The role of orexinergic system in the regulation of cataplexy

Loss of orexin/hypocretin causes serious sleep disorder; narcolepsy. Cataplexy is the most striking symptom of narcolepsy, characterized by abrupt muscle paralysis induced by emotional stimuli, and has been considered pathological activation of REM sleep atonia system. Clinical treatments for cataplexy/narcolepsy and early pharmacological studies in narcoleptic dogs tell us about the involvement of monoaminergic and cholinergic systems in the control of cataplexy/narcolepsy. Muscle atonia may be induced by activation of REM sleep-atonia generating system in the brainstem. Emotional stimuli may be processed in the limbic systems including the amygdala, nucleus accumbens, and medial prefrontal cortex. It is now considered that orexin/hypocretin prevents cataplexy by modulating the activity of different points of cataplexy-inducing circuit, including monoaminergic/cholinergic systems, muscle atonia-generating systems, and emotion-related systems. This review will describe the recent advances in understanding the neural mechanisms controlling cataplexy, with a focus on the involvement of orexin/hypocretin system, and will discuss future experimental strategies that will lead to further understanding and treatment of this disease.

Orexin recruits non-selective cationic conductance and endocannabinoid to dynamically modulate firing of caudal pontine reticular nuclear neurones

The neuropeptide orexin is involved in motor circuit function. However, its modulation on neuronal activities of motor structures, integrating orexin's diverse downstream molecular cascades, remains elusive. By combining whole-cell patch-clamp recordings and neuropharmacological methods, we revealed that both non-selective cationic conductance (NSCC) and endocannabinoids (eCBs) are recruited by orexin signalling on reticulospinal neurones in the caudal pontine reticular nucleus (PnC). The orexin-NSCC cascade provides a depolarizing force that proportionally enhances the firing-responsive gain of these neurones. Meanwhile, the orexin-eCB cascade selectively attenuates excitatory synaptic strength in these neurones by activating presynaptic cannabinoid receptor type 1. This cascade restrains the firing response of the PnC reticulospinal neurones to excitatory inputs. Intriguingly, non-linear or linear interactions between orexin postsynaptic excitation and presynaptic inhibition can influence the firing responses of PnC reticulospinal neurones in different directions. When presynaptic inhibition is in the lead, non-linear interactions can prominently downregulate or even gate the firing response. Conversely, linear interactions occur to promote the firing response, and these linear interactions can be considered a proportional reduction in the contribution of depolarization to firing by presynaptic inhibition. Through the dynamic employment of these interactions, adaptive modulation may be achieved by orexin to restrain or even gate the firing output of the PnC to weak/irrelevant input signals and facilitate those to salient signals. KEY POINTS: This study investigated the effects of orexin on the firing activity of PnC reticulospinal neurones, a key element of central motor control. We found that orexin recruited both the non-selective cationic conductances (NSCCs) and endocannabinoid (eCB)-cannabinoid receptor type 1 (CB1R) system to pontine reticular nucleus (PnC) reticulospinal neurones. The orexin-NSCC cascade exerts a postsynaptic excitation that enhances the firing response, whereas the orexin-eCB-CB1R cascade selectively attenuates excitatory synaptic strength that restrains the firing response. The postsynaptic and presynaptic actions of orexins occur in an overlapping time window and interact to dynamically modulate firings in PnC reticulospinal neurones. Non-linear interactions occur when presynaptic inhibition of orexin is in the lead, and these interactions can prominently downregulate or even gate firing responses in PnC reticulospinal neurones. Linear interactions occur when postsynaptic excitation of orexin is in the lead, and these interactions can promote the firing response. These linear interactions can be considered a proportional reduction of the contribution of depolarization to firing by presynaptic inhibition.

Dual Orexin Receptor Antagonist Attenuates Increases in IOP, ICP, and Translaminar Pressure Difference After Stimulation of the Hypothalamus in Rats

PURPOSE

Intraocular pressure (IOP) remains the only modifiable risk factor for glaucoma progression. Our previous discovery that stimulation of nuclei within the hypothalamus can modulate IOP, intracranial pressure (ICP), and translaminar pressure difference (TLPD) fluctuations led us to investigate this pathway further. Our purpose was to determine the role of orexin neurons, primarily located in the dorsomedial hypothalamus (DMH) and perifornical (PeF) regions of the hypothalamus, in modulating these pressures.

METHODS

Sprague Dawley rats were pretreated systemically with a dual orexin receptor antagonist (DORA-12) at 30 mg/Kg (n = 8), 10 mg/Kg (n = 8), or vehicle control (n = 8). The IOP, ICP, heart rate (HR), and mean arterial pressure (MAP) were recorded prior to and following excitation of the DMH/PeF using microinjection of the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline methiodide (BMI).

RESULTS

Administration of the DORA at 30 mg/Kg significantly attenuated peak IOP by 5.2 ± 3.6 mm Hg (P = 0.007). During the peak response period (8-40 minutes), the area under the curve (AUC) for the 30 mg/Kg DORA cohort was significantly lower than the control cohort during the same period (P = 0.04). IOP responses for peak AUC versus DORA dose, from 0 to 30 mg/Kg, were linear (R2 = 0.18, P = 0.04). The ICP responses during the peak response period (4-16 minutes) versus DORA dose were also linear (R2 = 0.24, P = 0.014). Pretreatment with DORA significantly decreased AUC for the TLPD following stimulation of the DMH/PeF (10 mg/kg, P = 0.045 and 30 mg/kg, P = 0.015).

CONCLUSIONS

DORAs have the potential to attenuate asynchronous changes in IOP and in ICP and to lessen the extent of TLPDs that may result from central nervous system (CNS) activation.

Neurobiology of cataplexy

Cataplexy is the pathognomonic and the most striking symptom of narcolepsy. It has originally been, and still is now, widely considered as an abnormal manifestation of rapid eye movement (REM) sleep during wakefulness due to the typical muscle atonia. The neurocircuits of cataplexy, originally confined to the brainstem as those of REM sleep atonia, now include the hypothalamus, dorsal raphe (DR), amygdala and frontal cortex, and its neurochemistry originally focused on catecholamines and acetylcholine now extend to hypocretin (HCRT) and other neuromodulators. Here, we review the neuroanatomy and neurochemistry of cataplexy and propose that cataplexy is a distinct brain state that, despite similarities with REM sleep, involves cataplexy-specific features.

The effects of orexin-A and orexin receptors on anxiety- and depression-related behaviors in a male rat model of post-traumatic stress disorder

Orexin neurons play an important role in stress-related mental disorders including post-traumatic stress disorder (PTSD). Anxiety- and depression-related symptoms commonly occur in combination with PTSD. However, the role of the orexin system in mediating alterations in these affective symptoms remains unclear. The medial prefrontal cortex (mPFC) is implicated in both cognitive and emotional processing. In the present study, we investigated anxiety- and depression-related behavioral changes using the elevated plus maze, the sucrose preference test, and the open field test in male rats with single prolonged stress (SPS) induced-PTSD. The expression of orexin-A in the hypothalamus and orexin receptors (OX1R and OX2R) in the mPFC was detected and quantified by immunohistochemistry, western blotting, and real-time polymerase chain reaction. We found that the SPS rats exhibited enhanced levels of anxiety, reduced exploratory activities, and anhedonia. Furthermore, SPS resulted in reductions in the expression of orexin-A in the hypothalamus and the increased the expression of OX1R in the mPFC. The intracerebroventricular administration of orexin-A alleviated behavioral changes in SPS rats and partly restored the increased levels of OX1R in the mPFC. These results suggest that the orexin system plays a role in the anxiety- and depression-related symptoms observed in PTSD.

Arousal and sleep circuits

The principal neurons of the arousal and sleep circuits are comprised by glutamate and GABA neurons, which are distributed within the reticular core of the brain and, through local and distant projections and interactions, regulate cortical activity and behavior across wake-sleep states. These are in turn modulated by the neuromodulatory systems that are comprised by acetylcholine, noradrenaline, dopamine, serotonin, histamine, orexin (hypocretin), and melanin-concentrating hormone (MCH) neurons. Glutamate and GABA neurons are heterogeneous in their profiles of discharge, forming distinct functional cell types by selective or maximal discharge during (1) waking and paradoxical (REM) sleep, (2) during slow wave sleep, (3) during waking, or (4) during paradoxical (REM) sleep. The neuromodulatory systems are each homogeneous in their profile of discharge, the majority discharging maximally during waking and paradoxical sleep or during waking. Only MCH neurons discharge maximally during sleep. They each exert their modulatory influence upon other neurons through excitatory and inhibitory receptors thus effecting a concerted differential change in the functionally different cell groups. Both arousal and sleep circuit neurons are homeostatically regulated as a function of their activity in part through changes in receptors. The major pharmacological agents used for the treatment of wake and sleep disorders act upon GABA and neuromodulatory transmission.

