GABAergic regulation of REM sleep in reticularis pontis oralis and caudalis in rats

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.

Hippocampal theta power pressure builds over non-REM sleep and dissipates within REM sleep episodes

The theta rhythm during waking has been associated with voluntary motor activity and learning processes involving the hippocampus. Theta also occurs continuously during rapid eye movement (REM) sleep where it likely serves memory consolidation. Theta amplitude builds across wakefulness and is the best indicator of the homeostatic need for non-REM (NREM) sleep. Although REM sleep is homeostatically regulated independently of NREM sleep, the drivers of REM sleep regulation are under debate. The dynamics of theta within REM sleep bouts have not been thoroughly explored. We equipped 20 male rats with sleep instrumentation and hippocampal electrodes to measure theta across normal sleep/waking periods over the first 4 h of the sleep phase on two consecutive days. We found that theta power decreased by a third, on average, within individual REM sleep bouts, but recovered between bouts. Thus, there was no general decline in theta power across the duration of the recording period or between days. The time constant of theta power decline within a REM sleep bout was the same whether the bout was short, midlength, or long, and did not predict the behavioral state immediately following the REM sleep bout. Interestingly, the more time spent in NREM sleep prior to REM sleep, the larger the decline in theta power during REM sleep, indicating that REM sleep theta may be homeostatically driven by NREM sleep just as NREM delta power is driven by the length of prior waking and by waking theta. Potential causes and implications for this phenomenon are discussed.

Differential Anaesthetic Effects following Microinjection of Thiopentone and Propofol into the Pons of Adult Rats: A Pilot Study

Identifying the central nervous system sites of action of anaesthetics is important for understanding the link between their molecular actions and clinical effects. The aim of the present pilot study was to compare the anaesthetic effect of bilateral microinjections of propofol and thiopentone (both 200 μg/μl, in Intralipid and 0.9% saline respectively) into a recently discovered anaesthetic-sensitive region in the rat brainstem, the “mesopontine tegmental anaesthetic area” (MPTA). Microinjections (1 μl per side) were made into the MPTA of fifteen male Sprague-Dawley rats. The effect of each agent on spontaneous behaviour, postural control and nociceptive responsiveness was subjectively assessed according to established criteria. The main finding was that thiopentone induced an “anaesthesia-like” state, including complete atonia and loss of righting ability, in 20% of the subjects. Overall, thiopentone significantly reduced postural control and had a moderate antinociceptive effect compared to saline microinjections (P<0.01 and 0.05, respectively, Wilcoxon test). In contrast, propofol did not induce “anaesthesia” in any animal tested, although a similar antinociceptive effect to that of thiopentone was observed (P<0.05, Wilcoxon test). In summary, propofol and thiopentone have different effects when microinjected into the MPTA. While both agents reduced reflex withdrawal to a nociceptive stimulus, only thiopentone induced an “anaesthesia-like” state.

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

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

The anatomical, cellular and synaptic basis of motor atonia during rapid eye movement sleep

Rapid eye movement (REM) sleep is a recurring part of the sleep-wake cycle characterized by fast, desynchronized rhythms in the electroencephalogram (EEG), hippocampal theta activity, rapid eye movements, autonomic activation and loss of postural muscle tone (atonia). The brain circuitry governing REM sleep is located in the pontine and medullary brainstem and includes ascending and descending projections that regulate the EEG and motor components of REM sleep. The descending signal for postural muscle atonia during REM sleep is thought to originate from glutamatergic neurons of the sublaterodorsal nucleus (SLD), which in turn activate glycinergic pre-motor neurons in the spinal cord and/or ventromedial medulla to inhibit motor neurons. Despite work over the past two decades on many neurotransmitter systems that regulate the SLD, gaps remain in our knowledge of the synaptic basis by which SLD REM neurons are regulated and in turn produce REM sleep atonia. Elucidating the anatomical, cellular and synaptic basis of REM sleep atonia control is a critical step for treating many sleep-related disorders including obstructive sleep apnoea (apnea), REM sleep behaviour disorder (RBD) and narcolepsy with cataplexy.

Calcium imaging of sleep-wake related neuronal activity in the dorsal pons

The dorsal pons has long been implicated in the generation of rapid eye movement (REM) sleep, but the underlying circuit mechanisms remain poorly understood. Using cell-type-specific microendoscopic Ca²⁺ imaging in and near the laterodorsal tegmental nucleus, we found that many glutamatergic neurons are maximally active during REM sleep (REM-max), while the majority of GABAergic neurons are maximally active during wakefulness (wake-max). Furthermore, the activity of glutamatergic neurons exhibits a medio-lateral spatial gradient, with medially located neurons more selectively active during REM sleep.

Dreaming occurs in REM sleep, yet the neural mechanisms involved in generating it are not understood. Here Cox and colleagues show that glutamatergic neurons in the dorsal pons are activated most during transition to REM sleep while GABAergic neurons are more active during waking state.

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.

Interactions between hypocretinergic and GABAergic systems in the control of activity of neurons in the cat pontine reticular formation

