A function for REM sleep: regulation of noradrenergic receptor sensitivity

The REM sleep-memory consolidation hypothesis

It has been hypothesized that REM (rapid eye movement) sleep has an important role in memory consolidation. The evidence for this hypothesis is reviewed and found to be weak and contradictory. Animal studies correlating changes in REM sleep parameters with learning have produced inconsistent results and are confounded by stress effects. Humans with pharmacological and brain lesion-induced suppression of REM sleep do not show memory deficits, and other human sleep-learning studies have not produced consistent results. The time spent in REM sleep is not correlated with learning ability across humans, nor is there a positive relation between REM sleep time or intensity and encephalization across species. Although sleep is clearly important for optimum acquisition and performance of learned tasks, a major role in memory consolidation is unproven.

Gene expression in the brain across the sleep-waking cycle

Sleep and waking differ significantly in terms of behavior, metabolism, and neuronal activity. Recent evidence indicates that sleep and waking also differ with respect to the expression of certain genes. To systematically investigate such changes, we used mRNA differential display and cDNA microarrays to screen approximately 10000 transcripts expressed in the cerebral cortex of rats after 8 h of sleep, spontaneous waking, or sleep deprivation. We found that 44 genes had higher mRNA levels after waking and/or sleep deprivation relative to sleep, while 10 were upregulated after sleep. Known genes that were upregulated in waking and sleep deprivation can be grouped into the following categories: immediate early genes/transcription factors (Arc, CHOP, IER5, NGFI-A, NGFI-B, N-Ras, Stat3), genes related to energy metabolism (glucose type I transporter Glut1, Vgf), growth factors/adhesion molecules (BDNF, TrkB, F3 adhesion molecule), chaperones/heat shock proteins (BiP, ERP72, GRP75, HSP60, HSP70), vesicle- and synapse-related genes (chromogranin C, synaptotagmin IV), neurotransmitter/hormone receptors (adrenergic receptor alpha(1A) and beta(2), GABA(A) receptor beta(3), glutamate NMDA receptor 2A, glutamate AMPA receptor GluR2 and GluR3, nicotinic acetylcholine receptor beta(2), thyroid hormone receptor TRbeta), neurotransmitter transporters (glutamate/aspartate transporter GLAST, Na(+)/Cl(-) transporter NTT4/Rxt1), enzymes (aryl sulfotransferase, c-jun N-terminal kinase 1, serum/glucocorticoid-induced serine/threonine kinase), and a miscellaneous group (calmodulin, cyclin D2, LMO-4, metallothionein 3). Several other genes that were upregulated in waking and all the genes upregulated in sleep, with the exception of the one coding for membrane protein E25, did not match any known sequence. Thus, significant changes in gene expression occur across behavioral states, which are likely to affect basic cellular functions such as RNA and protein synthesis, neural plasticity, neurotransmission, and metabolism.

Correlates of sleep and waking in Drosophila melanogaster

Drosophila exhibits a circadian rest-activity cycle, but it is not known whether fly rest constitutes sleep or is mere inactivity. It is shown here that, like mammalian sleep, rest in Drosophila is characterized by an increased arousal threshold and is homeostatically regulated independently of the circadian clock. As in mammals, rest is abundant in young flies, is reduced in older flies, and is modulated by stimulants and hypnotics. Several molecular markers modulated by sleep and waking in mammals are modulated by rest and activity in Drosophila, including cytochrome oxidase C, the endoplasmic reticulum chaperone protein BiP, and enzymes implicated in the catabolism of monoamines. Flies lacking one such enzyme, arylalkylamine N-acetyltransferase, show increased rest after rest deprivation. These results implicate the catabolism of monoamines in the regulation of sleep and waking in the fly and suggest that Drosophila may serve as a model system for the genetic dissection of sleep.

A passion of the soul: an introduction to pain for consciousness researchers

Pain is an important focus for consciousness research because it is an avenue for exploring somatic awareness, emotion, and the genesis of subjectivity. In principle, pain is awareness of tissue trauma, but pain can occur in the absence of identifiable injury, and sometimes substantive tissue injury produces no pain. The purpose of this paper is to help bridge pain research and consciousness studies. It reviews the basic sensory neurophysiology associated with tissue injury, including transduction, transmission, modulation, and central representation. In addition, it highlights the central mechanisms for the emotional aspects of pain, demonstrating the physiological link between tissue trauma and mechanisms of emotional arousal. Finally, we discuss several current issues in the field of pain research that bear on central issues in consciousness studies, such as sickness and sense of self.

The effect of sleep state on electroretinographic (ERG) activity during early human development

To assess the effects of sleep state on human retinal electric responses, full-field electroretinograms were obtained in a cross-sectional study from 123 preterm infants at 36, 40 and 56 weeks of post-gestational age. At each age, electroretinographic recordings were assigned to one of two groups according to whether the infants were in active sleep or quiet sleep. Both sleep states were determined behaviorally. Pure rod, maximal, 30 Hz flicker and light adapted single cone responses were evaluated when a sleep state was clearly established. Peak-to-peak amplitudes of most electroretinographic responses were significantly larger in active sleep relative to quiet sleep at 36 and 40 weeks of post-gestational age. We speculate that larger amplitudes during active sleep may play a role in the maturation of the visual system.

Heterogeneous effects of rapid eye movement sleep deprivation on binding to alpha- and beta-adrenergic receptor subtypes in rat brain

Quantitative receptor autoradiography was used to map alterations in binding to alpha1-, alpha2-, beta1- and beta2-adrenergic receptors throughout the brain of rats deprived of rapid eye movement sleep for 96 h. Binding of [3H]prazosin to alpha1 sites, while not significantly different in any of 46 brain regions examined, showed a clear overall tendency towards decreased values after sleep deprivation. [3H]UK-14,314-labeled alpha2 binding sites were not significantly affected by sleep deprivation in any of 91 brain regions analysed, despite a trend towards increased values. In contrast, beta-adrenergic binding was significantly reduced throughout the brain. Binding to beta1 sites labeled by [125I]iodopindolol in the presence of ICI-11855 was significantly reduced in 13 of 69 brain areas examined; binding to beta2 sites labeled by [125I]iodopindolol in the presence of CGP-20712A was likewise reduced throughout the brain and significantly so in 25 of the 72 brain areas analysed. Rank ordering of the binding changes indicated that reductions in beta1 vs beta2 binding were maximal in different brain areas. This pattern of results may reflect a particular configuration of effects specifically associated with sleep loss stress. The results are consistent with evidence of persisting noradrenergic cell activity during sleep deprivation. The observed heterogeneity of effects suggests that not all norepinephrine receptors are equally affected by rapid eye movement sleep deprivation.

