Rostral brainstem contributes to medullary inhibition of muscle tone

Neuronal structures controlling locomotor behavior during active and inactive motor states

Animal behaviors can be divided into two states according to their motor activity: the active motor state, which involves significant body movements, and the inactive motor state, which refers to when the animal is stationary. The timing and duration of these states are determined by the activity of the neuronal circuits involved in motor control. Among these motor circuits, those that generate locomotion are some of the most studied neuronal networks and are widely distributed from the spinal cord to the cerebral cortex. In this review, we discuss recent discoveries, mainly in rodents using state-of-the-art experimental approaches, of the neuronal mechanisms underlying the initiation and termination of locomotion in the brainstem, basal ganglia, and prefrontal cortex. These findings is discussed with reference to studies on the neuronal mechanism of motor control during sleep and the modulation of cortical states in these structures. Accumulating evidence has unraveled the complex yet highly structured network that controls the transition between motor states.

Breathing during sleep

The depth, rate, and regularity of breathing change following transition from wakefulness to sleep. Interactions between sleep and breathing involve direct effects of the central mechanisms that generate sleep states exerted at multiple respiratory regulatory sites, such as the central respiratory pattern generator, respiratory premotor pathways, and motoneurons that innervate the respiratory pump and upper airway muscles, as well as effects secondary to sleep-related changes in metabolism. This chapter discusses respiratory effects of sleep as they occur under physiologic conditions. Breathing and central respiratory neuronal activities during nonrapid eye movement (NREM) sleep and REM sleep are characterized in relation to activity of central wake-active and sleep-active neurons. Consideration is given to the obstructive sleep apnea syndrome because in this common disorder, state-dependent control of upper airway patency by upper airway muscles attains high significance and recurrent arousals from sleep are triggered by hypercapnic and hypoxic episodes. Selected clinical trials are discussed in which pharmacological interventions targeted transmission in noradrenergic, serotonergic, cholinergic, and other state-dependent pathways identified as mediators of ventilatory changes during sleep. Central pathways for arousals elicited by chemical stimulation of breathing are given special attention for their important role in sleep loss and fragmentation in sleep-related respiratory disorders.

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

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

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.

Neuroscience: A Distributed Neural Network Controls REM Sleep

How does the brain control dreams? New science shows that a small node of cells in the medulla - the most primitive part of the brain - may function to control REM sleep, the brain state that underlies dreaming.

The Neurobiology of Sleep and Wakefulness

Cortical electroencephalographic activity arises from corticothalamocortical interactions, modulated by wake-promoting monoaminergic and cholinergic input. These wake-promoting systems are regulated by hypothalamic hypocretin/orexins, while GABAergic sleep-promoting nuclei are found in the preoptic area, brainstem and lateral hypothalamus. Although pontine acetylcholine is critical for REM sleep, hypothalamic melanin-concentrating hormone/GABAergic cells may "gate" REM sleep. Daily sleep-wake rhythms arise from interactions between a hypothalamic circadian pacemaker and a sleep homeostat whose anatomical locus has yet to be conclusively defined. Control of sleep and wakefulness involves multiple systems, each of which presents vulnerability to sleep/wake dysfunction that may predispose to physical and/or neuropsychiatric disorders.

Control of REM sleep by ventral medulla GABAergic neurons

Rapid eye movement (REM) sleep is a distinct brain state characterized by activated electroencephalogram (EEG) and complete skeletal muscle paralysis, and it is associated with vivid dreams^1-3. Transection studies by Jouvet first demonstrated that the brainstem is both necessary and sufficient for REM sleep generation², and the neural circuits in the pons have since been studied extensively^4-8. The medulla also contains neurons that are active during REM sleep^9-13, but whether they play a causal role in REM sleep generation remains unclear. Here we show that a GABAergic pathway originating from the ventral medulla (vM) powerfully promotes REM sleep. Optogenetic activation of vM GABAergic neurons rapidly and reliably initiated REM sleep episodes and prolonged their durations, whereas inactivating these neurons had the opposite effects. Optrode recordings from channelrhodopsin 2 (ChR2)-tagged vM GABAergic neurons showed that they were most active during REM sleep (REM-max), and during wakefulness they were preferentially active during eating and grooming. Furthermore, dual retrograde tracing showed that the rostral projections to the pons and midbrain and caudal projections to the spinal cord originate from separate vM neuron populations. Activating the rostral GABAergic projections was sufficient for both the induction and maintenance of REM sleep, which are likely mediated in part by inhibition of REM-suppressing GABAergic neurons in the ventrolateral periaqueductal gray (vlPAG). These results identify a key component of the pontomedullary network controlling REM sleep. The capability to induce REM sleep on command may offer a powerful tool for investigating its functions.

