Binding of [3H]cimetidine to rat brain tissue

Histamine H2 receptor radioligands: triumphs and challenges

Since the discovery of the histamine H₂ receptor (H₂R), radioligands were among the most powerful tools to investigate its role and function. Initially, radiolabeling was used to investigate human and rodent tissues regarding their receptor expression. Later, radioligands gained increasing significance as pharmacological tools in in vitro assays. Although tritium-labeling was mainly used for this purpose, labeling with carbon-14 is preferred for metabolic studies of drug candidates. After the more-or-less successful application of numerous labeled H₂R antagonists, the recent development of the G protein-biased radioligand [³H]UR-KAT479 represents another step forward to elucidate the widely unknown role of the H₂R in the central nervous system through future studies.

Inhibition of brain [(3)H]cimetidine binding by improgan-like antinociceptive drugs

[(3)H]cimetidine, a radiolabeled histamine H(2) receptor antagonist, binds with high affinity to an unknown hemoprotein in the brain which is not the histamine H(2) receptor. Improgan, a close chemical congener of cimetidine, is a highly effective pain-relieving drug following CNS administration, yet its mechanism of action remains unknown. To test the hypothesis that the [(3)H]cimetidine-binding site is the improgan antinociceptive target, improgan, cimetidine, and 8 other chemical congeners were studied as potential inhibitors of [(3)H]cimetidine binding in membrane fractions from the rat brain. All compounds produced a concentration-dependent inhibition of [(3)H]cimetidine binding over a 500-fold range of potencies (K(i) values were 14.5 to >8000nM). However, antinociceptive potencies in rats did not significantly correlate with [(3)H]cimetidine-binding affinities (r=0.018, p=0.97, n=10). These results suggest that the [(3)H]cimetidine-binding site is not the analgesic target for improgan-like drugs.

High-affinity binding of [3H]cimetidine to a heme-containing protein in rat brain

[(3)H]Cimetidine (3HCIM) specifically binds to an unidentified site in the rat brain. Because recently described ligands for this site have pharmacological activity, 3HCIM binding was characterized. 3HCIM binding was saturable, heat-labile, and distinct from the histamine H(2) receptor. To test the hypothesis that 3HCIM binds to a cytochrome P450 (P450), the effects of nonselective and isoform-selective P450 inhibitors were studied. The heme inhibitor KCN and the nonselective P450 inhibitor metyrapone both produced complete, concentration-dependent inhibition of 3HCIM binding (K(i) = 1.3 mM and 11.9 muM, respectively). Binding was largely unaffected by inhibitors of CYP1A2, 2B6, 2C8, 2C9, 2D6, 2E1, and 19A1 but was eliminated by inhibitors of CYP2C19 (tranylcypromine) and CYP3A4 (ketoconazole). Synthesis and testing of CC11 [4(5)-(benzylthiomethyl)-1H-imidazole] and CC12 [4(5)-((4-iodobenzyl)-thiomethyl)-1H-imidazole] confirmed both drugs to be high-affinity inhibitors of 3HCIM binding. On recombinant human P450s, CC12 was a potent inhibitor of CYP2B6 (IC(50) = 11.7 nM), CYP2C19 (51.4 nM), and CYP19A1 (140.7 nM) and had a range of activities (100-494 nM) on nine other isoforms. Although the 3HCIM binding site pharmacologically resembles some P450s, eight recombinant human P450s and three recombinant rat P450s did not exhibit 3HCIM binding. Inhibition by KCN and metyrapone suggests that 3HCIM binds to a heme-containing brain protein (possibly a P450). However, results with selective P450 inhibitors, recombinant P450 isoforms, and a P450 antibody did not identify a 3HCIM-binding P450 isoform. Finally, CC12 is a new, potent inhibitor of CYP2B6 and CYP2C19 that may be a valuable tool for P450 research.

