Search PubMed⌕ Search

Biomedical subjects

M Garbarg

Publications and source records attributed to M Garbarg.

At least 73 records · Page 4Linked to original sources
PubMed1985

Actions of betahistine at histamine receptors in the brain.

The actions of betahistine (N alpha-methyl-2-pyridylethylamine) on brain histamine receptors were investigated in a series of biological models. [3H]Mepyramine binding to H1-receptors in membranes from guinea-pig cerebellum was inhibited by betahistine with a Ki value of 31 microM. The binding of [3H]mepyramine in brain of the living mouse was inhibited by betahistine in high dosages (150-300 mg/kg). In slices from mouse cerebral cortex, betahistine induced [3H]glycogen hydrolysis in a concentration-dependent manner with an EC50 value of 9.0 microM with a maximal effect 57% that of histamine. Mepyramine and triprolidine, two H1-receptor antagonists, inhibited the betahistine-induced glycogenolysis with Ki values of 28 nM and 7 nM respectively. In slices from guinea-pig hippocampus, betahistine stimulated the accumulation of cyclic AMP in the presence of 5 microM impromidine, a H2-receptor agonist. The maximal effect represented 22% of that elicited by histamine at the H1-receptor and the EC50 value was 32.4 microM. Mepyramine at 0.1 microM partially blocked the response to betahistine. Together these various observations indicate that betahistine is a partial agonist at cerebral H1-receptors. Finally, betahistine was not an agonist at histamine H3-autoreceptors but was a rather potent antagonist of the inhibitory effect of exogenous histamine on [3H]histamine release elicited by K+ depolarisation in slices from rat cerebral cortex (Ki = 6.9 microM).

Animals↗
PubMed1985

Autoregulation of histamine release in brain by presynaptic H3-receptors.

Regulation of histamine release was studied mainly on brain slices prelabeled with L-[3H]-histidine and depolarized by increased extracellular K+ concentration or veratridine in a non-superfused system. The released 3H-labeled amines, isolated by ion-exchange chromatography from a large excess of 3H-labeled precursor consisted by more than 95% of unchanged [3H]histamine. Exogenous histamine reduced the release of neosynthesized [3H]histamine via stimulation of previously characterized H3-receptors whereas it did not modify the 3H-labeled amine release from slices prelabeled with preformed [3H]histamine. The maximal inhibitory effect of exogenous histamine progressively diminished as the strength of the depolarizing stimulus or the external Ca2+ concentration were elevated. On the contrary H3-receptor antagonists like impromidine or burimamide enhanced the depolarization-induced release of [3H]histamine, an effect which was particularly marked when slices were loaded with histamine by preincubation with [3H]histidine in high concentration. These results suggest that the inhibition of [3H]histamine release by exogenous histamine acting via H3-receptor stimulation is mediated by a restricted access of Ca2+ and that its extent is influenced by the degree of autostimulation by endogenous histamine as well as, possibly, by actual internal Ca2+ concentration. In addition the decrease in external Ca2+ concentration shifted rightwards the concentration-response curve to histamine. The autoinhibitory effect of exogenous histamine was found on slices from various regions, known from lesion studies to contain terminals of extrinsic histaminergic neurons. It did not apparently involve interneurones, not being prevented in slices in which the traffic of action potentials was blocked by tetrodotoxin. It also remained unaffected in striatal slices in which the neuronal cell-bodies were selectively destroyed by prior local infusion of kainic acid. Finally exogenous histamine inhibited [3H]histamine release from depolarized synaptosomes of rat cerebral cortex, with an EC50 value similar to that found with slices and was antagonised by impromidine with an apparent Ki value similar to that displayed at H3-receptors. It is concluded that histamine modulates its own release from cerebral neurones by interacting with H3-presynaptic autoreceptors and via mechanisms similar to those previously evidenced on other aminergic systems.

Animals↗
PubMed1985

Histamine receptors in the brain.

In mammalian brain, neuronal histamine is likely to act as a neurotransmitter and is recognized by the two classes of histamine receptors (H1 and H2) previously characterized in peripheral organs. Cerebral H1 receptors can be selectively labeled by a tritiated antagonist mepyramine, in particulate fractions or in the living animal. Cerebral H1 receptors mediate the glycogen hydrolysis and the breakdown of inositol phospholipids elicited by the amine. They are indirectly involved in the histamine-mediated accumulation of cyclic AMP. All these biochemical responses mediated by H1 receptors are calcium-dependent. H2 receptors are coupled to an adenylate cyclase. In addition, a novel class of histamine receptors (H3) are presynaptic autoreceptors and modulate the release of neuronal histamine.

