Effect of DSP-4, a noradrenergic neurotoxin, on sleep and wakefulness and sensitivity to drugs acting on adrenergic receptors in the rat

When the Locus Coeruleus Speaks Up in Sleep: Recent Insights, Emerging Perspectives

For decades, numerous seminal studies have built our understanding of the locus coeruleus (LC), the vertebrate brain’s principal noradrenergic system. Containing a numerically small but broadly efferent cell population, the LC provides brain-wide noradrenergic modulation that optimizes network function in the context of attentive and flexible interaction with the sensory environment. This review turns attention to the LC’s roles during sleep. We show that these roles go beyond down-scaled versions of the ones in wakefulness. Novel dynamic assessments of noradrenaline signaling and LC activity uncover a rich diversity of activity patterns that establish the LC as an integral portion of sleep regulation and function. The LC could be involved in beneficial functions for the sleeping brain, and even minute alterations in its functionality may prove quintessential in sleep disorders.

Heightened Hippocampal β-Adrenergic Receptor Function Drives Synaptic Potentiation and Supports Learning and Memory in the TgF344-AD Rat Model during Prodromal Alzheimer's Disease

The central noradrenergic (NA) system is critical for the maintenance of attention, behavioral flexibility, spatial navigation, and learning and memory, those cognitive functions lost first in early Alzheimer's disease (AD). In fact, the locus coeruleus (LC), the sole source of norepinephrine (NE) for >90% of the brain, is the first site of pathologic tau accumulation in human AD with axon loss throughout forebrain, including hippocampus. The dentate gyrus is heavily innervated by LC-NA axons, where released NE acts on β-adrenergic receptors (ARs) at excitatory synapses from entorhinal cortex to facilitate long-term synaptic plasticity and memory formation. These synapses experience dysfunction in early AD before cognitive impairment. In the TgF344-AD rat model of AD, degeneration of LC-NA axons in hippocampus recapitulates human AD, providing a preclinical model to investigate synaptic and behavioral consequences. Using immunohistochemistry, Western blot analysis, and brain slice electrophysiology in 6- to 9-month-old wild-type and TgF344-AD rats, we discovered that the loss of LC-NA axons coincides with the heightened β-AR function at medial perforant path-dentate granule cell synapses that is responsible for the increase in LTP magnitude at these synapses. Furthermore, novel object recognition is facilitated in TgF344-AD rats that requires β-ARs, and pharmacological blockade of β-ARs unmasks a deficit in extinction learning only in TgF344-AD rats, indicating a greater reliance on β-ARs in both behaviors. Thus, a compensatory increase in β-AR function during prodromal AD in TgF344-AD rats heightens synaptic plasticity and preserves some forms of learning and memory.SIGNIFICANCE STATEMENT The locus coeruleus (LC), a brain region located in the brainstem which is responsible for attention and arousal, is damaged first by Alzheimer's disease (AD) pathology. The LC sends axons to hippocampus where released norepinephrine (NE) modulates synaptic function required for learning and memory. How degeneration of LC axons and loss of NE in hippocampus in early AD impacts synaptic function and learning and memory is not well understood despite the importance of LC in cognitive function. We used a transgenic AD rat model with LC axon degeneration mimicking human AD and found that heightened function of β-adrenergic receptors in the dentate gyrus increased synaptic plasticity and preserved learning and memory in early stages of the disease.

Optogenetic Investigation of Arousal Circuits

Modulation between sleep and wake states is controlled by a number of heterogeneous neuron populations. Due to the topological proximity and genetic co-localization of the neurons underlying sleep-wake state modulation optogenetic methods offer a significant improvement in the ability to benefit from both the precision of genetic targeting and millisecond temporal control. Beginning with an overview of the neuron populations mediating arousal, this review outlines the progress that has been made in the investigation of arousal circuits since the incorporation of optogenetic techniques and the first in vivo application of optogenetic stimulation in hypocretin neurons in the lateral hypothalamus. This overview is followed by a discussion of the future progress that can be made by incorporating more recent technological developments into the research of neural circuits.

