Histaminergic Neurotransmission as a Gateway for the Cognitive Effect of Oleoylethanolamide in Contextual Fear Conditioning

Background

The integrity of the brain histaminergic system is necessary for the unfolding of homeostatic and cognitive processes through the recruitment of alternative circuits with distinct temporal patterns. We recently demonstrated that the fat-sensing lipid mediator oleoylethanolamide indirectly activates histaminergic neurons to exerts its hypophagic effects. The present experiments investigated whether histaminergic neurotransmission is necessary also for the modulation of emotional memory induced by oleoylethanolamide in a contextual fear conditioning paradigm.

Methods

We examined the acute effect of i.p. administration of oleoylethanolamide immediately posttraining in the contextual fear conditioning test. Retention test was performed 72 hours after training. To test the participation of the brain histaminergic system in the cognitive effect of oleoylethanolamide, we depleted rats of brain histamine with an i.c.v. injection of alpha-fluoromethylhistidine (a suicide inhibitor of histidine decarboxylase) or bilateral intra-amygdala infusions of histamine H1 or H2 receptor antagonists. We also examined the effect of oleoylethanolamide on histamine release in the amygdala using in vivo microdialysis.

Results

Posttraining administration of oleoylethanolamide enhanced freezing time at retention. This effect was blocked by both i.c.v. infusions of alpha-fluoromethylhistidine or by intra-amygdala infusions of either pyrilamine or zolantidine (H1 and H2 receptor antagonists, respectively). Microdialysis experiments showed that oleoylethanolamide increased histamine release from the amygdala of freely moving rats.

Conclusions

Our results suggest that activation of the histaminergic system in the amygdala has a "permissive" role on the memory-enhancing effects of oleoylethanolamide. Hence, targeting the H1 and H2 receptors may modify the expression of emotional memory and reduce dysfunctional aversive memories as found in phobias and posttraumatic stress disorder.

Histamine is a modulator of metamorphic competence in Strongylocentrotus purpuratus (Echinodermata: Echinoidea)

BACKGROUND

A metamorphic life-history is present in the majority of animal phyla. This developmental mode is particularly prominent among marine invertebrates with a bentho-planktonic life cycle, where a pelagic larval form transforms into a benthic adult. Metamorphic competence (the stage at which a larva is capable to undergo the metamorphic transformation and settlement) is an important adaptation both ecologically and physiologically. The competence period maintains the larval state until suitable settlement sites are encountered, at which point the larvae settle in response to settlement cues. The mechanistic basis for metamorphosis (the morphogenetic transition from a larva to a juvenile including settlement), i.e. the molecular and cellular processes underlying metamorphosis in marine invertebrate species, is poorly understood. Histamine (HA), a neurotransmitter used for various physiological and developmental functions among animals, has a critical role in sea urchin fertilization and in the induction of metamorphosis. Here we test the premise that HA functions as a developmental modulator of metamorphic competence in the sea urchin Strongylocentrotus purpuratus.

RESULTS

Our results provide strong evidence that HA leads to the acquisition of metamorphic competence in S. purpuratus larvae. Pharmacological analysis of several HA receptor antagonists and an inhibitor of HA synthesis indicates a function of HA in metamorphic competence as well as programmed cell death (PCD) during arm retraction. Furthermore we identified an extensive network of histaminergic neurons in pre-metamorphic and metamorphically competent larvae. Analysis of this network throughout larval development indicates that the maturation of specific neuronal clusters correlates with the acquisition of metamorphic competence. Moreover, histamine receptor antagonist treatment leads to the induction of caspase mediated apoptosis in competent larvae.

CONCLUSIONS

We conclude that HA is a modulator of metamorphic competence in S. purpuratus development and hypothesize that HA may have played an important role in the evolution of settlement strategies in echinoids. Our findings provide novel insights into the evolution of HA signalling and its function in one of the most important and widespread life history transitions in the animal kingdom--metamorphosis.

Carnosine inhibits pentylenetetrazol-induced seizures by histaminergic mechanisms in histidine decarboxylase knock-out mice

In the present study, we used both histidine decarboxylase-deficient (HDC-KO) mice and wild-type (WT) mice to elucidate the possible role of carnosine in pentylenetetrazol (PTZ)-induced seizures. In the acute PTZ challenge study, PTZ (75 mg/kg) was injected intraperitoneally (i.p.) to induce seizures. Carnosine (200, 500 or 1000 mg/kg, i.p.) significantly decreased seizure stage, and prolonged the latency for myoclonic jerks in WT mice in a dose-dependent manner. The effects of carnosine (500 mg/kg) were time-dependent and reached a peak at 1h. However, it had no significant effect on HDC-KO mice. Carnosine (500 mg/kg) also significantly elevated the thresholds in WT mice but not HDC-KO mice following intravenous (tail vein) administration of PTZ. We also found that alpha-fluoromethylhistidine substantially reversed the protective effects of carnosine in WT mice. In addition, carnosine pretreatment reduced the cortical EEG activity induced by PTZ (75 mg/kg, i.p.). These results indicate that carnosine can protect against PTZ-induced seizures and its action is mainly through the carnosine-histidine-histamine metabolic pathway. This suggests that carnosine may be an endogenous anticonvulsant factor in the brain and may be used as a new antiepileptic drug in the future.

Neuroprotective effect of carnosine on necrotic cell death in PC12 cells

The nervous tissue of many vertebrates, including humans, can synthesize beta-alanyl-L-histidine (carnosine). The biological functions of carnosine are still open to question, although several theories supported by strong experimental data have been proposed. The objective of this study was to examine the effects of carnosine on neurotoxicity in differentiated rat pheochromocytoma (PC12) cells. Neurotoxicity was induced by N-methyl-D-aspartate (NMDA), which caused time- and concentration-dependent cell death as measured by MTT and LDH assays. Pretreatment with carnosine significantly prevented the neurotoxicity in a concentration-dependent manner. The protective effect of carnosine was antagonized by the H1 receptor antagonist pyrilamine, but not by the H2 receptor antagonist cimetidine. In addition, alpha-fluoromethylhistidine, a histidine decarboxylase inhibitor, slightly reversed the protective action of carnosine. These results indicate that carnosine can effectively protect against NMDA-induced necrosis in PC12 cells, and its protection may in part be due to the activation of the postsynaptic histamine H1 receptor. The study suggests that carnosine may be an endogenous protective factor and calls for its further study as a new anti-excitotoxic agent.

Histaminergic neurons protect the developing hippocampus from kainic acid-induced neuronal damage in an organotypic coculture system

The central histaminergic neuron system inhibits epileptic seizures, which is suggested to occur mainly through histamine 1 (H1) and histamine 3 (H3) receptors. However, the importance of histaminergic neurons in seizure-induced cell damage is poorly known. In this study, we used an organotypic coculture system and confocal microscopy to examine whether histaminergic neurons, which were verified by immunohistochemistry, have any protective effect on kainic acid (KA)-induced neuronal damage in the developing hippocampus. Fluoro-Jade B, a specific marker for degenerating neurons, indicated that, after the 12 h KA (5 microM) treatment, neuronal damage was significantly attenuated in the hippocampus cultured together with the posterior hypothalamic slice containing histaminergic neurons [HI plus HY (POST)] when compared with the hippocampus cultured alone (HI) or with the anterior hypothalamus devoid of histaminergic neurons. Moreover, alpha-fluoromethylhistidine, an inhibitor of histamine synthesis, eliminated the neuroprotective effect in KA-treated HI plus HY (POST), and extracellularly applied histamine (1 nM to 100 microM) significantly attenuated neuronal damage only at 1 nM concentration in HI. After the 6 h KA treatment, spontaneous electrical activity registered in the CA1 subregion contained significantly less burst activity in HI plus HY (POST) than in HI. Finally, in KA-treated slices, the H3 receptor antagonist thioperamide enhanced the neuroprotective effect of histaminergic neurons, whereas the H1 receptor antagonists triprolidine and mepyramine dose-dependently decreased the neuroprotection in HI plus HY (POST). Our results suggest that histaminergic neurons protect the developing hippocampus from KA-induced neuronal damage, with regulation of neuronal survival being at least partly mediated through H1 and H3 receptors.

