Tricyclic antidepressants and histamine H1 receptors.

Tricyclic antidepressants and some structurally related compounds were tested for their ability to antagonize histamine H1 and muscarinic acetylcholine receptors of cultured mouse neuroblastoma cells. As a group, tertiary amine tricyclic antidepressants tended to be more potent than secondary amine drugs at both receptors. The most potent antihistamine, doxepin hydrocholoride, was about 4 times more potent than amitriptyline hydrochloride, about 800 times more potent than diphenhydramine hydrochloride, and about 8,000 times more potent than desipramine hydrochloride, the least potent tricyclic antidepressant at both the histamine H1 and the muscarinic acetylcholine receptors. All tricyclic drugs except desipramine hydrochloride were more potent as antihistamines than as anticholinergics. Doxepin hydrochloride and amitriptyline hydrochloride may be the most potent antihistamines known, and the antihistaminic potencies of these and the other tricyclic antidepressant drugs may relate directly to their ability to cause sedation and drowsiness in patients.     

Mechanism of action of doxepin in the treatment of chronic urticaria

The present study examined 15 patients previously resistant to conventional antihistamines, in which doxepin at doses in the range of 50-75 mg/day was shown to be effective in treatment of chronic urticaria and without significant adverse side effects. However, some controversy remains about its mechanism of action in this particular disease. The aim of the present study was to examine the muscarinic, H1 and H2 blocking activity of doxepin. The following methods were used: a) gastric acid hypersecretion induced by histamine and carbachol in the pylorus-ligated rat preparation; b) contractile dose-response curves to histamine and carbachol in the guinea pig ileum; c) dimaprit-stimulated guinea pig atrium in vitro. pA2 values were determined for atropine, mepyramine, cimetidine and doxepin. As regards histamine, doxepin (50 mg/kg, po) increased gastric pH and decreased secretion volume, gastric acid concentration and total acid output; with carbachol, doxepin weakly antagonized those values. In the ileum, doxepin competitively antagonized carbachol (pA2 = 7.08) and histamine (pA2 = 9.72); pA2 values for atropine and mepyramine against carbachol and histamine were 9.11 and 8.82, respectively. In the atria, the dose-response curve to dimaprit was also competitively displaced by cimetidine (pA2 = 6.69) and doxepin (pA2 = 6.00). Doxepin displayed a very high affinity for H1 histamine receptor, being 8-fold more potent than mepyramine. Doxepin showed significant H2 blocking activity which was 5 times less potent than that of cimetidine. Doxepin competitively antagonized carbachol in the guinea pig ileum, and was 107 times less potent than atropine. The combined H1, H2 and muscarinic blocking activities of doxepin may contribute towards explaining its clinical efficacy in the treatment of chronic urticaria.     

Species difference in the muscarinic receptors of thoracic aorta

Muscarinic receptors in rabbit aortic membranes were characterized by binding assays using I-[3H]quinuclidinyl benzilate, and compared with muscarinic receptors in dog and bovine aortas. In the rabbit aortic membranes, I-[3H]quinuclidinyl benzilate specifically bound with high affinity (KD = 28.1 pM) to a single class of sites (maximum number of binding sites = 57.2 fmol/mg of protein) which have characteristics of muscarinic receptors. Drug competition curves indicated a single class of binding sites for antagonists (e.g., K1 = 160 nM for pirenzepine), but suggested the presence of two affinity binding sites (or states) for agonists (e.g., K1 = 8.29 nM and K2 = 0.32 microM for oxotremorine, respectively). I-[3H]quinuclidinyl benzilate also bound to a single class of sites in the membranes of dog and bovine aortas, with similar affinities to the case of rabbit aorta. But the maximum number of binding site values were significantly lower than the value in the rabbit aorta. Furthermore, bovine aorta exhibited a higher affinity (K1 = 8.7-22.3 nM) for pirenzepine, but a lower affinity (K1 = 0.62-1.46 microM) for oxotremorine, as compared with rabbit and dog aortas. Accordingly, muscarinic receptors in the aorta are different between animal species, regarding its density and interactions with muscarinic agents. The receptors in bovine aorta may be M1-subtype but receptors in rabbit and dog aortas may be relatively M2-subtypes.     

