Interaction of histamine H2-receptor antagonists with GABA and benzodiazepine binding sites in the CNS

Additive effect of histamine and muscimol upon induction of antinociceptive and antidepressant effects in mice

We investigated the effects of histamine and GABA A receptor agents on pain and depression-like behaviors and their interaction using a tail-flick test and the forced swimming test (FST) in male mice. Our data revealed that intraperitoneal administration of muscimol (0.12 and 0.25 mg/kg) increased the percentage of maximum possible effect (%MPE) and area under the curve (AUC) of %MPE, indicating an antinociceptive response. Intraperitoneal injection of bicuculline (0.5 and 1 mg/kg) decreased %MPE and AUC of %MPE, suggesting hyperalgesia. Moreover, muscimol by reducing the immobility time of the FST elicited an antidepressant-like response but bicuculline by enhancing the immobility time of the FST caused a depressant-like response. Intracerebroventricular (i.c.v.) microinjection of histamine (5 µg/mouse) enhanced %MPE and AUC of %MPE. i.c.v. infusion of histamine (2.5 and 5 µg/mouse) decreased immobility time in the FST. Co-administration of different doses of histamine along with a sub-threshold dose of muscimol potentiated antinociceptive and antidepressant-like responses produced by histamine. Cotreatment of different doses of histamine plus a noneffective dose of bicuculline reversed antinociception and antidepressant-like effects elicited by histamine. Cotreatment of histamine, muscimol, and bicuculline reversed antinociceptive and antidepressant-like behaviors induced by the drugs. The results demonstrated additive antinociceptive and antidepressant-like effects between histamine and muscimol in mice. In conclusion, our results indicated an interaction between the histaminergic and GABAergic systems in the modulation of pain and depression-like behaviors.

GABA-ergic agents modulated the effects of histamine on male mice behavior in the elevated plus-maze

NEW FINDINGS

What is the main question of this study? Is there an interaction between histamine and GABAergic system on modulation of anxiety in mice? What is the main finding and its importance? There is a synergistic anxiogenic effect between histamine and bicuculline in mice. This effect may be due to a direct or indirect effect of the histaminergic system on the GABAergic system.

ABSTRACT

There are documents that both histaminergic and GABAergic systems are participated in the neurobiology of anxiety behavior. In the current research, we investigated the effects of the histaminergic system and GABAA receptor agents on anxiety-related behaviors and their interaction using the elevated plus-maze (EPM) test in mice. Intraperitoneally (i.p.) administration of muscimol (0.12 and 0.25 mg/kg) increased the open arm time (OAT) (p < 0.001) without affecting the open arm entries (OAE) and locomotor activity, showing an anxiolytic effect. I.p. injection of bicuculline (0.5 and 1 mg/kg) decreased OAT (p < 0.001) but not OAE and locomotor activity, suggesting an anxiogenic behavior. Intracerebroventricularly (i.c.v.) microinjection of histamine (2.5 and 5 μg/mouse) declined OAT (p < 0.001) but not OAE and locomotor activity, indicating an anxiogenic response. Co-administration of histamine with GABAergic agents, muscimol (0.06 mg/kg; i.p.) and bicuculline (0.25 mg/kg; i.p.), decreased (p < 0.001) and increased (p < 0.05) the anxiogenic-like response of the effective dose (5 μg/mouse; i.c.v.) of histamine, respectively. In addition, co-treatment of effective doses of histamine (2.5 and 5 μg/mouse;i.c.v.) along with an effective dose of muscimol (0.12 mg/kg;i.p.) and not-effective dose of bicuculline (0.25 mg/kg; i.p.) significantly decreased OAT (p < 0.001), suggesting a likely interaction between the histaminergic and GABAergic systems on the regulation of anxiety. The results demonstrated a synergistic anxiogenic-like effect between histamine and bicuculline in mice. In conclusion, our results presented an interaction between the histaminergic and GABAergic systems on anxiolytic/anxiogenic-like behaviors in the EPM test. This article is protected by copyright. All rights reserved.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Effects of antihistamines on the function of human α7-nicotinic acetylcholine receptors

