Vitamin D hormone confers neuroprotection in parallel with downregulation of L-type calcium channel expression in hippocampal neurons

Although vitamin D hormone (VDH; 1,25-dihydroxyvitamin D-3), the active met abolite of vitamin D, is the major Ca2+-regulatory steroid hormone in the p eriphery, it is not known whether it also modulates Ca2+ homeostasis in bra in neurons. Recently, chronic treatment with VDH was reported to protect br ain neurons in both aging and animal models of stroke. However, it is uncle ar whether those actions were attributable to direct effects on brain cells or indirect effects mediated via peripheral pathways. VDH modulates L-type voltage-sensitive Ca2+ channels (L-VSCCs) in peripheral tissues, and an in crease in L-VSCCs appears linked to both brain aging and neuronal vulnerabi lity. Therefore, we tested the hypothesis that VDH has direct neuroprotecti ve actions and, in parallel, targets L-VSCCs in hippocampal neurons. Primary rat hippocampal cultures, treated for several days with VDH, exhibi ted a U-shaped concentration-response curve for neuroprotection against exc itotoxic insults: lower concentrations of VDH (1-100 nM) were protective, b ut higher, nonphysiological concentrations (500-1000 nM) were not. Parallel studies using patch-clamp techniques found a similar U-shaped curve in whi ch L-VSCC current was reduced at lower VDH concentrations and increased at higher (500 nM) concentrations. Real-time PCR studies demonstrated that VDH monotonically downregulated mRNA expression for the alpha (1C) and alpha ( 1D) pore-forming subunits of L-VSCCs. However, 500 nM VDH also nonspecifica lly reduced a range of other mRNA species. Thus, these studies provide the first evidence of (1) direct neuroprotective actions of VDH at relatively l ow concentrations, and (2) selective downregulation of L-VSCC expression in brain neurons at the same, lower concentrations.