Pharmacological specificity of brain histamine H2-receptors differs in intact cells and cell-free preparations

The Roles of Cardiovascular H2-Histamine Receptors Under Normal and Pathophysiological Conditions

This review addresses pharmacological, structural and functional relationships among H₂-histamine receptors and H₁-histamine receptors in the mammalian heart. The role of both receptors in the regulation of force and rhythm, including their electrophysiological effects on the mammalian heart, will then be discussed in context. The potential clinical role of cardiac H₂-histamine-receptors in cardiac diseases will be examined. The use of H₂-histamine receptor agonists to acutely increase the force of contraction will be discussed. Special attention will be paid to the potential role of cardiac H₂-histamine receptors in the genesis of cardiac arrhythmias. Moreover, novel findings on the putative role of H₂-histamine receptor antagonists in treating chronic heart failure in animal models and patients will be reviewed. Some limitations in our biochemical understanding of the cardiac role of H₂-histamine receptors will be discussed. Recommendations for further basic and translational research on cardiac H₂-histamine receptors will be offered. We will speculate whether new knowledge might lead to novel roles of H₂-histamine receptors in cardiac disease and whether cardiomyocyte specific H₂-histamine receptor agonists and antagonists should be developed.

Pharmacologically distinct phenotypes of α1B -adrenoceptors: variation in binding and functional affinities for antagonists

BACKGROUND AND PURPOSE

The pharmacological properties of particular receptors have recently been suggested to vary under different conditions. We compared the pharmacological properties of the α1B -adrenoceptor subtype in various tissue preparations and under various conditions.

EXPERIMENTAL APPROACH

[(3) H]-prazosin binding to α1B -adrenoceptors in rat liver (segments, dispersed hepatocytes and homogenates) was assessed and the pharmacological profiles were compared with the functional and binding profiles in rat carotid artery and recombinant α1B -adrenoceptors.

KEY RESULTS

In association and saturation-binding experiments with rat liver, binding affinity for [(3) H]-prazosin varied significantly between preparations (KD value approximately ten times higher in segments than in homogenates). The binding profile for various drugs in liver segments also deviated from the representative α1B -adrenoceptor profile observed in liver homogenates and recombinant receptors. L-765,314 and ALS-77, selective antagonists of α1B -adrenoceptors, showed high binding and antagonist affinities in liver homogenates and recombinant α1B -adrenoceptors. However, binding affinities for both ligands in the segments of rat liver and carotid artery were 10 times lower, and the antagonist potencies in α1B -adrenoceptor-mediated contractions of carotid artery were more than 100 times lower than the representative α1B -adrenoceptor profile.

CONCLUSIONS AND IMPLICATIONS

In contrast to the consistent profile of recombinant α1B -adrenoceptors, the pharmacological profile of native α1B -adrenoceptors of rat liver and carotid artery varied markedly under various receptor environments, showing significantly different binding properties between intact tissues and homogenates, and dissociation between functional and binding affinities. In addition to conventional 'subtype' characterization, 'phenotype' pharmacology must be considered in native receptor evaluations in vivo and in future pharmacotherapy.

Involvement of histamine receptors in the atypical antipsychotic profile of clozapine: a reassessment in vitro and in vivo

RATIONALE

The basis of the unique clinical profile of the antipsychotic clozapine is not yet elucidated. Brain histamine receptors may play a role in schizophrenia and its treatment, but their involvement in the profile of clozapine remained unknown.

OBJECTIVES

We explored the properties of clozapine and its two metabolites, N-desmethylclozapine (NDMC) and clozapine N-oxide, at the four human histaminergic receptors. We compared their active concentrations with their blood concentrations in patients treated by clozapine. We investigated the changes in receptor densities induced in rat brain by repeated administration of a therapeutic dose of clozapine.

RESULTS

Clozapine and NDMC behaved as very potent, and partial, H(1)-receptor inverse agonists, weak, and full, H(2)-receptor inverse agonists, moderate, and protean, H(3)-receptor agonists, and moderate, and partial, H(4)-receptor agonists. Taking into account their micromolar mean blood concentrations found in 75 treated patients, and assuming that they are enriched in human brain as they are in rat brain, a full occupation of H(1)-, H(3)-, and H(4)-receptors, and a partial occupation of H(2) receptors, is expected. In agreement, repeated administration of clozapine at a therapeutic dose (20 mg/kg/day for 20 days) induced an up-regulation of H(1)- and H(2)-receptors in rat brain.

CONCLUSIONS

Clozapine and its active metabolite NDMC interact with the four human histamine receptors at clinically relevant concentrations. This interaction may substantiate, at least in part, the atypical antipsychotic profile of clozapine, as well as its central and peripheral side effects such as sedation and weight gain.

