Organization of the histaminergic system in the brain of the turtle Chinemys reevesii

Different Peas in the Same Pod: The Histaminergic Neuronal Heterogeneity

The histaminergic neuronal system is recently receiving increasing attention, as much has been learned over the past 25 years about histamine role as a neurotransmitter. Indeed, this amine is crucial in maintaining arousal and provides important contributions to regulate circadian rhythms, energy, endocrine homeostasis, motor behavior, and cognition. The extent to which these distinct physiological functions are operated by independent histamine neuronal subpopulation is unclear. In the rat brain histamine neuronal cell bodies are grouped within the tuberomamillary nucleus of the posterior hypothalamus in five clusters, E1-E5, each sending overlapping axons throughout the entire central nervous system with no strict topographical pattern. These features lead to the concept that histamine regulation of a wide range of functions in the central nervous system is achieved by the histaminergic neuronal system as a whole. However, increasing experimental evidence suggesting that the histaminergic system is organized into distinct pathways modulated by selective mechanisms challenges this view. In this review, we summarized experimental evidence supporting the heterogeneity of histamine neurons, and their organization in functionally distinct circuits impinging on separate brain regions and displaying selective control mechanisms. This implies independent functions of subsets of histaminergic neurons according to their respective origin and terminal projections with relevant consequences for the development of specific compounds that affect only subsets of histamine neurons, thus increasing the target specificity.

Embryonic exposure to valproic acid affects the histaminergic system and the social behaviour of adult zebrafish (Danio rerio)

BACKGROUND AND PURPOSE

Histamine modulates several behaviours and physiological functions, and its deficiency is associated with neuropsychiatric disorders. Gestational intake of valproic acid (VPA) is linked to autism spectrum disorder (ASD), characterized by impaired sociability and stereotypies. VPA effects on the neurochemistry and functional morphology of the histaminergic system in ASD are unclear. Zebrafish are highly social, and given the similarities between zebrafish and human neurotransmitter systems, we have studied the effects of VPA on histamine in zebrafish.

EXPERIMENTAL APPROACH

Histaminergic, dopaminergic and noradrenergic systems of larval and adult zebrafish exposed to VPA from the end of gastrulation until neural tube formation were studied using HPLC, quantitative PCR, immunocytochemistry and in situ hybridization. Sociability, dark-flash response and locomotion were also studied.

KEY RESULTS

Zebrafish larvae exposed to VPA showed decreased locomotion and an abnormal dark-flash response. Additionally, a reduced number of histaminergic neurons, low histamine and altered mRNA expression of key genes of the monoaminergic systems were also detected. The reduced mRNA expression of genes of the studied systems persisted until adulthood. Furthermore, adult VPA-exposed animals presented lower brain levels of noradrenaline and 3,4-dihydroxyphenylacetic acid, along with impaired sociability.

CONCLUSIONS AND IMPLICATIONS

VPA exposure in early development causes molecular and neurochemical alterations in zebrafish, which persist into adulthood and accompany impaired sociability. These findings will highlight the possible involvement of the histaminergic system in outcomes related to neuropsychiatric disorders. Furthermore, it supports zebrafish as a tool to investigate mechanisms underlying these disorders.

Characterization of gastric and neuronal histaminergic populations using a transgenic mouse model

Histamine is a potent biogenic amine that mediates numerous physiological processes throughout the body, including digestion, sleep, and immunity. It is synthesized by gastric enterochromaffin-like cells, a specific set of hypothalamic neurons, as well as a subset of white blood cells, including mast cells. Much remains to be learned about these varied histamine-producing cell populations. Here, we report the validation of a transgenic mouse line in which Cre recombinase expression has been targeted to cells expressing histidine decarboxylase (HDC), which catalyzes the rate-limiting step in the synthesis of histamine. This was achieved by crossing the HDC-Cre mouse line with Rosa26-tdTomato reporter mice, thus resulting in the expression of the fluorescent Tomato (Tmt) signal in cells containing Cre recombinase activity. As expected, the Tmt signal co-localized with HDC-immunoreactivity within the gastric mucosa and gastric submucosa and also within the tuberomamillary nucleus of the brain. HDC expression within Tmt-positive gastric cells was further confirmed by quantitative PCR analysis of mRNA isolated from highly purified populations of Tmt-positive cells obtained by fluorescent activated cell sorting (FACS). HDC expression within these FACS-separated cells was found to coincide with other markers of both ECL cells and mast cells. Gastrin expression was co-localized with HDC expression in a subset of histaminergic gastric mucosal cells. We suggest that these transgenic mice will facilitate future studies aimed at investigating the function of histamine-producing cells.