Anatomical studies have demonstrated that hypocretinergic and GABAergic neurons innervate cells in the nucleus pontis oralis (NPO), a nucleus responsible for the generation of active (rapid eye movement (REM)) sleep (AS) and wakefulness (W). Behavioral and electrophysiological studies have shown that hypocretinergic and GABAergic processes in the NPO are involved in the generation of AS as well as W. An increase in hypocretin in the NPO is associated with both AS and W, whereas GABA levels in the NPO are elevated during W. We therefore examined the manner in which GABA modulates NPO neuronal responses to hypocretin. We hypothesized that interactions between the hypocretinergic and GABAergic systems in the NPO play an important role in determining the occurrence of AS or W. To determine the veracity of this hypothesis, we examined the effects of the juxtacellular application of hypocretin-1 and GABA on the activity of NPO neurons, which were recorded intracellularly, in chloralose-anesthetized cats. The juxtacellular application of hypocretin-1 significantly increased the mean amplitude of spontaneous EPSPs and the frequency of discharge of NPO neurons; in contrast, the juxtacellular microinjection of GABA produced the opposite effects, i.e., there was a significant reduction in the mean amplitude of spontaneous EPSPs and a decrease in the discharge of these cells. When hypocretin-1 and GABA were applied simultaneously, the inhibitory effect of GABA on the activity of NPO neurons was reduced or completely blocked. In addition, hypocretin-1 also blocked GABAergic inhibition of EPSPs evoked by stimulation of the laterodorsal tegmental nucleus. These data indicate that hypocretin and GABA function within the context of a neuronal gate that controls the activity of AS-on neurons. Therefore, we suggest that the occurrence of either AS or W depends upon interactions between hypocretinergic and GABAergic processes as well as inputs from other sites that project to AS-on neurons in the NPO.

Endogenous cholinergic input to the pontine REM sleep generator is not required for REM sleep to occur

Initial theories of rapid eye movement (REM) sleep generation posited that induction of the state required activation of the pontine subceruleus (SubC) by cholinergic inputs. Although the capacity of cholinergic neurotransmission to contribute to REM sleep generation has been established, the role of cholinergic inputs in the generation of REM sleep is ultimately undetermined as the critical test of this hypothesis (local blockade of SubC acetylcholine receptors) has not been rigorously performed. We used bilateral microdialysis in freely behaving rats (n = 32), instrumented for electroencephalographic and electromyographic recording, to locally manipulate neurotransmission in the SubC with select drugs. As predicted, combined microperfusion of D-AP5 (glutamate receptor antagonist) and muscimol (GABAA receptor agonist) in the SubC virtually eliminated REM sleep. However, REM sleep was not reduced by scopolamine microperfusion in this same region, at a concentration capable of blocking the effects of cholinergic receptor stimulation. This result suggests that transmission of REM sleep drive to the SubC is acetylcholine-independent. Although SubC cholinergic inputs are not majorly involved in REM sleep generation, they may perform a minor function in the reinforcement of transitions into REM sleep, as evidenced by increases in non-REM-to-REM sleep transition duration and failure rate during cholinergic receptor blockade. Cholinergic receptor antagonism also attenuated the normal increase in hippocampal θ oscillations that characterize REM sleep. Using computational modeling, we show that our in vivo results are consistent with a mutually excitatory interaction between the SubC and cholinergic neurons where, importantly, cholinergic neuron activation is gated by SubC activity.

GABAergic transmission in rat pontine reticular formation regulates the induction phase of anesthesia and modulates hyperalgesia caused by sleep deprivation

The oral part of the pontine reticular formation (PnO) contributes to the regulation of sleep, anesthesia and pain. The role of PnO γ-aminobutyric acid (GABA) in modulating these states remains incompletely understood. The present study used time to loss and time to resumption of righting response (LoRR and RoRR) as surrogate measures of loss and resumption of consciousness. This study tested three hypotheses: (i) pharmacologically manipulating GABA levels in rat PnO alters LoRR, RoRR and nociception; (ii) propofol decreases GABA levels in the PnO; and (iii) inhibiting GABA synthesis in the PnO blocks hyperalgesia caused by sleep deprivation. Administering a GABA synthesis inhibitor [3-mercaptopropionic acid (3-MPA)] or a GABA uptake inhibitor [nipecotic acid (NPA)] into rat PnO significantly altered LoRR caused by propofol. 3-MPA significantly decreased LoRR for propofol (-18%). NPA significantly increased LoRR during administration of propofol (36%). Neither 3-MPA nor NPA altered RoRR following cessation of propofol or isoflurane delivery. The finding that LoRR was decreased by 3-MPA and increased by NPA is consistent with measures showing that extracellular GABA levels in the PnO were decreased (41%) by propofol. Thermal nociception was significantly decreased by 3-MPA and increased by NPA, and 3-MPA blocked the hyperalgesia caused by sleep deprivation. The results demonstrate that GABA levels in the PnO regulate the time for loss of consciousness caused by propofol, extend the concept that anesthetic induction and emergence are not inverse processes, and suggest that GABAergic transmission in the PnO mediates hyperalgesia caused by sleep loss.

Alcohol and the sleeping brain

Alcohol acts as a sedative that interacts with several neurotransmitter systems important in the regulation of sleep. Acute administration of large amounts of alcohol prior to sleep leads to decreased sleep-onset latency and changes in sleep architecture early in the night, when blood alcohol levels are high, with subsequent disrupted, poor-quality sleep later in the night. Alcohol abuse and dependence are associated with chronic sleep disturbance, lower slow-wave sleep, and more rapid-eye-movement sleep than normal, that last long into periods of abstinence and may play a role in relapse. This chapter outlines the evidence for acute and chronic alcohol effects on sleep architecture and sleep electroencephalogram, evidence for tolerance with repeated administration, and possible underlying neurochemical mechanisms for alcohol's effects on sleep. Also discussed are sex differences as well as effects of alcohol on sleep homeostasis and circadian regulation. Evidence for the role of sleep disruption as a risk factor for developing alcohol dependence is discussed in the context of research conducted in adolescents. The utility of sleep-evoked potentials in the assessment of the effects of alcoholism on sleep and the brain and in abstinence-mediated recovery is also outlined. The chapter concludes with a series of questions that need to be answered to determine the role of sleep and sleep disturbance in the development and maintenance of problem drinking and the potential beneficial effects of the treatment of sleep disorders for maintenance of abstinence in alcoholism.

