The functional relevance of multiple &alpha-2-adrenergic receptor subtypes in signaling and behavior.

摘要

The nine Adrenergic Receptors (ARs) are divided into three groups of three subtypes, based on pharmacological and molecular cloning evidence: alpha 1-ARs, alpha2-ARs, and beta-ARs. The three alpha 2-ARs (alpha2A-AR, alpha2B-AR, and alpha 2C-AR) signal primarily via Gi/Go heterotrimeric G proteins to inhibit adenylyl cyclase and voltage-gated calcium channels, and stimulate receptor-operated potassium channels. I examined the functional relevance of multiple alpha 2-AR subtypes with similar signaling capabilities on both the cellular and in vivo behavioral levels.;The alpha2-AR subtypes differ in their endocytic response to agonist occupancy: alpha2B-ARs are rapidly removed from the cell surface upon agonist occupancy; alpha2A-ARs remain on the cell surface after agonist stimulation; alpha2C-ARs are substantially localized intracellularly, even in the absence of agonist. When reports in the literature suggested that endocytosis is required for some G protein-coupled receptors to stimulate MAP kinase, I postulated that the alpha2-ARs' differing internalization profiles would result in differing abilities to activate MAP kinase. My results show that this is not the case. In HEK 293 cells, alpha2A-ARs and alpha2B-ARs stimulate MAP kinase equally, with similar rates of activation. Additionally, blockade of the endocytosis of the alpha2B-AR using potassium depletion does not affect its ability to stimulate MAP kinase. Thus, differences in the endocytic profiles of the alpha2-ARs are not reflected in differing abilities to stimulate MAP kinase.;I also tested whether the different alpha2-AR subtypes perform different functions in behavioral models of antidepressant efficacy. We compared the behavior of wild type mice and knockout mice lacking the alpha 2A-AR in the Porsolt Forced Swim Test. In this test, the loss of the alpha 2A-AR has an overall depressant effect and renders the mice insensitive to the tricyclic antidepressant imipramine. This depressant effect is not confounded by a general lack of activity or mobility in alpha2A-AR-KO mice. Comparing our results with studies examining mice lacking the alpha 2C-AR, where the loss of the alpha2C-AR has an antidepressant effect, suggests that multiple alpha2-ARs, at least alpha 2A-ARs and alpha2C-ARs, exist to achieve balance in response to depression-inducing stimuli.