Pharmacological and biochemical characterization of the D-1 dopamine receptor mediating acetylcholine release in rabbit retina

Superfusion with dopamine (0.1 microM-10 mM) evokes calcium-dependent [3H]acetylcholine release from rabbit retina labeled in vitro with [3H]choline. This effect is antagonized by the D-1 dopamine receptor antagonist SCH 23390. Activation or blockade of D-2 dopamine, alpha-2 or beta receptors did not stimulate or attenuate the release of [3H]acetylcholine from rabbit retina. Dopamine receptor agonists evoke the release of [3H]acetylcholine with the following order of potency: apomorphine greater than or equal to SKF(R)82526 greater than SKF 85174 greater than SKF(R)38393 greater than or equal to pergolide greater than or equal to dopamine (EC50 = 4.5 microM) greater than SKF(S)82526 greater than or equal to SKF(S)38393. Dopamine receptor antagonists inhibited the dopamine-evoked release of [3H]acetylcholine: SCH 23390 (IC50 = 1 nM) greater than (+)-butaclamol greater than or equal to cis-flupenthixol greater than fluphenazine greater than perphenazine greater than trans-flupenthixol greater than R-sulpiride. The potencies of dopamine receptor agonists and antagonists at the dopamine receptor mediating [3H]acetylcholine release is characteristic of the D-1 dopamine receptor. These potencies were correlated with the potencies of dopamine receptor agonists and antagonists at the D-1 dopamine receptor in rabbit retina as labeled by [3H]SCH 23390, or as determined by adenylate cyclase activity. [3H]SCH 23390 binding in rabbit retinal membranes was stable, saturable and reversible. Scatchard analysis of [3H]SCH 23390 saturation data revealed a single high affinity binding site (Kd = 0.175 +/- 0.002 nM) with a maximum binding of 482 +/- 12 fmol/mg of protein. The potencies of dopamine receptor agonists to stimulate [3H]acetylcholine release were correlated with their potencies to stimulate adenylate cyclase (r = 0.784, P less than .05, n = 7) and with their affinities at [3H]SCH 23390 binding sites (r = 0.755, P greater than .05, n = 8). The potencies of antagonists to inhibit dopamine-evoked [3H]acetylcholine release were correlated with their potencies to inhibit the dopamine-stimulated adenylate cyclase (r = 0.759, P less than .05, n = 5) and with their affinities at [3H]SCH 23390 binding sites (r = 0.998, P less than .01, n = 7). We conclude that in rabbit retina dopamine evokes calcium-dependent [3H]acetylcholine release through activation of a site with the pharmacological characteristics of a D-1 dopamine receptor.     

A cholinergic antagonist identifies a subclass of muscarinic receptors in isolated rat pancreatic acini

Atropine, pirenzepine (PZ) and the novel antimuscarinic drug [11- [[2-(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H- pyrido[2,3-b][1,4]benzodiazepine-6-one (AF-DX 116) were used to subclassify the pancreatic muscarinic receptor by correlating their effects on carbachol-mediated amylase release with their actions on the binding of [3H]N-methylscopolamine in rat pancreatic acini. Maximal stimulation of amylase release occurred at 3 microM carbachol. Atropine, PZ and AF-DX 116 inhibited carbachol-mediated amylase release with the following pA2 values: atropine = 9.1, PZ = 6.5 and AF-DX 116 = 5.7. There was parallel inhibition of [3H]N-methylscopolamine binding, with the following inhibition constants: atropine = 2.38 nM, PZ = 426 nM and AF-DX 116 = 3660 nM. Using the same animals, these compounds inhibited [3H]N-methylscopolamine binding in homogenates from both cerebral cortex and heart. The order of potency was the same in the cerebral cortex as in the pancreas: atropine = 0.67 nM, PZ = 85 nM and AF-DX 116 = 440 nM. However, in the cortex, the binding data with PZ also exhibited a high-affinity site with a KH value of 11 nM. In the heart, the order of potency was shifted to atropine greater than AF-DX 116 greater than PZ, with inhibition constants of 1.55, 12 and 110 nM, respectively. Thus, the muscarinic receptors in the pancreas and the heart exhibited the characteristics of the putative M2 receptor subtype, having lower affinities for PZ than the muscarinic receptors in the cerebral cortex. However, the heart had a significantly higher affinity for AF-DX 116.(ABSTRACT TRUNCATED AT 250 WORDS)     