Dual regulatory effects of orexins on sympathetic nerve activity innervating brown adipose tissue in rats

This study examined how orexin regulates the activity of the sympathetic nerves that innervate brown adipose tissue (BAT) in rats. Infusion of orexin A at a dose of 0.3 nmol into the third cerebral ventricle decreased BAT sympathetic nerve activity, compared with the effect of PBS (P < 0.05), whereas infusion of orexin B at the same dose caused a significant increase (P < 0.05). Pretreatment with a third cerebral ventricle injection of 2.24 micromol/kg alpha-fluoromethylhistidine, an irreversible inhibitor of the histamine-synthesizing enzyme histidine decarboxylase, attenuated the orexin B-induced response of BAT sympathetic nerve activity, but not that induced by orexin A. These results indicate that orexins may regulate both BAT energy expenditure and thermogenesis through their dual effects on sympathetic nerve activity. In particular, orexin B regulates BAT sympathetic nerve activity via neuronal histamine in the hypothalamus.

Glucagon‐like peptide‐1, corticotropin‐releasing hormone, and hypothalamic neuronal histamine interact in the leptin‐signaling pathway to regulate feeding behavior

Glucagon-like peptide-1 (GLP-1), corticotropin-releasing hormone (CRH), and hypothalamic neuronal histamine suppress food intake, a target of leptin action in the brain. This study examined the interactions of GLP-1, CRH, and histamine downstream from the leptin-signaling pathway in regulating feeding behavior. Infusion of GLP-1 into the third cerebral ventricle (i3vt) at a dose of 1 mug significantly decreased the initial 1 h cumulative food intake in rats as compared with phosphate-buffered saline (PBS) controls. The GLP-1-induced suppression of feeding was partially attenuated by intraperitoneal pretreatment with alpha-fluoromethylhistidine (FMH), a specific suicide inhibitor of histidine decarboxylase, which depletes hypothalamic neuronal histamine. Pretreatment with alpha-helical CRH (10 microg/rat, i3vt), a nonselective CRH antagonist, abolished the GLP-1-induced suppression of feeding completely. I3vt infusion of GLP-1 increased the CRH content and histamine turnover assessed using the pargyline-induced accumulation of tele-methyl histamine (t-MH), a major metabolite of neuronal histamine, in the hypothalamus. The central infusion of CRH also induced the increase of histamine turnover and CRH receptor type 1 was localized on the cell body of histamine neuron. Pretreatment with exendin(9-39), a GLP-1 receptor antagonist, attenuated the leptin-induced increase in CRH content of the hypothalamus. Finally, i3vt infusion of leptin also increased histamine turnover in the hypothalamus. Pretreatment with exendin(9-39), alpha-helical CRH or both antagonists attenuated the leptin-induced responses of t-MH levels in the hypothalamus. These results suggest that CRH or hypothalamic neuronal histamine mediates the GLP-1-induced suppression of feeding behavior, that CRH mediates GLP-1 signaling to neuronal histamine and that a functional link from GLP-1 to neuronal histamine via CRH constitutes the leptin-signaling pathway regulating feeding behavior.

Ischemia of rat stomach mobilizes ECL cell histamine

Microdialysis was used to study how ischemia-evoked gastric mucosal injury affects rat stomach histamine, which resides in enterochromaffin-like (ECL) cells and mast cells. A microdialysis probe was inserted into the gastric submucosa, and the celiac artery was clamped (30 min), followed by removal of the clamp. Microdialysate histamine was determined by enzyme-linked immunosorbent assay. In addition, we studied the long-term effects of ischemia on the oxyntic mucosal histidine decarboxylase activity in omeprazole-treated rats. Gastric mucosal lesions induced by the ischemia were enlarged on removal of the clamp. The microdialysate histamine concentration increased immediately on clamping (50-fold rise within 30 min) and declined promptly after the clamp was removed. In contrast, histidine decarboxylase activity of the ECL cells was lowered by the ischemia and returned to preischemic values 9 days later. Mast cell-deficient rats responded to ischemia-reperfusion much like wild-type rats with respect to histamine mobilization. Pretreatment with the irreversible inhibitor of histidine decarboxylase, alpha-fluoromethylhistidine, which is known to eliminate histamine from ECL cells, prevented the rise in microdialysate histamine. Pharmacological blockade of acid secretion (cimetidine or omeprazole) prevented the lesions induced by ischemia-reperfusion insult but not the mobilization of histamine. In conclusion, ischemia of the celiac artery mobilizes large amounts of histamine from ECL cells, which occurs independently of the gross mucosal lesions. The prompt reduction of the mucosal histidine decarboxylase activity in response to ischemia probably reflects ECL cell damage. The lesions develop not because of mobilization of histamine per se but because of ischemia plus reperfusion plus gastric acid.

Role of the histamine system in nefopam-induced antinociception in mice

The present study explored the role of the histaminergic system in nefopam analgesia based on the structural relationship between nefopam and diphenhydramine. In vitro binding assays revealed that nefopam possesses moderate affinity for histamine H1 and H2 receptor subtypes, with IC50 of 0.8 and 6.9 microM, respectively, but no affinity for histamine H(3) receptor subtype until 100 microM. Subcutaneous nefopam administration dose-dependently inhibited pain in acetic acid-induced writhing (1-30 mg/kg) and formalin (1-10 mg/kg) tests in the mouse. Pretreatment with the histamine-depleting agent alpha-fluoromethylhistidine (alpha-FMH, 50 mg/kg), the histamine H1 receptor antagonist pyrilamine (3 or 10 mg/kg), or the histamine H2 receptor antagonists cimetidine (100 mg/kg) and zolantidine (10 or 30 mg/kg) did not significantly modify nefopam antinociception in both tests. The histamine H3 receptor agonist R(-)alpha-methylhistamine (RAMH, 10 mg/kg) did not significantly modify the nefopam analgesic activity in the writhing test. At 25 mg/kg, RAMH inhibited nefopam antinociception at 3 mg/kg, but not at 10 mg/kg in the formalin test. However, pretreatment with the histamine H3 receptor antagonist thioperamide (25 mg/kg) inhibited nefopam antinociception in the writhing test, but not in the formalin test. In conclusion, nefopam analgesic activity is not mediated by histamine H1 or H2 receptors, but can be slightly modulated by histamine H3 receptors in mouse pain tests.

Effects of carnosine on amygdaloid-kindled seizures in Sprague–Dawley rats

The effects of carnosine (beta-alanyl-L-histidine) on amygdaloid-kindled seizures were investigated in rats. I.p. injection of carnosine (500, 1000, 1500 mg/kg, i.p.) significantly decreased seizure stage, afterdischarge duration and generalized seizure duration, and significantly prolonged generalized seizure latency of amygdaloid-kindled seizures, in a dose-dependent, and time-related manner. The protective effect of carnosine (1500 mg/kg) was completely antagonized by histamine H1-antagonists pyrilamine (2, 5 mg/kg, i.p.) and diphenhydramine (5, 10 mg/kg, i.p.), but not by histamine H2-antagonist zolantidine even at a high dose of 10 mg/kg. Carnosine (1500 mg/kg, i.p.) caused a significant increase of carnosine and histidine levels in the hypothalamus, thalamus, hippocampus, amygdala and cortex, as well as histamine levels in the hippocampus and amygdala. I.c.v. injection of alpha-fluoromethylhistidine (50 microg, i.c.v.), a selective and irreversible histidine decarboxylase inhibitor, only partially reversed the inhibition of amygdaloid-kindled seizures induced by carnosine. In addition, carnosine significantly decreased glutamate contents in the amygdala and hippocampus. These results indicate that carnosine could protect against amygdaloid-kindled seizures in rats, and its action may be due to the activation of histamine postsynaptic H1-receptors via two different mechanisms, one being carnosine's direct action, and the other being indirectly mediated by histaminergic pathway. The study suggests that carnosine may be an endogenous anticonvulsant factor in the brain and could be used as a new antiepileptic drug in the future.

Regulation of Con A ? dependent cytokine production from CD4+ and CD8+ T lymphocytes by autosecretion of histamine

OBJECTIVES

Previously we have shown that both CD4+ T cells and CD8+ T cells produce histamine when activated with Con A. The aim of this study was to examine whether cytokine production by these cells is regulated by autosecretion of histamine.