Subdivisions of the turtle Pseudemys scripta hypothalamus based on the expression of regulatory genes and neuronal markers

The patterns of distribution of a set of conserved brain developmental regulatory transcription factors and neuronal markers were analyzed in the hypothalamus of the juvenile turtle, Pseudemys scripta. Combined immunohistochemical techniques were used for the identification of the main boundaries and subdivisions in the optic, paraventricular, tuberal, and mammillary hypothalamic regions. The combination of Tbr1 and Pax6 with Nkx2.1 allowed identification of the boundary between the telencephalic preoptic area, rich in Nkx2.1 expression, and the prethalamic eminence, rich in Tbr1 expression. In addition, at this level Nkx2.2 expression defined the boundary between the telencephalon and the hypothalamus. The dorsalmost hypothalamic domain was the supraoptoparaventricular region that was defined by the expression of Otp/Pax6 and the lack of Nkx2.1/Isl1. It is subdivided into rostral, rich in Otp and Nkx2.2, and caudal, only Otp-positive, portions. Ventrally, the suprachiasmatic area was identified by its catecholaminergic groups and the lack of Otp, and could be further divided into a rostral portion, rich in Nkx2.1 and Nkx2.2, and a caudal portion, rich in Isl1 and devoid of Nkx2.1 expression. The expressions of Nkx2.1 and Isl1 defined the tuberal hypothalamus, whereas only the rostral portion expressed Otp. Its caudal boundary was evident by the lack of Isl1 in the adjacent mammillary area, which expressed Nkx2.1 and Otp. All these results provide an important set of data on the interpretation of the hypothalamic organization in a reptile, and hence make a useful contribution to the understanding of hypothalamic evolution.

The histaminergic system in the brain: structural characteristics and changes in hibernation

Histaminergic neurons in adult vertebrate brain are confined to the posterior hypothalamic area, where they are comprised of scattered groups of neurons referred to as the tuberomammillary nucleus. Histamine regulates hormonal functions, sleep, food intake, thermoregulation and locomotor activity, for example. In the zebrafish, Danio rerio, histamine was detected only in the brain, where also the histamine synthesizing enzyme L-histidine decarboxylase (HDC) was expressed. It is possible that histamine has first evolved as a neurotransmitter in the central nervous system. We established sensitive quantitative in situ hybridization methods for histamine H(1) and H(2) receptors and HDC, to study the modulation of brain histaminergic system under pathophysiological conditions. A transient increase in H(1) receptor expression was seen in the dentate gyrus and striatum after a single injection of kainic acid, a glutamate analog. H(1) antagonists are known to increase duration of convulsions, and increased brain histamine is associated with reduced convulsions in animal models of epilepsy. No HDC mRNA was detected in brain vessels by in situ hybridization, which suggests lack of histamine synthesis by brain endothelial cells. This was verified by lack of HDC mRNA in a rat brain endothelial cell line, RBE4 cells. Both H(1) and H(2) receptor mRNA was found in this cell line, and the expression of both receptors was downregulated by dexamethasone. The findings are in agreement with the concept that histamine regulates blood-brain barrier permeability through H(1) and H(2) receptor mediated mechanisms. Hibernation is characterized by a drastic reduction of central functions. The activity of most transmitter systems is maintained at a very low level. Surprisingly, histamine levels and turnover were clearly elevated in hibernating ground squirrels, and the density of histamine-containing fibers was higher than in euthermic animals. It is possible that histamine actively maintains the low activity of other transmitters during the hibernation state.

