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Among animals with nervous systems, serotonin (5-hydroxytryptamine; 5-HT) is a ubiquitous neurotransmitter, and numerous classes and subclasses of G protein-coupled 5-HT receptors have evolved to transduce extracellular 5-HT signals to the intracellular milieu. In this chapter, we summarize naturally occurring variation in serotonin receptor sequences. These sequences vary by species and by class and subclass and are further modified from their canonical sequences by RNA editing, alternative splicing, and the existence of single-nucleotide polymorphisms. By the presence of 5-HT receptors in such relatively simple organisms as , it can be inferred that serotonergic signaling as a means of intracellular communication arose fairly early in evolutionary history. The multiple subclasses of 5-HT receptors and the various means to further modify receptor sequences, such as splicing and editing, presumably point to a biological requirement for very delicate “fine-tuning” of serotonergic signaling. How this fine-tuning is accomplished is likely to occupy and intrigue biologists for many years.

The purpose of this review is to examine experimental information concerning the structure and function of the G protein-coupled serotonin receptors in the three-dimensional context provided by the structure of rhodopsin. A critical examination of the suitability of rhodopsin as a template for serotonin receptor modeling from the level of sequence alignment to interpretation of biochemical experiments of relevance to the issues of structure-function relationships is presented.

This chapter first describes the structural changes involved in genetic polymorphisms, mRNA editing, and alternative mRNA splicing of 5-hydroxytryptamine (5-HT) receptors. These structural changes lead to modifications in the production and characteristics of 5-HT receptors and affect protein expression. Functionally, they affect radioligand binding, signal transduction, and receptor sensitivity, thus affecting interindividual variation in responses to therapeutic agents, particularly antipsychotics and antidepressants. Studies indicate that genetic polymorphic and posttranscriptional modifications of 5-HT receptor structure contribute also to pathological processes related to irritable bowel syndrome, cardiopulmonary problems, psychiatric illness (i.e., schizophrenia and mood disorders), Alzheimer’s disease, problems involving increased food and alcohol intake, and behavioral problems such as impulsivity, self-harm, and aggression. In the second part of this chapter, the 5HT, 5HT, and 5HT receptors are used to illustrate the structural changes involved in genetic polymorphisms, mRNA editing, and alternative mRNA splicing along with their functional consequences and pathological implications. Finally, this chapter describes the most salient posttranslational modifications of the 5HT receptors, which involve the chemical modification of the protein after its translation.

Many of the ligands in use today to investigate serotonin receptors and serotonin receptor pharmacology were serendipitous discoveries; this includes ligands that are commonly regarded as being “selective” for a given population of serotonin receptors. Nevertheless, there still remains a number of serotonin receptor types that lack a truly selective agonist and/or antagonist. Over the past 20 yr, there have been various attempts to rationally develop ligands with greater selectivity, or selective ligands for serotonin receptor types for which such agents were lacking. To this end, we describe some of our efforts to develop selective serotonergic agents by presenting a series of case studies. Several different strategies have been employed to achieve this goal. In particular, the “deconstruction-reconstruction-elaboration” approach is shown to be useful for aiding the development of selective ligands where a lead structure is already known, and the utility of the “standard series” approach is illustrated where a lead structure is not known. The discussion is focused on these and other methods that could have general applicability for the development of other selective serotonergic and nonserotonergic agents.

The purpose of this chapter is to summarize the main features of various signal transduction pathways utilized by the G protein-coupled 5-hydroxytryptamine (5-HT) receptors. Herein, we discuss major and secondary signals emanating from the major subtypes of 5-HT receptors (5-HT, 5-HT,5-HT,5-HT,5-HT, and 5-HT), as well as unique aspects of signaling for some of the subtypes. The 5-HT receptors, which are 5-HT gated ion channels, will not be discussed in this chapter. This chapter highlights the complexity of signaling from the diverse G protein-coupled 5-HT receptors and underscores the fundamental importance of understanding the nuances of the determinants of signaling specificity for these receptors.

For decades, pharmacologists have maintained that drugs have two properties: affinity, which describes a drug’s ability to bind to a receptor, and intrinsic efficacy, which describes the ability of a drug to change the behavior of a receptor toward cellular signaling machinery. Both affinity and intrinsic efficacy were defined to be ligand constants independent of response mechanisms and, thus, were of great predictive value for drug discovery efforts. Recently, clear evidence from many seven-transmembrane-spanning receptor systems has accumulated to indicate that ligands might not obey the tenets of classical receptor theory. In fact, intrinsic efficacy appears to be variable (changes with cell phenotype and physiological state) and dependent on cellular signaling machinery (response dependent), and ligands appear to have multiple intrinsic efficacies. Drugs can no longer be labeled as agonists (partial or full), inverse agonists (partial or full), or antagonists without reference to the receptor system (receptor and signaling pathway/response) and experimental conditions used. This newly discovered ligand behavior has been given many names by different groups: “agonist-directed trafficking of receptor stimulus,” “functional selectivity,” “stimulus trafficking,” and “biased agonism,” to name a few. Although the pharmacological actions of drugs have become considerably more complex, our pharmacological treasure chest might be much richer than previously thought, possibly heralding new and improved pharmacotherapy. In this chapter, we discuss the evidence for agonist-directed trafficking of serotonin receptor stimulus.