The determinants of long-term blood pressure stability: control of trough blood pressure during sleep

Long-term blood pressure stability in humans is not affected by prolonged hypotensive drug therapy. After withdrawal of long-term hypotensive therapy from essential hypertensives, their blood pressures soon return to pre-treatment levels. Since the well-studied stabilizing systems all adapt to prevailing blood pressure levels, they cannot account for long-term stability of blood pressure. There is a parallel between trough blood pressure and the lower limit of cerebral autoregulation. Blood pressure goes down to a trough level once sleep has been established. This level is maintained for 3-4 h, with minor perturbations. For normal and hypertensive humans and rats, the level is almost the same as the lower limit of cerebral autoregulation. In essential hypertension the nocturnal fall of blood pressure and the lower limit for cerebral autoregulation could be set by atheromatous narrowing or occlusion of large cerebral arteries, especially those supplying the brain stem, from which the Cushing response is initiated. Constriction and maldevelopment of smaller cerebral arteries are probably the main causes of increased cerebrovascular resistance in spontaneously hypertensive rats (SHR). The structural difference between the two situations might explain the failure of prolonged hypotensive therapy to reduce the lower limit of cerebral autoregulation in humans, which it is able to achieve in young SHR. If trough blood pressure stability were the anchor which prevents casual blood pressures from drifting, a primary increase of cerebrovascular resistance would be a plausible explanation for essential hypertension. It could also make a major causal neurogenic contribution to hypertension in rat models, especially SHR and stroke-prone SHR.

REM sleep deprivation increases the levels of tyrosine hydroxylase and norepinephrine transporter mRNA in the locus coeruleus

The present study was conducted to determine the effects of REM sleep deprivation on the levels of tyrosine hydroxylase (TH) and norepinephrine transporter (NET) mRNA in the locus coeruleus (LC) of rats. The animals were deprived of REM sleep for 1, 3 or 5 days, then killed and changes in the mRNA levels were determined using in situ hybridization. The levels of both TH and NET mRNA increased in animals deprived of REM sleep for 3 days or longer whereas no change in these messages were observed in the LC of control animals. REM sleep deprivation has been used as a mode of treatment for major depression. Others have shown that treatment with tricyclic antidepressants also results in increased levels of TH and NET mRNA in LC. Our results suggest that the antidepressant effect of REM sleep deprivation and tricyclic antidepressants may share similar molecular changes in the norepinephrine system.

Effect of amitriptyline on symptoms, sleep, and visceral perception in patients with functional dyspepsia

OBJECTIVE

Tricyclic antidepressants in low doses are widely used in the therapy of patients with functional gastrointestinal disorders, yet the mechanism(s) of action of these drugs in these disorders is not known. In the current study, we sought to determine in a group of patients with functional dyspepsia and associated poor sleep how amitryptiline affects digestive symptoms, perceptual responses to gastric distension, and subjective and objective measures of sleep.

METHODS

Patients were randomized to 4 wk of amitryptiline 50 mg taken at bedtime versus placebo. There was a 3-wk washout phase, followed by a cross-over to the alternate treatment. Perceptual sensitivity to gastric distention and sleep EEG were recorded at the end of each treatment period. Diaries of symptoms were maintained throughout.

RESULTS

Seven of seven patients reported significantly less severe gastrointestinal symptoms after 4 wk on amitryptiline compared to placebo. Five of seven patients had evidence for altered perception of gastric balloon distension during placebo. However, the subjective symptom improvement on amitryptiline was not associated with a normalization of the perceptual responses to gastric distension. Baseline sleep dysfunction in the form of reduced sleep efficiency, increased arousal, or abnormal amounts of REM sleep was found in all seven patients. Amitryptiline significantly reduced absolute and relative amounts of REM sleep, but had no effect on sleep parameters related to nonregenerative sleep.

CONCLUSION

The beneficial effect of low dose amitryptiline seen in functional dyspepsia is not related to changes in perception of gastric distension, or to measures of arousal from sleep. An increased tolerance to aversive visceral sensations may play a role in the therapeutic effect.

Dietary Tyrosine Protects Striatal Dopamine Receptors from the Adverse Effects of REM Sleep Deprivation

L-Tyrosine is a non-essential amino acid that is produced as an intermediary metabolite in the conversion of phenylalanine to 3,4-dihyroxyphenylalanine (DOPA), and is a precursor of the neurotransmitter dopamine. In previous studies, tyrosine pretreatment was shown to protect against the neurochemical and behavioral deficits of acute stress caused by tail shock or cold exposure in rodents. The present study addressed the hypothesis that tyrosine administration may be an effective counter-measure to dopamine-mediated behaviors induced by rapid eye-movement sleep deprivation (RSD). In order to test the hypothesis, Sprague-Dawley rats were divided into 9 treatment groups: RSD-treated rats on normal-protein diet (20% casein: 1% tyrosine, 1% valine); tank control (TC) rats on a normal diet; cage control (CC) rats on normal diet; RSD-treated rats on 4% tyrosine diet; TC rats on 4% tyrosine diet; CC rats on 4% tyrosine diet; RSD-treated rats on 4% valine diet; TC rats on 4% valine diet; CC rats on 4% valine diet. In the RSD group receiving tyrosine, there was no apparent change in Bmax for binding of the dopamine D2 receptor ligand [(3)H]YM-09151-2 in the striata as compared to the respective TC and CC groups; whereas RSD-treated rats maintained on the normal diet and valine supplementation demonstrated expected increases in Bmax for ligand binding. The TC group on the tyrosine diet showed attenuated catalepsy compared to the corresponding CC group, while the RSD group consuming tyrosine showed a catalepsy that was significantly increased, and similar to that of cage control animais on a control diet. These data suggest that the tyrosine-supplemented diet significantly attenuated RSD-induced changes in striatal dopamine D2 receptors, and the effect appeared sufficient to influence RSD-induced behaviors.

GABA release in the locus coeruleus as a function of sleep/wake state

GABA, glutamate, and glycine release in the locus coeruleus were measured as a function of sleep/wake state in the freely-behaving cat using the microdialysis technique. GABA release was found to increase during rapid-eye-movement sleep as compared to waking values. GABA release during slow-wave sleep was intermediate between that of waking states and rapid-eye-movement sleep. The concentration of glutamate and glycine in microdialysis samples was unchanged across sleep and wake states. Our findings are consistent with the hypothesis that GABAergic inhibition is responsible for the cessation of discharge in locus coeruleus neurons during REM sleep. The data suggest that a population of GABAergic neurons innervating the locus coeruleus are selectively active during rapid-eye-movement sleep.

