Antagonism of the stimulatory effects of efaroxan and glibenclamide in rat pancreatic islets by the imidazoline, RX801080

Binding Interactions of Ergotamine and Dihydroergotamine to 5-Hydroxytryptamine Receptor 1B (5-HT1b) Using Molecular Dynamics Simulations and Dynamic Network Analysis

Ergotamine (ERG) and dihydroergotamine (DHE), common migraine drugs, have small structural differences but lead to clinically important distinctions in their pharmacological profiles. For example, DHE is less potent than ERG by about 10-fold at the 5-hydroxytrptamine receptor 1B (5-HT_1B). Although the high-resolution crystal structures of the 5-HT_1B receptor with both ligands have been solved, the high similarity between these two complex structures does not sufficiently explain their activity differences and the activation mechanism of the receptor. Hence, an examination of the dynamic motion of both drugs with the receptor is required. In this study, we ran a total of 6.0 μs molecular dynamics simulations on each system. Our simulation data show the subtle variations between the two systems in terms of the ligand-receptor interactions and receptor secondary structures. More importantly, the ligand and protein root-mean-square fluctuations (RMSFs) for the two systems were distinct, with ERG having a trend of lower RMSF values, indicating it to be bound tighter to 5-HT_1B with less fluctuations. The molecular mechanism-general born surface area (MM-GBSA) binding energies illustrate this further, proving ERG has an overall stronger MM-GBSA binding energy. Analysis of several different microswitches has shown that the 5-HT_1B-ERG complex is in a more active conformation state than 5-HT_1B-DHE, which is further supported by the dynamic network model, with reference to mutagenesis data with the critical nodes and the first three low-energy modes from the normal mode analysis. We also identify Trp327^6.48 and Phe331^6.52 as key residues involved in the active state 5-HT_1B for both ligands. Using the detailed dynamic information from our analysis, we made predictions for possible modifications to DHE and ERG that yielded five derivatives that might have more favorable binding energies and reduced structural fluctuations.

THE CONCISE GUIDE TO PHARMACOLOGY 2019/20: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2019/20 is the fourth in this series of biennial publications. The Concise Guide provides concise overviews of the key properties of nearly 1800 human drug targets with an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. Although the Concise Guide represents approximately 400 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.14748. G protein-coupled receptors are one of the six major pharmacological targets into which the Guide is divided, with the others being: ion channels, nuclear hormone receptors, catalytic receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2019, and supersedes data presented in the 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

Investigation of morin-induced insulin secretion in cultured pancreatic cells

Morin is a flavonoid contained in guava that is known to reduce hyperglycemia in diabetes. Insulin secretion has been demonstrated to increase following the administration of morin. The present study is designed to investigate the potential mechanism(s) of morin-induced insulin secretion in the MIN6 cell line. First, we identified that morin induced a dose-dependent increase in insulin secretion and intracellular calcium content in MIN6 cells. Morin potentiated glucose-stimulated insulin secretion (GSIS). Additionally, we used siRNA for the ablation of imidazoline receptor protein (NISCH) expression in MIN6 cells. Interestingly, the effects of increased insulin secretion by morin and canavanine were markedly reduced in Si-NISCH cells. Moreover, we used KU14R to block imidazoline I₃ receptor (I-3R) that is known to enhance insulin release from the pancreatic β-cells. Without influence on the basal insulin secretion, KU14R dose-dependently inhibited the increased insulin secretion induced by morin or efaroxan in MIN6 cells. Additionally, effects of increased insulin secretion by morin or efaroxan were reduced by diazoxide at the dose sufficient to open K_ATP channels and attenuated by nifedipine at the dose used to inhibit L-type calcium channels. Otherwise, phospholipase C (PLC) is introduced to couple with imidazoline receptor (I-R). The PLC inhibitor dose-dependently inhibited the effects of morin in MIN6 cells. Similar blockade was also observed in protein kinase C (PKC) inhibitor-treated cells. Taken together, we found that morin increases insulin secretion via the activation of I-R in pancreatic cells. Therefore, morin would be useful to develop in the research and treatment of diabetic disorders.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Novel I1-Imidazoline Agonist S43126 Augment Insulin Secretion in Min6 Cells

The I₁-imidazoline receptor is a novel drug target for hypertension and insulin resistance which are major disorders associated with Type II diabetes. In the present study, we examined the effects of a novel imidazoline agonist S43126 on calcium fluxes and insulin secretion from Min6 β-cells. We also examined the effects of S43126 on the induction of IRAS, and phosphorylation of components in the I₁-imidazoline signaling pathways, namely ERK and PKB. Min6 β-cells were treated with varying doses of S43126 [10⁻⁸M to 10⁻⁵M] for various time (5–60mins). S43126 at higher dose [10⁻⁵M] stimulated insulin secretion under elevated glucose concentration compared to basal. In addition, insulin secretion and Ca²⁺ influx mediated by S43126 [10⁻⁵M] were decreased following co-treatment with efaroxan (I₁-antagonist) and nifedipine (L-type voltage-gated Ca²⁺-channel blocker) at various times (5–60mins). Furthermore, S43126 at [10⁻⁵M] increased Ca²⁺ oscillation, [Ca²⁺] and ⁴⁵Ca²⁺ uptake in a time and dose-dependent manner. Moreover, Western blot analysis of treated samples showed that S43126 caused an increased protein expression of IRAS as well as phosphorylation of both ERK1/2 and PKB in a concentration-dependent manner. We conclude that S43126 exerts its insulinotropic effect in a glucose dependent manner by a mechanism involving L-type calcium channels and imidazoline I₁-receptors.

Synthesis and characterization of a iodine-125-labeled pyrrolo[1,2-a]thieno[3,2-e]pyrazine and evaluation as a potential 5-HT4R SPECT tracer

In the aim to find new radiotracers for the in vivo imaging of 5-HT(4) receptors by ultra-high resolution quantitative SPECT, we have developed the synthesis of a radioiodinated 5-HT(4) ligand using an iododestannylation procedure. The [(125)I]-ligand was obtained in a high radiochemical yield. Preliminary autoradiographic and ex vivo studies failed to show a specific labeling of 5-HT(4) receptors.