MATERIALS

CD4+ and CD8+ T cells were separated from spleen cells of C57BL/6 mice and mice lacking the H1 receptor (H1R) or H2R, using anti-CD4+- and anti-CD8+-coupled magnetic beads, respectively.

RESULTS

Depletion of the H1R resulted in decreases in the release of IL-2 and IL-10 from both CD4+ and CD8+ cells and increases in the release of IL-4 from CD4+ T cells and IFN-gamma from CD8+ cells. Mice lacking the H2R showed up-regulation of IFN-gamma secretion from CD8+ cells and of IL-4 from CD4+ and CD8+ T cells. Release of IL-2 and IL-10 from CD4+ as well as CD8+ cells was down-regulated in these mice. Both CD4+ and CD8+ T cell fractions synthesized histamine, which was enhanced in the H1R-deficient CD8+ T cells. Treatment of the cells with alpha-fluoromethyl-histidine, a specific inhibitor of HDC, or histaminase increased IFN-gamma from CD8+ cells, whereas it had no appreciable effect on IL-4 secretion from CD4+ cells.

CONCLUSION

These results suggest that cytokine production by CD4+ and CD8+ T lymphocytes is regulated by autosecretion of histamine.

Possible role of L-carnosine in the regulation of blood glucose through controlling autonomic nerves

Mammalian muscles synthesize L-carnosine, but its roles were unknown. Previously, we found in rats that the administration of a certain amount of L-carnosine elicited an inhibition of the hyperglycemia induced by the injection of 2-deoxy-D-glucose (2DG) into the lateral cerebral ventricle (LCV), and that intravenous injection of L-carnosine inhibited sympathetic nerves and facilitated the parasympathetic nerve. Moreover, the suppressive effect of L-carnosine on the hyperglycemia induced by 2DG was eliminated by thioperamide, a histaminergic H3 receptor. These findings suggested that L-carnosine might control the blood glucose level through regulating autonomic nerves via H3 receptor. To further clarify the function of L-carnosine, we examined its role in the control of the blood glucose. In this experiment, the following results were observed in rats: (i) A certain amount (0.01% or 0.001%) but not a larger amount (0.1%) of L-carnosine given as a diet suppressed the hyperglycemia induced by LCV-injection of 2DG (2DG-hyperglycemia); (ii) LCV-injection but not the injection into the intraperitoneal space (IP) of a certain amount of L-histidine suppressed the 2DG-hyperglycemia; (iii) treatments of diphenhydramine, an H1 antagonist, and alpha-fluoromethylhistidine, an inhibitor of histamine-synthesizing enzyme, reduced the 2DG-hyperglycemia; (iv) the plasma L-carnosine concentration and carnosinase activity showed daily changes; (v) the plasma L-carnosine concentration was significantly lower in the streptozotocin-diabetic rats; (vi) exercise by a running wheel tended to increase carnosine synthase activity in the gastrocnemius muscle and elevated the plasma L-carnosine concentration in the dark (active) period, and enhanced the plasma carnosinase activity in the light period; (vii) IP-injection of certain amount of L-carnosine stimulated the feeding response to IP-injection of 2DG. These findings suggest a possibility that L-carnosine released from muscles due to exercise functions to reduce the blood glucose level through the regulation of the autonomic nerves.

Endogenous Central Histamine-Induced Reversal of Critical Hemorrhagic Hypotension in Rats: Studies with l-Histidine

Activation of the histaminergic system is characteristic of response to the action of adverse or potentially dangerous stimuli that disturb circulatory homeostasis, such as dehydration and changes in blood pressure. Previous study demonstrates that inhibition of histamine N-methyltransferase, which catabolizes histamine released from neurons, leads to the increase in endogenous central histamine concentrations and to the reversal of critical hemorrhagic hypotension. In the present study, the influence of intraperitoneal loading with histamine precursor L-histidine on central cardiovascular regulation was studied in a model of irreversible pressure-controlled hemorrhagic shock. Experiments were carried out in male Wistar rats anesthetized with ketamine/xylazine subjected to critical hemorrhagic hypotension of 20 to 25 mmHg, which resulted in the death of all control saline-treated animals within 30 min. L-histidine administered in 5 min of critical hypotension produced dose-dependent increases in mean arterial pressure and heart rate (100-500 mg/kg), and a 100% survival rate of 2 h (500 mg/kg), whereas in normotensive animals, it did not influence cardiovascular parameters. The resuscitating effect of L-histidine (500 mg/kg) was associated with increases in histamine concentrations in the cerebral cortex (0.97 +/- 0.11 nmol/g of wet tissue vs. 0.67 +/- 0.22 nmol/g of wet tissue; P<0.05), hypothalamus (4.78 +/- 0.58 nmol/g of wet tissue vs. 4.08 +/- 0.43 nmol/g of wet tissue; P<0.01), and medulla oblongata (0.55 +/- 0.18 nmol/g of wet tissue vs. 0.34 +/- 0.09 nmol/g of wet tissue; P<0.05), as well as with no changes in plasma histamine concentrations in comparison with the saline-treated group 20 min after injection. Pretreatment with (S)-alpha-fluoromethylhistidine (alpha-FMH, 0.5 mg intracerebroventricularly), an irreversible inhibitor of L-histidine decarboxylase, produced a decrease in central histamine concentrations and diminished volumes of blood required to achieve critical hypotension. Moreover, alpha-FMH inhibited L-histidine-induced increases in central histamine concentrations and its resuscitating effect. In conclusion, the increase in central histamine concentrations after loading with L-histidine in rats subjected to critical hemorrhagic hypovolemia leads to the reversal of hypotension and the improvement in the survival rate of 2 h. On the other hand, inhibition of L-histidine decarboxylase activity, and thus histamine synthesis, produces a decrease in hemodynamic stability in hypotension, which suggests the histaminergic system-induced activation of compensatory mechanisms in hemorrhagic shock.

Effects of endogenous histamine on seizure development of pentylenetetrazole-induced kindling in rats

This study was performed to investigate whether or not endogenous histamine can protect seizure development of pentylenetetrazole (PTZ)-induced kindling in rats. An intracerebroventricular (i.c.v.) injection with clobenpropit (5 and 10 microg), a representative H(3)-antagonist, significantly prolonged the onset of kindling and inhibited the seizure stages in a dose-dependent manner. Its action was significantly reversed by both immepip (2 microg, i.c.v.), an H(3)-agonist, and alpha-fluoromethylhistidine (alpha-FMH, 10 microg, i.c.v.), a selective histidine decarboxylase inhibitor. alpha-FMH (20 microg, i.c.v.) and pyrilamine (1 and 5 mg/kg i.p.), a classical H(1)-antagonist, markedly augmented the severity of seizure development of PTZ-induced kindling. Therefore, these results indicate that brain endogenous histamine plays a certain protective role on seizure development of PTZ-induced kindling in rats, and that its protective roles are mediated by H(1)-receptors.

Modulation of the histaminergic system and behaviour by alpha-fluoromethylhistidine in zebrafish

The functional role of histamine (HA) in zebrafish brains was studied. Zebrafish did not display a clear circadian variation in brain HA levels. Loading of zebrafish with l-histidine increased HA concentration in the brain. A single injection of the histidine decarboxylase (HDC) inhibitor, alpha-fluoromethylhistidine (alpha-FMH), gave rise to a rapid reduction in zebrafish brain HA. Low HDC activity in the brain after injections verified the effect of alpha-FMH. A reduction in the number of histaminergic fibres but not neurones and an increased expression of HDC mRNA was evident after alpha-FMH. Automated behavioural analysis after alpha-FMH injection showed no change in swimming activity, but abnormalities were detected in exploratory behaviour examined in a circular tank. No significant behavioural changes were detected after histidine loading. The time spent for performance in the T-maze was significantly increased in the first trial 4 days after alpha-FMH injections, suggesting that lack of HA may impair long-term memory. The rostrodorsal telencephalon, considered to correspond to the mammalian amygdala and hippocampus in zebrafish, is densely innervated by histaminergic fibres. These results suggest that low HA decreases anxiety and/or affects learning and memory in zebrafish, possibly through mechanisms that involve the dorsal forebrain.