Orexin neurons suppress narcolepsy via 2 distinct efferent pathways

The loss of orexin neurons in humans is associated with the sleep disorder narcolepsy, which is characterized by excessive daytime sleepiness and cataplexy. Mice lacking orexin peptides, orexin neurons, or orexin receptors recapitulate human narcolepsy phenotypes, further highlighting a critical role for orexin signaling in the maintenance of wakefulness. Despite the known role of orexin neurons in narcolepsy, the precise neural mechanisms downstream of these neurons remain unknown. We found that targeted restoration of orexin receptor expression in the dorsal raphe (DR) and in the locus coeruleus (LC) of mice lacking orexin receptors inhibited cataplexy-like episodes and pathological fragmentation of wakefulness (i.e., sleepiness), respectively. The suppression of cataplexy-like episodes correlated with the number of serotonergic neurons restored with orexin receptor expression in the DR, while the consolidation of fragmented wakefulness correlated with the number of noradrenergic neurons restored in the LC. Furthermore, pharmacogenetic activation of these neurons using designer receptor exclusively activated by designer drug (DREADD) technology ameliorated narcolepsy in mice lacking orexin neurons. These results suggest that DR serotonergic and LC noradrenergic neurons play differential roles in orexin neuron-dependent regulation of sleep/wakefulness and highlight a pharmacogenetic approach for the amelioration of narcolepsy.

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.

Are the pharmacology and physiology of α₂ adrenoceptors determined by α₂-heteroreceptors and autoreceptors respectively?

α(2)-Adrenoceptors are important mediators of physiological responses to the endogenous catecholamines noradrenaline and adrenaline. In addition, α(2)-adrenoceptors are pharmacological targets for the treatment of hypertension, sympathetic overactivity and glaucoma. α(2)-Adrenoceptors are also targeted to induce sedation and analgesia in anaesthesia and intensive care. α(2)-Adrenoceptors were first described as presynaptic receptors inhibiting the release of various transmitters from neurons in the central and peripheral nervous systems. In addition to these presynaptic neuronal receptors, α(2)-adrenoceptors were also identified in many non-neuronal cell types of the body. Gene-targeting in mice provided a comprehensive assignment of the physiological and pharmacological functions of these receptors to specific α(2A)-, α(2B) - and α(2C)-adrenoceptor subtypes. However, the specific cell types and signalling pathways involved in these subtype-specific α(2)-adrenoceptor functions were largely unexplored until recently. This review summarizes recent findings from transgenic mouse models, which were generated to define the role of α(2)-adrenoceptors in adrenergic neurons, that is, α(2)-autoreceptors, versus α(2)-adrenoceptors in non-adrenergic neurons, termed α(2)-heteroreceptors. α(2)-Autoreceptors are primarily required to limit release of noradrenaline from sympathetic nerves and adrenaline from adrenal chromaffin cells at rest. These receptors are desensitized upon chronic activation as it may for instance occur due to enhanced sympathetic activity during chronic heart failure. In contrast, pharmacological effects of acutely administered α(2)-adrenoceptor agonist drugs essentially require α(2)-heteroreceptors in non-adrenergic neurons, including analgesia, sedation, hypothermia and anaesthetic-sparing as well as bradycardia and hypotension. Thus a clear picture has emerged of the significance of auto- versus heteroreceptors in mediating the physiological functions of α(2)-adrenoceptors and the pharmacological functions of α(2)-adrenoceptor agonist drugs respectively.