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.

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.

State-dependent control of lumbar motoneurons by the hypocretinergic system

Neurons in the lateral hypothalamus (LH) that synthesize hypocretins (Hcrt-1 and Hcrt-2) are active during wakefulness and excite lumbar motoneurons. Because hypocretinergic cells also discharge during phasic periods of rapid eye movement (REM) sleep, we sought to examine their action on the activity of motoneurons during this state. Accordingly, cat lumbar motoneurons were intracellularly recorded, under alpha-chloralose anesthesia, prior to (control) and during the carbachol-induced REM sleep-like atonia (REMc). During control conditions, LH stimulation induced excitatory postsynaptic potentials (composite EPSP) in motoneurons. In contrast, during REMc, identical LH stimulation induced inhibitory PSPs in motoneurons. We then tested the effects of LH stimulation on motoneuron responses following the stimulation of the nucleus reticularis gigantocellularis (NRGc) which is part of a brainstem-spinal cord system that controls motoneuron excitability in a state-dependent manner. LH stimulation facilitated NRGc stimulation-induced composite EPSP during control conditions whereas it enhanced NRGc stimulation-induced IPSPs during REMc. These intriguing data indicate that the LH exerts a state-dependent control of motor activity. As a first step to understand these results, we examined whether hypocretinergic synaptic mechanisms in the spinal cord were state dependent. We found that the juxtacellular application of Hcrt-1 induced motoneuron excitation during control conditions whereas motoneuron inhibition was enhanced during REMc. These data indicate that the hypocretinergic system acts on motoneurons in a state-dependent manner via spinal synaptic mechanisms. Thus, deficits in Hcrt-1 may cause the coexistence of incongruous motor signs in cataplectic patients, such as motor suppression during wakefulness and movement disorders during REM sleep.

Cataplexy-active neurons in the hypothalamus: implications for the role of histamine in sleep and waking behavior

Noradrenergic, serotonergic, and histaminergic neurons are continuously active during waking, reduce discharge during NREM sleep, and cease discharge during REM sleep. Cataplexy, a symptom associated with narcolepsy, is a waking state in which muscle tone is lost, as it is in REM sleep, while environmental awareness continues, as in alert waking. In prior work, we reported that, during cataplexy, noradrenergic neurons cease discharge, and serotonergic neurons greatly reduce activity. We now report that, in contrast to these other monoaminergic "REM-off" cell groups, histamine neurons are active in cataplexy at a level similar to or greater than that in quiet waking. We hypothesize that the activity of histamine cells is linked to the maintenance of waking, in contrast to activity in noradrenergic and serotonergic neurons, which is more tightly coupled to the maintenance of muscle tone in waking and its loss in REM sleep and cataplexy.

Cessation of activity in red nucleus neurons during stimulation of the medial medulla in decerebrate rats

The pontine oral reticular nucleus, gigantocellular reticular nucleus (Gi) and dorsal paragigantocellular nucleus (DPGi) of the medulla are key elements of a brainstem-reticulospinal inhibitory system that participates in rapid eye movement (REM) sleep atonia. Our recent study has shown that excitation of these brainstem nuclei in decerebrate rats inhibits locus coeruleus cells and the midbrain locomotor region neurons related to muscle tone facilitation. In the present study we have examined the influences of electrical and chemical stimulation of Gi and DPGi inhibitory sites on the activity of neurons located in the magnocellular part of the red nucleus (RMC), a cell group that participates in both the tonic and phasic regulation of motor output. A total of 192 RMC neurons were recorded in precollicular-premammillary decerebrate rats with muscle rigidity and induced locomotion. Thirty-three RMC neurons were identified antidromically as rubrospinal (RMC-spinal) cells by stimulation of the contralateral dorsolateral funiculus at the L2 level. A total of 141 RMC neurons (88.7 %) and all RMC-spinal neurons were inhibited during electrical stimulation of Gi and DPGi inhibitory sites. This cessation of activity was correlated with bilateral muscle atonia or blockage of locomotion. Six RMC cells (3.8 %) were excited (224 +/- 50 %, n = 6, minimum = 98, maximum = 410, P < 0.05) and 11 cells (7 %) gave no response to Gi and DPGi stimulation. Microinjections of kainic acid (100 microM, 0.2 microl) into Gi and DPGi inhibitory sites, previously identified by electrical stimulation, produced a short-latency (35 +/- 3.5 s, n = 11) decrease of rigid hindlimb muscle tone and inhibition of all tested RMC (n = 7) and RMC-spinal (n = 5) neurons. These results, combined with our recent published data, suggest that inhibition of motor function during activation of the brainstem inhibitory system is related to both the descending inhibition of spinal motoneurons and suppression of activity in supraspinal motor facilitatory systems. These two mechanisms acting synergistically may cause generalized motor inhibition during REM sleep and cataplexy.

