5-HT4 receptors

Serotonin 4 receptors (5-HT(4)Rs) were discovered 15 years ago. They are coded by a very complex gene (700Kb, 38 exons) which generates eight carboxy-terminal variants (a, b, c, d, e, f, g, n). Their sequences differ after position L(358). Another variant is characterized by a 14 residue insertion within the extracellular loop 2. Highly selective potent 5-HT(4) receptor antagonists and partial agonists which cross the blood-brain barrier have been synthesized, but a specific full agonist for brain studies is still missing. Based on physiological and behavioral experiments, 5-HT(4)Rs may be targets to treat cognitive deficits, abdominal pain and feeding disorders. One 5-HT(4)R-directed drug (SL65.0155) is already in phase II to treat patients suffering from memory deficits or dementia.     

5-HT3 receptors

The 5-HT(3) receptor is a member of the Cys-loop family of ligand-gated ion channels. These receptors are located in both the peripheral and central nervous systems, where functional receptors are constructed from five subunits. These subunits may be the same (homopentameric 5-HT(3A) receptors) or different (heteropentameric receptors, usually comprising of 5-HT(3A) and 5-HT(3B) receptor subunits), with the latter having a number of distinct properties. The 5-HT(3) receptor binding site is comprised of six loops from two adjacent subunits, and critical ligand binding amino acids in these loops have been largely identified. There are a range of selective agonists and antagonists for these receptors and the pharmacophore is reasonably well understood. There are also a wide range of compounds that can modulate receptor activity. Studies have suggested many diverse potential disease targets that might be amenable to alleviation by 5-HT(3) receptor selective compounds but to date only two applications have been fully realised in the clinic: the treatment of emesis and irritable-bowel syndrome.     

5-HT1 receptors

Among the seven classes of serotonin (5-hydroxytryptamine, 5-HT) receptors which have been identified to date, the 5-HT(1) class is comprised of five receptor types, with the 5-HT(1A), 5-HT(1B) and 5-HT(1D) characterized by a high affinity for 5-carboxamido-tryptamine, the 5-HT(1E) and 5-HT(1F) characterized by a low affinity for this synthetic agonist, and all five having a nanomolar affinity for the endogenous indolamine ligand. The genes encoding 5-HT(1) receptors have been cloned in both human and rodents, allowing the demonstration that they all belong to the G-protein-coupled receptor super-family with the characteristic 7 hydrophobic (transmembrane) domain-containing amino acid sequence. All the 5-HT(1) receptor types actually interact with G alpha i/G alpha o proteins to inhibit adenylyl cyclase and modulate ionic effectors, i.e. potassium and/or calcium channels. Probes derived from the knowledge of amino acid sequence of the receptor proteins and of nucleotide sequence of their encoding mRNAs allowed the mapping of all the 5-HT(1) receptor types in the central nervous system and other tissues. For the last twenty years, both pharmacological investigations with selective agonists and antagonists and phenotypical characterization of knock-out mice have been especially informative regarding the physiological implications of 5-HT(1) receptor types. This research ends notably with the development of triptans, whose agonist activity at 5-HT(1B), 5-HT(1D) and 5-HT(1F) receptors underlies their remarkable efficacy as antimigraine drugs. Clear-cut evidence of the implication of 5-HT(1) receptors in anxiety- and depression-like behaviours and cognitive performances in rodents should hopefully promote research toward development of novel drugs with therapeutic potential in psychopathological and dementia-related diseases.     

5-HT3 receptors

5-HT(3)-receptor antagonists are highly selective competitive inhibitors of the 5-HT(3)-receptor with negligible affinity for other receptors. They are potent, rapidly absorbed and easily penetrate the blood-brain barrier; metabolized by the cytochrome P450-system with half-life varying from 3-10 hours. The compounds investigated so far do not modify normal behaviour in animals or man and are well tolerated over wide dose ranges, the most common side effects being headache or constipation. Clinical efficacy was first established in chemotherapy-induced emesis (and then in radiotherapy-induced and post-operative emesis), where 5-HT(3)-receptor antagonists set a new standard of antiemetic efficacy and tolerability. The 5-HT(3) receptor antagonists, via a central and / or peripheral action, have been shown to reduce secretion and motility in the gut and possess clinical utility in irritable bowel syndrome, and possibly other visceral pain disorders. Their value in fibromyalgia is being evaluated. In preclinical behavioural assays they induce effects consistent with anxiolysis, improved cognition, anti-dopaminergic activity and use in drug abuse and withdrawal. There is some evidence that ondansetron may reduce alcohol consumption in moderate alcohol abusers but overall, 5-HT(3) receptor antagonists seem to be of limited use in psychiatric disorders: where effects have been seen, they seem to be unusually sensitive to dose and stage of disease. Nevertheless, their antiemetic potential has been of great benefit to cancer patients and the possible extension of their use to bowel disorders may yet fulfil their initial exciting promise.     

