The 5-hydroxytryptamine 5-HT1D receptor subtype is negatively coupled to adenylate cyclase in calf substantia nigra

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

A short history of the 5-HT2C receptor: from the choroid plexus to depression, obesity and addiction treatment

This paper is a personal account on the discovery and characterization of the 5-HT_2C receptor (first known as the 5-HT_1C receptor) over 30 years ago and how it translated into a number of unsuspected features for a G protein-coupled receptor (GPCR) and a diversity of clinical applications. The 5-HT_2C receptor is one of the most intriguing members of the GPCR superfamily. Initially referred to as 5-HT_1CR, the 5-HT_2CR was discovered while studying the pharmacological features and the distribution of [³H]mesulergine-labelled sites, primarily in the brain using radioligand binding and slice autoradiography. Mesulergine (SDZ CU-085), was, at the time, best defined as a ligand with serotonergic and dopaminergic properties. Autoradiographic studies showed remarkably strong [³H]mesulergine-labelling to the rat choroid plexus. [³H]mesulergine-labelled sites had pharmacological properties different from, at the time, known or purported 5-HT receptors. In spite of similarities with 5-HT₂ binding, the new binding site was called 5-HT_1C because of its very high affinity for 5-HT itself. Within the following 10 years, the 5-HT_1CR (later named 5-HT_2C) was extensively characterised pharmacologically, anatomically and functionally: it was one of the first 5-HT receptors to be sequenced and cloned. The 5-HT_2CR is a GPCR, with a very complex gene structure. It constitutes a rarity in the GPCR family: many 5-HT_2CR variants exist, especially in humans, due to RNA editing, in addition to a few 5-HT_2CR splice variants. Intense research led to therapeutically active 5-HT_2C receptor ligands, both antagonists (or inverse agonists) and agonists: keeping in mind that a number of antidepressants and antipsychotics are 5-HT_2CR antagonists/inverse agonists. Agomelatine, a 5-HT_2CR antagonist is registered for the treatment of major depression. The agonist Lorcaserin is registered for the treatment of aspects of obesity and has further potential in addiction, especially nicotine/ smoking. There is good evidence that the 5-HT_2CR is involved in spinal cord injury-induced spasms of the lower limbs, which can be treated with 5-HT_2CR antagonists/inverse agonists such as cyproheptadine or SB206553. The 5-HT_2CR may play a role in schizophrenia and epilepsy. Vabicaserin, a 5-HT_2CR agonist has been in development for the treatment of schizophrenia and obesity, but was stopped. As is common, there is potential for further indications for 5-HT_2CR ligands, as suggested by a number of preclinical and/or genome-wide association studies (GWAS) on depression, suicide, sexual dysfunction, addictions and obesity. The 5-HT_2CR is clearly affected by a number of established antidepressants/antipsychotics and may be one of the culprits in antipsychotic-induced weight gain.

Structure and function of serotonin G protein-coupled receptors

Serotonin receptors are prevalent throughout the nervous system and the periphery, and remain one of the most lucrative and promising drug discovery targets for disorders ranging from migraine headaches to neuropsychiatric disorders such as schizophrenia and depression. There are 14 distinct serotonin receptors, of which 13 are G protein-coupled receptors (GPCRs), which are targets for approximately 40% of the approved medicines. Recent crystallographic and biochemical evidence has provided a converging understanding of the basic structure and functional mechanics of GPCR activation. Currently, two GPCR crystal structures exist for the serotonin family, the 5-HT1B and 5-HT2B receptor, with the antimigraine and valvulopathic drug ergotamine bound. The first serotonin crystal structures not only provide the first evidence of serotonin receptor topography but also provide mechanistic explanations into functional selectivity or biased agonism. This review will detail the findings of these crystal structures from a molecular and mutagenesis perspective for driving rational drug design for novel therapeutics incorporating biased signaling.

5-HT-stimulated [35S]guanosine-5'-O-(3-thio)triphosphate binding as an assay for functional activation of G proteins coupled with 5-HT1B receptors in rat striatal membranes

Receptor-mediated guanine nucleotide-binding regulatory protein (G protein) activation or functional coupling between receptors and G proteins has been investigated by means of agonist-induced [35S]guanosine-5'-O-(3-thio)triphosphate ([35S]GTPgammaS) binding, especially for the receptor subtypes negatively coupled to adenylyl cyclase through Gi type G proteins. In the present study, 5-HT-stimulated [35S]GTPgammaS binding to rat stritatal membranes was pharmacologically characterized in detail with the help of an extensive series of 5-HT receptor ligands. The optimum experimental conditions for the concentrations of GDP, MgCl2 and NaCl in the assay buffer were initially determined, and the standard assay was performed with 20 microM GDP, 5 mM MgCl2 and 100 mM NaCl. The specific [35S]GTPgammaS binding was stimulated by several compounds that had been shown to be agonists at 5-HT(1B/1D) receptors. The negative logarithmic values of the concentration eliciting half-maximal effect (pEC50) for these agonists were significantly correlated with their pKi's reported in the previous study of 5-HT1B receptor binding in rat frontal cortical membranes. The increase in specific [35S]GTPgammaS binding in response to 1 microM 5-HT was potently inhibited by several 5-HT(1B/1D) receptor antagonists as well as beta-adrenoceptor antagonists such as S(-)-cyanopindolol. On the other hand, 3-[4-(4-chlorophenyl)piperazin-1-yl]-1,1-diphenyl-2-propanol HCl (BRL15572), a selective antagonist against human 5-HT1D receptors, was inactive as an antagonist at least up to 1 microM. Additionally, the concentration-response curve for 2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-oxadiazol-5-yl]-1H-indol-3-yl]ethanamine (L694247) was shifted rightward in parallel by the addition of S(-)-cyanopindolol at concentrations of 10 and 100 nM, indicative of the competitive inhibitory manner. The specific [35S]GTPgammaS binding was reduced by 1'-methyl-5-([2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl)-2,3,6,7-tetrahydrospirospiro(furo[2,3-f]indole-3,4'-piperidine) (SB224289) and methiothepin in a concentration-dependent manner. The inhibitory curve by either compound was shifted to the right by 10 and 100 nM S(-)-cyanopindolol, suggesting that these two drugs behaved as inverse agonists at 5-HT1B receptors in the present functional assay system. 5-HT-stimulated [35S]GTPgammaS binding to rat striatal membranes serves as a simple but useful method of investigating the functional interaction between the native 5-HT1B receptors and their coupled G proteins in this brain region.

Polymorphic cytochrome P450 2D6: humanized mouse model and endogenous substrates

Cytochrome P450 2D6 (CYP2D6) is the first well-characterized polymorphic phase I drug-metabolizing enzyme, and more than 80 allelic variants have been identified for the CYP2D6 gene, located on human chromosome 22q13.1. Human debrisoquine and sparteine metabolism is subdivided into two principal phenotypes--extensive metabolizer and poor metabolizer--that arise from variant CYP2D6 genotypes. It has been estimated that CYP2D6 is involved in the metabolism and disposition of more than 20% of prescribed drugs, and most of them act in the central nervous system or on the heart. These drug substrates are characterized as organic bases containing one nitrogen atom with a distance about 5, 7, or 10 A from the oxidation site. Aspartic acid 301 and glutamic acid 216 were determined as the key acidic residues for substrate-enzyme binding through electrostatic interactions. CYP2D6 transgenic mice, generated using a lambda phage clone containing the complete wild-type CYP2D6 gene, exhibits enhanced metabolism and disposition of debrisoquine. This transgenic mouse line and its wild-type control are models for human extensive metabolizers and poor metabolizers, respectively, and would have broad application in the study of CYP2D6 polymorphism in drug discovery and development, and in clinical practice toward individualized drug therapy. Endogenous 5-methoxyindole- thylamines derived from 5-hydroxytryptamine were identified as high-affinity substrates of CYP2D6 that catalyzes their O-demethylations with high enzymatic capacity and specificity. Thus, polymorphic CYP2D6 may play an important role in the interconversions of these psychoactive tryptamines, including a crucial step in a serotonin-melatonin cycle.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Serotonergic modulation of hyperpolarization-activated current in acutely isolated rat dorsal root ganglion neurons

