H3-Receptors Control Histamine Release in Human Brain

The regulation of histamine release was studied on slices prepared from pieces of human cerebral cortex removed during neurosurgery and labeled with L-[3H]histidine. Depolarization by increased extracellular K+ concentration induced [3H]histamine release, although to a lesser extent than from rat brain slices. Exogenous histamine reduced by up to 60% the K+-evoked release, with an EC50 of 3.5 +/- 0.5 X 10(-8) M. The H3-receptor antagonists impromidine and thioperamide reversed the histamine effect in an apparently competitive manner and enhanced the K+-evoked release, indicating a participation of endogenous histamine in the release control process. The potencies of histamine and the H3-receptor antagonists were similar to those of these agents at presynaptic H3-autoreceptors controlling [3H]histamine release from rat brain slices. It is concluded that H3-receptors control histamine release in the human brain.     

Role of central histaminergic mechanism in behavioural depression (swimming despair) in mice

The role of the central histaminergic system in depression was studied by using swimming despair test in mice - a behavioural model of depression. In this test, immobility of mice reflects a state of depression. Intracerebral (ic) injection of histamine (50-200 micrograms) increased significantly the immobility. The H1-receptor blocker mepyramine (2.5-20 mg/kg ip) had no effect while H2-receptor blocker cimetidine (100-200 micrograms ic) caused a significant decrease in immobility. The histamine induced facilitation was blocked completely by cimetidine and antidepressant drugs-imipramine and desipramine, but remained unaffected in mice pretreated with mepyramine or atropine. The H2 agonist impromidine (20-40 micrograms ic) also enhanced significantly, the immobility which was blocked by cimetidine and antidepressant drugs. It has been concluded that central H2-receptors facilitate depression and antidepressant drugs block central H2-receptors.     

The H3 receptor is involved in cholecystokinin inhibition of food intake in rats.

We investigated the peripheral effects of an H3-receptor agonist and an H3-receptor antagonist (R)alpha-methylhistamine (Ralpha-MeHA) and thioperamide, respectively, on basal feeding and the CCK8-induced inhibition of food intake in rat. Intraperitoneal injection of thioperamide reduced food intake in a dose-dependent manner with maximal inhibition (35%, P<0.01 vs saline) at 3 mg/kg. (R)alpha-MeHA (0.3-3 mg/kg i.p.), an H3-receptor agonist alone had no effect on feeding but reversed the thioperamide-induced inhibition of food intake in a dose-dependent manner. The maximal feeding inhibitory dose of thioperamide (3 mg.kg i.p) increased by 40% and 22 % (P<0.01 vs saline) brain and stomach histamine contents, respectively. Histamine (0.3 - 6 mg/kg i.p.) and CCK-8 (3 - 30 microg/kg i.p) also inhibited food intake in a dose-dependent manner. Inhibition was 20% to 40% for histamine and 40% to 80% (P<0.01 vs saline) for CCK8. CCK-8 inhibition of feeding was increased by thioperamide and prevented by (R)alpha-MeHA in a dose-dependent way. In addition, CCK-8 did not reduce food intake if rats were pretreated with pyrilamine or ranitidine postsynaptic H1- and H2-receptor antagonists respectively. Our data suggest that the H3-receptor is involved in basal feeding. They also suggest that CCK satiety depends upon the release of histamine which acts on the H2- and H1-receptors, the final mediators of this effect.     

Histamine H3 receptors regulate acetylcholine release from the guinea pig ileum myenteric plexus.

The effect of selective histamine H3-receptor agonists and antagonists on the acetylcholine release from peripheral nerves was evaluated in the guinea pig longitudinal muscle-myenteric plexus preparations, preloaded with (3H)choline. In the presence of H1 and H2 blockade, histamine (10(-7)-10(-4) M) and (R)-alpha-methylhistamine (10(-8)-10(-6) M) inhibited the electrically-evoked acetylcholine release, being (R)-alpha-methylhistamine more active than histamine, but behaving as a partial agonist. The effect of histamine was completely reversed by selective H3-blocking drugs, thioperamide and impromidine, while only submaximal doses of (R)-alpha-methylhistamine were antagonized. Furthermore, thioperamide and impromidine enhanced the electrically-evoked acetylcholine release. On the contrary, the new H3-blocker, HST-7, was found substantially ineffective, both as histamine antagonist and as acetylcholine overflow enhancer. These data suggest that histamine exerts an inhibitory control on the acetylcholine release from intestinal cholinergic nerves through the activation of H3 receptors.     

