We previously identified spinophilin as a regulator of α2 adrenergic receptor (α2AR) trafficking and signaling in vitro and in vivo (Science 304:1940-1944, 2004). To assess the generalized role of spinophilin in regulating α2AR functions in vivo, the present study examined the impact of eliminating spinophilin on α2AR- evoked cardiovascular and hypnotic responses, previously demonstrated to be mediated by the α2AAR subtype, after systemic administration of the α2-agonists 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6- quinoxalinamine (UK14,304) and clonidine in spinophilin-null mice. Mice lacking spinophilin expression display dramatically enhanced and prolonged hypotensive, bradycardic, and sedative-hypnotic responses to α2AR stimulation. Whereas these changes in sensitivity to α2AR agonists occur independent of any changes in α2AAR density or intrinsic affinity for agonist in the brains of spinophilin-null mice compared with wild-type control mice, the coupling of the α2AAR to cognate G proteins is enhanced in spinophilin-null mice. Thus, brain preparations from spinophilin-null mice demonstrate enhanced guanine nucleotide regulation of UK14,304 binding and evidence of a larger fraction of α2AAR in the guanine-nucleotide-sensitive higher affinity state compared with those from wild-type mice. These findings suggest that eliminating spinophilin expression in native tissues leads to an enhanced receptor/G protein coupling efficiency that contributes to sensitization of receptor mediated responses in vivo. Copyright © 2010 The American Society for Pharmacology and Experimental Therapeutics.