1. The effect of serotonin (5-HT) on the hyperpolarization-activated cation current (IH) was studied in small-, medium- and large-diameter acutely isolated rat dorsal root ganglion (DRG) cells, including cells categorized as type 1, 2, 3 and 4 based on membrane properties. 5-HT increased IH in 91 % of medium-diameter DRG cells (including type 4) and in 67 % of large-diameter DRG cells, but not other DRG cell types. 2. The increase of IH by 5-HT was antagonized by spiperone but not cyanopindolol, and was mimicked by 5-carboxyamidotryptamine, but not (+)-8-hydroxydipropylaminotetralin (8-OH-DPAT) or cyanopindolol. These data suggested the involvement of 5-HT7 receptors, which were shown to be expressed by medium-diameter DRG cells using RT-PCR analysis. 3. 5-HT shifted the conductance-voltage relationship of IH by +6 mV without changing peak conductance. The effects of 5-HT on IH were mimicked and occluded by forskolin, but not by inactive 1,9-dideoxy forskolin. 4. At holding potentials negative to -50 mV, 5-HT increased steady-state inward current and instantaneous membrane conductance (fast current). The 5-HT-induced inward current and fast current were blocked by Cs+ but not Ba2+ and reversed at -23 mV, consistent with the properties of tonically activated IH. 5. In medium-diameter neurons recorded from in the current clamp mode, 5-HT depolarized the resting membrane potential, decreased input resistance and facilitated action potential generation by anode-break excitation. 6. The above data suggest that in distinct subpopulations of DRG neurons, 5-HT increases cAMP levels via activation of 5-HT7 receptors, which shifts the voltage dependence of IH to more depolarized potentials and increases neuronal excitability.

Serotonin-induced secretion of von Willebrand factor from human umbilical vein endothelial cells via the cyclic AMP-signaling systems independent of increased cytoplasmic calcium concentration

Endothelial cells are able to synthesize von Willebrand factor (vWf) protein, which is then either secreted in a constitutive way or stored within specific cellular secretory granules, the Weibel-Palade bodies. Stimulated secretion of vWf from these organelles is thought to be induced by agonists causing a transient increase in cytoplasmic calcium concentrations. Serotonin (5-hydroxytryptamine, 5-HT), a local transmitter substance released by activated platelets, has also recently been shown to induce the secretion of vWf. In experiments with human umbilical vein endothelial cells (HUVEC), we found that the 5-HT-induced secretion occurred without a significant increase in cellular calcium levels. The 5-HT 1(D) subtype-specific receptor agonist sumatriptan also induced the release of vWf without causing a calcium signal in HUVEC. Stimulation of endothelial cells with the adenylate cyclase inhibitor, MDL-12 A330, led to the secretion of vWf as well. Simultaneous addition of submaximal concentrations of histamine and 5-HT to HUVEC potentiated the effects of either agonist. Together, these results suggest that in HUVEC 5-HT-induced secretion of vWf is mediated by a decrease in cyclic AMP levels and is independent of changes in cytoplasmic calcium levels.

5-HT1-like receptors: a time to bid goodbye

It is exactly half a century ago that 5-hydroxytryptamine was discovered and over four decades ago two types of 5-HT receptors were described. In this article, Pramod Saxena, Peter De Vries and Carlos Villalón trace the development of the modern classification and nomenclature of 5-HT receptors, which now include more than a dozen subtypes. In doing so, they advocate that the so-called '5-HT1-like' receptors, having been shown to be a heterogeneous population of 5-HT1B, 5-HT1D and 5-HT7 receptors, are now redundant.

Serotonin inhibits trigeminal nucleus activity evoked by craniovascular stimulation through a 5HT1B/1D receptor: a central action in migraine?

The development of serotonin (5HT1B/1D) agonists as treatments for the acute attack of migraine has resulted in considerable interest in their mechanism of action and, to some extent, renewed interest in the role of serotonin (5-hydroxytryptamine; 5HT) in the disorder. The initial synthesis of this class of compounds was predicated on the clinical observation that intravenous 5HT terminated acute attacks of migraine. In this study the superior sagittal sinus was isolated in the alpha-chloralose (60 mg/kg i.p. and 20 mg/kg i.v. injection supplementary 2 hourly) anesthetized cat. The sinus was stimulated electrically (120V, 250 microsec duration, 0.3 Hz), and neurons of the trigeminocervical complex in the dorsal C2 spinal cord were monitored using electrophysiological methods. After baseline recordings in each animal, 5HT (15 microg/kg/min) was infused for 5 minutes in the presence of either vehicle (group A) or the 5HT1B/1D antagonist GR127935 (100 microg/kg i.v. injection; group B). The baseline probability of cell firing after sagittal sinus stimulation was 0.61 +/- 0.1 at a latency to the fastest peak of 11.1 +/- 0.4 msec. In group A, 5HT infusion alone had a small effect of increasing mean blood pressure (12 +/- 3 mm Hg), which in itself did not alter cell firing. In group A, 5HT alone had an inhibitory effect on evoked trigeminal activity, which developed 15 to 20 minutes after commencement of the infusion. The inhibition of cell firing lasted for 20 minutes, after which the activity returned to baseline. In group B, the combination of 5HT and GR127935 had no effect on trigeminal cell firing, although the small hypertensive effect was still present. These data indicate that 5HT inhibits evoked trigeminal nucleus firing via the 5HT1B/1D receptor at which GR127935 is an antagonist. It is likely that some part of the effect of 5HT in migraine relates to inhibition of trigeminal nucleus activity, just as it is likely that some part of the effect of the triptans is also mediated at this central site and may be complementary to their nonneuronal actions. Moreover, the data highlight the case for describing this class of headache as neurovascular headaches rather than vascular headaches, to recognize the implicit contribution of the trigeminovascular system to their pathophysiology.

Characterization of [3H]5-hydroxytryptamine and [3H]spiperone binding sites in clathrin-coated vesicles from bovine brain

Coated vesicles prepared from bovine brain cerebral cortex exhibited [3H]5-hydroxytryptamine (5-HT, serotonin) and [3H]spiperone binding activities. The binding activities were localized in the inner core vesicles. Binding reached an equilibrium level by 30-45 min at 30 degreesC, and was reversed by the addition of 100 microM 5-HT for [3H]5-HT binding or 10 microM ketanserin for [3H]spiperone binding. The saturation binding experiments indicated a single class of binding sites for [3H]5-HT and [3H]spiperone with apparent Kd values of 2.4 and 1.75 nM, respectively. The binding of [3H]5-HT was displaced by 5-HT and 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), but not by ketanserin. The binding of [3H]spiperone was displaced by spiperone and ketanserin but not by 5-HT or 8-OH-DPAT even at 1 mM. The coated vesicles were shown by immunoblotting assay to contain alpha-subunits of GTP-binding proteins, Galphas, Galphai2, Galphai3, Galphao and Galphaq/11. Forskolin-stimulated adenylate cyclase activity in the coated vesicles was inhibited to 80% of the control level by 5-HT or 8-OH-DPAT. These results suggested that 5-HT1A and 5-HT2A receptors are present in bovine brain coated vesicles and that the 5-HT1A receptors are coupled to adenylate cyclase activity via GTP binding proteins.

