Tom I. Bonner

International Union of Pharmacology. XXVII. Classification of Cannabinoid Receptors

Two types of cannabinoid receptor have been discovered so far, CB1 (2.1: CBD:1:CB1:), cloned in 1990, and CB2(2.1:CBD:2:CB2:), cloned in 1993. Distinction between these receptors is based on differences in their predicted amino acid sequence, signaling mechanisms, tissue distribution, and sensitivity to certain potent agonists and antagonists that show marked selectivity for one or the other receptor type. Cannabinoid receptors CB1 and CB2 exhibit 48% amino acid sequence identity. Both receptor types are coupled through G proteins to adenylyl cyclase and mitogen-activated protein kinase. CB1 receptors are also coupled through G proteins to several types of calcium and potassium channels. These receptors exist primarily on central and peripheral neurons, one of their functions being to inhibit neurotransmitter release. Indeed, endogenous CB1 agonists probably serve as retrograde synaptic messengers. CB2 receptors are present mainly on immune cells. Such cells also express CB1receptors, albeit to a lesser extent, with both receptor types exerting a broad spectrum of immune effects that includes modulation of cytokine release. Of several endogenous agonists for cannabinoid receptors identified thus far, the most notable are arachidonoylethanolamide, 2-arachidonoylglycerol, and 2-arachidonylglyceryl ether. It is unclear whether these eicosanoid molecules are the only, or primary, endogenous agonists. Hence, we consider it premature to rename cannabinoid receptors after an endogenous agonist as is recommended by the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. Although pharmacological evidence for the existence of additional types of cannabinoid receptor is emerging, other kinds of supporting evidence are still lacking.

Cloning and expression of the human and rat m5 muscarinic acetylcholine receptor genes.

The human and rat genes for a fifth muscarinic receptor have been cloned and expressed in mammalian cells. The 532 amino acid human protein has 89% sequence identity to the 531 amino acid rat protein and is most closely related to the m3 receptor. Both proteins are encoded by single exons. The receptor has intermediate affinity for pirenzepine and low affinity for AF-DX 116, and it increases metabolism of phosphatidylinositol when stimulated with carbachol. Expression of mRNA has yet to be observed in brain or selected peripheral tissues, suggesting that either it is substantially less abundant than m1-m4 or its distribution is quite different.

Immunomodulation by cannabinoids is absent in mice deficient for the cannabinoid CB2 receptor

Cannabinoids have immunomodulatory as well as psychoactive effects. Because the central cannabinoid receptor (cannabinoid CB(1) receptor) is highly expressed in many neuronal tissues and the peripheral cannabinoid receptor (cannabinoid CB(2) receptor) is highly expressed in immune cells, it has been suggested that the central nervous system effects of cannabinoids are mediated by cannabinoid CB(1) receptors and that the immune effects are mediated by cannabinoid CB(2) receptors. To test this hypothesis, we have generated the first mouse strain with a targeted mutation in the cannabinoid CB(2) receptor gene. Binding studies using the highly specific synthetic cannabinoid receptor agonist (-)-cis-3-¿2-Hydroxy-4-(1, 1-dimethylheptyl)phenyl-trans-4-(3-hydroxypropyl)cyclohexanol (¿3HCP 55,940) revealed no residual cannabinoid binding sites in the spleen of the cannabinoid CB(2) receptor knockout mice, while binding in the central nervous system was unchanged. Cannabinoid CB(2) receptor knockout mice, which appear healthy, are fertile and care for their offspring. Fluorescence activated cell sorting (FACS) analysis showed no differences in immune cell populations between cannabinoid CB(2) receptor knockout and wildtype mice. We investigated the immunomodulatory effects of cannabinoids in cannabinoid CB(2) receptor deficient mice using a T cell co-stimulation assay. Delta(9)Tetrahydrocannabinol inhibits helper T cell activation through macrophages derived from wild type, but not from knockout mice, thus indicating that this effect is mediated by the cannabinoid CB(2) receptor. In contrast, central nervous system effects of cannabinoids were not altered in these mice. Our results suggest that cannabinoid CB(2) receptor-specific ligands may be clinically useful in the modulation of macrophage immune function while exhibiting no central nervous system activity. Furthermore, we conclude that the cannabinoid CB(2) receptor knockout mouse is a useful animal model in which to study the role of the cannabinoid system in immunoregulation.

