Chronic low-amplitude electrical stimulation of the laterodorsal tegmental nucleus of freely moving cats increases REM sleep

The mesopontine tegmentum in reward and aversion: From cellular heterogeneity to behaviour

The mesopontine tegmentum, comprising the pedunculopontine tegmentum (PPN) and the laterodorsal tegmentum (LDT), is intricately connected to various regions of the basal ganglia, motor systems, and limbic systems. The PPN and LDT can regulate the activity of different brain regions of these target systems, and in this way are in a privileged position to modulate motivated behaviours. Despite recent findings, the PPN and LDT have been largely overlooked in discussions about the neural circuits associated with reward and aversion. This review aims to provide a timely and comprehensive resource on past and current research, highlighting the PPN and LDT's connectivity and influence on basal ganglia and limbic, and motor systems. Seminal studies, including lesion, pharmacological, and optogenetic/chemogenetic approaches, demonstrate their critical roles in modulating reward/aversive behaviours. The review emphasizes the need for further investigation into the associated cellular mechanisms, in order to clarify their role in behaviour and contribution for different neuropsychiatric disorders.

Rapid eye movement sleep loss associated cytomorphometric changes and neurodegeneration

Rapid eye movement sleep (REMS) is essential for leading normal healthy living at least in higher-order mammals, including humans. In this review, we briefly survey the available literature for evidence linking cytomorphometric changes in the brain due to loss of REMS. As a mechanism of action, we add evidence that REMS loss elevates noradrenaline (NA) levels in the brain, which affects neuronal cytomorphology. These changes may be a compensatory mechanism as the changes return to normal after the subjects recover from the loss of REMS or if during REMS deprivation, the subjects are treated with NA-adrenoceptor antagonist prazosin (PRZ). We had proposed earlier that one of the fundamental functions of REMS is to maintain the level of NA in the brain. We elaborate on this idea to propose that if REMS loss continues without recovery, the sustained level of NA breaks down neurophysiologically active compensatory mechanism/s starting with changes in the neuronal cytomorphology, followed by their degeneration, leading to acute and chronic pathological conditions. Identification of neuronal cytomorphological changes could prove to be of significance for predicting future neuronal (brain) damage as well as an indicator for REMS health. Although current brain imaging techniques may not enable us to visualize changes in neuronal cytomorphology, given the rapid technological progress including use of artificial intelligence, we are optimistic that it may be a reality soon. Finally, we propose that maintenance of optimum REMS must be considered a criterion for leading a healthy life.

Sleep-Wake and Arousal Dysfunctions in Post-Traumatic Stress Disorder:Role of Orexin Systems

Post-traumatic stress disorder (PTSD) is a trauma-related condition that produces distressing fear memory intrusions, avoidance behaviors, hyperarousal/startle, stress responses and insomnia. This review focuses on the importance of the orexin neural system as a novel mechanism related to the pathophysiology of PTSD. Orexinergic neurons originate in the lateral hypothalamus and project widely to key neurotransmitter system neurons, autonomic neurons, the hypothalamic-pituitaryadrenal (HPA) axis, and fear-related neural circuits. After trauma or stress, the basolateral amygdala (BLA) transmits sensory information to the central nucleus of the amygdala (CeA) and in turn to the hypothalamus and other subcortical and brainstem regions to promote fear and threat. Orexin receptors have a prominent role in this circuit as fear conditioned orexin receptor knockout mice show decreased fear expression while dual orexin receptor antagonists (DORAs) inhibit fear acquisition and expression. Orexin activation of an infralimbic-amygdala circuit impedes fear extinction while DORA treatments enhance it. Increased orexin signaling to the amygdalocortical- hippocampal circuit promotes avoidance behaviors. Orexin has an important role in activating sympathetic nervous system (SNS) activity and the HPA axis stress responses. Blockade of orexin receptors reduces fear-conditioned startle responses. In PTSD models, individuals demonstrate sleep disturbances such as increased sleep latency and more transitions to wakefulness. Increased orexin activity impairs sleep by promoting wakefulness and reducing total sleep time while DORA treatments enhance sleep onset and maintenance. The orexinergic neural system provides important mechanisms for understanding multiple PTSD behaviors and provides new medication targets to treat this often persistent and debilitating illness.

Laterodorsal tegmentum-ventral tegmental area projections encode positive reinforcement signals

The laterodorsal tegmentum (LDT) is a brainstem nucleus classically involved in REM sleep and attention, and that has recently been associated with reward‐related behaviors, as it controls the activity of ventral tegmental area (VTA) dopaminergic neurons, modulating dopamine release in the nucleus accumbens. To further understand the role of LDT–VTA inputs in reinforcement, we optogenetically manipulated these inputs during different behavioral paradigms in male rats. We found that in a two‐choice instrumental task, optical activation of LDT–VTA projections shifts and amplifies preference to the laser‐paired reward in comparison to an otherwise equal reward; the opposite was observed with inhibition experiments. In a progressive ratio task, LDT–VTA activation boosts motivation, that is, enhances the willingness to work to get the reward associated with LDT–VTA stimulation; and the reverse occurs when inhibiting these inputs. Animals abolished preference if the reward was omitted, suggesting that LDT–VTA stimulation adds/decreases value to the stimulation‐paired reward. In addition, we show that LDT–VTA optical activation induces robust preference in the conditioned and real‐time place preference tests, while optical inhibition induces aversion. The behavioral findings are supported by electrophysiological recordings and c‐fos immunofluorescence correlates in downstream target regions. In LDT–VTA ChR2 animals, we observed an increase in the recruitment of lateral VTA dopamine neurons and D1 neurons from nucleus accumbens core and shell; whereas in LDT–VTA NpHR animals, D2 neurons appear to be preferentially recruited. Collectively, these data show that the LDT–VTA inputs encode positive reinforcement signals and are important for different dimensions of reward‐related behaviors.

Neural circuitry underlying REM sleep: A review of the literature and current concepts

As one of the fundamental sleep states, rapid eye movement (REM) sleep is believed to be associated with dreaming and is characterized by low-voltage, fast electroencephalographic activity and loss of muscle tone. However, the mechanisms of REM sleep generation have remained unclear despite decades of research. Several models of REM sleep have been established, including a reciprocal interaction model, limit-cycle model, flip-flop model, and a model involving γ-aminobutyric acid, glutamate, and aminergic/orexin/melanin-concentrating hormone neurons. In the present review, we discuss these models and summarize two typical disorders related to REM sleep, namely REM sleep behavior disorder and narcolepsy. REM sleep behavior disorder is a sleep muscle-tone-related disorder and can be treated with clonazepam and melatonin. Narcolepsy, with core symptoms of excessive daytime sleepiness and cataplexy, is strongly connected with orexin in early adulthood.

REM sleep loss-induced elevated noradrenaline could predispose an individual to psychosomatic disorders: a review focused on proposal for prediction, prevention, and personalized treatment

Historically and traditionally, it is known that sleep helps in maintaining healthy living. Its duration varies not only among individuals but also in the same individual depending on circumstances, suggesting it is a dynamic and personalized physiological process. It has been divided into rapid eye movement sleep (REMS) and non-REMS (NREMS). The former is unique that adult humans spend the least time in this stage, when although one is physically asleep, the brain behaves as if awake, the dream state. As NREMS is a pre-requisite for appearance of REMS, the latter can be considered a predictive readout of sleep quality and health. It plays a protective role against oxidative, stressful, and psychopathological insults. Several modern lifestyle activities compromise quality and quantity of sleep (including REMS) affecting fundamental physiological and psychopathosomatic processes in a personalized manner. REMS loss-induced elevated brain noradrenaline (NA) causes many associated symptoms, which are ameliorated by preventing NA action. Therefore, we propose that awareness about personalized sleep hygiene (including REMS) and maintaining optimum brain NA level should be of paramount significance for leading physical and mental well-being as well as healthy living. As sleep is a dynamic, multifactorial, homeostatically regulated process, for healthy living, we recommend addressing and treating sleep dysfunctions in a personalized manner by the health professionals, caregivers, family, and other supporting members in the society. We also recommend that maintaining sleep profile, optimum level of NA, and/or prevention of elevation of NA or its action in the brain must be seriously considered for ameliorating lifestyle and REMS disturbance-associated dysfunctions.

