A review of central 5-HT receptors and their function

It is evident that in the last decade or so, a vast amount of new information has become available concerning the various 5-HT receptor types and their characteristics. This derives from two main research approaches, operational pharmacology, using selective ligands (both agonists and antagonists), and, more recently, molecular biology. Although the scientific community continues to deliberate about the hierarchy of criteria for neurotransmitter receptor characterisation, there seems good agreement between the two approaches regarding 5-HT receptor classification. In addition, the information regarding transduction mechanisms and second messengers is also entirely consistent. Thus, on the basis of these essential criteria for receptor characterisation and classification, there are at least three main groups or classes of 5-HT receptor: 5-HT1, 5-HT2, and 5-HT3. Each group is not only operationally but also structurally distinct, with each receptor group having its own distinct transducing system. The more recently identified 5-HT4 receptor almost undoubtedly represents a fourth 5-HT receptor class on the basis of operational and transductional data, but this will only be definitively shown when the cDNA for the receptor has been cloned and the amino acid sequence of the protein is known. Although those 5-HT receptors that have been fully characterised and classified to date (and, hence, named with confidence) would seem to mediate the majority of the actions of 5-HT throughout the mammalian body, not all receptors for 5-HT are fully encompassed within our scheme of classification. These apparent anomalies must be recognised and need further study. They may or may not represent new groups of 5-HT receptor or subtypes of already known groups of 5-HT receptor. Even though the cDNAs for the 5-ht1E, 5-ht1F, 5-ht5, 5-ht6, and 5-ht7 receptors have been cloned and their amino acid sequence defined, more data are necessary concerning their operational and transductional characteristics before one can be confident of the suitability of their appellations. Therefore, it is important to rationalise in concert all of the available data from studies involving both operational approaches of the classical pharmacological type and those from molecular and cellular biology.(ABSTRACT TRUNCATED AT 400 WORDS)

International Union of Pharmacology classification of receptors for 5-hydroxytryptamine (Serotonin).

Cyclic nucleotide phosphodiesterases (PDEs) are enzymes that regulate the cellular levels of the second messengers, cAMP and cGMP, by controlling their rates of degradation. There are 11 different PDE families, with each family typically having several different isoforms and splice variants. These unique PDEs differ in their three-dimensional structure, kinetic properties, modes of regulation, intracellular localization, cellular expression, and inhibitor sensitivities. Current data suggest that individual isozymes modulate distinct regulatory pathways in the cell. These properties therefore offer the opportunity for selectively targeting specific PDEs for treatment of specific disease states. The feasibility of these enzymes as drug targets is exemplified by the commercial and clinical successes of the erectile dysfunction drugs, sildenafil (Viagra), tadalafil (Cialis), and vardenafil (Levitra). PDE inhibitors are also currently available or in development for treatment of a variety of other pathological conditions. In this review the basic biochemical properties, cellular regulation, expression patterns, and physiological functions of the different PDE isoforms will be discussed. How these properties relate to the current and future development of PDE inhibitors as pharmacological agents is especially considered. PDEs hold great promise as drug targets and recent research advances make this an exciting time for the field of PDE research.

Cyclic Nucleotide Phosphodiesterases: Molecular Regulation to Clinical Use

Adenosine receptors are major targets of caffeine, the most commonly consumed drug in the world There is growing evidence that they could also be promising therapeutic targets in a wide range of conditions, including cerebral and cardiac ischaemic diseases, sleep disorders, immune and inflammatory disorders and cancer After more than three decades of medicinal chemistry research, a considerable number of selective agonists and antagonists of adenosine receptors have been discovered, and some have been clinically evaluated, although none has yet received regulatory approval However, recent advances in the understanding of the roles of the various adenosine receptor subtypes, and in the development of selective and potent ligands, as discussed in this review, have brought the goal of therapeutic application of adenosine receptor modulators considerably closer

Adenosine receptors as therapeutic targets

G protein-coupled receptors (GPCRs) are the largest family of membrane-bound receptors and also the targets of many drugs. Understanding of the functional significance of the wide structural diversity of GPCRs has been aided considerably in recent years by the sequencing of the human genome and by structural studies, and has important implications for the future therapeutic potential of targeting this receptor family. This article aims to provide a comprehensive overview of the five main human GPCR families--Rhodopsin, Secretin, Adhesion, Glutamate and Frizzled/Taste2--with a focus on gene repertoire, general ligand preference, common and unique structural features, and the potential for future drug discovery.

