Comparative anatomy of the histaminergic and other aminergic systems in zebrafish (Danio rerio).

The histaminergic system and its relationships to the other aminergic transmitter systems in the brain of the zebrafish were studied by using confocal microscopy and immunohistochemistry on brain whole-mounts and sections. All monoaminergic systems displayed extensive, widespread fiber systems that innervated all major brain areas, often in a complementary manner. The ventrocaudal hypothalamus contained all monoamine neurons except noradrenaline cells. Histamine (HA), tyrosine hydroxylase (TH), and serotonin (5-HT) -containing neurons were all found around the posterior recess (PR) of the caudal hypothalamus. TH- and 5-HT-containing neurons were found in the periventricular cell layer of PR, whereas the HA-containing neurons were in the surrounding cell layer as a distinct boundary. Histaminergic neurons, which send widespread ascending and descending fibers, were all confined to the ventrocaudal hypothalamus. Histaminergic neurons were medium in size (approximately 12 microm) with varicose ascending and descending ipsilateral and contralateral fiber projections. Histamine was stored in vesicles in two types of neurons and fibers. A close relationship between HA fibers and serotonergic raphe neurons and noradrenergic locus coeruleus neurons was evident. Putative synaptic contacts were occasionally detected between HA and TH or 5-HT neurons. These results indicate that reciprocal contacts between monoaminergic systems are abundant and complex. The results also provide evidence of homologies to mammalian systems and allow identification of several previously uncharacterized systems in zebrafish mutants.     

The histaminergic tuberomammillary nucleus is critical for motivated arousal

Obtaining food, shelter or water, or finding a mating partner are examples of motivated behaviors, which are essential to preserve the species. The full expression of such behaviors requires a high but optimal arousal state. We tested the idea that tuberomammillary nucleus (TMN) histamine neurons are crucial to generate such motivated arousal, using a model of the appetitive phase of feeding behavior. Hungry rats enticed with food within a wire mesh box showed intense goal‐directed motor activity aimed at opening the box, an increase in core temperature, a fast histamine release in the hypothalamus and an early increase in Fos immunoreactivity in TMN and cortical neurons. Enticing with stronger‐tasting food induced stronger motor, temperature and Fos immunoreactivity brain responses than ordinary food pellets. TMN lesion greatly decreased all of those responses. We conclude that histamine neurons increase arousal and vegetative activity, allowing the normal unfolding of voluntary, goal‐directed behavior such as obtaining food.     

Comparative mapping of GABA‐immunoreactive neurons in the central nervous systems of nudibranch molluscs

The relative simplicity of certain invertebrate nervous systems, such as those of gastropod molluscs, allows behaviors to be dissected at the level of small neural circuits composed of individually identifiable neurons. Elucidating the neurotransmitter phenotype of neurons in neural circuits is important for understanding how those neural circuits function. In this study, we examined the distribution of γ‐aminobutyric‐acid;‐immunoreactive (GABA‐ir) neurons in four species of sea slugs (Mollusca, Gastropoda, Opisthobranchia, Nudibranchia): Tritonia diomedea, Melibe leonina, Dendronotus iris, and Hermissenda crassicornis. We found consistent patterns of GABA immunoreactivity in the pedal and cerebral‐pleural ganglia across species. In particular, there were bilateral clusters in the lateral and medial regions of the dorsal surface of the cerebral ganglia as well as a cluster on the ventral surface of the pedal ganglia. There were also individual GABA‐ir neurons that were recognizable across species. The invariant presence of these individual neurons and clusters suggests that they are homologous, although there were interspecies differences in the numbers of neurons in the clusters. The GABAergic system was largely restricted to the central nervous system, with the majority of axons confined to ganglionic connectives and commissures, suggesting a central, integrative role for GABA. GABA was a candidate inhibitory neurotransmitter for neurons in central pattern generator (CPG) circuits underlying swimming behaviors in these species, however none of the known swim CPG neurons were GABA‐ir. Although the functions of these GABA‐ir neurons are not known, it is clear that their presence has been strongly conserved across nudibranchs. J. Comp. Neurol. 522:794–810, 2014. © 2013 Wiley Periodicals, Inc.     