5-HT2 receptors

5-HT(2) receptors are G-protein coupled receptors that currently comprise three subtypes: 5-HT(2A), 5-HT(2B) and 5-HT(2C) receptors. The subtypes are related in their molecular structure, amino acid sequence and signaling properties. 5-HT(2A) and 5-HT(2C) receptors have a widespread distribution and function in the central nervous system. 5-HT(2A)and 5-HT(2C) receptor antagonism is a property of certain antipsychotic and antidepressant drugs. 5-HT(2B) receptors have a restricted expression in the central nervous system. They have an important role in embryogenesis and in the periphery. In this article, selected aspects of 5-HT(2) receptor research are reviewed for each subtype under three main headings : (i) genes, protein structure and receptor signaling; (ii) receptor localization with emphasis on the CNS and (iii) compounds. The general discussion reflects on the reasons for the limited success in the clinic of 5-HT(2) receptor subtype selective drugs.     

5-HT7 receptors

Following the cloning of the 5-HT(7) receptor in 1993, studies to investigate 5-HT(7) receptor function in native tissues focused on identifying functional correlates that matched the pharmacological profile determined for the cloned receptor. Studies in peripheral tissues established that the 5-HT(7) receptor mediates the relaxation of smooth muscle, including the gastrointestinal tract and cardiovascular systems. Although a number of studies provided preliminary evidence for a role for the 5-HT(7) receptor in the circadian pacemaker function of the suprachiasmatic nucleus (SCN), additional studies to investigate 5-HT(7) receptor function in other brain regions have, until recently, been hindered by the absence of 5-HT(7) receptor-selective ligands. More recently, a number of 5-HT(7) receptor-selective antagonists including, SB-269970-A and SB-656104-A have been developed. Studies utilising these compounds suggest that the 5-HT(7) receptor modulates neuronal function in a number of brain areas including the hippocampus and thalamus. In turn, these findings suggest that 5-HT(7) receptor-selective ligands might prove therapeutically useful for the treatment of psychiatric disorders. In this respect there is increasing evidence to suggest that the 5-HT(7) receptor plays a role in the control of both circadian rhythms and sleep and might therefore represent a therapeutic target for the treatment of those disorders in which disturbances in circadian rhythms and sleep architecture are thought to be contributory factors. Furthermore, there is evidence to suggest that the receptor may play a role in other CNS disorders including, anxiety, cognitive disturbances and also migraine probably via both peripheral and central mechanisms. Although further studies are required to confirm the potential role of the receptor in such disorders, findings to date suggest there are exciting opportunities for the development of novel therapeutic agents acting either selectively at the 5-HT(7) receptor or whose profile of action includes an interaction with this receptor.     

Piglet sinoatrial 5-HT receptors resemble human atrial 5-HT4-like receptors

Summary 5-Hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT) and the gastrointestinal kinetic benzamides renzapride and cisapride caused tachycardia in spontaneously beating right atria of piglet in the presence of 400 nmol/l(±)-propranolol and 6 mol/l cocaine. The maximum tachycardia caused by agonists, compared to that evoked by 200 mol/l(–)-isoprenaline, was 63% for 5-HT, 50% for 5-CT, 50% for renzapride and 28% for cisapride. The rank order of potency was 5-HT > renzapride > cisapride > 5-CT. The effects of the agonists, but not those of (–)-isoprenaline, were antagonised by 3-tropanyl-1H-indole-3-carboxylic acid (ICS 205930); the pK_B of ICS 205930 (vs 5-HT) was 6.9. These characteristics suggest that piglet sinoatrial 5-HT receptors are similar to so-called 5-HT₄ receptors previously described in mouse colliculi neurons. Piglet sinoatrial 5-HT₄-like receptors resemble the human atrial 5-HT receptors that mediate positive isotropic effects of 5-HT.Send of fprint requests to A. J. Kaumann at the above address     

