We identified the cDNAs of three functional rat H3 receptor isoforms (H3A, H3B, and H3C) and one nonfunctional truncated H3 receptor (H3T). The H3A, H3B, and H3C receptor isoforms vary in the length of their third intracellular loop; the H3B and H3C receptor lack 32 and 48 amino acids, respectively. Transient expression of the H3A, H3B, and H3C receptors in COS-7 cells results in high affinity binding for the H3 antagonist [125I]iodophenpropit, which is displaced by selective H3 agonists and antagonists. The three isoforms differentially couple to the Gi protein-dependent inhibition of adenylate cyclase or stimulation of p44/p42 mitogen activated protein kinase (MAPK), a new signaling pathway for the H3 receptor. Whereas the H3A receptor was less effective in inhibiting forskolin-induced cAMP production compared with the H3B or H3C receptor, this isoform was more effective in the stimulation of p44/p42 MAPK. The H3 receptor isoforms also displayed differential CNS expression in key areas involved in regulation of sensory, endocrine, and cognitive functions. A differential H3 receptor isoform expression was seen in, for example, hippocampus, where a characteristic dorsoventral distribution was revealed. Differential H3 receptor expression was also characteristic for the cerebellum, indicating possible histaminergic regulation of motor functions. The identification of these new H3 receptor isoforms and their specific signaling properties adds a new level of complexity to our understanding of the role of histamine, and the H3 receptor in brain function. The heterogeneous distribution of the isoforms suggests that H3 receptor isoform-specific regulation is important in several brain functions.

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.

We described previously the cDNA cloning of three functional rat histamine H 3 receptor (rH 3 R) isoforms as well as the differential brain expression patterns of their corresponding mRNAs and signaling properties of the resulting rH 3A , rH 3B , and rH 3C receptor isoforms (Mol Pharmacol 59:1-8). In the current report, we describe the cDNA cloning, mRNA localization in the rat central nervous system, and pharmacological characterization of three additional rH 3 R splice variants (rH 3D , rH 3E , and rH 3F ) that differ from the previously published isoforms in that they result from an additional alternative-splicing event. These new H 3 R isoforms lack the seventh transmembrane (TM) helix and contain an alternative, putatively extracellular, C terminus (6TM-rH 3 isoforms). After heterologous expression in COS-7 cells, radioligand binding or functional responses upon the application of various H 3 R ligands could not be detected for the 6TM-rH 3 isoforms. In contrast to the rH 3A receptor (rH 3A R), detection of the rH 3D isoform using hemagglutinin antibodies revealed that the rH 3D isoform remains mainly intracellular. The expression of the rH 3D-F splice variants, however, modulates the cell surface expression-levels and subsequent functional responses of the 7TM H 3 R isoforms. Coexpression of the rH 3A R and the rH 3D isoforms resulted in the intracellular retention of the rH 3A R and reduced rH 3A R functionality. Finally, we show that in rat brain, the H 3 R mRNA expression levels are modulated upon treatment with the convulsant pentylenetetrazole, suggesting that the rH 3 R isoforms described herein thus represent a novel physiological mechanism for controlling the activity of the histaminergic system.

Histamine is implicated in the regulation of brain functions through three distinct receptors. Endogenous histamine in the brain is derived from mast cells and neurons, but the importance of these two pools during early postnatal development is still unknown. The expression of histamine H1-receptor in the rat brain was examined using in situ hybridization during postnatal development and in adults. For comparison, the expression of L-histidine decarboxylase (HDC) in the two pools was revealed. H1-receptor was evenly expressed throughout the brain on the first postnatal days, but resembled the adult, uneven pattern already on postnatal day 5 (P5). HDC was expressed in both mast cells and tuberomammillary neurons from birth until P5, after which the mast cell expression was no more detectable. In adult rat brain, high or moderate levels of H1-receptor expression were found in the hippocampus, zona incerta, medial amygdaloid nucleus and reticular thalamic nucleus. In most areas of the adult brain the expression of H1-receptor mRNA correlates well with binding data and histaminergic innervation. A notable exception is the hypothalamus, with high fibre density but moderate or low H1-receptor expression. Systemic kainic acid administration induced increased expression of H1-receptor mRNA in the caudate-putamen and dentate gyrus, whereas no change was seen in the hippocampal subfields CA1-CA3 or in the entorhinal cortex 6 h after kainic acid injections. This significant increase supports the concept that histaminergic transmission, through H1-receptor, is involved in the regulation of seizure activity in the brain.

Histaminergic neurotransmission seems to be necessary for the stimulatory effect of ethanol to occur, whereas lack of histamine leads to changes that enhance the conditioned reward by ethanol. Our findings also suggest a role for histamine H3 receptor in modulation of the ethanol stimulation and reward.

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The histaminergic system is thought to regulate brain reward, but the exact mechanisms are poorly understood. This study aims to reveal the pathophysiological differences in the brain histaminergic system between selectively outbred alcohol-preferring AA and alcohol-avoiding ANA rats by using a combination of biochemical, immunohistochemical, and molecular biological methods. We also want to test the functional significance of the system by using operant ethanol self-administration method. We show that alcohol-preferring AA rats, when compared with alcohol-avoiding ANA rats, display higher levels of brain histamine and its first metabolite as measured by HPLC and mass spectrometry, a higher density of histamineimmunoreactive nerve fibers, and lower H 1 receptor mRNA expression and H 1 and H 3 receptor binding as studied by using in situ hybridization and autoradiography. Two H 3 receptor antagonists, thioperamide and clobenpropit, reduced and an agonist, R-α-methylhistamine, increased operant responding for ethanol by the alcohol-preferring rats, whereas an H 1 receptor ligand, mepyramine, was inefficient. The results indicate that high alcohol preference is correlated with enhanced histaminergic mechanisms, most likely via H 3 receptor-mediated processes. Central histaminergic mechanisms may thus participate also in other addictive behaviors.Key words: histamine H 3 receptor • addiction • tuberomammillary nucleus • alcohol preference he role of dopamine and the mesocorticolimbic pathway in promoting reward functions concerning natural stimuli and drugs of abuse is well established (1, 2). Lesions of the histamine-containing tuberomammillary neurons (TM), the only known source of neuronal histamine in the brain (3), increase ipsilateral hypothalamic self-stimulation in rats (4, T 5), which indicates that the histaminergic TM neurons are implicated in the inhibitory control of reward functions (6). Furthermore, histamine, via its H 3 heteroreceptors, reduces the release and synthesis of dopamine (7,8), which should reduce reward (9). Therefore, histamine can be expected to strongly reduce reinforcement by natural stimuli and drugs of abuse, and to counteract the effects of the dopaminergic system in reward.Indeed, histamine has been implicated in the regulation of food intake and water balance. Histamine depletion by α-fluoromethylhistidine (α-FMH) increases feeding and body weight (10, 11), and icv injections of histamine or H 1 receptor agonist depress feeding behavior (12,13). In contrast to the food intake, histamine depletion by α-FMH does not affect water consumption (11), but H 1 and H 3 receptor agonists evoke a dose-dependent increase in it (14, 15).However, there have been no studies on brain histamine system in the central regulation of voluntary ethanol drinking and behavioral effects of ethanol by using well-characterized animal models displaying aberrant reward-related behavior. We therefore undertook this study on alcohol-preferring AA and alcohol-avoiding ANA rats that were developed by selective o...

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