Additive wake-promoting actions of medial basal forebrain noradrenergic alpha1- and beta-receptor stimulation

Heterogeneous organization of Locus coeruleus: An intrinsic mechanism for functional complexity

Locus coeruleus (LC) is a small nucleus located deep in the brainstem that contains the majority of central noradrenergic neurons, which provide the primary source of noradrenaline (NA) throughout the entire central nervous system (CNS).The release of neurotransmitter NA is considered to modulate arousal, sensory processing, attention, aversive and adaptive stress responses as well as high-order cognitive function and memory, with the highly ramified axonal arborizations of LC-NA neurons sending wide projections to the targeted brain areas. For over 30 years, LC was thought to be a homogeneous nucleus in structure and function due to the widespread uniform release of NA by LC-NA neurons and simultaneous action in several CNS regions, such as the prefrontal cortex, hippocampus, cerebellum, and spinal cord. However, recent advances in neuroscience tools have revealed that LC is probably not so homogeneous as we previous thought and exhibits heterogeneity in various aspects. Accumulating studies have shown that the functional complexity of LC may be attributed to its heterogeneity in developmental origin, projection patterns, topography distribution, morphology and molecular organization, electrophysiological properties and sex differences. This review will highlight the heterogeneity of LC and its critical role in modulating diverse behavioral outcomes.

Behavioral and Transcriptomic Changes Following Brain-Specific Loss of Noradrenergic Transmission

Noradrenaline (NE) plays an integral role in shaping behavioral outcomes including anxiety/depression, fear, learning and memory, attention and shifting behavior, sleep-wake state, pain, and addiction. However, it is unclear whether dysregulation of NE release is a cause or a consequence of maladaptive orientations of these behaviors, many of which associated with psychiatric disorders. To address this question, we used a unique genetic model in which the brain-specific vesicular monoamine transporter-2 (VMAT2) gene expression was removed in NE-positive neurons disabling NE release in the entire brain. We engineered VMAT2 gene splicing and NE depletion by crossing floxed VMAT2 mice with mice expressing the Cre-recombinase under the dopamine β-hydroxylase (DBH) gene promotor. In this study, we performed a comprehensive behavioral and transcriptomic characterization of the VMAT2DBHcre KO mice to evaluate the role of central NE in behavioral modulations. We demonstrated that NE depletion induces anxiolytic and antidepressant-like effects, improves contextual fear memory, alters shifting behavior, decreases the locomotor response to amphetamine, and induces deeper sleep during the non-rapid eye movement (NREM) phase. In contrast, NE depletion did not affect spatial learning and memory, working memory, response to cocaine, and the architecture of the sleep-wake cycle. Finally, we used this model to identify genes that could be up- or down-regulated in the absence of NE release. We found an up-regulation of the synaptic vesicle glycoprotein 2c (SV2c) gene expression in several brain regions, including the locus coeruleus (LC), and were able to validate this up-regulation as a marker of vulnerability to chronic social defeat. The NE system is a complex and challenging system involved in many behavioral orientations given it brain wide distribution. In our study, we unraveled specific role of NE neurotransmission in multiple behavior and link it to molecular underpinning, opening future direction to understand NE role in health and disease.

Rapid Effects of Vagus Nerve Stimulation on Sensory Processing Through Activation of Neuromodulatory Systems

After sensory information is encoded into neural signals at the periphery, it is processed through multiple brain regions before perception occurs (i.e., sensory processing). Recent work has begun to tease apart how neuromodulatory systems influence sensory processing. Vagus nerve stimulation (VNS) is well-known as an effective and safe method of activating neuromodulatory systems. There is a growing body of studies confirming VNS has immediate effects on sensory processing across multiple sensory modalities. These immediate effects of VNS on sensory processing are distinct from the more well-documented method of inducing lasting neuroplastic changes to the sensory pathways through repeatedly delivering a brief VNS burst paired with a sensory stimulus. Immediate effects occur upon VNS onset, often disappear upon VNS offset, and the modulation is present for all sensory stimuli. Conversely, the neuroplastic effect of pairing sub-second bursts of VNS with a sensory stimulus alters sensory processing only after multiple pairing sessions, this alteration remains after cessation of pairing sessions, and the alteration selectively affects the response properties of neurons encoding the specific paired sensory stimulus. Here, we call attention to the immediate effects VNS has on sensory processing. This review discusses existing studies on this topic, provides an overview of the underlying neuromodulatory systems that likely play a role, and briefly explores the potential translational applications of using VNS to rapidly regulate sensory processing.

Adrenergic α2 receptors are implicated in seizure-induced respiratory arrest in DBA/1 mice

AIMS

Sudden unexpected death in epilepsy (SUDEP) is a serious and underestimated public health burden. Both clinical and animal studies show that seizure-induced respiratory arrest (S-IRA) is the primary cause of death in SUDEP. Our previous studies demonstrated that atomoxetine, a norepinephrine reuptake inhibitor (NRI), suppresses S-IRA in DBA/1 mice, suggesting that noradrenergic neurotransmission modulates S-IRA. However, it remains unclear which adrenoceptors are implicated in S-IRA in DBA/1 mice.

MATERIALS AND METHODS

Naïve DBA/1 mice exhibit a low incidence of S-IRA, but after primed by acoustic stimulation, they become consistently susceptible to S-IRA. Atomoxetine, adrenoceptor agonists, antagonists or vehicle was intraperitoneally (i.p.) administered alone or in combination, and the effects of drug treatments on S-IRA incidence and seizure behaviors were examined.

KEY FINDINGS

The incidence of S-IRA in primed DBA/1 mice was significantly reduced by clonidine, an α2 adrenoceptor agonist, as compared with that of the vehicle control. However, compared with the vehicle control, S-IRA was not altered by cirazoline, an α1 agonist. Consistent with previous reports, atomoxetine reduced S-IRA in primed DBA/1 mice. The suppressing effect of atomoxetine on S-IRA was prevented by injection of an α2 adrenoceptor antagonist, yohimbine or atipamezole, but not by prazosin, an α1 antagonist. Administration of α1 or α2 antagonists alone did not promote the incidence of S-IRA in nonprimed DBA/1 mice.

