Elevated CREB activity in embryonic fibroblasts of gene-targeted histamine deficient mice

OBJECTIVES

Histamine is a known inducer of cAMP-responsive element binding protein (CREB), which plays a key role in initiation of adipogenesis. Our present goal was to study how histamine deficiency impacts CREB signalling and adipogenesis.

METHODS

We used a histidine-decarboxylase gene-targeted (HDC KO) mice model lacking endogenous histamine. We measured CREB activity and expression by EMSA, Western blot and real-time RT-PCR, as well as cAMP levels by ELISA in primary embryonic fibroblasts derived from WT and HDC KO mice. The ability of these cells to form adipocytes was also tested in preliminary experiments.

RESULTS

We found that in the absence of the histamine, cells show higher constitutive CREB activity and greatly increased intracellular cAMP levels, as well as that in contrast to WT cells, HDC KO fibroblasts are more prone to differentiate into adipocytes.

CONCLUSION

These data suggest a newly recognised inhibitory role for histamine in CREB activity and draws attention to the potential role of histamine in adipocyte differentiation.

Histamine-induced ion secretion across rat distal colon: involvement of histamine H1 and H2 receptors

The aim of the present study was to investigate the effect of histamine, a product of e.g. mast cells, on short-circuit current (I(sc)) across rat distal colon. Histamine concentration-dependently stimulated an increase in I(sc), which often was preceded by a transient negative current. Neither a release of neurotransmitters nor a release of prostaglandins contributed to the histamine response. The histamine-induced increase in I(sc) was blocked by the histamine H(1) antagonist, pyrilamine, but was resistant against the histamine H(2) antagonist, cimetidine. Conversely, the histamine H(1) agonist, TMPH (2-(3-trifluoromethylphenyl)histamine), exclusively evoked an increase in I(sc), whereas the histamine H(2) agonist, amthamine, evoked only a decrease in I(sc) suggesting that stimulation of different types of histamine receptors is responsible for the two phases of the response evoked by native histamine. Histamine induces the opening of glibenclamide-sensitive Cl(-) channels and of charybdotoxin-sensitive K(+) channels in the apical membrane as demonstrated by experiments at basolaterally depolarized epithelia. A further action site is the basolateral membrane, because histamine stimulates a charybdotoxin- and tetrapentylammonium-sensitive K(+) conductance in this membrane as observed in tissues, in which the apical membrane was permeabilized with an ionophore, nystatin. The increase in I(sc) evoked by histamine was blocked after depletion of intracellular Ca(2+) stores with cyclopiazonic acid and after blockade of inositol 1,4,5-trisphosphate (IP(3)) receptors, suggesting a release of stored Ca(2+). This was confirmed by the observation that the histamine H(1) agonist TMPH induced an increase in the fura-2 ratio signal of epithelial cells within isolated colonic crypts. Consequently, the mediator histamine seems to stimulate both histamine H(1) and H(2) receptors, from which the former seems to be prominently involved in the induction of epithelial chloride secretion.

Selective expression of histamine receptors H1R, H2R, and H4R, but not H3R, in the human intestinal tract

BACKGROUND AND AIMS

Histamine is known as a regulator of gastrointestinal functions, such as gastric acid production, intestinal motility, and mucosal ion secretion. Most of this knowledge has been obtained from animal studies. In contrast, in humans, expression and distribution of histamine receptors (HR) within the human gastrointestinal tract are unclear.

METHODS

We analysed HR expression in human gastrointestinal tissue specimens by quantitative reverse transcription-polymerase chain reaction and immunostaining.

RESULTS

We found that H1R, H2R, and H4R mRNA were expressed throughout the gastrointestinal tract, while H3R mRNA was absent. No significant differences in the distribution of HR were found between different anatomical sites (duodenum, ileum, colon, sigma, and rectum). Immunostaining of neurones and nerve fibres revealed that H3R was absent in the human enteric nervous system; however, H1R and H2R were found on ganglion cells of the myenteric plexus. Epithelial cells also expressed H1R, H2R and, to some extent, H4R. Intestinal fibroblasts exclusively expressed H1R while the muscular layers of human intestine stained positive for both H1R and H2R. Immune cells expressed mRNA and protein for H1R, H2R, and low levels of H4R. Analysis of endoscopic biopsies from patients with food allergy and irritable bowel syndrome revealed significantly elevated H1R and H2R mRNA levels compared with controls.

CONCLUSIONS

We have demonstrated that H1R, H2R and, to some extent, H4R, are expressed in the human gastrointestinal tract, while H3R is absent, and we found that HR expression was altered in patients with gastrointestinal diseases.

Histamine, mast cells, and the enteric nervous system in the irritable bowel syndrome, enteritis, and food allergies

There is altered expression of histamine H₁ and H₂ receptor subtypes in mucosal biopsies from the terminal ileum and large intestine of patients with symptoms of food allergy and/or irritable bowel syndrome

Immunohistochemical Localization of Histamine Receptors in Rat Cochlea

OBJECTIVE

Histamine may have physiologic functions in the inner ear. The locations of histamine receptors, however, have not yet been identified in the mammalian cochlea. The aim of this study was to investigate the localization of histamine receptor subtypes (H1, H2, and H3 receptors) in rat cochlea.

METHODS

Immunohistochemistry was performed with antibodies specific for each of the histamine receptors (H1, H2, and H3). To identify the type I and II spiral ganglion cells in the cochlea, some cryostat sections were double stained with antibodies to both a histamine receptor and neurofilament 200 kD, which predominantly stains type II spiral ganglion cells in the cochlea.

RESULTS

All H1, H2, and H3 receptor immunoreactive staining was limited to the spiral ganglion cells of the cochlea. Spiral ganglion cells with positive immunoreactivity to the neurofilament 200 kD antibody were stained only slightly by histamine H1, H2, and H3 receptor antibodies, indicating that histamine receptor immunoreactivity is specific to type I ganglion cells.

CONCLUSIONS

These findings indicate that histamine receptors are present in the cochlea and support the hypothesis that histamine plays a physiologic role in the cochlea.