Stress-induced sleep deprivation modifies corticotropin releasing factor (CRF) levels and CRF binding in rat brain and pituitary

Electroencephalographic (EEG) studies have shown that corticotropin-releasing factor (CRF) administration induces EEG activation, decreases sleep time both in rats and humans and modifies the sleep pattern in sleep deprived rats. In the present study we have investigated whether CRF neuronal activity could be altered in a situation of disrupted sleep-wake cycle. Sleep deprivation (SD) was induced by keeping the rat for 72 h on a small platform (7 cm) surrounded by water. Immediately after the SD period rats were killed and CRF levels and CRF receptor binding were evaluated in different brain areas. A marked increase in CRF levels was present in the striatum (+224%), limbic areas (+144%) and pituitary (+42%) whereas the hypothalamic CRF content was reduced (-57%). A significant decrease in CRF binding was found in the striatum (-33%) and pituitary (-38%) of sleep deprived rats. These results indicate that CRF neuronal activity is stimulated by SD, suggesting that CRF might play an important role in the physiological regulation of the sleep-wake cycle and that an altered CRF neuronal activity might be involved in behavioral modifications related to sleep disturbances.

Effect of sleep deprivation on the growth hormone response to the alpha-3 adrenergic receptor agonist, clonidine, in normal subjects

One night's sleep deprivation (SD) increased the growth hormone (GH) response to clonidine (20 ug/kg i.v.) in 11 normal men ( p < 0.005). This finding may indicate that SD enhances alpha-2 adrenergic receptor function or that the GH response to GH releasing factor in increased by SD.

The echidna Tachyglossus aculeatus combines REM and non-REM aspects in a single sleep state: implications for the evolution of sleep

Placental and marsupial mammals exist in three states of consciousness: waking, non-REM sleep, and REM sleep. We now report that the echidna Tachyglossus aculeatus, a representative of the earliest branch of mammalian evolution (the monotremes), does not have the pattern of neuronal activity of either of the sleep states seen in nonmonotreme mammals. Echidna sleep was characterized by increased brainstem unit discharge variability, as in REM sleep. However, the discharge rate decreased and the EEG was synchronized, as in non-REM sleep. Our results suggest that REM and non-REM sleep evolved as a differentiation of a single, phylogenetically older sleep state. We hypothesize that the physiological changes that occur during postnatal sleep development parallel certain aspects of the changes that have occurred during the evolution of sleep-waking states in mammals.

Changes in monoamines and their metabolite concentrations in REM sleep-deprived rat forebrain nuclei

Rapid eye movement sleep deprivation (REMSD) is a potent stressor in rats. Behavioral abnormalities such as passive and active avoidance, locomotor activity, problem solving, sensory information processing, and the development of adaptive copping strategy in response to repeated stress are among the earliest obvious symptoms of REMSD, the mechanism for which remain largely unknown. The aim of this study was to determine whether 96 h of REMSD causes changes in monoamine neurotransmitters concentrations in rat forebrain regions (frontal cortex, FC; parietal cortex, PC, and striatum) that are involved in mediating higher brain functions such as attentional mechanisms, sensory information processing, and locomotor activity, which are severely affected in REMSD conditions. Rats were subjected to 96 h of REMSD using inverted flower pot water tank technique. To account for the stress associated with water tanks, a tank control group (TC) was included where the animals could reside comfortably on a large pedestal in the water tank. Regional brain concentrations of norepinephrine (NE), dopamine (DA), dihydroxyphenyacetic acid (DOPAC), L-3,4-dihydroxyphenylalanine (L-DOPA), homovanillic acid (HVA), 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid (HIAA) were determined by electrochemical detection using high-performance liquid chromatography. The concentrations of serotonin and its metabolite, HIAA, was reduced in the frontal and parietal cortexes of REMSD rats compared with TC or cage control (CC) group. NE, DA, DOPAC, and HVA concentrations in FC and PC of REMSD animals were remained unchanged compared with TC or CC rats. A significant increase in the concentrations of DA metabolites was observed in the striatum of REMSD rats when compared with CC and TC rats. There was a 29 and 31% increase in the concentration of striatal DA in REMSD group compared to the TC and CC groups, respectively; however, these percentages were not statistically different. Striatal NE, 5-HT, and HIAA concentrations were not significantly different among the three groups. These results suggest that 96 h of REMSD alters dopaminergic and serotonergic systems in different locations in rat brain. The effect of REMSD on the serotonergic systems are localized in the cerebral cortex, whereas dopaminergic metabolism is increased in the striatum.

The echidna Tachyglossus aculeatus combines REM and non-REM aspects in a single sleep state: implications for the evolution of sleep

Placental and marsupial mammals exist in three states of consciousness: waking, non-REM sleep, and REM sleep. We now report that the echidna Tachyglossus aculeatus, a representative of the earliest branch of mammalian evolution (the monotremes), does not have the pattern of neuronal activity of either of the sleep states seen in nonmonotreme mammals. Echidna sleep was characterized by increased brainstem unit discharge variability, as in REM sleep. However, the discharge rate decreased and the EEG was synchronized, as in non-REM sleep. Our results suggest that REM and non-REM sleep evolved as a differentiation of a single, phylogenetically older sleep state. We hypothesize that the physiological changes that occur during postnatal sleep development parallel certain aspects of the changes that have occurred during the evolution of sleep-waking states in mammals.

Mild electrical stimulation of pontine tegmentum around locus coeruleus reduces rapid eye movement sleep in rats

The norepinephrinergic neurons in the locus coeruleus (LC) cease firing during REM sleep (REMS) and increase firing during REMS deprivation. Most of the earlier studies used lesion and transection techniques which could not confirm the role of LC in REMS generation and/or its maintenance, if at all. Hence, in this study it was hypothesized that if the LC REM-off neurons must cease firing before the onset of REMS, its continuous activation should eliminate or at least reduce REMS. Electrophysiological parameters characterizing sleep-wakefulness-REMS were recorded in freely moving male albino rats. In an attempt not to allow the REM-off LC neurons to cease firing, low intensity (200 microA), low frequency (2 Hz) rectangular (300 microseconds) pulses were continuously delivered to the LC bilaterally through chronically implanted electrodes, and the effects on sleep-wakefulness-REMS were investigated. Although the stimulation did not affect sleep state of the animals, it reduced REMS significantly. The effect on REMS was similar to that of REMS deprivation. Total duration of REMS was significantly reduced during stimulation and showed a rebound increase during the post stimulation period. This reduction in REMS duration was primarily due to a significant reduction in the REMS frequency/h while the mean REMS duration/episode was not affected. Thus, the results of this study suggest that the stimulated area (LC) affects REMS, most likely by suppression of REMS generation process.

