Influence of hypnogenic brain areas on wakefulness- and rapid-eye-movement sleep-related neurons in the brainstem of freely moving cats

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.

Pedunculo-pontine tegmentum cholinergic REM-ON neurons modulate ventral tegmental neurons to modulate rapid eye movement sleep in rats

The interaction among the acetylcholine (ACh)-ergic REM-ON neurons in the pedunculo-pontine area (PPT), noradrenergic REM-OFF neurons in locus coeruleus (LC) and GABA-ergic neurons in the regulation of rapid eye movement sleep (REMS) have been studied in relative details; however, many questions including the role of dopamine (DA) remain unanswered. The ventral tegmental area (VTA) is rich in DA-ergic neurons, which have been implicated with schizophrenia and depression, when REMS is significantly affected. Also, some of the symptoms of REMS and these diseases are common. As the ACh-ergic REM-ON neurons in the PPT project to VTA, we proposed that such inputs might affect REMS, dreams and hallucinations. We recorded sleep-wake-REMS in freely moving, chronically prepared rats under three controlled experimental conditions. In different sets of experiments, either the ACh-ergic inputs to the VTA were blocked by local microinjection of Scopolamine (Scop) alone, or, the PPT neurons were bilaterally stimulated by Glutamate (Glut), or, the PPT neurons were stimulated by Glut in presence of Scop into the VTA. It was observed that Glut into PPT and Scop into the VTA significantly increased and decreased REMS, respectively. Additionally, PPT stimulation induced increased REMS was prevented in the presence of Scop into the VTA. Based on these findings we propose that inputs from ACh-ergic REM-ON neurons to VTA increase REMS and it could be a possible circuitry for expressions of hallucinations and dreams.

Dopaminergic- and cholinergic-inputs from substantia nigra and pedunculo-pontine tegmentum, respectively, converge in amygdala to modulate rapid eye movement sleep in rats

Dreams appear intermittently during phasic rapid eye movement sleep (REMS). Although reasonable progress has been made about neuro-physio-pharmacological mechanism of appearance of REMS, appearance of dreams is a mystery. Isolated studies have reported that substantia nigra (SN) withdraws inhibition from pedunculo-pontine tegmentum (PPT) acetylcholine (ACh)-ergic REM-ON neurons to trigger REMS; some REM-ON neurons become phasically active during REMS; amygdala (Amyg), a limbic structure associated with emotions, may be related with dreaming like state; Amyg receives projections from both SN-Dopamine (DA)-ergic and PPT-ACh-ergic neurons. Collating these isolated findings, we proposed that on the background of REMS, SN-DA-ergic and PPT-ACh-ergic inputs phasically activate Amyg-neurons to manifest dreams. In the absence of better criteria, we recorded electrophysiological characteristics of REMS as the closest objective read-out for dreams in surgically prepared, chronic, freely moving rats. Microinjection of either DA-ergic or ACh-ergic agonist [Quinpirole (Qnp) or Carbachol (Carb)] bilaterally into Amyg increased, while antagonists [Haloperidol (Hal) or Scopolamine (Scop)] reduced REMS. Electrical stimulation of either bilateral SN or PPT increased REMS, which however, was prevented when stimulated in presence of Hal or Scop, respectively into the Amyg. These findings confirm and support our contention that SN-DA-ergic and PPT-ACh-ergic inputs integrate in Amyg for REMS regulation. Further, subject to confirmation in humans, we propose that on the background of REMS, some phasic PPT-ACh-ergic-REM-ON neurons intermittently trigger some neurons in Amyg, the area known to be associated with memory and emotions, causing intermittent appearance of REMS-associated dreams and in REMS behavior disorder.

Interplay of dopamine and GABA in substantia nigra for the regulation of rapid eye movement sleep in rats

Substantia nigra (SN) is rich in dopamine (DA)-ergic and GABA-ergic neurons, which project to and receive inputs from locus coeruleus (LC) and pedunculo-pontine tegmentum (PPT) possessing REM-OFF and REM-ON neurons, respectively. Loss of DA-ergic neurons and disturbed REM sleep (REMS) are associated with Parkinson's disease, depression and REMS behavior disorder. GABA-ergic projections from SN act pre-synaptically on the noradrenaline (NA)-ergic terminals coming from the LC-REM-OFF neurons onto the REM-ON neurons in PPT and play a critical role in initiating REMS. However, it was unknown how SN neurons get activated and whether the SN-DA-ergic neurons interact with the SN-GABA-ergic neurons for REMS regulation. In freely moving chronically prepared rats, neurons in SN (bilateral) were stimulated by local microinjection of Glutamate (Glut), sparing the fibers of passage, in the presence and absence of i.p. Haloperidol (Hal, DA-antagonist). In other sets, either Hal or Bicuculine (Bic, GABA-antagonist) alone or simultaneously was microinjected bilaterally into the SN and the effects on sleep-wakefulness were recorded. We observed that Glut in SN significantly increased REMS, which was prevented by Hal. REMS was decreased and increased by Hal and Bic, respectively; while their co-injection neutralized (ineffective) the individual effects. Combining these findings with previous reports suggest that the SN-DA-ergic neurons act on the SN-GABA-ergic to regulate REMS. The results advance our understanding of the neuro-anatomo-chemical connections and pharmaco-physiological regulation of REMS in health and diseases.