The role of hypothalamic histamine in leptin-induced suppression of short-term food intake in fasted rats

OBJECTIVE

Leptin suppresses food intake; however, the precise mechanism is not fully understood. Histamine (HA), which acts as a neurotransmitter in the central nervous system, has also been shown to be involved in feeding and exerts an inhibitory effect through activation of H(1) receptors. Therefore, we studied the possible role of HA in short-term leptin-induced suppression of food intake.

METHODS

We studied the 6-h feeding response of overnight-fasted adult (200 g) male Wistar rats to leptin and the HA synthesis inhibitor alpha-fluoromethylhistidine (alpha-FMH). Levels of transcription for neuropeptide Y (NPY) and corticotropin-releasing hormone (CRH), as well as hypothalamic content of HA and the HA metabolite telemethyl-HA were investigated.

RESULTS

Central administration of leptin (3, 5 and 10 microg at 09:00 h) in fasted rats caused a decrease in food intake. In contrast, central administration of alpha-FMH (11, 22 and 112 microg at 09:00 h) increased food intake. Prior administration of alpha-FMH prevented the leptin-induced decrease in food intake. Leptin decreased hypothalamic histamine content, while increasing the ratio between telemethyl-HA and HA, indicating that leptin reduces HA metabolism. Finally, alpha-FMH suppressed basal and leptin-induced CRH expression while stimulating NPY expression in fasted rats.

CONCLUSION

Histamine is involved in leptin-induced inhibition of food intake. The role of histamine may be mediating, i.e. leptin may directly activate and/or change the metabolism of the histaminergic system. Alternatively, the histaminergic system may be involved in a permissive manner.

Glucoprivic regulation of estrous cycles in the rat

Previous research has shown that glucoprivation induced by chronic 2-deoxy-D-glucose (2DG) treatment extends estrous cycle length and disrupts reproductive behaviors in female hamsters, similar to food deprivation. Such treatment also suppresses food intake, which is reversed in male rats by reducing brain histamine levels prior to 2DG treatment. We, therefore, determined if 2DG extends estrous cycles in the female rat and if this is due to elevated brain histamine levels. We measured estrous cycle length during 2DG-induced glucoprivation, in the presence and absence of alpha-fluoromethylhistidine (FMH), a treatment that reduces brain histamine levels. Adult female rats were treated for 72 h with either saline (n = 8), 2DG (200 mg/kg S.C. every 6 h; n = 9), or FMH (100 mg/kg i.p. daily) + 2DG (200 mg/kg; n = 7). An additional group was treated with FMH (100 mg/kg i.p.; n = 5) alone. To determine if 2DG extends estrous cycles due to glucoprivation or to decreased caloric intake, a group of rats (n = 7) received a reduced diet equal to the mean daily food intake of rats receiving 2DG alone. 2DG induced more long estrous cycles compared to rats receiving saline, FMH + 2DG, or FMH alone. In rats treated with FMH + 2DG, the percentage of 4-5-day cycles was similar to that of saline-treated rats, and a high percentage of 4-5-day cycles was also observed in rats receiving a reduced diet. These data suggest that 2DG does not suppress estrous cycles through a decrease in total calorie intake, but rather by inducing glucoprivation. In addition, during 2DG-induced glucoprivation, elevated brain histamine levels contribute to the mechanism that suppresses reproductive function.

Anatomical, physiological, and pharmacological characteristics of histidine decarboxylase knock-out mice: evidence for the role of brain histamine in behavioral and sleep-wake control

The hypothesis that histaminergic neurons are involved in brain arousal is supported by many studies. However, the effects of the selective long-term abolition of histaminergic neurons on the sleep-wake cycle, indispensable in determining their functions, remain unknown. We have compared brain histamine(HA)-immunoreactivity and the cortical-EEG and sleep-wake cycle under baseline conditions or after behavioral or pharmacological stimuli in wild-type (WT) and knock-out mice lacking the histidine decarboxylase gene (HDC-/-). HDC-/-mice showed an increase in paradoxical sleep, a decrease in cortical EEG power in theta-rhythm during waking (W), and a decreased EEG slow wave sleep/W power ratio. Although no major difference was noted in the daily amount of spontaneous W, HDC-/-mice showed a deficit of W at lights-off and signs of somnolence, as demonstrated by a decreased sleep latencies after various behavioral stimuli, e.g., WT-mice placed in a new environment remained highly awake for 2-3 hr, whereas HDC-/-mice fell asleep after a few minutes. These effects are likely to be attributable to lack of HDC and thus of HA. In WT mice, indeed, intraperitoneal injection of alpha-fluoromethylhistidine (HDC-inhibitor) caused a decrease in W, whereas injection of ciproxifan (HA-H3 receptor antagonist) elicited W. Both injections had no effect in HDC-/-mice. Moreover, PCR and immunohistochemistry confirmed the absence of the HDC gene and brain HA-immunoreactive neurons in the HDC-/-mice. These data indicate that disruption of HA-synthesis causes permanent changes in the cortical-EEG and sleep-wake cycle and that, at moments when high vigilance is required (lights off, environmental change em leader ), mice lacking brain HA are unable to remain awake, a prerequisite condition for responding to behavioral and cognitive challenges. We suggest that histaminergic neurons also play a key role in maintaining the brain in an awake state faced with behavioral challenges.

Effect of alpha-FMH and DPPE on colony-forming properties of human peripheral progenitor cells

Endogenous histamine regulates the haematopoiesis. Histidine decarboxylase inhibitor decreases the histamine level, and its intracellular antagonist decreases the histamine effect. The effect of histidine decarboxylase inhibitor (alpha-fluoromethyl histidine) and the intracellular antagonist of histamine [N'N-diethyl-2-4-(phenylmethyl) phenoxyethan-amine-HCl] was investigated on the colony-forming ability of human peripheral progenitor cells. Semi-solid culture medium was used both in the presence and in the absence of 3 U/ml erythropoietin. alpha-Fluoromethyl histidine was used in the range of 50 through 150 micro Mol/ml, the concentration of N'N-diethyl-2-4-(phenylmethyl) phenoxyethanamine-HCl was between 5 and 25 micro Mol/ml. The number of both the erythroide and the granulocyte macrophage colony was significantly decreased in a concentration dependent manner by the presence of both N'N-diethyl-2-4-(phenylmethyl) phenoxyethanamine-HCl (in all concentrations used) and &alpha-fluoromethyl histidine (at higher concentration). The inhibitory effect was decreased by erythropoietin.

Low brain histamine content affects ethanol-induced motor impairment

The effect of ethanol on motor performance in humans is well established but how neural mechanisms are affected by ethanol action remains largely unknown. To investigate whether the brain histaminergic system is important in it, we used a genetic model consisting of rat lines selectively outbred for differential ethanol sensitivity. Ethanol-sensitive rats had lower levels of brain histamine and lower densities of histamine-immunoreactive fibers than ethanol-insensitive rats, although both rat lines showed no changes in histamine synthesizing neurons. Lowering the high brain histamine content of the ethanol-insensitive rats with alpha-fluoromethylhistidine before ethanol administration increased their ethanol sensitivity in a behavioral motor function test. Higher H3 receptor ligand binding and histamine-induced G-protein activation was detected in several brain regions of ethanol-naive ethanol-sensitive rats. Brain histamine levels and possibly signaling via H3 receptors may thus correlate with genetic differences in ethanol-induced motor impairment.

Histidine suppresses food intake through its conversion into neuronal histamine

Hypothalamic neuronal histamine has been shown to regulate feeding behavior and energy metabolism as a target of leptin action in the brain. The present study aimed to examine the involvement of L-histidine, a precursor of neuronal histamine, in the regulation of feeding behavior in rats. Intraperitoneal (ip) injection of L-histidine at doses of 0.35 and 0.70 mmol/kg body weight significantly decreased the 24-hr cumulative food and water intakes compared to phosphate buffered saline injected controls (P < 0.05 for each). This suppression of feeding was mimicked dose-dependently by intracerebroventricular infusion of histidine at doses of 0.5, 1.0, and 2.0 micromol/rat (P < 0.05 for each). Pretreatment of the rats with an ip bolus injection of alpha-fluoromethylhistidine, a suicide inhibitor of a histidine decarboxylase (HDC), at a dosage of 224 micromol/kg blocked the conversion of histidine into histamine and attenuated the suppressive effect of histidine on food intake from 64.2% to 88.1% of the controls (P < 0.05). Administration of 0.35 mmol/kg histidine ip increased the concentration of hypothalamic neuronal histamine compared with the controls (P < 0.05). HDC activity was increased simultaneously by histidine administration compared with the controls (P < 0.05). The present findings indicate that L-histidine suppresses food intake through its conversion into histamine in the hypothalamus.

