Acute optogenetic silencing of orexin/hypocretin neurons induces slow-wave sleep in mice

The lateral hypothalamic area controls paradoxical (REM) sleep by means of descending projections to brainstem GABAergic neurons

It has recently been shown that the ventrolateral part of the periaqueductal gray (VLPAG) and the adjacent dorsal deep mesencephalic nucleus (dDpMe) contain GABAergic neurons gating paradoxical sleep (PS) onset by means of their projection to the glutamatergic PS-on neurons of the sublaterodorsal tegmental nucleus (SLD). To determine the mechanisms responsible for the cessation of activity of these GABAergic PS-off neurons at the onset and during PS, we combined the immunostaining of c-FOS, a marker of neuronal activation, with cholera toxin b subunit (CTb) retrograde tracing from the VLPAG/dDpMe in three groups of rats (control, PS deprived, and PS hypersomniac). We found that the lateral hypothalamic area (LH) is the only brain structure containing a very large number of neurons activated during PS hypersomnia and projecting to the VLPAG/dDpMe. We further demonstrated that 44% of these neurons express the neuropeptide melanin concentrating hormone (MCH). We then showed that bilateral injections in the LH of two inhibitory compounds, clonidine (an α-2 adrenergic agonist) and muscimol (a GABAa agonist) induce an inhibition of PS. Furthermore, after muscimol injections in the LH, the VLPAG/dDpMe contained a large number of activated neurons, mostly GABAergic, and projecting to the SLD. Altogether, our results indicate for the first time that the activation of a population of LH neurons, in part MCH containing, is necessary for PS to occur. Furthermore, our results strongly suggest that these neurons trigger PS by means of their inhibitory projection to the PS-off GABAergic neurons located in the VLPAG/dDpMe.

The physiological role of orexin/hypocretin neurons in the regulation of sleep/wakefulness and neuroendocrine functions

The hypothalamus monitors body homeostasis and regulates various behaviors such as feeding, thermogenesis, and sleeping. Orexins (also known as hypocretins) were identified as endogenous ligands for two orphan G-protein-coupled receptors in the lateral hypothalamic area. They were initially recognized as regulators of feeding behavior, but they are mainly regarded as key modulators of the sleep/wakefulness cycle. Orexins activate orexin neurons, monoaminergic and cholinergic neurons in the hypothalamus/brainstem regions, to maintain a long, consolidated awake period. Anatomical studies of neural projections from/to orexin neurons and phenotypic characterization of transgenic mice revealed various roles for orexin neurons in the coordination of emotion, energy homeostasis, reward system, and arousal. For example, orexin neurons are regulated by peripheral metabolic cues, including ghrelin, leptin, and glucose concentration. This suggests that they may provide a link between energy homeostasis and arousal states. A link between the limbic system and orexin neurons might be important for increasing vigilance during emotional stimuli. Orexins are also involved in reward systems and the mechanisms of drug addiction. These findings suggest that orexin neurons sense the outer and inner environment of the body and maintain the proper wakefulness level of animals for survival. This review discusses the mechanism by which orexins maintain sleep/wakefulness states and how this mechanism relates to other systems that regulate emotion, reward, and energy homeostasis.

Optogenetics in neuroscience: what we gain from studies in mammals

Optogenetics is a newly-introduced technology in the life sciences and is gaining increasing attention. It refers to the combination of optical technologies and genetic methods to control the activity of specific cell groups in living tissue, during which high-resolution spatial and temporal manipulation of cells is achieved. Optogenetics has been applied to numerous regions, including cerebral cortex, hippocampus, ventral tegmental area, nucleus accumbens, striatum, spinal cord, and retina, and has revealed new directions of research in neuroscience and the treatment of related diseases. Since optogenetic tools are controllable at high spatial and temporal resolution, we discuss its applications in these regions in detail and the recent understanding of higher brain functions, such as reward-seeking, learning and memory, and sleep. Further, the possibilities of improved utility of this newly-emerging technology are discussed. We intend to provide a paradigm of the latest advances in neuroscience using optogenetics.