H1 and H2 histamine receptors are absent on Langerhans cells and present on dermal dendritic cells

Human monocyte-derived dendritic cells (MoDC) have both histamine H1 and H2 receptors and can induce CD86 expression by histamine. Nevertheless, it has not been reported whether human epidermal Langerhans cells (LC) have histamine receptors or not. In this study, using RT-PCR, we investigated the expression of H1 and H2 receptor mRNA on DC with the features of LC (LC-like DC) that were generated in vitro from peripheral blood monocytes, LC derived from CD34+ hematopoietic progenitor cells, and LC obtained from human epidermis. We compared the histamine-induced CD86 expression among these cells. In contrast to MoDC, LC and LC-like DC did not express H1 or H2 receptors. In addition, they could not augment the CD86 expression by histamine. Interestingly, when transforming growth factor-beta1 (TGF-beta1) was added to the culture of MoDC, the expression of H1 and H2 receptors and the histamine-induced CD86 expression were abrogated in a concentration-dependent fashion. Finally, in the assessment of the cell surface expression of histamine receptors using fluorescence-labeled histamine, histamine could bind to MoDC and dermal dendritic cells obtained from the skin, whereas there was no specific binding of histamine to LC-like DC or LC obtained from the skin. These data suggest that LC do not express either H1 or H2 receptors, mainly because of the effect of TGF-beta1. This made a striking contrast with the expression of the functional H1 and H2 receptors on MoDC and dermal dendritic cells.

New concepts of histamine receptors and actions

Histamine and antihistamines are so deeply woven into the fabric of allergic diseases that it is sometimes difficult to see how this field could advance beyond our current, potent histamine H1-receptor drugs. Investigations of other actions of histamine and the identification of H2, H3, and now H4 receptors have suddenly reignited the search for new mono- and multi-receptor-specific agonists and antagonists. There is great excitement due to preliminary findings that H3 receptors act as neural inhibitory autoreceptors, and H4 receptors might modulate immune cell functions.

Distribution of the histamine H(2) receptor in monkey brain and its mRNA localization in monkey and human brain

The distribution of histamine H(2) receptor mRNA was determined by in situ hybridization histochemistry in human and monkey brain. In the case of monkey brain, we combined this technique with receptor ligand autoradiography to compare the distribution of mRNA and receptor binding sites. [(125)I]Iodoaminopotentidine ([(125)I]-APT), a reversible, high specific activity antagonist with high affinity and selectivity for the H(2) receptor, was used for receptor autoradiography. Radiolabeled oligonucleotides derived from the human mRNA sequence encoding this receptor were used as hybridization probes. The highest density of the H(2) receptor mRNA in human and monkey brain was found in caudate and putamen nuclei and external layers of cerebral cortex. Moderate levels were seen in the hippocampal formation and lower densities in the dentate nucleus of cerebellum. Areas such as globus pallidus, amygdaloid complex, cerebellar cortex, and substantia nigra were devoid of hybridization signal. The distribution of H(2) receptor mRNA in monkey brain is generally in good agreement with that of the corresponding binding sites: prominent in caudate, putamen, accumbens nuclei, and cortical areas. The hippocampus showed lower densities of receptors and low levels were detected in the globus pallidus pars lateralis. No binding sites were seen in amygdaloid complex and substantia nigra. The distribution of histaminergic innervation is in good correlation with the areas of high density for H(2) receptors: caudate, putamen, and external layers of cerebral cortex in monkey and human brain. The presence of mRNA in caudate and putamen nuclei, together with its absence from substantia nigra, suggests that the H(2) receptors found in the striatum are synthesized by intrinsic cells and not by nigral dopaminergic cells. These striatal H(2) receptors may be located on short circuit striatal interneurons or somatodendritically on striatal projection neurons which project to the globus pallidus pars lateralis. In conclusion, the present results, which constitute, to our knowledge, the first report of the regional distribution of mRNA encoding H(2) receptors detected by in situ hybridization, define the sites of synthesis of H(2) receptors and are the basis for future, more detailed studies that should result in a better understanding of H(2) receptor function.

Expression of histamine H1 and H2 receptors in human nasal mucosa

mRNA encoding histamine H1 and H2 receptors were detected in the human nasal mucosa using reverse transcriptase-polymerase chain reaction. The possibility of local release in connection with specific target receptors suggests a role for histamine in the regulation of vascular tone, glandular secretion and epithelial functions.

Effects of acute glucose overload on histamine H2 receptor-mediated Ca2+ mobilization in bovine cerebral endothelial cells

[Ca2+]i and whole-cell membrane current were measured in microvascular endothelial cells from bovine brain. The effects of histamine on [Ca2+]i were examined, and the acute effect of changing extracellular glucose concentration on Ca2+ homeostasis was investigated. Application of 10 micromol/l histamine evoked an initially transient and then sustained increase in [Ca2+]i in normal Krebs solution, but only the transient component in Ca2+-free solution, thereby indicating that histamine mobilizes Ca2+ both from intracellular store sites and extracellular space. The effects of histamine on [Ca2+]i were inhibited by the H2 antagonists, ranitidine and cimetidine, but not by the H1 antagonist, pyrilamine. Incubation of the cells for 2 h in solutions containing low (1.1 and 2.3 mmol/l) or high (23 mmol/l) concentrations of glucose did not influence the resting level of [Ca2+]i. Treatment with low concentrations of glucose did not impair histamine-induced Ca2+ mobilization. On the other hand, when histamine was applied to the cells pretreated with 23 mmol/l glucose, it failed to mobilize Ca2+ from both intracellular store sites and extracellular space. The effect of histamine was mimicked by dibutyryl cyclic AMP, but glucose overload failed to inhibit this, suggesting that glucose overload inhibits H2 receptor-mediated cyclic AMP production. Glucose overload-induced impairment of histamine action was reversed by pretreatment with staurosporine and calphostin C and mimicked by phorbol-12,13-dibutyrate, thereby suggesting the involvement of protein kinase C in the high glucose-induced inhibition of Ca2+ mobilization. Whole-cell membrane current measurement showed that there was no difference in the membrane currents between control and high glucose-treated cells. These results indicate that in bovine brain microvascular endothelial cells, histamine induces Ca2+ release from intracellular store sites and subsequent entry from the extracellular space through the activation of H2 receptors. Glucose overload acutely inhibits histamine-induced Ca2+ mobilization by the activation of protein kinase C.

Visualization of agonist-induced internalization of histamine H2 receptors

Histamine H2 receptors were tagged at the N-terminus with the eight amino acid Flag epitope to allow the immunological identification of the receptor peptide with the monoclonal anti-Flag M2 antibody. The introduction of the epitope did not modify the binding of several H2 ligands to the H2 receptor, nor the ability of histamine to stimulate the H2 receptor mediated cAMP production in HEK-293 cells. Western blots revealed a major protein band of 57 +/- 1 kDa, whereas a second band of 31 +/- 1 kDa was probably the result of a proteolytic breakdown of the 57 kDa band. Immunofluorescence measurements of stably transfected HEK-293 cells revealed the presence of anti-Flag-immunoreactivity in the plasma membrane. This immunoreactivity completely disappeared after a one hour treatment with histamine. The receptor internalization was reversible and blocked by the endocytosis inhibitor phenylarsine oxide. Forskolin did not induce H2 receptor internalization, indicating that histamine causes H2 receptor internalization via a cAMP-independent pathway.