Immunoelectron-microscopic demonstration of histamine depletion in the gastric enterochromaffin-like cells of rats treated with alpha-fluoromethylhistidine

We conducted an immunoelectron-microscopic study for histamine (HA) in the enterochromaffin-like (ECL) cells of normal rats and rats given alpha-fluoromethylhistidine (alpha-FMH, 3 mg/kg per hour) via osmotic minipumps over a period of 24 h. The indirect immunoperoxidase procedure utilized a mouse monoclonal antibody (mAb), AHA-2, which is produced against glutaraldehyde-conjugated HA. alpha-FMH is a potent and irreversible inhibitor of the HA-forming enzyme histidine decarboxylase and is known to reduce tissue HA concentrations in several tissues. The present study clearly demonstrated that HA immunoreactivity, which was found to a high degree in the cores of the granules and secretory vesicles and in the cytoplasm of ECL cells of control rats, was completely abolished from the corresponding compartments in the cells of alpha-FMH-treated rats. Furthermore, treatment with alpha-FMH drastically lowered the number of secretory vesicles and was associated with larger cores in the granules of the ECL cells. These results seem to support the idea of a HA-pathway mechanism, emphasizing that the granules in normal ECL cells take up HA from the cytosol during its transport from the Golgi zone to the more peripheral portion of the cell and condense it in their cores, thus forming mature secretory vesicles. However, the present study showed that not only the secretory vesicles but also almost all the granules seen in ECL cells were already loaded with HA in their cores, suggesting that the newborn granules very rapidly take up HA from the cytosol. Also suggested was the fact that HA depletion impairs the maturation of the granules into secretory vesicles.

Involvement of central histaminergic neurons in polypnea induced by hyperthermia in rabbits

A role of central histamine in the preoptic area/anterior hypothalamus (POA/AH) for the regulation of hyperthermia-induced polypnea was examined in anesthetized, paralyzed, vagotomized and artificially ventilated rabbits. Phrenic nerve activities were recorded to monitor respiratory neuronal output. Hyperthermia increased respiratory frequency by reductions of inspiratory time (T(I)) and expiratory time (T(E)). Pyrilamine, an H1 receptor antagonist, which was applied to the POA/AH reduced polypnea under hyperthermia. The effect of S+alpha-fluoromethylhistidine, a specific inhibitor of histidine decarboxylase, applied in a lateral ventricle was comparable to the effect of pyrilamine on polypnea. Moreover, histamine dihydrochloride applied into the POA/AH at a normal body temperature produced polypnea by reductions of T(I) and T(E). The results suggest that central histamine in the POA/AH contributes to the generation of polypnea in hyperthermia through H1 receptors.

Signaling mechanisms coupled to presynaptic A(1)- and H(3)-receptors in the inhibition of cholinergic contractile responses of the guinea pig ileum

The mechanisms coupled to adenosine A(1)- and histamine H(3)-receptors have been examined in the presynaptic inhibition of acetylcholine (ACh) release from the guinea pig ileum. Electrically evoked twitch contractions were used as a measure of neuronal ACh release. A(1)- and H(3)-receptors were activated by adenosine and R-(alpha)-methylhistamine (RAMH), respectively. The neuroinhibitory effect of adenosine and RAMH was augmented in the presence of the N-type Ca(2+) channel blocker, omega-conotoxin GVIA but unaffected by the L-type Ca(2+) channel blocker, nifedipine. The irreversible adenylyl cyclase inhibitor, MDL-12330A, potentiated the action of both adenosine and RAMH. Conversely, neither agonist was affected by the cAMP phosphodiesterase III and IV inhibitors, SKF-95654 and Ro-20-1724, respectively, or the cAMP antagonist, (R(p))-adenosine 3',5'-cyclic monophosphorothioate triethylamine. The neuromodulatory effect of adenosine, only, was potentiated by the cGMP phosphodiesterase V inhibitors, SKF-96231 and 1,3-dimethyl-6-(2-propoxy-5-methanesulfonylamidophenyl)- pyrazolo[3, 4-d]pyrimidin-4-(5H)-one but was unmodified by the cGMP analog, 8-bromo-cGMP or the guanylyl cyclase inhibitors, N-methylhydroxylamine and 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxaline-1-one (ODQ). N-Methylhydroxylamine reduced, and ODQ potentiated, the inhibitory action of H(3)-receptor activation, but 8-bromo-cGMP was without effect. The study suggests that presynaptic A(1)- and H(3)-receptors inhibit cholinergic neurotransmission in the guinea pig ileum by limiting the availability of intraneuronal Ca(2+) via inhibition of N-type Ca(2+) channels. The balance of evidence does not support the involvement of the adenylyl cyclase/cAMP or guanylyl cyclase/cGMP systems.

Central histamine contributed to temperature-induced polypnea in mice

Breathing pattern is influenced by body temperature. However, the central mechanism for changing breathing patterns is unknown. Central histamine is involved in heat loss mechanisms in behavioral studies, but little is known about its effect on breathing patterns. We examined first the effect of body temperature on breathing patterns with increasing hypercapnia in conscious mice and then that of the depletion of central histamine by S(+)-alpha-fluoromethylhistidine hydrochloride (alpha-FMH) (100 mg/kg ip), a specific inhibitor of histidine decarboxylase, at normal and raised body temperatures. A raised body temperature increased respiratory frequency with reductions in both inspiratory and expiratory time and decreased tidal volume. On the other hand, alpha-FMH lowered respiratory frequency with a prolongation of expiratory time at the raised temperature; however, this was not observed at a normal temperature. These results indicate that central histamine contributes to an increase in respiratory frequency as a result of a reduction in expiratory time when body temperature is raised.

alpha-fluoromethylhistidine depletes histamine from secreting but not from non-secreting rat stomach ECL cells

Histamine in the oxyntic mucosa of the rat stomach occurs in mast cells (10%) and ECL cells (90%). Unlike the mast cells, the ECL cells operate under the control of gastrin. alpha-Fluoromethylhistidine, an irreversible inhibitor of the histamine-forming enzyme, histidine decarboxylase depletes ECL-cell but not mast-cell histamine. This report shows that the effectiveness by which histidine decarboxylase inhibition depletes ECL-cell histamine depends on the rate of histamine secretion. Rats received alpha-fluoromethylhistidine by continuous subcutaneous infusion for 24 h. Maximally effective doses (>/=3 mg/kg/h) inhibited histidine decarboxylase and reduced oxyntic mucosal histamine in fed rats by 80-90%. In fasted rats, the reduction was 50%. alpha-Fluoromethylhistidine greatly reduced the number of histamine-immunoreactive ECL cells (immunocytochemistry) and of secretory vesicles in the ECL cells (electron microscopy) in fed but not in fasted rats. The half-life of oxyntic mucosal histamine (determined upon histidine decarboxylase inhibition) was 2.6 h in fed rats and 19.4 h in fasted rats. The amount of histamine secreted in response to gastrin (monitored by gastric submucosal microdialysis) was greatly reduced by alpha-fluoromethylhistidine in fed rats but not in fasted rats. ECL cells were isolated from rat stomach by elutriation (80% purity). Their histamine content was determined after culture, with or without alpha-fluoromethylhistidine, in the presence of varying concentrations of gastrin. In a medium containing 10 nM gastrin, ECL cells responded to a maximally effective concentration of alpha-fluoromethylhistidine (0.1 nM) with 80% reduction in histamine content. In the absence of gastrin, ECL cells responded to alpha-fluoromethylhistidine with 45% reduction of histamine; the releasable histamine pool was unaffected. In conclusion, the combination of histidine decarboxylase inhibition and a high rate of histamine secretion will promptly exhaust the ECL-cell histamine pool, while histidine decarboxylase inhibition and a low secretion rate will affect the histamine pool much less.