Adenylyl Cyclases↗
PubMed1985

N-ethylmaleimide-induced changes in agonist affinity for histamine H1-receptors in the guinea pig brain.

The effect of the thiol-alkylating agent, N-ethylmaleimide (NEM), on histamine (HA) H1-receptors from guinea pig cerebellum, labeled with [3H]mepyramine, was investigated. The properties of [3H]mepyramine binding (apparent dissociation constant and maximal number of sites) were not modified by prior treatment of the membranes with 2 or 5 mM NEM. This treatment did not change either the inhibition curves of d-chlorpheniramine or mianserin, two H1-receptor antagonists. In contrast, treatments of membranes with NEM significantly decreased the IC50 values of HA and the slope indexes (pseudo Hill coefficients) of HA inhibition curves, which became inferior to unity. These effects were irreversible, and their extent related to the NEM treatment duration and the NEM concentration. A computer analysis of the data indicated that part of the H1-receptors were converted from a state of low affinity for the amine (IC50 value of 75 microM) into a high agonist affinity state (IC50 value of 2 microM). The change was less marked for partial agonists than for HA. The NEM-induced change was observed in the presence and in the absence of Na+ ions, known to decrease the affinity of HA for H1-receptors. Agonists or antagonists did not protect against the modification of HA affinity induced by NEM. The digitonin-solubilized receptors retained their sensitivity to NEM. Among other thiol reagents, iodoacetamide and iodoacetic acid were ineffective, and organic mercurial agents strongly reduced the number of [3H]mepyramine-binding sites. NEM treatment might alkylate a critical thiol group located outside the ligand-binding domain of the H1 receptor and thereby stabilize the latter in a conformation distinct from that of the activated state.

Animals↗
PubMed1983

Auto-inhibition of brain histamine release mediated by a novel class (H3) of histamine receptor.

Although histaminergic neurones have not yet been histochemically visualized, there is little doubt that histamine (HA) has a neurotransmitter role in the invertebrate and mammalian central nervous system. For example, a combination of biochemical, electrophysiological and lesion studies in rats have shown that histamine is synthesized in and released from a discrete set of neurones ascending through the lateral hypothalamic area and widely projecting in the telencephalon. Histamine acts on target cells in mammalian brain via stimulation of two classes of receptor (H1 and H2) previously characterized in peripheral organs and probably uses Ca2+ and cyclic AMP, respectively, as second messengers. It is well established that several neurotransmitters affect neuronal activity in the central nervous system through stimulation not only of postsynaptic receptors, but also of receptors located presynaptically which often display distinct pharmacological specificity and by which they may control their own release. Such 'autoreceptors' have been demonstrated (or postulated) in the case of noradrenaline, dopamine, serotonin, acetylcholine and gamma-aminobutyric acid (GABA) neurones but have never been demonstrated for histamine. We show here that histamine inhibits its own release from depolarized slices of rat cerebral cortex, an action apparently mediated by a class of receptor (H3) pharmacologically distinct from those previously characterized, that is, the H1 and H2 receptors.

Animals↗
PubMed1983

Interaction between mianserin, an antidepressant drug, and central H1- and H2-histamine-receptors: in vitro and in vivo studies and radioreceptor assay.

Mianserin bimodally inhibited the stimulation of cyclic AMP accumulation mediated by histamine H1- and H2-receptors in slices from guinea-pig hippocampus with Ki values of 0.003 and 4 microM, respectively. Various treatments with mianserin were undertaken to determine whether the drug significantly interacted with cerebral histamine receptors in vivo in such a way that the response of the slice preparation could be modified. In hippocampal slices from animals treated with mianserin, the H2-receptor-mediated effect was estimated by constructing concentration-response curves to impromidine, a highly selective agonist, and that mediated by H1-receptors was measured by use of 0.5 mM 2-thiazolylethylamine (a predominantly H1-receptor agonist) in the presence of a maximal concentration of impromidine. After an acute treatment (10 mg/kg), the response mediated by H1-receptors was abolished whereas the response to impromidine in increasing concentrations was unchanged. After 1 week of drug administration (10 mg/kg twice daily), a 44% reduction in the response to 2-thiazolylethylamine was observed with no change in the response mediated by H2-receptors. When a dose of mianserin equivalent to a clinical dose (1 mg/kg, twice daily) was administered for 21 days, a partial but not significant decrease of the responsiveness to the H1-receptor agonist was accompanied by a significant increase of the maximal response to impromidine. Plasma levels of mianserin were estimated by a sensitive radioreceptor assay based upon inhibition of [3H]mepyramine binding. A good correlation was found between the concentration of mianserin in plasma and the tentative estimation of an equivalent concentration of mianserin in slices.