Increased and decreased muscle tone with orexin (hypocretin) microinjections in the locus coeruleus and pontine inhibitory area

Orexin-A (OX-A) and orexin-B (OX-B) (hypocretin 1 and hypocretin 2) are synthesized in neurons of the perifornical, dorsomedial, lateral, and posterior hypothalamus. The locus coeruleus (LC) receives the densest extrahypothalamic projections of the orexin (OX) system. Recent evidence suggests that descending projections of the LC have a facilitatory role in the regulation of muscle tone. The pontine inhibitory area (PIA), located ventral to LC, receives a moderate OX projection and participates in the suppression of muscle tone in rapid-eye-movement sleep. We have examined the role of OX-A and -B in muscle-tone control using microinjections (0.1 microM to 1 mM, 0.2 microl) into the LC and PIA in decerebrate rats. OX-A and -B microinjections into the LC produced ipsi- or bilateral hindlimb muscle-tone facilitation. The activity of LC units was correlated with the extent of hindlimb muscle-tone facilitation after OX microinjections (100 microM, 1 microl) into fourth ventricle. Microinjections of OX-A and -B into the PIA produced muscle-tone inhibition. We did not observe any significant difference in the effect of OX-A and -B on muscle tone at either site. Our data suggest that OX release activates LC units and increases noradrenergic tonus in the CNS. Moreover, OX-A and -B may also regulate the activity of pontine cholinoceptive and cholinergic neurons participating in muscle-tone suppression. Loss of OX function may therefore disturb both facilitatory and inhibitory motor processes.

Activation of pontine and medullary motor inhibitory regions reduces discharge in neurons located in the locus coeruleus and the anatomical equivalent of the midbrain locomotor region

Activation of the pontine inhibitory area (PIA) including the middle portion of the pontine reticular nucleus, oral part (PnO), or the gigantocellular reticular nucleus (Gi) suppresses muscle tone in decerebrate animals. The locus coeruleus (LC) and midbrain locomotor region (MLR) have been implicated in the facilitation of muscle tone. In the current study we investigated whether PIA and Gi stimulation causes changes in activity in these brainstem motor facilitatory systems. PIA stimulation evoked bilateral muscle tone suppression and inhibited 26 of 28 LC units and 33 of 36 tonically active units located in the anatomical equivalent of the MLR (caudal half of the cuneiform nucleus and the pedunculopontine tegmental nucleus). Gi stimulation evoked bilateral suppression of hindlimb muscle tone and inhibited 20 of 35 LC units and 24 of 24 neurons located in the MLR as well as facilitated 11 of 35 LC units. GABA and glycine release in the vicinity of LC was increased by 20-40% during ipsilateral PnO stimulation inducing hindlimb muscle tone suppression on the same side of the body. We conclude that activation of pontine and medullary inhibitory regions produces a coordinated reduction in the activity of the LC units and neurons located in the MLR related to muscle tone facilitation. The linkage between activation of brainstem motor inhibitory systems and inactivation of brainstem facilitatory systems may underlie the reduction in muscle tone in sleep as well as the modulation of muscle tone in the isolated brainstem.

Activation of pontine and medullary motor inhibitory regions reduces discharge in neurons located in the locus coeruleus and the anatomical equivalent of the midbrain locomotor region