Effects of the histamine H₁ receptor (H1R) antagonists (antihistamines), promethazine (PMZ), orphenadrine (ORP), chlorpheniramine (CLP), pyrilamine (PYR), diphenhydramine (DPH), citerizine (CTZ), and triprolidine (TRP) on the functional properties of the cloned α7 subunit of the human nicotinic acetylcholine receptor expressed in Xenopus oocytes were investigated. Antihistamines inhibited the α7-nicotinic acetylcholine receptor in the order PYR>CLP>TRP>PMZ>ORP≥DPH≥CTZ. Among the antihistamines, PYR showed the highest reversible inhibition of acetylcholine (100 µM)-induced responses with IC₅₀ of 6.2 µM. PYR-induced inhibition was independent of the membrane potential and could not be reversed by increasing the concentration of acetylcholine. Specific binding of [¹²⁵I] α-bungarotoxin, a selective antagonist for α7-nicotinic acetylcholine receptor, was not changed in the presence of PYR suggesting a non-competitive inhibition of nicotinic receptors. In line with functional experiments, docking studies indicated that PYR can potentially bind allosterically with the α7 transmembrane domain. Our results indicate that the H₂-H₄ receptor antagonists tested in this study (10 µM) showed negligible inhibition of α7-nicotinic acetylcholine receptors. On the other hand, H₁ receptor antagonists inhibited the function of human α7-nicotinic acetylcholine receptor, with varying potencies. These results emphasize the importance of α7-nicotinic acetylcholine receptor for future pharmacological/toxicological profiling.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Retrograde study of projections from the tuberomammillary nucleus to the dorsal raphe and the locus coeruleus in the rat

In the first series of experiments, a retrograde tracer, WGA-apo-HRP-gold (WG), was injected into the dorsal raphe (DR) or the locus coeruleus (LC) and adenosine deaminase immunostaining was subsequently performed for the tuberomammillary nucleus (TMN) in order to investigate projections from the TMN to the two brainstem monoaminergic nuclei. Following rostral DR injections, the majority of retrogradely labeled cells were located in the dorsomedial and ventrolateral subdivisions of the TMN. At middle DR levels, midline injections resulted in labeling mainly in the ventrolateral subdivision, whereas lateral wing injections produced labeling mostly in ventral and caudal TMN subdivisions. When injections were made in the caudal DR, only a few cells were observed along the rostro-caudal extent of the TMN. On the other hand, following rostral LC injections, labeled neurons were observed mainly in ventrolateral and ventral subdivisions of TMN. For principal LC injections, labeled cells were observed mostly in ventrolateral, ventral, and caudal TMN subdivisions, whereas for injections at caudal pole of LC, only a few cells were located along the rostro-caudal extent of the TMN. In the second series of experiments, an iontophoretic injection of fluorogold (FG) into the DR was paired with a pressure injection of WG into the LC to investigate the collateral distribution of TMN axonal fibers to DR and LC. Double-labeled cells were observed in ventrolateral, ventral, and caudal TMN subdivisions. The present study indicated that there exists a robust projection from the TMN to the DR or the LC and that some TMN neurons have axon collaterals projecting to both DR and LC. The TMN neurons with such axon collaterals might provide simultaneous, possibly more efficient, way of controlling the brainstem monoaminergic nuclei, thus influencing various sleep and arousal states of the animal.

Effects of cimetidine-like drugs on recombinant GABAA receptors

Even though conventional systemic doses of cimetidine and other histamine H(2) antagonists display minimal brain penetration, central nervous system (CNS) effects (including seizures and analgesia) have been reported after administration of these drugs in animals and man. To test the hypothesis that cimetidine-like drugs produce these CNS effects via inhibition of GABA(A) receptors, the actions of these drugs were studied on seven different, precisely-defined rat recombinant GABA(A) receptors using whole-cell patch clamp recordings. The H(2) antagonists famotidine and tiotidine produced competitive and reversible inhibition of GABA-evoked currents in HEK293 cells transfected with various GABA(A) receptor subunits (IC(50) values were between 10-50 microM). In contrast, the H(2) antagonist ranitidine and the cimetidine congener improgan had very weak (if any) effects (IC(50) > 50 microM). Since the concentrations of cimetidine-like drugs required to inhibit GABA(A) receptors in vitro (greater than 50 microM) are considerably higher than those found during analgesia and/or seizures (1-2 microM), the present results suggest that cimetidine-like drugs do not appear to produce seizures or analgesia by directly inhibiting GABA(A) receptors.

The cardiovascular effects of thyrotropin-releasing hormone (TRH) are attenuated by cimetidine in rats

The modulation of cardioventilator effects of thyrotropin-releasing hormone (TRH) by histaminergic mechanisms was studied in anaesthetized rats pretreated with histamine receptor antagonists. TRH (1-100 nmol/kg) into the lateral cerebral ventricle dose-dependently elevated mean arterial pressure, heart rate and stimulated respiration. The respiratory stimulating effect of TRH remained unchanged after pretreatments with histamine H1-receptor antagonist diphenhydramine or H2-receptor antagonists cimetidine and ranitidine, while the TRH-induced hypertension and tachycardia were attenuated by cimetidine. This antagonism was not due to an interaction between TRH and cimetidine at their central binding sites, since there was no displacement of [3H]MeTRH binding in the presence of cimetidine nor did TRH displace [3H]cimetidine in rat brain homogenates. Inability of diphenhydramine to modify the cardiovascular effects of TRH indicates that these effects are not due to histamine liberation, as cardiovascular stimulation after central administration of histamine is mainly mediated via H1-receptors. The antagonism of the cardiovascular responses to TRH by cimetidine was not due to blockade of H2-receptors, since another potent H2-receptor antagonist ranitidine was unable to affect the cardiovascular effects of TRH. Therefore, we suggest that cimetidine exerted antagonism of TRH by some non-specific action.