Ipsilateral feeding-specific circuits between the nucleus accumbens shell and the lateral hypothalamus: regulation by glutamate and GABA receptor subtypes

The nucleus accumbens shell (AcbSh) and the lateral hypothalamus (LH) are both involved in the control of food intake. Activation of GABA(A) receptors or blockade of AMPA and kainate receptors within the AcbSh induces feeding, as does blockade of GABA(A) receptors or activation of NMDA receptors in the LH. Further, evidence suggests that feeding induced via the AcbSh can be suppressed by LH inhibition. However, it is unclear if this suppression is specific to feeding. Adult male Sprague-Dawley rats with 3 intracranial guide cannulas, one unilaterally into the AcbSh and two bilaterally into the LH, were used to explore this issue. DNQX (1.25 μg) or muscimol (100 ng) infused into the AcbSh unilaterally elicited feeding, and this elicited intake was suppressed by bilateral LH injection of d-AP5 (2 μg) or muscimol (25 ng). The effectiveness of d-AP5 or muscimol infusion into either the LH site ipsilateral or contralateral to the AcbSh injection was compared. Ipsilateral LH injection of d-AP5 or muscimol was significantly more effective than contralateral injection in suppressing food intake initiated by AcbSh injection of DNQX or muscimol. These results add to the prior evidence that inhibition of the LH through pharmacological modulation of NMDA or GABA(A) receptors specifically suppresses feeding initiated by AcbSh inhibition, and that these two regions communicate via an ipsilateral circuit to specifically regulate feeding.

Optogenetic probing of fast glutamatergic transmission from hypocretin/orexin to histamine neurons in situ

Hypothalamic hypocretin/orexin (hcrt/orx) neurons coordinate sleep-wake cycles, reward seeking, and body energy balance. Neurochemical data suggest that hcrt/orx cells contain several transmitters, but what hcrt/orx cells release onto their projection targets is unknown. A major pathway by which hcrt/orx neurons are thought to promote arousal is through projections to tuberomammillary histamine (HA) neurons. To study the impact of the electrical activity in hcrt/orx cells on HA neurons, we genetically targeted the light-activated excitatory ion channel channelrhodopsin-2 (ChR2) to the plasma membrane of hcrt/orx cells, and performed patch-clamp recordings from HA cells in acute mouse brain slices. Stimulation of ChR2-containing fibers with millisecond flashes of blue light produced fast postsynaptic currents in HA neurons, with a high connection probability (≈60% of HA cells were connected to ≈40% of hcrt/orx cells expressing ChR2). These inputs depended on tetrodotoxin-sensitive action potentials, had kinetics typical of glutamatergic responses mediated by AMPA receptors, were blocked by the AMPA receptor blocker CNQX, and displayed multiple forms of short-term plasticity (depression in ≈70% trials, facilitation in ≈30% trials, both often in the same cell). Furthermore, stimulation of hcrt/orx axons at physiological frequencies rapidly and reversibly increased action potential firing in HA cells, an effect that was abolished by blockade of AMPA receptors. These results provide the first functional evidence that hcrt/orx neurons are capable of fast glutamatergic control of their projection targets, and suggest that variations in electrical activity of hcrt/orx axons can induce rapid changes in long-range signals generated by HA neurons.

Recent developments in optical neuromodulation technologies

The emergence of optogenetics technology facilitated widespread applications for interrogation of complex neural networks, such as activation of specific axonal pathways, previously found impossible with electrical stimulation. Consequently, within the short period of its application in neuroscience research, optogenetics has led to findings of significant importance both during normal brain function as well as in disease. Moreover, the optimization of optogenetics for in vivo studies has allowed the control of certain behavioral responses such as motility, reflex, and sensory responses, as well as more complex emotional and cognitive behaviors such as decision-making, reward seeking, and social behavior in freely moving animals. These studies have produced a wide variety of animal models that have resulted in fundamental findings and enhanced our understanding of the neural networks associated with behavior. The increasing number of opsins available for this technique enabled even broader regulation of neuronal activity. These advancements highlight the potential of this technique for future treatment of human diseases. Here, we provide an overview of the recent developments in the field of optogenetics technology that are relevant for a better understanding of several neuropsychiatric and neurodegenerative disorders and may pave the way for future therapeutic interventions.

Physiology of the orexinergic/hypocretinergic system: a revisit in 2012

The neuropeptides orexins and their G protein-coupled receptors, OX(1) and OX(2), were discovered in 1998, and since then, their role has been investigated in many functions mediated by the central nervous system, including sleep and wakefulness, appetite/metabolism, stress response, reward/addiction, and analgesia. Orexins also have peripheral actions of less clear physiological significance still. Cellular responses to the orexin receptor activity are highly diverse. The receptors couple to at least three families of heterotrimeric G proteins and other proteins that ultimately regulate entities such as phospholipases and kinases, which impact on neuronal excitation, synaptic plasticity, and cell death. This article is a 10-year update of my previous review on the physiology of the orexinergic/hypocretinergic system. I seek to provide a comprehensive update of orexin physiology that spans from the molecular players in orexin receptor signaling to the systemic responses yet emphasizing the cellular physiological aspects of this system.