Activation of the pontine inhibitory area (PIA) including the middle portion of the pontine reticular nucleus, oral part (PnO), or the gigantocellular reticular nucleus (Gi) suppresses muscle tone in decerebrate animals. The locus coeruleus (LC) and midbrain locomotor region (MLR) have been implicated in the facilitation of muscle tone. In the current study we investigated whether PIA and Gi stimulation causes changes in activity in these brainstem motor facilitatory systems. PIA stimulation evoked bilateral muscle tone suppression and inhibited 26 of 28 LC units and 33 of 36 tonically active units located in the anatomical equivalent of the MLR (caudal half of the cuneiform nucleus and the pedunculopontine tegmental nucleus). Gi stimulation evoked bilateral suppression of hindlimb muscle tone and inhibited 20 of 35 LC units and 24 of 24 neurons located in the MLR as well as facilitated 11 of 35 LC units. GABA and glycine release in the vicinity of LC was increased by 20-40% during ipsilateral PnO stimulation inducing hindlimb muscle tone suppression on the same side of the body. We conclude that activation of pontine and medullary inhibitory regions produces a coordinated reduction in the activity of the LC units and neurons located in the MLR related to muscle tone facilitation. The linkage between activation of brainstem motor inhibitory systems and inactivation of brainstem facilitatory systems may underlie the reduction in muscle tone in sleep as well as the modulation of muscle tone in the isolated brainstem.

Reticulospinal systems mediate atonia with short and long latencies

The pontomedullary region is responsible for both the tonic and phasic reduction of muscle activity in rapid-eye-movement sleep and contributes to the control of muscle tone in waking. This study focused on determining the time course of activity in the pontomedullary systems mediating atonia. Short-train stimulations (3 0.2-ms pulses at 330 Hz) of the pons and medulla suppressed neck and hindlimb muscle activity in decerebrate cats. We identified two distinct phases of suppression, early and late. The anatomic sites that produced each suppression were intermixed. We estimated the dividing value of the conduction velocity for reticulospinal projections responsible for early and late phases of hindlimb muscle tone suppression to be 22.8 m/s. In the medial medulla, 238 reticulospinal units, which send axons to the L1 level of the spinal cord, were identified. Pontine stimulation that suppressed hindlimb muscle tone increased the firing rate of 138 units (type I). Sixteen type I units showed a delayed response to the pontine stimulation with a latency of 10 ms or longer (type Id), whereas 122 type I units exhibited an earlier response (type Ie). Seven type Ie units had an axonal conduction velocity of <22.8 m/s, whereas the remaining 115 conducted at faster than 22.8 m/s. Early and late hindlimb muscle tone suppressions were hypothesized to be mediated through fast and slow conducting type Ie reticulospinal units. The activity of type Id neurons may contribute to the cessation of the early-phase suppression as well as to the induction, maintenance, or cessation of the late-phase suppression.

Autonomic reaction to vestibular damage

The vestibular system provides inputs to many neurons in the brain stem that participate in autonomic control. This multiplicity of vestibular-autonomic connections plays a variety of roles. Whereas it has been known for decades that unilateral vestibular lesions can result in motion sickness, recent data suggest that the vestibular system participates in making adjustments in blood pressure and respiration that are necessary to maintain homeostasis during movement and changes in posture. Animals with bilateral vestibular lesions are more susceptible to posturally related hypotension than vestibularly intact animals, and it is also possible that orthostatic hypotension after space flight is caused in part by microgravity-related changes in otolith function. Patients with vestibular lesions could also be more vulnerable to respiratory disturbances related to posture, such as obstructive apnea. Vestibular dysfunction has additionally been linked with anxiety disorders, such as agoraphobia, which may result from alteration of vestibular inputs to brain stem monoaminergic neurons (which are known to process these signals). Even sleep disturbances might be connected with vestibular disorders because neurons in the pontine reticular formation that are critical in switching between sleep states may be influenced by labyrinthine inputs. Thus it is likely that vestibular damage will result in a number of parallel disturbances in autonomic function.

Motor inhibition from the brainstem is normal in torsion dystonia during REM sleep

The maintenance of axial atonia during REM sleep was monitored in 14 patients with primary torsion dystonia, 10 patients with secondary torsion dystonia, and 10 normal subjects using submental EMG and video EEG telemetry. The excitability of the corticospinal tract during REM sleep was also assessed using scalp magnetic stimulation in seven patients and three controls. During REM sleep dystonic patients had well maintained atonia evidenced by infrequent bursts of submental activity, no episodes of complex semi-purposeful behaviour and reduced motor responses to magnetic stimulation. These findings suggest that the inhibitory centres in the region of the locus coeruleus and their descending pathways to the spinal alpha motor neurons are intact in torsion dystonia.

