Differing temporal roles of Ca2+ and cAMP in nicotine-elicited elevation of tyrosine hydroxylase mRNA

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Abstract
The involvement of cAMP- and Ca2+-mediated pathways in the activation of tyrosine hydroxylase (TH) gene expression by nicotine was examined in PC-12 cells. Extracellular Ca2+ and elevations in intracellular Ca2+ concentration ([Ca2+]i) were required for nicotine to increase TH mRNA. The nicotine-elicited rapid rise in [Ca2+]iwas inhibited by blockers of either L-type or N-type, and to a lesser extent P/Q-, but not T-type, voltage-gated Ca2+ channels. With continual nicotine treatment, [Ca2+]ireturned to basal levels within 3–4 min. After a lag of ∼5–10 min, there was a smaller elevation in [Ca2+]ithat persisted for 6 h and displayed different responsiveness to Ca2+ channel blockers. This second phase of elevated [Ca2+]iwas blocked by an inhibitor of store-operated Ca2+ channels, consistent with the observed generation of inositol trisphosphate. 1,2-Bis(2-aminophenoxy)ethane- N, N, N′, N′-tetraacetic acid-AM (BAPTA-AM), when added before or 2 h after nicotine, prevented elevation of TH mRNA. Nicotine treatment significantly raised cAMP levels. Addition of the adenylyl cyclase inhibitor 2′,5′-dideoxyadenosine (DDA) prevented the nicotine-elicited phosphorylation of cAMP response element binding protein. DDA also blocked the elevation of TH mRNA only when added after the initial transient rise in [Ca2+]iand not after 1 h. This study reveals that several temporal phases are involved in the induction of TH gene expression by nicotine, each of them with differing requirements for Ca2+ and cAMP.