Influence of tissue integrity on pharmacological phenotypes of muscarinic acetylcholine receptors in the rat cerebral cortex

Distinct pharmacological phenotypes of muscarinic acetylcholine receptors (mAChRs) have been proposed. We compared the pharmacological profiles of mAChRs in intact segments and homogenates of rat cerebral cortex and other tissues by using radioligand binding assays with [(3)H]N-methylscopolamine ([(3)H]NMS). Recombinant M(1) and M(3) mAChRs were also examined. The density of mAChRs detected by [(3)H]NMS binding to rat cerebral cortex segments and homogenates was the same (approximately 1400 fmol/mg tissue protein), but the dissociation constant of [(3)H]NMS was significantly different (1400-1700 pM in segments and 260 pM in homogenates). A wide variation in [(3)H]NMS binding affinity was also observed among the segments of other tissues (ranging from 139 pM in urinary bladder muscle to 1130 pM in the hippocampus). The mAChRs of cerebral cortex were composed of M(1), M(2), M(3), and M(4) subtypes, which showed typical subtype pharmacology in the homogenates. However, in the cortex segments the M(3) subtype showed a low selectivity for M(3) antagonists (darifenacin, solifenacin) and was not distinguished by the M(3) antagonists from the other subtypes. Recombinant M(1) and M(3) mAChRs showed high affinity for [(3)H]NMS and subtype-specific pharmacology for each tested ligand. The present binding study under conditions where tissue integrity was kept demonstrates a wide variation in [(3)H]NMS binding affinity among mAChRs of many rat tissues and the presence of an atypical M(3) phenotype in the cerebral cortex, suggesting that the pharmacological properties of mAChRs are not necessarily constant, rather they may be significantly modified by tissue integrity and tissue type.

Histamine and Schizophrenia

With the availability of an increased number of experimental tools, for example potent and brain-penetrating H1-, H2-, and H3-receptor ligands and mutant mice lacking the histamine synthesis enzyme or the histamine receptors, the functional roles of histaminergic neurons in the brain have been considerably clarified during the recent years, particularly their major role in the control of arousal, cognition, and energy balance. Various approaches tend to establish the implication of histaminergic neurons in schizophrenia. A strong hyperactivity of histamine neurons is induced in rodent brain by administration of methamphetamine or NMDA-receptor antagonists. Histamine neuron activity is modulated by typical and atypical neuroleptics. H3-receptor antagonists/inverse agonists display antipsychotic-like properties in animal models of the disease. Because of the limited predictability value of most animal models and the paucity of drugs affecting histaminergic transmission that were tried so far in human, the evidence remains therefore largely indirect, but supports a role of histamine neurons in schizophrenia.

Changes in mu opioid receptors and rheological properties of erythrocytes among opioid abusers

Opioids, reported originally to bind to specific receptors in the brain, now also appear to bind to receptors on blood cells. The high prevalence of anemia among chronic opioid users leads us to propose that chronic opiate use results in elevated mu opioid receptor levels on human erythrocytes and that these receptor changes may affect erythrocyte membrane properties. Blood samples from 17 opioid-dependent subjects (based on the Diagnostic and Statistical Manual of Mental Disorders, 4th edition or DSM-IV) and 15 drug-free controls were assayed for mu opioid receptors on erythrocytes using a flow cytometry immunoassay. Deformability and the hydration status of erythrocytes were studied by ektacytometry. Data were analyzed by independent t-tests, tests of correlation, chi square and cluster analyses. As expected, the percentage of erythrocytes from opioid-dependent subjects with opioid receptors (opioid receptor levels) was significantly higher (47.4 +/- 38.3%) than controls (22.8 +/- 30.1%) (t = 2.01, df = 30, p < 0.05). Also, the opioid-dependent patients showed a wide variation in the percentage of erythrocytes bearing opioid receptors and data analyses of these patients showed two strongly defined clusters. One subgroup consisted of nine individuals with very high receptor levels (mean = 81.5%) while the other had eight patients with low receptor levels (mean = 9.1%) that were not significantly different than the receptor levels of controls. Ektacytometry of opioid dependent patients with high opioid receptor levels showed changes in rheological parameters of erythrocytes, such as deformability index and cellular hydration. For example, a positive correlation was observed between opioid receptor levels and deformability indices among opioid-dependent patients (r = 0.74, p < 0.005). Our findings indicate that the mu opioid receptor is present on human erythrocytes, although with considerable variation in receptor levels, and that the levels of this receptor are significantly elevated with chronic opioid exposure. Moreover, erythrocytes with high opioid receptor levels from chronic opiate users seem to have high deformability. This study may offer clues to the biological properties of peripheral blood cells that may be mediated by mu opioid receptors and lead to a better understanding of some of the clinical effects of opioid use.

