Clinical and experimental evidence suggest that statins decrease sympathetic activity, but whether peripheral mechanisms involving direct actions on post-ganglionic sympathetic neurons contribute to this effect is not known. Because tonic activity of these neurons is directly correlated with the size of their dendritic arbor, we tested the hypothesis that statins decrease dendritic arborization in sympathetic neurons. Oral administration of atorvastatin (20 mg/kg/day for 7 days) significantly reduced dendritic arborization in vivo in sympathetic ganglia of adult male rats. In cultured sympathetic neurons, statins caused dendrite retraction and reversibly blocked bone morphogenetic protein-induced dendritic growth without altering cell survival or axonal growth. Supplementation with mevalonate or isoprenoids, but not cholesterol, attenuated the inhibitory effects of statins on dendritic growth, whereas specific inhibition of isoprenoid synthesis mimicked these statin effects. Statins blocked RhoA translocation to the membrane, an event that requires isoprenylation, and constitutively active RhoA reversed statin effects on dendrites. These observations that statins decrease dendritic arborization in sympathetic neurons by blocking RhoA activation suggest a novel mechanism by which statins decrease sympathetic activity and protect against cardiovascular and cerebrovascular disease.

NGF activates several signaling cascades in sympathetic neurons. We examined how activation of one of these cascades, the ERK/MAP (extracellular signal-regulated kinase/mitogen-activated protein) kinase pathway, affects dendritic growth in these cells. Dendritic growth was induced by exposure to NGF and BMP-7 (bone morphogenetic protein-7). Exposure to NGF increased phosphorylation of ERK1/2. Unexpectedly, two MEK (MAP kinase kinase) inhibitors (PD 98059 and U 0126) enhanced dendritic growth, and a ligand, basic FGF, that activates the ERK pathway inhibited the growth of these processes. The enhancement of dendritic growth by PD 98059 was associated with an increase in the number of axo-dendritic synapses, and it appeared to represent a specific morphogenic effect because neither axonal growth nor cell survival was affected. In addition, increased dendritic growth was not observed after exposure to inhibitors of other signaling pathways, including the phosphatidylinositol-3-kinase inhibitor LY 294002. Dendritic growth was also increased in cells transfected with dominant-negative mutants of MEK1 and ERK2 but not with dominant-negative mutants of MEK5 and ERK5, suggesting that ERK1/2 is the primary mediator of this effect. Exposure to BMP-7 induces nuclear translocation of Smad1 (Sma-and Mad-related protein 1), and PD 98059 treatment potentiated nuclear accumulation of Smad-1 induced by BMP-7 in sympathetic neurons, suggesting a direct enhancement of BMP signaling in cells treated with an MEK inhibitor. These observations indicate that one of the signaling cascades activated by NGF can act in an antagonistic manner in sympathetic neurons and reduce the dendritic growth induced by other NGF-sensitive pathways.

Clinical and experimental observations suggest that statins may be useful for treating diseases presenting with predominant neurogenic inflammation, but the mechanism(s) mediating this potential therapeutic effect are poorly understood. In this study, we tested the hypothesis that statins act directly on sensory neurons to decrease expression of proinflammatory neuropeptides that trigger neurogenic inflammation, specifically calcitonin gene-related peptide (CGRP) and substance P. Reverse transcriptase-polymerase chain reaction, radioimmunoassay, and immunocytochemistry were used to quantify CGRP and substance P expression in dorsal root ganglia (DRG) harvested from adult male rats and in primary cultures of sensory neurons derived from embryonic rat DRG. Systemic administration of statins at pharmacologically relevant doses significantly reduced CGRP and substance P levels in DRG in vivo.In cultured sensory neurons, statins blocked bone morphogenetic protein (BMP)-induced CGRP and substance P expression and decreased expression of these neuropeptides in sensory neurons pretreated with BMPs. These effects were concentration-dependent and occurred independent of effects on cell survival or axon growth. Statin inhibition of neuropeptide expression was reversed by supplementation with mevalonate and cholesterol, but not isoprenoid precursors. BMPs signal via Smad activation, and cholesterol depletion by statins inhibited Smad1 phosphorylation and nuclear translocation. These findings identify a novel action of statins involving down-regulation of proinflammatory neuropeptide expression in sensory ganglia via cholesterol depletion and decreased Smad1 activation and suggest that statins may be effective in attenuating neurogenic inflammation.