Circadian rhythm, sleep, and epilepsy

This review article: (1) describes the circadian distribution of ictal and interictal events; (2) differentiates transitional arousal, non-rapid eye movement and rapid eye movement sleep components and their substrates; (3) suggests the means by which the neural generators of these seizure-prone vs. seizure-resistant sleep and arousal states modulate the timing of different seizure manifestations; (4) considers clinical and mechanistic findings for the reciprocal effects of seizures and antiepileptic drugs upon the sleep-wake cycle; and (5) assesses clinical and basic mechanisms of sleep deprivation effects upon seizures.

The sequential hypothesis of the function of sleep

In addition to modulatory roles concerning bodily functions, sleep is assumed to play a main processing role with regard to newly acquired neural information. Elaboration of memory traces acquired during the waking period is assumed to require two sequential steps taking place during slow wave sleep (SWS) and eventually during paradoxical sleep (PS). This view is suggested by several considerations, not the least of which concerns the natural sequence of appearance of SWS and PS in the adult animal. While the involvement of PS in memory processing is well documented, the involvement of SWS is supported by the results of baseline and post-trial EEG analyses carried out in rats trained for a two-way active avoidance task or a spatial habituation task. Together with control analyses, these data indicate that the marked increase in the average duration of post-trial SWS episodes does not reflect the outcome of non-specific contingent factors, such as sleep loss or stress, but is related to memory processing events. Several considerations have furthermore led to the proposal that, during SWS, after a preliminary selection step, the first processing operation consists in the weakening of non-adaptative memory traces. The remaining memory traces would then be stored again under a better configuration during the ensuing PS episode. This view is in agreement with several relevant features of sleep, including the EEG waveforms prevailing during SWS and PS, as well as the ontogenetic sequence of appearance of SWS and PS. Some theoretical considerations on the role of sleep are also in agreement with the sequential hypothesis. More recent data indicate that the learning capacity of rats is correlated with several baseline EEG features of sleep and wakefulness. They include the average duration of PS episodes and of SWS episodes followed by wakefulness (longer in fast learning rats), and the waking EEG power spectrum of fast learning rats whose output is more balanced in the frequency range below 10 Hz than in slow learning and in non-learning rats. Additional EEG data suggest that fast learning rats may accomplish 'on line' processing of newly acquired information according to a sequence of events not dissimilar from the one proposed by the sequential hypothesis.

Processing of learned information in paradoxical sleep: relevance for memory

After a short review of the post-learning paradoxical sleep (PS) deprivation effects and of the PS changes induced by learning, we present a set of electrophysiological and behavioural experiments showing that: (1) processing of relevant information is possible during PS; (2) new associations can be formed during PS; (3) previously learned information can be reprocessed during PS; and (4) the effects of information processed during PS can be transferred to the awake state and be expressed in behaviour. Altogether, these results support the idea that dynamic processes occurring during post-learning PS can contribute to the effectiveness of memory processing and facilitate memory retrieval in wakefulness.

The function of fetal/neonatal rapid eye movement sleep

The hypothesis is put forward that rapid eye movement (REM) sleep in early life serves as (1) an indicator for the degree of brain maturation and (2) the promoter of further brain development. This hypothesis, although not exclusive, differs (a) from the theory of Roffwarg et al. that REM sleep substitutes for 'wakefulness' during the period (early life) in which wakefulness is limited, (b) from the theory of Crick and Mitchson, i.e., the 'unlearning' hypothesis of REM sleep, (c) from the theory of Jouvet, i.e., that REM sleep is a time for genetic read-out and (d) from the theory of Freud, i.e., that dreams fulfil our wishes (in other words, activation of neuronal systems that were disproportionally activated during wakefulness).

Sleep deprivation in the rat by the disk-over-water method

Chronic sleep deprivation may be required to reveal the most serious physiological consequences of sleep loss, but it usually requires strong stimulation which can obscure the interpretation of effects. The disk-over-water method permits chronic sleep deprivation of rats with gentle physical stimulation that can be equally applied to yoked control rats. A series of studies with this method has revealed little or no pathology in the control rats. The deprived rats show a reliable syndrome that includes temperature changes (which vary with the sleep stages that are lost); heat seeking behavior; increased food intake; weight loss; increased metabolic rate; increased plasma norepinephrine; decreased plasma thyroxine; an increased triiodothyronine-thyroxine ratio; and an increase of an enzyme which mediates thermogenesis by brown adipose tissue. The temperature changes are attributable to excessive heat loss and an elevated thermoregulatory setpoint, both of which increase thermoregulatory load, and the other changes are interpretable as responses to this increased load. This pattern indicates that sleep serves a thermoregulatory function in the rat. The sleep deprived rats also show stereotypic ulcerative and hyperkeratotic lesions localized to the tail and plantar surfaces of the paws, and they die within a matter of weeks; the mediation of these changes is unresolved.

Phylogeny and the function of REM sleep

Phylogenetic studies in placental and marsupial mammals have demonstrated three major correlates of increased REM sleep time across these species. These are high amounts of non-REM sleep time, safe sleep conditions and immaturity at birth. While these variables explain approximately 30% of the variance in REM sleep time across these orders, these relations are violated when animals other than placentals are included. Birds are small, many have safe sleeping situations and are certainly immature at birth, yet they have less REM sleep than the vast majority of mammals. The echidna is immature at birth, has high amounts of non-REM sleep and safe sleeping conditions, yet has been reported to have no REM sleep. Our recent studies in the echidna indicate that REM and non-REM sleep did not evolve sequentially, but rather evolved as a differentiation of a primitive state which held the seeds of both sleep states. The echidna sleeps with an activated brainstem and EEG synchronized forebrain. Future studies of sleep phylogeny need to compare the behavior of key neuronal groups across the sleep cycle, since these results indicate that EEG variables and sleep state durations may given an inadequate picture of the nature of brain activity during sleep.

