Double disruption of α_2A- and α_2C -adrenoceptors results in sympathetic hyperactivity and high-bone-mass phenotype

Evidence demonstrates that sympathetic nervous system (SNS) activation causes osteopenia via β₂-adrenoceptor (β2-AR) signaling. Here we show that female mice with chronic sympathetic hyperactivity owing to double knockout of adrenoceptors that negatively regulate norepinephrine release, α_2A-AR and α_2C-AR (α_2A/α_2C-ARKO), present an unexpected and generalized phenotype of high bone mass with decreased bone resorption and increased formation. In α_2A/α_2C-ARKO versus wild-type (WT) mice, micro-computed tomographic (μCT) analysis showed increased, better connected, and more plate-shaped trabeculae in the femur and vertebra and increased cortical thickness in the vertebra, whereas biomechanical analysis showed increased tibial and femoral strength. Tibial mRNA expression of tartrate-resistant acid phosphatase (TRACP) and receptor activator of NF-κB (RANK), which are osteoclast-related factors, was lower in knockout (KO) mice. Plasma leptin and brain mRNA levels of cocaine amphetamine-regulated transcript (CART), which are factors that centrally affect bone turnover, and serum levels of estradiol were similar between mice strains. Tibial β₂-AR mRNA expression also was similar in KO and WT littermates, whereas α_2A-, α_2B- and α_2C- AR mRNAs were detected in the tibia of WT mice and in osteoblast-like MC3T3-E1 cells. By immunohistochemistry, we detected α_2A-, α_2B-, α_2C- and β₂-ARs in osteoblasts, osteoclasts, and chondrocytes of 18.5-day-old mouse fetuses and 35-day-old mice. Finally, we showed that isolated osteoclasts in culture are responsive to the selective α₂-AR agonist clonidine and to the nonspecific α-AR antagonist phentolamine. These findings suggest that β₂-AR is not the single adrenoceptor involved in bone turnover regulation and show that α₂-AR signaling also may mediate the SNS actions in the skeleton. © 2010 American Society for Bone and Mineral Research. © 2011 American Society for Bone and Mineral Research.