Ascending and descending projections of the nucleus reticularis gigantocellularis in the cat demonstrated by the anterograde neural tracer, Phaseolus vulgaris leucoagglutinin (PHA-L)

Ascending and descending projections of the nucleus reticularis gigantocellularis (NRGc) were studied in the cat by the anterograde tracer, Phaseolus vulgaris leucoagglutinin. Ascending fibers from the left or the right NRGc coursed through the bilateral medial reticular formation and some of them reached the diencephalon. In the brainstem, PHA-L-labeled fibers and their terminals were observed in the medial reticular formation, the cranial motor nuclei (III, IV, V, VI, VII, XII), the vestibular complex, the LC complex, the raphe nuclei, the periaqueductal gray, the red nucleus, the Edinger-Westphal nucleus and the interstitial nucleus of Cajal. In the diencephalon, they were observed in the dorsal thalamus and the hypothalamic regions. In the caudal medulla, labeled fibers and their terminals were observed in the nucleus prepositus hypoglossi, the nucleus intercalatus and the inferior olive. Descending axons from the NRGc coursed bilaterally through the ventral and ventrolateral funiculi as far caudal as the upper thoracic cord. Single axon collaterals arising from the descending axons gave off terminal fibers to the left or the right gray matter. Their terminals were located in laminae V-X, mainly in laminae VII and VIII. In lamina IX, they were distributed mainly in the medial portion. A few fibers originating from the descending axons ipsilateral to the PHA-L injection side coursed through the anterior or posterior commissure, and ended in laminae VI, VII and VIII. The functional implications of these findings are discussed in relation to the behavioral state control and the generalized motor inhibition.

Neuronal structures of the brainstem participating in postural suppression in cats

Two histochemical tracer techniques (a horseradish peroxidase and a Phaseolus vulgaris leucoagglutinin axonal flow technique) were employed to determine the cells of origin, axonal trajectories and terminals of fibers of passage through the dorsal tegmental field (DTF) in the pons along the midline (Horsley-Clarke coordinates, P 3 to P 7, LR 0, H - 4.5 to H - 6.0). Stimulation of the DTF area results in the postural suppression in both acute decerebrate, reflexively standing cats and freely moving awake cats. With these techniques, we have been able to demonstrate that in addition to pontine reticular cells, many cells in the dorsolateral pontine tegmentum, including the LC-complex, project their axons to the rostral part of the midline DTF area, and that descending axons from the nucleus reticularis pontis oralis pass through the DTF area and terminate on cells in the nucleus reticularis pontis caudalis and the nucleus reticularis gigantocellularis. Among those cells in the nucleus reticularis gigantocellularis, a great number of giant cells project to the lumbar spinal cord. The functional roles of these neuronal structures are discussed in relation to the results obtained by chemical stimulation of the pontine structures.

Behavioral state-specific inhibitory postsynaptic potentials impinge on cat lumbar motoneurons during active sleep

High-gain intracellular records were obtained from lumbar motoneurons in intact, undrugged cats during naturally occurring states of wakefulness, quiet sleep, and active sleep. Spontaneous, discrete, inhibitory postsynaptic potentials (IPSPs) were found to impinge on lumbar motoneurons during all states of sleep and wakefulness. IPSPs which occurred during wakefulness and quiet sleep were of relatively low amplitude and had a low frequency of occurrence. During the state of active sleep there occurred a great increase in inhibitory input. This was the result of the appearance of large-amplitude IPSPs and of an increase in the frequency of low-amplitude IPSPs which were indistinguishable from those recorded during wakefulness and quiet sleep. In addition to a difference in amplitude, the time course of the large IPSPs recorded during active sleep further differentiated them from the smaller IPSPs recorded during wakefulness, quiet sleep, and active sleep; i.e., their rise-time and half-width were of longer duration and their rate-of-rise was significantly faster. We suggest that the large, active sleep-specific IPSPs reflect the activity of a group of inhibitory interneurons which are inactive during wakefulness and quiet sleep and which discharge during active sleep. These as yet unidentified interneurons would then serve as the last link in the brain stem-spinal cord inhibitory system which is responsible for producing muscle atonia during the state of active sleep.