Structural diversity of G protein-coupled receptors and significance for drug discovery

Selective inhibition of cyclic nucleotide phosphodiesterases of human, bovine and rat aorta.

Somatostatin (somatotropin release inhibiting factor, SRIF) was initially isolated from ovine hypothalamic extracts as a factor that inhibits growth hormone (GH) release from cultured pituitary cells.' Later it became evident that SRIF is an important regulator of endocrine and exocrine secretion and CNS functions. SRIF is derived from preprosomatostatin (116 amino acids) that is first cleaved to form prosomatostatin (92 amino acids) before being processed by tissue-specific cleavage to several shorter fragments, such as SRIF-28, SRIF-25, and SRIF-14, of which SRIF-14 was identified first. SRIF is found widely distributed throughout the intestinal tract; the D cells of the pancreas; specific endocrine cells of the thyroid, thymus, and brain; and also in neurons innervating the heart and r e t i ~ ~ a . ~ ~ SRIF not only inhibits the release of GH but also that of other hormones, such as insulin, glucagon, gastrin, and secretin, as well as exocrine secretion.2 SRIF is also present in various brain regions where it was shown to be a modulator of neural activity.@ It regulates the release of a variety of neurotransmitters, such as dopamine, 5-HT, and excitatory amino acids. In addition, SRIF and its metabolically stabilized analogues, such as octreotide (SMS 201-995), possess antiproliferative properties. Inhibitory effects on tumor-cell proliferation have been demonstrated both in vitro and in vivo.\"' All these observed regulatory effects of SRIF are mediated by specific, highaffinity membrane receptors on the target tissues, such as brain, pancreas, pituitary, gastrointestinal tract, and on tumor t i s s ~ e . ~ . ~ Results of radioligand binding and second-messenger studies, using selected SRIF analogues, have suggested for years the existence of at least two SRIF-receptor subtypes, SSl/SRIFl and SS2/SRIF2.12.13 Furthermore, various SRIF analogues differed in their inhibitory potency on the secretion of insulin, glucagon, growth hormone, and gastric acid, providing further support for the heterogeneity of SRIF re~ept0rs.l~ The final proof for SRIF receptor heterogeneity has been obtained during the last two years by the cloning of different genes coding for SRIF-receptor subtypes.

Molecular Pharmacology of Somatostatin‐receptor Subtypes

Our knowledge about 5-HT (serotonin, 5-hydroxytryptamine) receptors has gained significantly over the recent few years. The discovery of selective ligands and the use of new techniques have led to a significant increase in the number of recognised receptors subtypes. The present status of awareness is largely related to the use of radioligand binding studies, autoradiography, second messenger analysis and more recently, molecular biological techniques. Three main families of 5-HT receptors, of which subtypes have been described, are now accepted. This heterogeneity is further substantiated by the cloning of the cDNA's of three different 5-HT receptors. This article reviews some of the recent developments which led to the characterisation of 5-HT receptor subtypes.

5-HT receptors: subtypes and second messengers.

1)

The binding characteristics of [3H]5-HT (5-hydroxytryptamine, serotonin) were investigated in membrane preparations of several regions from calf, pig and human brain in the presence of 100 nmol/l 8-OH-DPAT (8-hydroxy-2[di-n-dipropylamino]tetralin) and 100 nmol/l mesulergine in order to mask 5-HT1A and 5-HT1C sites.




2)

[3H]5-HT bound rapidly, reversibly and stereo-selectively to a population of high affinity recognition sites in membranes from pig caudate, calf caudate and human cortex, caudate and substantia nigra.




3)

Saturation experiments carried out with [3H]5-HT in the presence of 100 nmol/l 8-OH-DPAT and 100 nmol/l mesulergine revealed that non-5-HT1A non-5-HT1C sites represented from 50 to more than 90% of the total 5-HT1 sites (determined with [3H]5-HT in the absence of 8-OHDPAT and mesulergine), depending on the tissue source.