Locus ceruleus and anterior cingulate cortex sustain wakefulness in a novel environment

Locus ceruleus (LC) neuronal activity is correlated with the waking state, yet LC lesions produce only minor alterations in daily wakefulness. Here, we report that sustained elevations in neurobehavioral and EEG arousal in response to exposure to an environment with novel stimuli, including social interaction, are prevented by selective chemical lesions of the LC in rats. Similar results are seen when the anterior cingulate cortex (ACC), which receives especially dense LC innervation, is selectively denervated of LC input or is ablated by the cell-specific neurotoxin ibotenic acid. Anterograde tracing combined with tyrosine hydroxylase immunohistochemistry demonstrates ACC terminals in apposition with the distal dendrites of LC neurons. Our data implicate the ACC as both a source of input to the LC as well as one of its targets and suggests that the two structures engage in a dialog that may provide a critical neurobiological substrate for sustained attention.

Developmental emergence of power-law wake behavior depends upon the functional integrity of the locus coeruleus

STUDY OBJECTIVES

Daily amounts of sleep and wakefulness are accumulated in discrete bouts that exhibit distinct statistical properties. In adult mammals, sleep bout durations follow an exponential distribution whereas wake bout durations follow a power-law distribution. In infant Norway rats, however, wake bouts initially follow an exponential distribution and only transition to a power-law distribution beginning around postnatal day 15 (P15). Here we test the hypothesis that the locus coeruleus (LC), one of several wake-active nuclei in the brainstem, contributes to this developmental transition.

DESIGN

At P7, rats were injected subcutaneously with saline or DSP-4, a neurotoxin that targets noradrenergic (NA) LC terminals. Then, at P21, sleep and wakefulness during the day and night were monitored. The effectiveness of DSP-4 treatment was verified by measuring NA, dopamine (DA), and serotonin (5-HT) concentration in cortical and non-cortical tissue using high performance liquid chromatography.

RESULTS

In relation to controls, subjects treated with DSP-4 exhibited significant reductions only in cortical and non-cortical NA concentration. Consistent with our hypothesis, the wake bout durations of DSP-4 subjects more closely followed an exponential distribution, whereas those of control subjects followed the expected power-law distribution. Sleep bout distributions were unaffected by DSP-4.

CONCLUSIONS

These results suggest that the fundamental developmental transition in the statistical structure of wake bout durations is effected in part by changes in noradrenergic LC functioning. Considered within the domain of network theory, the hub-like connectivity of the LC may have important implications for the maintenance of network function in the face of random or targeted neural degeneration.

Octopamine regulates sleep in drosophila through protein kinase A-dependent mechanisms

Sleep is a fundamental process, but its regulation and function are still not well understood. The Drosophila model for sleep provides a powerful system to address the genetic and molecular mechanisms underlying sleep and wakefulness. Here we show that a Drosophila biogenic amine, octopamine, is a potent wake-promoting signal. Mutations in the octopamine biosynthesis pathway produced a phenotype of increased sleep, which was restored to wild-type levels by pharmacological treatment with octopamine. Moreover, electrical silencing of octopamine-producing cells decreased wakefulness, whereas excitation of these neurons promoted wakefulness. Because protein kinase A (PKA) is a putative target of octopamine signaling and is also implicated in Drosophila sleep, we investigated its role in the effects of octopamine on sleep. We found that decreased PKA activity in neurons rendered flies insensitive to the wake-promoting effects of octopamine. However, this effect of PKA was not exerted in the mushroom bodies, a site previously associated with PKA action on sleep. These studies identify a novel pathway that regulates sleep in Drosophila.

Monoaminergic system lesions increase post-sigh respiratory pattern disturbance during sleep in rats