Pharmacological Characterization and Autoradiographic Localization of Histamine H2Receptors in Human Brain Identified with [125I]Iodoaminopotentidine

125I-Aminopotentidine (125I-APT), a reversible probe of high specific radioactivity and high affinity and selectivity for the H2 receptor, was used to characterize and localize this histamine receptor subtype in human brain samples obtained at autopsy. On membranes of human caudate nucleus, specific 125I-APT binding at equilibrium revealed a single component, with a dissociation constant of 0.3 nM and maximal capacity of about 100 fmol/mg of protein. At 0.2 nM, 125I-APT specific binding, as defined with tiotidine, an H2-receptor antagonist chemically unrelated to iodoaminopotentidine, represented 40-50% of the total. Specific 125I-APT binding was inhibited by a series of typical H2-receptor antagonists that displayed apparent dissociation constants closely similar to corresponding values at the reference biological system, i.e., guinea pig atrium. This indicates that the pharmacology of the H2 receptor is the same in the human brain as on this reference system. However, histamine was about 10-fold more potent in inhibiting 125I-APT binding to membranes of human brain than of guinea pig brain. 125I-APT binding was also inhibited by amitriptyline and mianserin, two antidepressant drugs, in micromolar concentrations corresponding to effective plasma concentrations of treated patients. The distribution of H2 receptors was established autoradiographically with 125I-APT on a series of coronal sections of human brain after assessing the pharmacological specificity of the labeling. The highest density of 125I-APT sites was found in the basal ganglia, various parts of the limbic system, e.g., hippocampus or amygdaloid complex, and the cerebral cortex. H2 receptors displayed a laminar distribution in cerebral cortex and hippocampal formation. A low density of sites was found in cerebellum as well as in hypothalamus, the brain area where all the perikarya and the largest number of axons of histaminergic neurons are found. The widespread distribution of H2 receptors in the human brain is consistent with the alleged modulatory role of histamine mediated by this subtype of receptor.

Expression of a cloned rat histamine H2 receptor mediating inhibition of arachidonate release and activation of cAMP accumulation

A DNA, cloned after screening a rat genomic bank with probes derived from the sequence of a putative dog histamine H2 receptor [Gantz, I., Schäffer, M., Delvalle, J., Logsdon, C., Campbell, V., Uhler, M. & Yamada, T. (1991) Proc. Natl. Acad. Sci. USA 88, 429-433], was used to prepare a probe for Northern blot analysis and to transfect Chinese hamster ovary (CHO) cells. Distribution of the gene transcripts in guinea pig tissues was consistent with that of H2 receptors. Transfected CHO cells expressed a high density of sites binding [125I]iodoaminopotentidine, a selective H2-receptor ligand. These sites were characterized as typical H2 receptors by using a series of competing agents that displayed apparent dissociation constants closely similar to corresponding values at a reference biological system. In transfected cells, histamine stimulated, with high potency and large receptor reserve, the accumulation of cAMP. In addition, in the same cells, histamine potently inhibited the release of arachidonic acid induced either by stimulation of constitutive purinergic receptors or by application of a Ca2+ ionophore. This inhibition was independent of either cAMP or Ca2+ levels. The results suggest that a single H2 receptor may be linked not only to adenylyl cyclase activation but also to reduction of phospholipase A2 activity. Because H1 receptors have been reported to stimulate arachidonic acid release, inhibition of this release, an unexpected signaling pathway for H2 receptors, may account for the opposing physiological responses elicited in many tissues by stimulation of these two receptors subtypes.

Interaction of mianserin, amitriptyline and haloperidol with guinea pig cerebral histamine H2 receptors studied with [125I]iodoaminopotentidine

Tricyclic antidepressants were previously shown to potently inhibit the histamine-induced activation of H2 receptors linked to adenylate cyclase in cerebral membranes, and this effect was postulated to represent the mechanism of their therapeutic activity. However, these drugs were found to be much less potent (up to more than hundred-fold) at H2 receptors mediating cyclic AMP responses in intact cells. We have presently assessed whether this large difference in potency of antidepressants, also observed with haloperidol, results from a modified pharmacological specificity of the H2 receptor according to the medium composition. We have studied the binding of [125I]iodoaminopotentidine (125I-APT) to striatal or hippocampal membranes under various experimental conditions. At equilibrium the Kd of 125I-APT, a highly selective ligand for H2 receptors, was six times higher in a supplemented Tris buffer used for adenylate cyclase assays in cell-free systems than in a Krebs-Ringer medium used in studies with intact cells. The medium composition also variously affected the Ki values of the four compounds studied amitriptyline, mianserin, haloperidol and tiotidine. Whereas the Ki value of amitriptyline was little affected, that of the other compounds was four to five times lower in the supplemented Tris buffer than in the Krebs-Ringer medium. With the exception of tiotidine, the Ki values of other compounds in the binding test performed in this medium, were intermediate between those derived from the antagonism of histamine-induced cyclic AMP responses in membranes and intact cells. These data indicate that the difference between the two test responses is diversely attributable to several factors according to the compounds, the main ones being medium composition and possibly cell disruption.