Animals↗
PubMed1981

Biochemical studies on histaminergic systems in mammalian brains.

The effects of (+/-) alpha-fluoromethylhistidine (alpha-FMH), a new histidine decarboxylase (HD) inhibitor, were investigated in vitro and in vivo. Following a preincubation with (+/-) alpha-FMH, brain HD-activity was progressively inhibited and could not be restored by dialysis, thus indicating the irreversible nature of this inhibition. Moreover, in vivo, a single intraperitoneal dose of 20 mg/kg of (+/-) alpha-FMH induced a complete and rapid loss of HD activity in gastric and brain tissues. The time-course of recovery was different according to the tissue studied. At a dose of 100 mg/kg (+/-) alpha-FMH did not modify histamine-N-methyl transferase (HMT), DOPA decarboxylase and glutamate decarboxylase activities. A high affinity binding of 3H-histamine was seen in particulate fractions from rat brains. The regional and subcellular distributions of these binding sites indicate that they are not related to HMT. They are likely to represent post-synaptic HA-receptors in view of their decrease after kainate-induced degeneration of neuronal perikarya in the striatum and their increase following interruption of the histaminergic inputs which suggested a denervation hypersensitivity. However, their pharmacological specificity was distinct from either H1- or H2-receptors and the possibility of a modified conformational state of HA-receptors was raised by the selective effect of guanylnucleotides.

Animals↗
PubMed1980

L-Histidine decarboxylase in the human brain: properties and localization.

The properties of the histamine-forming enzyme in human brain samples were studied utilizing a radiochromatographic procedure. The influence of postmortem conditions was checked with rat brains, and the results indicated that the enzyme activity is not altered in situ for a delay not exceeding 4 h at ambient temperature. Moreover, tissue blocks or homogenates can be stored at temperatures for up to 3 months with a good preservation of the enzyme activity. The data indicate that histamine synthesis in the human brain involves the "specific" histidine decarboxylase (HD, EC 4.1.1.22) and not the aromatic L-amino acid decarboxylase: (1) the optimum pH is 7.4 at 10(-6) M-L-histidine; (2) the apparent Km is about 3.10(-5) M; (3) it is inhibited by alpha-hydrazino histidine and brocresine but not affected by alpha-methyl DOPA. Moreover, a major portion of the enzyme is localized in a subcellular fraction containing nerve terminals and it shows an uneven regional distribution which parallels that observed in the brain of other mammalian species. Taken together these data strongly suggest that histamine could play a neurotransmitter role in the human brain.

Aged↗
PubMed1980

Inhibition of histamine synthesis in brain by alpha-fluoromethylhistidine, a new irreversible inhibitor: in vitro and in vivo studies.

alpha-Fluoromethylhistidine (alpha-FMH), a new potent inhibitor of histidine decarboxylase (HD), has been used for in vitro and in vivo studies of brain HD. Following a preincubation with (+)-alpha-FMH, brain HD activity was inhibited in a time-dependent and concentration-dependent manner. The enzyme activity was not restored by overnight dialysis against standard buffer. The (-) antimer of alpha-FMH was ineffective. When injected intraperitoneally in a single dose of 20 mg/kg, (+/-)-alpha-FMH induced a complete loss in HD activity in cerebral cortex and hypothalamus as well as in peripheral tissues, such as stomach. At a dosage of 100 mg/kg (+/-)-alpha-FMH did not alter histamine-N-methyltransferase, DOPA decarboxylase, and glutamate decarboxylase activities. The maximal decrease of HD activity occurred after 2 h in both cerebral cortex and hypothalamus, but the time course of the recovery of enzyme activity was slower in the cerebral cortex. The enzyme activity reached control value within 3 days in hypothalamus and was not fully restored after 4 days in cerebral cortex. Contrasting with the diminished HD activity, a substantial concentration of histamine remained present in five regions of mouse brain. Thus, alpha-FMH is a highly specific irreversible inhibitor of brain HD activity and its efficacy makes it useful to study the physiological role of brain histamine.

Animals↗