5-羟色胺及其受体与阿尔茨海默病的关系

5-羟色胺(5-hydroxytryptamine,5-HT)是一种重要的单胺类神经递质,其功能异常可能与包括阿尔茨海默病(Alzheimer disease,AD)在内的神经系统疾病密切相关。许多研究结果表明,5-HT受体在数量和分布等方面所发生的变化与AD的发生发展有关。本文就近年来5-HT与AD之间关系的研究进展进行综述,为药物治疗AD提供新思路。     

Serotonin 5-HT3, 5-HT4, and 5-HT-M receptors

Serotonin (5-hydroxytryptamine [5-HT]) receptors can be classified either pharmacologically, into 5-HT1, 5-HT2, 5-HT3 and 5-HT4 receptors, or functionally, into G-protein-coupled receptors (5-HT1, 5-HT2, and 5-HT4) and ligand-gated ion channels (5-HT3). This article concentrates on the pharmacology, distribution, receptor-effector coupling, possible subtypes, and species differences of 5-HT3 receptors, which are equivalent to Gaddum and Picarelli's 5-HT-M receptor. Also presented here are some of the prominent features of the recently characterized 5-HT4 receptor. Although pharmacologic similarities have been suggested between 5-HT-M and 5-HT4 receptors (some potent 5-HT3 antagonists are active, with lower potency, at 5-HT4 receptors), it is clear that 5-HT4 receptors are different from 5-HT-M receptors.     

Discriminating between 5-HT3A and 5-HT3AB receptors

The 5-HT3B subunit was first cloned in 1999, and co-expression with the 5-HT3A subunit results in heteromeric 5-HT₃AB receptors that are functionally distinct from homomeric 5-HT₃A receptors. The affinities of competitive ligands at the two receptor subtypes are usually similar, but those of non-competitive antagonists that bind in the pore often differ. A competitive ligand and allosteric modulator that distinguishes 5-HT₃A from 5-HT₃AB receptors has recently been described, and the number of non-competitive antagonists identified with this ability has increased in recent years. In this review, we discuss the differences between 5-HT₃A and 5-HT₃AB receptors and describe the possible sites of action of compounds that can distinguish between them.     

Molecular biology of 5-HT receptors

Within the past six months, isolation of cDNA or genomic clones has been reported for three 5-HT receptors, the 5-HT1C, 5-HT1A and 5-HT2 subtypes. As members of the G protein receptor superfamily, all three 5-HT receptor clones encode single-subunit proteins containing approximately 450 amino acids arrayed as seven interconnected transmembrane segments. Comparisons of 5-HT receptor sequence data with data from other G protein receptors provide suggestions for which amino acids may be involved in the binding of 5-HT and 5-HT antagonists to these receptors, and for certain key amino acids which may help confer 5-HT properties on these receptors. These rapid advances in our molecular understanding of 5-HT receptors also have significant implications for the ongoing debate over 5-HT receptor classification. In this article, Paul Hartig assembles the available molecular data and proposes a speculative model for the structure of G protein-coupled 5-HT receptors.     

5-HT receptors on identified Lymnaea neurones in culture: Pharmacological characterization of 5-HT2 receptors

1. Pressure ejection techniques were used to investigate the identity of receptors mediating 5-HT (5-Hydroxytryptamine) effects on the serotonin-containing cerebral giant cells (CGCs) of the cerebral ganglia and some of their follower motorneurones from the buccal ganglia of Lymnaea stagnalis in culture.2. The vertebrate 5-HT₂ receptor agonist α-methylserotonin maleate (10⁻⁴ M), inhibited most of the neurones inhibited by 5-HT (10⁻³ M). Others were excited by both agonists. In cells where 5-HT failed to evoke any effects, the 5-HT₂ agonist also lacked an effect.3. Bath application of the 5-HT₂ receptor antagonists ketanserin and methysergide (10⁻⁴ M), not only blocked spike generation, but also reduced both the excitatory and inhibitory responses to both 5-HT and α-methylserotonin maleate, while the 5-HT₃ antagonist MDL 72222 (10⁻⁴ M) failed to block α-methylserotonin maleate effects.4. At 10⁻³ M, α-methylserotonin maleate increased the amplitudes of the hyperpolarizing responses in a dose-dependent manner. These responses were blocked by ketanserin (10⁻⁴ M).5. The above results suggest that 5-HT₂ receptors are involved in the responses of the CGCs and the buccal motorneurones to 5-HT in Lymnaea stagnalis. The pharmacological characterization of these receptors indicates that the compounds that interact with the 5-HT₂ receptors in mammals also interact with the 5-HT₂ receptors in molluscs.     