5-HT 1B/D receptor antagonists

1. 5-Hydroxytryptamine-1B (5-HT 1B, formerly designated 5-HT 1D beta) and 5-hydroxy-tryptamine-1D (5-HT 1D, formerly designated 5-HT 1D alpha) receptors are distinct molecular entities that mediate serotonergic neurotransmission. Both are G-protein-coupled receptors without introns in their coding region, negatively coupled to adenylate cyclase; their precise function in human beings remains to be defined. In brain, they are highly enriched in the globus pallidus and the substantia nigra. 2. Presynaptic 5-HT 1B/D receptors take part in the control of the release not only of 5-HT itself, but also of other neurotransmitters-for example, acetylcholine, glutamate, dopamine, noradrenaline and gamma-aminobutyric acid. Selective blockade of central 5-HT 1B/D autoreceptors should facilitate 5-HT neurotransmission and may offer a novel approach to antidepressant therapy. Other 5-HT 1B/D receptors are located postsynaptically; those receptors may be supersensitive in the pathophysiology of obsessive-compulsive disorder and may be a potential target for its treatment. 3. Few if any ligands show selectivity for 5-HT 1B or 5-HT 1D receptors or both. Most pharmacological studies have been performed with nonselective antagonists-for example, metergoline, I-naphthylpiperazine, methiothepin, ketanserin and ritanserin. Recently, a novel series of benzanilides have been reported as the first examples of selective 5-HT 1B/D receptor antagonists. GR 127935, a representative compound of this series, displays mixed agonist-antagonist properties both in vitro and in vivo. It induces upon systemic administration in the guinea pig either an opposite (decrease) effect or a small increase (65%, 5 mg/kg) in the concentration of cortical extracellular 5-HT compared with fluoxetine (218%, 10 mg/kg). The importance of blockade of 5-HT 1B/D receptors in the raphé region, their possible interaction with 5-HT 1A receptors, and consequent inhibition of 5-HT release in terminal 5-HT 1B/D receptor-containing regions are discussed. 4. To find out whether the available so-called 5-HT 1B/D receptor antagonists are indeed antagonists and not partial agonists, efficacy was measured at recombinant human 5-HT 1B and 5-HT 1D receptor sites by using a [35S]-GTP gamma S binding assay to membrane preparations of stably transfected rat C6-glial cell lines. Metergoline and the selective 5-HT 1B/D receptor ligands GR 127935 as well as GR 125743 showed significant intrinsic activity (43% to 69%) at the 5-HT 1D receptor subtype, whereas the nonselective ligand 1-naphthylpiperazine yielded less (15% to 19%) intrinsic activity at both receptor subtypes. In contrast, the nonselective ligands methiothepin, ketanserin and ritanserin are inverse agonists because they displayed negative efficacy (-14% to -28%). Differential blockade of 5-HT 1B/D receptors by neutral antagonists and inverse agonists is discussed in relation to the 5-HT tone on 5-HT 1B/D receptors. 5. It can concluded that 5-HT 1B/D receptor ligands modulate 5-HT neurotransmission through a terminal 5-HT 1B/D receptor. Future work should be directed toward the identification of selective 5-HT 1B and 5-HT 1D receptor ligands that display either neutral antagonist or inverse agonist properties to evaluate the therapeutic potential of 5-HT 1B/D receptor blockade.

Operational characteristics of the 5-HT1-like receptors mediating external carotid vasoconstriction in vagosympathectomized dogs. Close resemblance to the 5-HT1D receptor subtype

It has recently been shown that the external carotid vasoconstrictor response to 5-HT in the dog is primarily mediated by sumatriptan-sensitive 5-HT1-like receptors; however, the fact that these receptors are not blocked by metergoline, a 5-HT1D ligand, raises questions about their possible correlation with the 5-HT1D receptor subtype. Since a number of drugs display high affinity for the 5-HT1D (GR127935) and 5-HT1F (e.g. methysergide and oxymetazoline) receptor subtypes, in this study we have used these drugs to determine whether the above vasoconstrictor 5-HT1-like receptors correlate with the 5-HT1D and/or 5-HT1F receptor subtypes. One-minute intracarotid infusions of 5-HT (0.3-30 micrograms/min), sumatriptan (1-30 micrograms/min), oxymetazoline (0.03-3 micrograms/min) and noradrenaline (0.3-3 micrograms/min) resulted in dose-dependent decreases in external carotid blood flow without changes in arterial blood pressure or heart rate. These vasoconstrictor responses remained unaltered after i.v. administration of physiological saline (0.015, 0.05 and 0.15 ml/kg; n = 4) or ritanserin (1 mg/kg; n = 5). In contrast, GR127935 (1, 3 and 10 micrograms/kg, n = 6) potently blocked the responses to 5-HT (unmasking a dose-dependent vasodilator component) and sumatriptan without affecting those to oxymetazoline or noradrenaline. Interestingly, methysergide (10, 30 and 100 micrograms/kg, n = 5) also blocked the vasoconstrictor responses to 5-HT and sumatriptan, but unlike GR127935, did not revert the vasoconstrictor response to 5-HT; the responses to oxymetazoline remained unaffected, but those to noradrenaline were apparently attenuated by the highest dose. Taken together, the above findings suggest that the sumatriptan-sensitive 5-HT1-like receptors mediating canine external carotid vasoconstriction resemble 5-HT1D receptors, probably of the 5-HT1D beta subtype on the basis of the resistance to blockade by ritanserin. The pharmacological profile of these receptors could be similar (bovine and human cerebral arteries, porcine carotid arteriovenous anastomoses and human coronary arteries) to other putative 5-HT1D receptors mediating vascular responses.

Functional characterization of 5-HT1D autoreceptors on the modulation of 5-HT release in guinea-pig mesencephalic raphe, hippocampus and frontal cortex

1. The aims of the present study were (i) to characterize further the pharmacology of 5-HT1D autoreceptors modulating 5-HT release in guinea-pig mesencephalic raphe, hippocampus and frontal cortex; (ii) to determine whether 5-HT1D receptors in the mesencephalic raphe are located on 5-HT neurones; (iii) to determine whether 5-HT1D autoreceptors are coupled to G proteins; and (iv) to assess their sensitivity following long-term 5-HT reuptake blockade and inhibition of type-A monoamine oxidase. 2. In mesencephalic raphe, hippocampus and frontal cortex slices, the 5-HT1D/1B receptor agonist, sumatriptan and the 5-HT1 receptor agonist, 5-methoxytryptamine (5-MeOT) but not the 5-HT1B receptor agonist, CP93129, inhibited electrically the evoked release of [3H]-5-HT in a concentration-dependent manner. This effect was antagonized by the 5-HT1D/1B receptor antagonist GR127935 in the three structures, but not by the 5-HT1A receptor antagonist, (+)-WAY100635 in mesencephalic raphe slices. These results confirm the presence of functional 5-HT1D autoreceptors controlling 5-HT release within the mesencephalic raphe as well as in terminal regions. 3. The inhibitory effect of sumatriptan on K(+)-evoked release of [3H]-5-HT was not reduced by the addition of the Na+ channel blocker, tetrodotoxin to the superfusion medium, suggesting that these 5-HT1D receptors in the mesencephalic raphe are located on 5-HT neurones and may be considered autoreceptors. 4. The in vitro treatment with the alkylating agent N-ethylmaleimide (NEM) was used to determine whether these 5-HT1D autoreceptors are coupled to G proteins. The inhibitory effect of sumatriptan on electrically evoked release of [3H]-5-HT was attenuated in NEM-pretreated slices from mesencephalic raphe, hippocampus and frontal cortex, indicating that the 5-HT1D autoreceptors activated by sumatriptan are coupled to G proteins in these three structures. Taken together with our previous results, this suggests that, in addition to the 5-HT1D autoreceptor activated by sumatriptan, another subtype of 5-HT autoreceptor is activated by 5-MeOT in the hippocampus. 5. Following a 3-week treatment with the selective 5-HT reuptake inhibitor, paroxetine (10 mg kg-1 day-1) and a 48 h washout period, the electrically evoked release of [3H]-5-HT was enhanced in mesencephalic raphe, hippocampus and frontal cortex slices. There was an attenuation of the capacity of sumatriptan to inhibit the evoked release of [3H]-5-HT from mesencephalic raphe slices but not from frontal cortex and hippocampus slices. Only in the latter structure was the suppressant effect of 5-MeOT attenuated. After a 3-week treatment with the reversible type-A monoamine oxidase inhibitor, befloxatone (0.75 mg kg-1 day-1) and 48 h washout period, the effectiveness of sumatriptan and 5-MeOT on the evoked release of [3H]-5-HT was unaltered in the same brain structures. 6. The enhancement of [3H]-5-HT release by long-term paroxetine treatment is possibly due to a desensitization of 5-HT1D autoreceptors activated by sumatriptan in mesencephalic raphe and by terminal 5-HT autoreceptors activated by 5-MeOT in hippocampus. In the case of the frontal cortex, it appears that 5-MeOT and sumatriptan may act on the same 5-HT1D autoreceptor which is not desensitized either after paroxetine or befloxatone treatment, as previously reported.