A comparison of the binding profiles of dextromethorphan, memantine, fluoxetine and amitriptyline: Treatment of involuntary emotional expression disorder

We compared the binding profiles of medications potentially useful in the treatment of involuntary emotional expression disorder at twenty-six binding sites in rat brain tissue membranes. Sites were chosen based on likelihood of being target sites for the mechanism of action of the agents in treating the disorder or their likelihood in producing side effects experienced by patients treated with psychoactive agents. We used radioligand binding assays employing the most selective labeled ligands available for sites of interest. Concentrations of labeled ligand were used at or below the K(i) value of the ligand for the target site. Compounds were initially screened at 1 muM. For compounds that competed for greater than 20-30% of specific binding at target sites of interest, full concentration curves were constructed. Dextromethorphan, amitriptyline and fluoxetine competed for binding to sigma(1) receptors and to serotonin transporters with high to moderate affinity. Of the target sites tested, these are the most likely to contribute to the therapeutic benefit of the various agents. In addition, all three drugs showed some activity at alpha(2) and 5-HT(1B/D) sites. Of the drugs tested, dextromethorphan bound to the fewest sites unlikely to be target sites. Although the mechanism of action of dextromethorphan or any drug that has been used in the treatment of involuntary emotional expression disorder is currently unknown, our data support that the affinity of the drug for sigma(1) receptors is consistent with its possible action through this receptor type in controlling symptoms of the disorder.

CC12, a high-affinity ligand for [3H]cimetidine binding, is an improgan antagonist

Improgan, a chemical congener of cimetidine, is a highly effective non-opioid analgesic when injected into the CNS. Despite extensive characterization, neither the improgan receptor, nor a pharmacological antagonist of improgan has been previously described. Presently, the specific binding of [(3)H]cimetidine (3HCIM) in brain fractions was used to discover 4(5)-((4-iodobenzyl)thiomethyl)-1H-imidazole, which behaved in vivo as the first improgan antagonist. The synthesis and pharmacological properties of this drug (named CC12) are described herein. In rats, CC12 (50-500nmol, i.c.v.) produced dose-dependent inhibition of improgan (200-400nmol) antinociception on the tail flick and hot plate tests. When given alone to rats, CC12 had no effects on nociceptive latencies, or on other observable behavioral or motor functions. Maximal inhibitory effects of CC12 (500nmol) were fully surmounted with a large i.c.v. dose of improgan (800nmol), demonstrating competitive antagonism. In mice, CC12 (200-400nmol, i.c.v.) behaved as a partial agonist, producing incomplete improgan antagonism, but also limited antinociception when given alone. Radioligand binding, receptor autoradiography, and electrophysiology experiments showed that CC12's antagonist properties are not explained by activity at 25 sites relevant to analgesia, including known receptors for cannabinoids, opioids or histamine. The use of CC12 as an improgan antagonist will facilitate the characterization of improgan analgesia. Furthermore, because CC12 was also found presently to inhibit opioid and cannabinoid antinociception, it is suggested that this drug modifies a biochemical mechanism shared by several classes of analgesics. Elucidation of this mechanism will enhance understanding of the biochemistry of pain relief.

Effects of metronidazole and its metabolites on histamine immunosuppression activity

We have previously reported that metronidazole treatment increases human lymphocyte proliferation showing individual differences. This drug and its metabolites are imidazole compounds like histamine and cimetidine. The first is an endogenous amine that inhibits T-helper lymphocyte proliferation, and the second is a histamine antagonist. We presently report the in vitro effects of histamine, cimetidine, imidazole, metronidazole and its two principal metabolites (the acetic acid and hydroxy forms), on the mitogenic response to phytohemagglutinin (PHA) stimulation of human peripheral blood lymphocytes. Histamine decreased lymphocyte proliferation while (in order of potency) cimetidine, the hydroxy metabolite of metronidazole, imidazole and metronidazole, increased the mitogenic response to PHA in a dose-response fashion. The acetic acid metabolite lacked immunomodulatory effects. Competitive studies showed that cimetidine, metronidazole, and the hydroxy metabolite blocked the inhibitory effect of histamine on lymphocyte proliferation in a dose-related manner. This blockage was non-competitive, suggesting that the target of the imidazole compounds was not the active site of the H2 receptor.