Evidence for histamine as a neurotransmitter in the cardiac sympathetic nervous system

The colocalization of histamine (HA) and norepinephrine (NE) immunoreactivities was identified within the superior cervical ganglia neurons of the guinea pig. HA and NE immunoreactivity levels were significantly attenuated after chemical sympathectomy with 6-hydroxydopamine (6-OHDA). Coexistence of NE and HA was also visualized in the cardiac sympathetic axon and varicosities labeled with anterograde tracer biotinylated dextran amine. Depolarization of cardiac sympathetic nerve endings (synaptosomes) with 50 mM potassium stimulated endogenous HA release, which was significantly attenuated by 6-OHDA or a vesicular monoamine transporter 2 (VMAT2) inhibitor reserpine pretreatments. Compound 48/80, a mast cell releaser, did not affect cardiac synaptosome HA exocytosis. Furthermore, K+ -evoked HA release was abolished by the N-type Ca2+ -channel blocker omega-conotoxin but was not affected by the L-type Ca2+ -channel blocker lacidipine. Cardiac synaptosome HA exocytosis was augmented by the enhanced synthesis of HA or the inhibition of HA metabolism. HA H3-receptor activation by (R)-alpha-methylhistamine inhibited high K+ -evoked histamine release. The HA H3 receptor antagonist thioperamide enhanced K+ -evoked HA release and blocked the (R)-alpha-methylhistamine effect. The K+ -evoked endogenous NE release was attenuated by preloading the cardiac synaptosomes with L-histidine or quinacrine. These inhibitory effects were reversed by thioperamide or antagonized by alpha-fluoromethylhistidine. Our findings indicate that high K+ -evoked corelease of NE and HA may be inhibited by endogenous HA via activation of presynaptic HA H3-receptors. The H3-receptor may function as an autoreceptor, rather than a heteroreceptor, in the regulation of sympathetic neurotransmission and HA may be a novel sympathetic neurotransmitter.     

Selective stimulation of histamine H3 autoreceptors

A simple derivative of histamine, alpha-methylhistamine i.e. 4-(2-aminopropyl)-imidazole, was shown to potently inhibit the K+-induced release of [3H] histamine from slices of rat cerebral cortex previously incubated in the presence of [3H] histidine. The maximal inhibition elicited by alpha-methylhistamine was of about 60% i.e. similar to that elicited by exogenous histamine. The effect occurred with an EC50 value of 4.3 +/- 1.1 X 10(-9) M about 10 times lower than that of histamine and was reversed by a H3-receptor antagonist. Since alpha-methylhistamine is known to display negligible potency at H1- and H2-receptors, this compound appears to be the first highly potent and selective H3-receptor agonist to be identified.     

Effects of the Histamine H3-Agonist (R)-α-Methylhistamine and the Antagonist Thioperamide on Histamine Metabolism in the Mouse and Rat Brain

To study the feedback control by histamine (HA) H3-receptors on the synthesis and release of HA at nerve endings in the brain, the effects of a potent and selective H3-agonist, (R)-alpha-methylhistamine, and an H3-antagonist, thioperamide, on the pargyline-induced accumulation of tele-methylhistamine (t-MH) in the brain of mice and rats were examined in vivo. (R)-alpha-Methylhistamine dihydrochloride (6.3 mg free base/kg, i.p.) and thioperamide (2 mg/kg, i.p.), respectively, significantly decreased and increased the steady-state t-MH level in the mouse brain, whereas these compounds produced no significant changes in the HA level. When administered to mice immediately after pargyline (65 mg/kg, i.p.), (R)-alpha-methylhistamine (3.2 mg/kg, i.p.) inhibited the pargyline-induced increase in the t-MH level almost completely during the first 2 h after treatment. Thioperamide (2 mg/kg, i.p.) enhanced the pargyline-induced t-MH accumulation by approximately 70% 1 and 2 h after treatment. Lower doses of (R)-alpha-methylhistamine (1.3 mg/kg) and thioperamide (1 mg/kg) induced significant changes in the pargyline-induced t-MH accumulation in the mouse brain. In the rat, (R)-alpha-methylhistamine (3.2 mg/kg, i.p.) and thioperamide (2 mg/kg, i.p.) also affected the pargyline-induced t-MH accumulation in eight brain regions and the effects were especially marked in the cerebral cortex and amygdala. These results indicate that these compounds have potent effects on HA turnover in vivo in the brain.     

