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 D₃), the active metabolite of vitamin D, is the major Ca²⁺-regulatory steroid hormone in the periphery, it is not known whether it also modulates Ca²⁺ homeostasis in brain neurons. Recently, chronic treatment with VDH was reported to protect brain neurons in both aging and animal models of stroke. However, it is unclear 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 increase in L-VSCCs appears linked to both brain aging and neuronal vulnerability. Therefore, we tested the hypothesis that VDH has direct neuroprotective actions and, in parallel, targets L-VSCCs in hippocampal neurons. Primary rat hippocampal cultures, treated for several days with VDH, exhibited a U-shaped concentration-response curve for neuroprotection against excitotoxic insults: lower concentrations of VDH (1-100 nM) were protective, but higher, nonphysiological concentrations (500-1000 nM) were not. Parallel studies using patch-clamp techniques found a similar U-shaped curve in which 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 α_1C and α_1D pore-forming subunits of L-VSCCs. However, 500 nM VDH also nonspecifically reduced a range of other mRNA species. Thus, these studies provide the first evidence of (1) direct neuroprotective actions of VDH at relatively low concentrations, and (2) selective downregulation of L-VSCC expression in brain neurons at the same, lower concentrations.