Alpha2-adrenergic receptor subtypes:: Subtle mutation of the α2A-adrenergic receptor in vivo by gene targeting strategies reveals the role of this subtype in multiple physiological settings

Alpha(2)-adrenergic receptors (alpha (2A)ARs) are coupled by pertussis-toxin sensitive G proteins to various effecters, including adenylyl cyclase and ion channels. The alpha (2)ARs respond to endogenous norepinephrine and epinephrine to elicit a variety of physiological responses, including inhibition of neurotransmitter release, suppression of insulin release from pancreatic beta cells, activation of platelet a,aggregation, and contraction of arteriolar smooth muscle. Three distinct alpha (2)AR subtypes (alpha (2A), alpha (2B), alpha (2C)) have been characterized by both pharmacological and molecular biological approaches; however, the lack of subtype-specific ligands has precluded an understanding of the physiological relevance of each subtype. Previous studies demonstrated that mutation of a conserved aspartate residue in the a,,AR to asparagine (D79N alpha (2A)AR) resulted in a receptor that retained its ability to inhibit voltage-gated Ca2+ channels and cAMP production but was unable to activate K+ currents in AtT20 cells (Surprenant er at., 1992). To explore the physiological role of the alpha (2A)AR subtype and to evaluate the selectivity of alpha (2A)AR effects with respect to various signal transduction pathways, we used gene targeting in embryonic stem cells to create a mouse line that expresses the mutant D79N alpha (2A)AR instead of the wild-type alpha (2A)AR We established a D79N alpha (2A)AR mouse line and characterized various alpha (2A)AR-mediated physiological functions in these mutant mice. Because the in vivo D79N alpha (2A)AR is expressed at a reduced density relative to wild-type alpha (2A) and is not selectively uncoupled from a single signal transduction pathway, our findings of losses of alpha (2A)AR-mediated functions in the D79N mice reflect a requirement for the alpha (2A)AR subtype but do not reveal the importance of a specific signal transduction pathway. The alpha (2A)AR subtype appears to mediate reduction in blood pressure following alpha (2A) agonist administration as well as sedative, anesthetic-sparing, and analgesic responses to alpha (2A)AR agonists. Therefore, the alpha (2A)AR subtype appears to mediate a majority of the clinically relevant responses associated with alpha (2A)AR agonist treatment.