Intracellular lithium and cyclic AMP levels are mutually regulated in neuronal cells

In this work, we studied the effect of intracellular 3',5'-cyclic adenosine monophosphate (cAMP) on Li+ transport in SH-SY5Y cells. The cells were stimulated with forskolin, an adenylate cyclase activator, or with the cAMP analogue, dibutyryl-cAMP. It was observed that under forskolin stimulation both the Li+ influx rate constant and the Li+ accumulation in these cells were increased. Dibutyryl-cAMP also increased Li+ uptake and identical results were obtained with cortical and hippocampal neurons. The inhibitor of the Na+/Ca2+ exchanger, KB-R7943, reduced the influx of Li+ under resting conditions, and completely inhibited the effect of forskolin on the accumulation of the cation. Intracellular Ca2+ chelation, or inhibition of N-type voltage-sensitive Ca2+ channels, or inhibition of cAMP-dependent protein kinase (PKA) also abolished the effect of forskolin on Li+ uptake. The involvement of Ca2+ on forskolin-induced Li+ uptake was confirmed by intracellular free Ca2+ measurements using fluorescence spectroscopy. Exposure of SH-SY5Y cells to 1 mm Li+ for 24 h increased basal cAMP levels, but preincubation with Li+, at the same concentration, decreased cAMP production in response to forskolin. To summarize, these results demonstrate that intracellular cAMP levels regulate the uptake of Li+ in a Ca(2+)-dependent manner, and indicate that Li+ plays an important role in the homeostasis of this second messenger in neuronal cells.

Effect of Li+ upon the Mg2+-dependent activation of recombinant Gialpha1

Although lithium salts have been used in the treatment and prophylaxis of manic-depressive or bipolar patients for 50 years, the mechanism of the pharmacologic action of Li+ is unknown. Based on activity studies of inhibitory and stimulatory guanine-binding (G) proteins in rat cortical membranes, it was proposed that Li+ inhibition of G-proteins may account for its pharmacologic action. We used the purified alpha subunit of the recombinant inhibitory G-protein, rGialpha1, and found that Li+ at therapeutic levels significantly inhibited the formation of the GDP.AlF4-.rGialpha1 complex. Because our studies were conducted with a purified, metal-reconstituted G-protein rather than with cell membrane suspensions, our Li+ inhibition results lend additional support to the G-protein hypothesis for Li+ action.

Li+/Mg2+ competition at therapeutic intracellular Li+ levels in human neuroblastoma SH-SY5Y cells

OBJECTIVES

One proposed mechanism of lithium action in the treatment of bipolar disorder is that Li+ competes with Mg2+ for Mg2+ binding sites within the cell. In this study, we investigated this competition at therapeutic intracellular Li+ levels in human neuroblastoma SH-SY5Y cells.

METHODS

We used fluorescence spectroscopy and a Mg2+ indicator, furaptra, to investigate this competition in human neuroblastoma SH-SY5Y cells. Atomic absorption spectrophotometry was used for determination of the intracellular Li+ levels.

RESULTS

The neuroblastoma cells, incubated in 15 mM or 30 mM Li+-containing buffer, showed a significant increase in free intracellular Mg2+ levels [using a positive linear within-groups contrast t-test, the 15 mM condition produced t(2) = 5.0, one-tailed p < 0.02, and the 30 mM Li+-incubation conditions gave t(2) = 9.2, one-tailed p < 0.006] but did not significantly increase over time in the Li+-free condition [t(2) = 0.1, one-tailed p > 0.96]. At the earlier times during the incubation (1 or 10 min for the 15 mM or 30 mM Li+-containing buffers), the intracellular Li+ concentrations were 0.6-2.5 mM, values which are comparable to those reached in the brain of Li+-treated patients.

CONCLUSION

We demonstrated that competition between Li+ and Mg2+ can occur at therapeutic intracellular Li+ levels.

Comparison of fluorescence, (31)P NMR, and (7)Li NMR spectroscopic methods for investigating Li(+)/Mg(2+) competition for biomolecules

The biochemical action of lithium in the treatment of manic-depressive illness is still unknown. One hypothesis is that Li(+) competes for Mg(2+)-binding sites in biomolecules. We report here our studies on metal ion competition by three distinct methods: fluorescence, (31)P NMR, and (7)Li NMR spectroscopy, using ATP as a model ligand. By fluorescence spectroscopy, we used the dye, furaptra, by measuring the increases in Mg(2+) levels in an ATP solution as Li(+) levels were increased in the solution. This increase in Mg(2+) levels was indicated by increases in the fluorescence intensity ratio (335/370) of furaptra. By (31)P NMR spectroscopy, this competition was demonstrated by changes in the (31)P NMR spectrum of ATP. The Li(+)/Mg(2+) competition was indicated by predictable changes in the separation between the alpha and beta resonances of the phosphates of ATP. For (7)Li NMR spectroscopy, spin-lattice relaxation measurements were used, which provided free Li(+) concentrations that could be used for determining the free Mg(2+) values in ATP solutions. The values of the free Mg(2+) concentrations obtained by all three methods were in good agreement. The fluorescence and (7)Li NMR methods, however, proved to be more sensitive to low concentrations of Li(+) than the (31)P NMR method.

Competition between Li+ and Mg2+ in neuroblastoma SH-SY5Y cells: a fluorescence and 31P NMR study

Because Mg2+ and Li+ ions have similar chemical properties, we have hypothesized that Li+/Mg2+ competition for Mg2+ binding sites is the molecular basis for the therapeutic action of lithium in manic-depressive illness. By fluorescence spectroscopy with furaptra-loaded cells, the free intracellular Mg2+ concentration within the intact neuroblastoma cells was found to increase from 0. 39 +/- 0.04 mM to 0.60 +/- 0.04 mM during a 40-min Li+ incubation in which the total intracellular Li+ concentration increased from 0 to 5.5 mM. Our fluorescence microscopy observations of Li+-free and Li+-loaded cells also indicate an increase in free Mg2+ concentration upon Li+ incubation. By 31P NMR, the free intracellular Mg2+ concentrations for Li+-free cells was 0.35 +/- 0. 03 mM and 0.80 +/- 0.04 mM for Li+-loaded cells (final total intracellular Li+ concentration of 16 mM). If a Li+/Mg2+ competition mechanism is present in neuroblastoma cells, an increase in the total intracellular Li+ concentration is expected to result in an increase in the free intracellular Mg2+ concentration, because Li+ displaces Mg2+ from its binding sites within the nerve cell. The fluorescence spectroscopy, fluorescence microscopy, and 31P NMR spectroscopy studies presented here have shown this to be the case.