Central histaminergic stimulation of pituitary--adrenocortical response in the rat

In conscious rats histamine, the H1-receptor agonist 2-pyridylethylamine (PEA), and the H2-receptor agonists dimaprit and impromidine given intracerebroventriculary (i.c.v.) increased the hypophyseal-adrenocortical response, evaluated indirectly through the corticosterone concentration in the blood serum. On a molar basis histamine was the most potent drug whereas its agonists were less potent in inducing an increased corticosterone response. Impromidine however, was far more active than dimaprit and PEA. The effect of histamine was significantly yet not totally antagonized by either mepyramine, a H1-receptor antagonist, or cimetidine, a H2-receptor blocker. The combination of mepyramine and cimetidine caused a considerably stronger inhibition than that induced by either antagonist given separately. Mepyramine impaired the corticosterone response to PEA, and the responses to impromidine and dimaprit were significantly diminished by cimetidine. The results suggest that i.c.v. histamine increases the pituitary-adrenocortical activity via both H1- and H2-receptors, and there seems to be no significant prevalence of either of these receptors in mediating this action of histamine.     

Effects of thioperamide, a histamine H3 receptor antagonist, on locomotor activity and brain histamine content in mast cell-deficient W/Wv mice.

The purpose of this study was to examine the effects of thioperamide, a histamine H3 antagonist, on the locomotor activity and the brain histamine content in mast-cell-deficient W/Wv mice. Thioperamide (12.5 and 25 mg/kg) showed significant increase in the locomotor activity of W/Wv mice, measured by a photo-beam system, 1 hr after the intraperitoneal injection. However, more than 75 mg/kg of thioperamide showed not only the reduction of the locomotor activity but also the inhibition of motor coordination measured by the rotarod performance. The increase in the locomotor activity by thioperamide was blocked by i. p. pretreatment with (R)-alpha-methyl-histamine, an H3 agonist, or pyrilamine, an H1 antagonist, or zolantidine, an H2 antagonist. The brain histamine content was decreased by thioperamide (12.5-75.0 mg/kg), 1 hr after administration. Thus, the blockade of histamine H3 receptor by thioperamide showed the activation of locomotor activity of mice, which may be mediated by H1 and/or H2 receptors. The present data support the hypothesis that central histaminergic neurons may be involved in the control of state of wakefulness.     

Activation of histamine H3 receptors inhibits renal noradrenergic neurotransmission in anesthetized dogs

To investigate the possible involvement of histamine H(3) receptors in renal noradrenergic neurotransmission, effects of (R)alpha-methylhistamine (R-HA), a selective H3-receptor agonist, and thioperamide (Thiop), a selective H3-receptor antagonist, on renal nerve stimulation (RNS)-induced changes in renal function and norepinephrine (NE) overflow in anesthetized dogs were examined. RNS (0.5-2.0 Hz) produced significant decreases in urine flow and urinary sodium excretion and increases in NE overflow rate (NEOR), without affecting renal hemodynamics. When R-HA (1 microg x kg(-1) x min(-1)) was infused intravenously, mean arterial pressure and heart rate were significantly decreased, and there was a tendency to reduce basal values of urine flow and urinary sodium excretion. During R-HA infusion, RNS-induced antidiuretic action and increases in NEOR were markedly attenuated. Thiop infusion (5 microg x kg(-1) x min(-1)) did not affect basal hemodynamic and excretory parameters. Thiop infusion caused RNS-induced antidiuretic action and increases in NEOR similar to the basal condition. When R-HA was administered concomitantly with Thiop infusion, R-HA failed to attenuate the RNS-induced antidiuretic action and increases in NEOR. However, in the presence of pyrilamine (a selective H1-receptor antagonist) or cimetidine (a selective H2-receptor antagonist) infusion, R-HA attenuated the RNS-induced actions, similarly to the case without these antagonists. Thus functional histamine H3 receptors, possibly located on renal noradrenergic nerve endings, may play the role of inhibitory modulators of renal noradrenergic neurotransmission.     

