Role of G-gamma-7 in striatal cannabinoid and D2 dopamine

Virginia Commonwealth University, Richmond VA

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Abstract

DESCRIPTION (provided by applicant): Cannabinoid CB1 receptors in the CNS mediate psychoactive effects of marijuana and are a critical component of the endogenous cannabinoid system, which is involved in motivation and reward, motor control, appetite, learning and memory, and pain and thermoregulation. Control of motor and motivational behaviors by the endocannabinoid system is thought to be largely mediated by CB1 receptors in the striatum/basal ganglia (striatum/BG). Within this system, CB1 receptors have been found to interact with dopaminergic neurotransmission, and are colocalized with D2 receptors in subpopulations of medium spiny GABAergic neurons. Both CB1 and D2 receptors are G-protein-coupled receptors (GPCRs) that inhibit adenylyl cyclase (AC) activity via Gi/o activation, and under certain circumstances can stimulate adenylyl cyclase through Gs/olf activation. The G-protein gamma subunit type 7 (Gng7) is highly expressed in striatum/BG relative to other brain regions, and has been shown to play a role in D1 receptor stimulation of adenylyl cyclase and to regulate G-alpha-olf expression as determined in Gng7 knockout (KO) mice. However, potential involvement of Gng7 in inhibitory regulation of AC by GPCRs has not been examined. Our preliminary findings indicate that AC inhibition by striatal CB1 and D2 receptors is diminished in Gng7 knockout mice, suggesting a role for this G-gamma subunit in inhibitory Gi/o signaling. Alternatively, Gng7 KO could, by concomitant reduction in G-olf, alter the proportion of striatal AC activity that can be inhibited by Gi/o. The proposed R03 project will test the hypothesis that CB1 and D2-mediated inhibition of AC is attenuated in Gng7 knockout mice because G-gamma-7 plays a role in inhibitory Gi/o signaling. Aim 1 will determine the extent of loss of adenylyl cyclase inhibition by CB1 and D2 receptors in subregions of the striatum/BG in Gng7 knockout mice, including caudate-putamen, nucleus accumbens and globus pallidus, as well as other CB1 and D2 containing regions such as prefrontal cortex and hippocampus. Cerebellum, which expressed CB1 receptors but does not express Gng7, will serve as negative control region. Aim 1 will also determine whether the effect of loss of Gng7 on AC inhibition is limited to CB1 and D2 receptors or is seen with other striatal GPCRs (e.g. opioid). Aim 2 will determine whether CB1- or D2-stimulated Gi/o-protein activity is diminished in regions in which AC inhibition is diminished in Gng7 KO mice, as determined by agonist-stimulated [35S]GTPgammaS binding. Whether any reduction in G-protein activation is due to a loss in receptor expression will be tested by immunoblotting of CB1 and D2 receptors. Aim 2 will also test the alternative hypothesis that attenuated AC inhibition is due to concomitant reduction in G-olf in Gng7 KO mice, by determining G-protein activation and AC inhibition by CB1 and D2 receptors in heterozygous G-alpha-olf KO mice. These studies will serve as a basis for a future a R01 application that will more extensively investigate the relationship between CB1 and D2 receptors in the regulation of striatal function, and the contribution of this regulation to control of motor and motivational behavior. These studies will help to determine molecular mechanisms contributing to neuropsychiatric disorders such as substance abuse and schizophrenia, as well as motor disorders. PUBLIC HEALTH RELEVANCE: Marijuana (cannabis) and other abused drugs produce their rewarding effects through the regulation biochemical signaling pathways in a brain region known as the striatum. This region is also involved in control of voluntary movement and habitual behavior, and is also under the control of a chemical messenger known as dopamine. A key protein in these signaling pathways in striatum is the G-protein gamma type 7 subunit. This project will determine the role of this protein in the biochemical effects of cannabinoid and dopamine receptor activation by studying mice that have been genetically altered to not express this protein. These studies will provide information that contributes to our understanding of the neural mechanisms of drug addiction and possibly neuropsychiatric and movement disorders, such as schizophrenia and Parkinsonism.