Blockade of porcine carotid vascular response to sumatriptan by GR 127935, a selective 5-HT1D receptor antagonist

1. It has previously been shown that the antimigraine drug, sumatriptan, a putative 5-HT1D receptor agonist, decreases porcine common carotid and arteriovenous anastomotic blood flows, but slightly increases the arteriolar (capillary) blood flow to the skin and ears. Interestingly, such responses, being mediated by 5-HT1-like receptors, are resistant to blockade by metergoline, which, in addition to displaying a very high affinity for (and occasionally intrinsic efficacy at) the 5-HT1D receptor subtypes, blocks (with lower potency than methiothepin) some 5-HT1D receptor-mediated vascular responses. These findings raise doubts whether sumatriptan-sensitive 5-HT1-like receptors mediating changes in the distribution of porcine carotid blood flow are identical to cloned 5-HT1D receptors. With the recent advent of the potent and selective 5-HT1D receptor antagonist, GR127935, we have examined in the present study whether the carotid vascular effects of sumatriptan in the pig are amenable to blockade by GR127935. 2. In animals pretreated with saline, sumatriptan (30, 100 and 300 micrograms kg-1, i.v.) reduced the total carotid and arteriovenous anastomotic blood flows in a dose dependent manner. In contrast, sumatriptan increased blood flow to the skin, ears and fat, although the total capillary fraction was not significantly affected. 3. While GR127935 pretreatment (0.25 and 0.5 mg kg-1) itself slightly reduced the total carotid and arteriovenous anastomotic blood flows, carotid vasoconstrictor responses to sumatriptan were either partly (0.25 mg kg-1) or completely (0.5 mg kg-1) blocked by the compound. In GR127935 pretreated animals, the sumatriptan-induced increases in blood flow to the skin, ears and fat were also attenuated. 4. Taken together, the results suggest that arteriovenous anastomotic constriction and, possibly, arteriolar dilatation in the skin, ears and fat by sumatriptan are mediated by 5-HT1D receptors. Therefore, vascular 5-HT1-like receptors in the porcine carotid bed appear to be identical to 5-HT1D receptors.

Phospholipase A2 and protein kinase C contribute to myofilament sensitization to 5-HT in the rabbit mesenteric artery

1. Calcium (Ca2+, 0.1-100 microM) stimulated concentration-dependent contractions in small strips from the rabbit mesenteric artery in which the smooth muscle cells had been permeabilized with Staphylococcus aureus alpha-toxin. 2. 5-Hydroxytryptamine (5-HT) and phenylephrine, each in the presence of 10 microM guanosine 5'-triphosphate (GTP), concentration-dependently stimulated additional contractions in strips sub-maximally contracted by the presence of a buffered concentration of calcium (0.3 microM). All the additional contraction was abolished with the selective inhibitor of protein kinase C, Ro 31-8220 (10 microM). 3. Quinacrine (10-50 microM), an inhibitor of phospholipase A2, selectively inhibited the sensitization to 5-HT, but did not alter the sensitization to either phenylephrine or GTP. 4. Myofilament sensitization to calcium was mimicked by exogenous arachidonic acid (300 microM, in the presence of indomethacin, miconazole and BW755c) and the stable analogue of arachidonic acid, 5,8,11,14-eicosatetrayonic acid (ETYA, 100 microM), and in both cases did not require the additional presence of GTP. Ro 31-8220, but not quinacrine, reduced the sensitization to arachidonic acid by around 30%. 5. These results indicate that G protein-linked myofilament sensitization to calcium in the mesenteric artery that follows the activation of 5-HT receptors, but not alpha 1-receptors, involves phospholipase A2. The sensitization stimulated by each of these different receptors, and a component of the response to arachidonic acid, also appears to involve the activation of protein kinase C.

Inhibition of noradrenaline release via presynaptic 5-HT1D alpha receptors in human atrium

In segments of human right atrial appendages preincubated with [3H]noradrenaline and superfused with physiological salt solution containing desipramine and corticosterone, we determined the effects of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists on tritium overflow evoked by transmural electrical stimulation (2 Hz). Tritium overflow was inhibited by 5-HT, 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine (5-MeOT), 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)-1H indole succinate (RU 24969) and sumatriptan. Yohimbine and oxymetazoline (in the presence of idazoxan) also inhibited tritium overflow. The inhibitory potency of the drugs was significantly correlated with their affinity for 5-HTID receptors in human brain and for cloned human 5-HT1D alpha and 5-HT1D beta receptors, but not with their affinity for 5-HT1B, 5-HT1E, 5-HT1F, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT3, 5-HT5A, 5-HT5B and 5-HT7 receptors. The potency order 5-CT > 5-HT > 5-MeOT is opposite to the order of affinities reported for 5-HT6 binding sites. The preferential 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetraline (up to 0.3 microM) and the selective 5-HT4 receptor agonist cisapride (up to 1 microM) failed to inhibit tritium overflow. L-694,247, a potent 5-HT1D beta receptor agonist, did not inhibit tritium overflow, but counteracted the inhibitory effect of 5-HT. Ketanserin at a concentration which should block 5-HT1D alpha but not 5-HT1D beta receptors and methiothepin at a concentration which may be assumed to block both 5-HT1D alpha and 5-HT1D beta receptors antagonized the inhibitory effect of 5-HT. Propranolol and ondansetron did not modify the 5-HT-induced inhibition of release. In conclusion, noradrenaline release in human right atrial appendages is inhibited via 5-HT receptors which are located on the noradrenergic axon terminals. These inhibitory presynaptic 5-HT receptors belong to the 5-HT1D subfamily. The ability of ketanserin to antagonize the inhibitory effect induced by activation of these receptors suggests that they can be subclassified as 5-HT1D alpha.

The activity of 5-HT1D receptor ligands at cloned human 5-HT1D alpha and 5-HT1D beta receptors

The present study has examined the functional activity of the 5-HT1D receptor agonist, sumatriptan, and antagonists, GR127935 (2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxyl ic acid [4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide), GR55562 (3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl] benzamide), metergoline and methiothepin in HeLa cells, stably transfected with either 5-HT1D alpha or 5-HT1D beta receptor subtypes. Sumatriptan, GR127935 and metergoline (each 0.01-1 microM) behaved as agonists, producing a concentration-dependent inhibition of forskolin-stimulated adenosine 3',5'-cyclic monophosphate (cAMP) production at both 5-HT1D alpha and 5-HT1D beta receptor subtypes (mean pIC50 values of 8.4 and 8.3 for sumatriptan, 7.9 and 8.0 for GR127935, and 7.9 and 8.3 for metergoline, respectively). In contrast, GR55562 and methiothepin behaved as competitive 5-HT1D receptor antagonists and were devoid of any agonist activity. GR55562 (10 microM) caused a rightward displacement of the GR127935 and metergoline concentration-response curves. The agonist activity of GR127935 and metergoline, observed in the present study, contrasts with their recognised 5-HT1D receptor antagonist profiles in animal isolated tissue and behavioural models. Unlike GR127935, GR55562 behaved as a silent antagonist at the cloned human 5-HT1D alpha and 5-HT1D beta receptors in the study.

Functional serotonin 5-HT1D receptors and 5-HT1D beta receptor mRNA expression in human umbilical vein endothelial cells

Pharmacological evidence has suggested the presence of 5-hydroxytryptamine (5-HT, serotonin), 5-HT(1D) receptors on endothelial cells but these receptors have never been identified unambiguously on this type of cells. We now report that human umbilical vein endothelial cells (HUVEC) express 5-HT(1D) receptors coupled to inhibition of cyclic AMP formation. 5-HT and 5-HT(1D) receptor agonists 5-carboxamidotryptamine (5-CT) and sumatriptan were approximately equipotent at inhibiting forskolin-stimulated cyclic AMP accumulation in HUVEC (mean pEC50 7.6-8.2, maximal effect 30% inhibition). The 5-HT(1A) receptor antagonist, 8-OH-DPAT was clearly less potent (pEC50 6.2) and less efficacious. The selective 5-HT(1D) receptor agonist, GR127935 (1 nM) markedly inhibited the effect of 5-HT (apparent pK(B) 10.8). Reverse transcription-polymerase chain reaction analysis showed the mRNA for 5-HT(1D beta) receptors to be expressed in HUVEC. These results demonstrate the presence of functional 5-HT(1D) receptors and the expression of 5-HT(1D beta) receptor mRNA in HUVEC. They support the involvement of 5-HT(1D beta) receptors in endothelial-mediated responses to 5-HT.

Selective 5-HT1D alpha serotonin receptor gene expression in trigeminal ganglia: implications for antimigraine drug development

Pharmacological data suggest that the actions of antimigraine drugs such as sumatriptan may be mediated by 5-HT1D-like serotonin receptors on trigeminovascular nerve endings. We sought molecular evidence for the expression of an mRNA species encoding the 5-HT1D receptor subtype in guinea pig and human trigeminal ganglia, using the polymerase chain reaction with oligonucleotides uniquely homologous to the coding sequences of the 5-HT1B/D family (human 5-HT1D alpha and 5-HT1D beta; rat 5-HT1B). A single band of predicted size was observed in samples from guinea pig trigeminal ganglia; sequence analysis revealed the presence of a single message, which was 85% and 71% identical to the human 5-HT1D alpha and 5-HT1D beta receptor DNA sequences, respectively. Similar analyses of postmortem human trigeminal ganglia revealed the presence of 5-HT1D alpha, but not 5-HT1D beta, receptor message. Inasmuch as one recent report found that mRNA encoding only the 5-HT1D beta receptor subtype was expressed by vascular smooth muscle of the central nervous system, the present findings suggest the importance of developing selective 5-HT1D alpha receptor agonists as a strategy to reduce the risk of myocardial infarction and possibly stroke that complicates the acute treatment of migraine headache.