Effect of combined treatment of thioperamide with some antiepileptic drugs on methionine-sulfoximine induced convulsions in mice

Methionine-sulfoximine (MSO), a convulsant is known to increase the activity of histamine N-methyl transferase. The effect of a selective H3 receptor agonist R- (alpha) methylhistamine (RAMH) and antagonist (thioperamide, THP) and some antiepileptic drugs (gabapentin and sodium valproate) have been evaluated on MSO-induced convulsions in mice. The effect of THP was also evaluated in combination with these antiepileptic drugs. Sodium valproate (300 mg/kg, po) and gabapentin (400 mg/kg, po) offered protection against MSO-induced convulsions as evidenced by a significant prolongation of latency to abnormal dorsoflexion and complete protection against mortality within 6 h of administration. THP (15 mg/kg, ip) alone and in combination with sub-effective doses of gabapentin (75 mg/kg, po) and sodium valproate (75 mg/kg, po) revealed no significant differences from the control group or either drug alone. Hence, the convulsant action of MSO does not appear to be mediated via histaminergic mechanisms.     

Histamine H1 receptors mediate the anorectic action of the pancreatic hormone amylin

We investigated the role of histamine H1 receptors in mediating the anorectic effect of intraperitoneally injected amylin (5 and 20 microg/kg), the amylin agonist salmon calcitonin (sCT; 10 microg/kg), leptin (1.3 mg/kg), and cholecystokinin (CCK; 20 microg/kg). The experiments were performed with mice lacking functional H1 receptors (H1Rko) and wild-type (WT) controls. The mice were also injected with the H3 antagonist thioperamide (20 mg/kg), which reduces feeding by enhancing the release of endogenous histamine through presynaptic H3 receptors. The feeding-suppressive effect of thioperamide was abolished in H1Rko mice. The anorectic effects of amylin and sCT were significantly reduced in 12-h food-deprived H1Rko mice compared with WT mice [1-h food intake: WT-NaCl 0.51 +/- 0.05 g vs. WT-amylin (5 microg/kg) 0.30 +/- 0.06 g (P < 0.01); H1Rko-NaCl 0.45 +/- 0.05 g vs. H1Rko-amylin 0.40 +/- 0.04 g; WT-NaCl 0.40 +/- 0.09 g vs. WT-sCT (10 microg/kg) 0.14 +/- 0.10 g (P < 0.05); H1Rko-NaCl 0.44 +/- 0.08 g vs. H1Rko-sCT 0.50 +/- 0.06 g]. The anorectic effect of leptin was absent in ad libitum-fed H1Rko mice, whereas CCK equally reduced feeding in WT and H1Rko animals. This suggests that the histaminergic system is involved in mediating the anorectic effects of peripheral amylin and sCT via histamine H1 receptors. The same applies to leptin but not to CCK. H1Rko mice showed significantly increased body weight gain compared with WT mice, supporting the role of endogenous histamine in the regulation of feeding and body weight.     

Participation of cyclic nucleotides and phospholipidis in signal transduction of histamin receptor H3 of the gastric mucosa parietal cells in rats with experimental ulcer

The goal of the work was to evaluate plasma membrane phospholipid composition in rat gastric parietal cells under the histamine H3 receptor activation. The content of cyclic nucleotides was also studied. It was shown that H3-receptor agonist (R)-alpha-methylhistamine increases the phosphatidylcholine (PC) and phosphatidylethanolamine (PE) level and decreases the phoshatidylinositole level in plasma membrane of rat gastric parietal cells and leads to attenuation of the cGMP production and enhancement of the cAMP production under the experimental stress--the induced stomach ulcer formation. The histamine H3-receptor antagonist, thioperamide, insignificantly increases the PC and PE level and increases the phoshatidylinositole level in the plasma membrane of rat gastric parietal cells and leads to cAMP production attenuation, and cGMP contents decreases in the above-stated cells. Thus it was shown that histamine H3 receptor activation causes different effects on polyphosphoinositide and adenylatcyclase cascades in parietal cells under stomach ulcer conditions.     