Development of the histaminergic neurons and expression of histidine decarboxylase mRNA in the zebrafish brain in the absence of all peripheral histaminergic systems

The histamine-storing neural system in adult and developing zebrafish (Danio rerio) was studied with immunocytochemical and chromatographical methods. Furthermore, the gene for histidine decarboxylase was partially cloned and its expression mapped with in situ hybridization. The histamine-storing neurons were only seen in the caudal hypothalamus, around the posterior recess of the diencephalic ventricle. Almost all parts of the brain, except the cerebellum, contained at least some histamine-immunoreactive fibres. The ascending projections had the rostral part of the dorsal telencephalon as a major target. Descending projections terminated in the torus semicircularis, central grey and inferior olive. A prominent innervation of the optic tectum, which has not been reported in other fish, was seen. The in situ hybridization gave a strong signal in cells with the same anatomical position as the histamine-immunoreactive neurons. The first histamine-immunoreactive neurons appeared in the ventral hypothalamus at about 85 h post-fertilization, and at 90 h, immunoreactive fibres terminated in the dorsal telencephalon. The embryonic histamine production described in mammals was lacking in this species. Both immunocytochemical and chromatographical studies indicated that histamine is absent in all other parts of the zebrafish body, and no specific hybridization was seen in any other part of the fish than the hypothalamus. The zebrafish could therefore be a very useful model for pharmacological in vivo studies of the histaminergic system of the brain, since the powerful peripheral actions of histamine should be lacking in this species.

Histamine-immunoreactive neurons in the brain of the teleost Gasterosteus aculeatus L. Correlation with hypothalamic tyrosine hydroxylase- and serotonin-immunoreactive neurons

The distribution of putative histaminergic neurons in the brain of a teleost, the three-spined stickleback, was investigated by means of immunocytochemistry using specific antibodies against histamine (HA), and conventional microscopy as well as confocal laser scanning microscopy. Histamine-immunoreactive (HAir) neurons form discrete populations ventral to the nucleus of the posterior recess (NRP) and in the nucleus saccus vasculosus (NSV), which belong to the periventricular hypothalamic nuclei. The neuronal somata are subependymally located, and do not possess apical neurites contacting the cerebrospinal fluid. They give rise to both long-range and local axonal projections. The local projections give rise to a field of dense punctate immunoreaction dorsal to the NRP and lateral to the NSV. Long-range projections are comprised of ascending projections to the thalamus, habenula, preoptic area and dorsal telencephalon; and descending projections via the posterior tuberal nucleus, ventrally to the nucleus interpeduncularis, and dorsally into the central gray. HAir neurons occur together with serotoninergic cerebrospinal fluid-contacting (CSFc) neurons in the NRP, and with tyrosine hydroxylase-immunoreactive (THir) neurons in the NSV. Although HAir elements occur together with THir ones in many brain areas, direct contacts between the two neurotransmitter systems are rare. The putative histaminergic neurons in the brain of the three-spined stickleback constitute a very discrete neuronal system, with a major projection area in the dorsal telencephalon in a region which is considered homologous with the dorsal pallium of land vertebrates.

Connections of the basal telencephalic areas c and d in the turtle brain

Tracer substances were injected into the basal telencephalic areas c and d of the turtle brain. These areas (Acd) have recently been shown to be connected reciprocally with the dorsal spino-medullary region, though the particular subregions involved in these projections remained unclear. We demonstrated that the efferent projections of area d terminate predominantly within or immediately adjacent to the trigeminal nuclear complex and in the high cervical spinal gray. The dendritic domain of the vagus-solitarius complex and the dorsal column nuclear complex might also receive some basal telencephalic efferents. The afferent projections to Acd, on the other hand, arise predominantly in the dorsal column nuclei as defined according to cytoarchitectural and hodological criteria. A few retrogradely labeled cells were found in the vagus-solitarius complex, the principal trigeminal nucleus and the high cervical spinal cord. Numerous labeled cells were found in the dorsolateral isthmo-rhombencephalic tegmentum, especially the n. visceralis secundarius, the n. vestibularis superior and parts of the lateral lemniscal complex. Aminergic cell populations projecting to Acd were the n. raphes inferior and superior, the locus coeruleus, the substantia nigra, pars compacta and the ventral tegmental area. Other meso-diencephalic cell groups were the griseum centrale (including the n. laminaris of the torus semicircularis), the n. interpeduncularis dorsalis, the nucleus of the fasciculus longitudinalis medialis, the nucleus and the nucleus interstitialis of flm, the n. interstitialis commissuralis posterior and then n. caudalis. Several hypothalamic regions, the reuniens complex and the perirotundal region of the thalamus also appeared to project heavily to Acd. Telencephalic areas retrogradely labeled after injection of tracer into Acd and its immediate surroundings were the rostral part of the lateral (olfactory) cortex, adjacent regions of the basal dorsal ventricular ridge and the n. centralis amygdalae, the n. tractus olfactorius lateralis as well as the areas g and h. The data suggest that areas c and d may correlate best with the 'extended' amygdala in mammals; further correlation with structures similar to the ventral striopallidum, however, cannot be excluded. Homostrategies are discussed with regard to the processing of higher-order somatovisceral information in turtles, birds and mammals.