Assay of dopamine receptors with [alpha-3H]flupenthixol

Two major classes of dopamine receptors, called D-1 receptors and D-2 receptors, have been identified. [alpha-3H]Flupenthixol has been used as a radioligand for the study of D-1 receptors, which are thought to act through stimulation of adenylate cyclase activity. Previous studies in our laboratory have shown that the D-2 receptors in rat caudate labeled by [3H]spiroperidol also have a high affinity for alpha-flupenthixol. The present experiments show that although Scatchard analysis of the binding of [alpha-3H]flupenthixol is consistent with the presence of a homogeneous population of receptors, subpopulations of these sites can be distinguished by their differing affinities for spiroperidol, which has a Kd for D-1 receptors of about 0.3 microM and a Kd for D-2 receptors of approximately 50 pM. The number of D-1 receptors in rat striatum is approximately 4 times the number of D-2 receptors. D-1 receptors can be studied by including 10 nM spiroperidol in assays carried out with [alpha-3H]flupenthixol, thus blocking the binding of [alpha-3H]flupenthixol to D-2 receptors. The affinity of these receptors for dopamine is decreased by GTP, as has been observed in studies of other receptors whose effects are mediated through changes in adenylate cyclase activity. In the presence of spiroperidol, the Hill coefficients determined from dose-response curves of the inhibition of the binding of [alpha-3H]flupenthixol by antagonists or by agonists in the presence of GTP suggest that the binding reaction obeys simple Michaelis-Menten kinetics for a single class of binding sites.     

Suppression of histamine-induced adrenocorticotropic hormone release by antihistamines and antidepressants

The antihistaminic activity of many antidepressant drugs is well documented in vitro but has not been investigated as thoroughly in vivo. In the course of an investigation of the roles of H1 and H2 receptors in histamine-induced adrenocorticotropic hormone (ACTH) release in rats, it was observed that several antidepressants were potent inhibitors of this response. Male Sprague-Dawley rats were injected with test drugs and then with histamine or histamine agonists. Serum ACTH concentrations were determined by radioimmunoassay. ACTH secretion was induced by both H1 and H2 receptor stimulation. Histamine-induced ACTH release was markedly attenuated by several H1 antihistamines, whereas the H2 antagonists were not as effective. The antidepressants imipramine, doxepin, mianserin, desipramine and amitriptyline suppressed histamine-induced ACTH release. However, iprindole and the antianxiety agent diazepam were without effect. ACTH release induced by histamine agonists was also diminished by pretreatment with some of these blocking agents.     

Identification and pharmacological characterization of a series of new 1H-4-substituted-imidazoyl histamine H3 receptor ligands