Monoamines are important regulators of behavioral state and respiratory pattern, and the impact of monoaminergic control during sleep is of particular interest for the stability of breathing regulation. The aim of this study was to test the effects of systemically induced chemical lesions to noradrenergic and serotonergic efferent systems, on the expression of sleep-wake states, pontine wave activity, and sleep-related respiratory pattern and its variability. In chronically instrumented male adult Sprague-Dawley rats we lesioned noradrenergic terminal axonal branches by a single intraperitoneal dose of DSP-4 (N-(2-chloroethyl)-N-ethyl-2-brombenzilamine; 50 mg/kg, i.p.), and serotonergic axonal terminals by two intraperitoneal doses, 24 h apart, of PCA (p-chloroamphetamine; 6 mg/kg, i.p.). In each animal, we recorded sleep, pontine waves (P-waves) and breathing at baseline, following sham injection, and every week for 5 weeks following injection of either systemic neurotoxin. Distinct responses were observed to the two lesions. DSP-4 lesions were associated with a trend toward increased NREM sleep (p < 0.06), decreased wakefulness (p < 0.05) and increased respiratory tidal volume during NREM (p = 0.0002) and REM (p = 0.0001) sleep with respect to baseline. None of these effects, however, were observed during the first 14 days after injection. No significant changes were observed in the frequency of apneas or sighs, nor in the coupling between these two, at any time after DSP-4 injection. Conversely, selective serotonergic lesion by PCA produced no change in the baseline respiratory frequency or tidal volume during sleep or wakefulness, nor was the expression of Wake, NREM or REM sleep affected. Instead, PCA injection resulted in a sustained increase in the frequency and duration of post-sigh apneas (PS) during NREM sleep (p = 0.002). This reflected increased coupling between sighs and apneas, because neither the frequency nor the amplitude of spontaneous sighs was altered by PCA.

Behavioral and sleep/wake characteristics of mice lacking norepinephrine and hypocretin

We investigated the interaction between norepinephrine (NE) and orexin/hypocretin (Hcrt) in the control of sleep behavior and narcoleptic symptoms by creating mice that were deficient in both neurotransmitters. Mice with a targeted disruption of the dopamine beta-hydroxylase (Dbh) gene (deficient in NE and epinephrine) or the Hcrt gene were bred to generate double knockouts (DKOs), each single KO (Dbh-KO and Hcrt-KO), and control mice. The duration of wake, non-rapid eye movement (NREM) and REM sleep were monitored by electroencephalogram (EEG)/electromyogram (EMG) recording over a 24-h period, and the occurrence of behavioral arrests was monitored by video/EEG recording for 4 h. Overall, there was very little interaction between the two genes; for most parameters that were measured, the DKO mice resembled either Dbh-KO or Hcrt-KO mice. REM sleep was increased in both DKO and Hcrt-KO mice at night relative to the other groups, but DKO mice had significantly more REM sleep during the day than the other three groups. Sleep latency in response to saline or amphetamine injections was reduced in Dbh-KO and DKO mice relative to other groups. Behavioral arrests, that are frequent in Hcrt-KO mice, were not exacerbated in DKO mice.

Locus ceruleus control of slow-wave homeostasis

Sleep intensity is regulated by the duration of previous wakefulness, suggesting that waking results in the progressive accumulation of sleep need (Borbely and Achermann, 2000). In mammals, sleep intensity is reflected by slow-wave activity (SWA) in the nonrapid eye movement (NREM) sleep electroencephalogram, which increases in proportion to the time spent awake. However, the mechanisms responsible for the increase of NREM SWA after wakefulness remain unclear. According to a recent hypothesis (Tononi and Cirelli, 2003), the increase in SWA occurs because during wakefulness, many cortical circuits undergo synaptic potentiation, as evidenced by the widespread induction of long-term potentiation (LTP)-related genes in the brain of awake animals. A direct prediction of this hypothesis is that manipulations interfering with the induction of LTP-related genes should result in a blunted SWA response. Here, we examined SWA response in rats in which cortical norepinephrine (NA) was depleted, a manipulation that greatly reduces the induction of LTP-related genes during wakefulness (Cirelli and Tononi, 2004). We found that the homeostatic response of the lower-range SWA was markedly and specifically reduced after NA depletion. These data suggest that the wake-dependent accumulation of sleep need is causally related to cellular changes dependent on NA release, such as the induction of LTP-related genes, and support the hypothesis that sleep SWA homeostasis may be related to synaptic potentiation during wakefulness.

