The subcellular localization of histamine and histamine methyltransferase in rat brain

Abstract— We have examined the subcellular localization of histamine and histamine methyl-transferase (S-adenosylmethionine: histamine 7V-methyltransferase; EC 2.1.1.8) in rat brain. The highest levels of histamine and histamine methyltransferase activity were found in the hypothalamus. A large proportion of hypothalamic histamine and histamine methyltransferase activity was found in particles with sedimentation properties in sucrose gradients similar to synaptosomes storing norepinephrine and serotonin. Histamine displayed a bimodal distribution in sucrose gradients. A substantial amount of a tracer dose of [³H]histamine added to hypothalamic homogenates at 4°C was bound to particulate fractions, suggesting that endogenous histamine may redistribute and bind to subcellular fractions during homogenization. The second, lighter peak of histamine in sucrose gradients was thought to be due to histamine that redistributed during homogenization.     

Histamine in the Insect Nervous System: Distribution, Synthesis and Metabolism

Abstract: The distribution of histamine in the nervous systems of the locust, the cockroach, and the sphinx moth was mapped and the capacity of locust nervous tissue to synthesise and metabolise histamine was assessed. In all three species the highest levels of histamine were present in the retina and in the lamina neuropil of the optic lobe. Lower levels of histamine were detectable throughout the nervous system. In the locust the retina was shown to synthesise considerable quantities of histamine. The optic lobe and metathoracic ganglion synthesised smaller, though significant, amounts of histamine. Metabolic in activation of histamine in locust nervous tissue was shown to occur primarily via oxidation to imidazole-4-acetic acid and via N-acetylation to N -acetyl histamine. Whereas the retina and the optic lobe formed the two metabolic products in approximately equal proportions, the metathoracic ganglion produced almost three times as much N- acetyl histamine as imidazole-4-acetic acid.     

Characterization of Extracellular Histamine in the Striatum and Bed Nucleus of the Stria Terminalis of the Rat: An In Vivo Microdialysis Study

The intracerebral microdialysis technique, coupled with a sensitive radioenzymatic assay, was employed to study histamine release in the striatum and in the bed nucleus of the stria terminalis (BNST) in conscious, freely moving rats. In these brain regions, extracellular histamine concentrations decreased by 20% when calcium was omitted from the perfusion solution. Extracellular histamine was insensitive to the addition of tetrodotoxin to the perfusion medium. In striatum, extracellular histamine concentrations declined in an apparent biexponential manner after the administration of alpha-fluoromethylhistidine, an inhibitor of histamine synthesis. The half-lives for the disappearance of histamine were 32 min and 7.7 h, indicating the presence of at least two histamine pools. Histidine loading resulted in a nearly twofold increase in histamine outflow in striatum. In the BNST, yohimbine increased the extracellular histamine content by 50%, suggesting that histamine release is subject to alpha 2-adrenergic regulation in vivo. The extent to which histamine detected in cerebral microdialysis samples is of neurogenic origin remains to be established.     

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.     

Histamine release and SOD, allopurinol and ranitidine pretreatment in haemorrhagic shock in the rat.

Histamine release have been demonstrated in haemorrhagic shock. There are some observations that oxygen free radicals can cause histamine release. Oxygen free radicals play a role in the pathogenesis of gastric mucosal lesions. The goal of this study was to determine whether ranitidine or SOD and allopurinol pretreatment modify the histamine release during and after the haemorrhagic shock in the rat. In the anaesthetized rat 0.1 N HCl was instilled into the stomach and the rat was bled to reduce the blood pressure to 30 mmHg for 20 min. The shed blood was reinfused. Twenty min later the stomach was removed. The area of gastric mucosal lesions were measured, histological grading was made. Blood samples taken from the carotid artery were examined by radioimmunoassay (IMMUNOTECH) to determine the plasma histamine level. Plasma histamine level did not change significantly during the preparative surgery, but there was a significant increase of histamine level by the end of shock period. After the reinfusion of the blood the plasma histamine remained essentially at the same level for five min. Oxygen free radicals did not cause an important histamine release. By the end of the experiment the histamine level decreased dramatically. Ranitidine, allopurinol and SOD pretreatment provided significant protection against the gastric mucosal lesions. Allopurinol and SOD did not influence significantly the histamine level. Ranitidine caused significant histamine release immediately after the injection and every histamine value was significantly higher in this group except for the final value which was lower than the control one. The oxygen free radicals were not found as endogenous histamine releasers in this study.     