Circuit mechanisms and computational models of REM sleep

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REM sleep was discovered in the 1950s.

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Many hypothalamic and brainstem areas have been found to contribute to REM sleep.

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An up-to-date picture of REM-sleep-regulating circuits is reviewed.

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A brief overview of computational models for REM sleep regulation is provided.

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Outstanding issues for future studies are discussed.

Rapid eye movement (REM) sleep or paradoxical sleep is an elusive behavioral state. Since its discovery in the 1950s, our knowledge of the neuroanatomy, neurotransmitters and neuropeptides underlying REM sleep regulation has continually evolved in parallel with the development of novel technologies. Although the pons was initially discovered to be responsible for REM sleep, it has since been revealed that many components in the hypothalamus, midbrain, pons, and medulla also contribute to REM sleep. In this review, we first provide an up-to-date overview of REM sleep-regulating circuits in the brainstem and hypothalamus by summarizing experimental evidence from neuroanatomical, neurophysiological and gain- and loss-of-function studies. Second, because quantitative approaches are essential for understanding the complexity of REM sleep-regulating circuits and because mathematical models have provided valuable insights into the dynamics underlying REM sleep genesis and maintenance, we summarize computational studies of the sleep-wake cycle, with an emphasis on REM sleep regulation. Finally, we discuss outstanding issues for future studies.

Cholinergic, Glutamatergic, and GABAergic Neurons of the Pedunculopontine Tegmental Nucleus Have Distinct Effects on Sleep/Wake Behavior in Mice

The pedunculopontine tegmental (PPT) nucleus has long been implicated in the regulation of cortical activity and behavioral states, including rapid eye-movement (REM) sleep. For example, electrical stimulation of the PPT region during sleep leads to rapid awakening, whereas lesions of the PPT in cats reduce REM sleep. Though these effects have been linked with the activity of cholinergic PPT neurons, the PPT also includes intermingled glutamatergic and GABAergic cell populations, and the precise roles of cholinergic, glutamatergic, and GABAergic PPT cell groups in regulating cortical activity and behavioral state remain unknown. Using a chemogenetic approach in three Cre-driver mouse lines, we found that selective activation of glutamatergic PPT neurons induced prolonged cortical activation and behavioral wakefulness, whereas inhibition reduced wakefulness and increased non-REM (NREM) sleep. Activation of cholinergic PPT neurons suppressed lower-frequency electroencephalogram rhythms during NREM sleep. Last, activation of GABAergic PPT neurons slightly reduced REM sleep. These findings reveal that glutamatergic, cholinergic, and GABAergic PPT neurons differentially influence cortical activity and sleep/wake states.

SIGNIFICANCE STATEMENT

More than 40 million Americans suffer from chronic sleep disruption, and the development of effective treatments requires a more detailed understanding of the neuronal mechanisms controlling sleep and arousal. The pedunculopontine tegmental (PPT) nucleus has long been considered a key site for regulating wakefulness and REM sleep. This is mainly because of the cholinergic neurons contained in the PPT nucleus. However, the PPT nucleus also contains glutamatergic and GABAergic neurons that likely contribute to the regulation of cortical activity and sleep-wake states. The chemogenetic experiments in the present study reveal that cholinergic, glutamatergic, and GABAergic PPT neurons each have distinct effects on sleep/wake behavior, improving our understanding of how the PPT nucleus regulates cortical activity and behavioral states.

Evaluating the Evidence Surrounding Pontine Cholinergic Involvement in REM Sleep Generation

Rapid eye movement (REM) sleep - characterized by vivid dreaming, motor paralysis, and heightened neural activity - is one of the fundamental states of the mammalian central nervous system. Initial theories of REM sleep generation posited that induction of the state required activation of the "pontine REM sleep generator" by cholinergic inputs. Here, we review and evaluate the evidence surrounding cholinergic involvement in REM sleep generation. We submit that: (i) the capacity of pontine cholinergic neurotransmission to generate REM sleep has been firmly established by gain-of-function experiments, (ii) the function of endogenous cholinergic input to REM sleep generating sites cannot be determined by gain-of-function experiments; rather, loss-of-function studies are required, (iii) loss-of-function studies show that endogenous cholinergic input to the PTF is not required for REM sleep generation, and (iv) cholinergic input to the pontine REM sleep generating sites serve an accessory role in REM sleep generation: reinforcing non-REM-to-REM sleep transitions making them quicker and less likely to fail.

Optogenetic activation of cholinergic neurons in the PPT or LDT induces REM sleep

Rapid eye movement (REM) sleep is an important component of the natural sleep/wake cycle, yet the mechanisms that regulate REM sleep remain incompletely understood. Cholinergic neurons in the mesopontine tegmentum have been implicated in REM sleep regulation, but lesions of this area have had varying effects on REM sleep. Therefore, this study aimed to clarify the role of cholinergic neurons in the pedunculopontine tegmentum (PPT) and laterodorsal tegmentum (LDT) in REM sleep generation. Selective optogenetic activation of cholinergic neurons in the PPT or LDT during non-REM (NREM) sleep increased the number of REM sleep episodes and did not change REM sleep episode duration. Activation of cholinergic neurons in the PPT or LDT during NREM sleep was sufficient to induce REM sleep.

Sleep deficits in mild cognitive impairment are related to increased levels of plasma amyloid-β and cortical thinning

Evidence suggests that amyloid-beta (Aβ) depositions parallel sleep deficits in Alzheimer's disease (AD). However, it remains unknown whether impaired sleep and changes in plasma Aβ levels are related in amnestic mild cognitive impairment (aMCI) subjects, and whether both markers are further associated with cortical thinning in canonical AD regions. To jointly address this issue, we investigated relationships between changes in physiological sleep and plasma Aβ concentrations in 21 healthy old (HO) adults and 21 aMCI subjects, and further assessed whether these two factors were associated with cortical loss in each group. aMCI, but not HO subjects, showed significant relationships between disrupted slow-wave sleep (SWS) and increased plasma levels of Aβ42. We also found that shortened rapid-eye movement (REM) sleep in aMCI correlated with thinning of the posterior cingulate, precuneus, and postcentral gyrus; whereas higher levels of Aβ40 and Aβ42 accounted for grey matter (GM) loss of posterior cingulate and entorhinal cortex, respectively. These results support preliminary relationships between Aβ burden and altered sleep physiology observed in animal models of AD amyloidosis, and provide precise cortical correlates of these changes in older adults with aMCI. Taken together, these findings open new research avenues on the combined role of sleep, peripheral Aβ levels and cortical integrity in tracking the progression from normal aging to early neurodegeneration.