[Cellular histamine and bradykinin receptors in the larynx and trachea. I]

Classic physiological and pharmacological experiments were made on 33 rabbits under hexenal anesthesia to determine the presence, topical concentration and activity of H2 and B2-receptors. These parameters were inferred by edema and other vascular responses of the mucosa to local application of histamine and bradykinin prior to and after usage of specific blocker of each receptor type. Both mediators and receptor blockers were applied to laryngeal and tracheal mucosa for 10 min (histamine 10(-4) and 10(-3) g/ml, H2-receptor blocker metiamide 0.1 and 1 mmol, bradykinin 10(-5) and 10(-4) g/ml, B2-receptor blocker pyridinolcarbamate 10(-2) and 10(-1) g/ml). The larynx and trachea were found to contain H2- and B2-receptors the concentrations of which were greater in the subvocal part of the larynx than in the vestibular part and trachea. B2-receptor activity and concentrations in the subvocal part of the larynx were 10 times while in the trachea 1.5-2 times greater than those of H2-receptors. These regularities were not influenced by individual varieties of receptor topical concentrations. The blocker density in the receptors and the duration of their binding were not universally proportional. This is also true for the relations mediator-receptor. Concentrations and activity of H2- and B2-receptors are thought important in pathogenesis and location of acute inflammatory and allergic conditions in the upper respiratory tract.

Histaminergic regulation of antibody-dependent cellular cytotoxicity of granulocytes, monocytes, and natural killer cells

We investigated effects of the biogenic diamine histamine on antibody-dependent cellular cytotoxicity (ADCC) against autologous anti-D-coated red blood cells mediated by human granulocytes, monocytes, and natural killer (NK) cells. Effector cells were separated from peripheral blood by countercurrent centrifugal elutriation. ADCC of monocytes and neutrophilic granulocytes was suppressed by histamine. ADCC of enriched CD3-/56+ NK cells was unchanged by histamine. ADCC of NK cells was effectively inhibited by elutriated monocytes or neutrophils. Histamine completely reversed the inhibition of NK cell-mediated ADCC induced by monocytes and partly reversed the inhibition induced by neutrophils; thereby, histamine augmented ADCC of NK cells in the presence of monocytes or neutrophils. The indirect effect of histamine on ADCC of NK cells and the effect of histamine on ADCC of monocytes/neutrophils were completely antagonized by the specific H2 receptor (H2R) blocker ranitidine. We conclude that activation of H2R suppresses ADCC reactivity of monocytes/neutrophils and, concomitantly, promotes ADCC reactivity of NK cells by abrogating a phagocyte-derived, suppressive signal.

Localization of the histamine H2 receptor and gene transcripts in rat stomach: back to parietal cells

In contrast with many physiological studies suggesting that histamine H2 receptors are present on acid-secreting parietal cells of the gastric epithelium, it was recently shown that immune cells in the lamina propria are the only cells expressing H2-receptor mRNAs (Mezey and Palkovits, Science, 1992, 258, 1662-1665). We have reinvestigated the cellular localization of H2 receptors in the rat stomach by visualizing both the H2 receptor mRNA and the H2-receptor protein itself. In situ hybridization histochemistry performed with an antisense riboprobe for the rat H2 receptor, and autoradiographic distribution of 125I-aminopotentidine binding sites, a highly selective H2-receptor ligand, did not show any labeling of the lamina propria. Signals were clearly and solely detected in the gastric epithelium, the strongest being observed in the upper part of the glands where the H2 receptor gene transcripts were only detected within parietal cells. In situ hybridization performed with an antisense riboprobe for L-histidine decarboxylase mRNA confirmed the basal localization of the histamine-synthetizing cells in the rat gastric gland, at some distance from parietal histamine-sensitive cells.

Histamine receptors on bovine peripheral blood lymphocytes

Histamine receptors on bovine peripheral blood lymphocytes (PBL) were detected by three different methods: a rosetting technique, binding to histamine-bearing Sepharose beads and immunofluorescence staining. The rosetting technique used histamine-rabbit serum albumin (H-RSA) conjugated to bovine red blood cells to detect histamine receptors and this showed that 10.8% of bovine PBL were positive. A method using H-RSA conjugate coupled Sepharose beads also detected histamine receptor bearing PBL but was not quantitative. The indirect immunofluorescence method, by which the subpopulation of histamine receptor bearing lymphocytes can be easily double stained to concurrently identify the B cell marker, revealed that PBL, the B cell and T cell fraction of bovine PBL contained 18.4, 52.8 and 9.3% histamine receptor bearing cells, respectively. This method was found to be more stable and more easily quantifiable than the other two methods. Blocking tests using the histamine H1 receptor antagonist diphenhydramine and the histamine H2 receptor antagonist cimetidine suggested that bovine PBL have both H1 and H2 receptors on their surfaces. Addition of histamine into cultures of PBL at the concentration range 10(-6) to 10(-3) M suppressed the response of PBL to the mitogen phytohemagglutinin. The histamine induced suppression of mitogenesis could be reduced partially by the H2 receptor antagonist cimetidine, but not by the H1 antagonist diphenhydramine. It is possible that histamine induced suppression of PBL mitogenesis was mediated by H2 receptors on T cells.

Loss of striatal histamine H2 receptors in Huntington's chorea but not in Parkinson's disease: comparison with animal models

Autoradiographic techniques were used to study the distribution of histamine H2-receptors as labeled with [125I]iodoaminopotentidine in the brains of patients affected by human neurodegenerative pathologies, as compared with control cases. The highest levels of histamine H2 binding sites in control cases were found in the caudate, putamen, and accumbens nuclei. In Huntington's chorea, the levels of histamine H2-receptor binding sites were found to be markedly decreased in virtually all regions examined, particularly in the putamen and globus pallidus lateralis. The loss of binding sites was related to the grade of the disease. Losses were more marked in grade III disease cases. The possible influence of neuroleptic treatment, commonly used in Huntington's patients, was studied by including samples from clinically treated schizophrenic patients. A moderate increase in the densities of [125I]iodoaminopotentidine was found in the globus pallidus of these patients. In Parkinson's disease, the levels of histamine H2-receptor binding sites were found not to be significantly different from those of control cases. These results were comparable with those obtained from unilaterally neurotoxin-lesioned guinea pigs. Similar losses of binding sites were observed in the quinolinic acid lesioned striatal intrinsic neurons in the guinea pig, whereas lesioning dopaminergic cell bodies in the substantia nigra with 6-hydroxydopamine did not produce any significant change. These results strongly suggest that histamine H2-receptors are expressed by striatal neurons, which degenerate in Huntington's chorea, but not by nigral dopaminergic neurons and may play a role in the regulation of the intact striato-nigral pathway.

