Obesity-driven synaptic remodeling affects endocannabinoid control of orexinergic neurons

Acute or chronic alterations in energy status alter the balance between excitatory and inhibitory synaptic transmission and associated synaptic plasticity to allow for the adaptation of energy metabolism to new homeostatic requirements. The impact of such changes on endocannabinoid and cannabinoid receptor type 1 (CB1)-mediated modulation of synaptic transmission and strength is not known, despite the fact that this signaling system is an important target for the development of new drugs against obesity. We investigated whether CB1-expressing excitatory vs. inhibitory inputs to orexin-A-containing neurons in the lateral hypothalamus are altered in obesity and how this modifies endocannabinoid control of these neurons. In lean mice, these inputs are mostly excitatory. By confocal and ultrastructural microscopic analyses, we observed that in leptin-knockout (ob/ob) obese mice, and in mice with diet-induced obesity, orexinergic neurons receive predominantly inhibitory CB1-expressing inputs and overexpress the biosynthetic enzyme for the endocannabinoid 2-arachidonoylglycerol, which retrogradely inhibits synaptic transmission at CB1-expressing axon terminals. Patch-clamp recordings also showed increased CB1-sensitive inhibitory innervation of orexinergic neurons in ob/ob mice. These alterations are reversed by leptin administration, partly through activation of the mammalian target of rapamycin pathway in neuropeptide-Y-ergic neurons of the arcuate nucleus, and are accompanied by CB1-mediated enhancement of orexinergic innervation of target brain areas. We propose that enhanced inhibitory control of orexin-A neurons, and their CB1-mediated disinhibition, are a consequence of leptin signaling impairment in the arcuate nucleus. We also provide initial evidence of the participation of this phenomenon in hyperphagia and hormonal dysregulation in obesity.

The vertebrate diencephalic MCH system: a versatile neuronal population in an evolving brain

Neurons synthesizing melanin-concentrating hormone (MCH) are described in the posterior hypothalamus of all vertebrates investigated so far. However, their anatomy is very different according to species: they are small and periventricular in lampreys, cartilaginous fishes or anurans, large and neuroendocrine in bony fishes, or distributed over large regions of the lateral hypothalamus in many mammals. An analysis of their comparative anatomy alongside recent data about the development of the forebrain, suggests that although very different, MCH neurons of the caudal hypothalamus are homologous. We further hypothesize that their divergent anatomy is linked to divergence in the forebrain - in particular telencephalic evolution.

Mechanism for Hypocretin-mediated sleep-to-wake transitions

Current models of sleep/wake regulation posit that Hypocretin (Hcrt)-expressing neurons in the lateral hypothalamus promote and stabilize wakefulness by projecting to subcortical arousal centers. However, the critical downstream effectors of Hcrt neurons are unknown. Here we use optogenetic, pharmacological, and computational tools to investigate the functional connectivity between Hcrt neurons and downstream noradrenergic neurons in the locus coeruleus (LC) during nonrapid eye movement (NREM) sleep. We found that photoinhibiting LC neurons during Hcrt stimulation blocked Hcrt-mediated sleep-to-wake transitions. In contrast, when LC neurons were optically stimulated to increase membrane excitability, concomitant photostimulation of Hcrt neurons significantly increased the probability of sleep-to-wake transitions compared with Hcrt stimulation alone. We also built a conductance-based computational model of Hcrt-LC circuitry that recapitulates our behavioral results using LC neurons as the main effectors of Hcrt signaling. These results establish the Hcrt-LC connection as a critical integrator-effector circuit that regulates NREM sleep/wake behavior during the inactive period. This coupling of distinct neuronal systems can be generalized to other hypothalamic integrator nuclei with downstream effector/output populations in the brain.

Somatostatin varicosities contain the vesicular GABA transporter and contact orexin neurons in the hypothalamus

Somatostatin (SST) is a neuropeptide with known inhibitory actions in the hypothalamus, where it inhibits release of growth hormone-releasing hormone (GHRH), while also influencing the sleep-wake cycle. Here we investigated in the rat whether SST neurons might additionally release GABA (gamma-aminobutyric acid) or glutamate in different regions and whether they might contact orexin neurons that play an important role in the maintenance of wakefulness. In dual-immunostained sections viewed by epifluorescence microscopy, we examined if SST varicosities were immunopositive for the vesicular transporter for GABA (VGAT) or glutamate (VGLUT2) in the posterolateral hypothalamus and neighboring arcuate nucleus and median eminence. Of the SST varicosities in the posterolateral hypothalamus, 18% were immunopositive for VGAT, whereas ≤ 1% were immunopositive for VGLUT2. In the arcuate and median eminence, 26 and 64% were VGAT+ and < 3% VGLUT2 + , respectively. In triple-immunostained sections viewed by epifluorescence and confocal microscopy, SST varicosities were seen in contact with orexin somata, and of these varicosities, a significant proportion (23%) contained VGAT along with synaptophysin, the presynaptic marker for small synaptic vesicles, and a similar proportion (25%) abutted puncta that were immunostained for gephyrin, the postsynaptic marker for GABAergic synapses. Our results indicate that a significant proportion of SST varicosities in the hypothalamus have the capacity to release GABA, to form inhibitory synapses upon orexin neurons, and accordingly through their peptide and/or amino acid, to inhibit orexin neurons, as well as GHRH neurons. Thus while regulating GHRH release, SST neurons could serve to attenuate arousal and permit progression through the sleep cycle.