Evidence that the novel imidazoline compound FT005 is an alpha(2)-adrenoceptor agonist

1: The aim of this study was to determine whether the hyperglycaemic action of the novel imidazoline compound FT005 could be mediated by activation of alpha(2)-adrenoceptors, using a variety of in vivo and in vitro methods including radioligand binding. 2: FT005 produced a dose-dependent increase in blood glucose levels of CBA/Ca mice (0.125-25 mg kg(-1), i.p.). The time course of this hyperglycaemic effect matched that of adrenaline (1 mg kg(-1)) more closely than glucagon (1 mg kg(-1)) or the K(ATP) channel opener diazoxide (25 mg kg(-1)). The hyperglycaemic effect of FT005 (1 mg kg(-1)) was significantly reduced by the alpha(2)-adrenoceptor antagonist rauwolscine (0.5 mg kg(-1)). 3: FT005 produced a significant reduction in plasma insulin levels of mice 30 min after administration. The hyperglycaemic effect of FT005 (25 mg kg(-1)), although still present, was significantly less in fasted mice in which insulin levels are lower, suggesting that a reduction of insulin secretion contributes to the action of FT005. 4: When studied in the mouse isolated vas deferens preparation, FT005 produced a complete inhibition of neurogenic contractions, which was blocked by rauwolscine. This is consistent with activation of pre-synaptic alpha(2)-adrenoceptors. 5: In radioligand binding studies FT005 completely displaced the alpha(2)-adrenoceptor antagonist [(3)H]-RX821002 from mouse whole brain homogenates. The displacement was best described by a two-site model of interaction comprising high and low affinity components. 6: The results indicate that FT005 is an agonist at alpha(2)-adrenoceptors. A reduction in insulin secretion contributes to the hyperglycaemic action of FT005, although an additional mechanism can not be excluded.

The potential antidiabetic activity of some alpha-2 adrenoceptor antagonists

The effect of alpha-2 adrenoceptor antagonists, yohimbine and efaroxan, on the plasma glucose and insulin levels was studied in non-diabetic control, type-I (insulin-dependent) and type-II (non-insulin-dependent) diabetic rats. Pretreatment with either yohimbine or efaroxan potentiated glucose-induced insulin release in non-diabetic control rats and produced an improvement of the oral glucose tolerance and potentiated glucose-induced insulin release in type-II but not in type-I diabetic rats. Treatment with either yohimbine or efaroxan reduced the plasma glucose level and increased the plasma insulin level of non-diabetic control and type-II diabetic rats but not of type-I diabetic rats. Effects of efaroxan were more marked. Pretreatment of non-diabetic control and type-II diabetic rats with either yohimbine or efaroxan inhibited clonidine-induced hyperglycaemia and suppressed or reversed clonidine-induced hypoinsulinaemia. Also, pretreatment of these animals with either yohimbine or efaroxan enhanced the hypoglycaemic and insulinotropic effects of glibenclamide. The combination of glibenclamide and efaroxan led to a synergistic increase in insulin secretion, while that of glibenclamide and yohimbine led to an additive increase. The hyperglycaemic effect of diazoxide in non-diabetic control and type-II diabetic rats was inhibited by pretreatment with either yohimbine or efaroxan. The hypoinsulinaemic effect of diazoxide in these animals was antagonized and reversed by pretreatment with yohimbine and efaroxan, respectively. In type-I diabetic rats, there was no change in the plasma glucose and insulin levels induced by the treatment of animals with each of clonidine or diazoxide alone or in combination with either yohimbine or efaroxan. Glibenclamide produced a slight decrease in the plasma glucose level of type-I diabetic rats, at the end of the 120 min period of investigation but there was no change in the plasma insulin level. Pretreatment of these animals with either yohimbine or efaroxan produced no change in glibenclamide effects. Additionally, bath application of efaroxan or glibenclamide inhibited the relaxant effects of different concentrations of diazoxide on the isolated norepinephrine-contracted aortic strips, while the application of yohimbine produced insignificant changes. The combination of glibenclamide and efaroxan led to complete inhibition of the relaxant effects of different concentrations of diazoxide, while that of glibenclamide and yohimbine did not produce such an effect. It is concluded that yohimbine, via blockade of postsynaptic alpha-2 adrenoceptors, and efaroxan, via blockade of postsynaptic alpha-2 adrenoceptors and adenosine triphosphate-sensitive potassium channels in the pancreatic beta-cell membrane, produce insulinotropic and subsequent hypoglycaemic effects.

Stimulation of insulin secretion in clonal BRIN-BD11 cells by the imidazoline derivatives KU14r and RX801080

The imidazoline derivatives KU14R and RX801080 have each been reported to antagonize imidazoline-stimulated insulin secretion. This study investigated the effects of a range of concentrations of both KU14R and RX801080 on insulin secretion from the clonal pancreatic beta cell line, BRIN-BD11. In the presence of a stimulatory (8.4 m m) glucose concentration, both KU14R (50-200 microm;P< 0.01 to P< 0.001) and RX801080 (50-200 microm;P< 0.01 to P< 0.001) were found to dose-dependently stimulate insulin secretion. The imidazoline efaroxan (200 microm) stimulated insulin secretion (P< 0.001) from BRIN-BD11 cells. This insulinotropic effect was significantly augmented by KU14R (100-200 microm;P< 0.01 to P< 0.001) and RX801080 (200 microm;P< 0.05). Insulin secretion from BRIN-BD11 cells was also stimulated by the novel guanidine derivative BTS 67 582 (200 microm;P< 0.001). This secretagogue action was augmented both by KU14R (25-200 microm;P< 0.001) and by RX801080 (25-200 microm;P< 0.05 to P< 0.001). It is concluded that, rather than acting as antagonists of imidazoline-induced insulin secretion, the imidazoline derivatives KU14R and RX801080 are themselves potent insulinotropic agents.

