Histamine and tyrosine hydroxylase in developing rat brain

Histamine receptors, agonists, and antagonists in health and disease

Histamine in the brain is produced by a group of tuberomamillary neurons in the posterior hypothalamus and a limited number of mast cells in different parts of the brain. Four G-protein-coupled receptors mediate the effects of histamine. Two of these receptors, H3 and H4 receptors, are high-affinity receptors in the brain and immune system, respectively. The two classic histamine receptors, H1 receptor and H2 receptor, are well known as drug targets for allergy and gastric ulcer, respectively. These receptors have lower affinity for histamine than the more recently discovered H3 and H4 receptors. The H1 and H2 receptors are important postsynaptic receptors in the brain, and they mediate many of the central effects of histamine on, e.g., alertness and wakefulness. H3 receptor is a pre- and postsynaptic receptor, which regulates release of histamine and several other neurotransmitters, including serotonin, GABA, and glutamate. H4 receptor is found in cerebral blood vessels and microglia, but its expression in neurons is not yet well established. Pitolisant, a H3 receptor antagonist, is used to treat narcolepsy and hypersomnia. H1 receptor antagonists have been used to treat insomnia, but its use requires precautions due to potential side effects. H2 receptor antagonists have shown efficacy in treatment of schizophrenia, but they are not in widespread clinical use. H4 receptor ligands may in the future be tested for neuroimmunological disorders and potentially neurodegenerative disorders in which inflammation plays a role, but clinical tests have not yet been initiated.

Developmental roles of brain histamine

Histamine appears early during brain development, has been shown to regulate fetal and adult brain-derived stem cells in a receptor type-dependent manner, and has widespread actions on systems involved in arousal and movement. Developmental studies in both rodents and zebrafish have elucidated the spatiotemporal patterning of the histaminergic system and, in zebrafish, have revealed the mechanisms whereby histamine regulates the number of hypocretin/orexin (hcrt) neurons, which in turn may regulate the number of histaminergic cells. Recent demonstrations of increased numbers of histaminergic neurons in patients with narcolepsy highlight the importance, for our understanding of both normal and pathological brain function, of understanding these interactions. Here, we review recent research into the developmental roles of histamine and suggest key areas for future research.

Intracranial transplantation and survival of tuberomammillary histaminergic neurons

Investigations were undertaken to determine whether fetal histaminergic neurons in the tuberomammillary nucleus of the posterior hypothalamus survive intracranial transplantation to adult hosts. Two methods of transplantation were utilized. Grafts were placed either into the delayed cavity of a fimbria-fornix lesion or directly into the hippocampus using stereotaxic techniques. The tissue was taken from rat fetuses at embryonic days 16-17 and grafted into adult rats of either the Sprague-Dawley or the Fischer 344 strain. Routine histology and immunohistochemistry were used to evaluate the grafts. All transplants to Sprague-Dawley rats showed signs of rejection, while no signs of rejection were seen in any of the Fischer 344 rats. Transplants placed directly into the delayed fimbria-fornix cavity did not grow as well or contain as many surviving neurons as the intraparenchymal grafts. The largest number of surviving histamine-positive neurons was obtained with grafts of posterolateral blocks of hypothalamus from fetal day 17 placed directly into the CA1 region of the rostral hippocampal formation of Fischer 344 hosts. Histamine-immunoreactive cell bodies with neuritic outgrowth were found in all Fischer 344 rats that received hypothalamic grafts. Cell bodies exhibited histamine immunoreactivity evenly throughout the cytoplasm and had morphological characteristics resembling histaminergic neurons in situ. Axonal outgrowth extended throughout the grafted hypothalamic tissue, and was sometimes seen in the host hippocampal tissue as well. It is concluded that fetal histaminergic neurons survive transplantation to the adult hippocampal formation, and that this allograft procedure can supplement current strategies to investigate the function of histaminergic tuberomamillary neurons in the central nervous system.

Distribution of histamine-, 5-hydroxytryptamine-, and tyrosine hydroxylase-immunoreactive neurons and nerve fibers in developing rat brain

Although the general patterns of the developing histaminergic system in the rat brain are known, no comparative studies between the development of the brain histaminergic system and the development of other neuroactive substances have yet been published. Interestingly, separate immunohistochemical studies on the development of the 5-HT system and on the catecholaminergic system in the rat imply common features in the different aminergic systems. Therefore, the spatial distribution of histamine-immunoreactive (HA-ir) neurons and nerve fibers was compared to the distribution of 5-hydroxytryptamine (5-HT)-, and tyrosine hydroxylase-immunoreactive (TH-ir) ones in the developing rat brain between embryonic days 12 (E12) and 20 (E20) by using a double-immunostaining method. The high-pressure liquid chromatography (HPLC) fluorometric method was used for determination of histamine concentration in different brain regions during the same period of development and synthetic oligonucleotide probes complementary to the rat histidine decarboxylase (HDC) to determine the origin of HA in the brain during the development with in situ hybridization. The immunohistochemical results revealed co-localization of HA and 5-HT within a subgroup of cells in the developing raphe nuclei between E14 and E18. From E18 onwards HA immunoreactivity started to gradually disappear from the rhombencephalon, and was totally abolished by E20, while 5-HT-ir cells continued to establish their adult positions. No significant colocalization of HA and TH immunoreactivities was detected. The biochemical results were in agreement with the immunohistochemical ones and confirmed that histamine detected in the early developing brain is authentic. A positive in situ hybridization signal for HDC was detected in a small area in the ventrolateral pons in the same areas as HA- and HDC-ir cell bodies at E16, suggesting that at least some HA may be synthesized locally. These results confirm that HA is one of the first neurotransmitters to appear in the developing brain. In addition, the transient co-localization of HA and 5-HT immunoreactivities and the transient HDC expression at E16 within the developing pontine raphe nuclei may imply an interesting and a more general role for HA in modification of brain development.