Cannabinoid (CB1) receptors in brain mediate the effects of delta 9-tetrahydrocannabinol (THC) and endocannabinoids, primarily by activation of inhibitory G-proteins of the Gi/Go family. However, it is our premise that CB1 receptor- G-protein coupling is not uniform throughout the brain, and CB1 receptors may couple to multiple G-proteins to account for the multiplicity of cannabinoid effects. Indeed, our progress to date demonstrates regional differences in CB1 receptor-mediated G-protein activity in both normal brain and in adaptive responses to chronic THC administration. Moreover, the pattern of tolerance development is not identical for all THC effects, a finding we hypothesize is related to these regional differences in CB1 receptor-G-protein coupling. The proposed studies will investigate the relationship between cannabinoid tolerance and CB1 receptor desensitization and downregulation, and examine whether selective coupling of CB1 receptors to specific G-protein subtypes is correlated with regional differences in CB1 receptor adaptation. We propose examining in a systematic manner the role of CB1 receptor- G-protein coupling in neuroadaptation not only as a means of understanding cannabinoid tolerance but as a way of characterizing the endocannabinoid system. In order to address the role of receptor occupancy in adaptation, the magnitude of THC tolerance will be varied by administering different doses of THC. We will then assess 1) tolerance to cannabinoid-mediated hypoactivity, antinociception, hypothermia and memory impairment in behavioral assays and 2) CB1 receptor downregulation and desensitization using radiolabeled ligand and agonist- stimulated [35S]GTPgammaS autoradiography. We also hypothesize that differences in CB1 receptor-G-protein coupling throughout the brain account for differences in recovery of tolerance to separate THC-mediated behavioral effects. Therefore, the temporal relationship between recovery of tolerance and CB1 receptor function will be evaluated by treating mice with THC, then evaluating tolerance and downregulation/desensitization at different times after cessation of treatment. We will also conduct experiments to determine whether CB1 receptor coupling to different G-protein subtypes is responsible for variations in cannabinoid actions in different brain regions. We will examine co-localization of CB1 receptors and specific G-beta and G-gamma subtypes using immunocytochemistry to determine whether there is selective co-localization of CB1 receptors and specific subunits in different regions. We will then examine whether chronic THC administration selectively alters CB1 receptor coupling to specific G-alpha subtypes using agonist- stimulated [35S]GTPgammaS binding with subsequent immunprecipitation of activated G-alpha subtypes. These studies will contribute to elucidation of the mechanisms of action of CB1 receptors in brain, as well as determine the effects of chronic cannabinoid administration on cellular function during tolerance.
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