Relation of pontine choline acetyltransferase immunoreactive neurons with cells which increase discharge during REM sleep

The purpose of this study was to determine whether neurons in the medial pontine reticular formation with high discharge rates during REM sleep could be localized in regions of the brainstem having neurons displaying choline acetyltransferase immunoreactivity. Six cats were implanted with sleep recording electrodes and microwires to record extracellular potentials of neurons in the pontine reticular formation. Single-units with a S:N ratio greater than 2:1 were recorded for at least two REM sleep cycles. A total of 49 units was recorded from the pontine reticular formation at medial-lateral planes ranging from 0.8 to 3.7 mm. The greatest proportion of the units (28.6%) showed highest discharge during active waking and phasic REM sleep compared to quiet waking, non-REM sleep, transition into REM sleep or quiet REM sleep periods. A percentage (20.4%) of the cells had high discharge associated with phasic REM sleep periods while 8.2% of the cells showed a progressive increase in discharge from waking to REM sleep. Subsequent examination of the distribution of choline acetyltransferase immunoreactive cells in the PRF revealed that cells showing high discharge during REM sleep were not localized near presumed cholinergic neurons. Indeed, we did not find any ChAT immunoreactive somata in the medial PRF, an area which has traditionally been implicated in the generation of REM sleep. These results suggest that while increased discharge of PRF cells may be instrumental to REM sleep generation, these cells are not cholinergic.

Behavioral states in the chronic medullary and midpontine cat

Behavioral state organization was studied in the caudal portion of chronically maintained cats with transections at the ponto-medullary junction or midpontine level. The cats spent most of their time in a 'quiescent state.' This state was periodically interrupted by 'phasic activations.' During quiescence, ECG and reticular unit activity rates were low and regular. EMG levels resembled those seen during non-REM sleep in intact cats. During phasic activations, unit activity in the nucleus gigantocellularis and neck EMG activity increased to levels seen in the intact cat during active waking. Gross postural changes, vestibular slow phase head nystagmus and head shake reflexes could be observed at these times. No periods of neck muscle atonia were observed in either state. No periods of brain-stem controlled rapid eye movements (REMs) occurred. Unit activity patterns similar to those seen in the intact cat during REM sleep were never observed. Physostigmine administration did not produce REM sleep signs, but rather, triggered an aroused state. Phasic activations occurred in a regular ultradian rhythm, with a period similar to that seen in the REM sleep cycle. We conclude that the chronic medullary cat retains primitive aroused and quiescent states, but does not have any of the local signs of REM sleep. However, the medulla does have the capability of generating ultradian rhythmicities which may contribute to the control of the basic rest activity cycle and the REM, non-REM sleep cycle.

Cerebral and local cerebral metabolism in the cat during slow wave and REM sleep

[14C]2-deoxyglucose autoradiography was used to show cerebral and regional cerebral metabolism during slow-wave sleep (SWS) and rapid-eye-movement sleep (REM) in the cat. Lower levels of mean cerebral metabolism, reflecting cerebral energy conservation, were associated with SWS. A clear link between REM and mean cerebral metabolism was not observed. At the regional level, SWS was associated with markedly low metabolism in thalamic sensory relays and in cortex. REM was associated with relatively low metabolism in the cerebellum, but with relatively high metabolism in the hippocampus, and in some 'motor' regions including the trigeminal and red nuclei. Thus, SWS was linked to cerebral energy conservation and to particularly low levels of functional activity in cortical and sub-cortical sensory regions. REM was unlike SWS in that: REM did not appear to be strongly linked to cerebral energy conservation; REM was linked to metabolism in fewer brain regions than was SWS; and most REM-linked regions exhibited relatively high levels of metabolism. In addition, while SWS was most clearly associated with functional activity in sensory regions, REM was linked to functional activity in a small number of limbic and motor regions. In sum, SWS and REM are associated with distinctive cerebral metabolic and functional states.

Site-specific enhancement and suppression of desynchronized sleep signs following cholinergic stimulation of three brainstem regions

The behavioral effects of carbachol microinjection into the pontine, midbrain and medullary reticular formation (RF) have been studied. Enhancement of electrographic desynchronized (D) sleep signs and D sleep-like behavior was specific to the pons; these parameters were contrastingly suppressed following cholinergic stimulation of the midbrain and medullary RF. Polygraphically defined wakefulness (W) was increased from the midbrain and medulla and was characterized by stereotyped motor activity, whereas slow wave (S) sleep was reduced or eliminated when carbachol was administered into all 3 brainstem regions. In addition, as a function of the site of carbachol administration within the pons, some qualitative electrographic differences between carbachol-induced D sleep and the physiological D sleep state were observed. These results are consistent with the hypotheses that the pontine RF is involved in the cholinergic initiation and/or maintenance of D sleep and that the midbrain RF mediates arousal via cholinoceptive reticular cells. The hypothesis that the medullary RF plays a major role in the generation of S sleep and the atonia of D sleep is not supported by these data, but non-cholinergic mechanisms of these proposed roles remain to be investigated. Preliminary reports of these data have been published.