New arylpiperazine derivatives as antagonists of the human cloned 5-HT(4) receptor isoforms

New derivatives of arylpiperazine 9 were designed from ML 10302, a potent 5-HT(4) receptor agonist in the gastrointestinal system. Compounds were synthesized by condensation of a number of available arylpiperazines or heteroarylpiperazines with 2-bromoethyl 4-amino-5-chloro-2-methoxybenzoate. They were evaluated in binding assays on the recently cloned human 5-HT(4(e)) isoform stably expressed in C6 glial cells with [(3)H]GR 113808 as the radioligand. The affinity values (K(i)) depended upon the substituent on the aromatic ring. A chlorine atom produced a marked drop in activity (K(i) > 100 nM), while a m-methoxy group gave a compound with nanomolar affinity (K(i) = 3 nM). The most potent compounds were the heterocyclic derivatives with pyrimidine, pyrazine, pyridazine, or pyridine moieties (compounds 9r, 9t, 9u, 9x, respectively). K(i) values for 9a and 9r were determined for the 5-HT(4(a)), 5-HT(4(b)), 5-HT(4(c)), and 5-HT(4(d)) receptor isoforms transiently expressed in COS cells. The results indicated that the compounds were not selective. They produced an inhibition of the 5-HT-stimulated cyclic AMP synthesis in the C6 glial cells stably expressing the 5-HT(4(e)) receptor and shifted the 5-HT concentration-effect curve on adenylyl cyclase activity with pK(D) values of 7.44 and 8.47, respectively. In isolated human atrial myocytes, 9r antagonized the stimulatory effect of 5-HT on the L-type calcium current (I(Ca)) with a K(D) value of 0.7 nM.

2,3-Dihydro-2-oxo-1H-benzimidazole-1-carboxamides with selective affinity for the 5-HT(4) receptor: synthesis and structure-affinity and structure-activity relationships of a new series of partial agonist and antagonist derivatives

A series of 2,3-dihydro-2-oxo-1H-benzimidazole-1-carboxamide derivatives bearing a piperazine moiety was synthesized. Their in vitro 5-HT(4), 5-HT(3), and D(2) receptors affinities were evaluated by radioligand binding assay. For selected compounds functional studies at the 5-HT(4) receptor were made by using precontracted (by carbachol) preparations of rat esophageal tunica muscularis mucosae (TMM). The influence of the 3-substituent of the benzimidazole ring, the 4-substituent of the piperazine moiety, and the alkylene spacer was studied. Compounds with an ethyl or a cyclopropyl substituent in the 3-position of the benzimidazole ring showed moderate to high affinity (K(i) = 6.7-75.4 nM) for the 5-HT(4) receptor with selectivity over 5-HT(3) and D(2) receptors and moderate antagonist activity (pK(b) = 6.19-7.73). Compounds with an isopropyl substituent in the 3-benzimidazole position exhibited moderate and selective 5-HT(4) affinity (K(i) >/= 38.9 nM) and a partial agonist activity (5a, i.a. = 0.94) higher than that of the reference compound BIMU 8 (i.a. = 0.70). This reversal of the pharmacological activity due only to a small structural difference might confirm the existence of two binding sites on the 5-HT(4) receptor. In the alkylene spacer, a two-methylene chain is favorable to optimize the affinity and the antagonist or the partial agonist activity. In the ethyl and cyclopropyl series, 5-HT(4) antagonist activity seems to be unrelated to the size of the 4-substituent of the piperazine moiety, whereas a methyl group is optimal for high partial agonist activity in the isopropyl series; however, the presence of a butyl substituent is a favorable pattern for 5-HT(4) antagonism and even causes a reversal of the pharmacological profile in the isopropyl series (5h, pK(b) = 7.94). N-Butyl quaternization of 5a led to an improvement in affinity for the 5-HT(4) receptor and mantained the high partial agonist activity (5r, K(i) = 66.3 nM, i.a. = 0.93).

Effect of the new imidazoline derivative S-22068 (PMS 847) on insulin secretion in vitro and glucose turnover in vivo in rats

We have investigated the possible mechanisms underlying the antihyperglycaemic effect of the imidazoline derivative S-22068. In vitro, in the presence of 5 mmol/l glucose, S-22068 (100 micromol/l) induced a significant and sustained increase in insulin secretion from isolated, perifused, rat islets and a marked sensitization to a subsequent glucose challenge (10 mmol/l). S-22068 (100 micromol/l was able to antagonize the stimulatory effect of diazoxide on 86Rb efflux from preloaded islets incubated in the presence of 20 mmol/l glucose. Experiments were also performed to investigate whether S-22068 can alter glucose turnover and peripheral insulin sensitivity in vivo in mildly diabetic rats and obese, insulin resistant, Zucker rats. Neither glucose production nor individual tissue glucose utilization was modified by S-22068 in either group of rats. Similar results were obtained whether the studies were performed under basal conditions or during euglycaemic/hyperinsulinemic clamps. The results suggest that S-22068 exerts part of its antihyperglycaemic effect by promoting insulin secretion without alteration of peripheral insulin sensitivity.

Imidazolines and pancreatic hormone secretion

A range of imidazoline derivatives are known to be effective stimulators of insulin secretion, and this response correlates with closure of ATP-sensitive potassium channels in the pancreatic beta-cell. However, mounting evidence indicates that potassium channel blockade may form only part of the mechanism by which imidazolines exert their effects on insulin secretion. Additionally, it remains unclear whether members of this class of drugs can bind directly to potassium channel components and whether occupation of a single binding site accounts for their functional activity. This review considers recent developments in the field and highlights evidence that does not fit readily with the concept that a single mechanism of action is sufficient to mediate the effects of imidazolines on pancreatic hormone secretion.