SIGNIFICANCE

These data demonstrate that noradrenergic neurotransmission modulates S-IRA predominantly via α2 adrenoceptors in DBA/1 mice, indicating that selective activation of α2 adrenoceptors can potentially prevent SUDEP.

Noninvasive vagus nerve stimulation in Parkinson's disease: current status and future prospects

INTRODUCTION

Parkinson's disease (PD) is a common progressive neurodegenerative disorder with multifactorial etiology. While dopaminergic medication is the standard therapy in PD, it provides limited symptomatic treatment and non-pharmacological interventions are currently being trialed.

AREAS COVERED

Recent pathophysiological theories of Parkinson's suggest that aggregated α-synuclein form in the gut and spread to nuclei in the brainstem via autonomic connections. In this paper, we review the novel hypothesis that noninvasive vagus nerve stimulation (nVNS), targeting efferent and afferent vagal projections, is a promising therapeutic tool to improve gait and cognitive control and ameliorate non-motor symptoms in people with Parkinson's. We conducted an unstructured search of the literature for any studies employing nVNS in PD as well as for studies examining the efficacy of nVNS on improving cognitive function and where nVNS has been applied to co-occurring conditions in PD.

EXPERT OPINION

Evidence of nVNS as a novel therapeutic to improve gait in PD is preliminary, but early signs indicate the possibility that nVNS may be useful to target dopa-resistant gait characteristics in early PD. The evidence for nVNS as a therapeutic tool is, however, limited and further studies are needed in both brain health and disease.

Noradrenergic Transmission at Alpha1-Adrenergic Receptors in the Ventral Periaqueductal Gray Modulates Arousal

BACKGROUND

Dysregulation of arousal is symptomatic of numerous psychiatric disorders. Previous research has shown that the activity of dopamine (DA) neurons in the ventral periaqueductal gray (vPAG) tracks with arousal state, and lesions of vPAGDA cells increase sleep. However, the circuitry controlling these wake-promoting DA neurons is unknown.

METHODS

This study combined designer receptors exclusively activated by designer drugs (DREADDs), behavioral pharmacology, electrophysiology, and immunoelectron microscopy in male and female mice to elucidate mechanisms in the vPAG that promote arousal.

RESULTS

Activation of locus coeruleus projections to the vPAG or vPAGDA neurons induced by DREADDs promoted arousal. Similarly, agonist stimulation of vPAG alpha1-adrenergic receptors (α1ARs) increased latency to fall asleep, whereas α1AR blockade had the opposite effect. α1AR stimulation drove vPAGDA activity in a glutamate-dependent, action potential-independent manner. Compared with other dopaminergic brain regions, α1ARs were enriched on astrocytes in the vPAG, and mimicking α1AR transmission specifically in vPAG astrocytes via Gq-DREADDS was sufficient to increase arousal. In general, the wake-promoting effects observed were not accompanied by hyperactivity.

CONCLUSIONS

These experiments revealed that vPAG α1ARs increase arousal, promote glutamatergic input onto vPAGDA neurons, and are abundantly expressed on astrocytes. Activation of locus coeruleus inputs, vPAG astrocytes, or vPAGDA neurons increase sleep latency but do not produce hyperactivity. Together, these results support an arousal circuit whereby noradrenergic transmission at astrocytic α1ARs activates wake-promoting vPAGDA neurons via glutamate transmission.

Organization of the locus coeruleus-norepinephrine system

The release of the neurotransmitter norepinephrine throughout the mammalian brain is important for modulating attention, arousal, and cognition during many behaviors. Furthermore, disruption of norepinephrine-mediated signaling is strongly associated with several psychiatric and neurodegenerative disorders in humans, emphasizing the clinical importance of this system. Most of the norepinephrine released in the brain is supplied by a very small, bilateral nucleus in the brainstem called the locus coeruleus. The goal of this minireview is to emphasize the complexity of the locus coeruleus beyond its primary definition as a norepinephrine-producing nucleus. Several recent studies utilizing innovative technologies highlight how the locus coeruleus-norepinephrine system can now be targeted with increased accuracy and resolution, in order to better understand its role in modulating diverse behaviors.

Norepinephrine at the nexus of arousal, motivation and relapse

Arousal plays a critical role in cognitive, affective and motivational processes. Consistent with this, the dysregulation of arousal-related neural systems is implicated in a variety of psychiatric disorders, including addiction. Noradrenergic systems exert potent arousal-enhancing actions that involve signaling at α1- and β-noradrenergic receptors within a distributed network of subcortical regions. The majority of research into noradrenergic modulation of arousal has focused on the nucleus locus coeruleus. Nevertheless, anatomical studies demonstrate that multiple noradrenergic nuclei innervate subcortical arousal-related regions, providing a substrate for differential regulation of arousal across these distinct noradrenergic nuclei. The arousal-promoting actions of psychostimulants and other drugs of abuse contribute to their widespread abuse. Moreover, relapse can be triggered by a variety of arousal-promoting events, including stress and re-exposure to drugs of abuse. Evidence has long-indicated that norepinephrine plays an important role in relapse. Recent observations suggest that noradrenergic signaling elicits affectively-neutral arousal that is sufficient to reinstate drug seeking. Collectively, these observations indicate that norepinephrine plays a key role in the interaction between arousal, motivation, and relapse. This article is part of a Special Issue entitled SI: Noradrenergic System.

Functional neuroanatomy of the central noradrenergic system

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

α(2A) adrenoceptor-mediated presynaptic inhibition of GABAergic transmission in rat tuberomammillary nucleus neurons

Histaminergic neurons within the tuberomammillary nucleus (TMN) play an important role in the regulation of sleep-wakefulness. Here, we report the adrenergic modulation of GABAergic transmission in rat TMN histaminergic neurons using a conventional whole-cell patch clamp technique. Norepinephrine (NE) reversibly decreased the amplitude of action potential-dependent GABAergic inhibitory post-synaptic currents (IPSCs) and increased the paired pulse ratio. The NE-induced inhibition of GABAergic IPSCs was mimicked by clonidine, a selective α2 adrenoceptor agonist. However, cirazoline and isoproterenol, nonselective α1 and β adrenoceptor agonists, respectively, had no effect on GABAergic IPSCs. The NE-induced inhibition of GABAergic IPSCs was significantly blocked by BRL44408, a selective α2A adrenoceptor antagonist, but not imiloxan or JP1302, a selective α2B and α2C adrenoceptor antagonists. The extent of NE-induced inhibition of GABAergic IPSCs was inversely proportional to the extracellular Ca(2+) concentration. Pharmacological agents affecting the activities of adenylyl cyclase or G-protein-coupled inwardly rectifying K(+) channels did not affect the NE-induced inhibition of GABAergic IPSCs. However, NE had no effect on the frequency and amplitude of GABAergic miniature IPSCs. These results suggest that NE acts on presynaptic α2A adrenoceptor to inhibit action potential-dependent GABA release via the inhibition of Ca(2+) influx from the extracellular space to GABAergic nerve terminals, and that this α2A adrenoceptor-mediated modulation of GABAergic transmission may be involved in regulating the excitability of TMN histaminergic neurons.