Are the effects of androgens on male sexuality noradrenergically mediated? Some consideration of the human

The effects of androgens on the sexuality of male rodents is likely to be mediated, at least in part, by central noradrenergic (NA) mechanisms. Davidson's group has shown this by restoring behaviour in castrated males with alpha-2 adrenoceptor antagonists. As yet, there are no comparable studies of drug administration to hypogonadal men, but relevant evidence from recent studies in eugonadal men is considered. The evidence is consistent with a role for NA mediation in human sexual arousal, but suggests that more than one NA mediated system exist, involving both central arousal and inhibition of peripheral responses such as erection. The study of spontaneous erections during REM sleep is of particular interest, as REM is accompanied by virtual cessation of peripheral sympathetic activity in relevant parts of the body. "Psychogenic" erectile dysfunction may involve a high level of central alpha-2 inhibitory tone which reduces the capacity for central arousal. On the other hand, there may be an age related loss of responsiveness to the central arousing effects of NA, suggesting that the aetiology of psychogenic erectile dysfunction may vary with age. NA mechanisms may well be involved in the mediation of androgenic effects in humans, but no simple relationship between NA and sexual response should be expected.

Brain distribution of c-fos expression as a result of prolonged rapid eye movement (REM) sleep period duration

Auditory stimulation (AS) or recovery from sleep deprivation (SD) has been shown to increase REM sleep periods in rats, cats and humans. This increment in REM has been credited to an amplified level of excitability in a widely distributed neuronal network throughout the brain. Fos-like immunostaining (FLI) has been useful in constructing maps of post-synaptic neuronal activity with single cell resolution, and has been proposed to be tightly related with progressing neuronal activation. This study utilized FLI as a marker to determine the number of neurons and structures which express c-fos in broadly distributed areas of the brain in animals with REM periods prolonged by either AS or SD. The results indicated that the brain stem and diencephalon present FLI increases in a variety of structures that possibly share various functional aspects of the REM sleep mechanism. These results are discussed in terms of the possibility that REM maintenance is related to an increase in the recruitment of REM-on neurons.

Rapid eye movement sleep deprivation decreases membrane fluidity in the rat brain

In this study we examined the effects of rapid eye movement sleep (REMS) deprivation on synaptosomal and microsomal membrane fluidity by studying 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence polarization in control as well as REMS-deprived rats. The flower pot technique was used to perform 24, 48 and 96 h REMS deprivation. Suitable control experiments were carried out to rule out the nonspecific effects. The results showed that DPH fluorescence polarization increased both in the microsome as well as in the synaptosome in REMS-deprived animals, except in the cerebellum, indicating that there was a generalized decrease in membrane fluidity in the rat brain. The alterations in membrane fluidity returned to baseline upon recovery from REMS deprivation. Control experiments suggested that the alterations were primarily caused by REMS deprivation and not due to nonspecific effects. This finding supports REMS deprivation induced other changes reported earlier. This increase in membrane rigidity could be at least one of the possibilities for REMS loss induced alterations in physiological phenomena including membrane bound enzyme activities and receptor densities.

Sleep deprivation in the rat: an animal model of mania

The model of sleep deprivation in rats by the platform method has been extensively studied in our laboratory as a possible animal model of mania. At the end of the period of sleep deprivation, the rat does not fall asleep as soon as it is returned to its home cage, but shows a period of wakefulness of about 30 min, during which the animal presents a cohort of symptoms that appear to mimic those present in idiopathic mania. In particular, during this period the animal displays insomnia, a high degree of hyperactivity, irritability, aggressiveness, hypersexuality and stereotypy. Haloperidol (0.2 mg/kg) was effective in reducing latency to sleep, while L-sulpiride was much weaker (< 50 mg/kg). The dopamine D1 receptor antagonist SCH 23390 exhibited an extremely high potency and efficacy in reducing sleep latency, a significant effect being observed with 3 micrograms/kg. The administration of the specific D1 receptor agonist SKF 38393 markedly prolonged the period of insomnia with the correlated behavioral syndrome. When lithium was added to the diet and consumed during the sleep deprivation period in adequate amounts to produce serum lithium levels of 0.7-1.0 mEq/l, sleep latency and locomotor activity were significantly reduced. The administration of naloxone (1-10 mg/kg) reduced the latency to sleep in a dose-related manner. By contrast, morphine (1 and 5 mg/kg, i.p.), beta-endorphin and [D-Ala2,D-Leu5]enkephalin (i.c.v., 2 and 1 micrograms, respectively) markedly prolonged the insomnia. The model not only represents a confirmation in the rat that sleep loss often precedes and may trigger a manic episode in man, but suggests that an opioid-dopamine interaction may play a pathogenetic role in mania.

Monoamine depletion attenuates the REM sleep deprivation-induced increase in clonidine response in the forced swimming test

Effect of monoamine depletion on the REM sleep (REMs) deprivation-induced increase in clonidine response in the forced swimming test was investigated. Mice were deprived of REMs by the small pedestal method. Clonidine HCl (10-1000 micrograms/kg, IP), an alpha 2-adrenoceptor agonist, dose dependently increased swimming activities in group-housed and socially isolated mice used as the control groups. The dose-response relationship shifted to the left following REMs deprivation (ED50 values in the group-housed, isolated, and REMs-deprived mice were 250, 200, and 27 micrograms/kg, respectively). Monoamine depletion, induced by reserpine (5 mg/kg, IP) plus alpha-methyl-p-tyrosine (250 mg/kg, IP), did not produce any changes in the effects of clonidine in the control groups. However, in REMs-deprived mice, monoamine depletion significantly decreased the effect of 100 micrograms/kg clonidine, but not that of 300 micrograms/kg clonidine on swimming activity. These results indicate that clonidine-induced increase in swimming activity in the forced swimming test is mainly mediated by postsynaptic alpha 2-adrenoceptor, and that endogenous noradrenaline in the brain plays an important role in the increase of clonidine response following REMs deprivation treatment. The neuronal mechanism of the increase in clonidine response is discussed.

Stress-related behavior and central norepinephrine concentrations in the REM sleep-deprived rat

Rapid eye movement sleep deprivation (REMd) is a potent stressor in the rat. Behavioral abnormalities are among the earliest overt symptoms of REMd, the mechanisms for which remain largely unknown. The phenomena of hyperphagia and weight loss that are associated with REMd may contribute to its later morbidity; however, little is known about the onset of these phenomena or the neurotransmitter mechanisms that are involved. The aim of this study was to determine whether the earliest effects of REMd on consumatory behavior in the rat and its performance in the swimming cylinder of Porsolt are related to changes in norepinephrine (NE) concentrations in the cerebral cortex and selected areas of the hypothalamus. Sprague-Dawley rats were divided into three groups (n = 6): the REMd group resided in a water tank on 6.5-cm diameter pedestals for 96 h; the tank control (TC) group resided in the water tank on 15-cm pedestals for 96 h; the cage controls (CC) remained in their home cages for the duration of the study. In the first series of experiments, body weights and caloric intake were recorded daily, along with the performance of all animals in the swimming cylinder of Porsolt. In the second series of experiments, body weights and caloric intake were recorded, but the Porsolt test was not employed and the brains were dissected after 96 h for NE analysis by HPLC. It was observed that the REMd group had lower immobility times (p < 0.05) in the Porsolt test after only 24 h, compared to groups TC and CC.(ABSTRACT TRUNCATED AT 250 WORDS)