Sleep disturbances in Alzheimer's and Parkinson's diseases

Alzheimer's disease (AD) and Parkinson's disease (PD) are the two most common neurodegenerative disorders and exact a burden on our society greater than cardiovascular disease and cancer combined. While cognitive and motor symptoms are used to define AD and PD, respectively, patients with both disorders exhibit sleep disturbances including insomnia, hypersomnia and excessive daytime napping. The molecular basis of perturbed sleep in AD and PD may involve damage to hypothalamic and brainstem nuclei that control sleep-wake cycles. Perturbations in neurotransmitter and hormone signaling (e.g., serotonin, norepinephrine and melatonin) and the neurotrophic factor BDNF likely contribute to the disease process. Abnormal accumulations of neurotoxic forms of amyloid β-peptide, tau and α-synuclein occur in brain regions involved in the regulation of sleep in AD and PD patients, and are sufficient to cause sleep disturbances in animal models of these neurodegenerative disorders. Disturbed regulation of sleep often occurs early in the course of AD and PD, and may contribute to the cognitive and motor symptoms. Treatments that target signaling pathways that control sleep have been shown to retard the disease process in animal models of AD and PD, suggesting a potential for such interventions in humans at risk for or in the early stages of these disorders.

A glass-coated tungsten microelectrode enclosing optical fibers for optogenetic exploration in primate deep brain structures

The optogenetic approach to primate brain circuitry has unparalleled potential for uncovering genetically and temporally resolved neuronal mechanisms of higher brain functions. In order to optogenetically investigate the large and complex primate brain, an optical-/electrical probe, or "optrode", must be inserted deeply, which requires the optrode to be not only long and stiff, but also sharp and smooth to reduce possible tissue damage. This study presents a tungsten microelectrode-based optrode that encloses optical fibers within its insulation glass. Optical fibers and a tungsten wire were tightly bound to each other and integrally coated with a smooth, thin layer of glass. This design satisfied the structural requirements for use in deep brain structures. The performance of the optrode was then examined in the thalamus of the rat and macaque monkeys which were injected with lentiviral vectors carrying the channelrhodopsin-2-enhanced yellow fluorescent protein (ChR2-EYFP) transgene. With fluorescence measurements via the optical fiber, ChR2-EYFP expression was detected clearly in vivo, which was confirmed by histological analysis in the rat. With photostimulation and extracellular recording, photo-responsive single-unit activities were isolated in the monkeys. The depth distribution of these units and the peak of the EYFP fluorescence profile overlapped consistently with each other. Thus, by developing a new probe, optogenetic methodology was successfully applied to a primate subcortical structure. This smooth glass-coated optrode is a promising tool for chronic in vivo experiments with various research targets including deep brain structures in behaving monkeys.

Hypocretin-1 receptors regulate the reinforcing and reward-enhancing effects of cocaine: pharmacological and behavioral genetics evidence

Considerable evidence suggests that transmission at hypocretin-1 (orexin-1) receptors (Hcrt-R1) plays an important role in the reinstatement of extinguished cocaine-seeking behaviors in rodents. However, far less is known about the role for hypocretin transmission in regulating ongoing cocaine-taking behavior. Here, we investigated the effects of the selective Hcrt-R1 antagonist SB-334867 on cocaine intake, as measured by intravenous (IV) cocaine self-administration in rats. The stimulatory effects of cocaine on brain reward systems contribute to the establishment and maintenance of cocaine-taking behaviors. Therefore, we also assessed the effects of SB-334867 on the reward-enhancing properties of cocaine, as measured by cocaine-induced lowering of intracranial self-stimulation (ICSS) thresholds. Finally, to definitively establish a role for Hcrt-R1 in regulating cocaine intake, we assessed IV cocaine self-administration in Hcrt-R1 knockout mice. We found that SB-334867 (1-4 mg/kg) dose-dependently decreased cocaine (0.5 mg/kg/infusion) self-administration in rats but did not alter responding for food rewards under the same schedule of reinforcement. This suggests that SB-334867 decreased cocaine reinforcement without negatively impacting operant performance. SB-334867 (1-4 mg/kg) also dose-dependently attenuated the stimulatory effects of cocaine (10 mg/kg) on brain reward systems, as measured by reversal of cocaine-induced lowering of ICSS thresholds in rats. Finally, we found that Hcrt-R1 knockout mice self-administered far less cocaine than wildtype mice across the entire dose-response function. These data demonstrate that Hcrt-R1 play an important role in regulating the reinforcing and reward-enhancing properties of cocaine and suggest that hypocretin transmission is likely essential for establishing and maintaining the cocaine habit in human addicts.

