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Biomedical subjects

M Garbarg

Publications and source records attributed to M Garbarg.

At least 19 recordsLinked to original sources
PubMed1992

Synthesis, absolute configuration, stereoselectivity, and receptor selectivity of (alpha R, beta S)-alpha,beta-dimethylhistamine, a novel high potent histamine H3 receptor agonist.

Depending on the selected synthetic pathway, structural variations of the neurotransmitter histamine led to mixtures of alpha,beta-dimethylhistamines as well as to the corresponding pure optical isomers. One of these isomers, namely (alpha R,beta S)-alpha,beta-dimethylhistamine, proved to be a highly potent H3 receptor agonist with exceptional receptor selectivity. The absolute configuration of the compound was determined by X-ray structure analysis of its dihydrobromide using the anomalous dispersion of bromine. The optical purity of both enantiomers of erythro-alpha,beta-dimethylhistamine was checked by 1HNMR investigations after acylation of the amines with (R)-2-methoxy-2-phenylacetyl chloride. As expected H3 receptors distinguish in a very strong way between the title compound and its alpha S,beta R-configured enantiomer. The agonistic potency of the latter is 2 orders of magnitude lower than the potency of (alpha R,beta S)-alpha,beta-dimethylhistamine.

Animals↗
PubMed1992

Design of an affinity matrix for purification of the histamine H1 receptor from guinea pig cerebellum.

H1 receptors from guinea pig cerebellum were solubilized using digitonin, and [125I]iodobolpyramine was used as a probe. [125I]Iodobolpyramine binding to this solubilized preparation occurred with a KD of 0.1 nM and a Bmax of 220 fmol/mg of protein and was inhibited by various H1 ligands with the expected potencies. Using a gel filtration procedure, a very sensitive radioassay was set up for detecting H1 activity in the solubilized preparation: 0.1 nM [125I]iodobolpyramine specific binding represented greater than 90% of total binding. Moreover, the synthesis is described of potent H1 antagonists that are mepyramine derivatives with an amino alkyl acylamido alkyl spacer arm. One of them, UCL 1057 (Ki = 0.5 nM), has been coupled to a Sepharose epoxy-activated resin. The resulting affinity matrix adsorbed selectively [125I]iodobolpyramine binding sites from the guinea pig cerebellum soluble preparation. In contrast, a Sepharose-glycine matrix was not able to adsorb these sites.

Adsorption↗
PubMed1992

S-[2-(4-imidazolyl)ethyl]isothiourea, a highly specific and potent histamine H3 receptor agonist.

The effects of a new agonist of histamine (HA) H3 receptors, Imetit (S-[2-(4-(imidazolyl)ethyl]isothiourea) were investigated in vitro and in vivo and compared to those of (R)-alpha-methylhistamine [(R)-alpha-MeHA], a prototypic drug. Imetit inhibited the binding of [3H](R-alpha-MeHA to rat brain membranes with a Ki value of 0.1 +/- 0.01 nM. The release of endogenously synthesized [3H]HA induced by K(+)-depolarization from rat brain slices and synaptosomes was inhibited by Imetit with EC50 values of 1.0 +/- 0.3 and 2.8 +/- 0.7 nM, respectively. Imetit behaved as a full agonist and was about 4 times more potent than (R)-alpha-MeHA and 60 times more potent than HA. Thioperamide, a selective H3 receptor antagonist, elicited a parallel rightward shift of the concentration-response curve for Imetit with an apparent Ki value of 5.6 +/- 1.4 nM. Imetit potencies relative to HA were less than 0.1% and only 0.6% at HA H1 and H2 receptor reference systems, respectively. Imetit was found not to be a substrate or an inhibitor of HMT. After p.o. administration to mice or rats, Imetit decreased (by approximately 50%) the tele-MeHA level in the cerebral cortex with ED50 values of 1.0 +/- 0.3 and 1.6 +/- 0.3 mg/kg, respectively. This effect was still maximal after 6 hr. The in vivo potency and duration of action of Imetit were in the same range as those of (R)-alpha-MeHA. It is therefore concluded that Imetit represents a new potent and selective HA H3 receptor agonist.