The molecular basis of muscarinic receptor diversity

The cloning of cDNAs and genes for five different muscarinic acetylcholine receptors provides a new basis for characterizing muscarinic receptor function. Studies of the cloned receptors when introduced into cells not expressing endogenous receptors have allowed the initial identification of two classes of functional response. The m1, m3 and m5 receptors belong to a class characterized by agonist-induced stimulation of phosphatidylinositol metabolism and are structurally more related to each other than they are to the m2 and m4 receptors, which belong to a class associated with agonist-induced inhibition of adenylate cyclase. While functional differences within these classes may yet be found, it appears likely that much of the difference between functionally similar receptors will be found to lie in their regulation.

International Union of Pharmacology. XLVI. G Protein-Coupled Receptor List

NC-IUPHAR (International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification) and its subcommittees provide authoritative reports on the nomenclature and pharmacology of G protein-coupled receptors (GPCRs) that summarize their structure, pharmacology, and roles in physiology and pathology. These reports are published in Pharmacological Reviews (http://www.iuphar.org/nciuphar_arti.html) and through the International Union of Pharmacology (IUPHAR) Receptor Database web site (http://www.iuphar-db.org/iuphar-rd). The essentially complete sequencing of the human genome has allowed the cataloging of all of the human gene sequences potentially encoding GPCRs. The IUPHAR Receptor List (http://www.iuphar-db.org/iuphar-rd/list/index.htm) presents this catalog giving IUPHAR-approved nomenclature (where available), known ligands, and gene names for all of these potential receptors (excluding sensory receptors and pseudogenes) together with links to curated sequence, descriptive information, and additional links in the Entrez Gene database (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=gene). This list is a major new initiative of NC-IUPHAR that, through continuing curation, defines the target of our ongoing receptor classification and invites further input from the scientific community.

Cannabinoid receptor binding and messenger RNA expression in human brain : an in vitro receptor autoradiography and in situ hybridization histochemistry study of normal aged and Alzheimer's brains

The distribution and density of cannabinoid receptor binding and messenger RNA expression in aged human brain were examined in several forebrain and basal ganglia structures. In vitro binding of [3H]CP-55,940, a synthetic cannabinoid, was examined by autoradiography in fresh frozen brain sections from normal aged humans (n = 3), patients who died with Alzheimers disease (n = 5) and patients who died with other forms of cortical pathology (n = 5). In the structures examined--hippocampal formation, neocortex, basal ganglia and parts of the brainstem--receptor binding showed a characteristic pattern of high densities in the dentate gyrus molecular layer, globus pallidus and substantia nigra pars reticulata, moderate densities in the hippocampus, neocortex, amygdala and striatum, and low densities in the white matter and brainstem. In situ hybridization histochemistry of human cannabinoid receptor, a ribonucleotide probe for the human cannabinoid receptor messenger RNA, showed a pattern of extremely dense transcript levels in subpopulations of cells in the hippocampus and cortex, moderate levels in hippocampal pyramidal neurons and neurons of the striatum, amygdala and hypothalamus, and no signal over dentate gyrus granule cells and most of the cells of the thalamus and upper brainstem, including the substantia nigra. In Alzheimers brains, compared to normal brains, [3H]CP-55,940 binding was reduced by 37-45% in all of the subfields of the hippocampal formation and by 49% in the caudate. Lesser reductions (20-24%) occurred in the substantia nigra and globus pallidus, internal segment. Other neocortical and basal ganglia structures were not different from control levels. Levels of messenger RNA expression did not differ between Alzheimers and control brains, but there were regionally discrete statistically significant losses of the intensely expressing cells in the hippocampus. The reductions in binding did not correlate with or localize to areas showing histopathology, estimated either on the basis of overall tissue quality or silver staining of neuritic plaques and neurofibrillary tangles. Reduced [3H]55,940 binding was associated with increasing age and with other forms of cortical pathology, suggesting that receptor losses are related to the generalized aging and/or disease process and are not selectively associated with the pathology characteristic of Alzheimers disease, nor with overall decrements in levels of cannabinoid receptor gene expression.