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.

Shining light on wakefulness and arousal

Alterations in arousal states are associated with multiple neuropsychiatric disorders, including generalized anxiety disorders, addiction, schizophrenia, and depression. Therefore, elucidating the neurobiological mechanisms controlling the boundaries between arousal, hyperarousal, and hypoarousal is a crucial endeavor in biological psychiatry. Substantial research over several decades has identified distinct arousal-promoting neural populations in the brain; however, how these nuclei act individually and collectively to promote and maintain wakefulness and various arousal states is unknown. We have recently applied optogenetic technology to the repertoire of techniques used to study arousal. Here, we discuss the recent results of these experiments and propose future use of this approach as a way to understand the complex dynamics of neural circuits controlling arousal and arousal-related behaviors.

A toolbox of Cre-dependent optogenetic transgenic mice for light-induced activation and silencing

Cell-type-specific expression of optogenetic molecules allows temporally precise manipulation of targeted neuronal activity. Here we present a toolbox of 4 knock-in mouse lines engineered for strong, Cre-dependent expression of channelrhodopsins ChR2-tdTomato and ChR2-EYFP, halorhodopsin eNpHR3.0, and archaerhodopsin Arch-ER2. All 4 transgenes mediate Cre-dependent, robust activation or silencing of cortical pyramidal neurons in vitro and in vivo upon light stimulation, with ChR2-EYFP and Arch-ER2 demonstrating light sensitivity approaching that of in utero or virally transduced neurons. We further show specific photoactivation of parvalbumin-positive interneurons in behaving ChR2-EYFP reporter mice. The robust, consistent, and inducible nature of our ChR2 mice represents a significant advancement over previous lines, whereas the Arch-ER2 and eNpHR3.0 mice are the first demonstration of successful conditional transgenic optogenetic silencing. When combined with the hundreds of available Cre-driver lines, this optimized toolbox of reporter mice will enable widespread investigations of neural circuit function with unprecedented reliability and accuracy.

Light-induced silencing of neural activity in Rosa26 knock-in mice conditionally expressing the microbial halorhodopsin eNpHR2.0

Temporally precise inhibition of genetically defined cell populations in intact nervous systems has been enabled by the microbial halorhodopsin NpHR, a fast, light-activated chloride pump. Here, we report the generation of new mouse strains that express eNpHR2-EYFP fusion proteins after Cre- and/or Flp-mediated recombination to silence neural activity in vivo. In these mouse strains, Cre/Flp recombination induced a high-level of eNpHR2-EYFP expression. Slice whole-cell patch clamp experiments confirmed that eNpHR2-EYFP-expressing neurons could be optically hyperpolarized and inhibited from firing action potentials. Thus, these mouse strains offer powerful tools for light-induced silencing of neural activity in genetically defined cell populations.

5-HT1A receptor-responsive pedunculopontine tegmental neurons suppress REM sleep and respiratory motor activity

Serotonin type 1A (5-HT(1A)) receptor-responsive neurons in the pedunculopontine tegmental nucleus (PPTn) become maximally active immediately before and during rapid eye movement (REM) sleep. A prevailing model of REM sleep generation indicates that activation of such neurons contributes significantly to the generation of REM sleep, and if correct then inactivation of such neurons ought to suppress REM sleep. We test this hypothesis using bilateral microperfusion of the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 10 μm) into the PPTn; this tool has been shown to selectively silence REM sleep-active PPTn neurons while the activity of wake/REM sleep-active PPTn neurons is unaffected. Contrary to the prevailing model, bilateral microperfusion of 8-OH-DPAT into the PPTn (n = 23 rats) significantly increased REM sleep both as a percentage of the total recording time and sleep time, compared with both within-animal vehicle controls and between-animal time-controls. This increased REM sleep resulted from an increased frequency of REM sleep bouts but not their duration, indicating an effect on mechanisms of REM sleep initiation but not maintenance. Furthermore, an increased proportion of the REM sleep bouts stemmed from periods of low REM sleep drive quantified electrographically. Targeted suppression of 5-HT(1A) receptor-responsive PPTn neurons also increased respiratory rate and respiratory-related genioglossus activity, and increased the frequency and amplitude of the sporadic genioglossus activations occurring during REM sleep. These data indicate that 5-HT(1A) receptor-responsive PPTn neurons normally function to restrain REM sleep by elevating the drive threshold for REM sleep induction, and restrain the expression of respiratory rate and motor activities.