Animals↗
PubMed1991

Novel chiral H3-receptor agonists.

Several alkyl-substituted histamine derivatives have been synthesized and investigated for their agonistic potency on three classes of histamine receptors. While all investigated compounds are full agonists at H3-receptors their relative potency vs. histamine is strongly varying and culminates in alpha R, beta S-dimethylhistamine which is a highly potent and selective H3-receptor agonist.

Animals↗
PubMed1991

The histamine H3-receptor: pharmacology, roles and clinical implications studied with agonists.

Among a series of methylated histamine derivatives, (R) alpha, (S) beta,-dimethylhistamine has been identified as a novel potent and selective H3-receptor agonist, even slightly more potent than (R) alpha-methylhistamine and displaying similar properties in vivo. Several studies suggesting that (R) alpha-methylhistamine might find various clinical applications, it was submitted to toxicological studies and initial clinical trials. The drug seems to display a very low animal toxicity, and, in humans, is well absorbed orally and metabolized via ring methylation before urinary excretion.

Animals↗
PubMed1990

Involvement of histaminergic neurons in arousal mechanisms demonstrated with H3-receptor ligands in the cat.

The effects of histamine H3-receptor ligands on sleep-waking parameters were studied in freely moving cats. Oral administration of (R)alpha-methylhistamine (alpha MHA), a H3-agonist, caused a significant increase in deep slow wave sleep while that of thioperamide, a H3-antagonist, enhanced wakefulness in a marked and dose-dependent manner. The arousal effects of thioperamide were prevented by pretreatment with alpha MHA or mepyramine, a H1-receptor antagonist. The findings support the hypothesis that the histaminergic neurons are critically involved in arousal mechanisms and suggest that H3-receptors play an active part in these mechanisms by regulating histamine transmission.

Animals↗
PubMed1989

Effects of histamine H3-receptor ligands on various biochemical indices of histaminergic neuron activity in rat brain.

The interaction of the potent histamine H3-receptor ligands i.e. (R)alpha-methylhistamine, an agonist, and thioperamide, an antagonist, with the three classes of cerebral histamine receptors was studied in vitro and in vivo. The histamine-induced stimulation of 3',5'-cyclic AMP accumulation in slices of guinea-pig hippocampus was not modified by thioperamide (up to 0.1 mM) and (R)alpha-methylhistamine stimulated cyclic AMP accumulation only at millimolar concentrations. Hence, both (R)alpha-methylhistamine and thioperamide were at least 100,000-fold more potent at H3- than at H1- or H2-receptors in brain. In vivo, the turnover of histamine in rat cerebral cortex, as determined from its depletion elicited by alpha-fluoromethylhistidine in a synaptosomal fraction was not modified by mepyramine and zolantidine but was markedly enhanced by thioperamide at a low dose (ED50 = 2 mg/kg). Thioperamide also elicited a long-lasting decrease in synaptosomal histamine and increase in radioimmunoassayable N tau-methylhistamine. In contrast, (R)alpha-methylhistamine markedly reduced cortical [3H]histamine synthesis (ED50 = 5 mg/kg). This long-lasting action was accompanied by an increase in synaptosomal histamine and a decrease in N tau-methylhistamine levels. These changes were compared with those in plasma drug levels. Hence the two H3-receptor ligands appear to modify the activity of cerebral histamine neurons markedly and in a long-lasting and opposite manner.

Animals↗
PubMed1989

Sensitive radioimmunoassays for histamine and tele-methylhistamine in the brain.

