Histamine does not play an essential role in electrocortical activation during waking behavior

Neural Circuits for Sleep-Wake Regulation

The neural mechanisms of sleep, a fundamental biological behavior from invertebrates to humans, have been a long-standing mystery and present an enormous challenge. Gradually, perspectives on the neurobiology of sleep have been more various with the technical innovations over the recent decades, and studies have now identified many specific neural circuits that selectively regulate the initiation and maintenance of wake, rapid eye movement (REM) sleep, and non-REM (NREM) sleep. The cholinergic system in basal forebrain (BF) that fire maximally during waking and REM sleep is one of the key neuromodulation systems related to waking and REM sleep. Here we outline the recent progress of the BF cholinergic system in sleep-wake cycle. The intricate local connectivity and multiple projections to other cortical and subcortical regions of the BF cholinergic system elaborately presented here form a conceptual framework for understanding the coordinating effects with the dissecting regions. This framework also provides evidences regarding the relationships between the general anesthesia and wakefulness/sleep cycle focusing on the neural circuitry of unconsciousness induced by anesthetic drugs.

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.

Brain inhibitory mechanisms involved in basic and higher integrated sleep processes

Brain function is supported by central activating processes that are significant during waking, decrease during slow wave sleep following waking and increase again during paradoxical sleep during which brain activation is as high as, or higher than, during waking in nearly all structures. However, inhibitory mechanisms are crucial for sleep onset. They were first identified by behavioral, neuroanatomical and electrophysiological criteria, then by pharmacological and neurochemical ones. During slow wave sleep, they are supported by GABAergic mechanisms located at midbrain, mesopontine and pontine levels but are induced and sustained by forebrain and hindbrain influences. GABAergic processes are also responsible for paradoxical sleep occurrence, particularly by suppression of noradrenaline and serotonin (5-HT) inhibition of paradoxical sleep-generating structures. Hindbrain and forebrain modulate these structures situated at the mesopontine level. For sleep mentation, the noradrenergic and serotonergic silence is thought, today, to be directly, or indirectly, responsible for dopamine predominance and glutamate decrease in the nucleus accumbens, which could be the background of the well-known psychotic-like mental activity of dreaming.

Histaminergic facilitation of electrocorticographic activation: role of basal forebrain, thalamus, and neocortex

The neuromodulator histamine plays an important role in the regulation of behavioural state and the neocortical electrocorticogram (ECoG). With the present experiments, we characterized the anatomical targets that mediate the cortical-activating effects of histamine. Urethane-anaesthetized rats displayed continuous large-amplitude, low-frequency oscillations with a maximal spectral power in the delta (0.5-3.9 Hz) frequency band. Electrical (100 Hz) stimulation of the pontine-tegmentum suppressed slow, large-amplitude oscillations and induced ECoG activation. Application of histamine (1 mm) into the basal forebrain cholinergic complex by reverse microdialysis enhanced ECoG activation elicited by tegmental stimulation without changing resting ECoG activity. Ventrolateral or central thalamic application of histamine had no effect on resting ECoG activity, and ventrolateral thalamic application produced only a slight enhancement of brainstem-induced activation. Neocortical application of histamine in close proximity (< 500 micro m) to the recording electrode reduced low-frequency delta power in the resting ECoG without affecting stimulation-induced ECoG activation. These data suggest that, under the present experimental conditions, histamine facilitates ECoG activation primarily by potentiating the excitatory influence of brainstem fibers at the level of the basal forebrain. Histamine release in some parts of the thalamus results in a minor enhancement of ECoG activation, and cortical histamine release produces a small but consistent suppression of slow delta oscillations in the resting ECoG. These concurrent subcortical and cortical actions probably permit histamine to effectively modulate cortical activation and excitability across different behavioural states.

The neurochemistry of waking and sleeping mental activity: the disinhibition-dopamine hypothesis

This paper describes a hypothesis related to the neurochemical background of sleep-waking mental activity which, although associated with subcortical structures, is principally generated in the cerebral cortex. Acetylcholine, which mainly activates cortical neurons, is released at the maximal rate during waking and rapid eye movement (REM) sleep dreaming stage. Its importance in mental functioning is well-known. However, brainstem-generated monoamines, which mainly inhibit cortical neurons, are released during waking. Both kinds of influences contribute to the organized mentation of waking. During slow wave sleep, these two types of influence decrease in intensity but maintain a sufficiently high level to allow mental activity involving fairly abstract pseudo-thoughts, a mode of activity modelled on the diurnal pattern of which it is a poor reply. During REM sleep, the monoaminergic neurons become silent except for the dopaminergic ones. This results in a large disinhibition and the maintained dopamine influence may be involved in the familiar psychotic-like mental activity of dreaming. Indeed, in this original activation-disinhibition state, the increase of dopamine influence at the prefrontal cortex level could explain the almost total absence of negative symptoms of schizophrenia during dreaming, while an increase in the nucleus accumbens is possibly responsible for hallucinations and delusions, which are regular features of mentation during this sleep stage.