Energy expenditure: role of orexin

The orexins/hypocretins are endogenous, modulatory and multifunctional neuropeptides with prominent influence on several physiological processes. The influence of orexins on energy expenditure is highlighted with focus on orexin action on individual components of energy expenditure. As orexin stabilizes and maintains normal states of arousal and the sleep/wake cycle, we also highlight orexin mediation of sleep and how sleep interacts with energy expenditure.

Orexin neurons and emotional stress

Stress increases cardiac function, ventilation, and body temperature and induces analgesia. These changes, which result in an increase in metabolic rate, oxygen supply, and the conduction velocity of nerve impulses, prepare the body for a fight-or-flight response. A part of the hypothalamus called the defense area has long been known to play a key role in these responses, but the precise mechanisms are largely unknown. Our recent findings suggest that orexin (hypocretin) neurons act as a master switch of the fight-or-flight response. In addition, our results, as well as those from other researchers, suggest that orexin neurons do not modulate specific behaviors such as the fight-or-flight responses but rather integrate the autonomic functions and behaviors in a broad sense or in a vigilance state-dependent manner. The orexin system seems to be a pivotal link between the subconscious and the conscious brain functions.

Genetically encoded molecular tools for light-driven silencing of targeted neurons

The ability to silence, in a temporally precise fashion, the electrical activity of specific neurons embedded within intact brain tissue, is important for understanding the role that those neurons play in behaviors, brain disorders, and neural computations. "Optogenetic" silencers, genetically encoded molecules that, when expressed in targeted cells within neural networks, enable their electrical activity to be quieted in response to pulses of light, are enabling these kinds of causal circuit analyses studies. Two major classes of optogenetic silencer are in broad use in species ranging from worm to monkey: light-driven inward chloride pumps, or halorhodopsins, and light-driven outward proton pumps, such as archaerhodopsins and fungal light-driven proton pumps. Both classes of molecule, when expressed in neurons via viral or other transgenic means, enable the targeted neurons to be hyperpolarized by light. We here review the current status of these sets of molecules, and discuss how they are being discovered and engineered. We also discuss their expression properties, ionic properties, spectral characteristics, and kinetics. Such tools may not only find many uses in the quieting of electrical activity for basic science studies but may also, in the future, find clinical uses for their ability to safely and transiently shut down cellular electrical activity in a precise fashion.

Mouse transgenic approaches in optogenetics

A major challenge in neuroscience is to understand how universal behaviors, such as sensation, movement, cognition, and emotion, arise from the interactions of specific cells that are present within intricate neural networks in the brain. Dissection of such complex networks has typically relied on disturbing the activity of individual gene products, perturbing neuronal activities pharmacologically, or lesioning specific brain regions, to investigate the network's response in a behavioral output. Though informative for many kinds of studies, these approaches are not sufficiently fine-tuned for examining the functionality of specific cells or cell classes in a spatially or temporally restricted context. Recent advances in the field of optogenetics now enable researchers to monitor and manipulate the activity of genetically defined cell populations with the speed and precision uniquely afforded by light. Transgenic mice engineered to express optogenetic tools in a cell type-specific manner offer a powerful approach for examining the role of particular cells in discrete circuits in a defined and reproducible way. Not surprisingly then, recent years have seen substantial efforts directed toward generating transgenic mouse lines that express functionally relevant levels of optogenetic tools. In this chapter, we review the state of these efforts and consider aspects of the current technology that would benefit from additional improvement.

Optogenetic tools for analyzing the neural circuits of behavior

In order to understand how the brain generates behaviors, it is important to be able to determine how neural circuits work together to perform computations. Because neural circuits are made of a great diversity of cell types, it is critical to be able to analyze how these different kinds of cell work together. In recent years, a toolbox of fully genetically encoded molecules has emerged that, when expressed in specific neurons, enables the electrical activity of the targeted neurons to be controlled in a temporally precise fashion by pulses of light. We describe this optogenetic toolbox, how it can be used to analyze neural circuits in the brain and how optogenetics is impacting the study of cognition.