The cholinergic agonist carbachol increases the frequency of spontaneous GABAergic synaptic currents in dorsal raphe serotonergic neurons in the mouse

Dorsal raphe nucleus (DRN) serotonin (5-HT) neurons play an important role in feeding, mood control and stress responses. One important feature of their activity across the sleep-wake cycle is their reduced firing during rapid-eye-movement (REM) sleep which stands in stark contrast to the wake/REM-on discharge pattern of brainstem cholinergic neurons. A prominent model of REM sleep control posits a reciprocal interaction between these cell groups. 5-HT inhibits cholinergic neurons, and activation of nicotinic receptors can excite DRN 5-HT neurons but the cholinergic effect on inhibitory inputs is incompletely understood. Here, in vitro, in DRN brain slices prepared from GAD67-GFP knock-in mice, a brief (3 min) bath application of carbachol (50 μM) increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in GFP-negative, putative 5-HT neurons but did not affect miniature (tetrodotoxin-insensitive) IPSCs. Carbachol had no direct postsynaptic effect. Thus, carbachol likely increases the activity of local GABAergic neurons which synapse on 5-HT neurons. Removal of dorsal regions of the slice including the ventrolateral periaqueductal gray (vlPAG) region where GABAergic neurons projecting to the DRN have been identified, abolished the effect of carbachol on sIPSCs whereas the removal of ventral regions containing the oral region of the pontine reticular nucleus (PnO) did not. In addition, carbachol directly excited GFP-positive, GABAergic vlPAG neurons. Antagonism of both muscarinic and nicotinic receptors completely abolished the effects of carbachol. We suggest cholinergic neurons inhibit DRN 5-HT neurons when acetylcholine levels are lower i.e. during quiet wakefulness and the beginning of REM sleep periods, in part via excitation of muscarinic and nicotinic receptors located on local vlPAG and DRN GABAergic neurons. Higher firing rates or burst firing of cholinergic neurons associated with attentive wakefulness or phasic REM sleep periods leads to excitation of 5-HT neurons via the activation of nicotinic receptors located postsynaptically and presynaptically on excitatory afferents.

Signals from the brainstem sleep/wake centers regulate behavioral timing via the circadian clock

Sleep-wake cycling is controlled by the complex interplay between two brain systems, one which controls vigilance state, regulating the transition between sleep and wake, and the other circadian, which communicates time-of-day. Together, they align sleep appropriately with energetic need and the day-night cycle. Neural circuits connect brain stem sites that regulate vigilance state with the suprachiasmatic nucleus (SCN), the master circadian clock, but the function of these connections has been unknown. Coupling discrete stimulation of pontine nuclei controlling vigilance state with analytical chemical measurements of intra-SCN microdialysates in mouse, we found significant neurotransmitter release at the SCN and, concomitantly, resetting of behavioral circadian rhythms. Depending upon stimulus conditions and time-of-day, SCN acetylcholine and/or glutamate levels were augmented and generated shifts of behavioral rhythms. These results establish modes of neurochemical communication from brain regions controlling vigilance state to the central circadian clock, with behavioral consequences. They suggest a basis for dynamic integration across brain systems that regulate vigilance states, and a potential vulnerability to altered communication in sleep disorders.

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.

Activation of inactivation process initiates rapid eye movement sleep

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

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.

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.

GABAergic actions on cholinergic laterodorsal tegmental neurons: implications for control of behavioral state

Cholinergic neurons of the pontine laterodorsal tegmentum (LDT) play a critical role in regulation of behavioral state. Therefore, elucidation of mechanisms that control their activity is vital for understanding of how switching between wakefulness, sleep and anesthetic states is effectuated. In vivo studies suggest that GABAergic mechanisms within the pons play a critical role in behavioral state switching. However, the postsynaptic, electrophysiological actions of GABA on LDT neurons, as well as the identity of GABA receptors present in the LDT mediating these actions is virtually unexplored. Therefore, we studied the actions of GABA agonists and antagonists on cholinergic LDT cells by performing patch clamp recordings in mouse brain slices. Under conditions where detection of Cl(-) -mediated events was optimized, GABA induced gabazine (GZ)-sensitive inward currents in the majority of LDT neurons. Post-synaptic location of GABA(A) receptors was demonstrated by persistence of muscimol-induced inward currents in TTX and low Ca(2+) solutions. THIP, a selective GABA(A) receptor agonist with a preference for δ-subunit containing GABA(A) receptors, induced inward currents, suggesting the existence of extrasynaptic GABA(A) receptors. LDT cells also possess GABA(B) receptors as baclofen-activated a TTX- and low Ca(2+)-resistant outward current that was attenuated by the GABA(B) antagonists CGP 55845 and saclofen. The tertiapin sensitivity of baclofen-induced outward currents suggests that a G(IRK) mediated this effect. Further, outward currents were never additive with those induced by application of carbachol, suggesting that they were mediated by activation of GABA(B) receptors linked to the same G(IRK) activated in these cells by muscarinic receptor stimulation. Activation of GABA(B) receptors inhibited Ca(2+) increases induced by a depolarizing voltage step shown previously to activate VOCCs in cholinergic LDT neurons. Baclofen-mediated reductions in depolarization-induced Ca(2+) were unaltered by prior emptying of intracellular Ca(2+) stores, but were abolished by low extracellular Ca(2+) and pre-application of nifedipine, indicating that activation of GABA(B) receptors inhibits influx of Ca(2+) involving L-type Ca(2+) channels. Presence of GABA(C) receptors is suggested by the induction of inward current by (E)-4- amino-2-butenoic acid (TACA) and its inhibition by 1,2,5,6-tetrahydropyridine-4-ylmethylphosphinic (TPMPA), a relatively selective agonist and antagonist, respectively, of GABA(C) receptors. All of these GABA-mediated actions were found to occur in histochemically-identified cholinergic neurons. Taken together, these data indicate for the first time that cholinergic neurons of the LDT exhibit functional GABA(A, B and C) receptors, including extrasynaptically located GABA(A) receptors, which may be tonically activated by synaptic overflow of GABA. Accordingly, the activity of cholinergic LDT neurons is likely to be significantly affected by GABAergic tone within the nucleus, and so, demonstrated effects of GABA on behavioral state may be mediated, in part, via direct actions on cholinergic neurons in the LDT.

Interleukin-1 inhibits putative cholinergic neurons in vitro and REM sleep when microinjected into the rat laterodorsal tegmental nucleus

STUDY OBJECTIVES

REM sleep is suppressed during infection, an effect mimicked by the administration of cytokines such as interleukin-1 (IL-1). In spite of this observation, brain sites and neurochemical systems mediating IL-1-induced suppression of REM sleep have not been identified. Cholinergic neurons in the brainstem laterodorsal tegmental nucleus (LDT) are part of the neuronal circuitry responsible for REM sleep generation. Since IL-1 inhibits acetylcholine synthesis and release, the aim of this study was to test the two different, but related hypotheses. We hypothesized that IL-1 inhibits LDT cholinergic neurons, and that, as a result of this inhibition, IL-1 suppresses REM sleep.

DESIGN, MEASUREMENT, AND RESULTS

To test these hypotheses, the electrophysiological activity of putative cholinergic LDT neurons was recorded in a rat brainstem slice preparation. Interleukin-1 significantly inhibited the firing rate of 76% of recorded putative cholinergic LDT neurons and reduced the amplitude of glutamatergic evoked potentials in 60% of recorded neurons. When IL-1 (1 ng) was microinjected into the LDT of freely behaving rats, REM sleep was reduced by about 50% (from 12.7% +/- 1.5% of recording time [after vehicle] to 6.1% +/- 1.4% following IL-1 administration) during post-injection hours 3-4.

CONCLUSIONS

Results of this study support the hypothesis that IL-1 can suppress REM sleep by acting at the level of the LDT nucleus. Furthermore this effect may result from the inhibition of evoked glutamatergic responses and of spontaneous firing of putative cholinergic LDT neurons.