Vascular receptors for 5-hydroxytryptamine: distribution, function and classification

The complex actions of 5-hydroxytryptamine (5-HT) on blood vessels result from interactions with a multiplicity of specific 5-HT receptors. Dramatic advances recently have been made in defining the receptor subtypes involved in terms of their pharmacology and biochemistry, as well as their molecular biology, provoking an evolution of the criteria used for classifying and naming them. This article reviews the distribution and function of different 5-HT receptor types present on vascular smooth muscle, endothelium and perivascular neurones, and considers ways in which they can be positively defined and differentiated using traditional pharmacological approaches. The characteristics of each receptor type are also considered in terms of current biochemical and molecular perspectives on 5-HT receptor classification.

Characterization of a serotonin receptor endogenous to frog melanophores

The response of a cell line of Xenopus laevis melanophores to serotonin was examined. Serotonin increased intracellular levels of cAMP and induced pigment dispersion in the cells. The responses depended on both the concentration of serotonin applied and on the time for which the cells were exposed to serotonin. Using a recently described, microtiter-plate-based bioassay, a series of serotonin receptor ligands were evaluated as agonists or antagonists at the melanophore serotonin receptor. The pharmacological profile suggests the presence of a receptor which shares some properties with but appears different from other previously described serotonin receptors.

Actions of lithium on the cyclic AMP signalling system in various regions of the brain--possible relations to its psychotropic actions. A study on the adenylate cyclase in rat cerebral cortex, corpus striatum and hippocampus

It has been estimated that in most industrialized countries 1 person out of every 1000 in the population is undergoing lithium treatment to stabilize their episodic mood disturbances due to manic-depressive illness. Lithium may stabilize mood swings by altering the action of certain neurotransmitters at the synaptic level in the brain. Recent research suggests that lithium alters neurotransmission by affecting neurotransmitter-coupled second messenger systems. A major second messenger system is the adenylate cyclase, which generates intracellular cAMP from ATP. The adenylate cyclases (type I-IV) are regulated by stimulatory and inhibitory receptors, which either stimulate or inhibit the adenylate cyclase activity. The stimulatory and inhibitory neurotransmitter-receptor signals are transferred to the catalytic unit of the adenylate cyclase by Gs and Gi, respectively. The activated receptor induces GTP stimulation of the heterotrimeric G protein, leading to a dissociation of the protein into the active alpha*GTP and the beta gamma complex. The former stimulates the catalytic unit of adenylate cyclase. The stimulation is terminated by a GTPase located on the alpha subunit that converts GTP to inactive GDP. At present, G proteins are known to play a central role in coupling receptors to effector proteins. In addition to extracellular regulation due to neurotransmitters, some adenylate cyclases (type I, III) are regulated by CaM as a consequence of enhanced intracellular concentrations of free Ca2+. The Ca(2+)-dependent stimulation of adenylate cyclase by CaM is assumed to occur by a direct effect on the catalytic unit. The catalytic units sensitive to Ca(2+)-CaM are also subjected to regulation by stimulatory and inhibitory neurotransmitter stimuli. Magnesium is essential for adenylate cyclase activity, since MgATP2- is the enzyme substrate. Furthermore, one Mg2+ site located on the G protein regulates both the receptor agonist affinity and the dissociation of the G protein during the activation cycle. A second Mg2+ site on the catalytic unit is responsible for Mg2+ regulation of the catalytic activity. The present work aimed at investigating the mechanisms by which lithium in vitro and after chronic treatment (ex vivo) affects adenylate cyclase activities in various regions of the rat brain. Lithium in vitro and ex vivo inhibited the selective stimulation of adenylate cyclase by Ca(2+)-CaM in the cerebral cortex. Furthermore, lithium in vitro interacted directly with the catalytic unit of adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

Partial agonists, full agonists, antagonists: dilemmas of definition

The absence of selective antagonists makes receptor characterization difficult, and largely dependent on the use of agonists. However, there has been considerable debate as to whether certain drugs acting at G protein-coupled receptors are better described as agonists, partial agonists or antagonists. Indeed many agonists tend to display differences in intrinsic activity, depending on the preparation used to study receptor pharmacology. It has been argued that variations in intrinsic activity of drugs, may be a reflection of receptor subtypes rather than varying degrees of receptor-effector coupling. However, it has often been overlooked that classical receptor theory predicts that a drug acting at a given receptor type can display a range of intrinsic activities depending on the extent of receptor reserve. Furthermore, because any receptor can couple to different effector systems, different degrees of intrinsic activity may be observed for a given drug. Daniel Hoyer and Hendrikus Boddeke review recent data which provide explanations for why drugs display such variations in intrinsic activity.

Partial agonist properties of rauwolscine and yohimbine for the inhibition of adenylyl cyclase by recombinant human 5-HT1A receptors

Previous studies by another group have suggested that the alpha 2-adrenergic receptor antagonist rauwolscine may function as an agonist at the serotonin1A (5-HT1A) receptor expressed in human brain. To directly test that hypothesis, we transfected the human 5-HT1A receptor cDNA into CHO cells and examined the ability of rauwolscine and its isomer, yohimbine, to inhibit ligand binding of [3H]-(+/-)-8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) and the activity of adenylyl cyclase in membranes derived from a single transformant that stably expresses approximately 225 fmol of 5-HT1A receptor/mg of membrane protein. Both ligands competitively antagonized the binding of [3H]8-OH-DPAT (Ki = 158 +/- 69 nM for rauwolscine and 690 +/- 223 nM for yohimbine), yielding shallow displacement curves consistent with agonist activity (Hill values = 0.69 +/- 0.2 for rauwolscine and 0.63 +/- 0.06 for yohimbine). Both ligands also inhibited forskolin-stimulated adenylyl cyclase activity in membranes derived from transfected (but not nontransfected) cells. For rauwolscine, the IC50 was 1.5 +/- 0.2 microM, and for yohimbine 4.6 +/- 1.0 microM, with activity ratios of 0.70 and 0.59, respectively, when compared to the full agonist serotonin. These studies demonstrated that rauwolscine and yohimbine are partial agonists for the human 5-HT1A receptor.

Autoradiographic mapping of 5-HT1 receptors in the guinea-pig brain with particular reference to the 5-HT1D receptor sites

The anatomical distribution of 5-HT1 receptors in the guinea-pig brain was studied by means of in vitro quantitative autoradiography using [3H]-5-HT as ligand. The relative presence of the subtypes of the 5-HT1 binding site was investigated by adding selective concentrations of 8-OH-DPAT, (-)21,009, mesulergine and 5-CT. In addition, differentiation of 5-HT1D receptors was achieved by incubation of the tissues with [3H]-5-HT in the presence of 100 nmol/l 8-OH-DPAT together with 100 nmol/l mesulergine. Areas presenting high densities of 5-HT1A receptors included the neocortex (internal layers), hippocampal formation (dentate gyrus, CA1 field), septum and raphe nuclei, while 5-HT1C sites accounted for most of the [3H]-5-HT binding to the choroid plexus. Non 5-HT1A-non 5-HT1C sites (mainly 5-HT1D and, also probably, 5-HT1E receptors) were clearly predominant in the guinea-pig brain. These sites were mainly present in the neocortex (external layers), basal ganglia, hypothalamus and midbrain (substantia nigra, superior colliculus). As previously described, sites with the properties of 5-HT1B receptors could not be clearly identified in the guinea-pig brain. The present results, in addition to providing a detailed map of the 5-HT1 receptors in the guinea-pig brain, indicate that the guinea-pig is a useful laboratory animal for the study of 5-HT1D receptors.