Psychostimulants and motivated behavior: arousal and cognition

Motivated, goal-directed behavior requires the coordination of multiple behavioral processes that facilitate interacting with the environment, including arousal, motivation, and executive function. Psychostimulants exert potent modulatory influences on these processes, providing a useful tool for understanding the neurobiology of motivated behavior. The neural mechanisms underlying the reinforcing effects of psychostimulants have been extensively studied over the past 50 years. In contrast, the study of the neurobiology of the arousal-enhancing and executive-modulating actions of psychostimulants was only initiated relatively recently. This latter work identifies a series of dose-dependent actions of psychostimulants within a network of prefrontal cortical and subcortical sites that coordinate the arousal-promoting and cognition-modulating effects of these drugs. These actions are dependent on a variety of catecholamine receptor subtypes, including noradrenergic α1 and α2 receptors and dopaminergic D1 receptors. In the prefrontal cortex, psychostimulants exert inverted-U shaped modulatory actions that are apparent at the levels of the neuron and behavior. Collectively, these observations provide new insight into the neurobiology underlying motivated, goal-directed behavior.

Neuropharmacology of Sleep and Wakefulness: 2012 Update

The development of sedative/hypnotic molecules has been empiric rather than rational. The empiric approach has produced clinically useful drugs but for no drug is the mechanism of action completely understood. All available sedative/hypnotic medications have unwanted side effects and none of these medications creates a sleep architecture that is identical to the architecture of naturally occurring sleep. This chapter reviews recent advances in research aiming to elucidate the neurochemical mechanisms regulating sleep and wakefulness. One promise of rational drug design is that understanding the mechanisms of sedative/hypnotic action will significantly enhance drug safety and efficacy.

Loss of Gnas imprinting differentially affects REM/NREM sleep and cognition in mice

It has been suggested that imprinted genes are important in the regulation of sleep. However, the fundamental question of whether genomic imprinting has a role in sleep has remained elusive up to now. In this work we show that REM and NREM sleep states are differentially modulated by the maternally expressed imprinted gene Gnas. In particular, in mice with loss of imprinting of Gnas, NREM and complex cognitive processes are enhanced while REM and REM-linked behaviors are inhibited. This is the first demonstration that a specific overexpression of an imprinted gene affects sleep states and related complex behavioral traits. Furthermore, in parallel to the Gnas overexpression, we have observed an overexpression of Ucp1 in interscapular brown adipose tissue (BAT) and a significant increase in thermoregulation that may account for the REM/NREM sleep phenotypes. We conclude that there must be significant evolutionary advantages in the monoallelic expression of Gnas for REM sleep and for the consolidation of REM-dependent memories. Conversely, biallelic expression of Gnas reinforces slow wave activity in NREM sleep, and this results in a reduction of uncertainty in temporal decision-making processes.

Noradrenergic modulation of wakefulness/arousal

The locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. State-dependent neuronal discharge activity of locus coeruleus noradrenergic neurons has long-suggested a role of this system in the induction of an alert waking state. Work over the past two decades provides unambiguous evidence that the locus coeruleus, and likely other noradrenergic nuclei, exert potent wake-promoting actions via an activation of noradrenergic β- and α₁-receptors located within multiple subcortical structures, including the general regions of the medial septal area, the medial preoptic area and, most recently, the lateral hypothalamus. Conversely, global blockade of β- and α₁-receptors or suppression of norepinephrine release results in profound sedation. The wake-promoting action of central noradrenergic neurotransmission has clinical implications for treatment of sleep/arousal disorders, such as insomnia and narcolepsy, and clinical conditions associated with excessive arousal, such as post-traumatic stress disorder.

Catecholamine influences on prefrontal cortical function: relevance to treatment of attention deficit/hyperactivity disorder and related disorders

The primary symptoms of attention deficit/hyperactivity disorder (ADHD) include poor impulse control and impaired regulation of attention. Research has shown that the prefrontal cortex (PFC) is essential for the "top-down" regulation of attention, behavior, and emotion, and that this brain region is underactive in many patients with ADHD. The PFC is known to be especially sensitive to its neurochemical environment; relatively small changes in the levels of norepinephrine and dopamine can produce significant changes in its function. Therefore, alterations in the pathways mediating catecholamine transmission can impair PFC function, while medications that optimize catecholamine actions can improve PFC regulation of attention, behavior, and emotion. This article reviews studies in animals showing that norepinephrine and dopamine enhance PFC function through actions at postsynaptic α(2A)-adrenoceptors and dopamine D1-receptors, respectively. Stimulant medications and atomoxetine appear to enhance PFC function through increasing endogenous adrenergic and dopaminergic stimulation of α(2A)-receptors and D1-receptors. In contrast, guanfacine mimics the enhancing effects of norepinephrine at postsynaptic α(2A)-receptors in the PFC, strengthening network connectivity. Stronger PFC regulation of attention, behavior, and emotion likely contributes to the therapeutic effects of these medications for the treatment of ADHD.

Sleep neurobiology from a clinical perspective

Many neurochemical systems interact to generate wakefulness and sleep. Wakefulness is promoted by neurons in the pons, midbrain, and posterior hypothalamus that produce acetylcholine, norepinephrine, dopamine, serotonin, histamine, and orexin/hypocretin. Most of these ascending arousal systems diffusely activate the cortex and other forebrain targets. NREM sleep is mainly driven by neurons in the preoptic area that inhibit the ascending arousal systems, while REM sleep is regulated primarily by neurons in the pons, with additional influence arising in the hypothalamus. Mutual inhibition between these wake- and sleep-regulating regions likely helps generate full wakefulness and sleep with rapid transitions between states. This up-to-date review of these systems should allow clinicians and researchers to better understand the effects of drugs, lesions, and neurologic disease on sleep and wakefulness.