Uptake and methylation of histamine by dispersed gastric mucosal cells and its possible influence on acid secretion

Inactivation of histamine by gastric mucosal tissue was examined in dispersed rabbit gastric mucosal cells. Mucosal cells were incubated with [14C]histamine. The formed radioactive metabolites were separated and identified by thin layer co-chromatography and quantitated, in both the cellular and extracellular mediums. Gastric mucosal cells internalized histamine, most of which was immediately methylated primarily to N tau-methylhistamine and released. Cellular histamine product accumulation reached a plateau. The rate of histamine methylation increased with increasing extracellular histamine concentration, moving towards a plateau above 5 microM. Histamine methylation was greatly decreased but not abolished at 4 degrees C, in the absence of Na+ and by phlorizin (0.5 mM), an inhibitor of Na(+)-dependent co-transport. Inhibition of histamine N-methyltransferase decreased intracellular methylhistamine content dose dependently without increasing intracellular histamine. The secretagogues pentagastrin and carbachol did not influence histamine metabolism but ethanol inhibited methylation. The data suggest that gastric mucosal cells take up histamine by a Na(+)-dependent and Na(+)-independent process. The histamine uptake capacity appears to be linked to the methylation activity within the cell. The decrease in histamine uptake and metabolism caused by ethanol could potentially increase histamine concentrations near the target cells and be the reason for the stimulatory effect of ethanol on acid secretion.     

Dynamics of the regulation of histamine levels in mouse brain

Abstract— The intraperitoneal administration of L-histidine in a dose of 1000 mg/kg increased threefold the whole brain levels of histamine in the mouse. This increase was evident in all brain regions except the medulla oblongata-pons. The subcellular localization of histamine and histidine was the same in mice administered bhistidine as in salinetreated animals. Cold exposure and restraint further augmented the elevation of histamine elicited by histidine treatment. a-Hydrazino-histidine and 4-bromo-3-hydroxybenzyloxyamine (NSD-1055) but not a-methyl-DOPA inhibited histidine decarboxylase [EC 4.1.1.221 activity in mouse brain homogenates and prevented the increase in brain histamine after histidine administration. NSD-1055 and a-hydrazino-histidine also lowered brain levels of histamine by 50 per cent. NSD-1055 lowered whole brain levels of histamine rapidly, with a half-life for the depletable histamine pool of about 5 min. Assuming that inhibition of histidine decarboxylase accounted for the reduction in histamine, then the rate of histamine decline reflects the rate of histamine turnover, and our results suggest that a portion of mouse brain histamine turns over quite rapidly. Reserpine lowered brain levels of histamine by about 50 per cent, whereas the antihistaminic agent, dexbrompheniramine, and sodium pentobarbital elevated histamine levels.     