SDZ 216-525, a selective and potent 5-HT1A receptor antagonist

The pharmacological properties of SDZ 216-525, methyl 4-(4-[4-(1,1,3-trioxo-2H-1,2-benzoisothiazol-2-yl)butyl]-1-p iperazinyl)1H- indole-2-carboxylate, a new selective and potent 5-HT1A receptor antagonist, are described in vitro (and comparisons made with those of MDL 73005 and NAN 190, two putative 5-HT1A receptor antagonists) and in vivo. In radioligand binding studies, SDZ 216-525 showed high affinity and selectivity for 5-HT1A sites (pKD = 9.2) as compared to 5-HT1B, 5-HT1C, 5-HT1D, 5-HT2 and 5-HT3 sites (pKD = 6.0, 7.2, 7.5, 5.2 and 5.4, respectively). The affinity of the compound for alpha 1, alpha 2, beta 1 and beta 2 adrenoceptors, and dopamine D2 receptors was at least 50-100 times lower than for 5-HT1A sites. The effects of SDZ 216-525, MDL 73005 and NAN 190 on 5-HT1 receptor-linked second messengers were characterised in the following tests: inhibition of forskolin-stimulated adenylate cyclase activity in calf hippocampus (5-HT1A), rat substantia nigra (5-HT1B) and calf substantia nigra (5-HT1D) and stimulation of inositol phosphate production in pig choroid plexus (5-HT1C). SDZ 216-525 potently antagonised the effects of 8-OH-DPAT (8-hydroxy-2-[N-dipropyl-amino]-tetralin) on 5-HT1A receptors (pKB = 10) and displayed no intrinsic activity in this test, whereas it behaved at best as a weak antagonist on the other receptor models (pKB values < 6.9).(ABSTRACT TRUNCATED AT 250 WORDS)

Increased cyclic AMP reduces 5-HT1D receptor-mediated inhibition of [3H]5-hydroxytryptamine release from guinea-pig cortical slices

This study investigated the role of cyclic AMP in the inhibition of [3H]5-hydroxytryptamine ([3H]5-HT) release mediated through the 5-HT1D autoreceptor using slices of guinea-pig cerebral cortex. Forskolin, dibutyryl-cyclic AMP and rolipram, which increase intracellular cyclic AMP by different mechanisms, all attenuated the inhibitory effect of the autoreceptor agonist 5-carboxamidotryptamine. These results indicate that modulation of cyclic AMP production affects 5-HT1D autoreceptor function.

Cloning of another human serotonin receptor (5-HT1F): a fifth 5-HT1 receptor subtype coupled to the inhibition of adenylate cyclase

An intronless gene encoding an additional human serotonin (5-HT) 5-HT1-like receptor subtype was isolated from a human genomic library with probes obtained from degenerate PCR primers used to amplify 5-HT-receptor-specific sequences. The highest degree of homology was found with the 5-HT1E subtype (70%) and the 5-HT1D alpha (63%) and 5-HT1D beta (60%) receptors. RNA for this gene was detected in the human brain but was not detected in kidney, liver, spleen, heart, pancreas, and testes. High-affinity (Kd = 9.2 nM) 3H-labeled 5-HT binding was detected. Competition studies revealed the following rank order of potencies for serotonergic ligands: 5-HT > sumatriptan >> 5-carboxyamidotryptamine > 8-hydroxy-2(di-1-propylamino)tetralin > spiperone. 5-HT produced a dose-dependent inhibition of forskolin-stimulated cAMP accumulation (EC50 = 7.9 nM) in transfected cells. These properties distinguish this receptor from any previously characterized and establish a fifth 5-HT1-like receptor subtype (5-HT1F) coupled to the inhibition of adenylate cyclase.

The role of serotonin in schizophrenia: an overview of the nomenclature, distribution and alterations of serotonin receptors in the central nervous system

Serotonin is a neurotransmitter that is widely distributed throughout the central nervous system. The role of serotonin in schizophrenia is still unclear. Postmortem studies of serotonin receptor subtypes in schizophrenia have been inconclusive for the most part. The most promising findings involve a reduction in 5-HT2 receptors and 5-HT reuptake sites in the prefrontal cortex of schizophrenic patients. In this paper we review the function, distribution and pharmacological characteristics of serotonin receptors. Postmortem studies are also reviewed, focusing upon the role of these receptors in schizophrenia.

Characterization of the 5-Hydroxytryptamine Receptor Modulating the Release of 5-[3H]Hydroxytryptamine in Slices of the Human Neocortex

In the rat brain, the presynaptic 5-hydroxytryptamine (5-HT) autoreceptors located on 5-HT terminals correspond to the 5-HT1B subtype. The presence of a 5-HT receptor probably located on 5-HT nerve endings and modulating transmitter release in the human neocortex has been reported, but its detailed pharmacological characterization is not yet available. On the other hand, receptor binding and autoradiographic results indicate that the 5-HT1B receptor subtype is not present in the human brain. We, therefore, studied the modulation of the electrically evoked release of [3H]5-HT by various 5-HT receptor agonists and antagonists in preloaded slices of human neocortex obtained from 18 patients undergoing neurosurgery. The nonselective 5-HT1A/1B/1D receptor agonist 5-carboxamidotryptamine produced a potent inhibition (70% at 0.03 microM) of the electrically evoked release of [3H]5-HT which was blocked by 5-HT receptor antagonists with the following relative order of potency: methiothepin greater than metergoline = methysergide greater than propranolol. The selective 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin at 0.1 microM did not modify the electrically evoked release of [3H]5-HT. The 5-HT1A/1B receptor agonist RU 24969 was 10 times more potent at inhibiting [3H]5-HT overflow in the rat frontal cortex than in the human neocortex. The potent 5-HT1B receptor antagonist cyanopinodolol did not modify the 5-carboxamidotryptamine-induced inhibition of the electrically evoked release of [3H]5-HT in slices of the human neocortex, but produced by itself a small inhibition of [3H]5-HT overflow.(ABSTRACT TRUNCATED AT 250 WORDS)

"5-HT1R" or 5-HT1D sites? Evidence for 5-HT1D binding sites in rabbit brain

Radioligand binding studies were performed in membranes of rabbit whole brain and striatum using the novel iodinated radioligand for 5-hydroxytryptamine 5-HT1B and 5-HT1D sites, Serotonin-5-O-Carboxymethyl-Glycyl[125I]Tyrosinamide ([125I]GTI). [125I]GTI labelled a finite number of high affinity sites in rabbit brain membranes, Bmax = 191 +/- 47 fmol/mg protein, pKD (-log mol/l) = 8.50 +/- 0.13, n = 5. The pharmacological profile of [125I]GTI binding was fully comparable to that reported previously in human and other brain preparations known to possess 5-HT1D sites (using either [3H]5-HT or [125I]GTI) and displayed a characteristic rank order of affinity: 5-carboxamido-tryptamine greater than 5-HT = dihydroergotamine greater than or equal to ergotamine greater than or equal to sumatriptan greater than or equal to CGS 12066 greater than or equal to metergoline greater than yohimbine greater than or equal to methysergide greater than ICYP greater than 8-OH-DPAT greater than or equal to CP 93129 greater than (-)pindolol greater than ketanserin greater than isamoltane greater than mesulergine greater than corynanthine greater than buspirone greater than MDL 72222. Autoradiographic studies were performed on rabbit brain slices using [3H]5-HT in the presence of 100 nmol/l 8-OH-DPAT and mesulergine (in order to mask 5-HT1A and 5-HT1C binding sites) and [125I]CYP (iodocyanopindolol) in the presence of 3 mumol/l isoprenaline and 100 nmol/l 8-OH-DPAT (in order to mask beta adrenoceptor and 5-HT1A binding sites). There was no detectable specific binding of [125I]CYP through the brain, thus excluding the presence of 5-HT1B sites in rabbit brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Further characterization of the 5-hydroxytryptamine 5-HT1-like receptor mediating contraction of guinea-pig iliac artery

We recently described a 5-hydroxytryptamine 5-HT1-like receptor mediating contraction in guinea-pig isolated iliac artery. The present study was aimed at characterizing this receptor with respect to the currently recognized 5-HT1 receptor subtypes (5-HT1A, 5-HT1B, 5-HT1C and 5-HT1D). The potencies of 13 drugs tested as agonists correlated with their affinities for 5-HT1D binding sites only. The concentration-response curve for 5-carboxamidotryptamine (5-CT, a 5-HT1-like receptor agonist) was unaffected by propranolol (10 microM), which is reported to have affinity for 5-HT1A, 5-HT1B and 5-HT1C recognition sites. Yohimbine (3 microM) and metergoline (1 microM) antagonized 5-CT with pKB values of 6.15 and 6.96, respectively. These values are close to those found in a functional correlate of 5-HT1D sites in the same species, namely the presynaptic 5-HT autoreceptor in guinea-pig brain cortex. The overall results support the view that the receptor studied is of the 5-HT1D subtype. The receptor shares close similarities with other vascular 5-HT1-like receptors mediating contraction, for example the receptor present in dog saphenous vein.