The influence of alpha-fluoromethylhistidine and histamine receptor antagonists on the beta-endorphin-induced corticosterone response

Involvement of a central histaminergic mechanism in the stimulating effect of beta-endorphin (beta-End) on the pituitary-adrenocortical activity, measured indirectly through corticosterone secretion, was investigated in conscious rats. The rise in serum corticosterone levels, induced by beta-End injected intraventricularly (icv) was considerably impaired by pretreatment with naltrexone, an opioid receptor antagonist. The stimulating effect of beta-End was almost totally suppressed by a prior icv administration of mepyramine, a histamine H1-receptor antagonist, and also considerably reduced by pretreatment with cimetidine, an H2-receptor antagonist. The strongest suppression, by 83%; of the beta-End-induced corticosterone response was evoked by a prior administration of alpha-fluoromethylhistidine, an inhibitor of neuronal histamine synthesis in the brain. These results indicate that both the brain neuronal histamine and central histamine H1- and H2-receptors are considerably involved in the beta-endorphin-induced stimulation of the pituitary-adrenocortical activity.     

Dehydration-induced vasopressin secretion in humans: involvement of the histaminergic system

In rats, the hypothalamic neurotransmitter histamine participates in regulation of vasopressin secretion and seems to be of physiological importance, because blockade of the histaminergic system reduces dehydration-induced vasopressin secretion. We investigated whether histamine is also involved in regulation of vasopressin secretion during dehydration in humans. We found that 40 h of dehydration gradually increased plasma osmolality by 10 mosmol/kg and induced a fourfold increase in vasopressin levels. Pretreatment with the H(2)-receptor antagonists cimetidine or ranitidine significantly reduced the dehydration-induced increase in vasopressin levels approximately 40% after 34 and 37 h of dehydration, whereas this was not the case with the H(1)-receptor antagonist mepyramine. Dehydration reduced aldosterone secretion by approximately 50%. This effect of dehydration was reduced by both H(1)- and H(2)-receptor blockade after 16 and/or 34 h of dehydration. We conclude that vasopressin secretion in response to dehydration in humans is under the regulatory influence of histamine and that the effect seems to be mediated via H(2)-receptors. In addition, the regulation of aldosterone secretion during dehydration also seems to involve the histaminergic system via H(1) and H(2) receptors.     

Adenosine-angiotensin II interactions in pentylenetetrazol seizure threshold in mice.

The effects of adenosinergic and angiotensin IIergic agents and of their combinations on the seizure threshold in mice were determined by measuring the dose of timed-intravenous (tail vein) infused pentylenetetrazol (PTZ) required to elicit clonic seizures. All drugs were administered intracerebroventricularly (i.c.v.). Angiotensin II (ANG II), its peptide analogue sarmesin, the selective adenosine A1 receptor agonists N6-cyclopentyladenosine (CPA) and 2-chloroadenosine (2-ClAdo) significantly increased the PTZ seizure threshold. The selective AT1 receptor antagonist losartan blocked the anticonvulsant effect of ANG II, sarmesin and CPA. The selective AT2 receptor antagonist PD 123319 failed to block the effect of ANG II and sarmesin on the PTZ seizure threshold but reversed the threshold-increasing effect of CPA. The selective adenosine A1 receptor antagonist 8-(p-sulfophenyl)-theophylline (8-p-SPT) alleviated the threshold-increasing effect of CPA and ANG II. Concurrent injection of 2-ClAdo and ANG II as well as of 2-ClAdo and sarmesin, at doses which had no significant effect on the PTZ seizure threshold when given alone, acted synergistically, producing greater effect on the threshold. Taken together, the findings support the possibility of specific ANG II-adenosine A1 receptor interactions in the regulation of the PTZ seizure threshold.     

Influence of sex on the interaction between dizocilpine (MK-801) pretreatment and acute cold-restraint stress in epilepsy susceptibility in an animal study