Functions of corazonin and histamine in light entrainment of the circadian pacemaker in the Madeira cockroach,Rhyparobia maderae

The circadian pacemaker of the Madeira cockroach, Rhyparobia (Leucophaea) maderae, is located in the accessory medulla (AME). Ipsi‐ and contralateral histaminergic compound eyes are required for photic entrainment. Light pulses delay locomotor activity rhythm during the early night and advance it during the late night. Thus, different neuronal pathways might relay either light‐dependent delays or advances to the clock. Injections of neuroactive substances combined with running‐wheel assays suggested that GABA, pigment‐dispersing factor, myoinhibitory peptides (MIPs), and orcokinins (ORCs) were part of both entrainment pathways, whereas allatotropin (AT) only delayed locomotor rhythms at the early night. To characterize photic entrainment further, histamine and corazonin were injected. Histamine injections resulted in light‐like phase delays and advances, indicating that the neurotransmitter of the compound eyes participates in both entrainment pathways. Because injections of corazonin only advanced during the late subjective night, it was hypothesized that corazonin is only part of the advance pathway. Multiple‐label immunocytochemistry in combination with neurobiotin backfills demonstrated that a single cell expressed corazonin in the optic lobes that belonged to the group of medial AME interneurons. It colocalized GABA and MIP but not AT or ORC immunoreactivity. Corazonin‐immunoreactive (‐ir) terminals overlapped with projections of putatively light‐sensitive interneurons from the ipsi‐ and contralateral compound eye. Thus, we hypothesize that the corazonin‐ir medial neuron integrates ipsi‐ and contralateral light information as part of the phase‐advancing light entrainment pathway to the circadian clock. J. Comp. Neurol. 525:1250–1272, 2017. © 2016 Wiley Periodicals, Inc.     

Is the histaminergic neuron system a regulatory center for whole-brain activity?

Recent immunocytochemical studies have demonstrated the existence of histaminergic neurons in the brain, which are concentrated in the tuberomammillary nucleus of the posterior hypothalamus, and which project efferent fibers to almost all parts of the brain. Three subtypes of histamine receptors are widely distributed in the brain, not only on neurons but also on astrocytes and blood vessels. Consistent with its wide-ranging output, the histaminergic neuron system regulates various activities of the brain, such as the arousal state, brain energy metabolism, locomotor activity, neuroendocrine, autonomic and vestibular functions, feeding, drinking, sexual behavior, and analgesia--this regulation is possibly achieved by the histaminergic system as a whole.     

Histaminergic neuron system in the brain: Distribution and possible functions

Recent immunocytochemical studies have identified the histaminergic neuron system in the brain. In the rat brain, histaminergic neuronal cell bodies are located in the tuberomammillary nucleus in the posterior hypothalamus, while histaminergic fibers are distributed in almost all regions of the brain. Similar distributions of histaminergic neuronal cell bodies and fibers have been reported in the brains of other mammals and nonmammalian vertebrates. As expected from the widespread distributions of the efferent fibers, the central histaminergic neuron system seems to be involved in multiple functions in the brain. The results of intracerebral injection of histamine and administration of alpha-fluoromethylhistidine (FMH), which depletes brain histamine level, suggest that the central histaminergic system may modulate feeding, drinking and sexual behaviors, sleep-wakefulness and circadian rhythm, neuroendocrine and cardiovascular controls and thermoregulation.     