Electroencephalographic activation by tacrine, deprenyl, and quipazine: cholinergic vs. non-cholinergic contributions

Drugs that stimulate central cholinergic transmission can induce activated, high frequency electroencephalographic (EEG) activity in rats. Monoaminergic enhancement also produces EEG activation, either by a direct stimulation of monoaminergic transmission in cortex, or a transsynaptic excitation of cholinergic projection neurons receiving excitatory monoaminergic afferents. We examined the degree of cholinergic involvement in EEG activation produced by monoaminergic and cholinergic drugs in rats. All animals were pretreated with 10 mg/kg reserpine and either 1 or 5 mg/kg scopolamine to abolish EEG activation. The acetylcholinesterase inhibitor tacrine (5-20 mg/kg) restored EEG activation in the low dose scopolamine group, but was less effective against the high scopolamine dose. The monoamine oxidase inhibitor deprenyl and the serotonergic receptor agonist quipazine restored EEG activation, an effect that was largely unaffected by different scopolamine doses. These results confirm that tacrine produces EEG activation by means of cholinergic stimulation. In contrast, activation produced by deprenyl or quipazine does not appear to be mediated by a transsynaptic excitation of cholinergic neurons and likely depends on a direct enhancement of cortical monoaminergic neurotransmission.

Differential modulation of hippocampal signal transfer by tuberomammillary nucleus stimulation in freely moving rats dependent on behavioral state

Tuberomammillary histamine neurons (TM) in the posterior hypothalamus project to extensive parts of the brain, including the hippocampal formation. The purpose of the present experiments was to investigate whether activation of the TM modulates signal transfer from the perforant pathway (PP) or ventral hippocampal commissure (VHC) to the dentate gyrus (DG) in freely moving rats. Paired pulses of electrical stimulation were delivered to PP or VHC, and evoked field potentials (fEPSPs and pop spikes) were recorded in the DG. Before activating PP or VHC, the TM was triggered by electrical stimulation. Experimentation was performed during four behavioral conditions: exploration, grooming, awake immobility, and slow-wave sleep. Electrical activation of the TM was found to modify dentate fEPSPs evoked by PP or VHC stimulation without generating a field potential by itself. Train stimulation of the TM (100 Hz, 500 ms) preceding paired pulses on the hippocampus by 50 ms decreased dentate fEPSPs in dependence of the ongoing behavior and the pathway stimulated. During exploration but not consummatory behavior, the PP signal was reduced when preceded by TM stimulation; during consummatory behavior but not exploration, the VHC signal was reduced. In contrast to other hippocampal afferents which increase pop spikes but leave fEPSPs unchanged, TM stimulation decreased dentate fEPSPs without affecting pop-spike activity. Thus, the TM-histaminergic system seems to modulate signal processing in the dentate gyrus in a specific way, exerting an inhibitory action on the entorhinal input only during learning-related exploratory behavior.

The role of brain noradrenaline in cortical activation and behavior: a study of lesions of the locus coeruleus, medial thalamus and hippocampus-neocortex and of muscarinic blockade in the rat

Local injection of 6-hydroxydopamine in the locus coeruleus resulted in a 90% depletion of noradrenaline (NA) in the cerebral cortex as assessed by high-pressure liquid chromatography. This NA depletion had no effect on scopolamine-resistant hippocampal rhythmical slow activity and only an occasional effect on scopolamine-resistant neocortical low voltage fast activity. However, NE depletion resulted in a slight deficit in a behavioral swim-to-platform test and increased the deficit produced on the test by systemic treatment with scopolamine. Large surgical lesions of the medial thalamus or hippocampal formation plus posterior neocortex greatly increased the behavioral deficit produced by scopolamine. It is concluded that ascending noradrenergic projections play only a modest and possibly indirect role in the control of electrocortical activation and that a number of different brain lesions increase the behavioral impairment produced by central muscarinic blockade.

Long-term depletion of brain histamine induced by alpha-fluoromethylhistidine increases feeding-associated locomotor activity in mice with a modulation of brain amino acid levels

We examined the long-term effects of administration of (S)-alpha-fluoromethylhistidine (FMH), a specific inhibitor of histidine decarboxylase, on the spontaneous locomotor activity, food intake and brain contents of histamine, catecholamines, serotonin and amino acids of ICR mice. The distance of ambulation and number of rearings significantly increased from 8 to 15 h (20.00-03.00 h) after treatment with FMH (100 mg/kg, i.p.) and the 24-h food intake also increased significantly. On FMH treatment, the locomotor activity in movements of 3-15 cm/0.5 s was greater than that of control mice, whereas the number of slight movements (0-1 cm/0.5 s) decreased, suggesting that once a mouse treated with FMH is in motion, it moves a longer distance than a control mouse. We sacrificed mice 12 or 24 h after FMH treatment to measure the brain contents of histamine, monoamines and amino acids. Decrease of the brain histamine content to 35% of the control level was observed until 24 h after FMH treatment, but no significant changes in the brain catecholamine and serotonin contents were detected. However, the brain GABA content of ICR mice decreased to 85% of control 12 h after FMH treatment. Moreover, decrease of the brain GABA content after FMH treatment was greater in mast cell-deficient W/Wv mice, being 70 and 62% of the control level 12 and 24 h after treatment, respectively. The present experiments support the idea that the locomotor activity is affected by the central histaminergic system, directly and/or indirectly.