GABA in pedunculopontine tegmentum increases rapid eye movement sleep in freely moving rats: possible role of GABA-ergic inputs from substantia nigra pars reticulata

Pedunculopontine tegmentum (PPT) has GABA-ergic neurons and receives GABA-ergic projections from substantia nigra pars reticulata (SNrpr). Based on the recent studies from our and other laboratories, it was hypothesized that GABA in PPT promotes rapid eye movement (REM) sleep. In order to further study the role of GABA in PPT in REM sleep regulation, we microinjected GABA-A agonist, muscimol (200 nL, 3.5 mM), into the PPT. Muscimol in PPT significantly enhanced the amount of REM sleep by increasing the mean number of REM sleep bouts. Besides the local interneurons, GABA-ergic afferents from SNrpr are another source of GABA in PPT. In order to understand the contribution of GABA-ergic inputs from SNrpr into PPT for REM sleep regulation, SNrpr was electrically stimulated either alone or simultaneously along with the infusion of GABA-A antagonist, picrotoxin (200 nL, 0.86 mM), into the PPT. The experiment was designed with the premise that stimulation of SNrpr should increase GABA levels in PPT which should increase REM sleep comparable to that after muscimol microinjection in PPT. Further, the effect of stimulation of SNrpr on REM sleep should be antagonized by simultaneous infusion of picrotoxin into PPT. The electrical stimulation of SNrpr did not produce any significant change in sleep-wake states although it was sufficient to counter the effect of picrotoxin injection into the PPT. To overcome the limitations and confounds of electrical stimulation, SNrpr was pharmacologically stimulated by glutamate microinjection (200 nL, 5.34 mM). Infusion of glutamate into SNrpr enhanced REM sleep by increasing the mean number of REM sleep bouts, which was similar and comparable to the effect of muscimol injection into the PPT. The results confirm that GABA in PPT either from local neurons or from SNrpr promotes REM sleep.

Neurobiology of REM and NREM sleep

This paper presents an overview of the current knowledge of the neurophysiology and cellular pharmacology of sleep mechanisms. It is written from the perspective that recent years have seen a remarkable development of knowledge about sleep mechanisms, due to the capability of current cellular neurophysiological, pharmacological and molecular techniques to provide focused, detailed, and replicable studies that have enriched and informed the knowledge of sleep phenomenology and pathology derived from electroencephalographic (EEG) analysis. This chapter has a cellular and neurophysiological/neuropharmacological focus, with an emphasis on rapid eye movement (REM) sleep mechanisms and non-REM (NREM) sleep phenomena attributable to adenosine. The survey of neuronal and neurotransmitter-related brainstem mechanisms of REM includes monoamines, acetylcholine, the reticular formation, a new emphasis on GABAergic mechanisms and a discussion of the role of orexin/hypcretin in diurnal consolidation of REM sleep. The focus of the NREM sleep discussion is on the basal forebrain and adenosine as a mediator of homeostatic control. Control is through basal forebrain extracellular adenosine accumulation during wakefulness and inhibition of wakefulness-active neurons. Over longer periods of sleep loss, there is a second mechanism of homeostatic control through transcriptional modification. Adenosine acting at the A1 receptor produces an up-regulation of A1 receptors, which increases inhibition for a given level of adenosine, effectively increasing the gain of the sleep homeostat. This second mechanism likely occurs in widespread cortical areas as well as in the basal forebrain. Finally, the results of a new series of experimental paradigms in rodents to measure the neurocognitive effects of sleep loss and sleep interruption (modeling sleep apnea) provide animal model data congruent with those in humans.

GABAergic modulation of developing pedunculopontine nucleus

Rapid eye movement sleep decreases dramatically during development. We tested the hypothesis that some of this decrease may be due to GABAergic inhibition of reticular activating system neurons. Recordings of pedunculopontine neurons in vitro showed that the gamma-amino-butyric acid, receptor agonist muscimol depolarized noncholinergic cells early in the developmental decrease in rapid eye movement sleep, and hyperpolarized them later. Most cholinergic cells were hyperpolarized throughout the period tested. The gamma-amino-butyric acid b receptor agonist baclofen hyperpolarized both cholinergic and noncholinergic cells, although the degree of polarization decreased with age. Part of the gradual decrement in rapid eye movement sleep during development may be due in part to the increasing inhibition mediated by gamma-amino-butyric acid, a receptor on pedunculopontine neurons. This influence, however, appears to be mainly on noncholinergic cells.

Role of noradrenergic and GABA-ergic inputs in pedunculopontine tegmentum for regulation of rapid eye movement sleep in rats

Rapid eye movement (REM) sleep disturbance is associated with several psycho-behavioral disorders, hence, it is important to understand its neural mechanism of regulation. Although it was known that the noradrenergic (NA-ergic) neurons from locus coeruleus (LC) project to the pedunculopontine tegmentum (PPT), the role of noradrenaline (NA) alone and in association with GABA, an inhibitory neurotransmitter, in PPT for REM sleep regulation was not known and was investigated in this study in freely moving normally behaving rats. Rats were surgically prepared for electrophysiological sleep-wake recording and simultaneous bilateral microinjections into PPT. 200nl of prazosin (alpha1-antagonist) or clonidine (alpha2-agonist) or propranolol (beta-antagonist) or combination of picrotoxin (GABA-A antagonist) and clonidine or vehicle (control) was microinjected bilaterally into PPT using a remote-controlled pump and the effects on REM sleep compared. Prazosin, clonidine and propranolol increased the total time spent in REM sleep whereas co-injection of picrotoxin and clonidine did not affect REM sleep. The results suggest that NA in PPT tonically inhibits REM sleep, possibly by acting on the cholinergic REM-ON neurons, while GABA inhibits the release of NA for REM sleep regulation. A model of neural connections explaining such regulation has been presented.

Dialysis delivery of an adenosine A2Aagonist into the pontine reticular formation of C57BL/6J mouse increases pontine acetylcholine release and sleep

In vivo microdialysis in C57BL/6J (B6) mouse was used to test the hypothesis that activating adenosine A(2A) receptors in the pontine reticular formation (PRF) increases acetylcholine (ACh) release and rapid eye movement (REM) sleep. Eight concentrations of the adenosine A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS 21680; CGS) were delivered to the PRF and ACh in the PRF was quantified. ACh release was significantly increased by dialysis with 3 mum CGS and significantly decreased by dialysis with 10 and 100 microm CGS. Co-administration of the adenosine A(2A) receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385; 30 nM) blocked the CGS-induced increase in ACh release. In a second series of experiments, CGS (3 microm) was delivered by dialysis to the PRF for 2 h while recording sleep and wakefulness. CGS significantly decreased time in wakefulness (-51% in h 1; -54% in h 2), increased time in non-rapid eye movement (NREM) sleep (90% in h 1; 151% in h 2), and increased both time in REM sleep (331% in h 2) and the number of REM sleep episodes (488% in h 2). The enhancement of REM sleep is consistent with the interpretation that adenosine A(2A) receptors in the PRF of the B6 mouse contribute to REM sleep regulation, in part, by increasing ACh release in the PRF. A(2A) receptor activation may promote NREM sleep via GABAergic inhibition of arousal promoting neurons in the PRF.

Urotensin II acts as a modulator of mesopontine cholinergic neurons

Urotensin II (UII) is a vasomodulatory peptide that was not predicted to elicit CNS activity. However, because we have recently shown that the urotensin II receptor (UII-R) is selectively expressed in rat mesopontine cholinergic (MPCh) neurons, we hypothesize that UII may have a central function. The present study demonstrates that the UII system is able to modulate MPCh neuron activity. Brain slice experiments demonstrate that UII excites MPCh neurons of the mouse laterodorsal tegmentum (LDTg) by activating a slow inward current. Furthermore, microinfusion of UII into the ventral tegmental area produces a sustained increase in dopamine efflux in the nucleus accumbens, as measured by in vivo chronoamperometry. In agreement with UII activation of MPCh neurons, intracerebroventricular injections of UII significantly modulate ambulatory movements in both rats and mice but do not significantly affect startle habituation or prepulse inhibition. The present study establishes that UII is a neuromodulator that may be exploited to target disorders involving MPCh dysfunction.