Background: Stress is a part of our daily life, inducing neurochemical and neurophysiological changes in the central nervous system.Objective: The present study was designed to investigate the importance of sex differences in the interaction between dizocilpine (MK-801) pretreatment and acute cold-restraint stress (CRS) in pentylenetetrazole (PTZ)-induced seizures in Swiss albino mice.Methods: A CRS protocol was applied to mice to investigate the interaction between MK-801 pretreatment (30 min before CRS) and stress (followed by PTZ injection) in epilepsy susceptibility. For this purpose, 6 groups were designated: (1) PTZ control group (received only PTZ); (2) stress group (received stress and PTZ); (3) saline group (received saline and PTZ); (4) MK-801 group (received MK-801 and PTZ); (5) saline + stress group (received saline, stress, and PTZ); and (6) MK-801 + stress group (received MK-801, stress, and PTZ).Results: Pretreatment with MK-801 (0.125, 0.25, 0.50 mg/kg) significantly potentiated the protective effect of stress in PTZ-induced (65 mg/kg) seizures in both sexes by prolonging the onset of myoclonic jerks and clonic convulsions. Male mice had a significantly greater delay in the onset of myoclonic jerks (males, 66.7–295.5 sec; females, 54.0–247.5 sec; P < 0.05) and clonic convulsions (males, 123.5–789.8 sec; females, 94.5–757.2 sec; P < 0.05) compared with female mice in all groups (ie, PTZ control, stress, saline, MK-801, saline + stress, and MK-801 + stress groups).Conclusion: The findings of this study in mice suggest the involvement of sex hormones in the interaction between MK-801 pretreatment and acute CRS in PTZ-induced seizures.     

Suppression of pulmonary and systemic vascular histamine H2-receptors in allergic sheep

We have previously demonstrated a depression of airway H2-receptor function in sheep allergic to Ascaris suum antigen. To investigate whether this is a generalized defect, we studied the H1- and H2- histamine receptor functions in the pulmonary and systemic circulations of allergic and nonallergic sheep. Pulmonary arterial pressure, and cardiac output were measured for calculation of pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR) before and immediately after a rapid intrapulmonary infusion of histamine (10 micrograms/kg), with and without pretreatment with H1- (chlorpheniramine) and H2- (metiamide) antagonists. Histamine alone increased mean PVR to 435 and 401% of base line and decreased mean SVR by 51 and 54% in the nonallergic and allergic sheep, respectively (P less than 0.001). In the nonallergic sheep following pretreatment with chlorpheniramine (selective H2 stimulation) or metiamide (selective H1 stimulation), histamine decreased SVR by 18 and 36%, respectively, suggesting that approximately two-thirds of the vasodepressor response was mediated by H1-receptors and one-third by H2-receptors. Combined H1- and H2-antagonists completely blocked the histamine response. In allergic sheep the histamine-induced decrease in SVR was primarily mediated by H1-receptors, because the response was blocked by H1-antagonist, chlorpheniramine, and the H2-antagonist, metiamide, had no effect. In the pulmonary circulation selective H1-stimulation caused a similar increase in PVR in allergic (365%) and nonallergic sheep (424%), whereas selective H2-stimulation caused a significant decrease in PVR in the nonallergic group (14%) but not in the allergic group.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prostaglandin E receptor subtypes involved in stimulation of gastroduodenal bicarbonate secretion in rats and mice.

We investigated prostaglandin E (EP) receptor subtypes responsible for the HCO3- stimulatory action of prostaglandin E2 (PGE2) in the gastroduodental mucosa, by examining the effects of various prostanoids with subtype specific EP receptor agonists in rats and those of PGE2 in knockout mice lacking EP1 or EP3 receptors. In rats, gastric HCO3- secretion was stimulated by i.v. administration of PGE2, 17-phenyl PGE2 the selective EP1 agonist as well as sulprostone the EP1 and EP3 agonist, but was not affected by other EP agonists such as butaprost the selective EP2 agonist, ONO-NT-012 the selective EP3 agonist or 11-deoxy PGE1 the EP3 and EP4 agonist. In contrast, the HCO3- secretion in rat duodenums was stimulated by PGE2, sulprostone, ONO-NT-012 as well as 11-deoxy PGE1 but not affected by either 17-phenyl PGE2 or butaprost. The HCO stimulatory effect of sulprostone in the stomach was significantly inhibited by ONO-AE-829, the selective EP1 antagonist. On the other hand, PGE2 applied topically to the mucosa for 10 min caused a dose-dependent increase of HCO3- secretion in both the stomach and duodenum of wild-type mice. The HCO3- stimulatory action of PGE2 in the stomach was also observed dose-dependently in knockout mice lacking EP3-receptors but was absent in EP1-receptor knockout mice, while the stimulatory effect in the duodenum was observed in EP1-receptor knockout mice, similar to wild-type animals, but not in knockout mice lacking EP3-receptors. These results indicate that PGE2 stimulates HCO3- secretion via different EP receptor subtypes in the stomach and duodenum; the former is mediated by EP1-receptors, while the latter mediated by EP3-receptors.