Mood stabilizers and the cell biology of neuronal growth cones

Evaluation of the effects of propylisopropylacetic acid (PIA) on neuronal growth cone morphology

Propylisopropylacetic acid (PIA) is a constitutional isomer of valproic acid (VPA). It has previously been found to be a weak antiepileptic, but in common with mood stabilizers, causes inositol depletion and growth cone spreading, suggesting the basis of a new series of mood stabilizers. To assess this possibility, we have compared the effects of racemic (R,S)-PIA and its individual enantiomers to those of the mood stabilizers lithium (Li+), VPA and carbamazepine (CBZ). Unlike Li+ and VPA, but in common with CBZ and (R,S)-PIA, neither (R)-PIA nor (S)-PIA enantiomer induces T-cell factor (TCF)-mediated gene expression. However, as seen for other mood stabilizers, both enantiomers are potent inducers of growth cone spreading. To investigate the mechanism for these effects, we examined changes in the actin cytoskeleton following drug treatment with Li+, VPA, CBZ, (R,S)-PIA or its individual enantiomers. All exhibit a redistribution of F-actin to the growth cone periphery, a feature of spread growth cones. (R,S)-PIA has the strongest effect as it also elevates F-actin polymerization at the cell periphery. This change in the actin cytoskeleton is associated with a substantial increase in F-actin-rich protrusions on the surface of the growth cone and in its close vicinity. These results demonstrate an effect of (R,S)-PIA on the neuronal actin cytoskeleton shared in common with other mood stabilizers, and suggest a potential to induce structural changes within the CNS.

Lithium normalizes elevated intracellular sodium

BACKGROUND

Both mania and bipolar depression are characterized by elevations of intracellular sodium concentrations. This observation has been purported to be central to the pathophysiology of abnormal moods in bipolar illness. Reduction of sodium influx is a proposed shared mechanism of action of effective mood stabilizers, but direct documentation of this effect for lithium has never been demonstrated.

METHODS

Flame spectroscopic determinations of intracellular sodium concentration were performed in the human glioma cell line, LN292, after treatment with the sodium pump inhibitor, ouabain, and co-treatment with ouabain and lithium.

RESULTS

Ouabain 0.1 microM doubles the intracellular sodium concentration after 3 days. Pretreatment with lithium 1 mM for 1 week normalizes intracellular sodium.

CONCLUSION

This is the first demonstration that lithium can normalize abnormally elevated intracellular sodium levels. This may be an important mechanism of lithium action.

Specificity of mood stabilizer action on neuronal growth cones

OBJECTIVES

Lithium, valproic acid (VPA) and carbamazepine (CBZ) are commonly used mood stabilizers, but their therapeutic mechanism is unclear. These drugs all cause the same morphological effects on postnatal rat neuronal dorsal root ganglia (DRG) growth cones via an inositol-reversible mechanism. However, due to limitations in earlier analysis, the effects of combining drugs, drug specificity and inositol stereoisomer specificity are unknown. We devised an improved analytical method to address these issues.

METHODS

Dorsal root ganglia explants were cultured individually and incubated with combinations of psychotropic drugs and inositol stereoisomers. We recorded axonal growth cone morphology and calculated growth cone area per a modified method described by Williams et al. (Nature 2002; 417: 292-295). Statistically significant changes in area were calculated using non-parametric statistical testing.

RESULTS

(i) Lithium and VPA showed an additive effect on growth cone spreading. (ii) Among eight additional psychotropic drugs to those previously tested, only imipramine and chlorpromazine altered DRG growth cone morphology. As this effect was not reversed by myo-inositol, it arises from a different mechanism to the mood stabilizers lithium, VPA and CBZ. (iii) Myo-inositol, but not scyllo- or epi-inositol, causes a significant reversal of the lithium effect on the growth cones spreading, consistent with the inositol depletion hypothesis.

CONCLUSIONS

These results show that lithium, VPA and CBZ are unique in causing altered neuronal morphology via myo-inositol depletion.

Lithium and bipolar mood disorder: the inositol-depletion hypothesis revisited

Inositol, a simple six-carbon sugar, forms the basis of a number of important intracellular signaling molecules. Over the last 35 years, a series of biochemical and cell biological experiments have shown that lithium (Li(+)) reduces the cellular concentration of myo-inositol and as a consequence attenuates signaling within the cell. Based on these observations, inositol-depletion was proposed as a therapeutic mechanism in the treatment of bipolar mood disorder. Recent results have added significant new dimensions to the original hypothesis. However, despite a number of clinical studies, this hypothesis still remains to be either proven or refuted. In this review of our current knowledge, I will consider where the inositol-depletion hypothesis stands today and how it may be further investigated in the future.