Serotonin inhibits adenylate cyclase in human platelet membranes

Serotonin (5-HT), the non-specific 5-HT1 receptor agonist methysergide, and the 5-HT1B receptor agonist CGS 12066 dose-dependently inhibited forskolin-stimulated adenylate cyclase activity in membranes derived from human platelets. Buspirone and ipsapirone showed partial agonist activity. The effect of 5-HT to inhibit forskolin-stimulated activity was partially antagonised by spiperone, yohimbine and pindolol, with each of these compounds showing some partial agonist activity. It is concluded that human platelet membranes possess receptors for 5-HT coupled negatively to adenylate cyclase. The relationship of these receptors to other binding sites which have been shown for 5-HT on platelet membranes remains to be finally elucidated.

A human serotonin 1D receptor variant (5HT1D beta) encoded by an intronless gene on chromosome 6

An intronless gene encoding a serotonin receptor (5HT1D beta) has been cloned and functionally expressed in mammalian fibroblast cultures. Based on the deduced amino acid sequence, the gene encodes a 390-amino acid protein displaying considerable homology, within putative transmembrane domains (approximately 75% identity) to the canine and human 5HT1D receptors. Membranes prepared from CHO cells stably expressing the receptor bound [3H]serotonin with high affinity (Kd 4 nM) and displayed a pharmacological profile consistent, but not identical, with that of the characterized serotonin 5HT1D receptor. Most notably, metergoline and serotonergic piperazine derivatives, as a group, display 3- to 8-fold lower affinity for the 5HT1D beta receptor than for the 5HT1D receptor, whereas both receptors display similar affinities for tryptamine derivatives, including the antimigraine drug sumatriptan. Northern blot analysis revealed an mRNA of approximately 5.5 kilobases expressed in human and monkey frontal cortex, medulla, striatum, hippocampus and amygdala but not in cerebellum, olfactory tubercle, and pituitary. The 5HT1D beta gene maps to human chromosome 6. The existence of multiple neuronal 5HT1D-like receptors may help account for some of the complexities associated with [3H]serotonin binding patterns in native membranes.

An endothelial 5-HT receptor that mediates relaxation in guinea-pig isolated jugular vein resembles the 5-HT1D subtype

1. Endothelium-dependent and -independent, concentration-related, relaxations to 5-hydroxytryptamine (5-HT) are described in a preparation of guinea-pig isolated jugular vein. 2. An endothelial 5-HT receptor was studied in the presence of mesulergine (at 10.0 microM, a concentration sufficient to antagonize 5-HT2 receptor-mediated contractions and endothelium-independent relaxations to 5-HT). Relaxations mediated by the endothelial 5-HT receptor were resistant to antagonism by mesulergine. 3. Several 5-HT receptor agonists activated the endothelial receptor with the following rank order of potency: 5-carboxamidotryptamine (5-CT) greater than 5-HT greater than methysergide greater than or equal to alpha-methyl-5-HT greater than sumatriptan greater than 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) greater than 2-methyl-5-HT. 4. Relaxations to 5-HT were not blocked by (+/-)-pindolol (1.0 microM), (-)-propranolol (1.0 microM), spiperone (1.0 microM), ondansetron (1.0 microM) or ICS 205-930 (10.0 microM). 5. Both 5-HT and sumatriptan evoked endothelium-dependent relaxations which were sensitive to antagonism (pA2 and apparent pA2 values respectively) by methiothepin (8.1 and 8.6), metergoline (7.4 and 7.5), PAPP (8.2 and 8.2), yohimbine (7.1 and 6.8), rauwolscine (6.8 and 6.7), but not by corynanthine (10.0 microM). 6. These observations are consistent with a 5-HT1D receptor-mediated effect, and provide further support for the concept that differences exist between endothelial 5-HT receptors in different tissues and species.

A cyclic AMP mechanism mediates the serotonin-induced increase in glutamic acid decarboxylase activity in the preoptic area and hypothalamus

Previous studies have shown that the injection of 5-hydroxytryptamine (5-HT) into the third ventricle of rats on the afternoon of proestrus increases glutamic acid decarboxylase (GAD) activity in the preoptic area and the hypothalamus. In the present report we examine the adenylate cyclase-cyclic AMP (cAMP) system as mediator of that effect. The increase in GAD activity induced by intraventricular injection of 5-HT was completely blocked by injecting an antiserum against cAMP into the third ventricle 30 min earlier, whereas an injection of serum from normal rabbits was ineffective. On the contrary, activation of adenylate cyclase activity by intraventricular injection of forskolin increased GAD activity, an effect that was also blocked by anti-cAMP serum. Anti-cAMP serum also lowered GAD activity in the preoptic area and hypothalamus when injected on the morning of proestrus but not when injected in the afternoon, when the values of GAD activity were already low. The results suggest that a cAMP mechanism may be involved in the changes in preoptic-area and hypothalamic GAD activity such as the rise in enzyme activity induced by intraventricular injection of 5-HT.

5-Hydroxytryptamine receptor agonists for the abortive treatment of vascular headaches block mast cell, endothelial and platelet activation within the rat dura mater after trigeminal stimulation

Antidromic stimulation of small caliber trigeminal axons causes neurogenic inflammation in the dura mater and tongue as evidenced by marked increases in mast cell activation, protein extravasation, as well as in the numbers of endothelial cytoplasmic vesicles, endothelial microvilli and platelet aggregates within ipsilateral post-capillary venules. In this report, we examined the effects of pretreatment with serotonin1 receptor agonists, dihydroergotamine (50 micrograms/kg, i.v.) and sumatriptan (100 micrograms/kg, i.v.) on the light and electron microscopic changes which develop after trigeminal ganglion stimulation. Both dihydroergotamine and sumatriptan are useful in the acute treatment of vascular headaches and bind with high affinity to 5-HT1D receptors. Both drugs decreased significantly the number of dural vessels showing endothelial or platelet changes and the numbers of activated mast cells, but did not affect the neurogenic response in the tongue. The drugs also blocked the accumulation of horseradish peroxidase reaction product within the endothelium and perivascular space on the stimulated side. The receptor is not present on trigeminovascular fibers innervating extracranial cephalic tissues. Drug mechanism probably involves inhibition of a proximal step in the pathophysiological cascade (e.g., via activation of a prejunctional receptor) because (a) receptors for sumatriptan have not been identified on mast cells whereas the inflammatory response was attenuated in mast cells as well as within platelets and the endothelium and (b) previous work indicates that sumatriptan and dihydroergotamine block neurotransmitter release. Hence, constriction of vascular smooth muscle mediated by postjunctional 5-hydroxytryptamine receptors is unlikely to explain the anti-inflammatory actions of dihydroergotamine or sumatriptan reported here.

Role of 5-HT1-like receptors in the increase in intragastric pressure induced by 5-hydroxytryptamine in the rat

The study concerned the effects of 5-hydroxytryptamine (5-HT) on intragastric pressure in bilaterally vagotomized spinal rats. Intravenous (i.v.) bolus injections of 5-HT (2.5, 5.0 and 10 micrograms/kg) produced dose-dependent increases in intragastric pressure; these effects were not modified by atropine (up to 0.2 mg/kg) or mepyramine (1 mg/kg), but were blocked by the mixed 5-HT1-like and 5-HT2 receptor antagonists, methiothepin (0.1, 0.3 and 0.5 mg/kg i.v.) and methysergide (0.5, 1 and 2.5 mg/kg i.v.). However, metergoline (0.5, 1 and 2 mg/kg i.v.) did not markedly modify this effect of 5-HT; only the response induced by 5 micrograms/kg 5-HT was significantly antagonized by the highest dose of metergoline. In contrast, neither the 5-HT2 receptor antagonist, ketanserin (0.5, 1 and 1.5 mg/kg i.v.), nor the 5-HT3 receptor antagonist, ICS 205-930 (0.5, 1 and 3 mg/kg i.v.), influenced the 5-HT-induced increase in intragastric pressure. In addition, 5-carboxamidotryptamine (25, 50 and 100 micrograms/kg i.v.) and RU 24969 (50, 100 and 200 micrograms/kg i.v.) mimicked the aforementioned effects of 5-HT but were weaker than 5-HT. These data suggest that the 5-HT-induced increase in intragastric pressure in the spinal and bilaterally vagotomized rat is mediated by an atypical 5-HT1-like receptor, which, based on the low agonist potency of 5-carboxamidotryptamine and RU 24969 and the resistance to blockade by metergoline, does not seem to correspond to either the 5-HT1A, 5-HT1B, 5-HT1C or the 5-HT1D receptor subtypes.