Serum albumin conjugates of histamine or tele-methylhistamine, a major catabolite, were prepared using 1,4-benzoquinone as the coupling agent and used to raise polyclonal antibodies in rabbits. The same reagent was used to prepare the [125I]iodinated tracer and treat tissue extracts submitted to the radioimmunoassays. The IC50 values of prederivatized histamine and tele-methylhistamine in the radioimmunoassays were 0.3 nM and 0.5 nM, respectively, whereas nonderivatized histamine or tele-methylhistamine, histidine, a variety of histamine derivatives, amines, etc., had at least 1,000-fold higher IC50 values. Application of the radioimmunoassays to nonpurified extracts of rat brain allowed the quantification of the two amine immunoreactivities in samples corresponding to less than 1 mg of hypothalamus. The tissue immunoreactivity corresponded to authentic histamine or tele-methylhistamine, as shown by (a) the parallel 125I-tracer displacement curves, (b) the similar elution patterns from HPLC columns, (c) the regional levels of histamine and tele-methylhistamine in brain, similar to those obtained with other methods, and (d) the clearcut effects of treatments with inhibitors of L-histidine decarboxylase or monoamine oxidase. The two radioimmunoassays appear as simple and sensitive tools to evaluate steady-state levels and turnover rates of histamine and tele-methylhistamine.

Animals↗
PubMed1989

The third histamine receptor. Highly potent and selective ligands.

The third histamine receptor was first identified on brain neurons and seems also to be present in other cells such as the lung mast cells. Hence the novel and potent H3 receptor agonist (R)-alpha-methylhistamine might find therapeutic applications in allergic diseases.

Animals↗
PubMed1988

A detailed mapping of histamine H1-receptors in guinea-pig central nervous system established by autoradiography with [125I]iodobolpyramine.

[125I]Iodobolpyramine, a potent and selective histamine H1-receptor antagonist derived from mepyramine, was used to generate light microscopic autoradiograms on sections of guinea-pig brain and spinal cord. Histamine H1-receptors were labelled with high sensitivity over a low background as determined using mianserin or other H1-receptor antagonists as competing agents. An atlas of H1-receptors was established using five sagittal sections and 39 frontal sections, the latter serially prepared at 50 micron intervals. Labelled areas were identified by comparison with corresponding, classically stained sections and their density was rated according to an arbitrary scale. Autoradiographic grains were detected in a large variety of gray matter areas whereas they were generally absent from white matter areas. In the cerebral cortex, H1-receptors are present in all areas and layers with a higher density in lamina IV. In the hippocampal formation, H1-receptors display a laminated pattern of distribution and are the most abundant in the dentate gyrus (hilus and molecular layer) and in several areas of the subiculum and commissural complex. In the amygdaloid complex, the highest densities are found in the medial group of nuclei. In the basal forebrain, the striatum is moderately labelled whereas the nucleus accumbens, islands of Calleja and most septal nuclei are highly labelled. In the thalamus, H1-receptors are present in high density, particularly in the anterior, median and lateral groups of nuclei. In the hypothalamus the labelling is highly heterogeneous with high densities in, for example, medial preoptic area, dorsomedial, ventromedial and most posterior nuclei, including the tuberomammillary complex in which histaminergic perikarya and short axons are present.(ABSTRACT TRUNCATED AT 250 WORDS)

Aminopyridines↗
PubMed1988

Characterization of histamine H1-receptor binding peptides in guinea pig brain using [125I]iodoazidophenpyramine, an irreversible specific photoaffinity probe.

Aminophenpyramine--i.e., N-(5-[2-(4-aminophenyl)ethanamidopentyl])-N'-(4-methoxybenzyl)-N-m ethyl-N'- (2-pyridinyl)-1,2-ethanediamine, a derivative of mepyramine (pyrilamine), a typical antagonist of histamine at its H1 receptor--was synthesized and converted into [125I)iodoazidophenpyramine, a potential photoaffinity probe for the H1 receptor. In the dark, reversible binding of this probe to cerebellar membranes occurred with a Kd of 1.2 x 10(-11) M and a Bmax of 240 fmol/mg of protein and was inhibited by various H1-receptor antagonists with the expected potencies. These features establish the compound as one of the most potent H1-receptor antagonists known so far. Upon UV irradiation, 5% of the bound radioactivity was covalently incorporated into cerebellar membrane polypeptides as shown by standard NaDodSO4/PAGE. Two bands of 47 and 56 kDa were consistently labeled, labeling being prevented by various H1-receptor antagonists with the expected potencies and stereoselectivity. In the presence of protease inhibitors, labeling of the 56-kDa peptide increased at the expense of the 47-kDa peptide, suggesting that the latter was produced by hydrolysis of the former under the action of membrane proteases. In the absence of 2-mercaptoethanol, a band of 350-400 kDa appeared, apparently at the expense of the lighter bands, suggesting that the latter might be linked by one or more disulfide bridges to a higher molecular mass complex. We propose that at least part of the ligand binding domain of the histamine H1 receptor resides within a subunit of apparent molecular mass 56,000.

