Derek T. Chalmers

Regulation of Serotonin1A, Glucocorticoid, and Mineralocorticoid Receptor in Rat and Human Hippocampus: Implications for the Neurobiology of Depression

BACKGROUND Disturbances of the limbic-hypothalamic-pituitary-adrenal axis and the serotonin system are commonly found in depressive illness. Studying the effect of stress on these two neurobiological systems may give us important clues into the pathophysiology of affective illness and help us understand how stress and mood disorders are related. METHODS We studied the effect of chronic unpredictable stress and antidepressant treatment on serotonin 1A (5-HT1A), glucocorticoid (GR), anti mineralocorticoid (MR) receptor levels in rat hippocampus, using in situ hybridization and receptor autoradiography. We also used in situ hybridization to quantify hippocampal 5-HT1A, GR, and MR messenger (mRNA) levels in a small group of suicide victims with a history of depression, compared to matched controls (n = 6). RESULTS We found that rats subjected to chronic unpredictable stress showed a significant elevation of basal plasma corticosterone compared to nonstressed rats. Chronic stress also caused a decrease in 5-HT1A mRNA and binding in the hippocampus. In addition, chronic stress produced alterations on the MR/GR mRNA ratio in this same region. The decreases in 5-HT1A mRNA and binding, as well as the MR/GR alterations, were prevented in animals that received imipramine or desipramine antidepressant treatment. Zimelidine was unable to reverse stress-induced increases in corticosterone, and was only partially successful in preventing the stress-induced receptor changes in the hippocampus. Suicide victims with a history of depression showed changes that were very similar to the changes found in chronic stress. CONCLUSIONS Alterations in hippocampal 5-HT1A levels and in the MR/GR balance may be one of the mechanisms by which stress may trigger and/or maintain depressive episodes.

Corticotrophin-releasing factor receptors: from molecular biology to drug design

Corticotrophin-releasing factor (CRF) acts within both the brain and the periphery to coordinate the overall response of the body to stress. The involvement of the CRF systems in a variety of both CNS and peripheral disease states has stimulated great interest in this peptide as a potential site of therapeutic intervention. The recent cloning of multiple CRF receptor subtypes has precipitated a new era in CRF research that has allowed precise molecular, pharmacological and anatomical examination of mammalian CRF receptors. In this article, Derek Chalmers and colleagues highlight the major differences between the two classes of CRF receptors, CRF1 and CRF2, and a functionally related CRF-binding protein, and discuss the relevance of these sites to the ongoing development of CRF-based therapeutics.

Lorcaserin, a Novel Selective Human 5-Hydroxytryptamine2C Agonist: in Vitro and in Vivo Pharmacological Characterization

5-Hydroxytryptamine (5-HT)2C receptor agonists hold promise for the treatment of obesity. In this study, we describe the in vitro and in vivo characteristics of lorcaserin [(1R)-8-chloro-2,3,4,5-tetrahydro-1-methyl-1H-3 benzazepine], a selective, high affinity 5-HT2C full agonist. Lorcaserin bound to human and rat 5-HT2C receptors with high affinity (Ki = 15 ± 1 nM, 29 ± 7 nM, respectively), and it was a full agonist for the human 5-HT2C receptor in a functional inositol phosphate accumulation assay, with 18- and 104-fold selectivity over 5-HT2A and 5-HT2B receptors, respectively. Lorcaserin was also highly selective for human 5-HT2C over other human 5-HT receptors (5-HT1A, 5-HT3, 5-HT4C, 5-HT55A, 5-HT6, and 5-HT7), in addition to a panel of 67 other G protein-coupled receptors and ion channels. Lorcaserin did not compete for binding of ligands to serotonin, dopamine, and norepinephrine transporters, and it did not alter their function in vitro. Behavioral observations indicated that unlike the 5-HT2A agonist (±)-1-(2,5-dimethoxy-4-phenyl)-2-aminopropane, lorcaserin did not induce behavioral changes indicative of functional 5-HT2A agonist activity. Acutely, lorcaserin reduced food intake in rats, an effect that was reversed by pretreatment with the 5-HT2C-selective antagonist 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-yl-carbamoyl]indoline (SB242,084) but not the 5-HT2A antagonist (R)-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol (MDL 100,907), demonstrating mediation by the 5-HT2C receptor. Chronic daily treatment with lorcaserin to rats maintained on a high fat diet produced dose-dependent reductions in food intake and body weight gain that were maintained during the 4-week study. Upon discontinuation, body weight returned to control levels. These data demonstrate lorcaserin to be a potent, selective, and efficacious agonist of the 5-HT2C receptor, with potential for the treatment of obesity.

The use of constitutively active GPCRs in drug discovery and functional genomics.

The complete sequencing of the human genome has afforded researchers the opportunity to identify novel G-protein-coupled receptors (GPCRs) that are expressed in human tissues. The successful identification of hundreds of GPCRs represents the single greatest opportunity for novel drug development today. However, the lack of identified ligands for these GPCRs has limited their utility for traditional drug discovery approaches that focus on ligand-based assay methods to discover and pharmacologically characterize drug candidates. Here, we review the use of constitutively activated GPCRs in the discovery pathway, both as a means to overcome the limitations of traditional drug discovery at novel GPCRs and as a tool to investigate the functionality of these receptors.

Regulation of 5-HT Receptors and the Hypothalamic-Pituitary-Adrenal Axis

ABSTRACT: Disturbances in the serotonin (5‐HT) system is the neurobiological abnormality most consistently associated with suicide. Hyperactivity of the hypothalmic‐pituitary‐adrenal (HPA) axis is also described in suicide victims. The HPA axis is the classical neuroendocrine system that responds to stress and whose final product, corticosteroids, targets components of the limbic system, particularly the hippocampus. We will review resulsts from animal studies that point to the possibility that many of the 5‐HT receptor changes observed in suicide brains may be a result of, or may be worsened by, the HPA overactivity that may be present in some suicide victims. The results of these studies can be summarized as follows: (1) chronic unpredictable stress produces high corticosteroid levels in rats; (2) chronic stress also results in changes in specific 5‐HT receptors (increases in cortical 5‐HT2A and decreases in hipocampal 5‐HT1A and 5‐HT1B); (3) chronic antidepressant administration prevents many of the 5‐HT receptor changes observed after stress; and (4) chronic antidepressant administration reverses the overactivity of the HPA axis. If indeed 5‐HT receptors have a partial role in controlling affective states, then their modulation by corticosteroids provides a potential mechanism by which these hormones may regulate mood. These data may also provide a biological understanding of how stressful events may increase the risk for suicide in vulnerable individuals and may help us elucidate the neurobiological underpinnings of treatment resistance.

Global analysis of G‐protein‐coupled receptor signaling in human tissues

The G‐protein‐coupled receptor (GPCR) family mediates a host of cell–cell communications upon activation by diverse ligands. Numerous GPCRs have been shown to display anatomically selective patterns of gene expression, however, our understanding of the complexity of GPCR signaling within human tissues remains unclear. In an effort to characterize global patterns of GPCR signaling in the human body, microarray analysis was performed on a large panel of tissues to monitor the gene expression levels of the receptors as well as related signaling and regulatory molecules. Analysis of the data revealed complex signaling networks in many tissue types, with tissue‐specific patterns of gene expression observed for the majority of the receptors and a number of components and regulators of GPCR signaling.