The study of histamine H1- and H2-receptors in human lung cancer

Data on human lung histamine H1- and H2-receptors in cancer and chronic inflammatory processes are reported. It has been found that the number of histamine H1-receptors significantly increases both in cancer and chronic pneumonia and does not practically change in tuberculosis lung parenchyma. The binding parameters of histamine H2-receptors both in cancer and inflammatory processes were similar to those obtained for the normal tissue. The important role of parenchymal histamine H1-receptors in the neuromodulation of airways in human lung adenocarcinoma is discussed.

Coexistence of histamine H1 and H2 receptors coupled to distinct signal transduction pathways in isolated intestinal muscle cells

Histamine receptors were characterized in muscle cells isolated from the longitudinal muscle layer of guinea pig intestine. Histamine caused concentration-dependent contraction and stimulated significant increases in [Ca++]i (141 +/- 16 nM, P < .001) and cyclic AMP (cAMP) (10.3 +/- 0.5 pmol/10(6) cells, P < .001). The H1 receptor antagonist, mepyramine, inhibited contraction and the increase in [Ca++]i, whereas the H2 receptor antagonist, cimetidine, augmented contraction and the increase in [Ca++]i (277 +/- 31 nM, P < .01) and abolished the increase in cAMP. In cells maximally contracted with a nonhistamine agonist [cholecystokinin octapeptide (CCK)], histamine caused concentration-dependent relaxation when the contractile effect mediated by H1 receptors was blocked with mepyramine; relaxation was inhibited by cimetidine in a concentration-dependent fashion. The effects mediated by H1 and H2 receptors were characterized further in cells in which only one receptor type was preserved by selective receptor protection. In cells in which only H1 receptors were preserved, histamine caused contraction that was inhibited by mepyramine; no relaxation of CCK-induced contraction could be elicited by histamine in the presence of mepyramine. Conversely, in cells in which only H2 receptors were preserved, histamine had little or no contractile effect by itself but caused relaxation of CCK-induced contraction in the absence of mepyramine. We conclude that in, intestinal muscle cells, H1 receptors mediating Ca(++)-dependent contraction coexist with H2 receptors mediating cAMP-dependent relaxation. The effect of histamine reflects activation of dominant H1 receptors as well as of H2 receptors which act to attenuate [Ca++]i and contraction by stimulating an increase in intracellular cAMP.

Identification of endothelial H1, vascular H2 and cardiac presynaptic H3 receptors in the pithed rat

In pithed and vagotomized rats the effects of the H3 receptor agonist R-(-)-alpha-methylhistamine, the H1 receptor agonist 2-(2-thiazolyl)ethylamine and the H2 receptor agonist dimaprit on basal diastolic blood pressure, basal heart rate and the electrically induced rise in heart rate were examined. Basal diastolic blood pressure was not altered by low, but increased by high doses of R-(-)-alpha-methylhistamine; the latter effect was not affected by selective H1, H2 or H3 receptor antagonists and by prazosin, but was attenuated by rauwolscine. Rauwolscine also unmasked a vasodepressor response to R-(-)-alpha-methylhistamine not affected by the H3 receptor antagonist thioperamide, but counteracted by the H1 receptor antagonist dimetindene or the H2 receptor antagonist ranitidine. The vasodepressor responses to 2-(2-thiazolyl)ethylamine and dimaprit were antagonized by dimetindene and ranitidine, respectively. The vasodepressor response to 2-(2-thiazolyl)ethylamine was not altered by indomethacin, but reduced by an inhibitor of endothelial nitric oxide synthase, N omega-nitro-L-arginine methyl ester (which, by itself, markedly increased blood pressure). Both drug tools did not alter the effect of dimaprit. Basal heart rate was not affected by 2-(2-thiazolyl)ethylamine (examined after administration of propranolol), dimaprit and R-(-)-alpha-methylhistamine. The electrically induced increase in heart rate (studied in animals which had received rauwolscine) was decreased by R-(-)-alpha-methylhistamine but not affected by 2-(2-thiazolyl)ethylamine and dimaprit. The effect of R-(-)-alpha-methylhistamine was abolished by thioperamide. R-(-)-alpha-methylhistamine did not influence the increase in heart rate produced by isoprenaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of histamine receptors in isolated pig basilar artery by functional and radioligand binding studies

Histamine receptors in pig basilar arteries were investigated in vitro by radioligand binding assays and by measuring the contractile and relaxant responses to histamine. Histamine and 2-pyridylethylamine (H1-agonist) induced concentration-dependent contractions, whereas impromidine (H2-agonist) induced concentration-dependent relaxations. These responses were independent of the presence of endothelial cells. Diphenhydramine (H1-antagonist) partially reversed the histamine-induced contractions to relaxations. Cimetidine (H2-antagonist) potentiated the contraction in a concentration-dependent manner. In the presence of cimetidine, the pEC50 value of histamine for the contraction was 6.30, and diphenhydramine competitively antagonized the histamine-induced contractions (pA2, 7.77). In the presence of diphenhydramine, the pEC50 value of histamine for the relaxation was 5.93, and cimetidine competitively antagonized the histamine-induced relaxations (pA2, 6.62). In the binding studies, the Kd value of [3H]mepyramine was 2.1 nM and the Bmax value was 95.6 fmol/mg protein. A competition experiment with diphenhydramine showed that the pKi value (7.51) was similar to the pA2 value. The Kd value for [3H]cimetidine was 126.0 nM and the Bmax value was 459.8 fmol/mg protein. The pKd (6.90) for [3H]cimetidine was similar to the pA2 for cimetidine. The Hill coefficients for these experiments were not significantly different from unity. The present findings indicate that the number of H1-receptors, in terms of the Bmax value for [3H]mepyramine, is smaller than that of H2-receptors, in terms of the Bmax value for [3H]cimetidine. However, the contractile response to histamine is predominantly mediated through stimulation of H1-receptors on vascular smooth muscle cells in pig basilar artery.