A new series of 1H-4-substituted imidazole compounds were synthesized and identified as potent and selective histamine (HA) H3 receptor ligands. These ligands establish that HA H3 antagonists exhibit stereoselective and conformational preferences in their binding to the HA H3 receptor. Structure-activity relationships were determined in vitro by HA H3 receptor-binding affinities using [3H]Nalpha-methylhistamine and rat cerebral cortical tissue homogenates. Several derivatives containing olefin, amide, and acetylene functional groups were identified as potent HA H3 receptor ligands. In the olefin series, GT-2227 (4-(6-cyclohexylhex-cis-3-enyl)imidazole) was identified as a potent HA H3 receptor ligand with a Ki of 4.2 +/- 0.6 nM, while the trans isomer (GT-2228) displayed a reduced potency (Ki = 15.2 +/- 2.4 nM). GT-2227 was also found to have excellent central nervous system penetration in an ex vivo binding paradigm (ED50 = 0.7 mg/kg i.p.). In the acetylene series, GT-2260 and GT-2286 both exhibited high affinity (Ki = 2.9 +/- 0.2 and 0.95 +/- 0.3 nM) and excellent central nervous system penetration profiles (ED50 = 0.43 and 0.48 mg/kg i.p., respectively). As a prototype for the series, GT-2227 showed high affinity for the human HA H3 receptor (3.2 nM) and minimal affinity for the human HA H1 (Ki = 13,407 +/- 540 nM) and H2 (Ki = 4,469 +/- 564 nM) receptor subtypes. GT-2227 also showed good selectivity for the HA H3 receptor over a broad spectrum of other neurotransmitter receptors (IC50 >/= 1 microM). Furthermore, GT-2227 improved acquisition in a cognitive paradigm without behavioral excitation or effect on spontaneous locomotor activity. In summary, the present studies demonstrate the development of novel HA H3-selective ligands, and lend support for the use of such agents in the treatment of disorders associated with cognitive or attentional deficits.     

Comparison of human, mouse, rat, and guinea pig histamine H4 receptors reveals substantial pharmacological species variation

The recently identified histamine H4 receptor has revealed a potential new complexity for the role of histamine in the immune system. To begin to understand the role of this receptor in humans, one must first determine whether homologs exist and can be studied in lower species. To address this, we cloned the rat, mouse, and guinea pig cDNAs corresponding to the recently identified human histamine H4 receptor. The rat, mouse, and guinea pig H4 sequences are significantly different from the human H4 sequence at 69, 68, and 65% homology, respectively. The tissue distribution of the rat, mouse, and guinea pig H4 receptors is similar to human in that bone marrow and spleen are the most abundant sources of expression. The human and guinea pig H4 receptors display the highest binding affinity for [3H]histamine (KD = 5 nM each), whereas the affinities for rat and mouse receptors are substantially lower at 136 and 42 nM, respectively. With respect to the pharmacological profile of known H3/H4 ligands, even greater differences in binding affinities exist among the species homologs. There are also substantial differences in the signal transduction response to each of the four species of H4 receptor. This work demonstrates the existence of histamine H4 receptors in lower species and demonstrates that a clear knowledge of each species pharmacological profile will be essential to elucidate the role of this receptor subtype in vivo.     

Neuroleptics and learning: effects of haloperidol, molindone, mesoridazine and thioridazine on the behavior of pigeons under a repeated acquisition procedure

The purpose of the present study was to examine the effects of haloperidol (0.3-10 mg/kg), molindone (0.3-5.6 mg/kg), mesoridazine (0.3-10) and thioridazine (0.3-25 mg/kg) on the behavior of pigeons exposed to a repeated acquisition procedure. At sufficiently high doses, each of these neuroleptics increased error rates (interfered with learning) and reduced rate of responding. When the drugs were compared on the basis of absolute doses administered, haloperidol disrupted behavior at doses considerably lower than the other drugs. If, however, chlorpromazine equivalent doses were examined, haloperidol was the least disruptive of the four drugs. Comparing the degree of behavioral disruption produced by the four drugs with their relative neuroreceptor affinities for dopamine D-2, cholinergic muscarinic, histamine H1, alpha-1 adrenergic and alpha-2 adrenergic receptors suggests that behavioral disruption cannot be attributed in any simple way to dopamine or acetylcholine receptor blockade. The relationship between the behavioral effects of neuroleptics and their simple neuropharmacological actions must be considered as highly tentative.     