Discharge and Role of Acetylcholine Pontomesencephalic Neurons in Cortical Activity and Sleep-Wake States Examined by Optogenetics and Juxtacellular Recording in Mice

Acetylcholine (ACh) neurons in the pontomesencephalic tegmentum (PMT) are thought to play an important role in promoting cortical activation with waking (W) and paradoxical sleep [PS; or rapid eye movement (REM)], but have yet to be proven to do so by selective stimulation and simultaneous recording of identified ACh neurons. Here, we employed optogenetics combined with juxtacellular recording and labeling of neurons in transgenic (TG) mice expressing ChR2 in choline acetyltransferase (ChAT)-synthesizing neurons. We established in vitro then in vivo in anesthetized (A) and unanesthetized (UA), head-fixed mice that photostimulation elicited a spike with short latency in neurons which could be identified by immunohistochemical staining as ACh neurons within the laterodorsal (LDT)/sublaterodorsal (SubLDT) and pedunculopontine tegmental (PPT) nuclei. Continuous light pulse stimulation during sleep evoked tonic spiking by ACh neurons that elicited a shift from irregular slow wave activity to rhythmic θ and enhanced γ activity on the cortex without behavioral arousal. With θ frequency rhythmic light pulse stimulation, ACh neurons discharged in bursts that occurred in synchrony with evoked cortical θ. During natural sleep-wake states, they were virtually silent during slow wave sleep (SWS), discharged in bursts during PS and discharged tonically during W. Yet, their bursting during PS was not rhythmic or synchronized with cortical θ but associated with phasic whisker movements. We conclude that ACh PMT neurons promote θ and γ cortical activity during W and PS by their tonic or phasic discharge through release of ACh onto local neurons within the PMT and/or more distant targets in the hypothalamus and thalamus.

Homeostatic Changes in GABA and Acetylcholine Muscarinic Receptors on GABAergic Neurons in the Mesencephalic Reticular Formation following Sleep Deprivation

We have examined whether GABAergic neurons in the mesencephalic reticular formation (RFMes), which are believed to inhibit the neurons in the pons that generate paradoxical sleep (PS or REMS), are submitted to homeostatic regulation under conditions of sleep deprivation (SD) by enforced waking during the day in mice. Using immunofluorescence, we investigated first, by staining for c-Fos, whether GABAergic RFMes neurons are active during SD and then, by staining for receptors, whether their activity is associated with homeostatic changes in GABA_A or acetylcholine muscarinic type 2 (AChM2) receptors (Rs), which evoke inhibition. We found that a significantly greater proportion of the GABAergic neurons were positively stained for c-Fos after SD (∼27%) as compared to sleep control (SC; ∼1%) and sleep recovery (SR; ∼6%), suggesting that they were more active during waking with SD and less active or inactive during sleep with SC and SR. The density of GABA_ARs and AChM2Rs on the plasma membrane of the GABAergic neurons was significantly increased after SD and restored to control levels after SR. We conclude that the density of these receptors is increased on RFMes GABAergic neurons during presumed enhanced activity with SD and is restored to control levels during presumed lesser or inactivity with SR. Such increases in GABA_AR and AChM2R with sleep deficits would be associated with increased susceptibility of the wake-active GABAergic neurons to inhibition from GABAergic and cholinergic sleep-active neurons and to thus permitting the onset of sleep and PS with muscle atonia.

Homeostatic regulation through GABA and acetylcholine muscarinic receptors of motor trigeminal neurons following sleep deprivation

Muscle tone is regulated across sleep-wake states, being maximal in waking, reduced in slow wave sleep (SWS) and absent in paradoxical or REM sleep (PS or REMS). Such changes in tone have been recorded in the masseter muscles and shown to correspond to changes in activity and polarization of the trigeminal motor 5 (Mo5) neurons. The muscle hypotonia and atonia during sleep depend in part on GABA acting upon both GABA_A and GABA_B receptors (Rs) and acetylcholine (ACh) acting upon muscarinic 2 (AChM2) Rs. Here, we examined whether Mo5 neurons undergo homeostatic regulation through changes in these inhibitory receptors following prolonged activity with enforced waking. By immunofluorescence, we assessed that the proportion of Mo5 neurons positively stained for GABA_ARs was significantly higher after sleep deprivation (SD, ~65%) than sleep control (SC, ~32%) and that the luminance of the GABA_AR fluorescence was significantly higher after SD than SC and sleep recovery (SR). Although, all Mo5 neurons were positively stained for GABA_BRs and AChM2Rs (100%) in all groups, the luminance of these receptors was significantly higher following SD as compared to SC and SR. We conclude that the density of GABA_A, GABA_B and AChM2 receptors increases on Mo5 neurons during SD. The increase in these receptors would be associated with increased inhibition in the presence of GABA and ACh and thus a homeostatic down-scaling in the excitability of the Mo5 neurons after prolonged waking and resulting increased susceptibility to muscle hypotonia or atonia along with sleep.

Orexin Neurons Respond Differentially to Auditory Cues Associated with Appetitive versus Aversive Outcomes

Orexin (Orx) neurons are known to be involved in the promotion and maintenance of waking because they discharge in association with cortical activation and muscle tone during waking and because, in their absence, waking with muscle tone cannot be maintained and narcolepsy with cataplexy ensues. Whether Orx neurons discharge during waking in association with particular conditions, notably with appetitive versus aversive stimuli or positive versus negative emotions, is debated and considered important in understanding their role in supporting particular waking behaviors. Here, we used the technique of juxtacellular recording and labeling in head-fixed rats to characterize the discharge of Orx neurons during the performance of an associative discrimination task with auditory cues for appetitive versus aversive outcomes. Of 57 active, recorded, and neurobiotin-labeled neurons in the lateral hypothalamus, 11 were immunohistochemically identified as Orx-positive (Orx(+)), whereas none were identified as melanin-concentrating hormone-positive. Orx(+) neurons discharged at significantly higher rates during the tone associated with sucrose than during the tone associated with quinine delivered upon licking. They also discharged at high rates after the tone associated with sucrose. Across periods and outcomes, their discharge was positively correlated with EEG gamma activity and EMG activity, which is indicative of cortical activation and behavioral arousal. These results suggest that Orx neurons discharge in a manner characteristic of reward neurons yet also characteristic of arousal neurons. Accordingly, the Orx neurons may respond to and participate in reward processes while modulating cortical activity and muscle tone to promote and maintain arousal along with learned adaptive behavioral responses.

SIGNIFICANCE STATEMENT

Orexin neurons play a critical role in promoting and maintaining a waking state because, in their absence, narcolepsy with cataplexy ensues. Known to discharge during waking and not during sleep, they have also been proposed to be selectively active during appetitive behaviors. Here, we recorded and labeled neurons in rats to determine the discharge of immunohistochemically identified orexin neurons during performance of an associative discrimination task. Orexin neurons responded differentially to auditory cues associated with appetitive sucrose versus aversive quinine, indicating that they behave like reward neurons. However, correlated discharge with cortical and muscle activity indicates that they also behave like arousal neurons and can thus promote cortical activation with behavioral arousal and muscle tone during adaptive waking behaviors.