Discrete forebrain neuronal networks supporting noradrenergic regulation of sensorimotor gating

Prepulse inhibition (PPI) refers to the reduction in the startle response when a startling stimulus is preceded by a weak prestimulus, and is an endophenotype of deficient sensorimotor gating in several neuropsychiatric disorders. Emerging evidence suggests that norepinephrine (NE) regulates PPI, however, the circuitry involved is unknown. We found recently that stimulation of the locus coeruleus (LC), the primary source of NE to the forebrain, induces a PPI deficit that is a result of downstream NE release. Hence, this study sought to identify LC-innervated forebrain regions that mediate this effect. Separate groups of male Sprague-Dawley rats received a cocktail solution of the α1-NE receptor agonist phenylephrine plus the β-receptor agonist isoproterenol (equal parts of each; 0, 3, 10, and 30 μg) into subregions of the medial prefrontal cortex (mPFC), nucleus accumbens (NAcc), extended amygdala, mediodorsal thalamus (MD-thalamus), or the dorsal hippocampus (DH) before PPI testing. NE agonist infusion into the posterior mPFC, NAcc shell, bed nucleus of the stria terminalis, basolateral amygdala, and the MD-thalamus disrupted PPI, with particularly strong effects in MD-thalamus. Sites in which NE receptor stimulation did not disrupt PPI (anterior mPFC, NAcc core, central amygdala, and DH) did support PPI disruptions with the dopamine D2 receptor agonist quinpirole (0, 10 μg). This pattern reveals new pathways in the regulation of PPI, and suggests that NE transmission within distinct thalamocortical and ventral forebrain networks may subserve the sensorimotor gating deficits that are seen in disorders such as schizophrenia, Tourette syndrome, and post-traumatic stress disorder.

Neuropharmacology of Sleep and Wakefulness

The development of sedative/hypnotic molecules has been empiric rather than rational. The empiric approach has produced clinically useful drugs but for no drug is the mechanism of action completely understood. All available sedative/hypnotic medications have unwanted side effects and none of these medications creates a sleep architecture that is identical to the architecture of naturally occurring sleep. This chapter reviews recent advances in research aiming to elucidate the neurochemical mechanisms regulating sleep and wakefulness. One promise of rational drug design is that understanding the mechanisms of sedative/hypnotic action will significantly enhance drug safety and efficacy.

Functional neuroanatomy of the noradrenergic locus coeruleus: its roles in the regulation of arousal and autonomic function part I: principles of functional organisation

The locus coeruleus (LC) is the major noradrenergic nucleus of the brain, giving rise to fibres innervating extensive areas throughout the neuraxis. Recent advances in neuroscience have resulted in the unravelling of the neuronal circuits controlling a number of physiological functions in which the LC plays a central role. Two such functions are the regulation of arousal and autonomic activity, which are inseparably linked largely via the involvement of the LC. The LC is a major wakefulness-promoting nucleus, resulting from dense excitatory projections to the majority of the cerebral cortex, cholinergic neurones of the basal forebrain, cortically-projecting neurones of the thalamus, serotoninergic neurones of the dorsal raphe and cholinergic neurones of the pedunculopontine and laterodorsal tegmental nucleus, and substantial inhibitory projections to sleep-promoting GABAergic neurones of the basal forebrain and ventrolateral preoptic area. Activation of the LC thus results in the enhancement of alertness through the innervation of these varied nuclei. The importance of the LC in controlling autonomic function results from both direct projections to the spinal cord and projections to autonomic nuclei including the dorsal motor nucleus of the vagus, the nucleus ambiguus, the rostroventrolateral medulla, the Edinger-Westphal nucleus, the caudal raphe, the salivatory nuclei, the paraventricular nucleus, and the amygdala. LC activation produces an increase in sympathetic activity and a decrease in parasympathetic activity via these projections. Alterations in LC activity therefore result in complex patterns of neuronal activity throughout the brain, observed as changes in measures of arousal and autonomic function.

Narcolepsy and depression and the neurobiology of gammahydroxybutyrate

A voluminous literature describes the relationship between disturbed sleep and depression. The breakdown of sleep is one of the cardinal features of depression and often also heralds its onset. Frequent arousals, periods of wakefulness and a short sleep onset REM latency are typical polysomnographic features of depression. The short latency to REM sleep has been attributed to the combination of a monoaminergic deficiency and cholinergic supersensitivity and these irregularities have been proposed to form the biological basis of the disorder. A similar imbalance between monoaminergic and cholinergic neurotransmission has been found in narcolepsy, a condition in which frequent awakenings, periods of wakefulness and short sleep onset REM latencies are also characteristic findings during sleep. In many cases of narcolepsy, this imbalance appears to result from a deficiency of hypocretin but once established, whether in depression or narcolepsy, this disequilibrium sets the stage for the dissociation or premature appearance of REM sleep and for the dissociation of the motor inhibitory component of REM sleep or cataplexy. In the presence of this monoaminergic/cholinergic imbalance, gammahydroxybutyrate (GHB) may acutely further reduce the latency of REM sleep and induce cataplexy, in both patients with narcolepsy or depression. On the other hand, the repeated nocturnal application of GHB in patients with narcolepsy improves the continuity of sleep, prolongs the latency to REM sleep and prevents cataplexy. Evidence to date suggests that GHB may restore the normal balance between monoaminergic and cholinergic neurotransmission. As such, the repeated use of GHB at night and the stabilization of sleep over time makes GHB an effective treatment for narcolepsy and a potentially effective treatment for depression.