REM sleep deprivation potentiates the effects of imipramine and desipramine but not that of clomipramine in the forced swimming test

Effects of REM sleep (REMs) deprivation on the basal swimming activity and the tricyclic anti-depressants-induced increase in swimming activity in the forced swimming test were investigated. Immediately after a 48-hr period of REMs deprivation, the basal swimming activity in REMs-deprived mice was significantly higher than those in group-housed and socially isolated animals used as the control groups. The REMs deprivation-induced increase in the swimming activity was not changed by adrenoceptor antagonists and it returned to the control levels 3 hr after the REMs deprivation treatment. Moreover, imipramine and desipramine but not clomipramine further increased the swimming activity enhanced by REMs deprivation at doses that did not affect the activity in the control groups. The enhancing effect of REMs deprivation on the sensitivity to imipramine and desipramine remained unchanged even at 3 hr after the REMs deprivation treatment, and it was blocked by the alpha 2-adrenoceptor antagonist yohimbine. These results suggest that the REMs deprivation-induced increase in basal swimming activity in the forced swimming test is not mediated by adrenoceptor mechanisms, whereas the enhancing effect of REMs deprivation on the sensitivity to imipramine and desipramine may be mediated by the functional changes in alpha 2-adrenoceptors in the brain.

Effect of rapid eye movement sleep deprivation on rat brain monoamine oxidases

Monoamine oxidase, monoamine oxidase-A, and monoamine oxidase-B activities were compared in free moving, rapid eye movement sleep-deprived, recovered, and control rat brains. The activities were estimated in the whole brain, cerebrum, cerebellum, whole brainstem, medulla, pons, and midbrain. The flowerpot method was used for continuing deprivation for one, two, or four days. Monoamine oxidase activity decreased significantly in the cerebrum and the cerebellum of the sleep-deprived rats, whereas monoamine oxidase-A and monoamine oxidase-B were differentially affected. Medullary MAO-A was the first to be affected, showing an increase after just one day of rapid eye movement sleep deprivation, while longer deprivation decreased its activity. The activity of monoamine oxidase-B was not significantly affected in any brain areas of the deprived rats until after two days of rapid eye movement sleep deprivation. All the altered enzyme activities returned to control levels after recovery. Control experiments suggest that the decrease was primarily caused by the rapid eye movement sleep deprivation and was not due to nonspecific effects. These findings are consistent with past studies and may help to explain earlier observations. The results support the involvement of aminergic mechanisms in rapid eye movement sleep. The plausible reasons for the changes in the activities of monoamine oxidases, after rapid eye movement sleep deprivation, are discussed.

A neuronal group theory of sleep function

A new theory of sleep function is presented within the context of the neuronal group selection hypothesis, which emphasizes that neuronal groups compete for neurons via use-dependent synaptic formation and atrophy. It is hypothesized that sleep serves to stabilize these competitive processes by providing a pattern of stimulation that serves to maintain a synaptic infrastructure upon which wakefulness-driven synaptic changes are superimposed. Sleep is 'quantal' in nature in that sleep is a statistical property of a population of neuronal groups in different states. The theory unifies past theories of sleep function yet simultaneously provides a fundamental new paradigm for sleep research.

The stuff that dreams aren't made of: why wake-state and dream-state sensory experiences differ

It is adaptive for individuals to be continuously alert and responsive to external stimuli (such as the sound and odor of an approaching predator or the cry of an infant), even during sleep. Natural selection thus has disfavored the occurrence during sleep of hallucinations that compromise external vigilance. In the great majority of mammalian species, including Homo sapiens, closed eyes and immobility are basic aspects of sleep. Therefore, (a) visual and movement sensory modalities (except kinesthesis) do not provide the sleeper with accurate information about the external environment or the sleeper's relationship to that environment; (b) the sleeper's forebrain "vigilance mechanism" does not monitor these modalities; hence (c) visual and movement hallucinations--similar or identical to percepts--can occur during sleep without compromising vigilance. In contrast, the other sensory modalities do provide the sleeper with a continuous flow of information about the external environment or the sleeper's relationship to that environment, and these modalities are monitored by the vigilance mechanism. Hallucinations of kinesthesis, pain, touch, warmth, cold, odor, and sound thus would compromise vigilance, and their occurrence during sleep has been disfavored by natural selection. This vigilance hypothesis generates novel predictions about dream phenomenology and REM-state neurophysiology and has implications for the general study of imagery.

The biology and function of rapid eye movement sleep

Rapid eye movement sleep, a stage of sleep that appears to be present in all marsupial and terrestrial placental mammals, was first identified in 1953. Although the brainstem mechanisms responsible for its generation have been clarified, the function of rapid eye movement sleep remains elusive. Recent findings suggest a role in memory processing.

Effects of chronic total sleep deprivation on central noradrenergic receptors in rat brain

The effect of chronic total sleep deprivation (TSD) on the regulation of central noradrenergic receptors was evaluated. Rats were subjected to 10 days of TSD by the disk-over-water method. As in previous TSD studies, these rats showed greater increases in food intake and energy expenditure and greater eventual declines in waking body temperature than their yoked-control (TSC) rats. After sacrifice, alpha 1-, alpha 2-, and beta-adrenoceptors were determined in 11 brain regions using radioligand binding assays with [3H]prazosin, [3H]rauwolscine, and 125I-iodocyanopindolol, respectively. Adrenoceptor density and affinity values were significantly different among TSD, TSC, and normally caged control rat groups only for the cerebellum, which showed higher alpha 2-binding density concomitant with lower affinity and lower beta-binding density than cage control rats. Such changes are attributable to apparatus or stimulus effects common to TSD and TSC rats. Given the absence of firm evidence for a TSD-induced downregulation of central noradrenergic receptors, the present results offer no support for the hypothesis of Siegel and Rogawski that a major function of paradoxical sleep is to upregulate these receptors.