Alpha adrenergic receptor subtype associated with receptor binding, Ca++ influx, Ca++ release and contractile events in the rabbit aorta

The alpha-adrenergic receptor mechanism in rabbit aorta was examined for the involvement of alpha-1 or alpha-2 receptor subtypes. Agonists (phenylephrine, norepinephrine and clonidine) and antagonists (prazosin and yohimbine) with known receptor subtype selectivity were used to define the contribution of alpha-1 or alpha-2 receptors to receptor-initiated cellular Ca++ influx, intracellular release of Ca++ and overall contraction. The receptor content of isolated membranes was also measured in [3H]prazosin and [3H]yohimbine radioligand binding studies. Contraction-derived KB values for prazosin (3 nM) or yohimbine (1 microM) were similar for all three agonists, indicating that each acted on the same alpha-1 receptor. Prazosin (10(-7) M) was effective in causing inhibition of cellular Ca++ influx initiated by agonists whereas yohimbine (10(-6) M) had no effect. Prazosin but not yohimbine caused a partial reduction in phenylephrine or norepinephrine-induced stimulation of 45Ca efflux rate whereas the smaller clonidine-induced stimulation was totally inhibited by prazosin and partially inhibited by yohimbine. Alpha-1 specific binding of [3H]prazosin was observed with a KD of 3.5 nM and maximum binding site (Bmax) of 73 fmol/mg of protein. Although no alpha-2 specific binding of [3H]yohimbine was observed, binding to a low-affinity/high-capacity class of sites was found. The results indicate the sole presence and contribution of alpha-1 receptors to Ca++ flux and contractile events in the rabbit aorta.     

Characterization of the striatal M2 muscarinic receptor mediating inhibition of cyclic AMP using selective antagonists: a comparison with the brainstem M2 receptor

Pharmacological profiles of the striatal and brainstem M2 receptors were developed with a group of selective muscarinic antagonists. The striatal M2 muscarinic receptor was identified by its inhibition of [3H]cyclic AMP levels, whereas the brainstem M2 receptor was characterized using competition with [3H]quinuclidinyl benzilate binding. The potency of pirenzepine does not differentiate clearly between the striatal M2 receptor (Ki approximately 300 nM) and the brainstem M2 receptor (Ki = 219 nM) or peripheral M2 receptors. In the present study, we used 4-diphenylacetoxy-N-methylpiperidine methbromide, hexahydrosiladifenidol, AF-DX 116 and methoctramine to characterize the striatal and brainstem M2 receptors in more pharmacological detail. For comparison, the potencies of these antagonists were also measured at cortical M2 receptors (using competition with [3H]pirenzepine binding). The potencies of 4-diphenylacetoxy-N-methylpiperidine methbromide (KB = 0.19 nM) and hexahydrosiladifenidol (KB = 14 nM) in blocking the striatal M2 receptor suggested similarity to those M2 receptors localized in certain smooth muscles or in glands. However, AF-DX 116 (KB = 155 nM) and methoctramine (KB = 47 nM) were considerably more potent in blocking the striatal M2 receptor than as reported in functional studies in smooth muscle or glands. Thus, the profile of the striatal M2 receptor obtained with these antagonists did not match in all respects with either glandular (probable M4 gene product) or cardiac (probable M2 gene product) muscarinic receptors. In contrast, our data with the brainstem M2 receptor was highly correlated (r = 0.93) with literature data regarding the cardiac muscarinic system.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prejunctional inhibitory effects of prostanoids on sympathetic neurotransmission in the rabbit iris-ciliary body

Both naturally occurring and synthetic prostaglandins (PGs) caused concentration-dependent inhibition of electrically evoked [3H]norepinephrine (NE) overflow from the isolated, superfused rabbit iris-ciliary body without affecting basal tritium efflux. The rank order of potencies of the agonists was: sulprostone greater than 16, 16-dimethyl-PGE2 greater than PGE2 greater than 11-deoxy-PGE1 greater than iloprost (stable PGl2 analog) greater than PGF2 alpha greater than or equal to PGD2. However, the Tx-mimetic, U-46619, was without effect on transmitter release at concentrations up to 1 microM. The selective EP1-receptor antagonists, AH 6809 (30 microM) or SC-19220 (10 microM) had no effect on basal or field-stimulated [3H]NE secretion, nor did they antagonize the PGE2-mediated reduction of evoked [3H]NE overflow. Indomethacin (3 microM) and the 5-lipoxygenase inhibitor, BW A4C (1 microM) were without effect on basal or evoked [3H]NE release, suggesting that endogenously formed arachidonic acid metabolites have no significant modulatory role in this in vitro system. Inhibitory effects of submaximal or maximal concentrations of PGE2 combined with corresponding concentrations of clonidine or carbachol were not additive, suggesting that prejunctional PGE2 receptors coexist with alpha-2 adrenergic and muscarinic receptors at neurotransmitter release sites. In the presence of yohimbine (100 nM) and/or atropine (100 nM), however, the inhibition produced by PGE2 was enhanced markedly, suggesting that tonic activation of prejunctional alpha-2 adrenergic or muscarinic receptors by endogenously released transmitters may impair the response to exogenous PGE2.(ABSTRACT TRUNCATED AT 250 WORDS)     