Relevance of deprivation studies in understanding rapid eye movement sleep

Rapid eye movement sleep (REMS) is a unique phenomenon essential for maintaining normal physiological processes and is expressed at least in species higher in the evolution. The basic scaffold of the neuronal network responsible for REMS regulation is present in the brainstem, which may be directly or indirectly influenced by most other physiological processes. It is regulated by the neurons in the brainstem. Various manipulations including chemical, elec-trophysiological, lesion, stimulation, behavioral, ontogenic and deprivation studies have been designed to understand REMS genesis, maintenance, physiology and functional significance. Although each of these methods has its significance and limitations, deprivation studies have contributed significantly to the overall understanding of REMS. In this review, we discuss the advantages and limitations of various methods used for REMS deprivation (REMSD) to understand neural regulation and physiological significance of REMS. Among the deprivation strategies, the flowerpot method is by far the method of choice because it is simple and convenient, exploits physiological parameter (muscle atonia) for REMSD and allows conducting adequate controls to overcome experimental limitations as well as to rule out nonspecific effects. Notwithstanding, a major criticism that the flowerpot method faces is that of perceived stress experienced by the experimental animals. Nevertheless, we conclude that like most methods, particularly for in vivo behavioral studies, in spite of a few limitations, given the advantages described above, the flowerpot method is the best method of choice for REMSD studies.

Disciplined sleep for healthy living: Role of noradrenaline

Sleep is essential for maintaining normal physiological processes. It has been broadly divided into rapid eye movement sleep (REMS) and non-REMS (NREMS); one spends the least amount of time in REMS. Sleep (both NREMS and REMS) disturbance is associated with most altered states, disorders and pathological conditions. It is affected by factors within the body as well as the environment, which ultimately modulate lifestyle. Noradrenaline (NA) is one of the key molecules whose level increases upon sleep-loss, REMS-loss in particular and it induces several REMS-loss associated effects and symptoms. The locus coeruleus (LC)-NAergic neurons are primarily responsible for providing NA throughout the brain. As those neurons project to and receive inputs from across the brain, they are modulated by lifestyle changes, which include changes within the body as well as in the environment. We have reviewed the literature showing how various inputs from outside and within the body integrate at the LC neuronal level to modulate sleep (NREMS and REMS) and vice versa. We propose that these changes modulate NA levels in the brain, which in turn is responsible for acute as well as chronic psycho-somatic disorders and pathological conditions.

Targeting modulation of noradrenalin release in the brain for amelioration of REMS loss-associated effects

Rapid eye movement sleep (REMS) loss affects most of the physiological processes, and it has been proposed that REMS maintains normal physiological processes. Changes in cultural, social, personal traits and life-style severely affect the amount and pattern of sleep, including REMS, which then manifests symptoms in animals, including humans. The effects may vary from simple fatigue and irritability to severe patho-physiological and behavioral deficits such as cognitive and behavioral dysfunctions. It has been a challenge to identify a molecule(s) that may have a potential for treating REMS loss-associated symptoms, which are very diverse. For decades, the critical role of locus coeruleus neurons in regulating REMS has been known, which has further been supported by the fact that the noradrenalin (NA) level is elevated in the brain after REMS loss. In this review, we have collected evidence from the published literature, including those from this laboratory, and argue that factors that affect REMS and vice versa modulate the level of a common molecule, the NA. Further, NA is known to affect the physiological processes affected by REMS loss. Therefore, we propose that modulation of the level of NA in the brain may be targeted for treating REMS loss-related symptoms. Further, we also argue that among the various ways to affect the release of NA-level, targeting α₂ adrenoceptor autoreceptor on the pre-synaptic terminal may be the better option for ameliorating REMS loss-associated symptoms.

