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 Centre for Addictionand Mental Health |
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Research
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Molecular Pharmacology: Reseach Annual Report 2003Section Heads: Drs. Susan R. George and Brian F. O'Dowd In the Molecular Pharmacology section, we continued our research on the biology of neurotransmitter receptors for the G protein-coupled receptors (GPCRs) for dopamine, opioids and apelin. In this work, we focus on the ability of the receptors to interact directly with each other to alter pharmacology and signal transduction. We have also continued investigating D1, D3 and D5 receptor-gene-deleted mice to analyze the role of the individual receptors in discrete behaviours. Our search for novel human genes continues; in this past year we identified novel orphan receptors and a mutated receptor in the human population. These genes, potential candidate genes in neuropsychiatric disease, will now be included in the search for and development of diagnostic tests or novel drugs. We have also developed a novel cell-based assay, which has several components. This assay can, for example, perform rapid screening for compounds targeting GPCRs, including the many orphan GPCRs we have identified. The assay can also screen for receptors or proteins that interact with each other. During the past year, we have submitted 19 papers for publication; 11 of these papers have now been published. Our laboratory previously discovered that receptors for neurotransmitters, such as dopamine, function not as individual molecules, but as highly ordered complexes on the cell surface. This discovery has been shown to be true for many members of the family of GPCRs and is probably universal. We also discovered that individual receptors formed complicated higher-order structures with other receptors, greatly enlarging the complexity of novel functional therapeutic targets in the brain. We continue to investigate receptor-receptor interactions, and the sites of interaction between two receptors has been precisely identified to involve specific transmembrane regions. Previously, we studied dopamine and opioid receptors, finding that they form homodimeric and heterodimeric (i.e., mixtures of receptors) complexes. We have shown the existence of these receptor complexes in cultured living neurons and human and rat brain by immunocytochemistry and state-of-the-art confocal microscopy. Using confocal microscopy, we are studying further the colocalization of the receptors, not only within single neurons, but also within cellular microdomains of the neurons. Our work has revealed that hetero-oligomerization of receptors may generate novel pharmacological and functional properties and that hetero-oligomerization is a specific process, with rules governing which receptors participate in the hetero-oligomeric complex. We have developed a novel method incorporating a strategy suitable for the identification of chemicals interacting with or modifying the activity of both known and orphan GPCRs, transporters and other plasma membrane receptors. The method will also allow us to evaluate the ability of GPCRs and transporters to selectively oligomerize with other GPCRs, transporters or other proteins to generate novel heteromeric drug targets. We will use this assay method to screen for novel compounds and to identify the dimerization partners of various receptor proteins. We continue to discover novel GPCR genes and to identify novel orphan receptors. As orphan receptors we cloned are being identified, such as GPR 7 and 8, it is apparent that these are completely novel ligand-receptor systems. Identifying the receptor and its endogenous ligand will now allow us to elucidate its physiological effects and functions. One of the first receptors we identified was the apelin receptor, which is highly expressed in brain. We have recently completed studies showing localization of the apelin receptor in human brain regions, with a unique nuclear localization within the neuron, highly novel for GPCRs. This suggests unique functions for this receptor, as the vast majority of other GPCRs are located on the cell surface; this receptor will be the focus of detailed study in future. We are searching through genomic databases and DNA of people with neuropsychiatric diseases for mutations and polymorphisms in the receptor genes that may predispose humans to disease. Recently we discovered a mutated GPCR that was present in a highly significant percentage of the population, including those with and without a neuropsychiatric disease. The prevalence of this mutation is the highest among documented receptor mutations; we plan to studythe functional significance of this mutation and its role in mental disorders. More excitingly, as we uncover the functions of additional orphan receptors, we will be able to elucidate distinct novel CNS functions, the role of the receptors in CNS disorders may become apparent, and the receptors may be targets for the development of new therapeutics. Role of Receptors in Behaviour Our analysis of the effects of receptor gene deletions on specific brain functions has helped us understand the role of the
receptors in important higher level functioning, with specific corollaries to several human CNS disorders. To explore the
functional role of specific dopamine receptors, we studied various receptor gene knockout models for the functional consequence
of deletions of the D1, D3, D5 and D1+C3 receptor genes. Our results showed that their fear conditioning responses were intact, although the extinction of the fear memory was abnormally prolonged compared to their wild-type littermates. This finding suggested a role for the D1 receptor in promoting the extinction of fearful memories, with implications for human disorders such as abnormal fear/anxiety states and post-traumatic stress disorder. These mice also show a markedly reduced motivation to work for rewarding stimuli. To our knowledge, this was the first demonstration of a single gene disruption that has resulted in specific attenuation of
drug-seeking and pleasure-seeking behaviour and enhancement of fear memory. |
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Last updated: Aug 24, 2004 3:23 PM Published: Jul 01, 2011 5:56 AM ID#P5707 |