Essential roles of GABA transporter-1 in controlling rapid eye movement sleep and in increased slow wave activity after sleep deprivation

GABA is the major inhibitory neurotransmitter in the mammalian central nervous system that has been strongly implicated in the regulation of sleep. GABA transporter subtype 1 (GAT1) constructs high affinity reuptake sites for GABA and regulates GABAergic transmission in the brain. However, the role of GAT1 in sleep-wake regulation remains elusive. In the current study, we characterized the spontaneous sleep-wake cycle and responses to sleep deprivation in GAT1 knock-out (KO) mice. GAT1 KO mice exhibited dominant theta-activity and a remarkable reduction of EEG power in low frequencies across all vigilance stages. Under baseline conditions, spontaneous rapid eye movement (REM) sleep of KO mice was elevated both during the light and dark periods, and non-REM (NREM) sleep was reduced during the light period only. KO mice also showed more state transitions from NREM to REM sleep and from REM sleep to wakefulness, as well as more number of REM and NREM sleep bouts than WT mice. During the dark period, KO mice exhibited more REM sleep bouts only. Six hours of sleep deprivation induced rebound increases in NREM and REM sleep in both genotypes. However, slow wave activity, the intensity component of NREM sleep was briefly elevated in WT mice but remained completely unchanged in KO mice, compared with their respective baselines. These results indicate that GAT1 plays a critical role in the regulation of REM sleep and homeostasis of NREM sleep.

GABA(A) receptors implicated in REM sleep control express a benzodiazepine binding site

It has been reported that non-subtype-selective GABAA receptor antagonists injected into the nucleus pontis oralis (PnO) of rats induced long-lasting increases in REM sleep. Characteristics of these REM sleep increases were identical to those resulting from injection of muscarinic cholinergic agonists. Both actions were blocked by the muscarinic antagonist, atropine. Microdialysis of GABAA receptor antagonists into the PnO resulted in increased acetylcholine levels. These findings were consistent with GABAA receptor antagonists disinhibiting acetylcholine release in the PnO to result in an acetylcholine-mediated REM sleep induction. Direct evidence has been lacking for localization in the PnO of the specific GABAA receptor-subtypes mediating the REM sleep effects. Here, we demonstrated a dose-related, long-lasting increase in REM sleep following injection (60 nl) in the PnO of the inverse benzodiazepine agonist, methyl-6,7-dimethoxy-4-ethyl-β-carboline (DMCM, 10(-2)M). REM sleep increases were greater and more consistently produced than with the non-selective antagonist gabazine, and both were blocked by atropine. Fluorescence immunohistochemistry and laser scanning confocal microscopy, colocalized in PnO vesicular acetylcholine transporter, a presynaptic marker of cholinergic boutons, with the γ2 subunit of the GABAA receptor. These data provide support for the direct action of GABA on mechanisms of acetylcholine release in the PnO. The presence of the γ2 subunit at this locus and the REM sleep induction by DMCM are consistent with binding of benzodiazepines by a GABAA receptor-subtype in control of REM sleep.

Mild Transient Hypercapnia as a Novel Fear Conditioning Stimulus Allowing Re-Exposure during Sleep

Introduction

Studies suggest that sleep plays a role in traumatic memories and that treatment of sleep disorders may help alleviate symptoms of posttraumatic stress disorder. Fear-conditioning paradigms in rodents are used to investigate causal mechanisms of fear acquisition and the relationship between sleep and posttraumatic behaviors. We developed a novel conditioning stimulus (CS) that evoked fear and was subsequently used to study re-exposure to the CS during sleep.

Methods

Experiment 1 assessed physiological responses to a conditioned stimulus (mild transient hypercapnia, mtHC; 3.0% CO₂; n = 17)+footshock for the purpose of establishing a novel CS in male FVB/J mice. Responses to the novel CS were compared to tone+footshock (n = 18) and control groups of tone alone (n = 17) and mild transient hypercapnia alone (n = 10). A second proof of principle experiment re-exposed animals during sleep to mild transient hypercapnia or air (control) to study sleep processes related to the CS.

Results

Footshock elicited a response of acute tachycardia (30–40 bpm) and increased plasma epinephrine. When tone predicted footshock it elicited mild hypertension (1–2 mmHg) and a three-fold increase in plasma epinephrine. When mtHC predicted footshock it also induced mild hypertension, but additionally elicited a conditioned bradycardia and a smaller increase in plasma epinephrine. The overall mean 24 hour sleep–wake profile was unaffected immediately after fear conditioning.

Discussion

Our study demonstrates the efficacy of mtHC as a conditioning stimulus that is perceptible but innocuous (relative to tone) and applicable during sleep. This novel model will allow future studies to explore sleep-dependent mechanisms underlying maladaptive fear responses, as well as elucidate the moderators of the relationship between fear responses and sleep.

Evidence that adrenergic ventrolateral medullary cells are activated whereas precerebellar lateral reticular nucleus neurons are suppressed during REM sleep

Rapid eye movement sleep (REMS) is generated in the brainstem by a distributed network of neurochemically distinct neurons. In the pons, the main subtypes are cholinergic and glutamatergic REMS-on cells and aminergic REMS-off cells. Pontine REMS-on cells send axons to the ventrolateral medulla (VLM), but little is known about REMS-related activity of VLM cells. In urethane-anesthetized rats, dorsomedial pontine injections of carbachol trigger REMS-like episodes that include cortical and hippocampal activation and suppression of motoneuronal activity; the episodes last 4–8 min and can be elicited repeatedly. We used this model to determine whether VLM catecholaminergic cells are silenced during REMS, as is typical of most aminergic neurons studied to date, and to investigate other REMS-related cells in this region. In 18 anesthetized, paralyzed and artificially ventilated rats, we obtained extracellular recordings from VLM cells when REMS-like episodes were elicited by pontine carbachol injections (10 mM, 10 nl). One major group were the cells that were activated during the episodes (n = 10). Their baseline firing rate of 3.7±2.1 (SD) Hz increased to 9.7±2.1 Hz. Most were found in the adrenergic C1 region and at sites located less than 50 µm from dopamine β-hydroxylase-positive (DBH⁺) neurons. Another major group were the silenced or suppressed cells (n = 35). Most were localized in the lateral reticular nucleus (LRN) and distantly from any DBH⁺ cells. Their baseline firing rates were 6.8±4.4 Hz and 15.8±7.1 Hz, respectively, with the activity of the latter reduced to 7.4±3.8 Hz. We conclude that, in contrast to the pontine noradrenergic cells that are silenced during REMS, medullary adrenergic C1 neurons, many of which drive the sympathetic output, are activated. Our data also show that afferent input transmitted to the cerebellum through the LRN is attenuated during REMS. This may distort the spatial representation of body position during REMS.