REM sleep loss associated changes in orexin-A levels in discrete brain areas in rats

Rapid eye movement sleep (REMS) serves house-keeping function of the brain and its loss affects several pathophysiological processes. Relative levels of neurotransmitters including orexin A (Orx-A) in various parts of the brain in health and diseases are among the key factors for modulation of behaviors, including REMS. The level of neurotransmitter in an area in the brain directly depends on number of projecting neurons and their firing rates. The locus coeruleus (LC), the site of REM-OFF neurons, receives densest, while the pedunculo-pontine area (PPT), the site of REM-ON neurons receives lesser projections from the Orx-ergic neurons. Further, the Orx-ergic neurons are active during waking and silent during REMS and NREMS. Therefore, the level of Orx-A in discrete regions of the brain is likely to be different during normal and altered states, which in turn is likely to be responsible for altered behaviors in health and diseases, including in relation to REMS. Therefore, in the present study, we estimated Orx-A level in LC, cortex, posterior hypothalamus (PH), hippocampus, and PPT after 96 h REMSD, in post-deprivation recovered rats and in control rats. This is the first report of estimation of Orx-A in different brain regions after prolonged REMSD. It was observed that after REMSD the Orx-A level increased significantly in LC, cortex and PH which returned to normal level after recovery; however, the level did not change in the hippocampus and PPT. The Orx-A induced modulation of REMS could be secondary to increased waking.

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.

A mathematical model towards understanding the mechanism of neuronal regulation of wake-NREMS-REMS states

In this study we have constructed a mathematical model of a recently proposed functional model known to be responsible for inducing waking, NREMS and REMS. Simulation studies using this model reproduced sleep-wake patterns as reported in normal animals. The model helps to explain neural mechanism(s) that underlie the transitions between wake, NREMS and REMS as well as how both the homeostatic sleep-drive and the circadian rhythm shape the duration of each of these episodes. In particular, this mathematical model demonstrates and confirms that an underlying mechanism for REMS generation is pre-synaptic inhibition from substantia nigra onto the REM-off terminals that project on REM-on neurons, as has been recently proposed. The importance of orexinergic neurons in stabilizing the wake-sleep cycle is demonstrated by showing how even small changes in inputs to or from those neurons can have a large impact on the ensuing dynamics. The results from this model allow us to make predictions of the neural mechanisms of regulation and patho-physiology of REMS.

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.

REM sleep loss increases brain excitability: role of noradrenaline and its mechanism of action

Ever since the discovery of rapid eye movement sleep (REMS), studies have been undertaken to understand its necessity, function and mechanism of action on normal physiological processes as well as in pathological conditions. In this review, first, we briefly surveyed the literature which led us to hypothesise REMS maintains brain excitability. Thereafter, we present evidence from in vivo and in vitro studies tracing behavioural to cellular to molecular pathways showing REMS deprivation (REMSD) increases noradrenaline level in the brain, which stimulates neuronal Na-K ATPase, the key factor for maintaining neuronal excitability, the fundamental property of a neuron for executing brain functions; we also show for the first time the role of glia in maintaining ionic homeostasis in the brain. As REMSD exerts a global effect on most of the physiological processes regulated by the brain, we propose that REMS possibly serves a housekeeping function in the brain. Finally, subject to confirmation from clinical studies, based on the results reviewed here, it is being proposed that the subjects suffering from REMS loss may be effectively treated by reducing either noradrenaline level or Na-K ATPase activity in the brain.

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.

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.

Role of alpha and beta adrenoceptors in locus coeruleus stimulation-induced reduction in rapid eye movement sleep in freely moving rats

Based on the results of independent studies the involvement of norepinephrine in REM sleep regulation was known. Isolated studies showed that the effect could be mediated through either one or more subtypes of adrenoceptors. Earlier we have reported that REM-OFF neurons continue firing during REM sleep deprivation and mild but continuous stimulation of locus coeruleus (LC) or picrotoxin injection into the LC, that did not allow the REM-OFF neurons in the LC to stop firing, reduced REM sleep. However, the mechanism of action and type of adrenoreceptors involved in REM sleep regulation were unknown. The possible mechanism of action has been investigated in this study. It was proposed that if LC stimulation-induced decrease in REM sleep was due to norepinephrine, adrenergic antagonist must prevent the effect. Therefore, in this study, the effects of alpha1, alpha2 and beta-antagonists, viz. prazosin, yohimbine and propranolol, respectively, and alpha2 agonist, clonidine, on LC stimulation-induced reduction in REM sleep were investigated. The results showed that stimulation of LC inhibited REM sleep by reducing the frequency of generation of REM sleep, although the duration per episode remained unaffected. This decrease in the frequency of REM sleep was blocked by beta-antagonist propranolol while the duration of REM sleep per episode was blocked by alpha1-antagonist, prazosin. Also, a critical level of norepinephrine in the system was required for the generation of REM sleep, however, a higher level may be inhibitory. Based on the results of this study and our earlier studies, an interaction between neurons, containing different neurotransmitters and their subtypes of receptors for LC-mediated regulation of REM sleep has been proposed.

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.