The interaction of amitriptyline, doxepin, imipramine and their N-methyl quaternary ammonium derivatives with subtypes of muscarinic receptors in brain and heart

The interaction of amitriptyline, doxepin, imipramine and their N-methyl quaternary derivatives with muscarinic receptors was investigated in the brain and heart. The potency of the tricyclic derivatives for inhibiting the binding of 11[[2-[(diethylamino) methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido[2,3-b] [1,4] benzodiazepine-6-one to M2 muscarinic receptors in cerebral cortex was similar to that measured in competitive binding experiments with the nonselective muscarinic antagonist [3H]N-methylscopolamine in the corpus striatum and heart. Moreover, the tricyclic derivatives antagonized muscarinic receptor-mediated inhibition of adenylate cyclase activity with similar potency in the corpus striatum and heart, and there was good agreement between the affinities of the tricyclic derivatives when measured by radioligand binding and by antagonism of the adenylate cyclase response. Our results show that amitriptyline, doxepin and imipramine lack selectivity for subtypes of the muscarinic receptor.     

Real-time intracellular Ca2+ mobilization by travoprost acid,bimatoprost, unoprostone,and other analogs via endogenous mouse,rat, and cloned human FP prostaglandin receptors

The ability of a number of prostaglandin F 2 alpha (PGF 2 alpha) analogs to mobilize intracellular Ca2+[Ca2+]iand to compete for [3H]PGF 2 alpha binding to prostaglandin F 2 alpha receptors (FP) was evaluated. Radioligand binding studies measuring displacement of [3H]PGF 2 alpha by a variety of FP prostaglandin analogs yielded the following rank order of affinities: travoprost acid [(+)-16-m-trifluorophenoxy tetranor PGF 2 alpha; (+)-fluprostenol] > bimatoprost acid (17-phenyl-trinor PGF 2 alpha) > unoprostone (13,14-dihydro-15-keto-20-ethyl PGF 2 alpha) = bimatoprost (17-phenyl-trinor PGF 2 alpha ethyl amide) > or = Lumigan (bimatoprost ophthalmic solution). In FP functional studies, travoprost acid (EC50= 17.5-37 nM, n = 13), bimatoprost acid (EC50= 23.3-49.0 nM, n = 6-12), unoprostone (EC50= 306-1270 nM, n = 4-8), bimatoprost (EC50= 3070- 3940 nM, n = 4-9), and Lumigan (EC50= 1470-3190 nM, n = 5-9) concentration dependently stimulated [Ca2+]imobilization via the rat (A7r5 cells), mouse (3T3 cells), and cloned human ocular FP prostanoid receptors. The rank order of potency of these compounds at the FP receptor of the three species was similar and in good agreement with the determined binding affinities. The agonist effects of these compounds were concentration dependently blocked by the FP receptor-selective antagonist, AL-8810 (11beta-fluoro-15-epi-15-indanyl-tetranor PGF 2 alpha) (Ki= 0.6-1.3 microM). These studies have demonstrated that bimatoprost, unoprostone, and bimatoprost acid possess direct agonist activities at the rat, mouse, and human FP prostanoid receptor and that travoprost acid is the most potent of the synthetic FP prostaglandin analogs tested.     