Affinity Labels↗
PubMed1988

[Potential interest in powerful and specific ligands for the histamine H3 receptor].

In contrast to cerebral histamine H1 and H2-receptors, histamine H3-receptors are presynaptically located on histamine-synthesizing nerve terminals (autoreceptors) and control the synthesis and release of the amine in cerebral neurons. Two imidazole derivatives were designed to interact at H3-receptors: (R) alpha-methylhistamine (alpha-MeHA), a chiral agonist, and thioperamide, a competitive antagonist derived from imidazolyl piperidine, both display high selectivity and potency at nanomolar concentrations in vitro. (R) alpha-MeHA, being about 15 times as potent as histamine itself, constitutes, when tritiated, a suitable probe for the radioassay of H3-receptors. Outside the brain, H3-receptor sites could be detected in the lung. The availability of new ligands made it possible to assess in vivo the physiological role of central and peripheral H3-receptors. The agonist and the antagonist modified in opposite directions histamine synthesis in the lung as in the brain, confirming the presence of H3-receptors in this peripheral organ. A large part of lung histamine being present within mast-cells, it is likely that these cells are endowed with H3-receptors controlling the amine synthesis and, possibly, release. Therefore, the novel agents may be of great practical interest in the field of allergy and inflammation.

Animals↗
PubMed1988

Synergism between histamine H1- and H2-receptors in the cAMP response in guinea pig brain slices: effects of phorbol esters and calcium.

The synergism between H1- and H2-receptors in the histamine-induced stimulation of cAMP accumulation was studied in slices from guinea pig hippocampus. Since H1-receptors appear to be coupled to the phosphatidylinositol cycle, the participation of the two branches of the cycle in this synergism was assessed by using phorbol esters and/or by removing Ca2+ from the external medium. The protein kinase C activator, 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB), strongly potentiated, with an EC50 of 0.2 microM, the accumulation of cAMP elicited by dimaprit, an H2-receptor agonist used at supramaximal concentration (0.3 mM). The effect of 4 beta-PDB was also observed in the presence of impromidine, an H2-receptor agonist, and histamine. 4 beta-Phorbol 12-myristate, 13-acetate, another protein kinase C activator, also potentiated the effect of dimaprit in a concentration-dependent manner although less potently than 4 beta-PDB. In contrast, 4 alpha-phorbol or the phorbol esters, 4 alpha-phorbol 12,13-didecanoate or 4-O-methylphorbol 12-myristate, 13-acetate, all inactive on protein kinase C, had no potentiating effect. 2-Thiazolylethylamine (2-TEA), a predominantly H1-receptor agonist, increased the stimulation induced by dimaprit (0.3 mM), and this response was further enhanced in the presence of 4 beta-PDB in maximal concentration (1 microM). Mepyramine (0.1 microM) antagonized the H1-receptor-mediated effect in the absence as well as the presence of 4 beta-PDB. The phorbol ester did not significantly alter the EC50 of 2-TEA or the magnitude of its effect. In the absence of phorbol esters, removal of Ca2+ from the incubation medium did not change the response elicited by 0.3 mM dimaprit but reduced by 50% the response to a supramaximal concentration of 2-TEA. This effect was more marked when EGTA was added in the Ca2+-free medium. The EC50 value of 2-TEA was only slightly modified in the absence of Ca2+ (180 +/- 20 microM as compared with 70 +/- 4 microM in the presence of 2.6 mM Ca2+). In the presence of 4 beta-PDB (1 microM), removal of Ca2+, particularly in the presence of EGTA, did not affect or slightly increased the response to dimaprit, but still strongly reduced the response to 2-TEA. The Ca2+ ionophore A 23187 (10 microM) showed a tendency to mimic the potentiating effect of 2-TEA.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