Interactions between GABAergic and cholinergic processes in the nucleus pontis oralis: neuronal mechanisms controlling active (rapid eye movement) sleep and wakefulness

The cholinergic system within the nucleus pontis oralis (NPO) of the pontine tegmentum is critically involved in the generation of active (rapid eye movement) sleep. Previously, we demonstrated that a GABAergic system in the NPO also plays an important role in the control of the behavioral states of wakefulness as well as active sleep. The present study examined interactions between these two neuronal systems vis-a-vis the occurrence of these behavioral states. Accordingly, cholinergic and GABAergic agonists and antagonists were injected into the NPO, and their combined effects on sleep and waking states of chronic, unanesthetized cats were examined. Microinjections of carbachol into the NPO elicited active sleep with a short latency. However, a preinjection of muscimol (a GABA(A) agonist) completely blocked the active sleep-inducing effects of carbachol. The induction of active sleep by carbachol was also suppressed by a subsequent injection of muscimol. On the other hand, the microinjection of scopolamine (a muscarinic receptor antagonist) did not block the induction of active sleep by bicuculline (a GABA(A) antagonist). We conclude that the excitatory cholinergic control of NPO neurons that are involved in the generation of active sleep is gated by a pontine GABAergic system that exerts its effects postsynaptically by inhibiting NPO neurons, resulting in the suppression of active sleep and the generation of wakefulness. In the absence of the activation of this GABAergic gating mechanism, active sleep occurs. These results reveal that specific interactions between cholinergic and GABAergic processes in the NPO play a critical role in the generation of active sleep and wakefulness.

Laterodorsal tegmental projections to identified cell populations in the rat ventral tegmental area

Projections from the laterodorsal tegmentum (LDT) to the ventral tegmental area (VTA) contribute to the activity of dopamine (DA) and GABA cells and, hence, to the affective and cognitive functions of this region. LDT afferents arise from neurochemically diverse cell types and mediate multiple functional influences. However, the VTA cell populations that receive LDT afferents are unknown and were investigated here by anterograde and retrograde tract-tracing in combination with immunocytochemistry to distinguish DA and GABA cells. Approximately 50% of the LDT to VTA pathway formed asymmetric, presumably excitatory synapses that innervated DA and GABA cells in rough proportion to their representation within the VTA. This portion of the LDT innervation appeared to selectively target DA but not GABA mesoaccumbens neurons and provide a relatively nonselective input to both DA and GABA mesoprefrontal cells. The remaining LDT axons formed symmetric, presumably inhibitory synapses with a different pattern of cellular targets that included a preferential input to GABA neurons of both mesoaccumbens and mesoprefrontal populations and an apparently selective innervation of mesoprefrontal and not mesoaccumbens DA neurons. These data suggest that the LDT mediates a convergent excitatory and inhibitory influence on both mesoprefrontal DA and GABA cells but a divergent impact on mesoaccumbens neurons that is likely to excite DA cells and inhibit GABA neurons. Combined with our previous description of prefrontal cortical afferents, our data also indicate that mesoaccumbens DA neurons receive putative excitatory drive from the LDT, whereas mesoprefrontal DA cells receive convergent excitation from both cortical and brainstem sources.

GABA in pedunculo pontine tegmentum regulates spontaneous rapid eye movement sleep by acting on GABAA receptors in freely moving rats

REM-OFF and REM-ON neurons in the brainstem are reported to regulate REM sleep, however, the detailed mechanism of generation of REM sleep is unknown. The former are continuously active except during REM sleep and an inhibitory neurotransmitter, GABA, has been implicated in mediating the inhibition for the generation of REM sleep. The REM-ON neurons, on the other hand, remain inactive throughout but increase firing during REM sleep. This study was conducted to investigate if GABA in the brain area rich in cholinergic REM-ON neurons would modulate REM sleep as proposed earlier. Rats were surgically prepared for sleep-wake recording and two cannulae aiming pedunculopontine areas in the brainstem that are rich in REM-ON neurons, were implanted bilaterally. After recovery, picrotoxin, a GABA(A) antagonist, was simultaneously microinjected bilaterally into the pedunculopontine area in freely moving normally behaving rats using a remote dual syringe pump and the effects were studied on electrophysiological sleep and waking parameters. The results showed that picrotoxin significantly reduced REM sleep for 6h and the effect was due to reduction in the frequency of generation of REM sleep.

GABAA receptors inhibit acetylcholine release in cat pontine reticular formation: implications for REM sleep regulation

This study used in vivo microdialysis in cat (n=12) to test the hypothesis that gamma aminobutyric acid A (GABAA) receptors in the pontine reticular formation (PRF) inhibit acetylcholine (ACh) release. Animals were anesthetized with halothane to hold arousal state constant. Six concentrations of the GABAA receptor antagonist bicuculline (0.03, 0.1, 0.3, 1, 3, and 10 mM) were delivered to a dialysis probe in the PRF, and endogenously released ACh was collected simultaneously. Bicuculline caused a concentration dependent increase in ACh release (maximal increase=345%; EC50=1.3 mM; r2=0.997). Co-administration of the GABAA receptor agonist muscimol prevented the bicuculline-induced increase in ACh release. In a second series of experiments, the effects of bicuculline (0.1, 0.3, 1, and 3 mM) on ACh release were examined without the use of general anesthesia. States of wakefulness, rapid-eye-movement (REM) sleep, and non-REM sleep were identified polygraphically before and during dialysis delivery of bicuculline. Higher concentrations of bicuculline (1 and 3 mM) significantly increased ACh release during wakefulness (36%), completely suppressed non-REM sleep, and increased ACh release during REM sleep (143%). The finding that ACh release in the PRF is modulated by GABAA receptors is consistent with the interpretation that inhibition of GABAergic transmission in the PRF contributes to the generation of REM sleep, in part, by increasing pontine ACh release.

Role of the mesolimbic cholinergic projection to the septum in the production of 22 kHz alarm calls in rats

The role of the ascending cholinergic projection from the laterodorsal tegmental nucleus (LDT) to septum in the production of 22 kHz ultrasonic vocalization was studied in adult rats, using behavioral-pharmacological and anatomical tracing methods. Direct application of carbachol, a muscarinic agonist, into the lateral septal region induced species-typical 22 kHz alarm calls. The septum receives cholinergic input from LDT, thus, activation with glutamate of predominantly cholinergic neurons of the LDT induced comparable 22 kHz alarm calls in the same animals. This glutamate-induced response from LDT was significantly reduced when the lateral septum was pretreated with scopolamine, a cholinergic antagonist. To investigate the localization of the cell groups projecting to septum, the fluorescent retrograde tracer, fluorogold, was pressure injected into the lateral septum and sections from these brains were also immunostained against choline acetyltransferase (ChAT) to visualize cholinergic cell bodies. Several ChAT-fluorogold double-labeled cells within the boundaries of the LDT were found, while other fluorogold-labeled regions did not contain double-labeled cells. These results provide both direct and indirect evidence that at least a part of the mesolimbic ascending cholinergic projection from LDT to septum is involved in the initiation of the 22 kHz vocalization. It is concluded that the septum is an integral part of the medial cholinoceptive vocalization strip and the 22 kHz alarm vocalization is triggered from septum by the cholinergic input from the LDT.