Pharmacological control of the human gastric histamine H2 receptor by famotidine: comparison with H1, H2 and H3 receptor agonists and antagonists

Histamine 0.1 microM-0.1 mM increased adenylate cyclase activity five- to ten-fold in human fundic membranes, with a potency Ka = 3 microM. The histamine dose-response curve was mimicked by the H3 receptor agonist (R) alpha-MeHA, but at 100 times lower potency, Ka = 0.3 mM. Histamine-induced adenylate cyclase activation was abolished by H2, H1 and H3 receptor antagonists, according to the following order of potency IC50: famotidine (0.3 microM) greater than triprolidine (0.1 mM) thioperamide (2 mM), respectively. Famotidine has no action on membrane components activating the adenylate cyclase system, including the Gs subunit of the enzyme stimulated by forskolin and cell surface receptors sensitive to isoproterenol (beta 2-type), PGE2 and VIP. The Schild plot was linear for famotidine (P less than 0.01) with a regression coefficient r = 0.678. The slope of the regression line was 0.64 and differs from unity. Accordingly, famotidine showed a slow onset of inhibition and dissociation from the H2 receptor in human cancerous HGT-1 cells. The results demonstrate that famotidine is a potent and selective H2 receptor antagonist with uncompetitive actions in human gastric mucosa. Consequently, famotidine might be a suitable drug with long-lasting actions in the treatment of Zollinger-Ellison syndrome. The results also confirm and extend the previous observations that (R) alpha-MeHA and thioperamide are two selective ligands at histamine H3 receptor sites. In the human gastric mucosa, these drugs are respectively 330 and 6700 times less potent than histamine and famotidine on the adenylate cyclase system. The possible involvement of histamine H3 receptors in the regulation of gastric secretion is proposed.     

Assessment of the influence of histaminergic actions on cocaine-like effects of 3alpha-diphenylmethoxytropane analogs

Previous studies demonstrated that analogs of benztropine (BZT) possess high affinity for the dopamine (DA) transporter (DAT) but generally have behavioral effects different from those of cocaine, suggesting either unique actions at the DA transporter or that another action of these drugs interferes with cocaine-like effects. Because the parent compound has histamine-antagonistic effects, the affinity of its analogs for histamine H(1), H(2), and H(3) receptors were compared with DA transporter affinity to assess whether those differences predicted the amount of cocaine-like activity. All of the compounds displaced [(3)H]mepyramine from H(1), [(125)I]iodoaminopotentidine from H(2), and [(3)H]N-alpha-methylhistamine from H(3) histamine receptors with affinities ranging from 15.7 to 37,600, 218 to >4430, and 4040 to >150,000 nM, respectively. Affinities at histamine H(1) receptors were, respectively, approximately 25- or 300-fold greater than those at H(2) or H(3) histamine receptors. Relative affinities for H(1) and DAT binding did not reliably predict the degree of cocaine-like stimulation of locomotor activity. In addition, interactions of various histaminic agents with cocaine assessed whether an action at any of the histamine sites could interfere with cocaine-like effects. None of the histaminic agents fully substituted for cocaine in rats trained to discriminate 10 mg/kg cocaine from saline nor did any of the compounds antagonize or otherwise diminish the discriminative stimulus effects of cocaine. The results suggest that affinity for histamine receptors cannot account for the diminished cocaine-like effects of the BZT analogs and suggest alternatively that these compounds have actions different from those of cocaine but likely mediated by their interaction with the DAT.     

Different affinity states of alpha-1 adrenergic receptors defined by agonists and antagonists in bovine aorta plasma membranes