Dopaminergic-adrenergic interactions in the wake promoting mechanism of modafinil

Adrenergic signaling regulates the timing of sleep states and sleep state-dependent changes in muscle tone. Recent studies indicate a possible role for noradrenergic transmission in the wake-promoting action of modafinil, a widely used agent for the treatment of excessive sleepiness. We now report that noradrenergic projections from the locus coeruleus to the forebrain are not necessary for the wake-promoting action of modafinil. The efficacy of modafinil was maintained after treatment of C57BL/6 mice with N-(2-chloroethyl)-N-ethyl 2-bromobenzylamine (DSP-4), which eliminates all noradrenaline transporter-bearing forebrain noradrenergic projections. However, the necessity for adrenergic receptors in the wake-promoting action of modafinil was demonstrated by the observation that the adrenergic antagonist terazosin suppressed the response to modafinil in DSP-4 treated mice. The wake-promoting efficacy of modafinil was also blunted by the dopamine autoreceptor agonist quinpirole. These findings implicate non-noradrenergic, dopamine-dependent adrenergic signaling in the wake-promoting mechanism of modafinil. The anatomical specificity of these dopaminergic-adrenergic interactions, which are present in forebrain areas that regulate sleep timing but not in brain stem areas that regulate sleep state-dependent changes in muscle tone, may explain why modafinil effectively treats excessive daytime sleepiness in narcolepsy but fails to prevent the loss of muscle tone that occurs in narcoleptic patients during cataplexy.

Altered sleep latency and arousal regulation in mice lacking norepinephrine

Latency to sleep and the amount of sensory stimulation required to awaken an animal are measures of arousal threshold, which are ultimately modulated by an arousal regulation system involving many brain areas. Among these brain areas and network connections are wake-promoting nuclei of the brainstem and their corresponding neurotransmitters, including norepinephrine (NE). In this study, we used mice that are unable to produce NE to study its role in regulating sleep latency after a variety of interventions, and to study arousal from sleep after sleep deprivation (SD). Sleep latency was measured after gentle awakening or after injections of saline, caffeine or modafinil. Sleep latency was also measured before and after partial restoration of NE pharmacologically. Arousal threshold was measured by recording the number of decibels of white noise required to wake each mouse from NREM sleep after 0, 3 and 3 + 3 h SD (3 h SD followed by < 2 min sleep, followed by an additional 3 h SD). Results showed that when mice were awakened without being touched, there were no differences in sleep latency between the genotypes. However, after an injection of saline, the control mice increased their sleep latency, whereas the NE-deficient mice did not. There were no group differences in sleep latency after treatment with either stimulant. The sleep latency difference between the genotypes was ameliorated by partial restoration of NE. The arousal threshold experiments revealed that significantly more noise was required to wake the NE-deficient mice after 3 and 3 + 3 h of SD. These findings show that mice lacking NE fall asleep more rapidly only after a mild stressor, such as an intraperitoneal injection. NE-deficient mice are also more difficult to wake up using audio stimulation after SD. The results presented here suggest that NE promotes wakefulness during transitions between sleep and wake under conditions involving mild stress and SD, but not under baseline circumstances.

Adrenergic signaling plays a critical role in the maintenance of waking and in the regulation of REM sleep

Many experiments have suggested that the adrenergic system is important for arousal and the regulation of sleep/wake states. Electrophysiological studies have found strong correlations between the firing of adrenergic neurons and arousal state. Lesions of adrenergic neurons have been reported to cause changes in sleep/wake regulation, although findings have been variable and sometimes transient. To more specifically address the role of adrenergic signaling in sleep/wake regulation, we performed electroencephalographic and electromyographic recordings in mice with a targeted disruption of the gene for dopamine beta-hydroxylase, the enzyme that converts dopamine to norepinephrine. These mice are unable to synthesize the endogenous adrenergic ligands norepinephrine and epinephrine. The mutant mice sleep approximately 2 h more each day. The decrease in waking is due to a considerable decrease in the duration of waking bouts in spite of an increase in the number of waking bouts and transitions from sleep to waking. In contrast, the amount of rapid-eye-movement (REM) sleep is only half that in control mice due to a decrease in the number and duration of REM sleep bouts. Delta power is selectively increased in the mutant mice, and there is much less variation in non-REM sleep delta power over 24 h. After 6 h of total sleep deprivation during the first half of the light period, there is no rebound recovery of sleep time in the mutant mice. These results provide genetic evidence that adrenergic signaling acts to maintain waking and is important for the regulation of REM sleep and possibly sleep homeostasis.