Regional Characteristics of Histamine Uptake into Neonatal Rat Astrocytes

Histaminergic signalling constitutes an attractive target for treatment of neuropsychiatric disorders. One obstacle to developing new pharmacological options has been failure to identify putative specific histamine transporter responsible for histamine clearance. Although high-affinity histamine uptake was detected in neonatal cortical astrocytes, its existence in other brain regions remains largely unexplored. We investigated whether cerebellar and striatal astrocytes participate in histamine clearance and evaluated the role of organic cation transporters (OCT) in astroglial histamine transport. Kinetic and pharmacological characteristics of histamine transport were determined in cultured astrocytes derived from neonatal rat cerebellum, striatum and cerebral cortex. As well as astrocytes of cortical origin, cultured striatal and cerebellar astrocytes displayed temperature-sensitive, high-affinity histamine uptake. Exposure to ouabain or Na⁺-free medium, supplemented with choline chloride markedly depressed histamine transport in cortical astrocytes. Conversely, histamine uptake in striatal and cortical astrocytes was ouabain-resistant and was only partially diminished during incubation in the absence of Na⁺. Also, histamine uptake remained unaltered upon exposure to OCT inhibitor corticosterone, although OCTs were expressed in cultured astrocytes. Finally, histamine transport in cerebellar and striatal astrocytes was not sensitive to antidepressants. Despite common characteristics, cerebellar astrocytes had lower affinity, but markedly higher transport capacity for histamine compared to striatal astrocytes. Collectively, we provide evidence to suggest that cerebellar, striatal as well as cortical astrocytes possess saturable histamine uptake systems, which are not operated by OCTs. In addition, our data indicate that Na⁺-independent histamine carrier predominates in cerebellar and striatal astrocytes, whereas Na⁺-dependent transporter underlies histamine uptake in cortical astrocytes. Our findings implicate a role for histamine transporters in regulation of extracellular histamine concentration in cerebellum and striatum. Inhibition of histamine uptake might represent a viable option to modulate histaminergic neurotransmission.     

H2 antihistamines augment antigen-induced histamine release from human basophils in vitro

H2 antihistamines, including cimetidine, burimamide, metiamide, and tiotidine, consistently augmented antigen-induced histamine release from human basophils in vitro when control histamine release was less than 20% of total. This effect was specific to the H2-receptor blocking activity of these drugs: equivalent degrees of receptor blockade by four different H2 antihistamines resulted in equipotent enhancement; H1-receptor antagonists did not alter histamine release; and aminoguanidine and amodiaquine, agents that inhibit histamine metabolism but do not block H2 receptors, did not enhance histamine release. Cimetidine did not enhance release when present a) when basophils were "activated" but did not release histamine ("first stage"), or b) when basophils were no longer susceptible to histamine inhibition ("second stage"). Thus, H2 antagonists enhanced histamine release by blocking the capacity of released histamine to act on H2 receptors to inhibit release. Because it is likely that only small percentages of histamine are released in vivo, it is possible that H2 antihistamines amplify the inflammatory process by blocking the inhibitory effects of the released histamine.     

The influence of histamine on precursors of granulocytic leukocytes in murine bone marrow

The effect of histamine on granulocytic progenitor cells in murine bone marrow was studied in vitro. When bone marrow cells were cultured for three days with the drug, 10(-8) M to 10(-5) M of histamine stimulated differentiation and proliferation of myeloid precursor cells. Subsequently, the number of descendant cells, such as metamyelocytes and neutrophils, increased dose-dependently. Co-existence of equimolar H2 blockers such as cimetidine and ranitidine completely suppressed this effect of histamine, though this was not the case with an H1 blocker/histamine combination. Significant increase in 3H-thymidine incorporation was observed almost exclusively in myeloblasts, promyelocytes and myelocytes after exposure to histamine at concentrations higher than 10(-8) M. Also, selective incorporation of 3H-histamine into bone marrow cells was observed in myeloblasts and promyelocytes, but histamine incorporation was not influenced by the presence of either of histamine agonists or antagonists. While histamine, via H2 receptors, selectively increased the number of granulocytic colony forming units in culture (CFU-C), it had no such effect on macrophage colonies. Considering these findings, it was concluded that histamine promotes proliferation and differentiation of granulocytic myeloid cells via 1) H2 receptors in the CFU-C stage and 2) histamine receptors which are neither H1 nor H2 in the stages of myeloblast and promyelocyte differentiation.     