Molecular pharmacological aspects of histamine receptors

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

Clinical evaluation of H1-receptor and H2-receptor antagonists for acute postoperative pain

The acute analgesic activity of an H1-histamine antagonist, terfenadine 60 mg, and an H2-histamine antagonist, ranitidine 150 mg, were compared with ibuprofen 600 mg and placebo in a double-blind, placebo-controlled, parallel-group study. Treatments were administered to a total of 127 patients 1 hour before oral surgery. Analgesia was assessed every 30 minutes for 240 minutes after surgery. Analgesic efficacy was compared using the following standard pain intensity scales: visual analog scale, category, graphic rating, and global evaluation. Ibuprofen was significantly better than all other treatments for all measures of analgesic activity. The effects of terfenadine and ranitidine were similar to placebo. These data indicate that pretreatments with a single dose of a histamine receptor antagonist specific for either the H1- or H2-receptor does not produce analgesia in an oral surgery model of acute pain with overall assay sensitivity, suggesting that antihistamines that act primarily at peripheral sites are devoid of analgesic activity. These data contrast with other studies that have demonstrated analgesia using centrally acting antihistamines such as hydroxyzine, phenyltoloxamine, or orphenadrine.

Histamine receptors: subclasses and specific ligands

In this review the three main types of histamine receptors are discussed together with their specific ligands. For the classical H1-receptors much emphasis is put on the mechanism by which the receptor is stimulated. For the H1- and H2-receptor the review includes information on the several models available for establishing agonistic or antagonistic activity. In the section on the H3-receptor the ligands are discussed as well as the possible physiological role of this receptor. In the final paragraphs some less well defined activities are presented.

Histamine excites arcuate neurons in vitro through H1 receptors

The electrophysiological effects of histamine on neurons of the hypothalamic arcuate nucleus of the female rat were tested with extracellular single unit recordings in an in vitro slice preparation. Histamine increased the spontaneous neuronal firing rate in 63% of the arcuate cells tested. An inhibitory response to histamine was seen in only one of the 117 neurons tested. The excitatory response to histamine showed dose dependency and was stable during synaptic blockade (by high magnesium and low calcium concentrations) and across a temperature range of 29-37 degrees C. Administration of histaminergic type 1 (pyrilamine and chlorpheniramine) and type 2 (cimetidine) receptor blockers revealed that the excitatory responses to histamine were mediated by type 1 receptors. The same neurons were also tested for responses to norepinephrine, serotonin, acetylcholine and substance P. A significant correlation was found between responses to histamine and substance P: all units excited by substance P were also excited by histamine. This subclass of histamine-responsive arcuate neurons may play a role in the regulation of the anterior pituitary, since histamine and substance P have similar effects on LH and prolactin secretion.

3H-tiotidine binding to guinea-pig cortical and striatal membranes

3H-Tiotidine has been identified as a suitable radioligand for the H2-receptor. We have confirmed and extended structure-binding affinity studies in the guinea-pig cortex, and established a structure-binding affinity relationship consistent with the H2-receptor in guinea-pig striatum. Cimetidine-displaceable 3H-tiotidine binding was observed also in the nucleus accumbens.

Binding of [3H]ICIA 5165, an H2-receptor antagonist to guinea pig gastric mucosa

ICIA 5165, 2-guanidino-4-[4-(2-cyano-3-methylguanidino)butyl] thiazole, a selective histamine H2-receptor antagonist was radiolabelled with tritium to a specific activity of 50.8 Ci/mmol for use in binding studies. Radiolabelling did not impair bioactivity. Binding characteristics of [3H]ICIA 5165 to guinea pig gastric mucosa were determined. Ligand binding was rapid, reaching equilibrium within five minutes at 0 degrees C, reversible and saturable. Specific [3H]ICIA 5165 binding had an equilibrium dissociation constant of 1.29 X 10(-8) M, determined by Scatchard plot analysis, and of 1.02 X 10(-8) M, calculated from the ratio of the dissociation to association rate constants. A Hill number, nH, of 1.02 was determined for the specific binding component. Specific binding of [3H]ICIA 5165 to gastric mucosal supernatant was not inhibited by methapyrilene, diphenhydramine, mepyramine, d-chlorpheniramine or l-chlorpheniramine (all at 10(-7) M), or by atropine or propranolol (both at 10(-6) M). Specific [3H]ICIA 5165 binding was inhibited in a concentration dependent manner by non-radioactive ICIA 5165 and tiotidine, as well as by a variety of other agents, with H2 agonist or H2 antagonist properties. In competition experiments, however, difficulties encountered in accurately defining the degree of specific binding indicate some reservation should be observed in interpreting these results.

