Vasopressin-induced cytoplasmic and nuclear calcium signaling in cultured cortical astrocytes

We sought to determine whether vasopressin V_1a receptor (V_1aR) mRNA detected in cortical astrocytes [Brain Res. Mol. Brain Res. 45 (1997) 138] was translated into functional receptors by investigating the effector calcium signaling cascade associated with the vasopressin V_1a receptor subtype. Analysis of intracellular calcium dynamics using the calcium-sensitive dye fura-2 AM indicated that exposure of cortical astrocytes to V₁ vasopressin receptor agonist, [Phe²,Orn⁸]-oxytocin, induced a marked dose-dependent increase in intracellular calcium which was abolished by depletion of extracellular calcium. V₁ agonist treatment induced a rapid increase in calcium signal in both the cytoplasm and nucleus, which was followed by an accumulation of the calcium signal in the nucleus, suggesting translocation of cytoplasmic calcium into the nucleus. The nuclear calcium signal was sustained for several seconds followed by translocation back to the cytoplasm. Following the nuclear-to-cytoplasmic calcium translocation, total free intracellular calcium concentration decreased. The dynamic calcium cytoplasmic and nuclear localization was confirmed by laser scanning confocal microscopy coupled with the calcium-sensitive dye fluo-3 AM. To determine the source of calcium, V₁ agonist-induced ⁴⁵Ca²⁺ uptake and [³H]IP₁ accumulation were investigated. V₁ agonist induced significant and rapid uptake of ⁴⁵Ca²⁺ and a significant dose-dependent increase in [³H]IP₁ accumulation in cortical astrocytes. To our knowledge, this is the first documentation of a vasopressin receptor-induced calcium signaling cascade in cortical astrocytes and the first documentation of vasopressin receptor induction of nuclear calcium signaling.