Imidazolines and the pancreatic B-cell. Actions and binding sites

Stimulation of insulin secretion by imidazoline compounds displays variable characteristics. Phentolamine (10-100 microM) increased secretion of perifused mouse islets at nonstimulatory glucose concentrations (5 mM) and even in the absence of glucose. Idazoxan (20-100 microM) elicited a moderate increase in insulin secretion, which required the presence of a stimulatory glucose concentration (10 mM). Phentolamine is therefore a stimulator of secretion in its own right, whereas idazoxan may be termed an enhancer of secretion. Both compounds inhibited the activity of ATP-dependent K+ channels in inside-out patches from B-cells; however, idazoxan achieved only an incomplete block. Both compounds depolarized the B-cell plasma membrane to an extent that permitted the opening of voltage-dependent Ca2+ channels (-40 to -30 mV). An increase in cytoplasmic Ca2+ concentration was induced by phentolamine and much less so by idazoxan. Activation of protein kinase C, a possible mechanism to amplify Ca(2+)-induced secretion, could not be verified for phentolamine. It thus appears that stimulation of insulin secretion by phentolamine is due to its blocking effect on KATP channels, which may be the correlate of non-adrenergic imidazoline binding sites which were characterized in insulin-secreting HIT cells. Whether incomplete closure of KATP channels by idazoxan or additional effects are responsible for the requirement of high glucose to stimulate secretion remains to be clarified.

Different modes of action of the imidazoline compound RX871024 in pancreatic beta-cells. Blocking of K+ channels, mobilization of Ca2+ from endoplasmic reticulum, and interaction with exocytotic machinery

The imidazoline compound RX871024 glucose-dependently potentiates the release of insulin in pancreatic islets and beta-cell lines. This activity of the compound is not related to its action by stimulating alpha 2-adrenoceptors and I1- and I2-imidazoline receptors. There are at least three modes of action of RX871024 in beta-cells: (1) RX871024 blocks the ATP-dependent, Ca(2+)-activated, and delayed rectifier K+ channel activity; (2) RX871024 causes mobilization of Ca2+ from thapsigargin-sensitive intracellular stores, the effect probably controlled by cytochrome P450; and (3) the stimulatory activity of RX871024 on insulin release involves interaction of the compound with the exocytotic machinery, unrelated to the changes in membrane potential and cytoplasmic-free Ca2+ concentration, whereas protein phosphorylation plays an important role in this process. The maximal insulinotropic effect of RX871024 is much higher than that of the sulfonylurea glibenclamide. RX871024 stimulates insulin release and normalizes blood glucose levels in rats in vivo without affecting blood pressure and heart rate.

Affinity isolation of imidazoline binding proteins from rat brain using 5-amino-efaroxan as a ligand

We have employed an amino derivative of the imidazoline ligand, efaroxan, to isolate imidazoline binding proteins from solubilised extracts of rat brain, by affinity chromatography. A number of proteins were specifically retained on the affinity column and one of these was immunoreactive with an antiserum raised against the ion conducting pore component of the ATP-sensitive potassium channel. Patch clamp experiments confirmed that, like its parent compound, amino-efaroxan blocks ATP-sensitive potassium channels in human pancreatic beta-cells and can stimulate the insulin secretion from these cells. The results reveal that a member of the ion conducting pore component family is strongly associated with imidazoline binding proteins in brain and in the endocrine pancreas.

Imidazoline binding sites and receptors in cardiovascular tissue

1. Imidazoline binding sites and receptors and their endogenous ligands have been identified in cardiovascular tissue of various species including human beings. 2. I2- (but only exceptionally I1-)imidazoline binding sites have been shown to exist on cardiac myocytes and vascular smooth muscle cells; at present, their functional role is unknown. 3. The sympathetic nerves supplying the cardiovascular system are endowed with presynaptic inhibitory imidazoline receptors that may become of therapeutic relevance as targets of drugs. 4. ATP-sensitive K+ channels present in heart and blood vessels can be blocked by several imidazolines and guanidines; hence, those drugs can interfere with the cardioprotective effects resulting from K(ATP) channel activation by a decrease in the endogenous ligand ATP or by drugs. 5. Imidazoline derivatives exhibit antiarrhythmic properties that are due to a reduction of sympathetic tone by central and peripheral mechanisms and to blockade of postsynaptic alpha2-adrenoceptors in the heart and coronary arteries. 6. Agmatine and clonidine-displacing substance, which are endogenous ligands at imidazoline and alpha2-receptors, are present in the blood serum and appear to participate in vascular smooth muscle proliferation and blood pressure regulation.

Cellular electrophysiological effects of chronic fluoxetine and duloxetine administration on serotonergic responses in the aging hippocampus

The pharmacological and physiological effects of chronic administration of the selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitor (SSRI) fluoxetine and the dual 5-HT/norepinephrine (NE) reuptake inhibitor duloxetine were compared on 5-HT-mediated electrophysiological responses recorded in the hippocampus of young (3-5 months) and old (17-20 months) female Fischer 344 rats. Fluoxetine, duloxetine, or vehicle (saline) was administered once daily for 14 days (10 mg/kg, i.p.) and extracellular recordings of spontaneously firing CA1 and CA3 pyramidal neurons were conducted 24 h following the last injection using microiontophoretic drug application techniques in a chloral hydrate anesthetized preparation. The recovery times (RT50 values; sec) following 5-HT application on pyramidal neurons were significantly increased in the young and old chronic fluoxetine (FLX) treated groups (73% and 104%, respectively; P < 0.05), but not chronic duloxetine- (DLX) or vehicle- (VEH) treated groups. Following prolonged application of duloxetine (5-10 min), the 5-HT RT50 values were significantly increased in the young FLX groups as compared to the age-matched DLX- and VEH-treated groups. In contrast, a significant decline in the time to recovery produced by 5-HT (52%) was observed in the old vs. young FLX-treated group following the second co-application of 5-HT with duloxetine. Within each drug treatment and age group, co-application of duloxetine and 5-HT did not alter the inhibitory responses (IT50 values; nC) produced by the application of 5-HT alone. These results demonstrate cellular adaptive changes in serotonergic neuronal function occur following repeated exposure to 5-HT reuptake inhibitors in an age-dependent manner.