Behavioral responses of dopamine beta-hydroxylase knockout mice to modafinil suggest a dual noradrenergic-dopaminergic mechanism of action

Modafinil is approved for use in the treatment of excessive daytime sleepiness. The precise mechanism of modafinil action has not been elucidated, although both dopamine (DA) and norepinephrine (NE) systems have been implicated. To explore the roles of DA and NE in the mechanism of modafinil-induced arousal, dopamine beta-hydroxylase knockout (Dbh -/-) mice were examined in behavioral paradigms of arousal (photobeam breaks and behavioral scoring of sleep latency). Dbh -/- mice completely lack NE but have hypersensitive DA signaling. It was hypothesized that Dbh -/- mice would be unresponsive to modafinil if the compound acts primarily via NE, but would be hypersensitive to modafinil if it acts primarily via DA. Dbh -/- mice had increased sensitivity to the locomotor-activating and wake-promoting effects of modafinil. Paradoxically, the alpha1-adrenergic receptor antagonist, prazosin, attenuated the effects of modafinil in control mice, but not in Dbh -/- mice. Blockade of DA receptors with flupenthixol decreased modafinil-induced locomotion and wake in both control and Dbh -/- mice. These results suggest that both NE and DA are involved in the behavioral effects of modafinil in control mice, but the requirement for NE can be bypassed by hypersensitive DA signaling.

Noradrenergic modulation of arousal

Through a highly divergent efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. State-dependent neuronal discharge activity of locus coeruleus neurons has long-suggested a role of this system in the induction of an alert waking state. More recent work supports this hypothesis, demonstrating robust wake-promoting actions of the locus coeruleus-noradrenergic system. Norepinephrine enhances arousal, in part, via actions of beta- and alpha1-receptors located within multiple subcortical structures, including the general regions of the medial septal area and the medial preoptic areas. Recent anatomical studies suggest that arousal-enhancing actions of norepinephrine are not limited to the locus coeruleus system and likely include the A1 and A2 noradrenergic cell groups. Thus, noradrenergic modulation of arousal state involves multiple noradrenergic systems acting within multiple subcortical regions. Pharmacological studies indicate that the combined actions of these systems are necessary for the sustained maintenance of arousal levels associated with spontaneous waking. Enhanced arousal state is a prominent aspect of both stress and psychostimulant drug action and evidence indicates that noradrenergic systems likely play an important role in both stress-related and psychostimulant-induced arousal. These and other observations suggest that the dysregulation of noradrenergic neurotransmission could well contribute to the dysregulation of arousal associated with a variety of behavioral disorders including insomnia and stress-related disorders.

A final common pathway for depression? Progress toward a general conceptual framework

Functional neuroimaging studies of depressed patients have converged with functional brain mapping studies of depressed animals in showing that depression is accompanied by a hypoactivity of brain regions involved in positively motivated behavior together with a hyperactivity in regions involved in stress responses. Both sets of changes are reversed by diverse antidepressant treatments. It has been proposed that this neural pattern underlies the symptoms common to most forms of the depression, which are the loss of positively motivated behavior and increased stress. The paper discusses how this framework can organize diverse findings ranging from effects of monoamine neurotransmitters, cytokines, corticosteroids and neurotrophins on depression. The hypothesis leads to new insights concerning the relationship between the prolonged inactivity of the positive motivational network during a depressive episode and the loss of neurotrophic support, the potential antidepressant action of corticosteroid treatment, and to the key question of whether antidepressants act by inhibiting the activity of the stress network or by enhancing the activity of the positive motivational system.

A final common pathway for depression: implications for therapy

Depression in humans and animal models has been found to be accompanied by a hypoactivity of brain regions involved in positively motivated behavior together with a hyperactivity in regions involved in stress responses. Both sets of changes are reversed by diverse antidepressant treatments. It has been proposed that this neural pattern underlies the symptoms common to most forms of depression, which are the loss of positively motivated behavior and the increase in stress. The present paper discusses how this framework can organize diverse findings on the multiple factors associated with this disorder. The hypothesis suggests new therapeutic strategies involving treatment with low-dose corticosteroids to suppress the stress network or with antagonists of alpha(1A)- and agonists of alpha(1B)-adrenoceptors to disinhibit or activate the positive motivational network, respectively.

Is the cortisol awakening rise a response to awakening?

A distinct rise in cortisol levels that occurs after morning awakening is increasingly used as an indicator of adrenocortical activity which is associated with different pathologies. Although it was previously assumed that the transition from sleep to wake is essential for the occurrence of the cortisol morning rise, this has never been tested. Here, we examined 16 healthy young men (20-33 yrs) between 2300 and 0800 h under sleep laboratory conditions. Serum cortisol and plasma adrenocorticotropin (ACTH) as well as salivary cortisol levels (after subjects were woken up at 0700 h) were repeatedly assessed. In a supplementary study condition, salivary cortisol levels in the first hour after awakening were measured at the subjects' home on two consecutive days. Comparison of pre- and post awakening measurements revealed significantly steeper increases in cortisol and ACTH after awakening. The rise in cortisol upon awakening under laboratory conditions did not significantly differ from that observed at home. We conclude that the cortisol increase after awakening is a response to morning awakening that is distinct from the circadian rise in hypothalamo-pituitary-adrenal (HPA) activity in the morning hours. Although the cortisol awakening response is modulated by circadian influences, it primarily reflects phasic psychophysiological processes specific to the sleep-wake transition.

Neural substrates of psychostimulant-induced arousal

Extensive research has provided substantial insight into the neurobiological mechanisms underlying the reinforcing, locomotor-activating and stereotypy-inducing actions of psychostimulants. The diverse behavioral effects of these drugs are superimposed on potent arousal-enhancing actions. Psychostimulant-induced arousal is a prominent contributing factor to the widespread use and abuse of these drugs. Moreover, enhanced arousal may be a critical component of the reinforcing and other behavioral actions of these drugs. Although long overlooked, recent work begins to identify the neural mechanisms involved in psychostimulant-induced arousal. For example, microdialysis studies demonstrate a close relationship between amphetamine-induced waking/arousal and amphetamine-induced increases in norepinephrine and dopamine efflux. Additionally, it is now clear that both norepinephrine and dopamine exert robust wake-promoting actions. The wake-promoting effects of norepinephrine involve synergistic actions of alpha1- and beta-receptors, whereas dopamine-induced waking involves both D1 and D2 receptors. Finally, additional studies have identified subcortical regions involved in the wake-promoting actions of both norepinephrine and amphetamine. These regions include, but may not be limited to, the medial septal area, the medial preoptic area, and the lateral hypothalamus. Combined, these and other observations indicate a prominent involvement of both norepinephrine and dopamine in stimulant-induced arousal via actions within a network of subcortical regions. Although it is clear that both norepinephrine and dopamine contribute to psychostimulant-induced arousal, the degree to which each transmitter system is necessary for the expression of stimulant-induced arousal remains to be fully elucidated.