Histamine dependence of pentagastrin-stimulated gastric acid secretion in rats

Does gastrin stimulate gastric acid secretion by direct action on oxyntic cells, by releasing histamine, or by being potentiated by histamine? Previous studies in the mouse pointed to gastrin-regulated histamine release. Guinea pig and rat are well known to vary in their sensitivity to histamine. Therefore, the effects of histamine and pentagastrin were compared quantitatively on isolated, lumen-perfused, stomach preparations from these species in the absence and presence of histamine H2-receptor blockade. The loss of potency of histamine in the rat was mirrored by a loss of potency of pentagastrin consistent with the idea that pentagastrin acts by releasing histamine. In the rat, a well-defined pentagastrin curve was obtained in the presence of histamine H2-receptor block as though pentagastrin acts both directly on the oxyntic cell and indirectly by releasing histamine. It was not necessary to invoke a potentiating interaction between histamine and pentagastrin at the oxyntic cell; the two effects appeared simply to add. Potentiation was observed, however, between other combinations of stimuli, for example, between vagal nerve and pentagastrin stimulation. The physiological consequences of these results are discussed.

[A study of histamine receptors in peripheral lymphocytes in asthmatic subjects]

The identification and quantitative analysis of specific histamine H1 and H2 receptors on lymphocytes were acquired and histamine-induced suppressor cell activity was examined by using 3H-histamine radioligand-binding assay and co-culture technique. The quantitative analysis of H2 receptors decreased on the lymphocytes of extrinsic asthmatic subjects. Peripheral-lymphocytes from asthmatic subjects generated less histamine-induced suppressor activity than did those from normal individuals. In the asthmatics, the functional suppressor-cell abnormality positively correlated with the decreased quantity of H2 receptors on the lymphocytes.

Characterization of suppressor T cells for antibody production by chicken spleen cells. II. Comparison of CT8+ cells from concanavalin A-injected normal and bursa cell-injected agammaglobulinaemic chickens

The phenotypes of two different types of suppressor T cells in the chicken, both capable of inhibiting secondary antibody responses in vitro, were determined. The first of these, induced by injection of concanavalin A (Con A) into normal chickens, was CT8+, TcR2+ (alpha beta), CT4-, TcR1- (gamma delta). These cells appeared to exhibit histamine type 2 (H2) receptors, as they adhered to cimetidine-BSA-coated dishes. Moreover, cimetidine added to the medium at 2 x 10(-4) M completely prevented the suppression induced by these suppressor cells. The second type of 'suppressor' T-cell studied, induced in agammaglobulinaemic (A gamma) chickens by injection of bursa cells, exhibited the same phenotype, but was insensitive to cimetidine and did not adhere to cimetidine-BSA-coated dishes, indicating heterogeneity with respect to H2 receptor expression on CT8+ chicken T cells with suppressor activity. The results also showed that a relatively larger proportion of CT8+ than of CT4+ cells adhered to cimetidine-BSA-coated dishes and thus appeared to be H2 receptor positive. TcR1 (gamma delta) cells did not contribute significantly to the antigen non-specific suppressor effects examined in this study.

Inotropic, electrophysiological and biochemical responses to histamine in rabbit papillary muscles: evidence for coexistence of H1- and H2-receptors

This study was performed to determine the subtypes of histamine receptors that are involved in the electrophysiological, inotropic and biochemical responses to histamine in isolated rabbit papillary muscles. Histamine increased force of contraction and shortened action potential duration (APD) in a concentration-dependent manner. The former was antagonized by chlorpheniramine, a H1-antagonist, whereas the latter was blocked by cimetidine, a H2-antagonist. However, even when H1-receptors were blocked entirely by chlorpheniramine, histamine still produced a positive inotropic effect, an effect which was antagonized by cimetidine. On the other hand, when H2-receptors were eliminated by cimetidine, histamine caused a H1-receptor mediated APD prolongation. Carbachol attenuated the decrease in APD but not the increase in force of contraction caused by histamine. Cyclic AMP and cyclic GMP levels both were elevated significantly by histamine. The increase in cyclic AMP level induced by histamine was abolished by cimetidine, but not altered by chlorpheniramine, whereas the converse was true for the increase in cyclic GMP level. Additionally, histamine produced a significant stimulation of phosphoinositide hydrolysis as measured by [3H]inositol monophosphate accumulation, although its extent was far less than that produced by carbachol. The phosphoinositide response to histamine was blocked by chlorpheniramine. These data suggest that H1- and H2-receptors coexist in rabbit ventricles. Stimulation of H1- and H2-receptors with histamine independently sets off the biochemical responses linked specifically to the respective subtypes of histamine receptors. On the other hand, the inotropic and electrophysiological responses to histamine are governed predominantly by H1- and H2-receptors, respectively, and this results in an apparent restriction of the expression of the responses mediated by another subtype.

Synergistic activation of human natural killer cell cytotoxicity by histamine and interleukin-2

Interleukin-2 (IL-2) is recognized as a major activating signal for human natural killer (NK) cells. The presence of monocytes alters NK cell responsiveness to IL-2. Thus, while IL-2 effectively augments NK cell cytotoxicity (NKCC) in monocyte-depleted NK effector cells, it is relatively ineffective or even suppressive for NK cells in monocyte-containing, NK-cell-enriched mononuclear cell fractions. Here we report that concomitant treatment with IL-2 and the biogenic amine histamine synergistically augmented NKCC against K562 and Daudi target cells of CD3-/CD16+ human NK cells in the presence but not in the absence of monocytes. Addition of peripheral-blood monocytes, recovered by countercurrent centrifugal elutriation, to purified NK cells abrogated IL-2 induced NK cell activation, reconstituted the synergistic, NK-activating effects of histamine and IL-2, and strongly reduced baseline NKCC. The effects of histamine on baseline NKCC and on NK cell responsiveness to IL-2 were related to counteraction of monocyte-mediated NK cell suppression. By contrast, histamine did not affect baseline or IL-2-induced NKCC in mixtures of NK cells and monocytes when the latter cells were recovered after adherence. The effect of histamine on NK cell responsiveness to IL-2 was mediated by H2-type histamine receptors, as judged by mimicry exerted by the specific H2 receptor agonist dimaprit, but not by an H2-receptor-inactive derivative of this compound, N-methyldimaprit, and blocking by the H2 receptor antagonist ranitidine. Experiments in which monocytes and nonadherent NK cells were separately pretreated with ranitidine prior to histamine treatment suggested that NK-regulatory effects of histamine were mediated by H2 receptors on monocytes. Our data suggest that histamine may provide an important signal in the regulation of NK cell responsiveness to IL-2.

Biodistribution and radiation absorbed dose of (N-methyl[11C])pyrilamine: a histamine H-1 receptor radiotracer

The in vivo biodistribution for (N-methyl[11C])pyrilamine in mice is reported for various times after i.v. bolus injection, together with estimates of the radiation absorbed dose for the same radiotracer in man. More than 60% of the injected dose was excreted via the kidneys in urine after 90 min post injection. The brain regional distribution in mice showed a favorable hypothalamus-to-cerebellum ratio, indicative of H-1 receptor binding, encouraging in vivo histamine H-1 receptor imaging studies in man.