Dissociated histaminergic neuron cultures from the tuberomammillary nucleus of rats: culture methods and ghrelin effects

The tuberomammillary nucleus (TMN) in the hypothalamus is the sole source of histamine in the brain. This nucleus, by innervating various brain regions, plays an important role for vital functions such as arousal and appetite. We have developed dissociated primary histaminergic neuron cultures from TMN of postnatal (3 and 10-day-old) rats. More than 50% of our cultured neurons from the TMN were histaminergic as revealed by adenosine deaminase (AD) as well as histamine immunocytochemistry. Among large neurons (diameter, >22 microm), more than 88% were histaminergic. Such large neurons (mean diameter, 26.5 microm) were used for electrophysiology. Using about 2-month-old TMN cultures, we investigated the effects of ghrelin, a recently discovered appetite-stimulating endogenous peptide. In GTPgammaS-loaded neurons, ghrelin (3 microM) suppressed currents that had previously been activated by an inhibitory neuropeptide, nociceptin. The mean current suppression by ghrelin was 471+/-128 pA (S.E.M., n=7). The I-V relationship revealed that the ghrelin-suppressed current was inwardly rectifying with a reversal potential around E(K). These results suggest that ghrelin inhibits G protein-coupled inward rectifier K+ channels (Kir3, GIRK) of TMN neurons and that our TMN cultures are useful for investigating physiological properties of brain histaminergic neurons.     

Heterogeneity of histaminergic neurons in the tuberomammillary nucleus of the rat

Histaminergic neurons of the hypothalamic tuberomammillary nuclei (TMN) send projections to the whole brain. Early anatomical studies described histaminergic neurons as a homogeneous cell group, but recent evidence indicates that histaminergic neurons are heterogeneous and organized into distinct circuits. We addressed this issue using the double‐probe microdialysis in freely moving rats to investigate if two compounds acting directly onto histaminergic neurons to augment cell firing [thioperamide and bicuculline, histamine H₃‐ and γ‐aminobutyric acid (GABA)_A‐receptor (R) antagonists, respectively] may discriminate groups of histaminergic neurons impinging on different brain regions. Intra‐hypothalamic perfusion of either drug increased histamine release from the TMN and cortex, but not from the striatum. Thioperamide, but not bicuculline, increased histamine release from the nucleus basalis magnocellularis (NBM), bicuculline but not thioperamide increased histamine release from the nucleus accumbens (NAcc). Intra‐hypothalamic perfusion with thioperamide increased the time spent in wakefulness. To explore the local effects of H₃‐R blockade in the histaminergic projection areas, each rat was implanted with a single probe to simultaneously administer thioperamide and monitor local changes in histamine release. Thioperamide increased histamine release from the NBM and cortex significantly, but not from the NAcc or striatum. The presence of H₃‐Rs on histaminergic neurons was assessed using double‐immunofluorescence with anti‐histidine decarboxylase antibodies to identify histaminergic cells and anti‐H₃‐R antibodies. Confocal analysis revealed that all histaminergic somata were immunopositive for the H₃‐R. This is the first evidence that histaminergic neurons are organized into functionally distinct circuits that influence different brain regions, and display selective control mechanisms.     

Food-deprived activity stress decreased the activity of the histaminergic neuron system in rats

The hypothalamus, which is rich in histaminergic neurons, is highly sensitive to aversive stimuli such as stress. Histamine H3 receptors, which regulate histamine release from the presynaptic site, are associated with stress-induced brain activity. In this study, we investigated the changes of histamine content and histamine H1 and H3 receptors in the brains of rats subjected to stress induced through food deprivation and physical activity on a running wheel (food-deprived activity stress). For purposes of comparison, we also examined the stressful effects of forced swimming on the histaminergic neuron system of rats. The H3 receptor density rapidly declined in the acute phase of stress but gradually returned to the control level in the chronic phase. On the other hand, the H1 receptor slowly decreased and remained at a low level during the chronic phase. These results reveal that there is a discrepancy between the levels of H1 and H3 receptors in the acute and chronic phases of stress. Brain histamine content gradually increased during the late phase of both food-deprived activity stress and forced swimming stress. These changes presumably resulted in the inhibition of histaminergic neuronal activity in the chronic stress condition. In accordance with this hypothesis, the intraventricular administration of histamine significantly reduced the hyperactivity caused by food-deprived activity stress. Since extensive exercise and restricted feeding are thought to be associated with anorexia nervosa, the abnormalities in the histaminergic neuron system might contribute to trait status in anorexia nervosa.     