Extrasynaptic GABAA receptors in rat pontine reticular formation increase wakefulness

STUDY OBJECTIVES

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

DESIGN

Within/between subjects.

SETTING

University of Michigan.

PATIENTS OR PARTICIPANTS

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

INTERVENTIONS

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

MEASUREMENTS AND RESULTS

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

CONCLUSION

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

CITATION

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

Sleep and Anesthesia Interactions: A Pharmacological Appraisal

Anesthetics have been used in clinical practice for over a hundred years, yet their mechanisms of action remain poorly understood. One tempting hypothesis to explain their hypnotic properties posits that anesthetics exert a component of their effects by "hijacking" the endogenous arousal circuitry of the brain. Modulation of activity within sleep- and wake-related neuroanatomic systems could thus explain some of the varied effects produced by anesthetics. There has been a recent explosion of research into the neuroanatomic substrates affected by various anesthetics. In this review, we will highlight the relevant sleep architecture and systems and focus on studies over the past few years that implicate these sleep-related structures as targets of anesthetics. These studies highlight a promising area of investigation regarding the mechanisms of action of anesthetics and provide an important model for future study.

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.

Inhibition of A5 Neurons Facilitates the Occurrence of REM Sleep-Like Episodes in Urethane-Anesthetized Rats: A New Role for Noradrenergic A5 Neurons?

When rapid eye movement (REM) sleep occurs, noradrenergic cells become silent, with the abolition of activity in locus coeruleus (LC) neurons seen as a key event permissive for the occurrence of REM sleep. However, it is not known whether silencing of other than LC noradrenergic neurons contributes to the generation of REM sleep. In urethane-anesthetized rats, stereotyped REM sleep-like episodes can be repeatedly elicited by injections of the cholinergic agonist, carbachol, into a discrete region of the dorsomedial pons. We used this preparation to test whether inhibition of ventrolateral pontine noradrenergic A5 neurons only, or together with LC neurons, also can elicit REM sleep-like effects. To silence noradrenergic cells, we sequentially injected the α₂-adrenergic agonist clonidine (20–40 nl, 0.75 mM) into both A5 regions and then the LC. In two rats, successful bilateral clonidine injections into the A5 region elicited the characteristic REM sleep-like episodes (hippocampal theta rhythm, suppression of hypoglossal nerve activity, reduced respiratory rate). In five rats, bilateral clonidine injections into the A5 region and then into one LC triggered REM sleep-like episodes, and in two rats injections into both A5 and then both LC were needed to elicit the effect. In contrast, in three rats, uni- or bilateral clonidine injections only into the LC had no effect, and clonidine injections placed in another six rats outside of the A5 and/or LC regions were without effect. The REM sleep-like episodes elicited by clonidine had similar magnitude of suppression of hypoglossal nerve activity (by 75%), similar pattern of hippocampal changes, and similar durations (2.5–5.3 min) to the episodes triggered in the same preparation by carbachol injections into the dorsomedial pontine reticular formation. Thus, silencing of A5 cells may importantly enable the occurrence of REM sleep-like episodes, at least under anesthesia. This is a new role for noradrenergic A5 neurons.

Identification of the transmitter and receptor mechanisms responsible for REM sleep paralysis

During REM sleep the CNS is intensely active, but the skeletal motor system is paradoxically forced into a state of muscle paralysis. The mechanisms that trigger REM sleep paralysis are a matter of intense debate. Two competing theories argue that it is caused by either active inhibition or reduced excitation of somatic motoneuron activity. Here, we identify the transmitter and receptor mechanisms that function to silence skeletal muscles during REM sleep. We used behavioral, electrophysiological, receptor pharmacology and neuroanatomical approaches to determine how trigeminal motoneurons and masseter muscles are switched off during REM sleep in rats. We show that a powerful GABA and glycine drive triggers REM paralysis by switching off motoneuron activity. This drive inhibits motoneurons by targeting both metabotropic GABA(B) and ionotropic GABA(A)/glycine receptors. REM paralysis is only reversed when motoneurons are cut off from GABA(B), GABA(A) and glycine receptor-mediated inhibition. Neither metabotropic nor ionotropic receptor mechanisms alone are sufficient for generating REM paralysis. These results demonstrate that multiple receptor mechanisms trigger REM sleep paralysis. Breakdown in normal REM inhibition may underlie common sleep motor pathologies such as REM sleep behavior disorder.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

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.