Different types of norepinephrinergic receptors are involved in preoptic area mediated independent modulation of sleep-wakefulness and body temperature

The preoptic area is known to regulate sleep-wakefulness and body temperature. It was suggested earlier that though sleep-wakefulness and body temperature may affect each other, the preoptic area mediated influence on those two physiological phenomena is likely to be independent of alteration in each other. Since intrapreoptic area norepinephrine could modulate both those functions, study of that system was undertaken. It was hypothesized that since the preoptic area has different types of norepinephrinergic receptors (viz. alpha 1, alpha 2 and beta), independent modulation of those two functions was probably due to activation or inactivation of separate receptors. Hence, the effects of different agonist and antagonist of those receptors individually as well as in combination into the preoptic area were studied on those two functions in freely moving rats. The results suggest that norepinephrine induced preoptic area mediated influence on the body temperature is primarily regulated by the alpha 1 receptors while the sleep and wakefulness are regulated by alpha 2 and beta receptors, respectively. The finding should help in explaining several poorly understood observations reported earlier and it suggests that similar phenomena may possibly exist in other system involving other neurotransmitters as well.

c-fos proto-oncogene changes in relation to REM sleep duration

Auditory stimulation has been shown to increase REM sleep periods in cats and humans. This effect has been attributed to an elevation of the level of excitability in a variety of brain stem neuronal groups. Fos-like immunostaining (FLI) has been useful in constructing maps of post-synaptic neuronal activity with single cell resolution, and has been suggested to be tightly correlated with ongoing neuronal activity. This study used FLI to quantify neurons from structures expressing c-fos in brain stem areas in animals with normal REMs and compared them with those showing extended REM periods. The results basically indicated that brain stem areas which in other studies have been described as having REM-ON cells, showed an increase in FLI, while no FLI changes occurred in areas described as having REM-OFF cells. These results are discussed in terms of the possibility that REM maintenance is related to a widespread increase in brain stem excitability.

Pontine regulation of REM sleep components in cats: integrity of the pedunculopontine tegmentum (PPT) is important for phasic events but unnecessary for atonia during REM sleep

Transection, lesion and unit recording studies have localized rapid eye movement (REM) sleep mechanisms to the pons. Recent work has emphasized the role of pontine cholinergic cells, especially those of the pedunculopontine tegmentum (PPT). The present study differentiated REM sleep deficits associated with lesions of the PPT from other pontine regions implicated in REM sleep generation, including those with predominantly cholinergic vs non-cholinergic cells. Twelve hour polygraphic recordings were obtained in 18 cats before and 1-2 weeks after bilateral electrolytic or radio frequency lesions of either: (1) PPT, which contains the dorsolateral pontine cholinergic cell column; (2) laterodorsal tegmental nucleus (LDT), which contains the dorsomedial pontine cholinergic cell column; (3) locus ceruleus (LC), which contains mostly noradrenergic cells; or (4) subceruleus (LC alpha, peri-LC alpha and the lateral tegmental field), which also contains predominantly noncholinergic cells. There were three main findings: (i) Only lesions of PPT and subceruleus significantly affected REM sleep time. These lesions produced comparable reductions in REM sleep time but influenced REM sleep components quite differently: (ii) PPT lesions, estimated to damage 90 +/- 4% of cholinergic cells, reduced the number of REM sleep entrances and phasic events, including ponto-geniculooccipital (PGO) spikes and rapid eye movements (REMs), but did not prevent complete atonia during REM sleep: (iii) Subceruleus lesions eliminated atonia during REM sleep. Mobility appeared to arouse the cat prematurely from REM sleep and may explain the brief duration of REM sleep epochs seen exclusively in this group. Despite the reduced amount of REM sleep, the total number of PGO spikes and REM sleep entrances increased over baseline values. Collectively, the results distinguish pontine loci regulating phasic events vs atonia. PPT lesions reduced phasic events, whereas subceruleus lesions created REM sleep without atonia. Severe REM sleep deficits after large pontine lesions, including PPT and subceruleus, might be explained by simultaneous production of both REM sleep syndromes. However, extensive loss of ACh neurons in the PPT does not disrupt REM sleep atonia.

The functions of sleep: further analysis

Most theories addressed to the functions of sleep are proposed primarily according to the results in one area of sleep research and may not be compatible with the results in other areas of sleep research. This paper provides a new theory regarding the functions of sleep by integratively analyzing different areas of sleep research. First, it concludes from the phylogenetic studies and other related sleep research that sleep in mammals has at least one obligatory function which cannot be accomplished during waking. It also shows that the synchronized sleep (SS) period plays a critical role in accomplishing the obligatory functions of sleep and that the obligatory functions of sleep are related to the brain. Then it points out that adjusting and reorganizing emotional behaviors is a very important function of SS. Finally, this theory suggests that the gradual accumulation of various randomly learned memories in the limbic structures would inevitably imbalance and disorganize emotional behaviors so that sleep should be developed in evolution to adjust and reorganize emotions and so that the functions of SS for memory and emotional regulation are the obligatory functions of sleep. Although phylogenetic studies suggest that (PS) may not play obligatory functions across all mammals, there is no doubt that, in tight correlation with SS, PS also plays very important roles in memory and emotion which, however, are different from the corresponding SS roles in those mammals possessing PS.

REM sleep deprivation reduces auditory evoked inhibition of dorsolateral pontine neurons

In many dorsolateral pontine neurons, auditory stimulation produces an initial excitation followed by a sustained inhibition. We now report that rapid eye movement (REM) sleep deprivation, for periods of from 22-48 h, reduced this auditory evoked inhibition of unit discharge. Inhibition returned to baseline levels after recovery REM sleep. Prior work indicates that the auditory evoked inhibition seen in noradrenergic cells in this region is partially mediated by norepinephrine. We hypothesize that the reduction in inhibition that we see is a consequence of either downregulation/desensitization of norepinephrine receptors or reduced norepinephrine release resulting from REM sleep deprivation.

Sleep modulation of interictal spike configuration in untreated children with partial seizures

Spontaneous changes in interictal spike morphology were evaluated before initiation of antiepileptic drug (AED) therapy in 38 children with uncontrolled partial seizures. Nonrapid eye movement (NREM) sleep during the first third of the night was associated with spikes of higher amplitude, longer duration, and lower degree of sharpness than those observed in wakefulness or REM sleep. Spike amplitude decreased in subsequent NREM epochs, but duration and sharpness remained relatively consistent. A NREM/REM modulatory pattern was identified, with REM periods exhibiting spikes of decreased amplitude, shorter duration, and increased sharpness as compared with NREM periods in each third of the night. The spike changes associated with REM sleep are similar to the alterations previously described as occurring coincidentally with attainment of seizure control and thus may reflect inherent modulation of epileptogenicity in synchrony with sleep cycles.

