Enhanced Hypotensive, Bradycardia and Hypnotic Responses to α2-adrenergic Agonists in Spinophilin Null Mice Are Accompanied by Increased G Protein Coupling to the α2AAR

We previously identified spinophilin as a regulator of α 2 adrenergic receptor (α 2 AR) trafficking and signaling in vitro and in vivo (Wang Q., Zhao J., Brady A.E., Feng J., Allen P.B., Lefkowitz R.J., Greengard P. and Limbird L.E. (2004) Science 304, 1940-1944). To assess the generalized role of spinophilin in regulating α 2 AR functions in vivo , the present study examined the impact of eliminating spinophilin on α 2 AR-evoked cardiovascular and hypnotic responses, previously demonstrated to be mediated by the α 2A AR subtype, following systemic administration of the α 2 -agonists, UK14,304 and clonidine, in spinophilin null mice. Mice lacking spinophilin expression display dramatically enhanced and prolonged hypotensive, bradycardic as well as sedative-hypnotic responses to α 2 AR stimulation. Whereas these changes in sensitivity to α 2 AR agonists occur independent of any changes in α 2A AR density or intrinsic affinity for agonist in the brain of spinophilin null mice when compared to wild type controls, the coupling of the α 2A AR 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 2A AR in the guanine-nucleotide-sensitive higher affinity state when compared to 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 .