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

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

Brainstem mechanisms of paradoxical (REM) sleep generation

Paradoxical sleep (PS) is characterized by EEG activation with a disappearance of muscle tone and the occurrence of rapid eye movements (REM) in contrast to slow-wave sleep (SWS, also known as non-REM sleep) identified by the presence of delta waves. Soon after the discovery of PS, it was demonstrated that the structures necessary and sufficient for its genesis are restricted to the brainstem. We review here recent results indicating that brainstem glutamatergic and GABAergic, rather than cholinergic and monoaminergic, neurons play a key role in the genesis of PS. We hypothesize that the entrance to PS from SWS is due to the activation of PS-on glutamatergic neurons localized in the pontine sublaterodorsal tegmental nucleus. The activation of these neurons would be due to a permanent glutamatergic input arising from the lateral and ventrolateral periaqueductal gray (vlPAG) and the removal at the onset of PS of a GABAergic inhibition present during W and SWS. Such inhibition would be coming from PS-off GABAergic neurons localized in the vlPAG and the adjacent deep mesencephalic reticular nucleus. The cessation of activity of these PS-off GABAergic neurons at the onset and during PS would be due to direct projections from intermingled GABAergic PS-on neurons. Activation of PS would depend on the reciprocal interactions between the GABAergic PS-on and PS-off neurons, intrinsic cellular and molecular events, and integration of multiple physiological parameters.

Antagonizing corticotropin-releasing factor in the central nucleus of the amygdala attenuates fear-induced reductions in sleep but not freezing

STUDY OBJECTIVES

Contextual fear is followed by significant reductions in rapid eye movement sleep (REM) that are regulated by the central nucleus of the amygdala (CNA). Corticotropin-releasing factor (CRF) plays a major role in regulating the stress response as well as arousal, and CRF in CNA is implicated in stress-related behavior. To test the hypothesis that CRF regulation of CNA is involved in fear-induced alterations in REM, we determined the effects of microinjections into CNA of the CRF1 antagonist, antalarmin (ANT) on fear-induced reductions in REM. We also evaluated c-Fos activation in the hypothalamic paraventricular nucleus (PVN), locus coeruleus (LC), and dorsal raphe nucleus (DRN) to determine whether activation of these regions was consistent with their roles in regulating stress and in the control of REM.

DESIGN

On separate days, rats were subjected to baseline and 2 shock training sessions (S1 and S2). Five days later, the rats received bilateral microinjections of ANT (4.8 mM) or vehicle (VEH) prior to exposure to the fearful context. Sleep was recorded for 20 h in each condition. Freezing was assessed during S1, S2, and context. Separate groups of rats received identical training and microinjections or handling control (HC) only, but were sacrificed 2 h after context exposure to assess c-Fos expression.

SETTING

NA.

PATIENTS OR PARTICIPANTS

NA.

INTERVENTIONS

NA.

MEASUREMENTS AND RESULTS

Compared to baseline, S1 and S2 significantly reduced REM. Exposure to the fearful context reduced REM in VEH treated rats, whereas REM in ANT treated rats did not differ from baseline. ANT did not significantly alter freezing. Fear-induced c-Fos expression was decreased in PVN and LC after ANT compared to VEH.

CONCLUSIONS

The results demonstrate that CRF receptors in CNA are involved in fear-induced reductions in REM and neural activation (as indicated by c-Fos) in stress and REM regulatory regions, but not in fear-induced freezing.

Sleep duration varies as a function of glutamate and GABA in rat pontine reticular formation

The oral part of the pontine reticular formation (PnO) is a component of the ascending reticular activating system and plays a role in the regulation of sleep and wakefulness. The PnO receives glutamatergic and GABAergic projections from many brain regions that regulate behavioral state. Indirect, pharmacological evidence has suggested that glutamatergic and GABAergic signaling within the PnO alters traits that characterize wakefulness and sleep. No previous studies have simultaneously measured endogenous glutamate and GABA from rat PnO in relation to sleep and wakefulness. The present study utilized in vivo microdialysis coupled on-line to capillary electrophoresis with laser-induced fluorescence to test the hypothesis that concentrations of glutamate and GABA in the PnO vary across the sleep/wake cycle. Concentrations of glutamate and GABA were significantly higher during wakefulness than during non-rapid eye movement sleep and rapid eye movement sleep. Regression analysis revealed that decreases in glutamate and GABA accounted for a significant portion of the variance in the duration of non-rapid eye movement sleep and rapid eye movement sleep episodes. These data provide novel support for the hypothesis that endogenous glutamate and GABA in the PnO contribute to the regulation of sleep duration.

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

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

Eye movements and abducens motoneuron behavior after cholinergic activation of the nucleus reticularis pontis caudalis

STUDY OBJECTIVES

the aim of this work was to characterize eye movements and abducens (ABD) motoneuron behavior after cholinergic activation of the nucleus reticularis pontis caudalis (NRPC).

METHODS

six female adult cats were prepared for chronic recording of eye movements (using the scleral search-coil technique), electroencephalography, electromyography, ponto-geniculo-occipital (PGO) waves in the lateral geniculate nucleus, and ABD motoneuron activities after microinjections of the cholinergic agonist carbachol into the NRPC.

RESULTS

unilateral microinjections of carbachol in the NRPC induced tonic and phasic phenomena in the oculomotor system. Tonic effects consisted of ipsiversive rotation to the injected side, convergence, and downward rotation of the eyes. Phasic effects consisted of bursts of rhythmic rapid eye movements directed contralaterally to the injected side along with PGO-like waves in the lateral geniculate and ABD nuclei. Although tonic effects were dependent on the level of drowsiness, phasic effects were always present and appeared along with normal saccades when the animal was vigilant. ABD motoneurons showed phasic activities associated with ABD PGO-like waves during bursts of rapid eye movements, and tonic and phasic activities related to eye position and velocity during alertness.

CONCLUSION

the cholinergic activation of the NRPC induces oculomotor phenomena that are somewhat similar to those described during REM sleep. A precise comparison of the dynamics and timing of the eye movements further suggests that a temporal organization of both NRPCs is needed to reproduce the complexity of the oculomotor behavior during REM sleep.

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

STUDY OBJECTIVES

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

DESIGN

Within/between subjects.

SETTING

University of Michigan.