Central alpha1-adrenergic system in behavioral activity and depression

Central alpha(1)-adrenoceptors are activated by norepinephrine (NE), epinephrine (EPI) and possibly dopamine (DA), and function in two fundamental and opposed types of behavior: (1) positively motivated exploratory and approach activities, and (2) stress reactions and behavioral inhibition. Brain microinjection studies have revealed that the positive-linked receptors are located in eight to nine brain regions spanning the neuraxis including the secondary motor cortex, piriform cortex, nucleus accumbens, preoptic area, lateral hypothalamic area, vermis cerebellum, locus coeruleus, dorsal raphe and possibly the C1 nucleus of the ventrolateral medulla, whereas the stress-linked receptors are present in at least three areas including the paraventricular nucleus of the hypothalamus, central nucleus of the amygdala and bed nucleus of the stria terminalis. Recent studies utilizing c-fos expression and mitogen-activated protein kinase activation have shown that various diverse models of depression in mice produce decreases in positive region-neural activity elicited by motivating stimuli along with increases in neural activity of stress areas. Both types of change are attenuated by various antidepressant agents. This has suggested that the balance of the two networks determines whether an animal displays depressive behavior. A central unresolved question concerns how the alpha(1)-receptors in the positive-activity and stress systems are differentially activated during the appropriate behavioral conditions and to what extent this is related to differences in endogenous ligands or receptor subtype distributions.

Organization of noradrenergic efferents to arousal-related basal forebrain structures

Norepinephrine acts within select basal forebrain regions to modulate behavioral state and/or state-dependent processes, including the general regions encompassing the medial septal area, the medial preoptic area, and the substantia innominata. The present study examined the origin and organization of noradrenergic efferents to these basal forebrain regions by using combined immunohistochemical identification of noradrenergic neurons with retrograde tracing. Results indicate that the locus coeruleus provides the majority of noradrenergic input to these regions. Lesser, although at times substantial, contributions from the A1/C1 and A2/C2 adrenergic cell groups were also observed, particularly in the case of the medial preoptic region. Given the prominent state-modulating actions of the locus coeruleus, additional studies examined: 1) lateralization of locus coeruleus efferents to these regions; 2) the topographical organization of basal forebrain-projecting locus coeruleus neurons; and 3) the degree of collateralization of individual locus coeruleus neurons across these regions. Approximately 80-85% of locus coeruleus efferents to these regions project ipsilaterally. In general, basal forebrain-projecting neurons were distributed throughout the entire dorsoventral and rostrocaudal extent of the locus coeruleus. Additionally, a large proportion of locus coeruleus neurons project simultaneously to these basal forebrain terminal fields. Combined, these observations indicate coordinated actions of locus coeruleus neurons across these basal forebrain regions implicated in the regulation of behavioral state and/or state-dependent processes.

Sleep induction and temperature lowering by medial preoptic α1 adrenergic receptors

Changes in sleep-wakefulness (S-W) and body temperature (T(b)) on administration of alpha(1) agonist (methoxamine) and antagonist (prazosin) into the medial preoptic area (mPOA) were studied in rats. Presynaptic catecholaminergic terminals of the mPOA were destroyed by injecting 6-hydroxydopamine at the ventral noradrenergic bundle (VNA), before administration of the drugs. Microinjection of 0.05 microg methoxamine induced sleep, though 0.1 microg prazosin produced no change in S-W. On the other hand, in normal rats, the same dose of methoxamine produced no change, while prazosin produced arousal. Denervation hypersensitivity may be responsible for the appearance of hypnogenic response on methoxamine administration, in the VNA-lesioned rats. The VNA-lesioned animals (before administration of any drug) had higher pre-injection values of wake period than the normal rats. A reduction in the tonic activity of noradrenergic fibers to the mPOA, and resulting reduced activity of alpha(1) receptors, may be responsible for increased wake period in the VNA-lesioned rats. The action of prazosin was probably abolished in the absence of tonic activity of alpha(1) receptor in the VNA-lesioned rats. Reduction and increase in T(b) produced by methoxamine and prazosin, respectively, confirm the involvement of alpha(1) receptors in the thermal changes. Methoxamine was less effective, than in normal rats, in reducing T(b). So, the possibility of involvement of presynaptic receptors in the thermal response is suggested. The results suggest the involvement of separate sets of alpha(1) receptors (and neurons) in hypnogenesis and in lowering T(b). As sleep is associated with fall in T(b), the alpha(1) adrenergic receptors may be involved in interlinking sleep regulation and thermoregulation.

Tonic activity of alpha1 adrenergic receptors of the medial preoptic area contributes towards increased sleep in rats

Several studies have suggested that noradrenergic afferents to the medial preoptic area might be involved in hypnogenesis and in lowering the body temperature, and that the alpha1 adrenergic receptors might be mediating these responses. This study was undertaken to find out the changes in sleep-wakefulness and body temperature in rats, when these adrenergic receptors of the medial preoptic area are blocked by alpha1 selective antagonist, prazosin. Adult male Wistar rats were chronically implanted with electrooculogram, electroencephalogram and electromyogram electrodes for sleep-wakefulness assessment, and a bilateral guide cannula for microinjection of prazosin at the medial preoptic area. A radio-transmitter was implanted in the abdomen for telemetric measurement of body temperature in four groups of rats. Sleep-wakefulness was also assessed telemetrically in four other groups of rats. Sleep-wakefulness recordings from these rats were done in a specialized chamber, where they could move about freely and select the ambient temperature which they prefer. Prazosin induced a dose dependent increase in wake period and in body temperature, when microinjected into the medial preoptic area. Results suggest that preoptic alpha1 adrenergic receptors mediate hypnogenic and hypothermic responses. It is proposed that the noradrenergic afferents to the medial preoptic area, by tonic activation of alpha1 adrenergic receptors, contribute towards increase in sleep especially during the daytime.