Role of norepinephrine in the regulation of rapid eye movement sleep

Sleep and wakefulness are instinctive behaviours that are present across the animal species. Rapid eye movement (REM) sleep is a unique biological phenomenon expressed during sleep. It evolved about 300 million years ago and is noticed in the more evolved animal species. Although it has been objectively identified in its present characteristic form about half a century ago, the mechanics of how REM is generated, and what happens upon its loss are not known. Nevertheless, extensive research has shown that norepinephrine plays a crucial role in its regulation. The present knowledge that has been reviewed in this manuscript suggests that neurons in the brain stem are responsible for controlling this state and presence of excess norepinephrine in the brain does not allow its generation. Furthermore, REM sleep loss increases levels of norepinephrine in the brain that affects several factors including an increase in Na-K ATPase activity. It has been argued that such increased norepinephrine is ultimately responsible for REM sleep deprivation, associated disturbances in at least some of the physiological conditions leading to alteration in behavioural expression and settling into pathological conditions.

Selective activation of the extended ventrolateral preoptic nucleus during rapid eye movement sleep

We found previously that damage to a cluster of sleep-active neurons (Fos-positive during sleep) in the ventrolateral preoptic nucleus (VLPO) decreases non-rapid eye movement (NREM) sleep in rats, whereas injury to the sleep-active cells extending dorsally and medially from the VLPO cluster (the extended VLPO) diminishes REM sleep. These results led us to examine whether neurons in the extended VLPO are activated during REM sleep and the connectivity of these neurons with pontine sites implicated in producing REM sleep: the laterodorsal tegmental nucleus (LDT), dorsal raphe nucleus (DRN), and locus ceruleus (LC). After periods of dark exposure that triggered enrichment of REM sleep, the number of Fos-positive cells in the extended VLPO was highly correlated with REM but not NREM sleep. In contrast, the number of Fos-positive cells in the VLPO cluster was correlated with NREM but not REM sleep. Sixty percent of sleep-active cells in the extended VLPO and 90% of sleep-active cells in the VLPO cluster in dark-treated animals contained galanin mRNA. Retrograde tracing from the LDT, DRN, and LC demonstrated more labeled cells in the extended VLPO than the VLPO cluster, and 50% of these in the extended VLPO were sleep-active. Anterograde tracing showed that projections from the extended VLPO and VLPO cluster targeted the cell bodies and dendrites of DRN serotoninergic neurons and LC noradrenergic neurons but were not apposed to cholinergic neurons in the LDT. The connections and physiological activity of the extended VLPO suggest a specialized role in the regulation of REM sleep.

Effects of pedunculopontine nucleus (PPN) stimulation on caudal pontine reticular formation (PnC) neurons in vitro

Stimulation of the pedunculopontine nucleus (PPN) is known to induce changes in arousal and postural/locomotor states. Previously, PPN stimulation was reported to induce prolonged responses (PRs) in extracellularly recorded PnC neurons in the decerebrate cat. The present study used intracellular recordings in semihorizontal slices from rat brain stem (postnatal days 12-21) to determine responses in PnC neurons following PPN stimulation. Two-thirds (65%) of PnC neurons showed PRs after PPN stimulation. PnC neurons with PRs had higher amplitude afterhyperpolarizations (AHP) than non-PR (NPR) neurons. Both PR and NPR neurons were of mixed cell types characterized by "A" and/or "LTS," or neither of these types of currents. PnC cells showed decreased AHP duration with age, due mostly to decreased AHP duration in NPR cells. The longest mean duration PRs were induced by stimulation at 60 and 90 Hz compared with 10 or 30 Hz. Maximal firing rates in PnC cells during PRs were induced by PPN stimulation at 60 Hz compared with 10, 30, or 90 Hz. BaCl2 superfusion blocked PPN stimulation-induced PRs, suggesting that PRs may be mediated by blockade of potassium channels, in keeping with increased input resistance observed during PRs. Depolarizing pulses failed to elicit, and hyperpolarizing pulses failed to reset, PPN stimulation-induced PRs, suggesting that PRs may not be plateau potentials. Pharmacological testing revealed that nifedipine superfusion failed to block PPN stimulation-induced PRs; i.e., PRs may not be calcium channel-dependent. The muscarinic cholinergic agonist carbachol induced depolarization in most PR neurons tested, and the muscarinic cholinergic antagonist scopolamine reduced or blocked PPN stimulation-induced PRs in some PnC neurons, suggesting that some PRs may be due to muscarinic receptor activation. The nonspecific ionotropic glutamate receptor antagonist kynurenic acid failed to block PPN stimulation-induced PRs, as did the metabotropic glutamate receptor antagonist (R, S)-alphamethyl-4-carboxyphenylglycine, suggesting that PRs may not be mediated by glutamate receptors. These findings suggest that PPN stimulation-induced PRs may be due to increased excitability following closing of muscarinic receptor-sensitive potassium channels, allowing PnC neurons to respond to a transient, frequency-dependent depolarization with long-lasting stable states. PPN stimulation appears to induce PRs using parameters known best to induce locomotion. This mechanism may be related to switching from one state to another (e.g., locomotion vs. standing or sitting, waking vs. non-REM sleep or REM sleep).

Serotonin and sleep

For 50 years, serotonin has been in the centre of the search for the mechanisms and control of sleep. Serotonergic neurotransmission is related to the behavioural state of the animal and plays an important role in modulation of the behavioural state, by interacting with other brain areas modulating circadian rhythm, sleep and waking. Serotonergic activity may be accompanied by waking or sleep depending on the brain area and receptor type involved in the response, on the current behavioural state and on the concomitant agonism/antagonism of other neurotransmitter systems.

GABAergic neurons in prepositus hypoglossi regulate REM sleep by its action on locus coeruleus in freely moving rats

GABA in locus coeruleus modulates REM sleep. Apart from the presence of interneurons, locus coeruleus also receives GABAergic inputs from prepositus hypoglossi in the medulla, where the presence of REM-ON-like neurons have been reported. Therefore, it was hypothesized that GABAergic projections from prepositus hypoglossi to locus coeruleus may modulate REM sleep. The experiments were conducted on chronic rats prepared for recording EEG, EOG, and EMG in freely moving conditions. Bipolar stimulating electrodes were implanted in prepositus hypoglossi bilaterally, while chemitrodes were implanted bilaterally in locus coeruleus. The prepositus hypoglossi were bilaterally stimulated (3 Hz, 250 microsec, 100 microA) for 8 h in the presence and absence of picrotoxin (0.25 microg/250 nl) microinjection bilaterally in locus coeruleus, followed by poststimulation recording for 4 h. It was observed that stimulation of prepositus hypoglossi alone significantly increased REM sleep primarily by increasing the REM sleep duration per episode. However, when it was stimulated in the presence of picrotoxin in LC, REM sleep decreased, predominantly due to decreased REM sleep duration per episode. The results of this study suggest that GABAergic inputs from prepositus hypoglossi act on locus coeruleus and regulate REM sleep, possibly by inhibition of REM-OFF neurons.

Pedunculopontine stimulation induces prolonged activation of pontine reticular neurons

Extracellular and intracellular recordings were carried out from neurons in the region of the pontine reticular formation at the transition between the nucleus reticularis pontis oralis and caudalis, and in the pontis caudalis. Responses were studied after stimulation of the mesopontine cholinergic pedunculopontine nucleus in precollicular-postmammillary transected, paralyzed preparations. Recordings of neurographic activity in hindlimb flexor and extensor nerves served to detect changes in fictive locomotion and muscle tone induced by pedunculopontine nucleus stimulation or occurring spontaneously. Short duration trains of pedunculopontine nucleus stimulation induced long lasting responses, on average over 12s in duration, in one-third of pontine reticular neurons. These prolonged responses were stimulation frequency-dependent such that the longest durations were induced by stimulation at 20-60Hz. In some cells, stimulation at lower (10Hz) or higher (100Hz) frequencies induced responses of shorter duration or were absent, while in others, higher frequencies prolonged the excitatory effects of pedunculopontine nucleus stimulation. We conclude that these stimulation frequency-dependent effects may be related to the modulation of postural muscle tone and locomotion by the pedunculopontine nucleus.