PATIENTS OR PARTICIPANTS

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

INTERVENTIONS

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

MEASUREMENTS AND RESULTS

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

CONCLUSION

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

Neuropharmacology of Sleep and Wakefulness

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

GABA(A) receptors in the pontine reticular formation of C57BL/6J mouse modulate neurochemical, electrographic, and behavioral phenotypes of wakefulness

Drugs that potentiate transmission at GABA(A) receptors are widely used to enhance sleep and to cause general anesthesia. The mechanisms underlying these effects are unknown. This study tested the hypothesis that GABA(A) receptors in the pontine reticular nucleus, oral part (PnO) of mouse modulate five phenotypes of arousal: sleep and wakefulness, cortical electroencephalogram (EEG) activity, acetylcholine (ACh) release in the PnO, breathing, and recovery time from general anesthesia. Microinjections into the PnO of saline (vehicle control), the GABA(A) receptor agonist muscimol, muscimol with the GABA(A) receptor antagonist bicuculline, and bicuculline alone were performed in male C57BL/6J mice (n = 33) implanted with EEG recording electrodes. Muscimol caused a significant increase in wakefulness and decrease in rapid eye movement (REM) and non-REM (NREM) sleep. These effects were reversed by coadministration of bicuculline. Bicuculline administered alone caused a significant decrease in wakefulness and increase in NREM sleep and REM sleep. Muscimol significantly increased EEG power in the delta range (0.5-4 Hz) during wakefulness and in the theta range (4-9 Hz) during REM sleep. Dialysis delivery of bicuculline to the PnO of male mice (n = 18) anesthetized with isoflurane significantly increased ACh release in the PnO, decreased breathing rate, and increased anesthesia recovery time. All drug effects were concentration dependent. The effects on phenotypes of arousal support the conclusion that GABA(A) receptors in the PnO promote wakefulness and suggest that increasing GABAergic transmission in the PnO may be one mechanism underlying the phenomenon of paradoxical behavioral activation by some benzodiazepines.

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.

Benzodiazepine receptor agonists cause drug-specific and state-specific alterations in EEG power and acetylcholine release in rat pontine reticular formation

STUDY OBJECTIVES

Benzodiazepine (BDZ) and non-benzodiazepine (NBDZ) hypnotics enhance GABAergic transmission and are widely used for the treatment of insomnia. In the pontine reticular formation (PRF), GABA inhibits rapid eye movement (REM) sleep and acetylcholine (ACh) release. No previous studies have characterized the effects of BDZ and NBDZ hypnotics on ACh release in the PRF. This study tested 2 hypotheses: (1) that microdialysis delivery of zolpidem, eszopiclone, and diazepam to rat PRF alters ACh release in PRF and electroencephalographic (EEG) delta power and (2) that intravenous (i.v.) administration of eszopiclone to non-anesthetized rat alters ACh release in the PRF, sleep, and EEG delta power.

DESIGN

A within- and between-groups experimental design.

SETTING

University of Michigan.

PATIENTS OR PARTICIPANTS

Adult male Crl:CD*(SD) (Sprague-Dawley) rats (n = 57).

INTERVENTIONS

In vivo microdialysis of the PRF in rats anesthetized with isoflurane was used to derive the concentration-response effects of zolpidem, eszopiclone, and diazepam on ACh release. Chronically instrumented rats were used to quantify the effects of eszopiclone (3 mg/kg, i.v.) on ACh release in the PRF, sleep-wake states, and cortical EEG power.

MEASUREMENTS AND RESULTS

ACh release was significantly increased by microdialysis delivery to the PRF of zolpidem and eszopiclone but not diazepam. EEG delta power was increased by zolpidem and diazepam but not by eszopiclone administered to the PRF. Eszopiclone (i.v.) decreased ACh release in the PRF of both anesthetized and non-anesthetized rats. Eszopiclone (i.v.) prevented REM sleep and increased EEG delta power.

CONCLUSION

The concentration-response data provide the first functional evidence that multiple GABA(A) receptor subtypes are present in rat PRF. Intravenously administered eszopiclone prevented REM sleep, decreased ACh release in the PRF, and increased EEG delta power. The effects of eszopiclone are consistent with evidence that ACh release in the PRF is lower during NREM sleep than during REM sleep, and with data showing that cholinergic stimulation of the PRF activates the cortical EEG.

GABAergic antagonism of the central nucleus of the amygdala attenuates reductions in rapid eye movement sleep after inescapable footshock stress

STUDY OBJECTIVES

Rapid eye movement sleep (REM) appears to be especially susceptible to the effects of stress; inescapable footshock stress (IS) can produce reductions in REM that can occur without recovery sleep. The amygdala has well-established roles in stress and emotion; the central nucleus of the amygdala (CNA) projects to REM regulatory regions in the brainstem and has been found to play a key role in the regulation of REM. The objective of this study was to determine whether the reduction in REM induced by IS could be regulated by CNA and brainstem regions.

DESIGN

The GABAergic agonist muscimol (MUS) and GABAergic antagonist bicuculline (BIC) were microinjected into CNA before IS, and sleep was recorded for 20 h. In a second experiment using the same manipulations, sleep was recorded for 2 h, after which the rats were killed to evaluate Fos expression (a marker of neuronal activity) in the locus coeruleus (LC), a brainstem REM regulatory region.

SETTING

NA.

PATIENTS OR PARTICIPANTS

The subjects were male, outbred Wistar rats.

INTERVENTIONS

The rats were surgically implanted with standard electrodes or with telemetry transmitters for determining arousal state.

MEASUREMENTS AND RESULTS

IS preceded by control or MUS microinjections selectively reduced REM and increased Fos expression in LC. By comparison, microinjection of BIC into CNA prior to IS attenuated both the reduction in REM and Fos expression in LC to levels seen in non-shocked controls.

CONCLUSIONS

The results suggest that the effects of IS on REM may involve local GABAergic inhibition in CNA and activation of LC.