Demonstration of histamine H2 receptors on human melanoma cells

Histamine induced a concentration-dependent increase in intracellular cyclic-AMP of the two human melanoma cell lines SK23 and DX3.LT5.1; maximal stimulation was obtained with 17.8 microM histamine which consistently produced greater than 50-fold increases in the cyclic AMP content of both cell lines. The dose-response curve for histamine in each culture was progressively displaced to the right with increasing concentrations of the histamine H2 receptor antagonist cimetidine. Ranitidine, another H2 receptor antagonist also prevented the histamine-induced cyclic AMP elevation, but the H1 receptor antagonists mepyramine and tripelennamine had no significant effect. These findings indicate that human melanoma cells express histamine H2 receptors, stimulation of which activates adenylate cyclase with a subsequent rise in intracellular cyclic AMP. Mast cell:melanoma interactions mediated by histamine in vivo might therefore be expected to modify some aspects of melanoma cell behaviour.

Biochemical and pharmacological characterization of histamine-mediated up-regulation of human platelet serotonin uptake. Evidence for a subclass of histamine H2 receptors (H2h) highly sensitive to H2 receptor antagonists

The stimulatory effect of histamine: H (1.2 to 3-fold increase) on serotonin (5-HT) uptake by human platelets was observed after a 5 min incubation period in the presence of 2.5 X 10(-7) M histamine, followed by subsequent 5 min incubation of the platelets with 10(-7) M [3H] 5-HT. Methyl, ethyl and acetyl substituents in the side chain of H mimicked the stimulatory effect of H. In contrast, H analogs methylated at the position N-1 of the imidazole ring of H, as well as imidazole and histidine inhibited platelet 5-HT uptake. The cAMP-inducing agents forskolin and theophylline have no effect on 5-HT uptake when they are tested alone or in combinations with H. In contrast, the cGMP-inducing agent sodium nitroprusside (10(-7) M-10(-6) M) stimulated and potentiated H-mediated up-regulation of 5-HT uptake. Histamine H2 receptor agonists and antagonists are more potent than drugs acting on H1 receptors (H2 greater than H1). However, the inhibition constants Ki are not consistent with those determined for typical H1, H2, H3 receptors characterized in other tissues. This findings provide further evidence for the existence of multiple forms of H receptors and suggest the involvement of a subpopulation of H2 receptors, highly sensitive to H2 receptor antagonists (H2h), mediating 5-HT uptake in human platelets.

Histamine H2 receptor activity and histamine metabolism in human U-937 monocyte-like cells and human peripheral monocytes

Functional and specific histamine H2 receptors were characterized in human peripheral monocytes and in U-937 cells, before and after retinoic acid--induced differentiation into monocyte/macrophage-like cells. The relative potencies of histamine and the H1, H2 receptor agonists and antagonists studied are remarkably similar in U-937 cells and U-937 monocytes. There is no change in histamine concentration and activity of the enzymes forming and degrading histamine during monocytic-like differentiation. The results raise the possibility that histamine H2 receptors might be involved in pathophysiological regulations (proliferation/differentiation and biological function) of normal and leukemic monocytes.

A human T-T-cell hybridoma-derived lymphocyte chemoattractant factor

Human T-T hybridomas were developed as a strategy for obtaining lymphokines that alter T-lymphocyte motility. Mitogen-stimulated human T lymphocytes were fused with cells of the human CEM lymphoma line and the supernatants derived from these fusion products were assessed for chemoattractant activity in a modified Boyden chamber assay. Supernatants from hybridoma 41B2 enhanced lymphocyte migration to 198 +/- 13% (mean +/- SEM) of control. Characterization by Sephadex G-100 molecular sieve chromatography revealed a single peak of chemoattractant activity corresponding to a molecular weight (MW) of 56,000. This activity eluted from a Sephadex QAE anion-exchange column at 4-6 mS. Subsequent isoelectric focusing in sucrose revealed an isoelectric point of 9.0-9.2. Fractions with activity after sequential molecular sieve and anion-exchange chromatography were concentrated, radiolabeled with 125I, and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Autoradiography revealed a band which corresponded to a MW of 14,000 (representing four similar monomeric chains) and to the region from which chemoattractant activity could be detected in eluates from slices of unstained gels run in parallel. The biological activity of this hybridoma-derived lymphocyte chemoattractant was abolished by treatment with trypsin and neuraminidase but was unaffected by heating to 56 degrees C. We conclude that certain human T-T-cell hybridomas constitutively elaborate a lymphocyte chemoattractant that appears to be physicochemically identical to a previously described human lymphokine, lymphocyte chemoattractant factor.

Histamine H2-binding on guinea pig cerebral cortex

The Kd-values of some histamine H2-active compounds, obtained from radio-ligand-binding studies on a homogenate of the guinea-pig cerebral cortex with 3H-tiotidine as the labelled H2-ligand, were compared with the pA2/pD2-value of these compounds on the guinea-pig right atrium and guinea-pig isolated gastric fundus. A good correlation was found between the pKd of the H2-antagonists and their pA2 on the guinea-pig right atrium. A much poorer correlation however was obtained between the pKd of the agonists on the cerebral cortex and their pD2-values on the guinea-pig right atrium and the gastric fundus. This poor correlation between true affinity and H2-activity of the agonists might be explained by spare receptors as a much better correlation was obtained between pKd and pD2 of partial agonists.

Desensitization by histamine of H2 receptor activity in HGT-1 human cancerous gastric cells

Histamine produced a time-dependent (half-life: 20 min at 37 degrees C), temperature-dependent (no effect at 20 degrees C) and homologous desensitization of histamine H2 receptor activity (H2 R) in HGT-1 cells. Maximal and half-maximal desensitization were respectively observed at 10(-5) and 2 X 10(-7) M histamine. Decline of responsiveness in intact cells was related to a remarkable loss in histamine efficacy (from 15- to 2-fold stimulation in control and treated cells). The affinity of the H2R for histamine (EC50 = 10(-5) M) did not change during desensitization. Paradoxically, histamine treatment is associated with increased [3H] histamine binding capacity in intact HGT-1 cells, and no change in H2 receptor antagonist binding ([3H]-tiodine and [3H]-SKF 93479). Desensitization process was preferentially mimicked by H2 receptor agonists (impromidine greater than histamine greater than AET greater than PEA) and preferentially reversed by simultaneous addition of H2 receptor antagonists (cimetidine greater than DPH). We suggest that the desensitization of H2R activity by histamine presented here may be involved in the pathophysiological regulation and pharmacological control of gastric cell function in man.