Central effects of histamine H2-receptor agonists and antagonists on nociception in the rat

The effects of intracerebroventricular injection of histamine H2-receptor agonists (4-methylhistamine, 4-MeH; dimaprit, DIM), H2-antagonists (cimetidine, CIM; ranitidine, RAN; famotidine, FAM) and of the DIM chemical analogue SK&F 91487 on hot-plate latency in rats were examined. Both DIM (0.4-0.8 mumol/rat) and 4-MeH (0.4-0.8 mumol/rat) significantly enhanced the pain threshold, whereas SF&F 91487 (0.8 mumol/rat) had no effect, indicating that DIM antinociception is specifically due to its activity on histamine (HA) receptors. The H2-antagonists CIM (0.8 mumol/rat) and RAN (0.6 mumol/rat) also enhanced the pain threshold, while FAM (0.03 mumol/rat) did not modify pain latency. When injected before 4-MeH, FAM reduced the antinociceptive effect of 4-MeH. These findings suggest that the antinociceptive activity of CIM and RAN is not related to specific blockade of H2-receptors and that the activation of HA-H2-receptors is inhibitory to nociception.

Histamine and GABA. Hydrogen bonds and permeation of vesicles

Spectroscopic studies on sodium di(2-ethylhexyl)-sulfosuccinate (AOT) inverted micelles, films of AOT and L-alpha-lysolecithin and on dihexadecyl phosphate vesicles show that histamine and gamma-aminobutyric acid (GABA) act differently on these membrane models. Histamine increases the permeability of the membrane to ions through interactions with its polar sites. GABA, on the other hand, prefers self-association to association with the membrane. If these two neurotransmitters are applied jointly, the result is a decrease in the permeating effect of histamine. Possible mechanisms for these processes are discussed.

Further evidence for endogenous hypothalamic serotonergic neurons involved in the cimetidine-induced prolactin release in the rat

The aim of the present work was to further explore the possible relationship between the prolactin-releasing effect of cimetidine and hypothalamic serotonergic neurons controlling pituitary hormone secretion. In a first approach, the prolactin-releasing effect of the drug was determined in adult male rats with total deafferentation of the hypothalamus. Cimetidine injection (60 mg/kg) produced a significant rise in prolactin, but not in luteinizing hormone (LH), both in deafferented rat and in sham-operated controls; by 15 min there was a 5-6 fold increase in prolactin titers. Methysergide, a serotonin receptor blocker, used in a dose (2.5 mg/kg), route (i.p.) and time (50 min earlier) which did not modify the hormone basal level in rats with total deafferentation of the hypothalamus, was able to prevent completely the prolactin release evoked by cimetidine. The same preventive effect on prolactin release was observed with the serotonin receptor blocker ketanserin (5 mg/kg, i.p., 30 min earlier). It is concluded that the prolactin-releasing effect of cimetidine is located at a hypothalamic level related to serotonergic neurons.

Pressor responses to central injection of H2 antagonists not caused by GABA blockade

In awake rats, ranitidine was more effective than cimetidine in elevating blood pressure following intracerebroventricular (i.c.v.) injection, yet neither drug affected the hypotensive response to subsequent injections of muscimol (8.8 nmol i.c.v.). Bicuculline (0.01 nmol) microinjected into the inferior colliculus of rats caused clonic seizures whereas cimetidine (100 nmol) had no effect. The antihistamines did not prevent GABAB receptor-mediated inhibition of twitch responses in transmurally stimulated guinea-pig ileum. Ranitidine potentiated rather than inhibited GABAA receptor-mediated contractions of ileum longitudinal muscle. Cimetidine had no effect on these responses except at high concentrations (3 X 10(-4) M) which caused a slight dextral shift in the contractile response curve for GABA that may be attributed to antimuscarinic actions of cimetidine. Taken together, these data do not support the concept that the centrally mediated pressor effects of H2 antagonists are caused by GABA receptor blockade.