Evidence for a displaceable non-specific [3H]neurotensin binding site in rat brain

Levocabastine is a potent antihistamine drug, structurally unrelated to neurotensin. In rat and mouse brain but not in other animal species, it inhibited 60% of the [3H]neurotensin binding displaced by unlabelled neurotensin or neurotensin(8-13). The levocabastine-sensitive site or "site 1" displayed high affinity properties for levocabastine (IC50 = 25 nM) and was highly sterospecific (IC50-value higher than 10 microM for one of the isomers). Binding to the "site 1" in rat brain corresponded to the [3H]neurotensin binding displaceable by 1 microM levocabastine, whereas binding to the "site 2" corresponded to the binding displaced by 1 microM neurotensin when the "site 1" was occluded by 1 microM levocabastine. Both "site 1" and "site 2" appeared to be saturable. Scatchard plots obtained in rat bulbus olfactorius allowed to calculate a KD-values of 7.1 nM and a Bmax-values of 37.2 fmol/mg original tissue for "site 1", while "site 2" displayed a KD-value of 0.7 nM and a Bmax-value of 16.3 fmol/mg original tissue. The regional distributions of both sites showed marked differences. The "site 1" was homogeneously distributed throughout all rat brain areas, whereas the amount of "site 2" binding was markedly different in separate brain areas: bulbus olfactorius and substantia nigra had the highest amounts (8.9 and 7.8 fmol/mg tissue) while cerebellum had the lowest (0.4 fmol/mg tissue). In spite of its high affinity and stereospecificity, "site 1" has to be considered as an acceptor or recognition site for [3H]neurotensin because of its species-link, low saturability and homogeneous distribution in all rat brain areas.(ABSTRACT TRUNCATED AT 250 WORDS)

Histamine H2-binding on guinea pig cerebral cortex

The Kd-values of some histamine H2-active compounds, obtained from radio-ligand-binding studies on a homogenate of the guinea-pig cerebral cortex with 3H-tiotidine as the labelled H2-ligand, were compared with the pA2/pD2-value of these compounds on the guinea-pig right atrium and guinea-pig isolated gastric fundus. A good correlation was found between the pKd of the H2-antagonists and their pA2 on the guinea-pig right atrium. A much poorer correlation however was obtained between the pKd of the agonists on the cerebral cortex and their pD2-values on the guinea-pig right atrium and the gastric fundus. This poor correlation between true affinity and H2-activity of the agonists might be explained by spare receptors as a much better correlation was obtained between pKd and pD2 of partial agonists.

New horizons in the pharmacologic management of peptic ulceration

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

Effects of centrally administered H2 antagonists in the behavioral despair test

Injection of the histamine-2 (H2) receptor antagonist cimetidine into the lateral ventricles of mice produced a dose-related reduction in swimming in the behavioral despair test. This response can be attenuated by intracerebroventricular (ICV) injection of the histamine-1 (H1) receptor antagonist chlorpheniramine, or the H2 receptor agonist impromidine, given simultaneously with cimetidine. At doses which blocked cimetidine, neither chlorpheniramine nor impromidine alone had effects on swimming. A similar decrease in swimming behavior was also seen after ICV injections of the non-imidazole H2 antagonist, BMY 25,368. This effect of BMY 25,368 was also attenuated by chlorpheniramine and impromidine. These results suggest that H1 and H2 receptors in the brain may mediate opposing behavioral effects.