5-HT receptors on identified Lymnaea neurones in culture: Pharmacological characterization of 5-HT3 receptors

• 1.1. The selective agonist, 1-(m-chlorophenyl)-biguanide (m-CPBG) and antagonist, 3-tropanyl-3,5-dichlorobenzoate (MDL 72222) were used to characterize the 5-HT₃ receptors in cultured identified neurones; the serotonin-containing cerebral giant cells (CGCs) and some follower neurones in the buccal ganglia of Lymnaea stagnalis.
• 2.2. 5-HT and its agonists were pressure ejected, while the 5-HT antagonists were bath applied.
• 3.3. Although m-CPBG evoked mostly depolarizing responses, hyperpolarizing responses were sometimes evoked.
• 4.4. At 10⁻⁴ M, m-CPBG failed to mimic the responses of 5-HT, but at a concentration higher. 10⁻³ M, pressure-ejected m-CPBG mimicked most 5-HT responses.
• 5.5. The 5-HT₂ antagonist ketanserin failed to block the m-CPBG-evoked responses, whilst partially blocking the 5-HT responses.
• 6.6. These results suggest the presence of 5-HT₃ receptors similar to those found in mammalian neurones, and that multiple subtypes of these receptors may be present in Lymnaea neurones.

     

In the search for selective ligands of 5-HT5, 5-HT6 and 5-HT7 serotonin receptors

In recent years much attention has been focused on the functional importance of 5-HT5, 5-HT6 and 5-HT7 receptors in the pathogenesis of neuropsychiatric and other diseases. In this connection, intensive studies with ligands of these receptors are currently in progress. Recognition of the structural characteristics responsible for the binding of a ligand molecule to an appropriate receptor, and development of an active complex have reached an advanced stage in the search for selective compounds. This review was undertaken to summarize the results of structure-activity relationship studies with ligands of 5-HT5, 5-HT6 and 5-HT7 receptors. Additionally, some data on localization, pharmacological properties and the functional role of those receptors were reported.     

Central administration of 5-HT activates 5-HT1A receptors to cause sympathoexcitation and 5-HT2/5-HT1C receptors to release vasopressin in anaesthetized rats.

1. The effects of intracerebroventricular injections to the right lateral ventricle (i.c.v.) of 5-hydroxytryptamine (5-HT, 40 and 120 nmol kg-1), N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT; 3 nmol kg-1), 5-carboxamidotryptamine (5-CT; 3 nmol kg-1), 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT; 3, 40 and 120 nmol kg-1) and 1-(2,5-di-methoxy-4-iodophenyl)-2-aminopropane (DOI; 40 and 120 nmol kg-1) on renal sympathetic nerve activity, blood pressure, heart rate and phrenic nerve activity were investigated in normotensive rats anaesthetized with alpha-chloralose. 2. 5-HT caused a long lasting pressor response which was associated with an initial bradycardia and renal sympathoinhibition followed by a tachycardia and renal sympathoexcitation. Pretreatment with the 5-HT2/5-HT1C receptor antagonists, cinanserin (300 nmol kg-1, i.c.v.) or LY 53857 (300 nmol kg-1, i.c.v.) reversed the initial bradycardia and sympathoinhibition to tachycardia and sympathoexcitation. Combined pretreatment with LY 53857 (300 nmol kg-1, i.c.v.) and the 5-HT1A antagonist, spiroxatrine (300 nmol kg-1, i.c.v.), blocked the effects of 5-HT on all the above variables. 3. Pretreatment with the vasopressin V1-receptor antagonist, beta-mercapto-beta,beta-cyclopentamethylene-propionyl1, O-Me-Tyr2, Arg8-vasopressin [(d(CH2)5Tyr(Me)AVP, 10 micrograms kg-1, i.v.] did not affect the magnitude but reduced the duration of the pressor response produced by i.c.v. 5-HT and reversed the initial bradycardia and renal sympathoinhibition to tachycardia and sympathoexcitation. 4. 1-(2,5-Di-methoxy-4-iodophenyl)-2-aminopropane (DOI) caused a pressor effect which was associated with a bradycardia and sympathoinhibition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neuronal 5-HT receptors in the periphery