Ontogeny and Subcellular Distribution of Rat Brain Tele-Methylhistamine

Abstract: The whole brain content and subcellular distribution of histamine and its metabolite, tele-methylhistamine, were studied during postnatal development of the rat. Brain methylhistamine levels were similar to or greater than histamine levels, indicating that histamine methylation is a major metabolic pathway in neonatal brain, as it is in adults. When calculated per brain, histamine, methylhistamine, and histamine methyltransferase were all maximal 10 days after birth. In neonates, brain histamine was found almost entirely in nuclear fractions, whereas methylhistamine was found almost exclusively in supernatant fractions. By day 20, however, a greater proportion of both amines was localized in subcellular fractions containing synaptosomes, a finding consistent with histamine's suggested transmitter role. The ontogenic pattern of brain methylhistamine questions the mast cell origin of neonatal histamine, but may be consistent with a role for histamine in brain development.     

Histamine in stress ulcer prophylaxis in rats.

BACKGROUND: Gastrin and its analogues increase the gastric acid secretion, but also enhance mucosal defense mechanisms. On the other hand, increased formation of histamine leading to an increase in gastric acid secretion is accompanied with gastroprotection and acceleration of gastric ulcer healing. AIM: Of this study was to examine the effect of histamine on stress induced gastric ulcers in rats. METHODS: Male Wistar rats were exposed to water immersion and restrain stress (WRS) for 3.5 h at 23 degrees C. Before WRS rats were pretreated with saline, histamine, ranitidine or omeprazole. RESULTS: WRS produces gastric lesions which were strongly reduced by ranitidine or omeprazole. Also treatment with histamine markedly reduced ulcer area evoked by WRS. Addition of histamine to ranitidine or omeprazole caused an additional reduction in ulcer area. Gastroprotective effect of histamine was accompanied with the increase in gastric blood flow (GBF). Administration of omeprazole or ranitidine alone was without significant effect on GBF. Histamine caused an slight decrease in gastric luminal pH, whereas ranitidine or omeprazole significantly increased gastric luminal pH. Plasma interleukin-1beta was significantly reduced after administration of omeprazole, ranitidine, or histamine, however, the effect of histamine was less pronounced. DNA synthesis was increased after administration of omeprazole, ranitidine or histamine when compared with WRS alone. Administration of histamine in combination with ranitidine or omeprazole caused an additional increase in DNA synthesis. CONCLUSIONS: Histamine exhibits protective effect and increases gastroprotective effect of ranitidine and omeprazole against stress-induced gastric lesions. This effect of histamine seems to be independent on gastric acid secretion but related to the increase in gastric blood flow and the reduction in activation of cytokine cascade.     

Which lactic acid bacteria are responsible for histamine production in wine?

AIMS: To quantify the ability of 136 lactic acid bacteria (LAB), isolated from wine, to produce histamine and to identify the bacteria responsible for histamine production in wine. METHODS AND RESULTS: A qualitative method based on pH changes in a plate assay was used to detect wine strains capable of producing high levels of histamine. Two quantitative, highly sensitive methods were used, an enzymatic method and HPLC, to quantify the histamine produced by LAB. Finally, an improved PCR test was carried out to detect the presence of histidine decarboxylase gene in these bacteria. The species exhibiting the highest frequency of histamine production is Oenococcus oeni. However, the concentration of histamine produced by this species is lower than that produced by strains belonging to species of Lactobacillus and Pediococcus. A correlation of 100% between presence of histidine decarboxylase gene and histamine production was observed. Wines containing histamine were analysed to isolate and characterize the LAB responsible for spoilage. CONCLUSIONS: Oenococcus was able to synthesize low concentrations of histamine in wines, while Pediococcus parvulus and Lactobacillus hilgardii have been detected as spoilage, high histamine-producing bacteria in wines. SIGNIFICANCE AND IMPACT OF THE STUDY: Information regarding histamine-producing LAB isolated from wines can contribute to prevent histamine formation during winemaking and storage.     

H1 and H2 histamine actions on lung vessels; their relevance to hypoxic vasoconstriction.