Electron microscopic localization of M2-muscarinic receptors in cholinergic and noncholinergic neurons of the laterodorsal tegmental and pedunculopontine nuclei of the rat mesopontine tegmentum

Muscarinic m2 receptors (M2Rs) are implicated in autoregulatory control of cholinergic output neurons located within the pedunculopontine (PPT) and laterodorsal tegmental (LTD) nuclei of the mesopontine tegmentum (MPT). However, these nuclei contain many noncholinergic neurons in which activation of M2R heteroceptors may contribute significantly to the decisive role of the LTD and PPT in sleep-wakefulness. We examined the electron microscopic dual immunolabeling of M2Rs and the vesicular acetylcholine transporter (VAchT) in the MPT of rat brain to identify the potential sites for M2R activation. M2R immunogold labeling was predominately seen in somatodendritic profiles throughout the PPT/LTD complex. In somata, M2R immunogold particles were often associated with Golgi lamellae and cytoplasmic endomembrannes, but were rarely in contact with the plasma membrane, as was commonly seen in dendrites. Approximately 36% of the M2R-labeled somata and 16% of the more numerous M2R-labeled dendrites coexpressed VAchT. M2R and M2R/VAchT-labeled dendritic profiles received synapses from inhibitory- and excitatory-type axon terminals, over 88% of which were unlabeled and others contained exclusively M2R or VAchT immunoreactivity. In axonal profiles M2R immunogold was localized to plasmalemmal and cytoplasmic regions and showed a similar distribution in many VAchT-negative glial profiles. These results provide ultrastructural evidence suggestive of somatic endomembrane trafficking of M2Rs, whose activation serves to regulate the postsynaptic excitatory and inhibitory responses in dendrites of cholinergic and noncholinergic neurons in the MPT. They also suggest the possibility that M2Rs in this brain region mediate the effects of acetylcholine on the release of other neurotransmitters and on glial signaling. J. Comp. Neurol. 524:3084-3103, 2016. © 2016 Wiley Periodicals, Inc.

Expression of the M3 Muscarinic Receptor on Orexin Neurons that Project to the Rostral Ventrolateral Medulla

Activation of central cholinergic receptors causes a pressor response in rats, and the hypothalamus is important for this response. Projections from hypothalamic orexin neurons to the rostral ventrolateral medulla (RVLM) are involved in sympatho-excitation of the cardiovascular system. A small population of orexin neurons is regulated by cholinergic inputs through M3 muscarinic acetylcholine receptor (M3 R). To elucidate whether the M3 R on orexin neurons is involved in cardiosympathetic regulation through the RVLM, we examined the presence of the M3 R on retrograde-labeled RVLM-projecting orexin neurons. The retrograde tracer was unilaterally injected into the RVLM. Within the hypothalamus, retrograde-labeled neurons were located predominantly ipsilateral to the injection side. In the anterior hypothalamus (-1.5 to -2.3 mm to the bregma), retrograde-labeled neurons were densely distributed in the paraventricular nuclei and scattered in the retrochiasmatic area. At -2.3 to -3.5 mm from the bregma, labeled neurons were located in the regions where orexin neurons were situated, that is, the tuberal lateral hypothalamic area, perifornical area, and dorsomedial nuclei. Very few retrograde-labeled neurons were observed in the hypothalamus at -3.5 to -4.5 mm from the bregma. About 19.5% ± 1.6% of RVLM-projecting neurons in the tuberal hypothalamus were orexinergic. The M3 R was present on 18.7% ± 3.0% of RVLM-projecting orexin neurons. Injection of a muscarinic agonist, oxotremorine, in the perifornical area resulted in a pressor response, which was attenuated by a pretreatment of atropine. We conclude that cholinergic inputs to orexin neurons may be involved in cardiosympathetic regulation through the M3 R on the orexin neurons that directly project to the RVLM.

Cumulative effects of repetitive intermittent hypercapnic hypoxia on orexin in the developing piglet hypothalamus

Orexin neuropeptides (OxA and OxB) and their receptors (OX1R and OX2R) are involved in maintenance of sleep and wakefulness, and are regulated by various environmental stimuli. We studied piglets, in the early neonatal period, exposed to 48-min of intermittent hypercapnic hypoxia (IHH; 7% O2/8% CO2) alternating with air. Three groups of 13-14 day-old piglets with IHH exposure of 1-day (1D-IHH) (n=7), 2-days (2D-IHH) (n=7) and 4-days (4D-IHH) (n=8) were compared to controls (exposed only to air, n=8). Immunoreactivity of OxA and OxB was studied in the piglet hypothalamic regions of the dorsomedial hypothalamus (DMH), perifornical area (PeF) and lateral hypothalamic area (LH). Results showed that after 1D- and 2D-IHH, total OxA and OxB expression decreased by 20% (p ≤ 0.005) and 40% (p<0.001), respectively. After 4D-IHH, the decrease in OxA and OxB was 50% (p<0.001). These findings indicate that a chronic IHH exposure induces greater changes in orexin neuropeptide expression than an acute 1-day exposure in the hypothalamus. This may be causally related to the dysregulation of sleep.

Decreased orexin (hypocretin) immunoreactivity in the hypothalamus and pontine nuclei in sudden infant death syndrome

Infants at risk of sudden infant death syndrome (SIDS) have been shown to have dysfunctional sleep and poor arousal thresholds. In animal studies, both these attributes have been linked to impaired signalling of the neuropeptide orexin. This study examined the immunoreactivity of orexin (OxA and OxB) in the tuberal hypothalamus (n = 27) and the pons (n = 15) of infants (1-10 months) who died from SIDS compared to age-matched non-SIDS infants. The percentage of orexin immunoreactive neurons and the total number of neurons were quantified in the dorsomedial, perifornical and lateral hypothalamus at three levels of the tuberal hypothalamus. In the pons, the area of orexin immunoreactive fibres were quantified in the locus coeruleus (LC), dorsal raphe (DR), laterodorsal tegmental (LDT), medial parabrachial, dorsal tegmental (DTg) and pontine nuclei (Pn) using automated methods. OxA and OxB were co-expressed in all hypothalamic and pontine nuclei examined. In SIDS infants, orexin immunoreactivity was decreased by up to 21 % within each of the three levels of the hypothalamus compared to non-SIDS (p ≤ 0.050). In the pons, a 40-50 % decrease in OxA occurred in the all pontine nuclei, while a similar decrease in OxB immunoreactivity was observed in the LC, LDT, DTg and Pn (p ≤ 0.025). No correlations were found between the decreased orexin immunoreactivity and previously identified risk factors for SIDS, including prone sleeping position and cigarette smoke exposure. This finding of reduced orexin immunoreactivity in SIDS infants may be associated with sleep dysfunction and impaired arousal.

Spontaneously hypertensive rats have more orexin neurons in the hypothalamus and enhanced orexinergic input and orexin 2 receptor-associated nitric oxide signalling in the rostral ventrolateral medulla

NEW FINDINGS

What is the central question of this study? Our previous study demonstrates that elevated orexin 2 receptor (OX2R) activity within the rostral ventrolateral medulla (RVLM) contributes to hypertension in spontaneously hypertensive rats (SHRs), and a lower OX2R protein level was detected in their RVLM. The present study aims to explore the mechanisms underlying elevated orexinergic activity in the RVLM of SHRs, compared with their normotensive counterparts, Wistar-Kyoto rats. What is the main finding and its importance? Increased orexinergic input into the RVLM and enhanced OX2R responsiveness in the RVLM, which was mainly mediated by augmented OX2R-neuronal nitric oxide synthase signalling, may underlie the elevated OX2R activity within the RVLM of SHRs. Our previous study showed that elevated orexin 2 receptor (OX2R) activity within the rostral ventrolateral medulla (RVLM) contributes to hypertension in spontaneously hypertensive rats (SHRs). Herein, we investigated the mechanism(s) underlying the elevated OX2R activity. The following results were found. (i) More hypothalamic orexin A-immunoreactive (OXA-IR) cells existed in SHRs than in Wistar-Kyoto (WKY) rats at either 4 (2217 ± 43 versus 1809 ± 69) or 16 weeks of age (1829 ± 59 versus 1230 ± 84). The number of OXA-IR cells that project to the RVLM was higher in 16-week-old SHRs than in WKY rats (91 ± 11 versus 52 ± 11). (ii) Higher numbers of OXA-IR and RVLM-projecting OXA-IR cells were found in the dorsomedial and perifornical hypothalamus of 16-week-old SHRs. (iii) Spontaneously hypertensive rats had higher levels of orexin A and B in the hypothalamus and higher levels of orexin A in the RVLM than did WKY rats. (iv) Unilateral intra-RVLM application of OX2R agonist, orexin A or [Ala(11), d-Leu(15)]-orexin B (50 pmol) induced a larger pressor response in SHRs than in WKY rats. (v) Intra-RVLM pretreatment with a neuronal nitric oxide synthase (NOS) inhibitor, 7-nitro-indazole (2.5 pmol), or a soluble guanylate cyclase inhibitor, methylene blue (250 pmol), reduced the intra-RVLM [Ala(11), d-Leu(15) ]-orexin B-induced pressor response in both WKY rats and SHRs. In contrast, an inducible NOS inhibitor, aminoguanidine (100 pmol), was ineffective. (vi) Neuronal NOS was co-expressed with OX2R in RVLM neurons. In conclusion, increased orexinergic input and enhanced OX2R-neuronal NOS signalling may underlie elevated OX2R activity in the RVLM and contribute to the pathophysiology of hypertension in SHRs.

