Methylphenidate regulation of osteoclasts in a dose- and sex-dependent manner adversely affects skeletal mechanical integrity

Methylphenidate (MP) is the most prescribed psychostimulant for ADHD patients, with clinically demonstrated detrimental effects on bone quality, potentially leading to early onset osteoporosis and higher fracture risk. The underlying mechanism for the effects of MP on bone remains elusive. This study demonstrates that sex- and dose-dependent effects of MP on bone quality and quantity are mediated by osteoclast activity. Four-week-old male and female rats were treated with low and high dose MP for 13 weeks. Bone quality and quantity were analyzed using microCT, mechanical testing, histomorphometry, and TRAP staining. Male and female rat bone marrow-derived osteoclasts were treated in a dose-dependent manner (0–1000 ng/ml) and osteoclast activity was determined at days 5, 7, and 14 using TRAP staining, as well as a pit formation assay at day 18. Animal studies showed a dose- and a sex-dependent decrease in mechanical integrity in femora and increased TRAP staining in MP-treated rats. Primary cultures revealed that MP had direct dose- and sex-dependent effects on osteoclast activity, as seen by increased differentiation, activity, and resorption. This study demonstrates for the first time that osteoclasts are differentially regulated by MP in adolescent male and female rats, resulting in sex-dependent effects on the skeleton.