Pulmonary vasomotor actions of histamine and the possible relationship of histamine to hypoxic pulmonary vasconstriction were studied in anaesthetized cats with one lobe of lung perfused at constant flow and in isolated perfused rat and ferret lungs. In the cat histamine caused dilatation, biphasic responses and constriction with increasing doses. Histamine induced dilatation was better demonstrated during hypoxic vasoconstriction and was reduced by an H2 histamine antagonist; constriction with histamine was abolished by an H1 antagonist. Histamine also caused both vasodilatation and vasoconstriction in ferret lungs. A mast cell stabilizing agent had no effect on hypoxic pulmonary vasoconstriction in cats or rats. This response was unaffected in cats but greatly reduced in rats and ferrets by cyproheptadine, a combined histamine and 5-hydroxy-tryptamine inhibitor. It was unaffected in cats but abolished in ferrets an H1 histamine inhibitor. It was again unaffected in cats but greatly reduced in rats and ferrets by an H2 histamine inhibitor. These species differences may reflect differences in mechanism but more probably reflect non-specific effects of the inhibitors in certain circumstances. However, when drugs nearly abolished hypoxic vasoconstriction, ATP still caused vasoconstriction.     

Uptake of histamine by mouse peritoneal macrophages and a macrophage cell line,RAW264.7

We have previously demonstrated that dietary histamine is accumulated in the spleens of L-histidine decarboxylase (HDC)-deficient mice, which lack endogenous histamine synthesis. To characterize the clearance system for dietary histamine in mice, we investigated the cell type and mechanism responsible for histamine uptake in the spleens of HDC-deficient mice. Immunohistochemical analyses using an antihistamine antibody indicated that a portion of the CD14⁺ cells in the spleen is involved in histamine storage. Peritoneal macrophages obtained from Balb/c mice and a mouse macrophage cell line, RAW264.7, had potential for histamine uptake, which was characterized by a low affinity and high capacity for histamine. The histamine uptake by RAW264.7 cells was observed at physiological temperature and was potently inhibited by pyrilamine, chlorpromazine, quinidine, and chloroquine, moderately inhibited by N-methylhistamine, dopamine, and serotonin, and not affected by tetraethylammonium and 1-methyl-4-phenylpyridinium. Intracellular histamine was not metabolized in RAW264.7 cells and was released at physiological temperature in the absence of extracellular histamine. These results suggest that histamine uptake by macrophages may be involved in the clearance of histamine in the local histamine-enriched environment. cation transporter; chlorpromazine; pyrilamine; quinidine     

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.     

Gastric submucosal microdialysis: a method to study gastrin- and food-evoked mobilization of ECL-cell histamine in conscious rats

Rat stomach ECL cells are rich in histamine and chromogranin A-derived peptides, such as pancreastatin. Gastrin causes the parietal cells to secrete acid by flooding them with histamine from the ECL cells. In the past, gastric histamine release has been studied using anaesthetized, surgically manipulated animals or isolated gastric mucosa, glands or ECL cells. We monitored gastric histamine mobilization in intact conscious rats by subjecting them to gastric submucosal microdialysis. A microdialysis probe was implanted into the submucosa of the acid-producing part of the stomach (day 1). The rats had access to food and water or were deprived of food (48 h), starting on day 2 after implantation of the probe. On day 4, the rats received food or gastrin (intravenous infusion), and sampling of microdialysate commenced. Samples (flow rate 1.2 microl min(-1)) were collected every 20 or 60 min, and the histamine and pancreastatin concentrations were determined. The serum gastrin concentration was determined in tail vein blood. Exogenous gastrin (4-h infusion) raised microdialysate histamine and pancreastatin dose-dependently. This effect was prevented by gastrin receptor blockade (YM022). Depletion of ECL-cell histamine by alpha-fluoromethylhistidine, an irreversible inhibitor of the histamine-forming enzyme, suppressed the gastrin-evoked release of histamine but not that of pancreastatin. Fasting lowered serum gastrin and microdialysate histamine by 50%, while refeeding raised serum gastrin and microdialysate histamine and pancreastatin 3-fold. We conclude that histamine mobilized by gastrin and food intake derives from ECL cells because: 1) Histamine and pancreastatin were released concomitantly, 2) histamine mobilization following gastrin or food intake was prevented by gastrin receptor blockade, and 3) mobilization of histamine (but not pancreastatin) was abolished by alpha-fluoromethylhistidine. Hence, gastric submucosal microdialysis allows us to monitor the mobilization of ECL-cell histamine in intact conscious rats under various experimental conditions not previously accessible to study. While gastrin receptor blockade lowered post-prandial release of ECL-cell histamine by about 80%, unilateral vagotomy reduced post-prandial mobilization of ECL-cell histamine by about 50%. Hence, both gastrin and vagal excitation contribute to the post-prandial release of ECL-cell histamine.     