An investigation of the 5-HT1D receptor binding affinity of 5-hydroxytryptamine, 5-carboxyamidotryptamine and sumatriptan in the central nervous system of seven species

The ability of three 5-HT1 receptor agonists, 5-HT (5-hydroxytryptamine), 5-CT (5-carboxyamidotryptamine) and sumatriptan to inhibit the binding of [3H]5-HT, in the presence of cyanopindolol and mesulergine, from cerebral cortical and/or caudate membranes in seven species (dog, guinea-pig, rabbit, pig, human, hamster and calf) has been investigated. Under the experimental conditions used, 5-CT and sumatriptan consistently yielded displacement curves best fit to a two-site model whereas 5-HT always gave a monophasic displacement curve. The pIC50 values obtained with 5-HT displacement gave a mean of 8.1 +/- 0.1 (mean +/- S.E.M.). In contrast the biphasic displacement curves for 5-CT and sumatriptan yielded high and low affinity pIC50 values of 8.3 +/- 0.1, 5.5 +/- 0.1 and 7.6 +/- 0.1, 5.0 +/- 0.1, respectively. These data indicate that under these experimental conditions the high affinity component labelled by [3H]5-HT is the same receptor subtype, previously denoted the 5-HT1D receptor, in all seven species.

Receptor-mediated autocrine growth-stimulatory effect of 5-hydroxytryptamine on cultured human pancreatic carcinoid cells

5-hydroxytryptamine (5-HT) is a mitogen for fibroblasts, vascular smooth muscle cells, renal mesangial cells, and jejunal crypt cells. The human carcinoid cell line (termed BON) that we established in our laboratory from a pancreatic carcinoid tumor produces and secretes 5-HT. In this study, therefore, we examined the effect of 5-HT on growth of BON cells. Furthermore, by use of selective 5-HT receptor antagonists, we examined receptor and post-receptor mechanisms by which 5-HT-induced responses were produced. 5-HT stimulated growth of BON cells. 5-HT stimulated phosphatidylinositol (PI) hydrolysis in a dose-dependent fashion and inhibited cyclic AMP production in a dose-dependent fashion. The 5-HT1A/1B receptor antagonist, SDZ 21-009, prevented the reduction of cyclic AMP production evoked by 5-HT and inhibited the mitogenic action of 5-HT. The 5-HT1C/2 receptor antagonist, mesulergine, competitively inhibited PI hydrolysis, but did not affect the mitogenic action of 5-HT. The mitogenic action of 5-HT and the reduction of cyclic AMP production evoked by 5-HT were also inhibited by pertussis toxin. These results suggest that 5-HT is an autocrine growth factor for BON cells and that mitogenic mechanism of 5-HT involves receptor-mediated inhibition of the production of cyclic AMP which may be linked to pertussis toxin-sensitive GTP binding protein. 8-bromo-cyclic AMP inhibited growth of BON cells whereas 8-bromo-cyclic GMP had no effect on cell growth. Involvement of protein kinase A in BON cell growth regulation was confirmed by the observation that a cAMP-dependent protein kinase antagonist (Rp-cAMPS) could stimulate BON cell growth.

Homogeneous 5-HT1D recognition sites in the human substantia nigra identified with a new iodinated radioligand

The novel iodinated radioligand, serotonin-5-O-carboxymethyl-glycyl[125I] tyrosinamide, was used for binding studies with membranes of human substantia nigra. Evidence is provided for the existence in this tissue of a homogeneous population of recognition sites with the pharmacological profile of the 5-HT1D site.

Species differences in the distribution of central 5-HT1 binding sites: a comparative autoradiographic study between rat and guinea pig

In this study, we compared the localization of central 5-HT1 binding sites of rat and guinea pig. The 5-HT1B sites were absent in the guinea pig brain. Good correlations were found between species in the regional distribution of 5-HT1 sites labelled with [3H]5-HT (r = 0.73), 5-HT1A sites labelled with [3H]8-OH-DPAT (r = 0.87), and 5-HT1B versus 5-HT1D sites labelled with [3H]5-HT in the presence of ipsapirone and DOI (r = 0.76). Despite the overall similarities, species differences were observed in many brain regions. The CA1/CA2 fields of the hippocampus and the dorsal subiculum displayed significantly more 5-HT1A receptor binding in guinea pig than in rat. Conversely, the 5-HT1A binding in dorsolateral septum, cingulate cortex and laminae IV-V of the neocortex, was more pronounced in rat. Areas almost exclusively containing 5-HT1B or 5-HT1D sites, such as the ventral pallidum, globus pallidus and substantia nigra, expressed markedly more [3H]5-HT binding in rat as compared to guinea pig, while the opposite occurred in claustrum, dorsal endopiriform nucleus, lateral geniculate nucleus, and superficial grey layer of the superior colliculus. The implications of the species differences are illustrated by the binding of [3H]eltoprazine. The distribution of [3H]eltoprazine binding sites showed a good correlation with that of the 5-HT1B sites in rat (r = 0.89), and with that of the 5-HT1A sites in guinea pig (r = 0.97). The data give rise to the possibility that differences in the presence and distribution of 5-HT1 receptor sites are related to species differences in behavioural, neurochemical and physiological responses to drugs with 5-HT1 receptor affinity.

Potencies of antagonists indicate that 5-HT1C receptors mediate 1-3(chlorophenyl)piperazine-induced hypophagia

1. 1-3(Chlorophenyl)piperazine (mCPP) (5 mg kg-1, i.p.) inhibited 2 h food intake in rats previously deprived of food for one day. Ten 5-hydroxytryptamine (5-HT) antagonists given s.c. opposed this hypophagic response. Calculated ID50 values correlated significantly with reported affinities (r = 0.81, n = 10, P less than 0.01) for 5-HT1C but not for 5-HT2, 5-HT1A, 5-HT1B or 5-HT1D receptors. 2. ID50 values of the ten antagonists against 5-hydroxytryptophan (5-HTP) + carbidopa-induced head shakes (a 5-HT2-mediated response) correlated significantly (r = 0.81, n = 10, P less than 0.01) with their affinities for 5-HT2 but not for 5-HT1A, 5-HT1B, 5-HT1C or 5-HT1D receptors. 3. ID50 values for inhibition of hypophagia and head shakes did not correlate significantly with each other. 4. Ratios of ID50 values against hypophagia and 5-HT2-mediated head shakes gave indices of relative in vivo potencies independent of differences in drug metabolism and disposition. These ratios correlated highly significantly (r = 0.91, n = 10, P less than 0.001) with the ratios of the affinities of the drugs for 5-HT1C (but not for 5-HT1A, 5-HT1B or 5-HT1B or 5-HT1D receptors) and with their affinities for 5-HT2 receptors. These results strongly support the hypothesis that mediation of mCPP-induced hypophagia is by stimulation of 5-HT1C receptors and the mediation of 5-HTP-induced head twitches by 5-HT2 receptors.

5-HT receptors in human and animal cerebrovasculature

5-HT has been implicated in the pathogenesis of cerebral arterial vasospasm following subarachnoid haemorrhage and in the aetiology of migraine. Within the cerebrovasculature, 5-HT possesses a complex action dependent on the type of 5-HT receptors present. In this review, Andrew Parsons analyses the evidence demonstrating contractile and relaxant 5-HT receptors within different areas of the cerebrovasculature, and examines differences between human and animal cerebral arteries. The extensive regional and species differences are important considerations for the development of future therapeutic agents in cerebrovascular disease.

G proteins and G-protein-coupled receptors: structure, function and interactions

The G protein family continues to grow and at least 15 heterotrimeric G proteins have now been identified. This review deals with the nature of the functional domains of the members of the G-protein-coupled receptor family as well as the associated G proteins.

Interaction of the alpha-adrenoceptor agonist oxymetazoline with serotonin 5-HT1A, 5-HT1B, 5-HT1C and 5-HT1D receptors

Oxymetazoline was recognized with nanomolar affinity by 5-HT1A, 5-HT1B and 5-HT1D binding sites and mimicked the effects of 5-hydroxytryptamine with about the same potency and intrinsic activity as the endogenous amine in the corresponding functional tests. At 5-HT1C receptors, oxymetazoline behaved as a mixed agonist-antagonist. Clonidine had minimal activity. Methiothepin antagonized the effects of oxymetazoline (7.4 less than pKB less than 8.8). Thus, oxymetazoline is a full and potent agonist at 5-HT1A, 5-HT1B and 5-HT1D receptors and a partial agonist at 5-HT1C receptors.