H2 histaminic receptors in rat cerebral cortex. 2. Inhibition of [3H]histamine by H2 antagonists

Sites labeled by [3H]histamine in homogenates of rat cerebral cortex reveal a pharmacological specificity typical of H2 receptors. Fourteen H2 antagonists inhibit the specific binding of the radioligand to the same level; Hill coefficients are near or equal to one for five compounds and markedly lower for nine. The binding patterns of individual antagonists (A) are well described by the empirical expression Y = F1K1/(K1 + [A]) + F2K2/(K2 + [A]), in which F1 and F2 sum to 1; F2 is 0 for those drugs that reveal a Hill coefficient of 1. Concentrations of A that reduce specific binding by 50% (IC50) correlate well (r = 0.991; P less than 0.00001) and show good numerical agreement with potencies reported for inhibition of the response to histamine in H2-mediated systems. The correlation is poorer when IC50 is replaced by either K1 (r = 0.973) or K2 (r = 0.921) for those antagonists that reveal both; the antihistaminic activity of the drug thus appears not to be associated preferentially with one or other class of sites. Since F2 varies from 0.16 to 0.60 among those antagonists that discern heterogeneity, the antagonist appears to determine the distribution of sites between the two classes. Moreover, a correlation among antagonists between values of K1 and K2 (r = 0.975; P = 0.00001) suggests that the apparent heterogeneity reflects different conformers within an otherwise homogeneous population. H2 antagonists appear to be noncompetitive with respect to each other and to the radioligand: one antagonist has relatively little effect on the values of K1, K2, and F2 revealed by another; also, estimates of K1 and K2 are independent of the concentration of [3H]histamine between 1.3 and 10 nM, although the radioligand exhibits an apparent dissociation constant of 3.9 nM [Steinberg, G. H., Eppel, J. G., Kandel, M., Kandel, S. I., & Wells, J. W. (1985) Biochemistry (preceding paper in this issue)].

H2 histaminic receptors in rat cerebral cortex. 1. Binding of [3H]histamine

Saturable binding of [3H]histamine in equilibrium with homogenates of rat cerebral cortex reveals Hill coefficients between 0.4 and 1.0, depending upon the conditions. Data from individual experiments are well described assuming one or two classes of sites. Only the sites of higher affinity (KP1 = 3.9 +/- 0.5 nM) are observed when binding is measured by isotopic dilution at a low concentration of the radioligand (less than 1.5 nM) in the presence of magnesium or by varying the concentration of the radioligand. The sites of lower affinity (KP2 = 221 +/- 26 nM) appear during isotopic dilution at higher concentrations of the radioligand or at lower concentrations either upon the addition of guanylyl imidodiphosphate (GMP-PNP) or upon the removal of magnesium. Estimates of the second- and first-order rate constants for association and dissociation of [3H]histamine agree well with KP1. Apparent capacities corresponding to KP1 and KP2 are of the order of 100 ([R1]t) and 1300 pmol/g of protein ([R2]t), respectively. Simple interconversion cannot account for the changes in binding that occur upon adding GMP-PNP or removing magnesium, since the increase in [R2]t exceeds the decrease in [R1]t. Moreover, the apparent amount of high-affinity complex exhibits a biphasic dependence on the concentration of [3H]histamine; an increase at low concentrations is offset by a decrease that occurs at higher concentrations. The latter appears to be positively cooperative and concomitant with formation of the low-affinity complex. These and other observations indicate that the binding of histamine is inconsistent with models commonly invoked to rationalize the binding of agonists to neurohumoral receptors. GMP-PNP and magnesium reciprocally alter capacity at the sites of higher affinity, however, and the reduction caused by GMP-PNP reflects a substantial increase in the rate constant for dissociation at the sites that appear to be lost. The sites labeled by [3H]histamine thus reveal the properties of neurohumoral receptors linked to a nucleotide-specific G/F protein.

The binding of [3H]-tiotidine to homogenates of guinea-pig lung parenchyma

By use of a rapid filtration assay, the binding of [3H]-tiotidine to homogenates of guinea-pig lung parenchyma was found to be saturable and of a high affinity. Mean values for the KD and Bmax were calculated as 8.5 +/- 1.5 nM and 28 +/- 5 fmol mg-1 protein respectively. The association and dissociation rate constants for [3H]-tiotidine binding at 4 degrees C were calculated to be 0.81 +/- 0.06 microM min-1 and 0.063 +/- 0.005 min-1 respectively, yielding a kinetically derived KD of 7.8 nM. A wide range of H2-receptor agonist and antagonists displaced [3H]-tiotidine binding from lung parenchyma homogenates in a biphasic manner. Examination of the first phase of the displacement of [3H]-tiotidine yielded Ki values for the antagonists tested similar to those found in other binding studies using this ligand and similar to KB values calculated for the antagonists in pharmacological studies.