The dual origins of affect in nightmares: The roles of physiological homeostasis and memory

Strong negative affect is a key and distressing ingredient of nightmares. Affect in nightmares arises either from the new generation of affective states due to physiological imbalances that occur during sleep or from the reactivation of affect-laden memories. The disruption of physiological balance produces a negative hedonic state, restoration of this balance produces a positive hedonic state, and when balance is attained, a neutral hedonic state results. As a result, hedonic states provoke behaviors in defense of homeostasis, then guide and terminate them. When, due to inadvertent behavior, a pronounced disruption of homeostasis occurs after sleep onset, the resultant strong negative hedonic state is likely to precipitate a nightmare and may lead to awakening. During normal wakefulness, associations of the interplay between stimuli and behaviors that disrupt homeostasis, those that restore homeostasis, and the affective states generated in the process, are committed to memory as affecto-cognitive ensembles. Sleep serves to build or rebuild neural architecture to effect development or to compensate for use- or disease-related wear (e.g. repair oxidative damage). Dreaming serves to synchronize or resynchronize such modified neural circuits with each other and those not modified. Hence, during dreaming, affecto-cognitive ensembles may get reactivated as part of the synchronization process. Where such an ensemble contains strong negative affect (i.e., due to strong affect generated during the original experience), a nightmare may be precipitated. Although both can occur throughout life, the latter type of nightmare is more likely in adults and the former in young children. For the latter memory-based behavioral therapy and for the former education and care are expected to be useful. For both types of nightmare, because strong negative affect is deemed dependent on noradrenergic outflow from the locus coeruleus, the administration of alpha-adrenergic antagonists will provide relief subject to certain caveats.

Wake-promoting actions of medial basal forebrain beta2 receptor stimulation

The locus coeruleus-noradrenergic system exerts an activating influence on forebrain neuronal and behavioral activity states, in part, through the actions of noradrenergic beta receptors located within the medial septal (MS) and medial preoptic (MPOA) areas. The current study examined the extent to which beta2 receptors located within these medial basal forebrain regions modulate behavioral state. In this study, the sleep-wake effects of microinfusion of the beta2 agonist, clenbuterol, into the MS and MPOA were examined. Clenbuterol infusion into both MS and MPOA elicited a dose-dependent increase in time spent awake. These observations indicate that medial basal forebrain beta-sub-2 receptors participate in the noradrenergic-dependent modulation of behavioral state.

Dopaminergic-adrenergic interactions in the wake promoting mechanism of modafinil

Adrenergic signaling regulates the timing of sleep states and sleep state-dependent changes in muscle tone. Recent studies indicate a possible role for noradrenergic transmission in the wake-promoting action of modafinil, a widely used agent for the treatment of excessive sleepiness. We now report that noradrenergic projections from the locus coeruleus to the forebrain are not necessary for the wake-promoting action of modafinil. The efficacy of modafinil was maintained after treatment of C57BL/6 mice with N-(2-chloroethyl)-N-ethyl 2-bromobenzylamine (DSP-4), which eliminates all noradrenaline transporter-bearing forebrain noradrenergic projections. However, the necessity for adrenergic receptors in the wake-promoting action of modafinil was demonstrated by the observation that the adrenergic antagonist terazosin suppressed the response to modafinil in DSP-4 treated mice. The wake-promoting efficacy of modafinil was also blunted by the dopamine autoreceptor agonist quinpirole. These findings implicate non-noradrenergic, dopamine-dependent adrenergic signaling in the wake-promoting mechanism of modafinil. The anatomical specificity of these dopaminergic-adrenergic interactions, which are present in forebrain areas that regulate sleep timing but not in brain stem areas that regulate sleep state-dependent changes in muscle tone, may explain why modafinil effectively treats excessive daytime sleepiness in narcolepsy but fails to prevent the loss of muscle tone that occurs in narcoleptic patients during cataplexy.

Organization of hypocretin/orexin efferents to locus coeruleus and basal forebrain arousal-related structures

Hypocretin/orexin neurons give rise to an extensive projection system, portions of which innervate multiple regions associated with the regulation of behavioral state. These regions include the locus coeruleus, medial septal area, medial preoptic area, and substantia innominata. Evidence indicates that hypocretin modulates behavioral state via actions within each of these terminal fields. To understand better the circuitry underlying hypocretin-dependent modulation of behavioral state, the present study characterized the degree to which there exists: 1) lateralization of hypocretin efferents to basal forebrain and brainstem arousal-related regions, 2) topographic organization of basal forebrain- and brainstem-projecting hypocretin neurons, and 3) collateralization of individual hypocretin neurons to these arousal-related terminal fields. These studies utilized combined immunohistochemical identification of hypocretin neurons with single or double retrograde tracing from the locus coeruleus, medial preoptic area, medial septal area, and substantia innominata. Results indicate that approximately 80% of hypocretin efferents to basal forebrain regions project ipsilaterally, whereas projections to the locus coeruleus are more bilateral (65%). There was a slight preference for basal forebrain-projecting hypocretin neurons to be distributed within the medial half of the hypocretin cell group. In contrast, hypocretin neurons projecting to the locus coeruleus were located primarily within the dorsal half of the hypocretin cell group. Finally, a large proportion of hypocretin neurons appear to project simultaneously to at least two of the examined terminal fields. These latter observations suggest coordinated actions of hypocretin across multiple arousal-related regions.

Rate-dependent behavioral effects of stimulation of central motoric alpha(1)-adrenoceptors: hypothesized relation to depolarization blockade

AIM

The purpose of this review is to clarify how central alpha(1)-adrenoceptors control behavioral activity under varying conditions of activity and stress.

METHOD

The literature is reviewed regarding the behavioral actions of alpha(1)-agonists and antagonists, and alpha(2)-agonists and antagonists under conditions of high and low baseline activity and stress.

RESULTS

It was found that alpha(1)-receptor stimulation of active behavior has a number of similarities to rate dependency including: (1) a dependence on low-active, low-stress conditions or on the prior depletion of endogenous brain catecholamines; (2) a nonmonotonic dose-response relationship with high doses producing a fall-off or actual depression of activity; (3) a failure to be blocked at high agonist doses by alpha(1)-antagonists; and (4) a facilitation by alpha(2)-adrenoceptor agonists which produce an opposing hyperpolarization.