Perifornical orexinergic neurons modulate REM sleep by influencing locus coeruleus neurons in rats

Activation of the orexin (OX)-ergic neurons in the perifornical (PeF) area has been reported to induce waking and reduce rapid eye movement sleep (REMS). The activities of OX-ergic neurons are maximum during active waking and they progressively reduce during non-REMS (NREMS) and REMS. Apparently, the locus coeruleus (LC) neurons also behave in a comparable manner as that of the OX-ergic neurons particularly in relation to waking and REMS. Further, as PeF OX-ergic neurons send dense projections to LC, we argued that the former could drive the LC neurons to modulate waking and REMS. Studies in freely moving normally behaving animals where simultaneously neuro-chemo-anatomo-physio-behavioral information could be deciphered would significantly strengthen our understanding on the regulation of REMS. Therefore, in this study in freely behaving chronically prepared rats we stimulated the PeF neurons without or with simultaneous blocking of specific subtypes of OX-ergic receptors in the LC while electrophysiological recording characterizing sleep-waking was continued. Single dose of glutamate stimulation as well as sustained mild electrical stimulation of PeF (both bilateral) significantly increased waking and reduced REMS as compared to baseline. Simultaneous application of OX-receptor1 (OX1R) antagonist bilaterally into the LC prevented PeF stimulation-induced REMS suppression. Also, the effect of electrical stimulation of the PeF was long lasting as compared to that of the glutamate stimulation. Further, sustained electrical stimulation significantly decreased both REMS duration as well as REMS frequency, while glutamate stimulation decreased REMS duration only.

Discharge profiles across the sleep-waking cycle of identified cholinergic, GABAergic, and glutamatergic neurons in the pontomesencephalic tegmentum of the rat

Distributed within the laterodorsal tegmental and pedunculopontine tegmental nuclei (LDT and PPT), cholinergic neurons in the pontomesencephalic tegmentum have long been thought to play a critical role in stimulating cortical activation during waking (W) and paradoxical sleep (PS, also called REM sleep), yet also in promoting PS with muscle atonia. However, the discharge profile and thus precise roles of the cholinergic neurons have remained uncertain because they lie intermingled with GABAergic and glutamatergic neurons, which might also assume these roles. By applying juxtacellular recording and labeling in naturally sleeping-waking, head-fixed rats, we investigated the discharge profiles of histochemically identified cholinergic, GABAergic, and glutamatergic neurons in the LDT, SubLDT, and adjoining medial part of the PPT (MPPT) in relation to sleep-wake states, cortical activity, and muscle tone. We found that all cholinergic neurons were maximally active during W and PS in positive correlation with fast (γ) cortical activity, as "W/PS-max active neurons." Like cholinergic neurons, many GABAergic and glutamatergic neurons were also "W/PS-max active." Other GABAergic and glutamatergic neurons were "PS-max active," being minimally active during W and maximally active during PS in negative correlation with muscle tone. Conversely, some glutamatergic neurons were "W-max active," being maximally active during W and minimally active during PS in positive correlation with muscle tone. Through different discharge profiles, the cholinergic, GABAergic, and glutamatergic neurons of the LDT, SubLDT, and MPPT thus appear to play distinct roles in promoting W and PS with cortical activation, PS with muscle atonia, or W with muscle tone.

The hypocretins (orexins) mediate the "phasic" components of REM sleep: A new hypothesis

In 1998, a group of phenotypically distinct neurons were discovered in the postero-lateral hypothalamus which contained the neuropeptides hypocretin 1 and hypocretin 2 (also called orexin A and orexin B), which are excitatory neuromodulators. Hypocretinergic neurons project throughout the central nervous system and have been involved in the generation and maintenance of wakefulness. The sleep disorder narcolepsy, characterized by hypersomnia and cataplexy, is produced by degeneration of these neurons. The hypocretinergic neurons are active during wakefulness in conjunction with the presence of motor activity that occurs during survival-related behaviors. These neurons decrease their firing rate during non-REM sleep; however there is still controversy upon the activity and role of these neurons during REM sleep. Hence, in the present report we conducted a critical review of the literature of the hypocretinergic system during REM sleep, and hypothesize a possible role of this system in the generation of REM sleep.

Carbachol excites sublaterodorsal nucleus neurons projecting to the spinal cord

Considerable electrophysiological and pharmacological evidence has long suggested an important role for acetylcholine in the regulation of rapid-eye-movement (REM) sleep. For example, injection of the cholinergic agonist carbachol into the dorsomedial pons produces an REM sleep-like state with muscle atonia and cortical activation, both of which are cardinal features of REM sleep. Located within this region of the pons is the sublaterodorsal nucleus (SLD), a structure thought to be both necessary and sufficient for generating REM sleep muscle atonia. Subsets of glutamatergic SLD neurons potently contribute to motor inhibition during REM sleep through descending projections to motor-related glycinergic/GABAergic neurons in the spinal cord and ventromedial medulla. Prior electrophysiological and pharmacological studies examining the effects of acetylcholine on SLD neurons have, however, produced conflicting results. In the present study, we sought to clarify how acetylcholine influences the activity of spinally projecting SLD (SLDsp) neurons. We used retrograde tracing in combination with patch-clamp recordings and recorded pre- and postsynaptic effects of carbachol on SLDsp neurons. Carbachol acted presynaptically by increasing the frequency of glutamatergic miniature excitatory postsynaptic currents. We also found that carbachol directly excited SLDsp neurons by activating an Na(+)-Ca(2+) exchanger. Both pre- and postsynaptic effects were mediated by co-activation of M1 and M3 muscarinic receptors. These observations suggest that acetylcholine produces synergistic, excitatory pre- and postsynaptic responses on SLDsp neurons that, in turn, probably serve to promote muscle atonia during REM sleep.

Synaptic interactions between perifornical lateral hypothalamic area, locus coeruleus nucleus and the oral pontine reticular nucleus are implicated in the stage succession during sleep-wakefulness cycle

The perifornical area in the posterior lateral hypothalamus (PeFLH) has been implicated in several physiological functions including the sleep-wakefulness regulation. The PeFLH area contains several cell types including those expressing orexins (Orx; also known as hypocretins), mainly located in the PeF nucleus. The aim of the present study was to elucidate the synaptic interactions between Orx neurons located in the PeFLH area and different brainstem neurons involved in the generation of wakefulness and sleep stages such as the locus coeruleus (LC) nucleus (contributing to wakefulness) and the oral pontine reticular nucleus (PnO) nucleus (contributing to REM sleep). Anatomical data demonstrated the existence of a neuronal network involving the PeFLH area, LC, and the PnO nuclei that would control the sleep-wake cycle. Electrophysiological experiments indicated that PeFLH area had an excitatory effect on LC neurons. PeFLH stimulation increased the firing rate of LC neurons and induced an activation of the EEG. The excitatory effect evoked by PeFLH stimulation in LC neurons was blocked by the injection of the Orx-1 receptor antagonist SB-334867 into the LC. Similar electrical stimulation of the PeFLH area evoked an inhibition of PnO neurons by activation of GABAergic receptors because the effect was blocked by bicuculline application into the PnO. Our data also revealed that the LC and PnO nuclei exerted a feedback control on neuronal activity of PeFLH area. Electrical stimulation of LC facilitated firing activity of PeFLH neurons by activation of catecholaminergic receptors whereas PnO stimulation inhibited PeFLH neurons by activation of GABAergic receptors. In conclusion, Orx neurons of the PeFLH area seem to be an important organizer of the wakefulness and sleep stages in order to maintain a normal succession of stages during the sleep-wakefulness cycle.