Functional heterogeneity of the responses of histaminergic neuron subpopulations to various stress challenges

In rats, the cell bodies of the histaminergic neuronal system are clustered in five distinct cell groups (E1-E5) within the posterior hypothalamus. On the basis of tract tracing studies, these histaminergic subgroups have been regarded as one functional unit. In addition to its well-characterized role in arousal, locomotor activity, metabolism, feeding, drinking and behaviour, as well as in coordination of autonomic functions, histamine has been implicated in regulation of the hypothalamo-pituitary-adrenocortical axis during stress. To address the capacity of different histaminergic subgroups to respond to various challenges, we revealed c-Fos, the immediate early gene marker of activated neurons, in histamine synthesizing neurons by combining c-Fos immunocytochemistry with in situ hybridization of histidine decarboxylase (HDC) mRNA. Compared to the negligible colocalization of these markers in control rats, restraint, insulin-induced hypoglycaemia and foot shock resulted in specific activation of histamine synthesizing neurons of the E4 and E5 subgroup in the tuberomammillary region. Up to 36% of HDC mRNA-expressing cells show c-Fos immunoreactivity in the E5 region. In addition, some neurons of the E1, E2 and E3 histaminergic groups were activated after restraint stress. Many less c-Fos-positive histaminergic neurons were detected after immobilization and dehydration. Ether stress, acute hyperosmotic stimulus or injection of bacterial lipopolysaccharide did not activate hypothalamic HDC-positive neurons. These results suggest, for the first time, the functional heterogeneity of histaminergic neuron population, the components of which are recruited in a stressor- and subgroup-specific manner.     

Actions of a histaminergic/peptidergic projection neuron on rhythmic motor patterns in the stomatogastric nervous system of the crab Cancer borealis

Histamine is a neurotransmitter with actions throughout the nervous system of vertebrates and invertebrates. Nevertheless, the actions of only a few identified histamine-containing neurons have been characterized. Here, we present the actions of a histaminergic projection neuron on the rhythmically active pyloric and gastric mill circuits within the stomatogastric ganglion (STG) of the crab Cancer borealis. An antiserum generated against histamine labeled profiles throughout the C. borealis stomatogastric nervous system. Labeling occurred in several somata and neuropil within the paired commissural ganglia as well as in neuropil within the STG and at the junction of the superior oesophageal and stomatogastric nerves. The source of all histamine-like immunolabeling in the STG neuropil was one pair of neuronal somata, the previously identified inferior ventricular (IV) neurons, located in the supraoesophageal ganglion. These neurons also exhibited FLRFamide-like immunoreactivity. Activation of the IV neurons in the crab inhibited some pyloric and gastric mill neurons and, with inputs from the commissural ganglia eliminated, terminated both rhythms. Focal application of histamine had comparable effects. The actions of both applied histamine and IV neuron stimulation were blocked, reversibly, by the histamine type-2 receptor antagonist cimetidine. With the commissural ganglia connected to the STG, IV neuron stimulation elicited a longer-latency activation of commissural projection neurons which in turn modified the pyloric rhythm and activated the gastric mill rhythm. These results support the hypothesis that the histaminergic/peptidergic IV neurons are projection neurons with direct and indirect actions on the STG circuits of the crab C. borealis.     