Specific 3H-cimetidine binding to receptors in the submandibular gland of the rat

Recent studies indicate that the secretagogue action of histamine on the salivary gland is mediated via H2-receptors. In this study we attempt to characterize the histamine H2-receptors in the submandibular gland of the rat. The results show the presence of a specific 3H-cimetidine binding site in the gland. However, the binding constants of this site are not fully characteristic of the specific 3H-cimetidine-H2-receptor binding. They tend to indicate that the binding of 3H-cimetidine to the gland membrane is similar to the well-characterized 3H-cimetidine-imidazole-recognition-receptor binding found in the membrane of the brain, gastric mucosae and atrium of various mammalian species.

Criteria for receptor sites in binding studies

The study of binding is not an easy task especially because of the difficulty of interpreting the results in terms of binding on specific receptor sites. The problem is not new; what is new is the increasing amount of fanciful interpretation that such a technique has generated. The tendency to interpret anomalous or intriguing results in terms of new receptor subtypes seems to have reached its peak. The need to apply severe criteria becomes imperative before concluding that a binding site may be called a receptor site. Some pitfalls in binding studies will be discussed.

3H-cimetidine and the H2-receptor

The H2-antagonist cimetidine is widely employed in biochemical and pharmacological studies of the H2-receptor. These studies include the use of 3H-cimetidine in radioligand binding experiments. Confirming our previous finding as to the unsuitability of this ligand in these types of investigations, we now report data showing the lack of correlation between the displacement of specific 3H-cimetidine binding and histamine stimulated adenylate cyclase activity, and the displacement of specific binding by imidazoles devoid of H2-receptor activity. Results are also presented which question the use of copper ions in 3H-cimetidine binding studies. Our conclusions are discussed in relation to the work carried out by a number of laboratories where 3H-cimetidine is reported to label the H2-receptor.

Histamine and the hypothalamus

The chemical tools that could be used to examine the function of histamine in the brain are considered together with the evidence linking histamine specifically with the hypothalamus. The distribution of histamine and the enzymes responsible for its synthesis and metabolism is consistent with there being both mast cells and histaminergic nerve terminals within the hypothalamus. Iontophoresis, mepyramine binding and histamine-stimulated adenylate cyclase studies suggest that both histamine H1- and H2- receptors are present in the hypothalamus. In addition, intracerebroventricularly injected histamine receptor agonists and antagonists affect many functions associated with the hypothalamus such as cardiovascular control, food intake, body temperature control, and pituitary hormones whose release is mediated via the hypothalamus, such as corticotropin, growth hormone, thyroid stimulating hormone, prolactin, gonadotropins and vasopressin. However, only in the case of thyroliberin release, prolactin release, body fluid control and blood pressure control is there evidence yet that such effects are mediated via histamine receptors actually in the hypothalamus. The effects of enzyme inhibitors suggest endogenous histamine may be involved in the physiological control of thyroid stimulating hormone, growth hormone and blood pressure, and the effects of receptor antagonists support a role for endogenous histamine in prolactin control. Otherwise, there is little evidence for a physiological role for endogenous, as against exogenous, histamine whether it be from histaminergic terminals or mast cells. In addition, few studies have tried to distinguish possible effects on presynaptic receptors, postsynaptic receptors, hypothalamic blood vessels or the hypophyseal portal blood vessels. It is concluded that although there is good evidence now linking histamine and the hypothalamus more specific studies are required, for instance using microinjection or in vitro techniques and the more specific chemical tools now available, to enable a clearer understanding of the physiological role of histamine in the hypothalamus.