Distribution and intrinsic membrane properties of basal forebrain GABAergic and parvalbumin neurons in the mouse

The basal forebrain (BF) strongly regulates cortical activation, sleep homeostasis, and attention. Many BF neurons involved in these processes are GABAergic, including a subpopulation of projection neurons containing the calcium-binding protein, parvalbumin (PV). However, technical difficulties in identification have prevented a precise mapping of the distribution of GABAergic and GABA/PV+ neurons in the mouse or a determination of their intrinsic membrane properties. Here we used mice expressing fluorescent proteins in GABAergic (GAD67-GFP knock-in mice) or PV+ neurons (PV-Tomato mice) to study these neurons. Immunohistochemical staining for GABA in GAD67-GFP mice confirmed that GFP selectively labeled BF GABAergic neurons. GFP+ neurons and fibers were distributed throughout the BF, with the highest density in the magnocellular preoptic area (MCPO). Immunohistochemistry for PV indicated that the majority of PV+ neurons in the BF were large (>20 μm) or medium-sized (15-20 μm) GFP+ neurons. Most medium and large-sized BF GFP+ neurons, including those retrogradely labeled from the neocortex, were fast-firing and spontaneously active in vitro. They exhibited prominent hyperpolarization-activated inward currents and subthreshold "spikelets," suggestive of electrical coupling. PV+ neurons recorded in PV-Tomato mice had similar properties but had significantly narrower action potentials and a higher maximal firing frequency. Another population of smaller GFP+ neurons had properties similar to striatal projection neurons. The fast firing and electrical coupling of BF GABA/PV+ neurons, together with their projections to cortical interneurons and the thalamic reticular nucleus, suggest a strong and synchronous control of the neocortical fast rhythms typical of wakefulness and REM sleep.

Social and environmental contexts modulate sleep deprivation-induced c-Fos activation in rats

People often sleep deprive themselves voluntarily for social and lifestyle reasons. Animals also appear to stay awake longer as a result of their natural curiosity to explore novel environments and interact socially with conspecifics. Although multiple arousal systems in the brain are known to act jointly to promote and maintain wakefulness, it remains unclear whether these systems are similarly engaged during voluntary vs. forced wakefulness. Using c-Fos immunohistochemistry, we compared neuronal responses in rats deprived of sleep for 2 h by gentle sensory stimulation, exploration under social isolation, or exploration with social interaction, and rats under undisturbed control conditions. In many arousal, limbic, and autonomic nuclei examined (e.g., anterior cingulate cortex and locus coeruleus), the two sleep deprivation procedures involving exploration were similarly effective, and both were more effective than sleep deprivation with sensory stimulation, in increasing the number of c-Fos immunoreactive neurons. However, some nuclei (e.g., paraventricular hypothalamic nucleus and select amygdala nuclei) were more responsive to exploration with social interaction, while others (e.g., histaminergic tuberomammillary nucleus) responded more strongly to exploration in social isolation. In the rostral basal forebrain, cholinergic and GABAergic neurons responded preferentially to exploration with social interaction, whereas resident neurons in general responded most strongly to exploration without social interaction. These results indicate that voluntary exploration with/without social interaction is more effective than forced sleep deprivation with gentle sensory stimulation for inducing c-Fos in arousal and limbic/autonomic brain regions, and suggest that these nuclei participate in different aspects of arousal during sustained voluntary wakefulness.

Molecular regulation of GABAergic neuron differentiation and diversity in the developing midbrain

The midbrain GABAergic neurones control several aspects of behaviour, play important roles in psychiatric disease and are targets of medical treatments as well as drugs of abuse. However, their molecular diversity and regulation of development are only beginning to be understood. In this review, we briefly introduce distinct subpopulations of the midbrain GABAergic neurones and discuss knowledge on their development, including the developmental origins of midbrain GABAergic neurones as well as transcriptional regulatory mechanisms guiding their differentiation and identity. Important GABAergic neuron subpopulations are found within the dopaminergic (DA) nuclei in the ventral midbrain. GABAergic substantia nigra pars reticulata is the main output pathway of the basal ganglia system regulating voluntary movements. Recent studies have also highlighted importance of the GABAergic neurones associated with the ventral tegmental area for the control of DA neuron activity and motivated behaviours. Interestingly, the development of the GABAergic neurones associated with the DA nuclei is very different from the rest of the midbrain. Knowledge on developmental regulation can lead to insights into the molecular, structural and functional diversity of the midbrain GABAergic neurones and their subpopulations, cell groups of great physiological and medical interest.

Extrasynaptic GABAA receptors in rat pontine reticular formation increase wakefulness

STUDY OBJECTIVES

Gamma-aminobutyric acid (GABA) causes phasic inhibition via synaptic GABAA receptors and tonic inhibition via extrasynaptic GABAA receptors. GABA levels in the extracellular space regulate arousal state and cognition by volume transmission via extrasynaptic GABAA receptors. GABAergic transmission in the pontine reticular formation promotes wakefulness. No previous studies have determined whether an agonist at extrasynaptic GABAA receptors administered into the pontine reticular formation alters sleep and wakefulness. Therefore, this study used gaboxadol (THIP; agonist at extrasynaptic GABAA receptors that contain a δ subunit) to test the hypothesis that extrasynaptic GABAA receptors within the pontine reticular formation modulate sleep and wakefulness.

DESIGN

Within/between subjects.

SETTING

University of Michigan.

PATIENTS OR PARTICIPANTS

Adult male Crl:CD*(SD) (Sprague-Dawley) rats (n = 10).

INTERVENTIONS

Microinjection of gaboxadol, the nonsubtype selective GABAA receptor agonist muscimol (positive control), and saline (negative control) into the rostral pontine reticular formation.

MEASUREMENTS AND RESULTS

Gaboxadol significantly increased wakefulness and decreased both nonrapid eye movement sleep and rapid eye movement sleep in a concentration-dependent manner. Relative to saline, gaboxadol did not alter electroencephalogram power. Microinjection of muscimol into the pontine reticular formation of the same rats that received gaboxadol increased wakefulness and decreased sleep.

CONCLUSION

Tonic inhibition via extrasynaptic GABAA receptors that contain a δ subunit may be one mechanism by which the extracellular pool of endogenous GABA in the rostral pontine reticular formation promotes wakefulness.

CITATION

Vanini G; Baghdoyan HA. Extrasynaptic GABAA receptors in rat pontine reticular formation increase wakefulness. SLEEP 2013;36(3):337-343.

Hypocretinergic and non-hypocretinergic projections from the hypothalamus to the REM sleep executive area of the pons

Within the postero-lateral hypothalamus neurons that utilize hypocretin or melanin-concentrating hormone (MCH) as neuromodulators are co-distributed. These neurons have been involved in the control of behavioral states, and a deficit in the hypocretinergic system is the pathogenic basis of narcolepsy with cataplexy. In this report, utilizing immunohistochemistry and retrograde tracing techniques, we examined the hypocretinergic innervation of the nucleus pontis oralis (NPO), which is the executive site that is responsible for the generation of REM sleep in the cat. The retrograde tracer cholera toxin subunit b (CTb) was administered in pontine regions where carbachol microinjections induced REM sleep. Utilizing immunohistochemical techniques, we found that approximately 1% of hypocretinergic neurons in the tuberal area of the hypothalamus project to the NPO. In addition, approximately 6% of all CTb+ neurons in this region were hypocretinergic. The hypocretinergic innervation of the NPO was also compared with the innervation of the same site by MCH-containing neurons. More than three times as many MCHergic neurons were found to project to the NPO compared with hypocretinergic cells; both neuronal types exhibited bilateral projections. We also identified a group of non-hypocretinergic non-MCHergic neuronal group of neurons that were intermingled with both hypocretinergic and MCHergic neurons that also projected to this same brainstem region. These neurons were grater in number that either hypocretin or MCH-containing neurons; their soma size was also smaller and their projections were mainly ipsilateral. The present anatomical data suggest that hypocretinergic, MCHergic and an unidentified companion group of neurons of the postero-lateral hypothalamus participate in the regulation of the neuronal activity of NPO neurons, and therefore, are likely to participate in the control of wakefulness and REM sleep.