Clonidine-displacing substance and its putative role in control of insulin secretion: a minireview

1. Imidazoline-binding sites, or I-sites, are a class of recently defined nonadrenoceptor recognition sites whose most potent ligands are imidazolines and related compounds. 2. The pancreatic islet beta-cell I-site, which mediates imidazoline-induced stimulation of insulin release, appears to be the first site to be pharmacologically defined with selective agonists and antagonists. 3. The natural ligand for imidazoline recognition sites is still unknown. The strongest candidate is clonidine-displacing substance (CDS), originally identified in extracts of rat and bovine brain. However, the bioactive molecule has not been identified definitively. Agmatine, a decarboxylated derivative of arginine, also binds to both I-sites and alpha2-adrenoceptors (Li et al., 1994), and is, by definition, a CDS molecule. 4. In the endocrine pancreas, agmatine is a weak insulin secretagogue, which induces a slowly developing secretory response. However, this profile does not correlate with interaction at the islet I-site, and thus agmatine is unlikely to be an endogenous secretagogue acting functionally at the islet I-site. 5. Crude preparations of CDS from rat brain can potentiate glucose-induced insulin release and reverse the effects of diazoxide in rat and human islets of Langerhans. These two effects are also subject to blockade by the imidazoline antagonists RX801080 and KU14R. Furthermore, islets that were desensitized to the effects of the imidazoline secretagogue efaroxan (after 18-hr culture with imidazoline) were refractory to the actions of CDS. 6. Overall, CDS displays many characteristics expected of an endogenous regulator of insulin secretion acting through the islet beta-cell imidazoline site. This evidence strengthens the hypothesis that the islet beta-cell imidazoline site mediating control of insulin release in the endocrine pancreas is a biologically relevant receptor. Furthermore, a physiological role of CDS in the endocrine pancreas cannot be excluded.

Characterisation of new efaroxan derivatives for use in purification of imidazoline-binding sites

The insulin secretagogue activity of certain imidazoline compounds is mediated by a binding site associated with ATP-sensitive K+ (K(ATP)) channels in the pancreatic beta-cell. We describe the effects of a series of structural modifications to efaroxan on its activity at this site. Substitution of amino-, nitro- or azide- groups onto the 5-position of the benzene ring of efaroxan did not significantly affect the functional interaction of the ligand with the islet imidazoline binding site. Modification of the imidazoline ring to an imidazole to generate 2-(2-ethyl-2,3-dihydrobenzo[b]furan-2-yl)-1H-imidazole (KU14R) resulted in loss of secretagogue activity. Indeed, this reagent appeared to act as an imidazoline antagonist since it blocked the secretory responses to imidazoline compounds and also inhibited the blockade of beta-cell K(ATP) channels by efaroxan in patch clamp experiments. Application of KU14R alone resulted in a modest reduction in K(ATP) channel opening, suggesting that it may display weak partial agonism, at least in patch-clamp experiments.

Potent antihyperglycaemic property of a new imidazoline derivative S-22068 (PMS 847) in a rat model of NIDDM

Recent data suggest that some imidazoline derivatives can lower plasma glucose in experimental animal models of diabetes. We studied the activity of an imidazoline S-22068, in rat model of non-insulin-dependent diabetes mellitus (NIDDM) produced with a low dose of streptozotocin (35 mg kg(-1), i.v.) in the adult. The respective increase over basal value in glucose (deltaG) and insulin (deltaI), and the rate of glucose disappearance (K), were measured during a 30 min intravenous glucose tolerance test. After an intraperitoneal injection of S-22068 (24 mg kg(-1)), deltaG (mM min(-1)) was decreased (91.67+/-5.83 vs 120.5+/-3.65; P<0.001), whereas K was increased (1.74+/-0.09 vs 1.18+/-0.05; P<0.001). Although insulinaemia was increased at time-point 0 of the test, deltaI was unchanged. During oral glucose tolerance tests (OGTT), S-22068 (24 mg kg(-1), p.o.) improved glucose tolerance, and its efficiency was potentiated after chronic treatment (15 days). Basal glycaemia was unaffected by the treatment. Under the same conditions, a higher dose of S-22068 (40 mg kg(-1)) further improved glucose tolerance without causing hypoglycaemia. Binding experiments revealed that S-22068 displays no affinity for either adrenoceptors or the two imidazoline receptors I1 or I2. These results demonstrate that S-22068 improves glucose tolerance without causing hypoglycaemia. Thus S-22068 represents a new potential option in the treatment of NIDDM.

Sigma receptor ligands and imidazoline secretagogues mediate their insulin secretory effects by activating distinct receptor systems in isolated islets

The effects of two potent sigma receptor agonists (+)-3-PPP ((R)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine) and DTG (N,N'-di-(o-tolyl)guanidine) on the insulin secretory responses in rat islets of Langerhans were investigated. Both sigma receptor ligands were able to potentiate the insulin secretory response of islets incubated at 6 mM glucose, in a dose-dependent manner and were also able to reverse the effects of diazoxide on insulin release. When islets were treated with efaroxan, a well-characterised imidazoline insulin secretagogue, and either (+)-3-PPP or DTG together, there was an unexpected and profound absence of stimulation of insulin release as compared to when islets were incubated with each compound alone. Experiments performed with islets where there was desensitization of DTG/sigma receptor or efaroxan/imidazoline binding site mediated responses suggest that at least two distinct receptor systems appear to be involved. The complex interactions of these two classes of drug require further investigation.

The selective 5-HT1B receptor inverse agonist 1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2, 4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6,7-tetrahydro- spiro[furo[2,3-f]indole-3,4'-piperidine] (SB-224289) potently blocks terminal 5-HT autoreceptor function both in vitro and in vivo

5-HT1 receptors are members of the G-protein-coupled receptor superfamily and are negatively linked to adenylyl cyclase activity. The human 5-HT1B and 5-HT1D receptors (previously known as 5-HT1Dbeta and 5-HT1Dalpha, respectively), although encoded by two distinct genes, are structurally very similar. Pharmacologically, these two receptors have been differentiated using nonselective chemical tools such as ketanserin and ritanserin, but the absence of truly selective agents has meant that the precise function of the 5-HT1B and 5-HT1D receptors has not been defined. In this paper we describe how, using computational chemistry models as a guide, the nonselective 5-HT1B/5-HT1D receptor antagonist 4 was structurally modified to produce the selective 5-HT1B receptor inverse agonist 5, 1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2, 4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6, 7-tetrahydrospiro[furo[2,3-f]indole-3,4'-piperidine] (SB-224289). This compound is a potent antagonist of terminal 5-HT autoreceptor function both in vitro and in vivo.