The distribution of histaminergic axons in the superior colliculus of the cat

The histaminergic projection from the hypothalamus to the superior colliculus was examined immunohistochemically in the cat brain using an antibody to histamine. The source of histaminergic fibers in the brain is a group of neurons in the posterior hypothalamus, located primarily in ventrolateral and periventricular regions and collectively referred to as the tuberomammillary nucleus. All laminae of the superior colliculus--including the superficial, intermediate, and deep layers, as well as the central gray--were blanketed with histamine-immunoreactive axonal fibers. Overall, labeling in the superior colliculus was moderately dense compared to other locations in the cat brain, with some variation in fiber density. Individual labeled fibers resembled histaminergic fibers described previously in the brain. Labeled axonal fibers showed infrequent branching and were beaded with numerous en passant varicosities that were typically 1 micron or smaller, but as large as 2.5 micron in diameter. Varicosity size differed significantly at different depths in the colliculus. The histaminergic projection appears to be separate from a previously reported, apparently non-histaminergic projection from neurons in the dorsal hypothalamic area to discrete regions of intermediate and deep colliculus. These results indicate that the histaminergic projection from the tuberomammillary nucleus of the hypothalamus projects extensively throughout the superior colliculus. Histamine, which is believed to act as a neuromodulator in the brain, is in a position to influence sensory and motor-related processes in every layer of the cat superior colliculus.

Histaminergic system in the cat hypothalamus with reference to type B monoamine oxidase

It is known that histamine (HA) and type B monoamine oxidase (MAO-B), an enzyme involved in its metabolism, are present in the posterior hypothalamus, but the sites where MAO-B intervenes in HA metabolism remain uncertain. The present study examined and compared the detailed distribution and morphology of neurons immunoreactive to HA (HA-ir) and MAO-B (MAO-B-ir) in the cat hypothalamus. HA-ir neurons were localized almost exclusively in the posterior hypothalamus with the largest group in the tuberomammillary nucleus and adjacent areas. MAO-B-ir staining was detected in the vast majority of HA-ir neurons, suggesting that the degradation of tele-methylhistamine (t-MHA), the direct metabolite of HA, may occur within these cells. Nevertheless, a few HA-ir cells showed no detectable or very weak MAO-B-ir labeling; a small group of neurons containing MAO-B alone was detected in the area dorsolateral to the caudal part of the arcuate nucleus. Numerous HA-ir axons and terminal-like structures were distributed unevenly in virtually all hypothalamic regions. One of their principal trajectories ascended through the ventrolateral part of the hypothalamus and rostrally formed an axon column, which ascended into the preoptic area and contributed fibers to the diagonal band of Broca and bed nucleus of the stria terminalis. Other HA-ir axons passed laterally, dorsal to the zona incerta or ventrally through a narrow zone dorsal to the optic tract. Numerous long HA-ir axons coursed dorsomedially from the ventrolateral posterior hypothalamus to the dorsal hypothalamic area. Many are oriented vertically to the thalamus in the midline. MAO-B-ir axons and fibers were detectable throughout the hypothalamus and overlapped the areas distributing HA-ir fibers. They were, however, weaker in staining intensity and apparently fewer than the HA-ir fibers. MAO-B-ir glial cells were numerous in all hypothalamic structures rich in HA-ir fibers. These results suggest that the metabolism of t-MHA may also occur within HA terminals and glial cells.