Galanin gene expression and effects of its knock-down on the development of the nervous system in larval zebrafish

Despite the known importance of galanin in the nervous system of vertebrates, the galanin gene structure and expression and the consequences of galanin deficiency in developing zebrafish are unknown. We cloned the galanin gene and analyzed its expression by using in situ hybridization, PCR, and immunocytochemistry throughout the early development of zebrafish until the end of the first week of life. The single zebrafish galanin gene encoded for a single amidated galanin peptide and a galanin message‐associated peptide. Two forms resulting from alternative processing were identified. Galanin mRNA was maternally expressed and found in developing fish throughout early development. In situ hybridization showed the first positive neurons in three groups in the brain at 28 hours postfertilization. At 2 days postfertilization, three prosencephalic neuron groups were seen in the preoptic area and in rostral and caudal periventricular hypothalamus. In addition, two other groups of weakly stained neurons were visible, one in the midbrain and another in the hindbrain. Translation inhibition of galanin mRNA with morpholino oligonucleotides caused complete disappearance of galanin immunoreactivity in the brain until 7 dpf and did not induce known cascades of nonspecific pathways or morphological abnormalities. A minor disturbance of sensory ganglia was found. Galanin knockdown did not alter the expression of tyrosine hydroxylases 1 and 2, choline acetyltransferase, histidine decarboxylase, or orexin mRNA. The results suggest that galanin does not regulate the development of these key markers of specific neurons, although galanin‐expressing fibers were in a close spatial proximity to several neurons of these neuronal populations. J. Comp. Neurol. 520:3846–3862, 2012. © 2012 Wiley Periodicals, Inc.     

Gonadotropin‐inhibitory hormone identification,cDNA cloning,and distribution in rhesus macaque brain

Gonadotropin‐inhibitory hormone (GnIH) is a hypothalamic neuropeptide that modulates the reproductive physiology of birds and mammals by inhibiting gonadotropin secretion from the anterior pituitary gland. GnIH can also directly inhibit reproductive behaviors, possibly via action within the brain. Identification of the distribution of GnIH neurons and fibers may provide us with clues to how the brain controls reproductive activities of the animal. Here, we characterized the location and connectivity of GnIH neurons in the rhesus macaque (Macaca mulatta) brain. We determined the macaque GnIH precursor mRNA, and further identified a mature GnIH peptide (SGRNMEVSLVRQVLNLPQRF‐NH₂) by mass spectrometry combined with immunoaffinity purification. The majority of GnIH precursor mRNA‐positive and GnIH‐immunoreactive (GnIH‐ir) cell bodies were localized in the intermediate periventricular nucleus (IPe) in the hypothalamus, as determined by in situ hybridization and immunocytochemistry, respectively. Abundant GnIH‐ir fibers were observed in the nucleus of the stria terminalis in the telencephalon; habenular nucleus, paraventricular nucleus of the thalamus, preoptic area, paraventricular nucleus of the hypothalamus, IPe, arcuate nucleus of hypothalamus, median eminence and dorsal hypothalamic area in the diencephalon; medial region of the superior colliculus, central gray substance of the midbrain and dorsal raphe nucleus in the midbrain; and parabrachial nucleus in the pons. GnIH‐ir fibers were observed in close proximity to gonadotropin‐releasing hormone‐I, dopamine, β‐endorphin, and gonadotropin‐releasing hormone‐II neurons in the preoptic area, IPe, arcuate nucleus of hypothalamus, and central gray substance of midbrain, respectively. GnIH neurons might thus regulate several neural systems in addition to pituitary gonadotropin release. J. Comp. Neurol. 517:841–855, 2009. © 2009 Wiley‐Liss, Inc.     

Importance of histamine in modulatory processes, locomotion and memory.