Blastocyst H(2) receptor is the target for uterine histamine in implantation in the mouse

The process of implantation is a ‘two-way’ interaction between the blastocyst and uterus. It has long been suspected that histamine is an important mediator in embryo-uterine interactions during implantation, but its source, targets and mechanism of actions remained undefined. We have recently demonstrated that uterine epithelial cells are the source of histamine, which peaks on day 4 of pregnancy (the day of implantation) in the mouse. In searching for its target and site of action, we discovered that preimplantation blastocysts, which express histamine type 2 receptor (H(2)), is the target for histamine action. Using multiple approaches, we demonstrate herein that uterine-derived histamine interacts with embryonic H(2) receptors in a paracrine fashion to initiate the process of implantation.

L-Histidine decarboxylase protein and activity in rat brain microvascular endothelial cells

OBJECTIVE AND DESIGN

L-Histidine decarboxylase (HDC) is the primary enzyme regulating histamine biosynthesis. This study was carried out to examine whether the cultured rat brain microvascular endothelial cells (BMEC), which constitute the blood-brain barrier (BBB), have the ability to form histamine, and whether HDC mRNA is expressed in rat BMEC.

MATERIAL

Male, 3-week-old Wistar rats were used. For in vitro studies, rat BMEC were isolated from rat brains, and subculture cells were grown on collagen-coated culture flask and slide.

METHODS

HDC assay, immunofluorescence analysis and expression of HDC mRNA by RT-PCR were performed in rat BMEC.

RESULTS

The HDC activity of the BMEC was estimated to be 0.14 +/- 0.05 p mol/min/mg protein. This activity was completely inhibited by (S)-alpha-fluoromethylhistidine, a specific inhibitor of HDC. Using a polyclonal anti HDC antibody and immunofluorescence microscopy, we confirmed the presence of HDC protein in rat BMEC. RT-PCR also showed the expression of HDC mRNA in rat BMEC.

CONCLUSIONS

L-Histidine uptaken by rat BMEC was shown to be converted to histamine, suggesting that HDC plays an important role in BBB.

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.

Constitutive activity of the histamine H(1) receptor reveals inverse agonism of histamine H(1) receptor antagonists

Transient expression of the wild-type human histamine H(1) receptor in SV40-immortalised African green monkey kidney cells resulted in an agonist-independent elevation of the basal levels of the second messenger inositoltrisphospate. Several histamine H(1) receptor antagonists, including the therapeutically used anti-allergics cetirizine, loratadine and epinastine reduced this constitutive histamine H(1) receptor activity. Inverse agonism, i.e., stabilisation of an inactive conformation of the human histamine H(1) receptor, may therefore be a key component of the anti-allergic mechanism of action of clinically used antihistamines.

Histaminergic neurons modulate acetylcholine release in the ventral striatum: role of H3 histamine receptors

To investigate whether histaminergic neurons influence the activity of cholinergic neurons, the ventral striatum was superfused through a push-pull cannula and the release of endogenous acetylcholine was determined in the superfusate. Local inhibition of histamine synthesis by superfusion with alpha-fluoromethylhistidine (FMH) gradually decreased the release rate of acetylcholine. Superfusion with histamine increased the release of acetylcholine. The releasing effect of histamine was greatly inhibited when the striatum was simultaneously superfused with the D2/D3 agonist quinpirole and the D1 antagonist (+/-)-7-bromo-1-(fluoresceinylthioureido)phenyl-8-hydroxy-3-methyl -2,3,4,5-tetrahydro-1H-3-benzapine (SKF 83566). The effect of histamine on acetylcholine release was abolished by the GABA(A) receptor antagonist bicuculline. Superfusion with the H3 receptor agonists imetit or immepip increased acetylcholine release rate in the striatum. The releasing effects of the two H3 agonists were FMH resistant, while superfusion with quinpirole and SKF 83566 abolished the H3 receptor agonist-induced acetylcholine release. Superfusion with the H3 receptor antagonist thioperamide enhanced acetylcholine release rate. The releasing effect of thioperamide was abolished after inhibition of histamine synthesis by FMH. The release of acetylcholine by thioperamide was also abolished on simultaneous superfusion with quinpirole and SKF 83566. The findings show that, in the striatum, the activity of cholinergic neurons is permanently modulated by neighbouring histaminergic nerve terminals and axons. The release of acetylcholine is also permanently inhibited by neighbouring GABAergic neurons. The enhanced release of acetylcholine by the H3 receptor agonists imetit and immepip is due to stimulation of H3 heteroreceptors, while the increase of acetylcholine release by the H3 receptor antagonist thioperamide is elicited via blockade of H3 autoreceptors. Histamine released from histaminergic nerve terminals increases the release of acetylcholine in part by inhibition of dopamine release which, in turn, decreases GABAergic transmission. A dopamine-independent way seems also to be involved in the histamine-evoked acetylcholine release.

Effects of intracerebroventricular injection of alpha-fluoromethylhistidine on radial maze performance in rats

The effects of alpha-fluoromethylhistidine (alpha-FMH) on spatial cognition were investigated using the eight-arm radial maze paradigm in rats. Intracerebroventricular (ICV) injection of alpha-FMH resulted in spatial memory deficits characterized by an increase in the number of total errors (TE) and a decrease in the number of initial correct responses (ICR). There was a strong correlation between increases in the number of TE and decreases in histamine contents of the cortex and hippocampus regions of the brain, which are known to participate in learning and memory. On the other hand, both histamine (50-100 ng, ICV) and thioperamide (10 microg, ICV) significantly ameliorated the memory deficit induced by alpha-FMH. However, metoprine showed no significant effect on the alpha-FMH-induced memory deficit. Pyrilamine and R-(alpha)-methylhistamine enhanced the memory deficit induced by alpha-FMH, at doses that had no appreciable effect when administered alone. In contrast, no significant influence on alpha-FMH-induced memory deficit was observed with zolantidine.

Interactions between the thiol-group reagent N-ethylmaleimide and neutral and basic amino acid transporter-related amino acid transport

The neutral and basic amino acid transport protein (NBAT) expressed in renal and jejunal brush-border membranes is involved in amino acid and cystine absorption. NBAT mutations result in Type 1 cystinuria. A C-terminal myc-tagged NBAT (NBATmyc) retains the amino acid transport and protein-protein interaction properties of NBAT when expressed in Xenopus oocytes. Neutral amino acid (Ala, Phe)-cationic amino acid (Arg) heteroexchanges related to NBATmyc expression in oocytes are inactivated by treatment with the thiol-group reagent N-ethylmaleimide (NEM), although significant Arg-Arg and Ala-Ala homoexchanges persist. Inactivation of heteroexchange activity by NEM is accompanied by loss of >85% of alanine and cystine uptake, with smaller (<50%) inhibition of arginine and phenylalanine uptake. NEM-sensitive cystine uptake and arginine-alanine heteroexchange (system b(0,+) activity) are not expressed by an NBAT truncation mutant (NBATmyc-Sph1) lacking the 13 C-terminal amino acid residues, but the mutant expresses NEM-resistant transport activity (system y(+)L-like) equivalent to that of full-length NBATmyc. The deleted region of NBATmyc-Sph1 contains two cysteine residues (671/683) which may be the targets of NEM action. The synthetic amino acid 2-trifluoromethylhistidine (TFMH) stimulated alanine efflux at pH 7.5 and arginine at pH 5.5, but not vice versa, establishing the existence of distinct pathways for cationic and neutral amino acid homoexchange (TFMH is zwitterionic at pH 7.5 and cationic at pH 5.5). We suggest that NBAT expresses a combination of system b(0,+) and y(+)L-like activities, possibly by interacting with different light-chain subunits endogenous to oocytes (as does the homologous 4F2hc protein). The C-terminus of NBAT may also have an additional, direct role in the mechanism of System b(0,+) transport (the major transport activity that is defective in Type 1 cystinuria).

