Molecular Pharmacology: CAMH Research Annual Report 2002

During the past year, the Molecular Pharmacology Section continued work on the biology of neurotransmitter receptors for dopamine, opioid peptides, apelin and others that we identified, including the ability of the receptors to interact directly with other receptors to alter signal transduction. We have continued investigating receptor-gene-deleted mice models and models over-expressing neurotransmitter/neuropeptide to study the role of the individual receptors in brain function. Our search for novel human genes resulted in the identification of several novel receptors. Many of these are highly expressed in brain. They are potential targets as candidate genes in neuropsychiatric disease and may lead to the development of novel drugs. During the past year, we have published 13 peer-reviewed papers on our research in this section.

Receptor Biology

Almost 10 years ago, our laboratory 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 substantiated and shown to be true for all members of the family of G-protein-coupled receptors.

We also discovered that individual receptors can form complicated higher-order structures with other receptors, greatly enlarging the complexity of novel functional therapeutic targets in the brain.

We are now investigating receptor-receptor interactions, using functional assays we developed, to establish the physiological roles of this process in receptor and cell function.

We have determined the sites of interaction between two interacting receptors, and have narrowed this to a particular region of the receptor. We are intensively studying the dopamine (5 distinct receptors,D1 to D5) and opioid (3 distinct receptors) receptors and a novel receptor that we cloned, the apelin receptor. The dopamine receptors form homomeric and heteromeric (i.e., mixtures of receptors) complexes. The apelin receptors have a highly novel expression in the cell -- we are further investigating this. After activation by the specific neurotransmitter, the receptors on a cell or neuron undergo a process by which they become insensitive to further activation, termed desensitization. We have recently identified the specific amino acids within the D1 receptor responsible for this effect, in distinction from other amino acids that mediate the internalization of the receptor from the cell surface into the interior.

Novel Receptor Genes

Our successful work on the discovery of novel receptor genes has resulted in the identification of ~50 additional ones to date, many of which are expressed highly in human and rat brain and will form novel drug targets. Our laboratory and many others world-wide are conducting the physiological characterization of these receptors. 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.

Role of Receptors in Behaviour

Our development of several mouse models lacking individual or multiple receptor genes has proven to be an extremely valuable strategy to determine the complete repertoire of functions mediated in the brain.

Mice lacking the D1 dopamine receptor lose their preference for alcohol drinking and for sugar pellets. They appear unable to perceive rewarding stimuli and will not press a lever to obtain these substances. They also have a spatial learning deficit and an inability to forget fearful events.

We have developed a colony of D3 dopamine-receptor-lacking mice. These mice show less anxiety on some behavioural tests.

The loss of both D1 and D3 receptors prevents the manifestation of reduced anxiety, implicating an interaction between these two receptor systems in mediating the level of anxiety. We have engineered a mouse model overexpressing apelin. At present, we are characterizing the regions of brain where this is occurring and will investigate the behavioural consequences.