Robert Nienhuis

Reduced number of hypocretin neurons in human narcolepsy.

Murine and canine narcolepsy can be caused by mutations of the hypocretin (Hcrt) (orexin) precursor or Hcrt receptor genes. In contrast to these animal models, most human narcolepsy is not familial, is discordant in identical twins, and has not been linked to mutations of the Hcrt system. Thus, the cause of human narcolepsy remains unknown. Here we show that human narcoleptics have an 85%-95% reduction in the number of Hcrt neurons. Melanin-concentrating hormone (MCH) neurons, which are intermixed with Hcrt cells in the normal brain, are not reduced in number, indicating that cell loss is relatively specific for Hcrt neurons. The presence of gliosis in the hypocretin cell region is consistent with a degenerative process being the cause of the Hcrt cell loss in narcolepsy.

Sleep deprivation decreases superoxide dismutase activity in rat hippocampus and brainstem.

Sleep deprivation by the disk-over-water technique results in a predictable syndrome of physiological changes in rats. It has been proposed that reactive oxygen species (ROS) may be responsible for some of these effects. A variety of antioxidative enzymes such as superoxide dismutase (SOD) and glutathione peroxidase (GPx) help to regulate the level of ROS. In this study we investigated the effects of prolonged (5–11 days) sleep deprivation on the activities of SOD and GPx as well as the metabolic activity of the mitochondria (using alamar blue) in several brain regions (cortex, hippocampus, hypothalamus, brainstem and cerebellum). We show that prolonged sleep deprivation significantly decreased Cu/Zn-SOD activity in the hippocampus and brainstem, suggesting an alteration in the metabolism of ROS resulting in oxidative stress.

Pattern of hypocretin (orexin) soma and axon loss, and gliosis, in human narcolepsy

Human narcolepsy is correlated with a greatly reduced number of hypocretin (orexin) containing neurons and axons, and an elevated level of hypothalamic gliosis. We now report that the percentage loss of Hcrt cells and percentage elevation of GFAP staining are variable across forebrain and brainstem nuclei, and are maximal in the posterior and tuberomammillary hypothalamic region. Regional gliosis and percent loss of hypocretin axons in narcoleptics are not correlated with regional hypocretin cell soma density in normals or with regional percent soma loss in narcoleptics. Rather they are independently and strongly correlated with the regional density of hypocretin axons and the message density for hypocretin receptor 2, as quantified in the rat. These results are consistent with the hypotheses that the loss of hypocretin function in narcolepsy results from a cytotoxic or immunologically mediated attack focused on hypocretin receptor 2 or an antigen anatomically linked to hypocretin receptor 2, and that this process is intensified in regions of high axonal density.

Sleep in the platypus

We have conducted the first study of sleep in the platypus Ornithorhynchus anatinus. Periods of quiet sleep, characterized by raised arousal thresholds, elevated electroencephalogram amplitude and motor and autonomic quiescence, occupied 6-8 h/day. The platypus also had rapid eye movement sleep as defined by atonia with rapid eye movements, twitching and the electrocardiogram pattern of rapid eye movement. However, this state occurred while the electroencephalogram was moderate or high in voltage, as in non-rapid eye movement sleep in adult and marsupial mammals. This suggests that the low-voltage electroencephalogram is a more recently evolved feature of mammalian rapid eye movement sleep. Rapid eye movement sleep occupied 5.8-8 h/day in the platypus, more than in any other animal. Our findings indicate that rapid eye movement sleep may have been present in large amounts in the first mammals and suggest that it may have evolved in pre-mammalian reptiles.

REM sleep signs rostral to chronic transections at the pontomedullary junction

The brainstems of 3 cats were transected at the ponto-medullary junction and the cats maintained in stable condition for periods of from 16 to 31 days. After transection, all of these cats had periods in which forebrain sensorimotor cortex, olfactory bulb, hippocampus, eye movement and lateral geniculate recordings exhibited the pattern of activity seen only in REM sleep in the intact cat. We conclude that medullary regions are not required to generate these signs of REM sleep. The pons is necessary for REM sleep and is sufficient to produce REM sleep signs in rostral as well as caudal brain regions. However, the medulla may contribute to regulation of the duration and periodicity of REM sleep.

A Brief History of Hypocretin/Orexin and Narcolepsy

The hypothalamic peptides named the orexins, or hypocretins, were discovered in 1998. In 1999 it was established that genetic narcolepsy could be caused by mutations in the genes synthesizing these peptides or their receptors. In September of 2000 it was found that most human narcolepsy is caused by loss of hypocretin cells, most likely as a result of a degenerative process. This paper reviews these events and their implications for our understanding of brain arousal and motor control systems.

Rostral brainstem contributes to medullary inhibition of muscle tone

It has long been known that stimulation of the medial medulla in the decerebrate animal produces bilateral inhibition of muscle tone. In the present study we have found that transection of the brainstem at the ponto-medullary junction attenuates this inhibition. An interaction between medullary and rostal brainstem systems is responsible for the medullary inhibition phenomenon. A similar interaction may produce the inhibition of muscle tone seen in REM sleep.