CCK1 and CCK2 receptors regulate gastric pepsinogen secretion

The present study investigated (1) the pharmacological profile of cholecystokinin (CCK) receptor subtypes involved in the regulation of gastric pepsinogen secretion, (2) the influence of gastric acidity on peptic responses induced by CCK-8-sulfate (CCK-8S) or gastrin-I; and (3) the mechanisms accounting for the effects of CCK-like peptides on pepsinogen secretion. In anaesthetized rats, i.v. injection of CCK-8S or gastrin-I increased both pepsinogen and acid secretion. The pepsigogue effect of CCK-8S was higher than that of gastrin-I, whereas acid hypersecretion after CCK-8S was lower than that induced by gastrin-I. Peptic output following CCK-8S was partly blocked by i.v. injection of the CCK1 receptor antagonist, devazepide (-75.3%), or the CCK2 receptor antagonist, L-365,260 [3R(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepine-3 yl)-N'-(3-methyl-phenyl)urea; -27.9%], but was fully prevented by combined administration of devazepide and L-365,260. The gastric acid hypersecretory effect of CCK-8S was enhanced by devazepide (+84.5%) and blocked by L-365,260. In contrast, the gastric secretory actions of gastrin-I were insensitive to devazepide, but abolished by L-365,260. Excitatory effects of CCK-8S and gastrin-I were not modified by vagotomy or atropine, whereas cimetidine or alpha-fluoromethylhistidine (irreversible blocker of histidine decarboxylase) partly prevented acid hypersecretion induced by both peptides without affecting their pepsigogue effects. After pretreatment with omeprazole, both CCK-8S and gastrin-I failed to stimulate acid secretion, while they increased pepsinogen output. In rats with gastric perfusion of acid solutions, CCK-8S or gastrin-I increased peptic output in a pH-independent manner either with or without pretreatment with omeprazole. Ablation of capsaicin-sensitive sensory nerves as well as application of lidocaine to the gastric mucosa failed to modify the excitatory effects of CCK-8S or gastrin-I on pepsinogen and acid secretion. Blockade of the nitric oxide (NO) synthase pathway by N(G)-nitro-L-arginine-methyl ester prevented the pepsigogue actions of both CCK-8S and gastrin-I (-61.8% and -71.7%, respectively), without affecting the concomitant increase in acid output. In addition, both these peptides significantly increased the release of NO breakdown products into the gastric lumen. The present results suggest that: (1) both CCK1 and CCK2 receptors mediate the peptic secretory responses induced by CCK-like peptides; (2) the excitatory inputs of CCK-8S and gastrin-I to chief cells are not driven through acid-dependent mechanisms or capsaicin-sensitive afferent sensory nerves; and (3) under in vivo conditions, the stimulant actions of CCK-like peptides on pepsinogen secretion are mediated, at least in part, by an increase in NO generation.

Histaminergic influence on vestibular stimulation-induced locus coeruleus inhibition in rats

In previous reports we have shown that caloric stimulation (CS) of the vestibular apparatus inhibits locus coeruleus (LC)-noradrenergic neuronal activity in urethane-anaesthetized rats. The present study examined the effect of neural histamine depletion by alpha-fluoromethylhistidine (alpha-FMH) on CS-induced LC inhibition. In alpha-FMH treated rats, LC neuronal inhibition caused by CS was still observed. This finding indicates that the central histaminergic neuron system does not participate in the CS-induced LC-noradrenergic inhibition. It is suggested that the noradrenergic neuron system is involved in the development of vestibulo-autonomic response, independent of the histaminergic neuron system.

Histidine decarboxylase in peripheral lymphocytes of healthy individuals and chronic lymphoid leukemia patients

Histidine decarboxylase (HDC), the only enzyme capable of synthetizing histamine, has been found in many proliferating cells and tissues suggesting a role of histamine in cellular proliferation. In this study expression of HDC and the significance of histamine in the proliferation of peripheral lymphocytes of five healthy persons and six patients with chronic lymphoid leukemia (CLL) was examined. Expression of HDC mRNA and the protein was proved by reverse transcriptase polymerase chain reaction and by immunoblot, respectively. The role of histamine was studied in proliferation assays in the presence of irreversible inhibitor of the HDC (alpha-fluoromethylhistidine--aFMH) and also by competing for the intracellular binding sites of histamine using N,N-diethyl-2, 4-phenylmethyl-phenoxy-ethanamine-HCl (DPPE). By inhibiting the HDC enzyme activity by FMH and blocking the intracellular action of histamine by DPPE, a significant decrease in cell proliferation was observed in mitogen stimulated lymphocytes of healthy donors. In CLL patients the proliferation of leukemic lymphocytes was significantly inhibited by blocking the binding of histamine to intracellular binding sites by DPPE but not by FMH inhibiting only the de novo histamine formation. The observations suggest that HDC has functional relevance in lymphocytes, since mitogen induced lymphocyte proliferation of healthy donors is mainly enhanced by de novo synthesis and subsequent action of intracellular histamine. Alternatively, in constitutively proliferating chronic lymphoid leukemia cells we suggest that the preformed pool but not the de novo synthesized intracellular histamine interferes with cellular proliferation.

Production of monoclonal antibody against histamine and its application to immunohistochemical study in the stomach

A monoclonal antibody against histamine has been produced. A histamine-haemocyanin conjugate prepared using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide as a coupling agent was used for immunizing mice. Immunized mice were sacrificed to prepare monoclonal antibody using a hybridoma technique. On immunospot assay, the hybridoma culture supernatant containing a monoclonal antibody was capable of detecting 50 pmol of histamine. Using this antibody, we examined the cellular localization of histamine-like immunoreactivity in the stomach of normal or alpha-fluoromethylhistidine-treated rats and mice. Immunoreactive cells were abundant in the gastric mucosal layer. These positive cells were often located in the basal half of the fundic gland but were rare in the pyloric gland. The cells, small or medium in size, spindle or cone in shape, were intermingled with immunonegative epithelial cells. In the cytoplasm of the positive cells, granular reaction products were densely deposited. In addition, a few positive cells, identified as mast cells by Toluidine Blue staining, were distributed mainly in the submucosal and muscular layer. The antibody preabsorbed with 10 mM histamine gave no positive immunostaining. For pharmacological study, some rats were injected six times with c-fluoromethylhistidine every 8 h. In these rats, positive cells except mast cells were no longer detected. In conclusion, the monoclonal antibody produced appears to be highly specific for histamine. Its application in immunohistochemistry should provide a powerful tool for analysing the roles of histamine in enterochromaffin-like or mast cells in the stomach.

The role of histamine in platelet aggregation by physiological and immunological stimuli

BACKGROUND

Platelets participate in allergic and inflammatory processes beside their role in haemostasis and thrombosis. This paper reports the level, the uptake, the metabolism and the release of histamine in human platelets. The effects of exogenous histamine, as well as the receptor and signal transduction of these effects, are also described.

METHODS

Purified suspensions of platelets, prepared from healthy volunteers and from atopic patients, were exposed in vitro to physiological and immunological stimuli. Platelet aggregation was measured by the increase in light transmission; histamine content and release, as well as cytosolic free Ca2+ concentration, were measured fluorimetrically. Platelet histamine forming capacity, and the uptake of exogenous histamine, were measured with a radioisotopic method.

RESULTS

Human platelets contain 72.5 +/- 9.6pmoles of histamine x 10(9) platelets, and their capacity to form histamine is 18.7 +/- 3.5pmoles h(-1)g(-1) protein, which is reduced by alpha-fluoromethylhistidine (10(-5) M) a selective inhibitor of the specific histidine decarboxylase. Human platelets take up the preformed amine by a calcium and energy-dependent process, and the uptake of histamine is reduced by mepyramine, an H1-receptor antagonist, and N,N-diethyl-2-[4-(phenylmethyl) phenoxyl] ethanamine (10(-6) M), a blocker of intracellular histamine receptors. Histamine is also metabolized by human platelets. The exposure of platelets to thrombin (10-60 mUml(-1)) produced a progressive aggregation, associated with histamine release. The same is observed in platelets isolated from atopic patients exposed to anti-IgE antibodies. Exogenous histamine dose-dependently potentiates the aggregation induced by physiological and immunological stimuli. In resting platelets cytosolic calcium level (207 +/- 4.2 nM/10(8) platelets) is increased by thrombin as well as by anti-IgE; this effect is potentiated by 10(-5) M histamine.

CONCLUSIONS

The synergistic effect between histamine and other monoamines on platelet aggregation may explain some aspects of allergic vasculitis in which platelet aggregation is present.