Reversible and irreversible labelling of H1- and H2 -receptors using novel [125I] probes

We have recently designed the first 125I-labelled probes specific for the histamine H1 and H2 receptors. These reversible and irreversible antagonists are among the most potent H1 and H2 ligands and have enabled investigations into the biochemical and pharmacological properties of these two receptors. In various brain animal species, the ligand binding peptide of the H1 and H2 receptors, as determined by photoaffinity labeling, resides within 56-59 kDa peptides. In contrast, in guinea pig heart, the ligand binding domain of the H1 receptor is characterized by a higher molecular weight (68 kDa), suggesting the presence of an isoform of this protein, clearly differentiable by this biochemical property but not by its pharmacology. The reversible 125I-probes allowed us to extend the pharmacology of these receptors in several biological preparations and in human brain, and to establish their interaction with G-proteins. A detailed mapping of H1 and, for the first time, of H2 receptors, has been achieved in guinea pig brain, establishing their presence in almost all brain areas. These experiments show that there is no correlation between the density of H2 receptor and the activity of adenylate cyclase sensitive to histamine suggesting a molecular heterogeneity of this receptor.

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.

Non-selectivity of amitriptyline for subtypes of brain muscarinic receptors demonstrated in binding and functional assays

The characteristics of interaction of amitriptyline, a tricyclic antidepressant, with rat brain muscarinic receptors were assessed using both radioligand binding and functional assays. In competition studies, amitriptyline displaced muscarinic ligand binding from a single high-affinity site in homogenates of various brain regions which have a different distribution of M1 and M2 receptor subtypes. The affinity of amitriptyline for muscarinic receptors was also comparable in all brain regions. Furthermore, amitriptyline identified a single species of muscarinic receptors in intact cells dissociated from the cerebral cortex and in cerebrocortical slices. The non-selectivity of amitriptyline for muscarinic receptor subtypes in these preparations was in contrast to the selectivity exhibited by pirenzepine. This non-selective nature of amitriptyline was also evident in functional assays, since this antidepressant was equipotent at antagonizing M1-mediated increase in phosphoinositide hydrolysis and M2-mediated inhibition of cyclic AMP formation in dissociated cortical cells. Atropine was also equipotent at blocking both responses but was 20- to 30-fold more potent than amitriptyline. These results demonstrate that amitriptyline behaves as a non-selective muscarinic antagonist using both radioligand binding and functional measurements.

Zolantidine (SK&F 95282) is a potent selective brain-penetrating histamine H2-receptor antagonist

1. The novel benzthiazole derivative zolantidine (SK&F 95282) is a potent antagonist of histamine at H2-receptors in guinea-pig atrium and rat uterus. Only apparent pA2 values of 7.46 and 7.26 respectively could be calculated since the slopes of the Schild plots were significantly less than unity. 2. Zolantidine is equally potent as an antagonist at histamine H2-receptors in guinea-pig brain. The compound inhibited histamine stimulated adenylate cyclase (pKi 7.3) and dimaprit stimulated adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation (approx pA2 7.63), and competed with [3H]-tiotidine binding (pKi 7.17). 3. Zolantidine is at least 30 fold more potent at H2-receptors than at other peripheral and central receptors investigated. 4. Infusion of zolantidine into rats produces a brain concentration greater than the plateau blood concentration (brain/blood ratio 1.45). 5. Zolantidine is thus characterized as a potent selective brain-penetrating H2-receptor antagonist, and will be a valuable pharmacological tool for investigating possible physiological and pathological roles for histamine in the central nervous system.

Tissue slices in radioligand binding assays: studies in brain, pineal and muscle

The use of tissue homogenates in receptor binding assays raises serious questions as to the physiological value of a preparation which examines receptors (binding sites) in disrupted tissue. In order to usefully study the regulatory properties of neurotransmitter receptors under physiological conditions, the necessity for tissue preparations which retain some degree of cellular integrity is clear. We review here the experiments which have utilized intact tissue - largely in the form of thick slices - to perform radioligand binding assays. There are many reports which note marked differences between studies in intact versus broken cell preparations. For example, significant discrepancies in KD and Bmax values are apparent for [3H] quinuclidinyl benzilate (muscarinic) and [3H] ouabain (Na+/K+-ATP ase, sodium pump) sites in brain and muscle respectively. A further example is the well-described stimulatory effect of GABA on benzodiazepine binding sites which is not seen in tissue slices. Other examples are highlighted. For all ligands so far examined, binding to slices is reversible, stereospecific, saturable, displaceable by appropriate drugs and of high affinity (nM). The method developed in our own laboratory is inexpensive, rapid and involves a minimum of tissue preparation. The technique is so simple as to allow many workers to enter this field who would not otherwise have done so. We suggest that metabolically active tissue slices offer the simplest approach to the study of cell-surface receptor regulation in living tissue.

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).