Knockdown of orexin type 2 receptor in the lateral pontomesencephalic tegmentum of rats increases REM sleep

Dysfunction of the orexin/hypocretin neurotransmitter system causes the sleep disorder narcolepsy, characterized by intrusion of rapid eye movement (REM) sleep-like events into normal wakefulness. The sites where orexins act to suppress REM sleep are incompletely understood. Previous studies suggested that the lateral pontomesencephalic tegmentum (lPMT) contains an important REM sleep inhibitory area, and proposed that orexins inhibit REM sleep via orexin type 2 receptors (OxR2) in this region. However, this hypothesis has heretofore not been tested. We thus performed bilateral injection of small interfering RNAs (siRNAs) targeting Ox2R into the lPMT on two consecutive days. This led to a approximately 30% increase of time spent in REM sleep in both the dark and light periods for the first 2 days after injection, with a return to baseline over the next two post-injection days. This increase was mainly due to longer (> 120 s) REM episodes. Cataplexy-like episodes were not observed. The percentage of time spent in wakefulness and non-(N)REM sleep, as well as the power spectral profile of NREM and REM sleep, were unaffected. Control animals injected with scrambled siRNA had no sleep changes post-injection. Quantification of the knockdown revealed that unilateral microinjection of siRNAs targeting OxR2 into the lPMT induced a approximately 40% reduction of OxR2 mRNA 2 days following the injections when compared with the contralateral side receiving control (scrambled) siRNA. Orexin type 1 receptor mRNA level was unaffected. Our results indicate that removal of OxR2 neurotransmission in the lPMT enhances REM sleep by increasing the duration of REM episodes.

LY2033298, a positive allosteric modulator at muscarinic M₄ receptors, enhances inhibition by oxotremorine of light-induced phase shifts in hamster circadian activity rhythms

RATIONALE

Entrainment of circadian rhythms to the light-dark cycle is essential for restorative sleep, and abnormal sleep timing is implicated in central nervous system (CNS) disorders like depression, schizophrenia, and Alzheimer's disease. Many transmitters, including acetylcholine, that exerts its actions via muscarinic receptors modulate the suprachiasmatic nucleus, the master pacemaker.

OBJECTIVES

Since positive allosteric modulators of muscarinic M(4) receptors are candidates for treatment of mood and cognitive deficits of CNS disorders, it is important to evaluate their circadian actions.

MATERIALS AND METHODS

The effects of intraperitoneally applied muscarinic agents on circadian wheel-running rhythms were measured employing hamsters, a model organism for studying activity rhythms.

RESULTS

Systemic administration of the muscarinic receptor agonist oxotremorine (0.01-0.04 mg/kg) inhibited light-induced phase delays and advances of hamster circadian wheel-running rhythms. The M₄ positive allosteric modulator, LY2033298 (10-40 mg/kg), had no effect on light-induced phase shifts when administered alone, yet significantly enhanced (at 20 mg/kg) the inhibitory influence of oxotremorine on light-induced phase delays. In addition, the muscarinic receptor antagonist, scopolamine, which was without effect on light-induced phase shifts when administered alone (0.001-0.1 mg/kg), antagonized (at 0.1 mg/kg) the inhibitory effect of oxotremorine and LY2033298 on light-induced phase delays.

CONCLUSIONS

These results are the first to demonstrate that systemically applied muscarinic receptor agonists modulate circadian activity rhythms, and they also reveal a specific role for M₄ receptors. It will be of importance to evaluate circadian actions of psychotropic drugs acting via M₄ receptors, since they may display beneficial properties under pathological conditions.

Knockouts reveal overlapping functions of M(2) and M(4) muscarinic receptors and evidence for a local glutamatergic circuit within the laterodorsal tegmental nucleus

Cholinergic neurons in the laterodorsal tegmental (LDT) and peduncolopontine tegmental (PPT) nuclei regulate reward, arousal, and sensory gating via major projections to midbrain dopamine regions, the thalamus, and pontine targets. Muscarinic acetylcholine receptors (mAChRs) on LDT neurons produce a membrane hyperpolarization and inhibit spike-evoked Ca(2+) transients. Pharmacological studies suggest M(2) mAChRs are involved, but the role of these and other localized mAChRs (M(1-)-M(4)) has not been definitively tested. To identify the underlying receptors and to circumvent the limited receptor selectivity of available mAChR ligands, we used light- and electron-immunomicroscopy and whole cell recording with Ca(2+) imaging in brain slices from knockout mice constitutively lacking either M(2), M(4), or both mAChRs. Immunomicroscopy findings support a role for M(2) mAChRs, since cholinergic and noncholinergic LDT and pedunculopontine tegmental neurons contain M(2)-specific immunoreactivity. However, whole cell recording revealed that the presence of either M(2) or M(4) mAChRs was sufficient, and that the presence of at least one of these receptors was required for these carbachol actions. Moreover, in the absence of M(2) and M(4) mAChRs, carbachol elicited both direct excitation and barrages of spontaneous excitatory postsynaptic potentials (sEPSPs) in cholinergic LDT neurons mediated by M(1) and/or M(3) mAChRs. Focal carbachol application to surgically reduced slices suggest that local glutamatergic neurons are a source of these sEPSPs. Finally, neither direct nor indirect excitation were knockout artifacts, since each was detected in wild-type slices, although sEPSP barrages were delayed, suggesting M(2) and M(4) receptors normally delay excitation of glutamatergic inputs. Collectively, our findings indicate that multiple mAChRs coordinate cholinergic outflow from the LDT in an unexpectedly complex manner. An intriguing possibility is that a local circuit transforms LDT muscarinic inputs from a negative feedback signal for transient inputs into positive feedback for persistent inputs to facilitate different firing patterns across behavioral states.

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.

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.

Molecular fingerprint of neuropeptide S-producing neurons in the mouse brain

Neuropeptide S (NPS) has been associated with a number of complex brain functions, including anxiety-like behaviors, arousal, sleep-wakefulness regulation, drug-seeking behaviors, and learning and memory. In order to better understand how NPS influences these functions in a neuronal network context, it is critical to identify transmitter systems that control NPS release and transmitters that are co-released with NPS. For this purpose, we generated several lines of transgenic mice that express enhanced green-fluorescent protein (EGFP) under control of the endogenous NPS precursor promoter. NPS/EGFP-transgenic mice show anatomically correct and overlapping expression of both NPS and EGFP. A total number of ∼500 NPS/EGFP-positive neurons are present in the mouse brain, located in the pericoerulear region and the Kölliker-Fuse nucleus. NPS and transgene expression is first detectable around E14, indicating a potential role for NPS in brain development. EGFP-positive cells were harvested by laser-capture microdissection, and mRNA was extracted for expression profiling by using microarray analysis. NPS was found co-localized with galanin in the Kölliker-Fuse nucleus of the lateral parabrachial area. A dense network of orexin/hypocretin neuronal projections contacting pericoerulear NPS-producing neurons was observed by immunostaining. Expression of a distinct repertoire of metabotropic and ionotropic receptor genes was identified in both NPS neuronal clusters that will allow for detailed investigations of incoming neurotransmission, controlling neuronal activity of NPS-producing neurons. Stress-induced functional activation of NPS-producing neurons was detected by staining for the immediate-early gene c-fos, thus supporting earlier findings that NPS might be part of the brain stress response network.

Cholinergic modulation of narcoleptic attacks in double orexin receptor knockout mice

To investigate how cholinergic systems regulate aspects of the sleep disorder narcolepsy, we video-monitored mice lacking both orexin (hypocretin) receptors (double knockout; DKO mice) while pharmacologically altering cholinergic transmission. Spontaneous behavioral arrests in DKO mice were highly similar to those reported in orexin-deficient mice and were never observed in wild-type (WT) mice. A survival analysis revealed that arrest lifetimes were exponentially distributed indicating that random, Markovian processes determine arrest lifetime. Low doses (0.01, 0.03 mg/kg, IP), but not a high dose (0.08 mg/kg, IP) of the cholinesterase inhibitor physostigmine increased the number of arrests but did not alter arrest lifetimes. The muscarinic antagonist atropine (0.5 mg/kg, IP) decreased the number of arrests, also without altering arrest lifetimes. To determine if muscarinic transmission in pontine areas linked to REM sleep control also influences behavioral arrests, we microinjected neostigmine (50 nl, 62.5 µM) or neostigmine + atropine (62.5 µM and 111 µM respectively) into the nucleus pontis oralis and caudalis. Neostigmine increased the number of arrests in DKO mice without altering arrest lifetimes but did not provoke arrests in WT mice. Co-injection of atropine abolished this effect. Collectively, our findings establish that behavioral arrests in DKO mice are similar to those in orexin deficient mice and that arrests have exponentially distributed lifetimes. We also show, for the first time in a rodent narcolepsy model, that cholinergic systems can regulate arrest dynamics. Since perturbations of muscarinic transmission altered arrest frequency but not lifetime, our findings suggest cholinergic systems influence arrest initiation without influencing circuits that determine arrest duration.