Signaling and sites of interaction for RX-871024 and sulfonylurea in the stimulation of insulin release

The objective of this study was to compare effects of RX-871024, a compound with imidazoline structure, and the sulfonylurea glibenclamide, representatives of two groups of ATP-dependent potassium channel (KATP) blockers, on insulin secretion and cytoplasmic free calcium concentration ([Ca2+]i). Furthermore, we studied the interaction of the compounds on these two parameters. The experiments were performed in the perfused rat pancreas, isolated rat pancreatic islets, and dispersed beta-cells. At maximal effective concentrations of the compounds, RX-871024 had a more pronounced insulinotropic effect than glibenclamide, but the increase in [Ca2+]i was similar. Glibenclamide enhanced the insulinotropic effect of suboptimal concentrations of RX-871024 at 3.3 and 16.7 mM glucose. Notably, glibenclamide and RX-871024 also stimulated insulin secretion under Ca(2+)-clamped conditions, i.e., during plasma membrane depolarization with KCl and glucose or in permeabilized islets. The magnitudes of insulin stimulation under the latter types of conditions were similar for both compounds. It is concluded that RX-871024 and the sulfonylurea glibenclamide promote insulin secretion by two mechanisms, namely closure of KATP channels and a direct stimulation of exocytosis. At a similar increase in [Ca2+]i, the maximal stimulatory effect of RX-871024 on insulin secretion was stronger than that of glibenclamide, implying that RX-871024 also affects insulin secretion by a signal transduction pathway that is not activated by glibenclamide.

Phentolamine block of KATP channels is mediated by Kir6.2

The ATP-sensitive K+-channel (KATP channel) plays a key role in insulin secretion from pancreatic beta cells. It is closed both by glucose metabolism and the sulfonylurea drugs that are used in the treatment of noninsulin-dependent diabetes mellitus, thereby initiating a membrane depolarization that activates voltage-dependent Ca2+ entry and insulin release. The beta cell KATP channel is a complex of two proteins: Kir6.2 and SUR1. The former is an ATP-sensitive K+-selective pore, whereas SUR1 is a channel regulator that endows Kir6.2 with sensitivity to sulfonylureas. A number of drugs containing an imidazoline moiety, such as phentolamine, also act as potent stimulators of insulin secretion, but their mechanism of action is unknown. We have used a truncated form of Kir6.2, which expresses independently of SUR1, to show that phentolamine does not inhibit KATP channels by interacting with SUR1. Instead, our results argue that phentolamine may interact directly with Kir6.2 to produce a voltage-independent reduction in channel activity. The single-channel conductance is unaffected. Although the ATP molecule also contains an imidazoline group, the site at which phentolamine blocks is not identical to the ATP-inhibitory site, because phentolamine block of an ATP-insensitive mutant (K185Q) is normal. KATP channels also are found in the heart where they are involved in the response to cardiac ischemia: they also are blocked by phentolamine. Our results suggest that this may be because Kir6.2, which is expressed in the heart, forms the pore of the cardiac KATP channel.

Interactions between imidazoline compounds and sulphonylureas in the regulation of insulin secretion

1. Imidazoline alpha 2-antagonist drugs such as efaroxan have been shown to increase the insulin secretory response to sulphonylureas from rat pancreatic B-cells. We have investigated whether this reflects binding to an islet imidazoline receptor or whether alpha 2-adrenoceptor antagonism is involved. 2. Administration of (+/-)-efaroxan or glibenclamide to Wistar rats was associated with a transient increase in plasma insulin. When both drugs were administered together, the resultant increase in insulin levels was much greater than that obtained with either drug alone. 3. Use of the resolved enantiomers of efaroxan revealed that the ability of the compound to enhance the insulin secretory response to glibenclamide resided only in the alpha 2-selective-(+)-enantiomer; the imidazoline receptor-selective-(-)-enantiomer was ineffective. 4. In vitro, (+)-efaroxan increased the insulin secretory response to glibenclamide in rat freshly isolated and cultured islets of Langerhans, whereas (-)-efaroxan was inactive. By contrast, (+)-efaroxan did not potentiate glucose-induced insulin secretion but (-)-efaroxan induced a marked increase in insulin secretion from islets incubated in the presence of 6 mM glucose. 5. Incubation of rat islets under conditions designed to minimize the extent of alpha 2-adrenoceptor signalling (by receptor blockade with phenoxybenzamine; receptor down-regulation or treatment with pertussis toxin) abolished the capacity of (+)- and (+/-)-efaroxan to enhance the insulin secretory response to glibenclamide. However, these manoeuvres did not alter the ability of (+/-)-efaroxan to potentiate glucose-induced insulin secretion. 6. The results indicate that the enantiomers of efaroxan exert differential effects on insulin secretion which may result from binding to effector sites having opposite stereoselectivity. Binding of (-)-efaroxan (presumably to imidazoline receptors) results in potentiation of glucose-induced insulin secretion, whereas interaction of (+)-efaroxan with a second site leads to selective enhancement of sulphonylurea-induced insulin release.

Arylcarbamate derivatives of 1-piperidineethanol as potent ligands for 5-HT4 receptors

A series of carbamate derivatives (7) of 2-(1-piperidinyl)ethyl 4-amino-5-chloro-2-methoxybenzoates, which have been described as potent agonists and antagonists of 5-HT4 receptors, were synthesized. They were evaluated using radioligand binding assays with [3H]GR 113808, a 5-HT4 receptor selective ligand, in the rat striatum and the electrically stimulated myenteric plexus longitudinal muscle of the guinea pig. In contrast to the previously described ester derivatives, a drop in the affinity for 5-HT4 receptors was observed and the compounds were inactive as agonists in the guinea pig ileum preparation. Unexpectedly, the ortho-substituted carbamates 8b,c (R' = H, RO = MeO or EtO, R" = H) had nanomolar affinity for 5-HT4 receptors (Ki = 8.9 +/- 0.5 and 2.6 +/- 0.4 nM, respectively). As reported previously, the cis- or trans-3,5-dimethyl substitution of piperidine (8n,o) was particularly favorable (Ki = 1.1 +/- 0.6 nM for both isomers). 8c is an antagonist equipotent to the 5-HT4 receptor antagonist SDZ 205-557 (1).