5-Hydroxytryptamine (5-HT) induces responses in neurones from all branches of the mammalian peripheral nervous system. Responses may be excitatory or inhibitory and are mediated through at least four distinct receptor sites.One receptor mediates excitation in motoneurones and preganglionic sympathetic neurones and can be designated a D (or possibly 5-HT₂) receptor since “classical” antagonists such as methysergide, metergoline or cinanserin are potent and selective antagonists at this site. A second receptor mediating neuronal excitation can be positively identified on the basis of susceptibility to blockade by small concentrations of 1αH,3α,5αH-tropan-3-yl-3,5-dichlorobenzoate (MDL 72222) and the weak or negligible affinity, relative to 5-HT, of certain agonists such as 5-methoxytryptamine. Such sites mediate depolarization of sympathetic and parasympathetic neurones and excitation of both the cell bodies and terminals of primary afferent fibres. A third receptor, mediating neuronal excitation, is the classical M-receptor of Gaddum and Picarelli, at this stage clearly identified only on postganglionic parasympathetic neurones of the guinea-pig myenteric plexus. These sites can be differentiated from other excitatory 5-HT receptors since MDL 72222 is neither potent nor selective as an antagonist and 5-methoxytryptamine approaches the potency of 5-HT as an agonist. (3α-Homotropanyl)-1-methyl-5-fluoro-indole-3-carboxylic acid is a potent, surmountable antagonist of 5-HT at the M-receptor of the ileum, but is non-selective.Neuronal inhibitory responses have been observed using electrophysiological techniques or by monitoring the decrease in depolarization-evoked release of transmitter in enteric, parasympathetic and sympathetic neurones. Largely negative results, using selective agonists and antagonists, allow the receptor(s) mediating inhibition to be clearly differentiated from the three neuronal excitatory receptors for 5-HT. Comparison of relative potencies of agonists suggests similarities with the 5-HT₁ recognition site of the central nervous system; no selective antagonist has yet emerged to permit their positive identification.     

Cerebral cortical 5-HT1 and 5-HT2 receptors of morphine tolerant-dependent rats

The effect of chronic administration of morphine to rats on 5-HT₁ and 5-HT₂ receptors in the cerebral cortex was determined. Male Sprague-Dawley rats were implanted subcutaneously with 6 pellets of morphine (each containing 75 mg of morphine free base) during a 7 day period. Animals which served as controls were implanted with placebo pellets. The procedure for implantation of pellets produced a high degree of tolerance to and physical dependence on morphine in the rat. The tolerance to the analgesic and hyperthermic effects of morphine was demonstrated by decreased responses in the rats implanted with morphine pellet in comparison to the placebo-treated controls. The physical dependence was shown by the greater weight loss after removal of the pellet in the rats implanted with morphine pellets when compared to rats implanted with placebo pellets. The pellets were removed (withdrawn) and, after 6–8 h, the rats were sacrificed and the cerebral cortex was isolated. In another experiment the pellets were left in place (tolerant-dependent rats). The 5-HT₁ and 5-HT₂ receptors were characterized by using [^3H]5-HT and [^3H]spiroperidol as the ligands and unlabelled 5-HT and ketanserin, respectively, to determine non-specific binding. The [^3H]5-HT bound to 5-HT₁ receptors on membranes from the cerebral cortex of rats implanted with placebo pellets, at a single high affinity site, with a B_max of 102 ± 10 fmol/mg protein and a K_d of 6.02 ± 0.98 nM. Implantation of morphine pellets, followed by removal of the pellets resulted in a 50% increase in the B_max value of [^3H]5-HT but the K_d values did not change. In rats from which the pellets were not removed, the B_max and K_d values of [^3H]5-HT in placebo- and morphine-treated groups did not differ. [^3H]Spiroperidol bound to 5-HT₂ receptors on cortical membranes of rats implanted with placebo pellet at a single high affinity site with B_max and K_d values of 131 ± 5 fmol/mg protein and 0.22 ± 0.01 nM, respectively. The implantation of pellets of morphine followed by removal of the pellets did not alter the characteristics of 5-HT₂, receptors, however in rats with the pellets in place, the B_max for 5-HT₂ receptors in placebo- and morphine-treated groups did not differ but the K_d values were much smaller in morphine-treated rats compared to rats implanted with placebo pellets. It is concluded that the development of tolerance to, and physical dependence on, morphine by implantation of pellets results in up-regulation of 5-HT₂ receptors whereas in morphine-abstinent rats there is a selective up-regulation of 5-HT₁ receptors on the membranes in the cerebral cortex.