Endogenous GABA levels in the pontine reticular formation are greater during wakefulness than during rapid eye movement sleep

Studies using drugs that increase or decrease GABAergic transmission suggest that GABA in the pontine reticular formation (PRF) promotes wakefulness and inhibits rapid eye movement (REM) sleep. Cholinergic transmission in the PRF promotes REM sleep, and levels of endogenous acetylcholine (ACh) in the PRF are significantly greater during REM sleep than during wakefulness or non-REM (NREM) sleep. No previous studies have determined whether levels of endogenous GABA in the PRF vary as a function of sleep and wakefulness. This study tested the hypothesis that GABA levels in cat PRF are greatest during wakefulness and lowest during REM sleep. Extracellular GABA levels were measured during wakefulness, NREM sleep, REM sleep, and the REM sleep-like state (REM(Neo)) caused by microinjecting neostigmine into the PRF. GABA levels varied significantly as a function of sleep and wakefulness, and decreased significantly below waking levels during REM sleep (-42%) and REM(Neo) (-63%). The decrease in GABA levels during NREM sleep (22% below waking levels) was not statistically significant. Compared with NREM sleep, GABA levels decreased significantly during REM sleep (-27%) and REM(Neo) (-52%). Comparisons of REM sleep and REM(Neo) revealed no differences in GABA levels or cortical EEG power. GABA levels did not vary significantly as a function of dialysis site within the PRF. The inverse relationship between changes in PRF levels of GABA and ACh during REM sleep indicates that low GABAergic tone combined with high cholinergic tone in the PRF contributes to the generation of REM sleep.

Orexin/hypocretin plays a role in the response to physiological disequilibrium

In the decade since the discovery that pathology of the orexin/hypocretin system is causative for the sleep disorder narcolepsy, considerable progress has been made in understanding the functional role of the neuropeptide. Two, apparently separate functions of orexin have emerged as a consensus from studies to date. The first is the effect on vigilance state boundaries, as exemplified by narcolepsy. Thus the absence of orexin severely limits the ability to maintain prolonged periods of wakefulness or sleep and also allows the unregulated appearance of cataplexy as sudden muscle weakness during wakefulness. The second function is that orexin acts as a signaling molecule in transferring information about physiological disequilibrium to the central nervous system. Orexin activates the central arousal and motor systems during such disequilibrium and so may facilitate the necessary response and adaptation to restore equilibrium. A feasible relationship between these two functions is therefore that the maintenance of prolonged and active wakefulness is an integral part of this adaptive process. Furthermore, the limit placed on the onset of sleep by orexin suggests that these adaptive processes then continue during sleep to become integrated into the development of a coping strategy for the longer term.

Hypocretin and GABA interact in the pontine reticular formation to increase wakefulness

STUDY OBJECTIVES

Hypocretin-1/orexin A administered directly into the oral part of rat pontine reticular formation (PnO) causes an increase in wakefulness and extracellular gamma-aminobutyric acid (GABA) levels. The receptors in the PnO that mediate these effects have not been identified. Therefore, this study tested the hypothesis that the increase in wakefulness caused by administration of hypocretin-1 into the PnO occurs via activation of GABAA receptors and hypocretin receptors.

DESIGN

Within/between subjects.

SETTING

University of Michigan.

PATIENTS OR PARTICIPANTS

Twenty-three adult male Crl:CD*(SD) (Sprague Dawley) rats.

INTERVENTIONS

Microinjection of hypocretin-1, bicuculline (GABAA receptor antagonist), SB-334867 (hypocretin receptor-1 antagonist), and Ringer solution (vehicle control) into the PnO.

MEASUREMENTS AND RESULTS

Hypocretin-1 caused a significant concentration-dependent increase in wakefulness and decrease in rapid eye movement (REM) sleep and non-REM (NREM) sleep. Coadministration of SB-334867 and hypocretin-1 blocked the hypocretin-1-induced increase in wakefulness and decrease in both the NREM and REM phases of sleep. Coadministration of bicuculline and hypocretin-1 blocked the hypocretin-1-induced increase in wakefulness and decrease in NREM sleep caused by hypocretin-1.

CONCLUSION

The increase in wakefulness caused by administering hypocretin-1 to the PnO is mediated by hypocretin receptors and GABAA receptors in the PnO. These results show for the first time that hypocretinergic and GABAergic transmission in the PnO can interact to promote wakefulness.

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.

Immunohistochemical evidence for synaptic release of glutamate from orexin terminals in the locus coeruleus

Orexin (Orx or hypocretin) is critically important for maintaining wakefulness, since in its absence, narcolepsy with cataplexy occurs. In this role, Orx-containing neurons can exert their influence upon multiple targets through the brain by release of Orx but possibly also by release of other neurotransmitters. Indeed, evidence was previously presented to suggest that Orx terminals could utilize glutamate (Glu) in addition to Orx as a neurotransmitter. Using fluorescence and confocal laser scanning microscopy, we investigated whether Orx varicosities contain the presynaptic markers for synaptic release of Glu or GABA and come into contact with postsynaptic markers for excitatory synapses within the locus coeruleus of the rat brain. We found that a proportion of the Orx+ varicosities were immunostained for the vesicular transporter for Glu, VGluT2. None were immunostained for vesicular glutamate transporter 1 (VGluT1) or VGluT3 or for the vesicular transporter for GABA, vesicular GABA transporter (VGAT). Among the Orx+ varicosities, 4% of all and 28% of large varicosities contained VGluT2. A similar proportion of the large Orx+ varicosities contained synaptophysin (Syp), a presynaptic marker for synaptic vesicles. Orx+ varicosities also contacted elements immunostained for postsynaptic density protein-95 (PSD)-95, a postsynaptic marker for glutamatergic synapses. We thus conclude that synaptic release of Glu occurs from Orx terminals within the locus coeruleus and can thus be important for the engagement of noradrenergic neurons in stimulating and maintaining arousal.

Nicotine self-administration in the rat: effects of hypocretin antagonists and changes in hypocretin mRNA

RATIONALE

The hypocretin (hcrt) system has been implicated in addiction-relevant effects of several drugs, but its role in nicotine dependence has been little studied.

OBJECTIVES

These experiments examined the role of the hcrt system in nicotine reinforcement.

METHODS

Rats were trained for nicotine self-administration (NSA) on fixed-ratio schedules. The effects of acute, presession treatments with the hcrtR1 antagonist SB334867 and the hcrtR1/2 antagonist almorexant were examined on NSA maintained on a fixed-ratio (FR) 5 schedule. Gene expression for the hcrt system (mRNA for hcrt, hcrtR1, and hcrtR2) was measured in animals following NSA on a FR 1 schedule for a 19-day period.

RESULTS

The hcrtR1 antagonist SB334867 and the hcrtR1/2 antagonist almorexant both reduced NSA dose-dependently (significantly at doses of 30 and 300 mg/kg, respectively); SB334867 did not affect food-maintained responding whereas almorexant (at the 300 mg/kg) did. Tissue from animals collected 5 h after self-administration showed an increase in hcrtR1 mRNA in the arcuate nucleus compared to control subjects. In tissue collected immediately after a similar 19-day self-administration period, mRNA for hcrtR1 was decreased in the rostral lateral hypothalamus compared to controls.

CONCLUSIONS

These data confirm a previous report (Hollander et al., Proc Natl Acad Sci U S A 105:19480-19485, 2008) that the hypocretin receptor hcrtR1 is activated in nicotine reinforcement and in addition show that both the arcuate nucleus and lateral hypothalamus are sites at which hcrt receptor mechanisms may influence reinforcement. Different patterns of mRNA expression at different times after NSA suggest that changes in the hcrt system may be labile with time.

Sleep, activity, temperature and arousal responses of mice deficient for muscarinic receptor M2 or M4

AIMS

The type 2 muscarinic receptor (M2R) differs from the other G-protein-coupled muscarinic receptor (type 4, or M4R) in tissue distribution and physiologic effects. We studied the impact of these receptors on sleep and arousal by using M2R and M4R knock-out (KO) mice.

MAIN METHODS

M2R and M4R KO and genetically intact mice were compared in terms of normal patterns of sleep, responses to sleep loss, infectious challenge and acoustic startle, and acoustic prepulse inhibition of startle (PPI).