4)

The pharmacological profile of these sites was characterized in competition experiments performed with a variety of ligands in membranes of calf, pig and human caudate membranes. Under these conditions, [3H]5-HT labelled a population of “5-HT1-like” sites which display nanomolar affinity for tryptamines (5-carboxamidotryptamine > 5-HT >- 5-methoxytryptamine > tryptamine) and some ergolines (metergoline > methysergide). In contrast, these sites showed low affinity for drugs with high affinity and/or selectivity for 5-HT1A (8-OH-DPAT, buspirone), 5-HT1B (21-009, RU 24969), 5-HT1C (mesulergine, mianserin) and 5-HT2 sites (ketanserin, cinanserin).




5)

Non-5-HT1A non-5-HT1C sites from calf, pig and human caudate membranes displayed a closely similar affinity profile as illustrated by the statistically very significant correlation parameters obtained when they were compared to one another. The pharmacological profile of these sites is different from that of 5-HT1A, 5-HT1B, 5-HT1C, 5-HT2 and 5-HT3 sites but is consistent with the pharmacology of a “5-HT1-like” receptor. It is very similar to that of the 5-HT1D site recently described in bovine brain by Heuring and Peroutka (1987).




6)

The present findings demonstrate the presence and the pharmacological similarity of 5-HT1D sites in pig, calf and human brain.

Molecular pharmacology of 5-HT1D recognition sites: radioligand binding studies in human, pig and calf brain membranes.

The effects of several putative 5-HT1 receptorsubtype selective ligands were investigated in biochemical models for 5-HT1A, 5-HT1B, and 5-HT1D receptors (inhibition of forskolin-stimulated adenylate cyclase activity in calf hippocampus, rat and calf substantia nigra, respectively) and 5-HT1C receptors (stimulation of inositol phosphates production in pig choroid plexus). Following compounds were studied: 5-HT (5-hydroxytryptamine), TFMPP (1-(mtrifluoromethylphenyl)piperazine), mCPP (1-m-chlorophe-nyl)piperazine, 1 CGS 12066 (7-trifluoromethyl-4-(4-methyl1-piperazinyl)-pyrrolo[1,2-a]quinoxaline 1), isamoltane (CGP 361A, 1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propranol), quipazine, 1-NP (1-(1-naphthyl)piperazine), and PAPP (LY165163, 1-[2-(4-aminophenyl)ethyl]-4-(3-trifluoromethylphenyl)-piperazine). Among reported 5-HT1B receptor selective drugs, TFMPP had similar potency at 5HT1A, 5-HT1B and 5-HT1C receptors, mCPP did not separate between 5-HT1B and 5-HT1C receptors, CGS 12066 was equipotent at 5-HT1B and 5-HT1D receptors, and isamoltane was only slightly 5-HTIB versus 5-HT1A selective. Quipazine showed equal potency at 5-HTIB and 5-HT1C receptors and 1-NP did not discriminate between the four receptor subtypes. PAPP described as 5-HT1A receptor selective, was equally potent at 5-HT1A and 5-HT1D receptors. The potencies determined in second messenger studies were in good agreement with the affinity values determined in radioligand binding studies. Thus 5-HT1A, 5-HT1B, 5-HT1C and 5-HT1D receptors have different pharmacological profiles as predicted from radioligand binding studies. Despite claims to the contrary, none of the tested compounds had actual selectivity for a given 5-HT1 receptor subtype. Of interest were the properties of several of these drugs, which behaved as agonists at some receptors and as antagonists at others (e. g. quipazine, 1-NP, PAPP and isamoltane).

Philippe Schoeffter

Papers

Selective inhibition of cyclic nucleotide phosphodiesterases of human, bovine and rat aorta.

Molecular Pharmacology of Somatostatin‐receptor Subtypes

5-HT receptors: subtypes and second messengers.

Molecular pharmacology of 5-HT1D recognition sites: radioligand binding studies in human, pig and calf brain membranes.

Interaction of arylpiperazines with 5-HT1A, 5-HT1B, 5-HT1C and 5-HT1D receptors: do discriminatory 5-HT1B receptor ligands exist?