Inhibition by dexamethasone of histamine production in allergic inflammation in rats.

In an allergic inflammation model of air pouch type in rats, histamine level in the pouch fluid and histidine decarboxylase activity of pouch wall tissues in the postanaphylaxis phase were increased. Although treatment with dexamethasone failed to inhibit histamine release from mast cells in the anaphylaxis phase, histamine production in the postanaphylaxis phase was inhibited dose dependently. Histamine production-increasing activity in the pouch fluid collected 8 h after the Ag challenge, which was estimated by an activity to stimulate histamine production by bone marrow cells, was decreased by the administration of dexamethasone at the time of the Ag challenge. The addition of steroidal antiinflammatory drugs, dexamethasone, prednisolone, or hydrocortisone, into the incubation medium inhibited the pouch fluid-induced histamine production by bone marrow cells. Hydrocortisone mesylate antagonized the inhibitory effect of dexamethasone on histamine production by bone marrow cells. However, hydrocortisone mesylate failed to recover the decrease in histamine production-increasing activity of the pouch fluid collected from dexamethasone-treated rats. In addition, the dialyzed sample of pouch fluid obtained from dexamethasone-treated nonsensitized rats did not reduce the stimulated histamine production by the pouch fluid sample obtained from the sensitized rats. However, increase in histamine production of bone marrow cells stimulated by the pouch fluid was not inhibited by cyclosporin A that inhibited histamine production induced by Con A. This observation indicates that the pouch fluid has no effect to induce production of the histamine production-increasing factor by bone marrow cells. Consequently, it is suggested that dexamethasone inhibits not only the production of histamine production-increasing factor but also the response of histamine-producing cells to this factor.     

IN VIVO FORMATION AND CATABOLISM OF [14C]HISTAMINE IN MOUSE BRAIN

Abstract— The formation of histamine in brain was studied in mice injected with l -[¹⁴C]-histidine (ring 2-¹⁴C) intravenously (i.v.) or intracerebrally; [¹⁴C]histamine appeared rapidly and exhibited a rapid rate of turnover. Drugs known to block various pathways of histamine catabolism were tested for effects on brain–[¹⁴C]histamine and [¹⁴C]-methyl-histamine in mice given (1) [¹⁴C]histamine i.v., (2) [¹⁴C]histamine intracerebrally, and (3) l -[¹⁴C]histidine i.v. Blood-borne histamine did not enter brain; brain histamine was formed locally by decarboxylation of histidine Methylhistamine did cross the blood-brain barrier. Methylation was the major route of histamine catabolism in mouse brain and some of the methylhistamine formed was destroyed by monoamine oxidase. No evidence for catabolism by the action of diamine oxidase was found.     

Biosynthesis of carcinine (beta-alanyl-histamine) in vivo

Carcinine was biosynthesized by Carcinus maenas from [14C]beta-alanine, [14C] histidine and [14C] histamine. Since carnosine (beta-alanyl-histidine) could not be detected in crab tissues, biosynthesis of carcinine could only be effected by direct coupling of beta-alanine and histamine resulting from histidine decarboxylation. Biosynthesis of carcinine was weak when [14C]beta-alanine and [14C] histidine were used as precursors. On the contrary when [14C] histamine was used, synthesis was important. Thus carcinine appears to be a product of histamine catabolism. After injecting [14C] histamine, radioactive carcinine was concentrated mainly in the heart and nervous system; nonmetabolized [14C] histamine was recovered mainly in the latter. The nervous system might therefore be the seat of carcinine biosynthesis and thus the site of action of histamine.