Evidence that the ability of imidazoline compounds to stimulate insulin secretion is not due to interaction with sigma receptors

Recent studies have suggested that a variety of ion channels possess a binding site for ligands such as phencyclidine (PCP), dizocilpine and certain sigma ligands and that some imidazoline compounds can also bind to this site. We have investigated whether interaction with this binding site could account for the ability of imidazolines to stimulate insulin secretion from rat islets. Neither PCP nor dizocilpine shared the insulin secretory activity of the imidazoline efaroxan in rat islets suggesting that they do not have similar actions in the pancreatic B-cell. Further, we were able to define a new antagonist, KU14R (2(2-ethyl 2,3-dihydro-2-benzofuranyl)-2-imidazole), which selectively blocks the insulin secretory response to imidazolines. The results suggest that imidazolines do not stimulate insulin secretion by causing physical blockade of the K(+)-ATP channel in pancreatic B-cells and show that their effects are not reproduced by PCP or sigma receptor ligands.

New esters of 4-amino-5-chloro-2-methoxybenzoic acid as potent agonists and antagonists for 5-HT4 receptors

A number of benzoates derived from 4-amino-5-chloro-2-methoxybenzoic acid and substituted 1-piperidineethanol were synthesized and found to be potent 5-HT4 receptor agonists in the electrically-stimulated myenteric plexus and longitudinal muscle of the guinea pig ileum and the rat esophagus muscle. Monosubstitution of the piperidine ring with Me, OH, NH-Ac, or CONH2 groups gave compounds equipotent to 7a (ML 10302), a 5-HT4 receptor agonist previously reported to have nanomolar affinity. 7a,k were as potent as serotonin (5-HT) but had maximal responses which were only 60-80% of that of 5-HT, suggesting a partial agonist profile for these compounds. Binding assays were performed with [3H]GR 113808 in the rat striatum, and several of these compounds were found to have nanomolar affinity for 5-HT4 receptors (7a, Ki = 1.07 +/- 0.5 nM; 7k, Ki = 1.0 +/- 0.3 nM). The introduction of two methyl groups on the piperidine ring brought about a dramatic change in the pharmacological profile of 2-[(cis- and trans-3,5-dimethylpiperidinyl)ethyl]-4-amino-5-chloro-2- methoxybenzoate, 7g,h. 7g (Ki = 0.26 +/- 0.06 nM) inhibited the relaxant action of 5-HT in the rat esophagus muscle with a pA2 value of 8.6. The advantage of the ester function was demonstrated by comparing the activity of several such compounds at 5-HT4 receptors with those of the corresponding amidic derivatives. This difference was less marked when the basic moiety was sterically constrained as in the quinuclidine and tropane moieties. Structural analyses of 7a,g were performed by determining their X-ray crystal structures and by molecular modeling (SYBYL). A relatively limited number of minimum energy conformers was found for both compounds. They were characterized by the cis folded conformation of the ethyl chain and by the orientation of the lone pair of the nitrogen atom pointing out of the molecule as seen in conformationally-constrained benzamides such as zacopride and renzapride. A hypothetical model for the 5-HT4 receptor with two sites for the binding of agonist and antagonist molecules was proposed.

The effect of the putative endogenous imidazoline receptor ligand, clonidine-displacing substance, on insulin secretion from rat and human islets of Langerhans

1. The effects of a rat brain extract containing clonidine-displacing substance (CDS), a putative endogenous imidazoline receptor ligand, on insulin release from rat and human isolated islets of Langerhans were investigated. 2. CDS was able to potentiate the insulin secretory response of rat islets incubated at 6 mM glucose, in a dose-dependent manner. The magnitude of this effect was similar to that in response to the well-characterized imidazoline secretagogue, efaroxan. 3. CDS, like other imidazoline secretagogues, was also able to reverse the inhibitory action of diazoxide on glucose-induced insulin release, in both rat and human islets. 4. These effects of CDS on secretion were reversed by the imidazoline secretagogue antagonists, RX801080 and the newly defined KU14R, providing the first evidence that imidazoline-mediated actions of CDS can be blocked by specific imidazoline antagonists. 5. The effects of CDS on insulin secretion were unaffected when the method of preparation involved centri-filtration through a 3,000 Da cut-off membrane or when the extract was treated with protease. These results confirm that the active principle is of low molecular weight and is not a peptide. 6. Overall, the data suggest that CDS behaves as a potent endogenous insulin secretagogue acting at the islet imidazoline receptor.

Cloning, expression and pharmacology of the mouse 5-HT(4L) receptor

Since most of our knowledge on pharmacological properties of brain 5-HT4 receptors have been discussed for mouse colliculi neurons, we cloned the corresponding receptor using the RT-PCR approach. As expected, the homology with the already cloned rat 5-HT(4L) receptor was high, revealing only 16 differences at the amino-acid level. One of the differences, proline75 in mouse, alanine75 in the already published rat sequences was not confirmed. Therefore this proline is part of the consensus sequence present in all 5-HT receptor transmembrane domain II (LVMP). Comparing the affinities of 11 agonists and five antagonists for the cloned mouse receptor (5-HT(4L))expressed in LLCPK1 and the corresponding receptor in mouse colliculi shows an excellent correlation. The transfected mouse 5-HT(4L) receptor stimulated cAMP production. When expressed at high density, it exhibited intrinsic activity. In contrast to the previously described distribution, we found that mRNA encoding for both the short (5-HT(4S))and the long form (5-HT(4L)) of 5-HT4 receptors are expressed in all mouse and rat brain areas.