The histaminergic innervation of the lateral geniculate complex in the cat

The histaminergic innervation of the thalamic dorsal and ventral lateral geniculate nuclei and the perigeniculate nucleus of the cat was examined immunohistochemically by means of an antibody to histamine. We find histamine-immunoreactive neurons in the cat brain are concentrated in the ventrolateral portion of the posterior hypothalamus, confirming a previous report. However, this cell group also spreads into medial, dorsal, and extreme lateral regions of the posterior hypothalamus and extends as far rostral as the optic chiasm. Histamine-labeled fibers cover all regions of the lateral geniculate complex, but the density of labeling varies. The ventral lateral geniculate nucleus (vLGN) is most densely labeled, the A laminae of the dorsal lateral geniculate are sparsely labeled, and the geniculate C laminae and the perigeniculate nucleus show intermediate amounts of label. Thus, histaminergic fibers demonstrate a predilection for zones innervated by the W-cell system. Labeled fibers exhibit few branchings and numerous en passant swellings, lending a beaded appearance. The vLGN showed more instances of fibers with larger-sized swellings (up to 2 microns). Following injections of biotinylated tracers into the hypothalamus, we find labeled fibers throughout the lateral geniculate complex. The anterogradely labeled fibers resemble the histaminergic fibers in morphology, distribution, and relative bouton size. Thus, the hypothalamus appears to be the source of the histaminergic fibers in the lateral geniculate complex. Histamine-labeled fibers in the dorsal lateral geniculate nucleus (dLGN) exhibit uncommon ultrastructural morphology. Many extremely large, round, or elliptical vesicles fill the fiber swellings. Swellings are directly apposed to a variety of other dendritic and axonal profiles, but thus far no convincing synaptic contacts have been seen. The distribution and appearance of these histaminergic fibers resembles those reported for serotonergic fibers. Our results support the idea that histamine works nonsynaptically as a neuromodulator in the lateral geniculate complex, affecting the level of visual arousal.

Immunocytochemical and biochemical studies of histamine in the retina of the turtle Pseudemys scripta

A combination of immunocytochemical and biochemical methods was used to study histamine in the turtle retina. Histamine-like immunoreactivity was localized within paraboloids of certain cone photoreceptors by use of two different antisera directed against histamine. Preincubation of eyecups in Ringer's containing 10 microM histamine selectively increased the immunoreactivity of these photoreceptor paraboloids. The present localization of histamine in paraboloids indicated that, although histamine is in photoreceptors of the turtle retina, it may play some metabolic or neuromodulatory role, and not function as a neurotransmitter.

Regional distribution of histamine in the brain of non-mammalian vertebrates

The histamine contents in the brains of various species of non-mammalian vertebrates were determined by an HPLC-fluorometric method. The whole brain contents of histamine in birds (200-500 pmoles/g) were comparable to those in mammals, but were higher in reptiles (1000-13500 pmoles/g) and amphibia (1600-2200 pmoles/g) and lower in teleosts (10-50 pmoles/g). In all species, histamine was unevenly distributed, being present at highest concentrations in the diencephalon, except in teleosts, in which its content was highest in the telencephalon. The brain histamine contents were proportional to the reported densities of histamine-immunoreactive fibers.

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.

Organization of the histaminergic system in the brain of the teleost, Trachurus trachurus

To accumulate phylogenetic information on the central histaminergic system, we investigated the histaminergic system in the brain of a teleost, the jack mackerel (Trachurus trachurus), using the indirect immunofluorescent method with antiserum against histamine. A small number of histamine-immunoreactive cell bodies were observed in the posterior hypothalamus around the posterior recess. Histamine-immunoreactive fibers innervated the telencephalon, diencephalon, tegmentum, and rostral part of the medulla oblongata. The immunoreactive fibers were very sparse or absent in the olfactory bulb, optic tectum, cerebellum, caudal part of the medulla oblongata, spinal cord, and hypophysis. Ascending fiber bundles were seen in the basal hypothalamus, supplying fiber collaterals to the telencephalon and diencephalon, whereas descending fibers were observed in the midline of the lower brainstem. These findings suggest that the central histaminergic system of the jack mackerel is homologous to those of mammals, reptiles, and amphibians, although poorly developed compared with them. The histamine-immunoreactive neuronal cell bodies found in the border area between the mesencephalon and rhombencephalon of the river lamprey were not detected in the brain of the jack mackerel.