DISCUSSION

To explain these findings, it is proposed that high levels of stimulation of central alpha(1)-receptors produce, in host neurons, a depolarization block that impedes nerve impulse generation and inhibits active behavior. This effect is assumed to be precluded or mitigated by low-active, low-stress conditions, depletion of brain catecholamines, and by hyperpolarizing alpha(2)-agonists, and to be reversed at high agonist doses by alpha(1)-antagonists.

CONCLUSION

Because brain alpha(1)-receptors are not only involved in motor activity but also in the mechanism of action of antidepressant and stimulant drugs, arousal, anxiety, stress and psychosis, a depolarization block from intense stimulation of these receptors could have broad psychopharmacological consequences and underlie rate dependency to a variety of stimulant drugs.

Gross mapping of α1-adrenoceptors that regulate behavioral activation in the mouse brain

Brain alpha1-adrenoceptors that participate in behavioral activation were mapped in the mouse brain by determining where microinjection of the alpha1-antagonist, terazosin, inhibited behavioral activity in a novel cage test. A total of 5 out of 23 tested regions were shown to be involved including the dorsal pons/locus coeruleus region (DP/LC), the dorsal raphe/periaqueductal gray area (DR/PAG), the vermis cerebellum (CER), the nucleus accumbens (ACC) and the medial preoptic area (MPOA). Injection in the 4th ventricle was also effective perhaps by acting on several of these regions simultaneously. A partial inhibition was obtained from the motor cortex. Coinjection of the alpha1/2-agonist, 6-fluoronorepinephrine (6FNE) but not the alpha2-agonist, dexmedetomidine (DMT) reversed the behavioral inhibition in all regions. It is hypothesized that brain motoric alpha1-receptors elicit behavioral activation by coordinately exciting several monoaminergic, motor and motivational systems.

Immobility from administration of the alpha1-adrenergic antagonist, terazosin, in the IVth ventricle in rats

Brain alpha1-adrenoceptors have been shown to be essential for motor activity and movement in mice using intraventricular injection of alpha1-antagonists. To facilitate subsequent neuroanatomical mapping of these receptors, the present study was undertaken to replicate these effects in the rat. Rats were administered the alpha1-antagonist, terazosin, in the absence and presence of the alpha1-agonist, phenylephrine, in the IVth ventricle and were tested for their motor activity responses to an environmental change. Terazosin was found to produce a dose-dependent, virtually complete cessation of behavioral activity that was reversed by coinfusion of phenylephrine. The results could not be explained by sedation. It is concluded that central alpha1-adrenoceptors are essential for behavioral activation in rats as in mice.

Role of locus coeruleus ?1-adrenoceptors in motor activity in rats

The question of whether or not the locus coeruleus (LC) participates in the control of motor activity has been controversial due to difficulties in demonstrating permanent motor deficits after neurotoxic lesions of this nucleus or of the dorsal noradrenergic bundle (DNB). In the present experiments it was shown in rats that acute local blockade (with terazosin) or stimulation (with phenylephrine) of LC alpha(1)-adrenoceptors respectively blocked or stimulated exploratory behavior in a novel cage and the home cage. Moreover, previous lesion of the DNB by i.p. DSP4 abolished the behavioral changes to local LC alpha(1)-receptor manipulation but did not affect motor activity in the novel or home cage by itself. These findings are consistent with the hypothesis that the intact LC does contribute to motor activity control, exerted in part by its alpha(1)-receptors; however, the permanent loss of this nucleus is compensated for by remaining CNS motor structures.

Wake-promoting actions of dopamine D1 and D2 receptor stimulation

Multiple ascending neurotransmitter systems participate in the regulation of behavioral state. For example, noradrenergic, cholinergic, and serotonergic systems increase EEG and, in some cases, behavioral indices of arousal. The extent to which dopaminergic systems exert a similar activating influence on behavioral state remains unclear. The current studies examined the wake-promoting actions of centrally administered D1 and D2 receptor agonists. In these studies, intracerebroventricular infusions of a D1 (SKF-82958; 2.5 and 25 nmol) or D2 (quinpirole; 40 and 140 nmol)-agonist were made into sleeping animals. The effects of these infusions on electroencephalogram/electromyographic indices of sleep-wake state and behavior were examined. D1 agonist administration dose dependently increased time spent awake and suppressed rapid eye movement and slow-wave sleep in the 2 h immediately after infusion. D1 agonist administration also elicited modest increases in measures of locomotion and time spent grooming and eating. D2 agonist administration had similar wake-promoting actions, accompanied by modest effects on drinking and locomotion. Interestingly, D2 agonist administration also significantly increased time spent chewing on inedible material, an arousal/stress-related behavior. Overall, these results demonstrate that dopamine contributes to the alert waking state via actions of D1 and D2 receptors. Additionally or alternatively, these results further suggest a potential involvement of dopamine receptors in the induction of high-arousal states, including stress.

The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

Through a widespread efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. Initial studies provided critical insight into the basic organization and properties of this system. More recent work identifies a complicated array of behavioral and electrophysiological actions that have in common the facilitation of processing of relevant, or salient, information. This involves two basic levels of action. First, the system contributes to the initiation and maintenance of behavioral and forebrain neuronal activity states appropriate for the collection of sensory information (e.g. waking). Second, within the waking state, this system modulates the collection and processing of salient sensory information through a diversity of concentration-dependent actions within cortical and subcortical sensory, attention, and memory circuits. Norepinephrine-dependent modulation of long-term alterations in synaptic strength, gene transcription and other processes suggest a potentially critical role of this neurotransmitter system in experience-dependent alterations in neural function and behavior. The ability of a given stimulus to increase locus coeruleus discharge activity appears independent of affective valence (appetitive vs. aversive). Combined, these observations suggest that the locus coeruleus-noradrenergic system is a critical component of the neural architecture supporting interaction with, and navigation through, a complex world. These observations further suggest that dysregulation of locus coeruleus-noradrenergic neurotransmission may contribute to cognitive and/or arousal dysfunction associated with a variety of psychiatric disorders, including attention-deficit hyperactivity disorder, sleep and arousal disorders, as well as certain affective disorders, including post-traumatic stress disorder. Independent of an etiological role in these disorders, the locus coeruleus-noradrenergic system represents an appropriate target for pharmacological treatment of specific attention, memory and/or arousal dysfunction associated with a variety of behavioral/cognitive disorders.