Knockout of spinophilin, an endogenous antagonist of arrestin-dependent α₂-adrenoceptor functions, enhances receptor-mediated antinociception yet does not eliminate sex-related differences

We have previously shown gonadal steroid-dependent, gender specific modulation of nociception by α₂-adrenoceptors. Agonist activation of the receptor enhances its association with spinophilin that antagonizes arrestin functions both by diminishing receptor phosphorylation by G-protein-coupled receptor kinase 2 (GRK2) and by competing for receptor interactions with arrestin. Since spinophilin is highly enriched in dendritic spines, we investigated whether α₂-adrenoceptor-induced antinociception as well as sex-related differences are modified in spinophilin knockout mice. We evaluated α₂-adrenoceptor antinociception in a heat-evoked tail flick test in spinophilin wild type (Sp^+/+) and knockout (Sp^-/-) mice. Baseline tail flick latencies (TFLs) did not change between any groups. Interestingly, the α₂-adrenoceptor agonist, clonidine, increased TFL in male and diestrous (low estrogen) Sp^-/- as well as Sp^+/+ mice; in fact, this increase in TFL was significantly higher in Sp^-/- male and diestrous groups than in their Sp^+/+ counterparts. This unexpected finding is consistent with enhanced α₂-adrenoceptor-mediated sedation observed previously in Sp^-/- mice, presumably due to accelerated endocytosis of desensitized receptors and recycling of refreshed receptors when arrestin is not competed for by spinophilin in Sp^-/- mice. Despite modulation of α₂-adrenoceptor effects in Sp^-/- mice, sex-related differences were retained; thus, clonidine was ineffective in proestrous females (highest estrogen levels), in both Sp^-/- and Sp^+/+ mice, reaffirming that estrogen suppresses α₂-adrenoceptor-evoked antinociception. These findings show that elimination of spinophilin enhances α₂-adrenoceptor-evoked antinociception in estrogen-deprived physiological settings, suggesting a role for spinophilin to suppress these effects, and yet this enhanced response cannot overcome the absence of antinociception with elevated estrogen levels.