Differences in the interaction of histamine H2 receptor antagonists and tricyclic antidepressants with adenylate cyclase from guinea pig gastric mucosa

The interaction of adenylate cyclase with histamine H2 receptor agents and with tricyclic antidepressants was studied in guinea pig gastric mucosal membranes. The H2 receptor antagonist tiotidine acted as a competitive inhibitor of histamine-stimulated adenylate cyclase. The tricyclic antidepressants imipramine and amitryptyline were also competitive inhibitors. The dissociation constant of imipramine was the same whether histamine or dimaprit was used to activate the enzyme. In membrane preparations that had been stored frozen, there was a marked increase in the concentration of histamine or dimaprit required to cause half-maximal enzyme stimulation, and the dissociation constants of some classical H2 receptor antagonists were greatly increased. In contrast, the dissociation constants of the antidepressants were either unchanged or decreased. These results suggest that antidepressants are potent blockers of H2 receptors in gastric mucosal membranes, but there are differences between antidepressants and classical H2 receptor antagonists in their interaction with H2 receptors.

Pharmacological characterization of histamine H2 receptors on clonal cytolytic T lymphocytes. Evidence for histamine-induced desensitization

Cultured cytolytic T lymphocytes of clonal origin were screened for histamine-stimulated cyclic AMP production. Histamine caused a 2- to 8-fold elevation of cyclic AMP levels in five independent clones. The EC50 for histamine of 1.7 X 10(-5) M and the rank order of potencies of H1 and H2 agonists [impromidine greater than histamine greater than dimaprit greater than 4-methylhistamine greater than 2-methylhistamine greater than 2-(2-aminoethyl)-thiazole] were characteristic of the conventional histamine H2 receptor. H1 and H2 antagonists inhibited histamine-stimulated cyclic AMP elevation with inhibition constants typical for those found on other H2 receptor systems. Prior incubation of cells with histamine resulted in a marked loss in responsiveness to subsequent histamine challenge. We demonstrate that this desensitization is dose and time dependent and results in a change in the efficacy and not the potency of histamine. Although cyclic AMP increases could also be elicited with isoproterenol, prostaglandin E1 or forskolin, desensitization of histamine had no effect on the ability of these agents to stimulate cyclic AMP production. In contrast to the rapid rate of histamine-induced desensitization, recovery of histamine responsiveness could not be detected for several hours.

Action of tricyclic antidepressants on cyclic AMP in mucosal cells from guinea pig stomach

In dispersed mucosal cells from guinea pig stomach, tricyclic antidepressants (amitriptyline and imipramine) inhibited the increases in cyclic AMP caused by histamine or dimaprit. The inhibition by low concentrations of amitriptyline was competitive, with a pA2 value of 5.8 +/- 0.15 (Ki, 1.5 +/- 0.8 microM). At high concentrations, amitriptyline reduced the maximal response to histamine as well as inhibited the increase in cyclic AMP caused by theophylline and prostaglandin E1, and also reduced the basal respiratory rate of gastric cells. This inhibition was non-competitive (IC50, 100-250 microM). These results suggest that at low concentrations tricyclic antidepressants interact with the H2-receptors on gastric cells in a fashion similar to their interaction with the brain histamine H2-receptors.

Opiate ([3H]-naloxone) binding to hypothalamic and cerebral cortical slices of mouse brain

We have characterized the binding of [3H]-naloxone to thick (400 microns) slices of hypothalamus and cerebral cortex from mouse brain. Binding is reversible, saturable, stereospecific, thermolabile, readily displaceable by opiates and sensitive to phenoxybenzamine and phentolamine. Values for KD and Bmax are very close to published figures obtained in brain homogenates. Metabolic inhibitors (ouabain and azide) have no effect on specific binding. The assay is rapid, simple and involves minimal tissue preparation.

Benzodiazepine ([3H])-flunitrazepam) binding sites in cerebellar and cerebral cortical slices of mouse brain

We have characterized and quantified the specific binding of [3H]-flunitrazepam ( FNZ ) to thick (230 micron) slices of mouse brain. The binding site has the characteristics of a benzodiazepine receptor, i.e., binding of FNZ is reversible, stereospecific, saturable and of high affinity. Clonazepam, but not R05 -4864, readily displaces the label. In contrast to results from homogenate assays, neither GABA nor bicuculline has any effect on [3H]- FNZ binding. However, as previously reported, the slice assay confirms the lower number of benzodiazepine receptors in "emotional" mouse brain. In addition, we have confirmed that the neurotoxin DSP4 can modify [3H]- FNZ binding though in our hands this compound elevates rather than reduces binding. The speed, simplicity and minimal tissue preparation involved suggests that this slice assay could be a valuable addition to neurochemical studies of neurotransmitter receptors.

alpha-Noradrenergic potentiation of neurotransmitter-stimulated cAMP production in rat striatal slices

