Serotonin regulates cytosolic Ca2+ activity and membrane potential in a neuronal and in a glial cell line via 5-HT3 and 5-HT2 receptors by different mechanisms

The mechanisms of action of two different serotonin receptors, found in a neuronal cell line (neuroblastoma X glioma hybrid cells) and in a non-excitable glioma cell line, were explored. In both cell lines, serotonin induced a dose-dependent, transient rise of cytosolic Ca2+ activity (measured by fura-2 or indo-1 fluorescence). Ca2+ channel blockers (Ni2+ and La3+, not nifedipine) suppressed the Ca2+ response to serotonin in the hybrid cells but not in the glioma cells. After application of Ca2+ ionophores (ionomycin and A23187) in order to short-circuit internal Ca2+ stores, serotonin was still able to induce a Ca2+ response in the hybrid cells but not in the glioma cells. Serotonin dose-dependently stimulated the rate of 45Ca2+ uptake several-fold in the hybrid cells, but hardly at all in the glioma cells. Thus, in the neuronal cell line cytosolic Ca2+ activity is raised through enhancement of Ca2+ entry into the cells from the extracellular environment via 5-HT3 receptors (blocked by ICS 205–930, MDL 72222 and GR 38032 F). The depolarization response caused by serotonin in the hybrid cells is due to activation of cation conductance(s), obviously allowing entry of extracellular Ca2+. In contrast to the neuronal cell line, in the glial cell line the rise of Ca2+ activity is mediated by ketanserin-susceptible 5-HT2 receptors (not affected by treatment with pertussis toxin) mainly liberating Ca2+ from internal stores. In the glioma cells the release of Ca2+ from internal stores leads to opening of Ca2+-dependent K+ channels, responsible for the hyperpolarizing response. Thus, the neuronal and the glial cell lines might provide suitable systems in which to study the diverse cellular functions triggered by the rise of cytosolic Ca2+ activity, which is caused by different serotonin receptors.