Depletion of brain histamine induced by alpha-fluoromethylhistidine enhances radial maze performance in rats with modulation of brain amino acid levels

We examined the effects of repeated administration of (S)-alpha-fluoromethylhistidine (FMH), a specific inhibitor of histidine decarboxylase (HDC), on radial maze performance and brain contents of histamine and amino acids in rats. By daily subcutaneous (s.c.) administration of FMH (100 mg/kg), rats showed significant enhancement of a radial maze performance without changes in locomotion. Six days after FMH treatment, the histamine levels both in the cerebral cortex and diencephalon decreased significantly. However, the glutamate and glycine levels significantly increased in the cerebral cortex and hippocampus. These results suggest that FMH enhances the acquisition phase of radial maze study with the increases in glutamate and glycine levels in the cerebral cortex and hippocampus of rats.

Thioperamide, a histamine H3-receptor blocker, facilitates vasopressor response to footshocks

OBJECTIVE AND DESIGN

We assessed the functional role of the histamine H3-receptor in conscious intact rats during activation of the sympathoadrenal axis.

MATERIAL

Male Sprague-Dawley rats, with or without cerebroventricular cannula, were subjected to mild footshocks and mean arterial pressure (MAP) and heart rate were determined using a tail-cuff plethysmograph.

TREATMENTS

Saline, phentolamine (3 mg/kg, i.p.), (R)-alphafluoromethylhistidine (AFMH) (100 mg/kg, i.p., or 100 microg/5 microl, i.v.t.), (R)-alphamethylhistamine (AMH) (2 mg/kg, i.p. or 100 microg/5 microl, i.v.t.), thioperamide (THIO) (1 or 2 mg/kg, i.p., or 100 microg/5 microl, i.v.t.), mepyramine (10 mg/kg, i.p.), cimetidine (2 mg/kg, i.p.).

METHODS

Urinary catecholamines were determined by fluorometry. Statistical differences between experimental groups were evaluated by Student's t-test or one-way ANOVA.

RESULTS

Footshocks increased both MAP and heart rate. The vasopressor response to footshocks was facilitated (p < 0.001) by i.p. administration of AFMH, a histidine decarboxylase inhibitor, or THIO, a H3-receptor antagonist, but not by i.v.t. injection of these drugs. AMH, a H3-receptor agonist, given i.p., decreased the vasopressor response to footshocks (p < 0.001). This action of AMH was abolished by THIO but not by mepyramine or cimetidine. The MAP response to exogenous norepinephrine was not altered by i.p. administration of either AFMH or THIO.

CONCLUSIONS

Our results demonstrate an involvement of peripheral histamine H3 prejunctional receptors in the inhibitory modulation of peripheral noradrenergic responses during stress.

Evidence of a key-role for histamine from mast cells in the analgesic effect of clomipramine in rats

OBJECTIVE AND DESIGN

We investigated the role of neuronal and mast cell histamine in the analgesic effect of clomipramine.

SUBJECTS

Male Sprague-Dawley rats (4-6 weeks old) were used (n = 228).

TREATMENT

Clomipramine (10, 20 or 40 mg/kg i.p.) was injected in rats pretreated with [a] saline i.cv. or i.p., [b] alpha-fluoromethylhistidine (alpha-FMH, 200 microg i.cv.), [c] compound 48/80 (C48/80, 1 mg/kg i.p.). Other rats were pretreated with clomipramine, before C48/80.

METHODS

Antinociceptive responses were determined before and 30, 60, 90, 120 min after drug injection by tail-flick (TFT) and hot-plate (HPT) tests. Results for each treatment group are given as mean %MPE +/- SEM (Student's t-test, ANOVA).

RESULTS

Clomipramine produced no significant changes in TFT and HPT in saline- or alpha-FMH-pretreated rats. Following C48/80, clomipramine (10 and 20 mg/kg) produced a dose-related significant increase in latencies, between 30 and 120 min: 28.5 +/- 5.7 vs 8 +/- 1.6 (p < 0.05), 56 +/- 5 vs 9.2 +/- 1.9 (p < 0.01) in TFT; 31 +/- 4.3 vs 12 +/- 2.5 (p < 0.05), 46.2 +/- 6 vs 11.5 +/- 1.9 (p < 0.01) in HPT. Clomipramine (40 mg/kg, after C48/80) produced marked and persistent increase in latencies 83.2 +/- 4.2 vs 10.5 +/- 3 (p < 0.01) in TFT and 91.2 +/- 4.6 vs 10.5 +/- 3 (p < 0.01) in HPT, followed by symptoms of toxicity and death of some animals. In rats pretreated with clomipramine, C48/80 was unable to show antinociceptive effects on TFT and HPT.

CONCLUSIONS

Results suggest that the antinociceptive effect of clomipramine may depend on mast cell histamine levels.

ECL cells of the rat stomach: development of lipofuscin in response to sustained gastrin stimulation

Ageing cells, especially post-mitotic cells, are known to accumulate pigments, i.e. highly electron-dense material, referred to as ceroid or lipofuscin. This material is formed as a consequence of autophagocytosis and peroxidation of the products undergoing degradation. The present study describes the development of lipofuscin in the ECL cells of the rat stomach. These cells produce and secrete histamine in response to gastrin. They are rich in secretory vesicles, which fuse to form vacuoles in hypergastrinaemic rats. Hypergastrinaemia was induced by continuous infusion of human Leu15-gastrin-17 for 6 days or by daily treatment with omeprazole for 10 weeks. Either treatment caused both vacuoles and lipofuscin bodies to appear in large numbers; the vacuoles disappeared promptly after interruption of the hypergastrinaemia, whereas the lipofuscin bodies remained. Antrectomy-evoked hypogastrinaemia was associated with a reduced number and volume density of lipofuscin bodies. Treatment with alpha-fluoromethylhistidine, an irreversible inhibitor of the histamine-forming enzyme, resulted in depletion of ECL-cell histamine and was found to prevent the omeprazole-evoked formation of vacuoles and lipofuscin. The numbers of both vacuoles and lipofuscin bodies were well-correlated with the serum gastrin concentration, suggesting that gastrin stimulates the development not only of vacuoles but also of lipofuscin, perhaps through enhanced autophagocytosis and/or oxidative stress. Thus, lipofuscin bodies may develop from vacuoles, and both vacuoles and lipofuscin bodies may reflect the efforts of overstimulated ECL cells to cope with the excessive formation of secretory products.

Satiation and masticatory function modulated by brain histamine in rats

Both the ventromedial hypothalamus (VMH) and the mesencephalic trigeminal sensory nucleus (Me5) are densely innervated by histaminergic neurons. The depletion of neuronal histamine (HA) from the Me5 by the bilateral microinfusion of 448 nmol/rat alpha-fluoromethylhistidine (FMH), a specific suicide inhibitor of histidine decarboxylase, reduced the eating speed and prolonged meal duration, while leaving the meal size unaffected. HA depletion from the VMH increased the size of the meal and prolonged its duration, but not the eating speed. When the HA turnover rate was measured at 15 min after the scheduled feeding following fasting for less than 24 hr, the rate increased in the region including the Me5, but not in the hypothalamus. The turnover rate reached higher levels at 60 min in both regions. Gastric intubation of an isocaloric liquid diet or an equivolume of water with the liquid diet abolished the increase in HA turnover both in the Me5 region and the hypothalamus. The present findings indicate that brain HA thus modulates satiation through both the VMH and masticatory function as well as due to the action of the Me5. The HA function activated by mastication began earlier in the Me5 and later in the hypothalamus due to a signal originating from the oral proprioceptors and initiated by chewing.

Physiological significance of ECL-cell histamine

In the oxyntic mucosa of the mammalian stomach, histamine is stored in ECL cells and in mucosal mast cells. In the rat, at least 80 percent of oxyntic mucosal histamine resides in the ECL cells. Histamine is a key factor in the regulation of gastric acid secretion. Following depletion of ECL-cell histamine by treatment with alpha-fluoromethylhistidine (alpha-FMH), basal acid secretion was reduced, and gastrin-stimulated acid secretion was abolished. Vagally-induced acid secretion (by insulin injection or pylorus ligation) was unaffected by alpha-FMH treatment but inhibited by an H2 antagonist. These results suggest that gastrin stimulates acid secretion via release of ECL-cell histamine, whereas vagally-induced acid secretion--although histamine-dependent--does not rely on ECL-cell histamine. Gastrin is known to have a trophic effect on the oxyntic mucosa. By combining long-term hypergastrinemia with continuous infusion of alpha-FMH, we were able to show that gastrin-evoked trophic effects in the stomach do not depend on ECL-cell histamine.