Ultrastructural characterization of relationship between serotonergic and GABAergic structures in the ventral part of the oral pontine reticular nucleus

The ventral part of the oral pontine reticular nucleus (vRPO) is involved in the generation and maintenance of rapid eye movement (REM) sleep. Both GABAergic and serotonergic neurotransmission have been implicated in the control of the sleep-wakefulness cycle. Nevertheless, the synaptic organization of serotonergic terminals in the vRPO has not yet been characterized. We performed an electron microscope study of serotonin-immunoreactive (5-HT-IR) terminals using immunoperoxidase or immunogold-silver methods. In a second set of experiments, combining GABA immunoperoxidase and 5-HT immunogold-silver techniques, we examined inputs from GABA-immunoreactive (GABA-IR) terminals to serotonergic neurons. 5-HT-IR terminals were located primarily on dendrites and occasionally on somata of unlabeled and 5-HT-IR neurons. The majority of the synapses formed by 5-HT-IR terminals were of the symmetrical type, making contacts primarily with unlabeled dendritic profiles. Moreover, 5-HT-IR terminals contacted unlabeled axon terminals that formed asymmetric synapses on dendrites. Double immunolabeling experiments showed 5-HT-IR and GABA-IR afferents, in apposition to each other, making synapses with the same dendrites. Finally, GABA-IR terminals innervated 5-HT-IR and GABA-IR dendrites. Our findings indicate that serotonin would modulate the neuronal activity through inhibitory or excitatory influences, although the action of serotonin on the vRPO would predominantly be inhibitory. Moreover, the present results suggest that the serotonin modulation of vRPO neurons might involve indirect connections. In addition, GABA might contribute to the induction and maintenance of REM sleep by inhibiting serotonergic and GABAergic neurons in the vRPO.

A novel GABAergic afferent input to the pontine reticular formation: the mesopontine GABAergic column

Pharmacological manipulations of gamma-aminobutyric acid (GABA) neurotransmission in the nucleus pontis oralis (PnO) of the rat brainstem produce alterations in sleep/wake behavior. Local applications of GABA(A) receptor antagonists and agonists increase REM sleep and wake, respectively. These findings support a role for GABAergic mechanisms of the PnO in the control of arousal state. We have been investigating sources of GABA innervation of the PnO that may interact with local GABA(A) receptors in the control of state. Utilizing a retrograde tracer, cholera toxin-B subunit (CTb), injected into the PnO and dual-label immunohistochemistry with an antibody against glutamic acid decarboxalase-67 (GAD67), we report on a previously unidentified GABAergic neuronal population projecting to the contralateral PnO appearing as a column of cells, with long-axis in the sagittal plane, extending through the midbrain and pons. We refer to these neurons as the mesopontine GABAergic column (MPGC). The contiguous, columnar, anatomical distribution suggests operation as a functional neural system, which may influence expression of REM sleep, wake and other behaviors subserved by the PnO.

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

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

Gamma-aminobutyric acid-mediated neurotransmission in the pontine reticular formation modulates hypnosis, immobility, and breathing during isoflurane anesthesia

BACKGROUND

Many general anesthetics are thought to produce a loss of wakefulness, in part, by enhancing gamma-aminobutyric acid (GABA) neurotransmission. However, GABAergic neurotransmission in the pontine reticular formation promotes wakefulness. This study tested the hypotheses that (1) relative to wakefulness, isoflurane decreases GABA levels in the pontine reticular formation; and (2) pontine reticular formation administration of drugs that increase or decrease GABA levels increases or decreases, respectively, isoflurane induction time.

METHODS

To test hypothesis 1, cats (n = 5) received a craniotomy and permanent electrodes for recording the electroencephalogram and electromyogram. Dialysis samples were collected from the pontine reticular formation during isoflurane anesthesia and wakefulness. GABA levels were quantified using high-performance liquid chromatography. For hypothesis 2, rats (n = 10) were implanted with a guide cannula aimed for the pontine reticular formation. Each rat received microinjections of Ringer's (vehicle control), the GABA uptake inhibitor nipecotic acid, and the GABA synthesis inhibitor 3-mercaptopropionic acid. Rats were then anesthetized with isoflurane, and induction time was quantified as loss of righting reflex. Breathing rate was also measured.

RESULTS

Relative to wakefulness, GABA levels were significantly decreased by isoflurane. Increased power in the electroencephalogram and decreased activity in the electromyogram caused by isoflurane covaried with pontine reticular formation GABA levels. Nipecotic acid and 3-mercaptopropionic acid significantly increased and decreased, respectively, isoflurane induction time. Nipecotic acid also increased breathing rate.

CONCLUSION

Decreasing pontine reticular formation GABA levels comprises one mechanism by which isoflurane causes loss of consciousness, altered cortical excitability, muscular hypotonia, and decreased respiratory rate.

Differential localization of carbachol- and bicuculline-sensitive pontine sites for eliciting REM sleep-like effects in anesthetized rats

Carbachol, a cholinergic agonist, and GABA(A) receptor antagonists injected into the pontine dorsomedial reticular formation can trigger rapid eye movement (REM) sleep-like state. Data suggest that GABAergic and cholinergic effects interact to produce this effect but the sites where this occurs have not been delineated. In urethane-anesthetized rats, in which carbachol effectively elicits REM sleep-like episodes (REMSLE), we tested the ability of 10 nL microinjections of carbachol (10 mm) and bicuculline (0.5 or 2 mm) to elicit REMSLE at 47 sites located within the dorsal pontine reticular formation at the levels -8.00 to -10.80 from bregma (B) (Paxinos and Watson, The Rat Brain in Stereotaxic Coordinates, Academic Press, San Diego, 1997). At rostral levels, most carbachol and some bicuculline injections elicited REMSLE with latencies that gradually decreased from 242 to 12 s for carbachol and from 908 to 38 s for bicuculline for more caudal injection sites. As the latencies decreased, the durations of bicuculline-elicited REMSLE increased from 104 s to over 38 min, and the effect was dose dependent, whereas the duration of carbachol-elicited REMSLE changed little (104-354 s). Plots of REMSLE latency versus the antero-posterior coordinates revealed that both drugs were maximally effective near B-8.80. At levels caudal to B-8.80, carbachol was effective at few sites, whereas bicuculline-elicited REMSLE to at least B-9.30 level. Thus, the bicuculline-sensitive sites extended further caudally than those for carbachol and antagonism of GABA(A) receptors both triggered REMSLE and controlled their duration, whereas carbachol effects on REMSLE duration were small or limited by its concurrent REMSLE-opposing actions.