The present study examines the possible involvement of alpha-adrenergic receptors in catecholamine-stimulated cAMP production in intact slices of rat striatum. Norepinephrine (NE) produces a greater stimulation of cAMP levels than does the beta-adrenergic agonist, isoproterenol (ISO), and the NE response is inhibited by both the beta-adrenergic antagonist, propranolol, and the alpha-adrenergic antagonist, phentolamine. The alpha-adrenergic agonist, 6-fluoronorepinephrine (6-FNE), has little or no effect on basal cAMP levels; however, 6-FNE causes a marked potentiation of the cAMP response to ISO. Hence, NE stimulation of cAMP levels in striatal slices appears to involve a synergistic interaction between alpha- and beta-adrenergic receptors. alpha-Receptors also potentiate adenosine stimulation of cAMP levels in striatal slices. However, in contrast to results previously reported in cerebral cortical slices, the alpha-adrenergic component of the NE response in striatal slices is not dependent on endogenous adenosine. Finally, 6-FNE interactions with adenylate cyclase in striatal homogenates differ from those observed in the slice preparation. In homogenates, 6-FNE appears to directly stimulate adenylate cyclase through a D-1 receptor. D-1 receptor involvement in catecholamine responses in the striatal slice preparation, on the other hand, appears to be minimal.

Effects of antidepressant drugs on histamine-H1 receptors in the brain

The histamine-H1 receptor blocking properties of a number of structurally different antidepressant drugs have been evaluated using a 3H-mepyramine binding assay and a guinea-pig ileum preparation. The tricyclic antidepressants all inhibited the histamine-H1 receptor. Some newer antidepressant drugs, such as zimelidine and nomifensine were devoid of activity while others, such as iprindole and mianserin were very potent. It is concluded that antagonistic effects on the histamine-H1 receptor is not associated with the therapeutic efficacy in depression, but may contribute to the sedative effects of the antidepressant drugs.

Dopaminergic binding sites in rat striatal slices and the action of guanyl nucleotides

Dopaminergic binding sites were studied in slices from rat striatum incubated in a physiological medium and using the two highly selective ligands 3H-apomorphine and 3H-domperidone. The clearly biphasic or stretched inhibition of the specific binding of these two ligands by domperidone or apomorphine, respectively allowed to define three distinct classes of binding site. It was demonstrated, by comparing the binding of the 3H-ligand added at the beginning of slice incubation or just before homogenisation of tissue and filtration, that the "specific" bindings only occurred during the incubation of slices. The inhibition constants (Ki values) of dopaminergic agents for the three classes of binding site as also the dissociation constants (Kd values) of 3H-ligands and the maximal capacity (Bmax) of the three classes of binding site were closely similar to those of binding sites previously demonstrated on rat striatal membranes, namely D-2, D-3 and D-4 sites (Sokoloff et al. 1980 a, b). Their identification on a preparation in which the cellular organisation is largely preserved rules out the possibility that these sites represent an artifact due to membrane preparation. Unexpectedly the addition of guanyl nucleotides like GTP or GppNHp to the slice preparation decreased the binding of 3H-apomorphine to the high affinity sites (particularly to the D-2 sites) while D-4 site binding was correspondingly increased. The guanyl nucleotide effect apparently took place before cell disruption and occurred at concentrations similar to those required in striatal membrane preparations. These observations, together with those indicating the presence of high affinity binding sites for dopaminergic agonists in intact striatal cells, suggest that a putative nucleotide regulatory unit of dopamine receptors, is not fully occupied by intracellular GTP but could be interacted with from the external face of the cell membrane.

Modulation of low calcium induced field bursts in the hippocampus by monoamines and cholinomimetics

The influence of monoamine transmitter candidates, acetylcholine and related substances on rhythmic depolarization shifts (field bursts) in the CA 1 area of hippocampal slices from rats in low calcium (0.2 mmol X 1(-1) ) high magnesium (4 mmol X 1(-1) ) was investigated. Acetylcholine (ACh), histamine (HA) and H2-agonists, noradrenaline (NA) and beta-agonists at nano- to micromolar concentrations as well as dopamine (DA) and 8-bromo-cyclic AMP at 100 mumol X 1(-1) accelerated the field bursts. H2-antagonists blocked HA actions, beta-antagonists blocked NA actions selectively; muscarinic antagonists blocked ACh, HA and NA actions. H1-agonists, serotonin, dopamine and adenosine slowed the field bursts at micromolar concentrations. These effects parallel the action of the tested substances on afterhyperpolarizations in CA 1 pyramidal cells. High sensitivity and specificity make this response of the field bursts an excellent model to study postsynaptic transmitter actions in the central nervous system.

The effects of cimetidine on schedule-controlled responding and locomotor activity in rats

The effect of intraperitoneal injections of cimetidine, a selective histaminergic H2-receptor blocking agent, on operant behavior and locomotor activity were examined in rats. Cimetidine (1-100 mg/kg) failed to show any significant effect on responding maintained under a fixed-ratio (FR) 30 fixed-interval (FI) 5-min schedule of food presentation. A higher dose of cimetidine (300 mg/kg) produced decreases in both FR and FI rates of responding. In contrast, 100 mg/kg of cimetidine increased the response rate and decreased reinforcement rate in rats performing under a schedule requiring the temporal spacing of responses (DRL-10 sec). Cimetidine (10-300 mg/kg) did not induce significant changes in locomotor activity in the rat. These data suggest that cimetidine is more potent in altering the steady low rate of responding under a DRL schedule of food presentation, than responding maintained under a multiple FR FI schedule, and that cimetidine is even less potent in altering locomotor activity.

