Enhanced hypotensive, bradycardic, and hypnotic responses to α₂-adrenergic agonists in spinophilin-null mice are accompanied by increased G protein coupling to the α_2A-adrenergic receptor

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 α_2AAR 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 α_2AAR density or intrinsic affinity for agonist in the brains of spinophilin-null mice compared with wild-type control mice, the coupling of the α_2AAR 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 α_2AAR 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.