Acetylcholine modulates histaminergic transmission via M(1) receptors. On the other hand, cholinergic transmission is modulated by neighbouring histaminergic neurons via H(1), H(2) and H(3) receptors. Dopaminergic and GABAergic neurons are also involved in these modulatory mechanisms. Furthermore, the release of histamine is modulated by glutamatergic neurons and nitric oxide of neuronal origin. The release of histamine in the brain oscillates according to circadian, slow ultradian and fast ultradian rhythms. Ultradian fluctuations have also been observed in the theta- and delta-frequency bands of the EEG spectral power. Simultaneous recordings of histamine outflow and EEG in the hypothalamus revealed that the ultradian histamine release rhythm coincides temporally with ultradian fluctuations in the EEG spectral power. Histamine receptor ligands used in pharmacotherapy, like H(1) and H(2) antagonists, modify the frequency of the EEG fluctuations. Brain histamine seems to be involved in memory processes, since inhibition of histamine synthesis deteriorates, while H(3) antagonists, histamine and histidine improve short-term memory. The latter finding may open new horizons in pharmacological treatment of memory disorders.     

Differential effect of cannabinoid agonists and endocannabinoids on histamine release from distinct regions of the rat brain

Cannabinoids exert complex actions on neurotransmitter systems involved in cognition, locomotion, appetite, but no information was available so far on the interactions between the endocannabinoid system and histaminergic neurons that command several, similar behavioural states and memory. In this study, we investigated the effect of cannabimimetic compounds on histamine release using the microdialysis technique in the brain of freely moving rats. We found that systemic administration of the cannabinoid receptors 1 (CB1-r) agonist arachidonyl-2'chloroethylamide/N-(2chloroethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (ACEA; 3 mg/kg) increased histamine release from the posterior hypothalamus, where the histaminergic tuberomamillary nuclei (TMN) are located. Local infusions of ACEA (150 nm) or R(+)-methanandamide (mAEA; 1 microm), another CB1-r agonist, in the TMN augmented histamine release from the TMN, as well as from two histaminergic projection areas, the nucleus basalis magnocellularis and the dorsal striatum. When the endocannabinoid uptake inhibitor AM404 was infused into the TMN, however, increased histamine release was observed only in the TMN. The cannabinoid-induced effects on histamine release were blocked by co-administrations with the CB1-r antagonist AM251. Using double-immunofluorescence labelling and confocal laser-scanning microscopy, CB1-r immunostaining was found in the hypothalamus, but was not localized onto histaminergic cells. The modulatory effect of cannabimimetic compounds on histamine release apparently did not involve inhibition of gamma-aminobutyric acid (GABA)ergic neurotransmission, which provides the main inhibitory input to the histaminergic neurons in the hypothalamus, as local infusions of ACEA did not modify GABA release from the TMN. These profound effects of cannabinoids on histaminergic neurotransmission may partially underlie some of the behavioural changes observed following exposure to cannabinoid-based drugs.     

GABA‐ and serotonin‐expressing neurons take part in inhibitory as well as excitatory input pathways to the circadian clock of the Madeira cockroach Rhyparobia maderae

In the Madeira cockroach, pigment‐dispersing factor‐immunoreactive (PDF‐ir) neurons innervating the circadian clock, the accessory medulla (AME) in the brain′s optic lobes, control circadian behaviour. Circadian activity rhythms are entrained to daily light–dark cycles only by compound eye photoreceptors terminating in the lamina and medulla. Still, it is unknown which neurons connect the photoreceptors to the clock to allow for light entrainment. Here, we characterized by multiple‐label immunocytochemistry the serotonin (5‐HT)‐ir anterior fibre fan and GABA‐ir pathways connecting the AME‐ and optic lobe neuropils. Colocalization of 5‐HT with PDF was confirmed in PDF‐ir lamina neurons (PDFLAs). Double‐labelled fibres were traced to the AME originating from colabelled PDFLAs branching in accessory laminae and proximal lamina. The newly discovered GABA‐ir medial layer fibre tract connected the AME to the medulla′s medial layer fibre system, and the distal tract fibres connected the AME to the medulla. With Ca²⁺ imaging on primary cell cultures of the AME and with loose‐patch‐clamp recordings in vivo, we showed that both neurotransmitters either excite or inhibit AME clock neurons. Because we found no colocalization of GABA and 5‐HT in any optic lobe neuron, GABA‐ and 5‐HT neurons form separate clock input circuits. Among others, both pathways converged also on AME neurons that coexpressed mostly inhibitory GABA‐ and excitatory 5‐HT receptors. Our physiological and immunocytochemical studies demonstrate that GABA‐ and 5‐HT‐immunoreactive neurons constitute parallel excitatory or inhibitory pathways connecting the circadian clock either to the lamina or medulla where photic information from the compound eye is processed.     