[Occurrence of atypical histamine H2 receptors in the wall of the frog subclavian vein]

Selective H2-histamine agonist dimaprit was shown to produce relaxation of the isolated frog subclavian vein, with it persisting under the effect of selective H2-histamine antagonist cimetidine. Possible nonspecific mechanisms of relaxation produced by histamine are discussed. The data presented do not exclude that there are atypical H2-histamine receptors in subclavian vein of frogs, the activation of which initiates the attenuation of the active tension.

Cimetidine reverses tumor growth enhancement of plasmacytoma tumors in mice demonstrating conditioned immunosuppression

Mice were given repeated injections, at 21 day intervals, of cyclophosphamide paired with a novel taste (saccharin) in the drinking water. Subsequent challenge with a syngeneic plasmacytoma tumor led to elevated tumor growth and mortality only in conditioned mice that were reexposed to saccharin. This effect was abolished by conditioned exposure to a histamine type II-receptor antagonist.

Selective disappearance of histamine H2-receptor activity in the human gastric cancer cell line HGT-1 after short-term or chronic treatment by histamine or its H2-antagonists

Homologous loss of histamine H2-receptor activity (cAMP generation) was observed after short-term (10-20 min) or chronic treatment (6 days) of cultured HGT-1 cells with histamine (desensitization) or the H2-receptor antagonist SKF 93479. This inactivation process was not observed when HGT-1 cells were exposed to the classical H2-antihistamine cimetidine. The data show: (1) that the compound SKF 93479 has a very prolonged inhibitory action on histamine receptor activity, suggesting an irreversible interaction between the antagonist and the receptor; (2) that cimetidine is a reversible H2-receptor antagonist which can be removed without changing the the efficacy and the potency of histamine on gastric cells; (3) that the H2-receptor antagonists cimetidine and SKF 93479 specifically block histamine H2-receptor activity in HGT-1 cells since cAMP generation induced by other hormones such as vasoactive intestinal peptide (VIP), glucagon or gastric inhibitory peptide (GIP) was unchanged after treatment; (4) the first evidence for time-dependent (half-life: 20 min) desensitization of gastric H2-receptors.

Characteristics of histamine receptors present on suppressor T cells in "healthy individuals"

Twelve to 30% histamine receptor bearing cells were detectable in the peripheral blood mononuclear cells of healthy tuberculin sensitive individuals. The number of binding sites per cell ranged from 2.1 X 10(4) to 5.08 X 10(4) (mean 2.5 X 10(4)) with an affinity ranging from 2.5 X 10(-6) M to 10.9 X 10(-6) M (mean 3.6 X 10(-6) M). The histamine receptors on these cells were found to be of H2 type as indicated by the abrogation of binding of 3H-histamine by cimetidine. It was further confirmed that histamine receptor bearing cells in the peripheral blood belonged to a T cell subset which formed rosettes with AET treated sheep erythrocytes and had receptors for Fc portion of IgG and phenotype markers of T3 and T8. Deletion of such cells by means of affinity chromatography on histamine bound Sepharose columns, led to enhanced antigen induced lymphoproliferation indicating the suppressor nature of these T cells.

Histamine H2-binding studies in the guinea-pig brain

Using 3H-mepyramine and 3H-tiotidine to label, respectively, the histamine H1- and H2-receptors, we have assessed the distribution of these receptors in five distinct areas of guinea-pig brain. With regard to the H2-receptor, we could detect no binding in either the cerebellum or pons-medulla, whereas the distribution in the other brain areas was corpus striatum greater than cortex greater than hippocampus. This was in contrast to the distribution of the H1-receptor which was found to be in all areas studied but predominantly in the cerebellum.

Demonstration of histamine receptors on human platelets by flow cytometry

Fluoresceinated human albumin conjugated with histamine (FHA-HIS) has been used for the demonstration of histamine receptors on human platelets. Such receptors were demonstrated on 40-63% of peripheral blood platelets in 4 healthy donors. The binding of FHA-HIS was inhibited on 35-79% of the platelets by the histamine H1 receptor antagonists diphenhydramine and clemastine. The histamine H2 receptor antagonist cimetidine blocked the FHA-HIS binding on 14-37% of the platelets. It is concluded that histamine H1 as well as H2 receptors occur on human platelets but the receptors are not equally distributed in the platelet population.

Effect of cimetidine on basal and histamine-induced secretion of parathyroid hormone in vitro

The effect of cimetidine on basal and histamine-induced PTH secretion was tested using single cell suspensions obtained from (a) primary parathyroid adenomas, and (b) secondary hyperplastic parathyroid tissue from patients undergoing chronic hemodialysis. The histamine-stimulated hormone secretion was dose-dependent. Cimetidine suppressed both basal and histamine-stimulated hormone secretion. Within the therapeutic range the suppressive effect was identical for adenoma and hyperplasia. Both adenomata and secondary hyperplastic glands showed a histamine H2-receptor-related response. The role of histamine in the pathogenesis of hyperparathyroidism is not quite clear so that the possible benefits of cimetidine for medical treatment of primary and secondary hyperparathyroidism will have to be proven by careful clinical trials.

Extragastric H2-receptors

The discovery of H2-receptors by Black and coworkers opened a new era in the history of histaminology. Their importance became apparent when, soon after their discovery, the physiological role of histamine in the regulation of gastric secretion was clarified. In the last decade the explosion of research in the field of histamine and its receptors has demonstrated that H2-receptors have a much wider distribution than previously suspected. H2-receptors are found in the brain, the endocrine and exocrine glands, the pulmonary system, the cardiovascular system of different species, the gastrointestinal muscle, the genitourinary system, the immunological system and in the skin. In some instances stimulation of the various H2-receptors evokes responses that are opposite to those elicited by stimulation of H1-receptors. In other cases they are quite similar. Usually, activation of H2-receptors leads to an increased activity of the adenylate cyclase system with a consequent increase in intracellular cyclic AMP. Most H2-receptors are located postsynaptically on muscle or gland surfaces. However, there is recent evidence concerning the possibility of a presynaptic localization with a modulatory function on the release of different mediators. Finally, controversy exists over the possibility that H2-receptors do not represent a homogeneous population. In fact, several observations suggest that "anomalous" H2-receptors are characterized by different sensitivity to the H2-antagonists and/or antagonists in various tissues. If this is true, exact characterization of H2-receptors will be decidedly more difficult and will require "super selective" H2-agonists and antagonists which are not currently available.

