Serotonin reuptake inhibitors act centrally to cause bone loss in mice by counteracting a local antiresorptive effect

The use of selective serotonin reuptake inhibitors (SSRIs) has been associated with an increased risk of bone fracture, raising concerns about their increasingly broader usage. This deleterious effect is poorly understood and thus strategies to avoid this side effect remain elusive. We show here that fluoxetine (Flx), one of the most prescribed SSRI, acts on bone remodeling through two distinct mechanisms. Peripherally, Flx has antiresorptive properties, directly impairing osteoclast differentiation and function through a serotonin reuptake-independent Ca(2+-)calmodulin-NFATc1-dependent mechanism. With time, however, Flx also triggers a brain serotonin-dependent rise in sympathetic output that increases bone resorption sufficiently to counteract its local antiresorptive effect; thus leading to a net effect of impaired bone formation and bone loss. Accordingly, neutralizing this second mode of action through co-treatment with the β-blocker propranolol, while leaving the peripheral effect intact, prevents Flx-induced bone loss in mice. Hence, this study identifies a dual mode of action of SSRIs on bone remodeling and suggests a therapeutic strategy to block the deleterious effect on bone homeostasis from their chronic use.