Molecular mechanism of histamine clearance by primary human astrocytes

Histamine clearance is an essential process for avoiding excessive histaminergic neuronal activity. Previous studies using rodents revealed the predominant role of astrocytes in brain histamine clearance. However, the molecular mechanism of histamine clearance has remained unclear. We detected histamine N‐methyltransferase (HNMT), a histamine‐metabolizing enzyme, in primary human astrocytes and the astrocytes of human brain specimens. Immunocytochemical analysis and subcellular fractionation assays revealed that active HNMT localized to the cytosol, suggesting that histamine transport into the cytosol is crucial for histamine inactivation. We showed that primary human astrocytes transported histamine in a time‐dependent manner. Kinetics analysis showed that two low‐affinity transporters were involved in histamine transport. Histamine uptake by primary human astrocytes was not dependent on the extracellular Na⁺/Cl^? concentration. Histamine is reported to be a substrate for three low‐affinity and Na⁺/Cl^?‐independent transporters: organic cation transporter 2 (OCT2), OCT3, and plasma membrane monoamine transporter (PMAT). RT‐PCR analysis revealed that OCT3 and PMAT were expressed in primary human astrocytes. Immunohistochemistry confirmed OCT3 and PMAT expression in the astrocytes of human brain specimens. Drug inhibition assays and gene knockdown assays revealed the major contribution of PMAT and the minor contribution of OCT3 to histamine transport. The present study demonstrates for the first time that the molecular mechanism of histamine clearance is by primary human astrocytes. These findings might indicate that PMAT, OCT3 and HNMT in human astrocytes play a role in the regulation of extraneuronal histamine concentration and the activities of histaminergic neurons.     

Brain structures and mechanisms involved in the control of cortical activation and wakefulness, with emphasis on the posterior hypothalamus and histaminergic neurons

Wakefulness is a functional brain state that allows the performance of several "high brain functions", such as diverse behavioural, cognitive and emotional activities. Present knowledge at the whole animal or cellular level suggests that the maintenance of the cerebral cortex in this highly complex state necessitates the convergent and divergent activity of an ascending network within a large reticular zone, extending from the medulla to the forebrain and involving four major subcortical structures (the thalamus, basal forebrain, posterior hypothalamus and brainstem monoaminergic nuclei), their integral interconnections and several neurotransmitters, such as glutamate, acetylcholine, histamine and noradrenaline. In this mini-review, the importance of the thalamus, basal forebrain and brainstem monoaminergic neurons in wake control is briefly summarized, before turning our attention to the posterior hypothalamus and histaminergic neurons, which have been far less studied. Classical and recent experimental data are summarized, supporting the hypothesis that (1) the posterior hypothalamus constitutes one of the brain ascending activating systems and plays an important role in waking; (2) this function is mediated, in part, by histaminergic neurons, which constitute one of the excitatory sources for cortical activation during waking; (3) the mechanisms of histaminergic arousal involve both the ascending and descending projections of histaminergic neurons and their interactions with diverse neuronal populations, such as neurons in the pre-optic area and cholinergic neurons; and (4) other widespread-projecting neurons in the posterior hypothalamus also contribute to the tonic cortical activation during wakefulness and/or paradoxical sleep.