Specific binding of 3H-tiotidine to histamine H2 receptors in guinea pig cerebral cortex

Although the H2 subclass of histamine receptor has been revealed by classical pharmacological approaches, the direct identification of this adenylate cyclase-linked receptor has, despite much effort, remained elusive. Initial studies using 3H-metiamide and 3H-histamine and, subsequently, work from our own laboratory and others using 3H-cimetidine and 3H-ranitidine in various tissues, has shown the unsuitability of these ligands for labelling the H2 receptor. We report here our results using 3H-tiotidine, a more potent H2-antagonist than either cimetidine or ranitidine, and show that this ligand meets the criteria for labeling the H2 receptor in guinea pig cerebral cortex membranes.

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

The histamine H2-receptor antagonist cimetidine potently inhibited [3H]muscimol and enhanced [3H]flunitrazepam binding in membranes prepared from several brain regions in the rat, including the dorsal raphe nucleus. As further examined in cortical membranes, this effect on both GABA and benzodiazepine binding sites was specific for imidazole-derived H2-receptor antagonists (potency: cimetidine greater than metiamide greater than tiotidine) and not observed with either several H1-receptor antagonists or histamine. These data indicate a striking similarity between the actions of cimetidine (and other imidazole-derived H2-receptor antagonists) and GABA on binding parameters at the GABA receptor complex.

The effects of histamine H2 receptor antagonists on androgen action in vivo and dihydrotestosterone binding to the rat prostate androgen receptor in vitro

Several histamine H2 receptor antagonists have been tested for antiandrogenic activity by determining their effects on accessory sex organ weights in castrate testosterone propionate (TP) treated rats and on [3H] dihydrotestosterone (DHT) binding to the androgen receptor of the rat ventral prostate in vitro. When given in high doses cimetidine and metiamide possessed antiandrogenic activity whereas the other H2 receptor antagonists SK&F 92456, SK&F 92994, SK&F 92629 amd SK&F 93479 did not Cimetidine, metiamide and SK&F 92456 inhibited (3H] DHT binding to rat ventral prostate androgen receptor in vitro whereas SK&F 92629 and SK&F 93479 did not. SK&F 92994 affects DHT binding only slightly. Of the compounds that are similar to cimetidine in their potency as H2 receptor antagonists only metiamide was antiandrogenic. However SK&F 92994 and SK&F 93479 are not antiandrogenic despite being more potent than cimetidine as H2 antagonists. It is concluded that the antiandrogenicity of cimetidine and metiamide was not related to their activity as histamine H2 receptor antagonists.

Histamine H2 receptors in rat renal glomeruli

The aim of this study was to demonstrate histamine-H2 receptors in glomeruli isolated from rat renal cortex and to correlate binding to stimulation by histamine of glomerular cyclic AMP concentration. Binding studies were performed at 10-12 degrees C using [3H]cimetidine as a tracer. Specificity of binding relies on the following: inhibition of [3H]cimetidine binding by the unlabelled drug, other H2-antagonists and agonists in contrast with the very weak inhibitory effects of H1 agonists and antagonists; reversibility of steady-state binding after addition of unlabelled drug; half inhibition of the glomerular cyclic AMP response to histamine at concentrations of cimetidine close to the KD value derived from the binding studies (3 microM); calculated KD value in agreement with the therapeutical concentration of cimetidine and the physiological concentration of histamine. [3H]Cimetidine binding concentration of cimetidine and the physiological concentration of histamine. [3H]Cimetidine binding strikingly increased in the presence of copper chloride (20-300 microM) due to an increase both in number of sites and affinity. However this greater binding did not influence either the inhibitory effect of cimetidine on histamine-induced glomerular cyclic AMP concentration or the stimulatory effect of histamine itself. [3H]Cimetidine binding was temperature-dependent since it progressively diminished from 0 to 37 degrees. This was not due to [3H]cimetidine degradation as shown by thin layer chromatography but rather to a change in drug-receptor interaction at higher temperatures. Glumerular concentration of cyclic AMP increased progressively in the presence of histamine (0.1-1000 microM). This stimulatory effect was markedly inhibited by H2 antagonists. These data demonstrate the presence in rat glomeruli of H2 receptors linked to adenylate cyclase.