Antidepressants are weak competitive antagonists of histamine H2 receptors in dissociated brain tissue

Guinea pig hippocampus dissociated by mechanical means into uniform clumps of cells (approximately 100 micron in diameter) contains histamine receptors (H1 and H2) which mediate the formation of cyclic AMP. In this preparation, antidepressants are very potent antagonists of histamine H1 receptors but are weak antagonists of histamine H2 receptors. The latter result is contrary to data derived by others using homogenates of the guinea pig hippocampus and seems to dispel the idea that antidepressants derive their efficacy by blocking histamine H2 receptors in brain.

Opiate receptors on lymphocytes and platelets in man

Receptors for opiates, opiate-like substances, and their antagonists, such as naloxone (a close chemical and conformational congener of morphine), on brain cell homogenates and neuroblastoma X glioma hybrid cells in tissue culture have been reported. The present study on the binding of [3H]naloxone to lymphocytes and platelets freshly isolated from the peripheral blood of 39 healthy adult human volunteers showed that (1) [3H]naloxone bound to lymphocytes and platelets at 4 degrees C, reaching equilibrium in 30 min, and was not removed by washing (three times) with the suspending medium; (2) the binding of [3H]naloxone to cells decreased in the presence of increasing amounts of unlabeled naloxone, approaching a plateau; (3) significant amounts of the radioligand remained bound in the presence of micromolar quantities of the unlabeled ligand; and (4) in the absence of Na+ ions, 1 to 10 nmol of morphine hydrochloride for 10(6) lymphocytes, and 1 to 25 nmol of morphine hydrochloride for 10(8) platelets, decreased the binding of [3H]naloxone by 43 to 57%. It is concluded that at least some of the [3H]naloxone binding sites on human lymphocytes and platelets are specific opioid receptor sites of the mu type (Enkephalins define the delta sites.) The observations on the binding of naloxone to cells do not appear to be artifacts. Opioid receptor sites on lymphocyte and platelet membranes may have properties similar to those on nerve cell membranes.

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.

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.

Development of beta-adrenergic receptors and their function in glia-neuron communication in cultured chick brain

The beta-receptors of intact neuronal and glial cells of chick embryonic brain were studied via the specific binding of the beta-antagonist [3H]dihydro-L-alprenolol ( [3H]DHA). Cells were cultivated in either highly homogeneous or mixed populations; the neuronal cells were also grown under the influence of glial conditioned medium (GCM) or 10(-11)-10(-10) M L-norepinephrine or L-isoproterenol. The beta-receptors of both neuronal and glial cells proved to be positively cooperative (n = 2.5) and of high affinity, with a Kdapp of 98 and 44 pM, respectively. The Kdapp value was influenced only slightly by the different culture conditions. The receptor concentration was relatively low in the homogeneous neuronal and glial cultures (Bmax = 6.4 and 3.3 fmol/10(6) cells, respectively). It increased by a factor of 2-3 if development of the neuron-glia contacts in the culture was possible (mixed cultures). GCM and beta-agonists elevated the number of beta-receptors of the neuronal cells approximately 4-fold, even in the absence of glial cells. This receptor-number change was preceded by a well observable morphological differentiation. Both the morphological and the beta-receptor effects of L-norepinephrine were antagonized by L-propranolol. The beta-receptor number increased about 2-fold during a 10-day in vitro development, even in neuron-glia mixed cultures.

Clinical investigations into antidepressive mechanisms. I. Antihistaminic and cholinolytic effects: amitriptyline versus promethazine

It is assumed that established antidepressants exert their clinical efficacy by potentiation or decrease of central noradrenergic and serotonergic neurotransmission. However, recent experimental work suggests that antihistaminic and/or cholinolytic effects may also be involved. This double-blind controlled study compared amitriptyline (catecholamine potentiating, antihistaminic, cholinolytic) with promethazine (antihistaminic, cholinolytic) in 50 severely depressed inpatients over a 30-day treatment period. Analysis of the Hamilton depression rating scale revealed significant clinical superiority of amitriptyline over promethazine in such major depressive symptoms as depressed mood, suicidal ideation, psychic anxiety, and sleep disturbances. No significant difference was evident as far as autonomous side effects were concerned. Similar results were found by analysis of the AMP rating system. It is concluded that antihistaminic or cholinolytic effects per se do not explain the antidepressants' efficacy. However, potentiation of noradrenergic neurotransmission by cholinolytic activity might be the major antidepressive mechanism.