G protein-coupled receptors (GPCRs) not only interact with heterotrimeric G proteins but also with accessory proteins, called GPCR-interacting proteins (GIPs). GIPs are implicated in GPCR targeting to specific cellular compartments, in their assembling into large functional complexes called “receptosomes,” in their trafficking to and from the plasma membrane, as well as in the fine-tuning of their signaling properties. Here, we describe “receptosomes“ associated with the C-terminal tails of 5-hydroxytryptamines 5-HT, 5-HT, as well as 5-HT and 5-HT receptors. The three last residues of these receptor C-termini are canonical PDZ ligands interacting with type I PDZ domain-containing proteins (5-HT, 5-HT, 5-HT tails) and type II (5-HT). The entire C-terminal tails fused to glutathione-S-transferase or synthetic peptides encompassing the last 14 C-terminal residues of the receptors were used as baits to fish out GIPs from mouse brain. Controls were made with mutant bait (mutated in the PDZ ligand). Proteins, which were specifically retained on native PDZ ligand-containing peptides, were separated on two-dimensional gels and identified by MALDI-TOF mass spectrometry or immunoblotting. Ten and seven PDZ domain-containing proteins were found to bind to the 5-HT and 5-HT receptors, respectively. The sequences of the C-terminal PDZ ligands of 5-HT and 5-HT receptors are very similar (SSV and SCV, respectively). If some of the PDZ domain-containing proteins associated with these receptors were identical (ARIP-1/MAGI-2, SAP97, PSD-95, Dlgh3/MPP3), others were clearly different. The 5-HT but not the 5-HT receptor interacted with the SAP102 and Veli3/CASK/Mint1 ternary complex, whereas the 5-HT but not the 5-HT receptors interacted with CIPP. MUPP1, which was found to interact with the 5-HT receptor in a two-hybrid screen, was also fished out by the 5-HT and 5-HT C-termini. A few other non-PDZ domain-containing proteins were found in these “receptosomes.” Electron microscopy (EM) studies of 5-HT and 5-HT receptors and some of their interacting proteins led to the proposition of a presynaptic and postsynaptic localization of 5-HT receptors and a preferential postsynaptic localization of 5-HT receptors. In a similar manner, we identified 10 and 3 proteins that interacted specifically with the 5-HT and 5-HT receptor splice variants, respectively. Most of them are PDZ proteins. Among them, NHERF recruited 5-HT receptors in microvilli, where they localized with activated ezrin, consistent with a role of 5-HT receptors in cytoskeleton remodeling. The same variant interacted with both the constitutive and the inducible (upon metham-phetamine treatment) forms of SNX27 (SNX27a and SNX27b, respectively). SNX27a redirected part of the 5-HT receptors to early endosomes.

Dysfunction in the serotonergic system contributes to the etiology and pathophysiology of a variety of neuropsychiatric and systemic disorders, and 5-hydroxytryptamine(5-HT) receptors as a group represent important therapeutic targets for many of these debilitating diseases. Encompassing a diverse group of proteins that interact with 5-HT receptors, 5-HT receptor-associated proteins (FRAPs) coordinate receptor targeting to the appropriate subcellular compartments, organization of networks of signaling and structural proteins (known as receptosomes), and calibration of receptor-mediated signal transduction, including desensitization, resensitization, and crosstalk with other signaling pathways. The study of FRAPs opens a decidedly wide arena for novel pharmaceutical research approaches. We will provide an overview of the burgeoning field of FRAPs, with a focus on their role in modulating signal transduction.

Immunocytochemistry allows for a precise localization of neurotransmitter receptors in tissues and cells. This review summarizes much of the available data on the cellular and subcellular distribution of serotonin (5-hydroxytryptamine [5-HT]) receptors in the mammalian central nervous system. Among fourteen 5-HT receptor types, all cloned and sequenced, only a few have yet been amenable to detailed immunocytochemical visualization, not only at the light microscopic but particularly at the electron microscopic level. The 5-HT and 5-HT receptors have been the most thoroughly investigated and provide a meaningful demonstration of the wealth of information to be gained from this methodological approach, not only in terms of anatomical and cytological localization, and thus physiological role and eventual implication in health and disease, but also of functional properties and drug effects.

Serotonin (5-hydroxytryptamine [5-HT]) produces its effects through a variety of membrane-bound receptors, located in the central and peripheral nervous systems and in non-neuronal tissues in the gastrointestinal tract, cardiovascular system, and blood. The knowledge of the anatomical localization of the receptors is an important step forward for the understanding of their function. Several techniques allow the microscopic visualization of the sites of expression of these receptors in the tissues. In this review, we will attempt to summarize the available information on the neuroanatomical organization of each one of the 5-HT receptor subtypes.