Sulphonylureas do not increase insulin secretion by a mechanism other than a rise in cytoplasmic Ca2+ in pancreatic B-cells

The following sequence of events is thought to underlie the stimulation of insulin release by hypoglycaemic sulphonylureas. Interaction of the drugs with a high-affinity binding site (sulphonylurea receptor) in the B-cell membrane leads to closure of ATP-sensitive K+ channels, depolarization, opening of voltage-dependent Ca2+ channels, Ca2+ influx and rise in cytoplasmic [Ca2+]i. Recent experiments using permeabilized islet cells or measuring changes in B-cell membrane capacitance have suggested that sulphonylureas can increase insulin release by a mechanism independent of a change in [Ca2+]i. This provocative hypothesis was tested here with intact mouse islets. When B-cells were strongly depolarized by 60 mM K+, [Ca2+]i was increased and insulin secretion stimulated. Under these conditions, tolbutamide did not further increase [Ca2+]i or insulin release, whether it was applied before or after high K+, and whether the concentration of glucose was 3 or 15 mM. This contrasts with the ability of forskolin and phorbol 12-myristate 13-acetate (PMA) to increase release in the presence of high K+. Tolbutamide also failed to increase insulin release from islets depolarized with barium (substituted for extracellular Ca2+) or with arginine in the presence of high glucose. Glibenclamide and its non-sulphonylurea moiety meglitinide were also without effect on insulin release from already depolarized B-cells. In the absence of extracellular Ca2+, acetylcholine induced monophasic peaks of [Ca2+]i and insulin secretion which were both unaffected by tolbutamide. Insulin release from permeabilized islet cells was stimulated by raising free Ca2+ (between 0.1 and 23 microM). This effect was not affected by tolbutamide and inconsistently increased by glibenclamide. In conclusion, the present study does not support the proposal that hypoglycaemic sulphonylureas can increase insulin release even when they do not also raise [Ca2+]i in B-cells.

Azabicyclic indole esters as potent 5-HT4 receptor antagonists

The synthesis of a series of azabicyclic indole esters is described and their potency reported as 5-HT4 receptor antagonists. Optimization of the most potent compound (19) by preparing the corresponding oxazino[3,2-a]indole ester afforded 34, which had a pIC50 of 9.5 in the guinea pig distal colon longitudinal muscle myenteric plexus preparation.

Dynamic properties of I2-receptors

Evidence is presented that drug binding to I2-sites in the guinea pig kidney and liver is subjected to dynamic homeotropic allosteric regulation. Drugs have been found that are capable of inducing both positive and negative cooperativity on both kidney and liver I2-sites when studied in competition with [3H]idazoxan. Moreover the kinetics of [3H]idazoxan association with guinea pig kidney I2-sites was complex and occurred with three different rate constants. However, on dissociation of [3H]idazoxan from its binding sites in the kidney a substantial number of the slower sites had been converted to faster sites, thus supporting the idea of a dynamic interconversion of I2 sites. The guinea pig liver and kidney I2-sites appear to be similar in their pharmacological properties. They also bear some resemblance to the I2-sites of the smooth muscle of the guinea pig ileum, but they appear to be more distant to the guinea pig cerebral cortex I2-sites. Nevertheless, data indicate that considerable overall heterogeneity exists between the guinea pig kidney and liver I2-sites and the guinea pig ileum and cerebral cortex I2-sites.

Preparation and in vitro pharmacology of 5-HT4 receptor ligands. Partial agonism and antagonism of metoclopramide analogous benzoic esters

Alicyclic ester analogues of the gastroprokinetic benzamide metoclopramide (1) and its ester congener SDZ 205557 (2), a 5-HT4 receptor antagonist, were prepared by O-alkylation of 4-amino-5-chloro-2-methoxybenzoate with N-(2-chloroethyl) substituted alicyclic amines. The bromo and iodo analogue of compound 13b (2-(1-piperidinyl)ethyl 4-amino-5-chloro-2-methoxybenzoate) were obtained by halogenation of dechloro-13b with N-halogenated succinimides. The series was evaluated in functional in vitro assays with regard to affinity for serotoninergic 5-HT4, 5-HT3 and muscarinic M3 receptors. The affinities for 5-HT3 and M3 receptors were below 6.0 (pKB or pA2). On 5-HT4 receptors in guinea-pig ileal longitudinal muscle and rat oesophagus, the majority of compounds revealed partial 5-HT4 receptor agonism susceptible to blockade by SDZ 205557, a reference 5-HT4 receptor antagonist (pKB = 7.25-7.73 (guinea-pig ileum) and 7.09-7.43 (rat oesophagus)). The relative agonist potency was in the range of 5-303% (5-HT: 100%). Compound 13b and its bromo analogue 17 were the most potent esters of the series. The enantiomers of 13g ((R)- and (S)-2-(2-methyl-1-piperidinyl)ethyl 4-amino-5-chloro-2-methoxybenzoate) interacted stereoselectively with 5-HT4 receptors and displayed enantiomeric potency ratios (R)/(S) of 4.3-8.7. There was an excellent correlation between (a) antagonist affinity on guinea-pig ileum and rat oesophagus, (b) relative agonist potency on guinea-pig ileum and rat oesophagus, and (c) between antagonist affinity and relative agonist potency within each assay (r2 > 0.91). The new compounds may serve as academic tools in evaluating the functional role of 5-HT4 receptors. The selective partial 5-HT4 receptor agonists presented in this paper may be useful to restore physiological motility and secretion in the gut with reduced or absent propensity to elicit tachycardia and desensitization of the intestinal target receptor.

New players in the 5-HT receptor field: genes and knockouts

In recent years, cloning has revealed the existence of no less than 14 mammalian 5-HT receptors, as well as evidence that isoforms of the 5-HT4 and 5-HT7 receptors are generated by alternative splicing. In addition, molecular biology techniques have provided new tools with which to study the function of 5-HT receptors. For example, new technologies are emerging that will allow the generation of either inducible or tissue-specific knockouts (or both). In this review José Lucas and René Hen focus on the characteristics of the most recently cloned receptors and the contribution of molecular biology to the understanding of 5-HT receptor function.