KEY FINDINGS

Under basal conditions, M2R and M4R KO mice do not differ from the background strain or each other in the amount or diurnal pattern of sleep, locomotor activity, and body temperature. After enforced sleep loss, M2R KO mice, in contrast to the other two strains, show no rebound in slow-wave sleep (SWS) time, although their SWS is consolidated, and they show a greater rebound in time spent in REMS (rapid-eye-movement sleep) and REMS consolidation. During influenza infection, M2R KO mice, as compared with the other strains, show marked hypothermia and a less robust increase in SWS. During Candida albicans infection, M2R KO mice show a greater increase in SWS and a greater inflammatory response than do the other strains. M2R KO mice also show greater acoustic startle amplitude than does the background strain, although PPI was not different across the 3 strains over a range of stimulus intensities.

SIGNIFICANCE

Taken together, these findings support different roles for M2R and M4R in the modulation of sleep and arousal during homeostatic challenge.

Hypocretin mechanisms in nicotine addiction: evidence and speculation

BACKGROUND

The hypocretin/orexin system has been implicated in arousal mechanisms, sleep, and sleep disorders, including narcolepsy, and more recently in drug addiction. Theoretically, hypocretin (hcrt) mechanisms appear to be potential substrates for nicotine addiction: arousal and attentional mechanisms influence use and withdrawal symptoms, and hcrt systems overlap anatomically with a number of brain regions associated with nicotine addiction.

OBJECTIVE

This review summarizes the studies that have examined hcrt mechanisms in the effects of nicotine and describes hcrt innervation of, and effects in, several brain regions implicated in nicotine addiction. The review speculates on the possible mechanisms by which hcrt may contribute to nicotine addiction in these regions, with the objective of encouraging research in this area.

RESULTS

In a small literature, both experimenter-administered and self-administered nicotine have been shown to elicit or depend on hcrt signaling. However, although untested in experimental designs, there is compelling evidence that hcrt mechanisms in the ventral tegmental area, the pontine region, thalamocortical circuits, the prefrontal cortex, and the amygdala could have a broad influence on nicotine addiction.

CONCLUSIONS

Evidence reviewed leads to the conclusion that hcrt mechanisms could mediate several dimensions of nicotine addiction, including a multi-faceted regulation of mesocorticolimbic dopaminergic function, but beyond dopaminergic mechanisms, hcrt could influence nicotine use and relapse during abstinence through broadly based arousal/attentional effects. These speculative ideas need to be examined experimentally; the potential gains are a more thorough understanding of the pathophysiology of nicotine addiction, and the discovery of novel targets for the development of pharmacotherapeutics.

Hypocretin-2 saporin lesions of the ventrolateral periaquaductal gray (vlPAG) increase REM sleep in hypocretin knockout mice

Ten years ago the sleep disorder narcolepsy was linked to the neuropeptide hypocretin (HCRT), also known as orexin. This disorder is characterized by excessive day time sleepiness, inappropriate triggering of rapid-eye movement (REM) sleep and cataplexy, which is a sudden loss of muscle tone during waking. It is still not known how HCRT regulates REM sleep or muscle tone since HCRT neurons are localized only in the lateral hypothalamus while REM sleep and muscle atonia are generated from the brainstem. To identify a potential neuronal circuit, the neurotoxin hypocretin-2-saporin (HCRT2-SAP) was used to lesion neurons in the ventral lateral periaquaductal gray (vlPAG). The first experiment utilized hypocretin knock-out (HCRT-ko) mice with the expectation that deletion of both HCRT and its target neurons would exacerbate narcoleptic symptoms. Indeed, HCRT-ko mice (n = 8) given the neurotoxin HCRT2-SAP (16.5 ng/23nl/sec each side) in the vlPAG had levels of REM sleep and sleep fragmentation that were considerably higher compared to HCRT-ko given saline (+39%; n = 7) or wildtype mice (+177%; n = 9). However, cataplexy attacks did not increase, nor were levels of wake or non-REM sleep changed. Experiment 2 determined the effects in mice where HCRT was present but the downstream target neurons in the vlPAG were deleted by the neurotoxin. This experiment utilized an FVB-transgenic strain of mice where eGFP identifies GABA neurons. We verified this and also determined that eGFP neurons were immunopositive for the HCRT-2 receptor. vlPAG lesions in these mice increased REM sleep (+79% versus saline controls) and it was significantly correlated (r = 0.89) with loss of eGFP neurons. These results identify the vlPAG as one site that loses its inhibitory control over REM sleep, but does not cause cataplexy, as a result of hypocretin deficiency.

Unilateral headache associated with a pontine infarction

Brain stem structures are implicated in the generation of migraine and other types of headache. The patient described herein had chronic left hemicranial headaches associated with a left pontine infarction.

Dorsomedial pontine neurons with descending projections to the medullary reticular formation express orexin-1 and adrenergic alpha2A receptor mRNA

Neurons located in the dorsomedial pontine rapid eye movement (REM) sleep-triggering region send axons to the medial medullary reticular formation (mMRF). This pathway is believed to be important for the generation of REM sleep motor atonia, but other than that they are glutamatergic little is known about neurochemical signatures of these pontine neurons important for REM sleep. We used single-cell reverse transcription and polymerase chain reaction (RT-PCR) to determine whether dorsomedial pontine cells with projections to the mMRF express mRNA for selected membrane receptors that mediate modulatory influences on REM sleep. Fluorescein (FITC)-labeled latex microspheres were microinjected into the mMRF of 26-34-day-old rats under pentobarbital anesthesia. After 5-6 days, rats were sacrificed, pontine slices were obtained and neurons were dissociated from 400 to 600 microm micropunches extracted from dorsomedial pontine reticular formation. We found that 32 out of 51 FITC-labeled cells tested (63+/-7% (SE)) contained the orexin type 1 receptor (ORX1r) mRNA, 27 out of 73 (37+/-6%) contained the adrenergic alpha(2A) receptor (alpha(2A)r) RNA, and 6 out of 31 (19+/-7%) contained both mRNAs. The percentage of cells positive for the ORX1r mRNA was significantly lower (p<0.04) for the dorsomedial pontine cells that were not retrogradely labeled from the mMRF (32+/-11%), whereas alpha(2A)r mRNA was present in a similar percentage of FITC-labeled and unlabeled neurons. Our data suggest that ORX and adrenergic pathways converge on a subpopulation of cells of the pontine REM sleep-triggering region that have descending projections to the medullary region important for the motor control during REM sleep.

Activity profiles of cholinergic and intermingled GABAergic and putative glutamatergic neurons in the pontomesencephalic tegmentum of urethane-anesthetized rats

Cholinergic neurons in the pontomesencephalic tegmentum form part of the ascending activating system and are thought to participate in stimulating cortical activation. Yet in the laterodorsal tegmental and pedunculopontine tegmental nuclei (LDT and PPT), they lie intermingled with GABAergic and glutamatergic neurons, which could also modulate cortical activity and sleep-wake state. To characterize the discharge of these cell types in relation to cortical activity, we recorded neurons in urethane-anesthetized rats during spontaneous slow wave and somatosensory evoked fast electroencephalographic (EEG) activity, then labeled the cells by juxtacellular technique with Neurobiotin (Nb) and dual-immunostained them for vesicular acetylcholine transporter (VAChT) and glutamic acid decarboxylase (GAD). All cholinergic cells discharged minimally during prestimulation (approximately 0.5 Hz) and moderately in a tonic manner (approximately 4 Hz) during stimulation. Being heterogeneous, some GABAergic, called "On," cells (approximately 48%) increased their discharge (from approximately 4 to 7 Hz), whereas others, called "Off" cells (approximately 38%), decreased or ceased firing during stimulation. Similarly, some noncholinergic/non-GABAergic On cells increased (from approximately 2 to 6 Hz, approximately 49%), whereas other Off cells decreased firing ( approximately 35%) during stimulation. Putative glutamatergic On together with GABAergic On neurons could thus act in parallel with cholinergic cells to stimulate cortical activation. Possibly influenced by cholinergic On and glutamatergic Off cells, whose change in discharge precedes theirs, the GABAergic Off cells could oppose neighboring neurons such as noradrenergic cells, which discharge during waking and cease firing during sleep. By concerted activity, these heterogeneous cell groups can modulate cortical activity and behavioral state across the sleep-waking cycle.