Department of Pharmacology
Uniformed Services University of the Health Sciences
Department of Pharmacology
4301 Jones Bridge Road
Bethesda, Maryland 20814-4799
Phone: (301) 295-3223
FAX: (301) 295-3220
Thomas E. Côté, Ph.D.
Our laboratory studies the biochemical events that occur following the activation of the mu opioid receptor. It is known that morphine relieves severe pain by activating mu opioid receptors throughout the nervous system. It is also known that repeated exposure of mu opioid receptors to morphine results in an attenuation in the ability of morphine to relieve pain. Our laboratory is also interested in studying the biochemical events that result in the 'desensitization' of the mu receptor signalling system.
When the mu opioid receptor binds an opiate (such as morphine), a series of other proteins associated with the mu receptor signalling pathway become activated. The first protein activated by the morphine-bound mu receptor is a GTP-binding protein (G protein). The G protein requires guanosine triphosphate (GTP) to be active. When the GTP is hydrolyzed to guanosine diphosphate (GDP), the protein becomes inactive. There are a number of different types of G proteins in neurons, and the specific types of G proteins that associate with the mu opioid receptor have recently been identified by our laboratory. We are also investigating how chronic exposure of the mu receptor to morphine results in a diminished responsiveness of neurons to opiates (tolerance). Recently, a new group of proteins has been discovered in the nervous system. These proteins are capable of increasing the rate of hydrolysis of GTP that is bound to G proteins. When GTP is rapidly hydrolyzed, the signalling pathway shuts down. These new proteins that increase the hydrolysis of GTP are called RGS proteins (for Regulators of G protein Signalling). We are interested in determining if chronic morphine administration can increase the synthesis of these RGS proteins and/or cause these RGS proteins to become associated with the G proteins that mediate the effects of the mu receptor.
Chronic exposure of the mu opioid receptor to morphine results in the interaction of RGS proteins with the G proteins that normally mediate the effects of the mu opioid receptor. The RGS protein inhibits the activity of the G protein by stimulating the hydrolysis of GTP to GDP.
Our laboratory has recently developed antibodies that can selectively immunoprecipitate the mu opioid receptor in an active state in order to co-purify proteins that associate with the mu receptor. These antibodies were raised against fusion proteins that contained 50 to 61 amino acid regions that occur in the mu opioid receptor. These antibodies are currently being used to purify soluble, active mu opioid receptors in association with their interactive G proteins. During this past year, we have identified Gao and Gai1 as the most prominent G proteins that co-precipitate with the rat brain mu opioid receptor. We are also interested in determining if chronic morphine treatment can cause RGS proteins to become associated with the mu receptor/G protein complex. To accomplish this goal, we are also developing antibodies against the newly discovered RGS proteins. Finally, during the coming year, we will be transfecting cells that express the mu opioid receptor with selected RGS proteins to determine if expression of these proteins can diminish mu opioid receptor signalling.
Weems, H.B., Chalecka-Franaszek, E., and Cote, T.E. (1996) Solubilization and pharmacological characterization of high affinity, guanine nucleotide sensitive mu-opioid receptors from rat brain. J. Neurochem. 66, 1042-1050.
Chalecka-Franaszek, E., Weems, H.B., Crowder, A.T., Cox, B.M., and Cote, T.E. (2000) Immunoprecipitation of high-affinity, guanine nucleotide-sensitive, solubilized mu opioid receptors from rat brain: co-immunoprecipitation of the G proteins Gao, Gai1 and Gai3. J. Neurochem. 74:1068-78.