Catecholaminergic innervation of the hippocampus in the cynomolgus monkey

We studied the immunocytochemical distribution of catecholaminergic fibers in the hippocampal formation from two cynomolgus monkeys by using phenylethanolamine-N-methyltransferase, dopamine-beta-hydroxylase, and tyrosine-hydroxylase antibodies. There were no phenylethanolamine-N-methyltransferase immunoreactive fibers suggesting the lack of epinephrine containing fibers. In order to compare the distributions of tyrosine-hydroxylase and dopamine-beta-hydroxylase immunoreactive fibers, we counted fibers in four hippocampal regions, the dentate gyrus, CA3, CA1, and the subiculum at three different rostrocaudal levels. The distributions of dopamine-beta-hydroxylase and tyrosine-hydroxylase immunoreactive fibers were overlapping but clearly different, suggesting that the hippocampus receives both noradrenergic and dopaminergic inputs in primates. Dopamine-beta-hydroxylase-immunoreactive fibers were present in moderate density and roughly evenly distributed throughout the hippocampus. Tyrosine-hydroxylase immunoreactive fibers were found in high density in the dentate gyrus, in the stratum lacunosum-moleculare, and in the molecular layer of the subiculum. There were only minor side-side and rostrocaudal differences in the distribution of tyrosine-hydroxylase and dopamine-beta-hydroxylase immunoreactive fibers. The identification of a putative dopaminergic projection to primate hippocampus, which is more dense and widely distributed than in the rodent, parallels similar increases in dopaminergic projections reported for primate cerebral neocortex.

Peptidergic innervation of the locus coeruleus cells in the human brain

The immunocytochemical localization of corticotropin-releasing factor (CRF), thyrotropin-releasing hormone (TRH), substance P (SP), neuropeptide Y (NPY), and neurotensin (NT) were studied in the human locus coeruleus (LC) using the avidin-biotin complex (ABC) technique. The brains were obtained from 3 adult male human subjects of 36-61 years old. All of these peptides studied were present in nerve fibers and varicosities throughout the entire nucleus, where they distributed unevenly. The highest density of immunoreactive neuronal networks was generally observed in the middle and to a lesser extent in the caudal part of the nucleus, while only scattered fibers were seen in the rostral part of the LC. Among the investigated neuronal peptides, the CRF-immunopositive network was the most dense, less dense immunostaining with TRH and NPY antisera while only few SP- and NT-immunoreactive fibers were present in the nucleus. Cell bodies immunostained for the above peptides were not seen in the LC. Per analogiam with rats, fibers immunostained in the LC may be of both intrinsic and extrinsic origins.

Changes in pontine unit activity with REM sleep deprivation

Short term rapid eye movement (REM) sleep deprivation produced a decrease in walking discharge rates of presumably noradrenergic pontine 'REM sleep-off' cells and an increase in waking discharge rates of pontine 'REM sleep-on' cells. These changes can be viewed as a correlate of increased REM sleep pressure. Slowing of REM sleep-off cells in waking is hypothesized to counteract the functional effects of REM sleep loss on noradrenergic receptor sensitivity. This slowing and the resulting reduction in norepinephrine release may contribute to the loss of vigilance seen with sleep deprivation.

Corticotropin-releasing factor (CRF) increases paradoxical sleep (PS) rebound in PS-deprived rats

The intracerebroventricular injection (1 microgram) of murine corticotropin-releasing factor (r-CRF) to rats after 72 h of sleep deprivation markedly postponed the sleep onset, reduced slow wave sleep and prolonged the duration of paradoxical sleep (PS) episodes. Moreover, CRF is-to our knowledge-the first compound to be able to further prolong PS in a condition (sleep deprivation) in which its duration is already increased. Our findings raise the possibility that CRF might play a physiological role in the regulation of the sleep-wakefulness cycle.

Mechanisms of sleep control

Neurons in the dorsolateral pontine reticular formation, in interaction with other brainstem and forebrain regions, generate the basic phenomena of rapid-eye-movement (REM) sleep. Non-REM sleep is the product of multiple control mechanisms in the forebrain and brainstem. Despite considerable progress in the identification of generation mechanisms, the functions of sleep remain a mystery. I hypothesize that REM sleep has a role in the regulation of receptor sensitivity in the central and peripheral nervous systems.

The pathophysiology of the Rett syndrome from the standpoint of polysomnography

Twelve polysomnographies were performed on eight cases of the Rett syndrome and the results were compared between cases under and over 5 years of age. There observed an increase in the ratio of stage REM (sREM) against slow wave sleep (SWS) with age. The ratio of the number of mentalis twitch movement (ment TM) during sREM against the number of rapid eye movements (REMs) (reflecting dopaminergic activities) was below normal in the younger cases but increased markedly. The ratio of ment TM in the period of REMs burst against the total ment TM during sREM (reflecting the noradrenergic hypofunction) was high in the younger group, while it decreased later. The nocturnal variation became inapparent with age in % SWS and the phasic components of sREM, while that of % sREM exaggerated at later ages. The early hypofunctions of the noradrenergic and dopaminergic neurons later modified by the serotonergic dysfunction and dopaminergic postsynaptic supersensitivity are suggested.

Concomitant depression of locus coeruleus neurons and of flexor reflexes by an alpha 2-adrenergic agonist in rats: a possible mechanism for an alpha 2-mediated muscle relaxation

The alpha 2-agonist tizanidine, clinically used as an antispastic drug, also strongly reduces polysynaptic flexor reflexes. The hypothesis was tested that the noradrenergic coerulespinal system exerts a tonic facilitation on spinal reflexes and that the depressant effects of tizanidine may be explained by an alpha 2-mediated autoinhibition of the tonic activity of locus coeruleus neurons, resulting in a disfacilitation of the spinal reflexes. The following results support this working hypothesis: (1) systemic injections of tizanidine markedly decreased the spontaneous activity of locus coeruleus neurons, but not of non-locus coeruleus neurons. The alpha 2-antagonist yohimbine reversed this effect. (2) The time course of diminished locus coeruleus activity paralleled that of depressed flexor reflexes. (3) Flexor reflexes were also markedly depressed by the alpha 1-adrenergic antagonist prazosin, administered alone, which is in line with the proposition that the noradrenergic system exerts a tonic facilitation on spinal neurons by way of alpha 1-adrenergic receptor activation. (4) Flexor reflexes were facilitated by conditioning microstimulation of locus coeruleus neurons, and this effect was reversed by prazosin. (5) Flexor reflexes significantly diminished in size following placement of an irreversible lesion in the ipsilateral locus coeruleus. Although these results strongly support the above hypothesis regarding a descending modulatory function of the descending locus coeruleus system on spinal reflexes, possible additional mechanisms, perhaps also involving the ascending projection of the locus coeruleus to supraspinal motor structures, remain to be elucidated.