Pontine reticular formation (PnO) administration of hypocretin-1 increases PnO GABA levels and wakefulness

STUDY OBJECTIVES

GABAergic transmission in the oral part of the pontine reticular formation (PnO) increases wakefulness. The hypothalamic peptide hypocretin-1 (orexin A) promotes wakefulness, and the PnO receives hypocretinergic input. The present study tested the hypothesis that PnO administration of hypocretin-1 increases PnO GABA levels and increases wakefulness. This study also tested the hypothesis that wakefulness is either increased or decreased, respectively, by PnO administration of drugs known to selectively increase or decrease GABA levels.

DESIGN

Awithin-subjects design was used for microdialysis and microinjection experiments.

SETTING

University of Michigan.

PATIENTS OR PARTICIPANTS

Experiments were performed using adult male Crl:CD (SD)IGS BR (Sprague-Dawley) rats (n=46).

INTERVENTIONS

PnO administration of hypocretin-1, nipecotic acid (a GABA uptake inhibitor that increases extracellular GABA levels), 3-mercaptopropionic acid (a GABA synthesis inhibitor that decreases extracellular GABA levels; 3-MPA), and Ringer solution (vehicle control).

MEASUREMENTS AND RESULTS

Dialysis administration of hypocretin-1 to the PnO caused a statistically significant, concentration-dependent increase in PnO GABA levels. PnO microinjection of hypocretin-1 or nipecotic acid caused a significant increase in wakefulness and a significant decrease in non-rapid eye movement (NREM) sleep and REM sleep. Microinjecting 3-MPA into the PnO caused a significant increase in NREM sleep and REM sleep and a significant decrease in wakefulness.

CONCLUSIONS

An increase or a decrease in PnO GABA levels causes an increase or decrease, respectively, in wakefulness. Hypocretin-1 may promote wakefulness, at least in part, by increasing GABAergic transmission in the PnO.

Blockade of GABA, type A, receptors in the rat pontine reticular formation induces rapid eye movement sleep that is dependent upon the cholinergic system

The brainstem reticular formation is an area important to the control of rapid eye movement (REM) sleep. The antagonist of GABA-type A (GABA(A)) receptors, bicuculline methiodide (BMI), injected into the rat nucleus pontis oralis (PnO) of the reticular formation resulted in a long-lasting increase in REM sleep. Thus, one factor controlling REM sleep appears to be the number of functional GABA(A) receptors in the PnO. The long-lasting effect produced by BMI may result from secondary influences on other neurotransmitter systems known to have long-lasting effects. To study this question, rats were surgically prepared for chronic sleep recording and additionally implanted with guide cannulas aimed at sites in the PnO. Multiple, 60 nl, unilateral injections were made either singly or in combination. GABA(A) receptor antagonists, BMI and gabazine (GBZ), produced dose-dependent increases in REM sleep with GBZ being approximately 35 times more potent than BMI. GBZ and the cholinergic agonist, carbachol, produced very similar results, both increasing REM sleep for about 8 h, mainly through increased period frequency, with little reduction in REM latency. Pre-injection of the muscarinic antagonist, atropine, completely blocked the REM sleep-increase by GBZ. GABAergic control of REM sleep in the PnO requires the cholinergic system and may be acting through presynaptic modulation of acetylcholine release.

Characterization of GABAergic neurons in rapid-eye-movement sleep controlling regions of the brainstem reticular formation in GAD67-green fluorescent protein knock-in mice

Recent experiments suggest that brainstem GABAergic neurons may control rapid-eye-movement (REM) sleep. However, understanding their pharmacology/physiology has been hindered by difficulty in identification. Here we report that mice expressing green fluorescent protein (GFP) under the control of the GAD67 promoter (GAD67-GFP knock-in mice) exhibit numerous GFP-positive neurons in the central gray and reticular formation, allowing on-line identification in vitro. Small (10-15 microm) or medium-sized (15-25 microm) GFP-positive perikarya surrounded larger serotonergic, noradrenergic, cholinergic and reticular neurons, and > 96% of neurons were double-labeled for GFP and GABA, confirming that GFP-positive neurons are GABAergic. Whole-cell recordings in brainstem regions important for promoting REM sleep [subcoeruleus (SubC) or pontine nucleus oralis (PnO) regions] revealed that GFP-positive neurons were spontaneously active at 3-12 Hz, fired tonically, and possessed a medium-sized depolarizing sag during hyperpolarizing steps. Many neurons also exhibited a small, low-threshold calcium spike. GFP-positive neurons were tested with pharmacological agents known to promote (carbachol) or inhibit (orexin A) REM sleep. SubC GFP-positive neurons were excited by the cholinergic agonist carbachol, whereas those in the PnO were either inhibited or excited. GFP-positive neurons in both areas were excited by orexins/hypocretins. These data are congruent with the hypothesis that carbachol-inhibited GABAergic PnO neurons project to, and inhibit, REM-on SubC reticular neurons during waking, whereas carbachol-excited SubC and PnO GABAergic neurons are involved in silencing locus coeruleus and dorsal raphe aminergic neurons during REM sleep. Orexinergic suppression of REM during waking is probably mediated in part via excitation of acetylcholine-inhibited GABAergic neurons.