Evidence for a nonlinear relationship between alpha-1 adrenergic receptor occupancy and tissue responses, together with the finding of different affinity states for agonist binding, has raised the possibility of functional heterogeneity of alpha-1 adrenergic receptors. We have conducted studies to examine: 1) binding characteristics of [3H]prazosin, 2) competition of antagonists at these sites and 3) different affinity states of the receptor for agonists and modulation of these states by 5'-guanylylimidodiphosphate [Gpp(NH)p]. A plasma membrane-enriched vesicular fraction (F2; 15%/33% sucrose interphase) was prepared from the muscular medial layer of bovine thoracic aorta. [3H]Prazosin binding was characterized by a monophasic saturation isotherm (KD = 0.116 nM, Bmax = 112 fmol/mg of protein). Antagonist displacement studies yielded a relative potency order of prazosin greater than or equal to WB4104 much greater than phentolamine greater than corynanthine greater than yohimbine greater than or equal to idazoxan greater than rauwolscine. Competition curves for unlabeled prazosin, WB4101 (2-(2,6-dimethoxyphenoxyethyl)-aminomethyl-1,4 benzodioxane) and phentolamine were shallow and were best modeled to two binding sites with picomolar and nanomolar KD values. Gpp(NH)p was without effect on antagonist affinity. Agonist (epinephrine, norepinephrine and phenylephrine) competition with [3H]prazosin binding was biphasic with pseudo-Hill slopes less than 1.0. Binding was best described by a two-site model in which the average contribution of high affinity sites was 23% of total binding. KD values for the high affinity site ranged from 2.9 to 18 nM, and 3.9 to 5.0 microM for the low affinity site.(ABSTRACT TRUNCATED AT 250 WORDS)     

3H]Mianserin: differential labeling of serotonin and histamine receptors in rat brain

Mianserin, a tetracyclic antidepressant, is a potent serotonin (5-HT) and histamine H1 antagonist in peripheral smooth muscle systems. Mianserin was found to possess high affinity for 5-HT2 and histamine H1 receptor binding sites in brain membranes. By using [3H]mianserin, both 5-HT2 and histamine H1 receptors can be specifically labeled in rat cerebral cortex membranes. Simultaneous incubation of brain membranes with 300 nM triprolidine or 30 nM spiroperidol enables the selective labeling of 5-HT2 or histamine H1 receptors, respectively. In the guinea-pig cerebellum, [3H]mianserin exclusively labels histamine H1 receptors, since 5-HT2 sites are virtually absent in this area.     

Effects of histamine H2 receptor agonists and antagonists on the isolated guinea pig gallbladder

Histamine H2 receptor-mediated responses were examined on cholecystokinin-octapeptide (CCK-8)-precontracted guinea pig gallbladder in vitro, testing histamine and a series of specific histamine H2 receptor agonists and antagonists. Among the antagonists tested, zolantidine and compound SKF 92857 were previously shown to antagonize H2 receptor-mediated responses in the heart, but not in the stomach. Histamine, in the presence of the H2 receptor antagonist mepyramine (10 microM), and the H2 receptor agonists dimaprit, impromidine and amthamine, inhibited CCK-8 (3 nM)-induced contractions in a concentration-dependent fashion, with the following rank orders of potency: impromidine > amthamine > histamine > dimaprit (pD2 values were 6.73 +/- 0.04, 5.44 +/- 0.07, 4.64 +/- 0.04 and 3.71 +/- 0.05, respectively). The maximal relaxation produced by dimaprit was significantly lower than that produced by histamine, as well as by impromidine and amthamine, which behaved as full agonists. All the H2 receptor antagonists examined were able to inhibit amthamine-induced relaxation. Famotidine (pA2 = 7.09 +/- 0.26), zolantidine (pA2 = 6.54 +/- 0.11), compound SKF 92857 (pA2 = 6.58 +/- 0.13) and aminopotentidine (pA2 = 6.86 +/- 0.06) competitively antagonised the amthamine-induced effect, while iodoaminopotentidine produced surmountable antagonism only at low concentrations (1 microM, pA2 = 6.83 +/- 0.21). Finally, the slowly-dissociable antagonist loxtidine caused a non-parallel displacement to the right of the concentration--response curve to amthamine (pKB = 7.55 +/- 0.24), with a significant depression of the maximal response to the agonist, even at the lowest effective concentration (0.3 microM). In conclusion, histamine H2 receptors in guinea pig gallbladder resemble those of the heart, as regards their sensitivity to zolantidine and compound SKF 92857, which, by contrast, were unable to antagonize histamine effects at gastric H2 receptors in different experimental models.