Effects of clonidine and alpha-adrenoceptor antagonists on motor activity in DSP4-treated mice I: dose-, time- and parameter-dependency

In three experiments the acute effects of clonidine administration upon locomotor and rearing behaviour of mice pretreated with the selective noradrenaline (NA) neurotoxin, DSP4 (1 x 75 mg/kg, i.p.) 10-12 days previously, were studied. Clonidine (0.01, 0.05, 0.25, 1.25 and 3.0 mg/kg, i.p.) induced a dose-dependent reduction of motor activity during the initial 30 min of testing in both DSP4-treated and control mice; this effect was attenuated by DSP4 treatment in the 0.01, 0.05, 0.25 and 3.0 mg/kg dose groups. By the third 30-min period of testing (60-90 min), each clonidine dose group, except the highest (3.0 mg/kg) dose for locomotion and the two highest (1.25 and 3.0 mg/kg) doses for rearing, induced increases in motor activity in the control mice. In DSP4-treated mice, a large increase in locomotor counts was produced by the 0.05 mg/kg dose of clonidine with lesser increases induced by the 0.01 mg/kg dose group, whereas a lesser effect of the 0.05 mg/kg group (30-60 min) was obtained for rearing but a larger effect of the 0.25 mg/kg group (60-90 min). Yohimbine (0.5 mg/kg, i.p., 15 min before clonidine) attenuated the suppressive effects of clonidine (0.01 and 0.05 mg/kg) during the initial 30 min of testing and markedly increased locomotor and rearing counts, both by itself and in combination with each dose of clonidine, in both DSP4-treated and control mice over the following 90 min of testing. Yohimbine treatment attenuated the large increase in locomotor counts induced by the 0.05 mg/kg dose of clonidine in the NA-denervated mice. Dihydroergotamine (0.5 mg/kg, i.p., 15 min before clonidine) did not antagonise either the initial suppressive effect or the later supersensitivity effect of the 0.05 mg/kg dose of clonidine. DSP4 treatment by itself reduced motor activity. The effects of clonidine, dose- and time-dependently, by itself or in co-administration with alpha-adrenoceptor antagonists, in DSP4-treated or control mice displayed denervation-induced supersensitivity that appear to reflect mainly postsynaptic alpha2-adrenoceptor mediation.

Noradrenaline neurotoxin DSP-4 effects on sleep and brain temperature in the rat

N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) has a selective degenerative effect on noradrenergic fibers originating from locus coeruleus (LC) neurons. In the present study, we studied its effect on vigilance states and brain temperature by continuous recordings for periods of 1-5 days and 2-4 weeks following DSP-4 treatment. On the first day, paradoxical sleep duration was significantly decreased (-67%, P < 0.05), slow-wave sleep (SWS) duration increased (+16%, P < 0.05) up to 48 h after DSP-4 treatment (+8%, P < 0.05) and the wake period decreased (-8%, P < 0.05). The vigilance states returned to control values 4-5 days later. The brain temperature was decreased during the first night (-2 degrees C) and then recovered the control values. Two and 4 weeks after DSP-4 treatment, paradoxical sleep was still decreased (-18% and -23%, respectively, P < 0.05), while SWS was significantly increased only at night during the fourth week (+23%, P < 0.05). These results therefore provide evidence for a differential involvement of the noradrenergic LC system in sleep mechanisms depending on the light-dark cycle. Different hypotheses are proposed.