Discharge pattern of reticular formation unit pairs in waking and REM sleep

Abstract Interactions between simultaneously recorded pairs of neurons in the magnocellular and gigantocellular fields of the reticular formation were studied in unanesthetized, unrestrained cats. Each cell pair was recorded during both waking and REM sleep. Dependencies in discharge between spike trains were observed visually and with cross-correlation analyses. These dependencies were present at both short-latency and long-latency intervals. Dependencies were observed with equal frequency in waking and REM sleep. Short duration (1 to 3 ms) interactions were found in 40% of significant cross correlations and were most common in adjacent cells with related behavioral correlates. Patterns of discharge in REM sleep were similar to those in waking. These results suggest that there is common synaptic input to a large proportion of adjacent reticular cell pairs during both waking and REM sleep. Synchronized firing in local cell clusters may be a way in which reticular formation contributions to complex motor behavior are synthesized.

Ontogeny of Feline Temporal Lobe Epilepsy, II: Stability of Spontaneous Sleep Epilepsy in Amygdala-Kindled Kittens

Summary: We previously described a model of spontaneous “sleep epilepsy” in kindled kittens with temporal lobe epilepsy (TLE). We now describe the postkindling course of this model from preadolescence to maturity and suggest pathophysiologic mechanisms. Spontaneous epilepsy, particularly generalized tonic‐clonic convulsions (GTCs), developed 1h to 4 months after amygdala kindling and persisted to adulthood. At first, GTCs were detected only in sleep; later, convulsions also occurred during wakefulness. Two factors were consistently associated with the sequential onset of sleep and waking GTCs: seizure clusters and anatomic seizure localization. (1) Seizure clusters. Cats with infrequent or unclustered GTCs continued to exhibit “sleep epilepsy,” defined by convulsions occurring exclusively during sleep. In contrast, cats with frequent seizure clusters developed recurrent or terminal convulsive status in conjunction with GTCs during waking and sleep. Severe seizure manifestations therefore appeared to contribute to the dissociation of convulsions from the sleep‐wake cycle. (2) Anatomical seizure localization. Focal seizure origin appeared to differentiate sleep from waking GTCs. Onset during sleep was first recorded in the kindled amygdala, whereas onset during waking was initially detected outside the temporal lobe. Findings thus suggest secondary “kindling” of multifocal epilepsy. Secondary epileptogenesis is consistent with “transsynaptic” kindling effects. This phenomenon is defined in mature animals by rapid secondary site kindling (transfer) and subtle morphologic changes distal to the stimulating electrode. Transfer may be accentuated by youth, because kittens developed spontaneous seizure foci in previously unstimulated tissue. Moreover, multifocal interactions and diffuse cell loss were implicated as possible mechanisms. Collectively, the findings indicate complications with early onset TLE in kindled cats. Onset during youth can have an unfavorable prognosis, reflected by recurrent status epilepticus and multifocal epilepsy with convulsions distributed throughout the sleep‐wake cycle.

Role of the Hypocretin (Orexin) Receptor 2 (Hcrt-r2) in the Regulation of Hypocretin Level and Cataplexy

Hypocretin receptor-2 (Hcrt-r2)-mutated dogs exhibit all the major symptoms of human narcolepsy and respond to drugs that increase or decrease cataplexy as do narcoleptic humans; yet, unlike narcoleptic humans, the narcoleptic dogs have normal hypocretin levels. We find that drugs that reduce or increase cataplexy in the narcoleptic dogs, greatly increase and decrease, respectively, hypocretin levels in normal dogs. The effects of these drugs on heart rate and blood pressure, which were considerable, were not correlated with their effects on cataplexy. Administration of these drugs to Hcrt-r2-mutated dogs produced indistinguishable changes in heart rate and blood pressure, indicating that neither central nor peripheral Hcrt-r2 is required for these cardiovascular effects. However, in contrast to the marked Hcrt level changes in the normal dogs, these drugs did not alter hypocretin levels in the Hcrt-r2 mutants. We conclude that Hcrt-r2 is a vital element in a feedback loop integrating Hcrt, acetylcholine, and norepinephrine function. In the absence of functional Hcrt-r2, Hcrt levels are not affected by monoaminergic and cholinergic drugs, despite the strong modulation of cataplexy by these drugs. Conversely, strong transient reductions of Hcrt level by these drugs do not produce episodes of cataplexy in normal dogs. The Hcrt-r2 mutation causes drug-induced cataplexy by virtue of its long-term effect on the functioning of other brain systems, rather than by increasing the magnitude of phasic changes in Hcrt level. A similar mechanism may be operative in spontaneous cataplexy in narcoleptic dogs as well as in narcoleptic humans.