Chronic antidepressant therapy and associated changes in central monoaminergic receptor functioning

Acutely administered antidepressants possess a multiplicity of pharmacological actions. However, the fact that agents possessing similar pharmacological actions are devoid of antidepressant activity, together with the lack of correlation between doses required for acute pharmacological effects and clinical efficacy, suggest that the mechanism(s) of action of antidepressants cannot be directly attributed to the acute pharmacological properties of the drugs. The lag phase in onset of clinical effectiveness emphasizes the importance of adaptive changes following chronic antidepressant administration. A rapidly accelerating trend in attempting to delineate the precise molecular mechanisms of action of antidepressants is the shift in emphasis following chronic antidepressant therapies from alterations in uptake, storage, synthesis and release of neurotransmitters to adaptive changes in receptor functioning. These adaptations occur both pre- and postsynaptically. Examples of the former are alpha 2 and DA presynaptic receptors, both being down-regulated by certain forms of chronic antidepressant therapy. The fact that the NE-coupled adenylate cyclase system in rat brain slices is down-regulated by tricyclics, atypical antidepressants, MAO inhibitors and ECT emphasizes the importance of the system. Electrophysiological and behavioral studies point to the up-regulation of central alpha 1 and 5-HT receptor functioning following long-term antidepressant therapy. In contrast to the beta-adrenoceptor, these findings cannot be correlated with data from radioligand binding studies. In general central alpha 1-adrenoceptor binding remains unaltered. This is also true for 5-HT1 binding whereas cortical 5-HT2 binding is both increased and decreased depending on the type of antidepressant therapy being investigated. The relationship of these adaptive changes to the clinical efficacy of antidepressants in man is not clear since there is generally a lack of good models for studying human central receptor functioning. A review of current data from animal studies would tend to disfavour the view that all forms of antidepressant therapy possess a common mechanism of action. Perhaps multiple intervention sites exist. The introduction and evaluation of agents possessing a specificity of pharmacological action will undoubtedly aid psychotherapeutic research. The knowledge that peptides and 'classical' neurotransmitters can co-exist in the same neurone will undoubtedly generate studies of the significance and importance of the co-transmitter function of peptides in the mechanisms of action of antidepressant therapies.

Impromidine-induced hypothermia in rats: effect of cimetidine and mianserine

Impromidine, a highly potent histamine H2-receptor agonist, given i.v. at doses of 3.1-62 nmole, induced a dose-dependent hypothermia in the rat with a maximal effect after 15 min. Cimetidine, an H2-receptor antagonist, having no effect when administered alone, antagonized the hypothermic action of impromidine. Two antiserotoninergic agents, p-chlorophenylalanine and metergoline, and chronic treatment with an antidepressant mianserine reduced the impromidine-induced hypothermia. It is suggested that the impromidine-induced hypothermia is an H2-receptor-mediated phenomenon, and the antagonizing effect of mianserine is related to serotonin receptor blocking activity of the drug rather than to its direct H2-receptor antagonism.

Mechanism for gastric antisecretory effects of desmethylimipramine in rats

The mechanism for the gastric antisecretory action of desmethylimipramine (DMI) was studied using the pylorus-ligated rat preparation. DMI was approximately 40 times more potent in decreasing gastric acid secretion when given into the lateral ventricles of the brain than when administered intravenously. The antisecretory effects of DMI could be blocked by the alpha 2-adrenoceptor antagonists yohimbine and SK&F 72223 and mimicked by central administration of an alpha 2-agonist. It could not be blocked by the alpha 1-antagonist prazosin or mimicked by alpha 1-adrenoceptor agonists. SK&F 72223 and yohimbine themselves produced small increases in gastric acid, but the increase output by SK&F 72223 failed to reduce the antisecretory response to atropine. Since DMI is not an alpha 2-adrenoceptor agonist, but is a potent inhibitor of norepinephrine uptake, these data suggest that the effects of DMI on gastric acid secretion are mediated indirectly via inhibition of catecholamine uptake at central synapses containing alpha 2-adrenoceptors.

Cerebral circulation and histamine: 2. Responses of pial veins and arterioles to receptor agonists

H2-receptors predominantly mediate pial arteriolar dilatation in response to histamine, but the reaction of pial veins to histamine has not been clearly identified. In anesthetized cats, we examined responses of pial veins and arterioles to perivascular microapplication of histamine and specific histamine H1 and H2 receptor agonists. Arterioles were very sensitive to the H2-receptor agonist impromidine, with significant dilatation (+16%) occurring at concentrations as low as 10(-10) M. Arteriolar responses to H1 receptor stimulation by 2.2-pyridylethylamine were small, even at high concentrations. The order of potency and maximum dilatations found for the receptor agonists were: H2 (43%) greater than histamine (28%) greater than H1 (17%). By contrast, pial veins did not respond to histamine or the receptor agonists. The results indicate that pial venomotor activity to histamine is negligible, and suggest a sparse distribution of histamine receptors on the outer surfaces of pial veins.