Interactions between cholinergic and GABAergic neurotransmitters in and around the locus coeruleus for the induction and maintenance of rapid eye movement sleep in rats

The noradrenergic "REM-off" neurons in the locus coeruleus cease firing, whereas some cholinergic and non-cholinergic "REM-on" neurons increase firing during rapid eye movement sleep. A reciprocal interaction between these neurons was proposed. However, acetylcholine did not inhibit neurons in the locus coeruleus. Nevertheless, since GABA levels increase during rapid eye movement sleep and picrotoxin injections into the locus coeruleus reduced rapid eye movement sleep, it was hypothesized that GABA in the locus coeruleus might play an intermediary inhibitory role for rapid eye movement sleep regulation. Therefore, the effects of GABA or carbachol (a mixed cholinergic agonist receptor) alone, as well as an agonist of one in presence of an antagonist of the other, in the locus coeruleus were investigated on sleep-wakefulness and rapid eye movement sleep. The cholinergic agonist carbachol increased, while the muscarinic antagonist receptor scopolamine decreased, the frequency of induction of rapid eye movement sleep per hour. In contrast, GABA and picrotoxin increased and decreased, respectively, the duration of rapid eye movement sleep per episode. However, when carbachol was injected in the presence of picrotoxin or GABA was injected in the presence of scopolamine, the effect of GABA or picrotoxin was dominant. Microinjection of both scopolamine and picrotoxin in combination reduced both the frequency of initiation as well as the duration per episode of rapid eye movement sleep. From these results we suggest that in the locus coeruleus cholinergic input modulates the frequency of induction of rapid eye movement sleep and this action is mediated through GABA interneurons, whereas the length of rapid eye movement sleep per episode is maintained by the presence of an optimum level of GABA. A model of neural connections for initiation and maintenance of rapid eye movement sleep is proposed and discussed.

Lesions of the pedunculopontine tegmental nucleus reduce paradoxical sleep (PS) propensity: evidence from a short-term PS deprivation study in rats

Cholinergic neurons in the mesopontine tegmentum are thought to play a critical role in the generation of paradoxical sleep (PS). However, no study has yet examined whether lesions of these neurons cause deficits of PS in the rat. We describe here the effects of lesions of the pedunculopontine tegmental nucleus (PPT) on spontaneous PS and on PS propensity, expressed during and after a short period of PS deprivation. Lesions were induced by bilateral injections of ibotenate. PS deprivation was performed manually by gently waking rats each time they showed polygraphic signs of PS. Two weeks after lesions, an 8-h baseline recording was performed; the following day, rats were PS deprived for 6 h and polygraphic recordings were then continued for 2 h, to examine recovery sleep. The same protocol was repeated 1 week later. Compared with controls and with rats with limited PPT lesions, rats bearing > 60% NADPH-diaphorase-positive cell loss within the PPT showed unaffected PS under baseline conditions. However, they made fewer attempts to enter PS during deprivation and they exhibited an attenuated rebound increase in PS time after deprivation. The number of PS attempts and the magnitude of PS rebound were negatively correlated with the percent loss of diaphorase-positive neurons within the PPT. Thus, PS propensity that accumulated as a result of PS deprivation was reduced after extensive PPT lesions. In summary, although spontaneous PS was found to be unaltered, the PS deprivation procedure used in this study demonstrated the dysfunctioning of PS caused by PPT lesions.

GABAergic neurons of the laterodorsal and pedunculopontine tegmental nuclei of the cat express c-fos during carbachol-induced active sleep

The laterodorsal and pedunculopontine tegmental nuclei (LDT-PPT) are involved in the generation of active sleep (AS; also called REM or rapid eye movement sleep). Although the LDT-PPT are composed principally of cholinergic neurons that participate in the control of sleep and waking states, the function of the large number of GABAergic neurons that are also located in the LDT-PPT is unknown. Consequently, we sought to determine if these neurons are activated (as indicated by their c-fos expression) during active sleep induced by the microinjection of carbachol into the rostro-dorsal pons (AS-carbachol). Accordingly, immunocytochemical double-labeling techniques were used to identify GABA and Fos protein, as well as choline acetyltransferase (ChAT), in histological sections of the LDT-PPT. Compared to control awake cats, there was a larger number of GABAergic neurons that expressed c-fos during AS-carbachol (31.5+/-6.1 vs. 112+/-15.2, P<0.005). This increase in the number of GABA+Fos+ neurons occurred on the ipsilateral side relative to the injection site; there was a small decrease in GABA+Fos+ cells in the contralateral LDT-PPT. However, the LDT-PPT neurons that exhibited the largest increase in c-fos expression during AS-carbachol were neither GABA+ nor ChAT+ (47+/-22.5 vs. 228.7+/-14.0, P<0.0005). The number of cholinergic neurons that expressed c-fos during AS-carbachol was not significantly different compared to wakefulness. These data demonstrate that, during AS-carbachol, GABAergic as well as an unidentified population of neurons are activated in the LDT-PPT. We propose that these non-cholinergic LDT-PPT neurons may participate in the regulation of active sleep.

Chronic, low-level exposure to the cholinesterase inhibitor DFP. II. Time course of behavioral state changes in rats

Rats were repeatedly administered with low doses of diisopropylfluorophosphate (DFP; 0.2 mg/kg/day, SC), an irreversible cholinesterase (ChE) inhibitor. Control rats received a daily injection of oil vehicle or of saline. Recordings of the sleep-wake states were obtained in the 6 h following 1, 3, 6, 9, 13, 17, and 21 injections, as well as 2, 4, and 19 days after 9-day treatment. DFP administration increased waking at the expense of slow-wave sleep (SWS), but not of paradoxical sleep (PS); as a result, the PS/SWS ratio was strongly enhanced. These changes developed across days, were maximal after six to nine injections, and were then maintained at that level until cessation of treatment. This time course of behavioral state alterations paralleled the time course of ChE inhibition in the mesopontine cholinergic nuclei and the pontine reticular formation described in the companion article. In contrast, after DFP withdrawal, behavioral states returned to control values more rapidly (in 2-4 days) than did ChE activity. These results are discussed regarding the promoting role of cholinergic neurotransmission in brain-activated states.

Behavioral state control through differential serotonergic inhibition in the mesopontine cholinergic nuclei: a simultaneous unit recording and microdialysis study

Cholinergic neurons of the mesopontine nuclei are strongly implicated in behavioral state regulation. One population of neurons in the cholinergic zone of the laterodorsal tegmentum and the pedunculopontine nuclei, referred to as rapid eye movement (REM)-on neurons, shows preferential discharge activity during REM sleep, and extensive data indicate a key role in production of this state. Another neuronal group present in the same cholinergic zone of the laterodorsal tegmentum and the pedunculopontine nuclei, referred to as Wake/REM-on neurons, shows preferential discharge activity during both wakefulness and REM sleep and is implicated in the production of electroencephalographic activation in both of these states. To test the hypothesis of differential serotonergic inhibition as an explanation of the different state-related discharge activity, we developed a novel methodology that enabled, in freely behaving animals, simultaneous unit recording and local perfusion of neuropharmacological agents using a microdialysis probe adjacent to the recording electrodes. Discharge activity of REM-on neurons was almost completely suppressed by local microdialysis perfusion of the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), although this agonist had minimal or no effect on the Wake/REM-on neurons. We conclude that selective serotonergic inhibition is a basis of differential state regulation in the mesopontine cholinergic nuclei, and that the novel methodology combining neurophysiological and neuropharmacological information from the freely behaving animal shows great promise for further insight into the neural basis of behavioral control.