Functional characteristics of histamine receptor-bearing mononuclear cells. I. Selective production of lymphocyte chemoattractant lymphokines with histamine used as a ligand

Mitogens and antigens have been the traditional ligands for activating lymphocytes in vitro for the elaboration of lymphokines. Recently, histamine, by interaction with histamine-type 2 receptors on T lymphocytes, has been found to induce the production of one lymphokine, histamine-induced suppressor factor (HSF), that inhibits lymphocyte proliferation and lymphokine production in vitro. Because the biologic effects of HSF appear to be confined to alterations in lymphocyte function, we assessed the ability of soluble products of histamine-stimulated human blood mononuclear cells to affect another lymphocyte function, motility. Utilizing a modified Boyden chamber assay to assess lymphocyte migration, we identified chemoattractant activity for human blood and rat splenic T lymphocytes in histamine-induced mononuclear cell supernatants. No neutrophil or monocyte chemoattractant activity was present. Sephadex G-100 gel filtration of histamine-induced supernatants showed the lymphotactic activity eluted with a 56,000 m.w. This activity was cationic as determined by its elution pattern from a Sephadex QAE anion exchange matrix with a single pl of 9.0 to 9.4 determined by isoelectric focusing in sucrose. Its biologic activity is predominantly chemokinetic in nature, is stable to heating at 56 degrees C for 30 min, but is sensitive to the effects of trypsin and neuraminidase. These physicochemical and functional characteristics establish it as identical to a recently described concanavalin A-induced (Con A) lymphotactic lymphokine (LCF). Mononuclear cells that did not adhere to a histamine affinity matrix were unable to produce LCF when subsequently stimulated with histamine or Con A. Mononuclear cells incubated with histamine and diphenhydramine produced LCF; the addition of cimetidine eliminated LCF production. In fact, supernatants from cells incubated with histamine and cimetidine significantly inhibited lymphocyte migration, a phenomenon explainable by the two regions of lymphocyte migration inhibitory activity that were present in the Sephadex G-100 chromatography of crude histamine-induced supernatants. These data suggest that a subset of lymphocytes defined by the presence of histamine-type 2 receptors is capable of producing LCF while cells that bear histamine-type 1 receptors produce lymphocyte migration inhibitory activity.

Evaluation of presynaptic histamine receptors in the canine renal vascular bed

Histamine has been shown to increase renal blood flow via H1- and H2-receptors. Furthermore, H2-receptors have been demonstrated to attenuate stimulation-induced release of norepinephrine. The present studies examined whether histamine has a presynaptic effect on sympathetic nerves in the canine renal vascular bed. Renal blood flow was measured in anesthetized dogs, and vasoconstrictor responses to renal nerve stimulation and i.a. injections of norepinephrine were compared before and during i.a. infusions of histamine. Histamine increased renal blood flow and decreased stimulation-induced vasoconstriction to a greater degree than norepinephrine responses. 2-(2-pyridyl)ethylamine, an H1-agonist, did not produce consistent effects. Dimaprit, an H2-agonist, produced responses similar to histamine but to a lesser extent. The H1-antagonist tripelennamine and the H2-antagonist cimetidine each minimally antagonized the effect of histamine on nerve stimulation. When both blocking agents were infused together, maximum antagonism of histamine occurred. Thus, it appears that histamine will produce a neuroinhibitory effect in the canine renal vascular bed and this effect appears to be mediated by both H1- and H2-receptors because both receptor antagonists are necessary to block this effect.

Histamine and the hypothalamus

The chemical tools that could be used to examine the function of histamine in the brain are considered together with the evidence linking histamine specifically with the hypothalamus. The distribution of histamine and the enzymes responsible for its synthesis and metabolism is consistent with there being both mast cells and histaminergic nerve terminals within the hypothalamus. Iontophoresis, mepyramine binding and histamine-stimulated adenylate cyclase studies suggest that both histamine H1- and H2- receptors are present in the hypothalamus. In addition, intracerebroventricularly injected histamine receptor agonists and antagonists affect many functions associated with the hypothalamus such as cardiovascular control, food intake, body temperature control, and pituitary hormones whose release is mediated via the hypothalamus, such as corticotropin, growth hormone, thyroid stimulating hormone, prolactin, gonadotropins and vasopressin. However, only in the case of thyroliberin release, prolactin release, body fluid control and blood pressure control is there evidence yet that such effects are mediated via histamine receptors actually in the hypothalamus. The effects of enzyme inhibitors suggest endogenous histamine may be involved in the physiological control of thyroid stimulating hormone, growth hormone and blood pressure, and the effects of receptor antagonists support a role for endogenous histamine in prolactin control. Otherwise, there is little evidence for a physiological role for endogenous, as against exogenous, histamine whether it be from histaminergic terminals or mast cells. In addition, few studies have tried to distinguish possible effects on presynaptic receptors, postsynaptic receptors, hypothalamic blood vessels or the hypophyseal portal blood vessels. It is concluded that although there is good evidence now linking histamine and the hypothalamus more specific studies are required, for instance using microinjection or in vitro techniques and the more specific chemical tools now available, to enable a clearer understanding of the physiological role of histamine in the hypothalamus.

Characterization of histamine receptors in cat cerebral arteries in vitro and in situ

Histamine is probably a mediator of vascular responses in the brain, but there is little experimental evidence for its importance in this role. By using both in vitro and in situ techniques, we have studied responses of cat pial arteries to stimulation of histamine receptors by pharmacological agents. In vitro, histamine and 2,2-pyridylethylamine (PEA, H1 agonist) caused contraction of resting arteries while impromidine (H2 agonist) was without effect. The PEA-induced constriction was blocked by the histamine H1 antagonist, mepyramine. When the arteries were precontracted (by 3 X 10(-6) M prostaglandin F2 alpha), however, all three agents caused vascular relaxation with an order of effectiveness as follows: histamine = impromidine much much greater than PEA. The responses of histamine and impromidine were reduced by the H2-antagonists, metiamide or cimetidine. Schild plots for the H2 receptor antagonists resulted in pA2 values of 6.90 and 7.03 for metiamide and cimetidine, respectively. In situ, neither agonist caused pial arterial constriction. Impromidine was considerably more effective than PEA in producing arterial dilatation. Metiamide reduced the effect of impromidine, whereas the dilatation of PEA was reduced by mepyramine. Dilatations resulting from PEA persisted in the presence of metiamide. Our results are consistent with the hypothesis that histamine H2 receptors are present in cerebral vascular smooth muscle as identified both in vitro and in situ. Indications for the additional presence of H1 receptors are, however, weak.