Brain histamine regulation following chronic diazepam treatment and stress

Chronic diazepam treatment (5 mg/kg intragastrically, twice daily for 14 days) did not influence either hypothalamic, midbrain or cortical histamine (HA) levels or histidine decarboxylase (HD) activity in male Sprague-Dawley (200-220 g) rats. However, a small but significant decrease in hypothalamic HA concentration and significantly increased HD activity was seen following diazepam withdrawal. Air blast stress induced a significant elevation in hypothalamic HA levels and HD activity in vehicle-treated controls, diazepam-treated and diazepam-withdrawn rats, but the change in HD activity was significantly greater in the last group. The latter group also displayed the greatest elevation in plasma corticosterone levels in response to stress. Hence, diazepam withdrawal in rats results in some changes in the basal hypothalamic HA regulation and may influence the hypothalamic HA and corticosterone response to stress.

New horizons in the pharmacologic management of peptic ulceration

Greatly improved understanding of the cellular basis for gastric acid secretion and gastroduodenal mucosal defense has led to a dramatic improvement in the pharmacologic treatment of peptic ulcer disease. The advances produced by cimetidine and ranitidine are being continued by a new generation of histamine receptor antagonists, as well as by other anti-ulcer agents. These new drugs, when used appropriately, will greatly expand the surgeon's ability to treat patients with peptic ulcer disease. A knowledge of the pathophysiologic characteristics of peptic ulceration and of the inherent limitations of each agent will become increasingly important for surgeons who treat these patients.

Histaminergic axons in the neostriatum and cerebral cortex of the rat: a correlated light and electron microscopic immunocytochemical study using histidine decarboxylase as a marker

Histaminergic nerve fibers and their axonal varicosities in the neostriatum and cerebral cortex were light and electronmicroscopically examined by means of peroxidase-antiperoxidase immunocytochemistry with histidine decarboxylase (HDC) as a marker. A majority of HDC-like immunoreactive axonal varicosities observed in serial thin sections for electron microscopy exhibited no synaptic contacts in either the neostriatum or cerebral cortex. The remaining small proportion of immunoreactive axonal varicosities formed synaptic contacts with non-immunoreactive dendritic shafts and spines.

Ranitidine potentiates ileum contractions caused by GABA and electrical stimulation

GABA-evoked contractions of the guinea pig ileum were significantly potentiated by the histamine H2-receptor antagonist ranitidine in concentrations above 10 microM. To help define the mechanism of this interaction, the present study compared the effects of ranitidine on contractile responses of the guinea pig ileum to GABA, acetylcholine (A Ch) and electrical stimulation of intrinsic cholinergic neurons. Ranitidine, at concentrations that potentiated responses to GABA, also potentiated contractions induced by transmural electrical stimulation. The ability of ranitidine to amplify these latter responses was antagonized by atropine. Contractile responses to exogenous A Ch, however, were unaffected by ranitidine at any concentration. These results suggest that prejunctional, rather than postjunctional mechanisms, are of primary importance in the interaction between ranitidine and GABA.

Histamine-induced depression of serotoninergic dorsal raphe neurons: antagonism by cimetidine, a reevaluation

Microiontophoretic application of the histamine H2-receptor antagonist cimetidine selectively attenuated the histamine-induced, but not the gamma-aminobutyric acid (GABA)-induced, depression of serotoninergic cells recorded in the dorsal raphe nucleus of the rat. These data support the view that an H2-receptor mediates the effects of histamine on these serotoninergic neurons. We also now ascribe a previous report of GABA-like properties of cimetidine and metiamide to an impurity in the lots of these compounds used originally.

Ivermectin interactions with benzodiazepine receptors in rat cortex and cerebellum in vitro

The anthelmintic macrolide, ivermectin, enhances the binding of benzodiazepine agonist ( [3H]-diazepam) and antagonist ( [3H] beta-carboline ethyl ester) ligands to rat cortical and cerebellar membrane preparations. Enhancement of benzodiazepine agonist binding is partially additive with that of gamma-aminobutyric acid (GABA) and is inhibited by etazolate, bicuculline, and the steroid GABA antagonist R5135. Ivermectin-stimulated benzodiazepine antagonist binding is enhanced by bicuculline and inhibited by GABA and etazolate. The modulatory effects of bicuculline are chloride-dependent. The stimulatory effects of ivermectin, while quantitatively different in cortex and cerebellum, are qualitatively similar in both brain regions and are reduced in the presence of chloride. Ivermectin effects on benzodiazepine ligand binding to the benzodiazepine receptor complex and the differences in the effects of GABA, bicuculline, and R5135 on ivermectin-stimulated agonist and antagonist binding may provide evidence for distinct differences in the recognition sites for the two classes of benzodiazepine receptor ligand and their interactions with other components of the receptor complex.