Carbachol stimulates [35S]guanylyl 5'-(gamma-thio)-triphosphate binding in rapid eye movement sleep-related brainstem nuclei of rat

Carbachol enhances rapid eye movement (REM) sleep when microinjected into the pontine reticular formation of the cat and rat. Carbachol elicits this REM sleep-like state via activation of postsynaptic muscarinic cholinergic receptors (mAChRs). The present study used in vitro autoradiography of carbachol-stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate ([35S]GTPgammaS) binding to test the hypothesis that carbachol activates mAChRs to induce stimulation of G-proteins in brainstem nuclei contributing to REM sleep generation. The results demonstrate a heterogeneous increase in carbachol-stimulated G-protein activation across rat brainstem. Binding of [35S]GTPgammaS in the presence of carbachol, compared with basal binding, was significantly increased in the laterodorsal tegmental nucleus (75.7%), caudal pontine reticular nucleus (68.9%), oral pontine reticular nucleus (64.5%), pedunculopontine tegmental nucleus (55.7%), and dorsal raphe nucleus (54.0%) but not in the nucleus locus coeruleus. The activation of G-proteins by carbachol was concentration-dependent and antagonized by atropine, demonstrating that G-proteins were activated via mAChR stimulation. The results provide the first direct measures of mAChR-activated G-proteins in brainstem nuclei known to contribute to REM sleep generation.

Mesolimbic component of the ascending cholinergic pathways: electrophysiological-pharmacological study

The cholinergic input from the pontomesencephalic cholinergic neurons to the diencephalic and basal forebrain structures has been implicated in a number of limbically controlled overt behaviors. The cellular mechanism by which the cholinergic terminals initiate behavioral manifestations is not clear. The objective of this study was to investigate the effects of the ascending cholinergic projection from the laterodorsal tegmental nucleus (LDT) on neuronal firing in the anterior hypothalamic-medial preoptic region (AHMP), known to be involved in agonistic behavior. Experiments were performed on urethan-anesthetized rats. Iontophoretic application of carbachol (CCh) into the vicinity of single cells in the AHMP caused a dose-dependent decrease in the mean firing rate of 83% of units and an increase in 10% of units. The inhibitory effect of CCh, but not the excitatory effect, was reversed by iontophoretic pretreatment with scopolamine. The inhibition of the firing rate was repeatable for the same dose of CCh and dose dependent. Electrical stimulation of neurons in the LDT caused a comparable, current-dependent decrease in the mean firing rate of AHMP neurons that also was reversed by pretreatment of neurons in the AHMP with scopolamine. The antagonizing effects of scopolamine were reversible with time. The same units in the AHMP that inhibited their firing to stimulation of the LDT also responded with a similar inhibition to local iontophoretic CCh. Finally, the fluorescent carbocyanine dye, 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide, (DiA), has been used as a retrograde axonal tracer and was injected into the recording sites immediately after the electrophysiological recordings. After 1 wk, DiA dye was found in numerous neurons in the LDT as shown by the fluorescence confocal microscopy. Results of the study suggest that LDT cholinergic neurons project and terminate in the AHMP and that their activation causes a decrease in the mean firing rate of the AHMP neurons. It is postulated that this inhibitory effect is implicated in the initiation of some of the behavioral patterns like defensive or alarm vocalization and behavioral inhibition.

Cholinergic and non-cholinergic afferents of the caudolateral parabrachial nucleus: a role in the long-term enhancement of rapid eye movement sleep

A single microinjection of the cholinergic agonist carbachol into the feline caudolateral parabrachial nucleus produces an immediate increase in state-independent ipsilateral ponto-geniculooccipital waves, followed by a long-term rapid eye movement sleep enhancement lasting 7-10 days. Using retrogradely-transported fluorescent carbachol-conjugated nanospheres and choline acetyltransferase immunohistochemistry, afferent projections to this injection site for long-term rapid eye movement sleep enhancement were mapped and quantified. Six regions in the brain stem contained retrogradely-labelled cells: the raphe nuclei, locus coeruleus, laterodorsal tegmental nucleus, pedunculopontine tegmental nucleus, parabrachial nucleus, and the pontine reticular formation. The retrogradely-labelled (rhodamine+) cells in the pontine reticular formation and pedunculopontine tegmental nucleus contributed the predominant input to the parabrachial nucleus injection site (34.3 +/- 5.3% and 28.4 +/- 5.6%, respectively), compared to the laterodorsal tegmental nucleus (5.8 +/- 3.8%), parabrachial nucleus (13.5 +/- 3.1%), raphe nuclei (12.9 +/- 2.7%), and locus coeruleus (5.1 +/- 2.4%). By comparison with findings of afferent input to the induction site for short-latency rapid eye movement sleep in the anterodorsal pontine reticular formation, the parabrachial nucleus injection site is characterized by a similar proportion of afferents, except that the raphe nuclei were found to provide more than a two-fold greater input. Retrogradely-labelled neurons quantified in these nuclear regions consisted of 21.5% double-labelled (rhodamine+/choline acetyltransferase+) cholinergic and 78.5% noncholinergic (rhodamine+/choline acetyltransferase-) cells. The pedunculopontine tegmental nucleus contributed the predominant (51.7 +/- 8.2%) cholinergic input, compared to laterodorsal tegmental nucleus (20.7 +/- 10.2%), parabrachial nucleus (23.1 +/- 7.5%), and pontine reticular formation (4.4 +/- 2.1%). A comparative analysis of the total retrogradely-labelled cells within each nuclear region which were also double-labelled showed the highest proportion in the laterodorsal tegmental nucleus (76.2 +/- 7.5%) compared to pedunculopontine tegmental nucleus (39.4 +/- 3.6%), parabrachial nucleus (37.3 +/- 2.8%), and pontine reticular formation (3.2 +/- 2.1%). These data indicate that while pedunculopontine tegmental nucleus and laterodorsal tegmental nucleus neurons exert a powerful cholinergic influence on the injection site for long-term rapid eye movement enhancement, a major component of the afferent circuitry is non-cholinergic. Since the non-cholinergic input includes contributions from the locus coeruleus and raphe nuclei, it is probable that the caudolateral parabrachial nucleus contains cholinergic and aminergic afferent systems that participate in the long-term enhancement of rapid eye movement sleep.

The neuropsychology of REM sleep dreaming

Recent PET imaging and brain lesion studies in humans are integrated with new basic research findings at the cellular level in animals to explain how the formal cognitive features of dreaming may be the combined product of a shift in neuromodulatory balance of the brain and a related redistribution of regional blood flow. The human PET data indicate a preferential activation in REM of the pontine brain stem and of limbic and paralimbic cortical structures involved in mediating emotion and a corresponding deactivation of dorsolateral prefrontal cortical structures involved in the executive and mnemonic aspects of cognition. The pontine brainstem mechanisms controlling the neuromodulatory balance of the brain in rats and cats include noradrenergic and serotonergic influences which enhance waking and impede REM via anticholinergic mechanisms and cholinergic mechanisms which are essential to REM sleep and only come into full play when the serotonergic and noradrenergic systems are inhibited. In REM, the brain thus becomes activated but processes its internally generated data in a manner quite different from that of waking.