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

Abstract

We previously identified spinophilin as a regulator of ␣ 2 adrenergic receptor (␣ 2 AR) trafficking and signaling in vitro and in vivo (Science 304:1940(Science 304: -1944(Science 304: , 2004. To assess the generalized role of spinophilin in regulating ␣ 2 AR functions in vivo, the present study examined the impact of eliminating spinophilin on ␣ 2 AR-evoked cardiovascular and hypnotic responses, previously demonstrated to be mediated by the ␣ 2A AR subtype, after systemic administration of the ␣ 2 -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 ␣ 2 AR stimulation. Whereas these changes in sensitivity to ␣ 2 AR agonists occur independent of any changes in ␣ 2A AR density or intrinsic affinity for agonist in the brains of spinophilin-null mice compared with wild-type control mice, the coupling of the ␣ 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 ␣ 2A AR 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.The ␣ 2 -adrenergic receptor (AR) is a prototypical G protein-coupled receptor (GPCR) that couples to the G i/o subfamily of G proteins (Wang and Limbird, 2007). In native cells, stimulation of the ␣ 2 AR leads to inhibition of adenylyl cyclase and voltage-gated Ca 2ϩ currents and to activation of receptor-operated K ϩ currents and mitogen activated protein kinase (

Noradrenergic antidepressant responses to desipramine in vivo are reciprocally regulated by arrestin3 and spinophilin

“…Two-way ANOVA revealed significant drug × genotype interactions for both immobility (p=0.0006) and active swimming (p<0.0001). Unfortunately, we were unable to satisfactorily assay the antidepressant effect of clonidine in Sp -/- mice due to a predominance of confounding sedative effects; indeed, we have previously shown that Sp -/- mice have enhanced sensitivity to the sedative effects of α 2 AR agonist treatment (Wang et al, 2004; Lu et al, 2010). …”

“…We have previously shown that the dendritic scaffolding protein spinophilin serves as a functional antagonist to arrestin at the α 2A AR, both in vitro and in vivo (Wang et al, 2004), and that Sp-null mice exhibit enhanced α 2A AR-mediated responses to agonists in vivo (Lu et al, 2010) and in vitro 2 . Therefore, we tested Sp -/- mice for their responsiveness to DMI in the FST.…”

“…Although arrestin3-null mice are known to have altered behavioral responses to pharmacological manipulations (Schmid and Bohn, 2009) and we have shown that spinophilin-null mice have a range of enhanced responses to α 2 AR agonists in vivo (Lu et al, 2010), neither of these models has been used to investigate potential roles for these proteins in the antidepressant behavioral response. In particular, whether reciprocal interplay between arrestin and spinophilin applies to the α 2 AR-mediated antidepressant response to DMI is entirely unknown.…”

PDZ Protein Regulation of G Protein–Coupled Receptor Trafficking and Signaling Pathways

“…Conversely, spinophilin appears to promote RGS2-mediated inhibition of a 2 AR-evoked Ca 21 signaling and RGS2-mediated modulation of a 1 -adrenergic receptor-NMDAR crosstalk (Wang et al, 2005;Liu et al, 2006). In spinophilin knockout mice, the a 2A -adrenergic receptor (a 2A AR) exhibits increased G protein coupling and sensitized responses to a 2A AR agonists (Lu et al, 2010;Cottingham et al, 2012). Both spinophilin and neurabin-1 are implicated in the D 1 R-dependent regulation of AMPAR as well as long-term depression and potentiation, respectively .…”

Minireview: Role of Intracellular Scaffolding Proteins in the Regulation of Endocrine G Protein-Coupled Receptor Signaling

“…It has also been shown that spinophilin can actively regulate ␣ 1B AR function because it recruits RGS2 to the ␣ 1B AR/G q signaling complex to form a heterotetrameric complex that is implicated in the regulation of Ca 2ϩ signaling by ␣-agonists in cultured cells (137). In vivo studies with the ␣ 2A AR in spinophilin-null mice reveal the impact of spinophilin on ␣ 2 AR-evoked cardiovascular responses (138). In mice lacking spinophilin, ␣ 2 AR stimulation leads to an enhanced and prolonged hypotensive, bradycardic, and sedative-hypnotic responses.…”