Neuronal gene expression in the waking state: a role for the locus coeruleus

Several transcription factors are expressed at higher levels in the waking than in the sleeping brain. In experiments with rats, the locus coeruleus, a noradrenergic nucleus with diffuse projections, was found to regulate such expression. In brain regions depleted of noradrenergic innervation, amounts of c-Fos and nerve growth factor-induced A after waking were as low as after sleep. Phosphorylation of cyclic adenosine monophosphate response element-binding protein was also reduced. In contrast, electroencephalographic activity was unchanged. The reduced activity of locus coeruleus neurons may explain why the induction of certain transcription factors, with potential effects on plasticity and learning, does not occur during sleep.

Effects of clonidine and alpha-methyl-p-tyrosine on the carbachol stimulation of paradoxical sleep

Acetylcholine promotes paradoxical sleep (PS), but the role of noradrenaline in this stimulation is controversial. The relationship between cholinergic and noradrenergic systems in the production of PS was investigated in the rat implanted on a continuous basis for sleep recordings. Stimulation of PS was obtained with microinjections of carbachol (1 microgram) into the pontine reticular formation. In the presence of the alpha 2-agonist clonidine (5 micrograms/kg, IP), the carbachol activation of PS was abolished. This stimulation also disappeared when the animals were pretreated with alpha-methyl-paratyrosine (150 mg/kg, IP), an inhibitor of catecholamine synthesis. Thus, carbachol stimulation appeared inefficient when brain noradrenergic activation was decreased. This observation supports the view that the realization of PS by the cholinergic system requires a certain level of noradrenergic activity.

Alpha 2-adrenoceptors and vigilance in cats: antagonism of medetomidine sedation by atipamezole

In order to evaluate the effect of a specific alpha 2-adrenoceptor antagonist, atipamezole, on vigilance, adult cats with implanted electrodes for polygraphy were tested in a double-blind Latin square design. The standard clinical dose (0.1 mg/kg i.m.) of the specific alpha 2-adrenoceptor agonist, medetomidine, promptly induced stuporous sedation. Atipamezole, given 30 min later at 0.2, 0.4 or 0.8 mg/kg i.m., reversed the sedation within 3 min, resulting in complete awareness of the animal. After the small dose of atipamezole, arousal with some motor excitation continued for 6 h, whereas after the larger doses, the physiological sleep-wake cycle returned earlier. Used alone, the preferred dose, 0.4 mg/kg atipamezole i.m., allowed physiological sleep within 33 +/- 9 min, compared to 22 +/- 3 min after saline. Atipamezole thus proved to be a most effective antagonist to sedation with alpha 2-adrenoceptor agonist drugs, without disturbing excitatory effects. Specific alpha 2-adrenoceptor modulating drugs have evident clinical application, as antidotes to overdosage of alpha 2-adrenoceptor agonists, or to terminate their effect after surgical procedures.

The effect of selective noradrenergic denervation on thyrotropin secretion in the rat

The effect of DSP4 [N-(2-chloroethyl)-N-ethyl-2 bromobenzylamine], a neurotoxin which selectively lesions noradrenergic projections from the locus coeruleus, on thyrotropin (TSH) secretion was investigated in the rat. DSP4 treatment (60 mg/kg injected i.p. 10 days prior to experimentation) significantly decreased the noradrenaline (NA) content of the hippocampus, frontal cortex and hypothalamus of the rat brain. DSP4 treatment did not affect the clonidine (250 micrograms/kg, i.p ) or TSH-releasing-hormone (TRH 5 micrograms/kg i.v.) induced stimulation or the isoproterenol induced inhibition of TSH secretion in the rat. These results suggest that the noradrenergic projection from the locus coeruleus to the hypothalamus does not play a significant role in the regulation of TSH secretion. Furthermore, the noradrenergic deficiency did not give rise to the development of the abnormal TSH response to TRH administration which is frequently observed in depression.