Brain orexin promotes obesity resistance

Resistance to obesity is becoming an exception rather than the norm, and understanding mechanisms that lead some to remain lean in spite of an obesigenic environment is critical if we are to find new ways to reverse this trend. Levels of energy intake and physical activity both contribute to body weight management, but it is challenging for most to adopt major long-term changes in either factor. Physical activity outside of formal exercise, also referred to as activity of daily living, and in stricter form, spontaneous physical activity (SPA), may be an attractive modifiable variable for obesity prevention. In this review, we discuss individual variability in SPA and NEAT (nonexercise thermogenesis, or the energy expended by SPA) and its relationship to obesity resistance. The hypothalamic neuropeptide orexin (hypocretin) may play a key role in regulating SPA and NEAT. We discuss how elevated orexin signaling capacity, in the context of a brain network modulating SPA, may play a major role in defining individual variability in SPA and NEAT. Greater activation of this SPA network leads to a lower propensity for fat mass gain and therefore may be an attractive target for obesity prevention and therapy.

ATP-sensitive potassium channels mediate the thermosensory response of orexin neurons

High body temperatures are generally associated with somnolence, lethargy, hypophagia and anhedonia. Orexin neurons have been suggested to play a role in such sickness behaviours due to their known functions in appetite, behavioural and autonomic activation. Furthermore, the activity of orexin neurons is inhibited by lipopolysaccharide that induces fever. However, the cellular mechanism(s) underlying this suppression of orexin neurons was unknown. We used patch-clamp recordings in acute rat brain slices to demonstrate that orexin neurons, including those projecting to the wake-promoting locus coeruleus, are inhibited by increasing the ambient temperature by a 2-4°C increment between 26 and 40°C. This effect was not mediated by conventional thermosensing mechanisms but instead involved the activation of ATP-sensitive potassium (KATP) channels. Since KATP channels can also sense energy substrate levels and cellular metabolism, our results suggest that orexin neurons can integrate the state of energy balance and body temperature, and adjust their output accordingly. Thus, the thermosensitivity of orexin neurons may be an important part of maintaining energy homeostasis during hyperthermia and fever.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Activation of inactivation process initiates rapid eye movement sleep

Interactions among REM-ON and REM-OFF neurons form the basic scaffold for rapid eye movement sleep (REMS) regulation; however, precise mechanism of their activation and cessation, respectively, was unclear. Locus coeruleus (LC) noradrenalin (NA)-ergic neurons are REM-OFF type and receive GABA-ergic inputs among others. GABA acts postsynaptically on the NA-ergic REM-OFF neurons in the LC and presynaptically on the latter's projection terminals and modulates NA-release on the REM-ON neurons. Normally during wakefulness and non-REMS continuous release of NA from the REM-OFF neurons, which however, is reduced during the latter phase, inhibits the REM-ON neurons and prevents REMS. At this stage GABA from substantia nigra pars reticulate acting presynaptically on NA-ergic terminals on REM-ON neurons withdraws NA-release causing the REM-ON neurons to escape inhibition and being active, may be even momentarily. A working-model showing neurochemical-map explaining activation of inactivation process, showing contribution of GABA-ergic presynaptic inhibition in withdrawing NA-release and dis-inhibition induced activation of REM-ON neurons, which in turn activates other GABA-ergic neurons and shutting-off REM-OFF neurons for the initiation of REMS-generation has been explained. Our model satisfactorily explains yet unexplained puzzles (i) why normally REMS does not appear during waking, rather, appears following non-REMS; (ii) why cessation of LC-NA-ergic-REM-OFF neurons is essential for REMS-generation; (iii) factor(s) which does not allow cessation of REM-OFF neurons causes REMS-loss; (iv) the association of changes in levels of GABA and NA in the brain during REMS and its deprivation and associated symptoms; v) why often dreams are associated with REMS.

An animal model of panic vulnerability with chronic disinhibition of the dorsomedial/perifornical hypothalamus

Panic disorder (PD) is a severe anxiety disorder characterized by susceptibility to induction of panic attacks by subthreshold interoceptive stimuli such as sodium lactate infusions or hypercapnia induction. Here we review a model of panic vulnerability in rats involving chronic inhibition of GABAergic tone in the dorsomedial/perifornical hypothalamic (DMH/PeF) region that produces enhanced anxiety and freezing responses in fearful situations, as well as a vulnerability to displaying acute panic-like increases in cardioexcitation, respiration activity and "flight" associated behavior following subthreshold interoceptive stimuli that do not elicit panic responses in control rats. This model of panic vulnerability was developed over 15 years ago and has provided an excellent preclinical model with robust face, predictive and construct validity. The model recapitulates many of the phenotypic features of panic attacks associated with human panic disorder (face validity) including greater sensitivity to panicogenic stimuli demonstrated by sudden onset of anxiety and autonomic activation following an administration of a sub-threshold (i.e., do not usually induce panic in healthy subjects) stimulus such as sodium lactate, CO(2), or yohimbine. The construct validity is supported by several key findings; DMH/PeF neurons regulate behavioral and autonomic components of a normal adaptive panic response, as well as being implicated in eliciting panic-like responses in humans. Additionally, patients with PD have deficits in central GABA activity and pharmacological restoration of central GABA activity prevents panic attacks, consistent with this model. The model's predictive validity is demonstrated by not only showing panic responses to several panic-inducing agents that elicit panic in patients with PD, but also by the positive therapeutic responses to clinically used agents such as alprazolam and antidepressants that attenuate panic attacks in patients. More importantly, this model has been utilized to discover novel drugs such as group II metabotropic glutamate agonists and a new class of translocator protein enhancers of GABA, both of which subsequently showed anti-panic properties in clinical trials. All of these data suggest that this preparation provides a strong preclinical model of some forms of human panic disorders.