IDENTIFICATION OF A GABAB RECEPTOR SUBUNIT, GB2, REQUIRED FOR FUNCTIONAL GABAB RECEPTOR ACTIVITY

G protein-coupled receptors are commonly thought to bind their cognate ligands and elicit functional responses primarily as monomeric receptors. In studying the recombinant γ-aminobutyric acid, type B (GABAB) receptor (gb1a) and a GABAB-like orphan receptor (gb2), we observed that both receptors are functionally inactive when expressed individually in multiple heterologous systems. Characterization of the tissue distribution of each of the receptors by in situhybridization histochemistry in rat brain revealed co-localization of gb1 and gb2 transcripts in many brain regions, suggesting the hypothesis that gb1 and gb2 may interact in vivo. In three established functional systems (inwardly rectifying K+channel currents in Xenopus oocytes, melanophore pigment aggregation, and direct cAMP measurements in HEK-293 cells), GABA mediated a functional response in cells coexpressing gb1a and gb2 but not in cells expressing either receptor individually. This GABA activity could be blocked with the GABAB receptor antagonist CGP71872. In COS-7 cells coexpressing gb1a and gb2 receptors, co-immunoprecipitation of gb1a and gb2 receptors was demonstrated, indicating that gb1a and gb2 act as subunits in the formation of a functional GABAB receptor.

International Union of Pharmacology. LVI. Ghrelin Receptor Nomenclature, Distribution, and Function

Ghrelin is a 28-amino acid peptide originally isolated from rat stomach and is cleaved from a 117-amino acid precursor. The sequence of the mature peptide from rats and mice differs by two amino acids from that of human ghrelin. Alternative splicing of the ghrelin gene transcript can result in the translation of a second biologically active peptide, des-Gln14-ghrelin. Both peptides have a unique post-translational modification, octanoylation of Ser3, which is essential for the binding to receptors in hypothalamus and pituitary and stimulating the release of growth hormone from the pituitary. The growth hormone secretagogue receptor (GHS-R1a, Swiss-Prot code Q92847, LocusLink ID 2693), a rhodopsin-like seven transmembrane spanning G protein-coupled receptors belonging to Family A, was cloned in 1996 from the pituitary and hypothalamus and shown to be the target of growth hormone secretagogues (GHS), a class of synthetic peptide and nonpeptide compounds causing growth hormone release from the anterior pituitary. In 1999, ghrelin was identified as the endogenous cognate ligand for this receptor. The purpose of this review is to propose an official International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR) nomenclature designating GHS-R1a as the ghrelin receptor to follow the convention of naming receptors after the endogenous agonist, abbreviated where necessary to GRLN.

IUPHAR-DB: the IUPHAR database of G protein-coupled receptors and ion channels

The IUPHAR database (IUPHAR-DB) integrates peer-reviewed pharmacological, chemical, genetic, functional and anatomical information on the 354 nonsensory G protein-coupled receptors (GPCRs), 71 ligand-gated ion channel subunits and 141 voltage-gated-like ion channel subunits encoded by the human, rat and mouse genomes. These genes represent the targets of approximately one-third of currently approved drugs and are a major focus of drug discovery and development programs in the pharmaceutical industry. IUPHAR-DB provides a comprehensive description of the genes and their functions, with information on protein structure and interactions, ligands, expression patterns, signaling mechanisms, functional assays and biologically important receptor variants (e.g. single nucleotide polymorphisms and splice variants). In addition, the phenotypes resulting from altered gene expression (e.g. in genetically altered animals or in human genetic disorders) are described. The content of the database is peer reviewed by members of the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR); the data are provided through manual curation of the primary literature by a network of over 60 subcommittees of NC-IUPHAR. Links to other bioinformatics resources, such as NCBI, Uniprot, HGNC and the rat and mouse genome databases are provided. IUPHAR-DB is freely available at http://www.iuphar-db.org.

Dysregulated Cannabinoid Signaling Disrupts Uterine Receptivity for Embryo Implantation

The mechanisms by which synchronized embryonic development to the blastocyst stage, preparation of the uterus for the receptive state, and reciprocal embryo-uterine interactions for implantation are coordinated are still unclear. We show in this study that preimplantation embryo development became asynchronous in mice that are deficient in brain-type (CB1) and/or spleen-type (CB2) cannabinoid receptor genes. Furthermore, whereas the levels of uterine anandamide (endocannabinoid) and blastocyst CB1 are coordinately down-regulated with the onset of uterine receptivity and blastocyst activation prior to implantation, these levels remained high in the nonreceptive uterus and in dormant blastocysts during delayed implantation and in pregnant, leukemia inhibitory factor (LIF)-deficient mice with implantation failure. These results suggest that a tight regulation of endocannabinoid signaling is important for synchronizing embryo development with uterine receptivity for implantation. Indeed this is consistent with our finding that while an experimentally induced, sustained level of an exogenously administered, natural cannabinoid inhibited implantation in wild-type mice, it failed to do so inCB1 −/− /CB2 −/−double mutant mice. The present study is clinically important because of the widely debated medicinal use of cannabinoids and their reported adverse effects on pregnancy.