Elsevier

Brain Research

Volume 880, Issues 1–2, 13 October 2000, Pages 84-91
Brain Research

Research report
Chronic lithium and sodium valproate both decrease the concentration of myo-inositol and increase the concentration of inositol monophosphates in rat brain

https://doi.org/10.1016/S0006-8993(00)02797-9Get rights and content

Abstract

One of the mechanisms underlying lithium’s efficacy as a mood stabilizer in bipolar disorder has been proposed to be via its effects on the phosphoinositol cycle (PI-cycle), where it is an inhibitor of the enzyme converting inositol monophosphates to myo-inositol. In contrast, sodium valproate, another commonly used mood stabilizer, appears to have no direct effects on this enzyme and was thus believed to have a different mechanism of action. In the present study, high resolution nuclear magnetic resonance (NMR) spectroscopy was used to study the chronic effects of both lithium and sodium valproate on the concentrations of myo-inositol and inositol monophosphates in rat brain. As predicted, lithium-treated rats exhibited a significant increase in the concentration of inositol monophosphates and a significant decrease in myo-inositol concentration compared to saline-treated controls. However, unexpectedly, sodium valproate administration produced exactly the same results as lithium administration. These novel findings suggest that both lithium and sodium valproate may share a common mechanism of action in the treatment of bipolar disorder via actions on the PI-cycle.

Introduction

Bipolar disorder is a serious psychiatric illness affecting at least 1% of the population and is characterized by fluctuations between manic and depressed mood states [11]. While the physiological basis of this condition remains to be elucidated, the search for common mechanisms of action between mood stabilizers may provide insight into the causes of bipolar disorder.

Although lithium has remained the primary treatment of bipolar disorder for decades, other medications like sodium valproate are now commonly used as mood stabilizers [29]. One widely accepted hypothesis used to explain lithium’s mechanism of action is the inositol-depletion hypothesis [3] outlined in Fig. 1. This hypothesis proposes that uncompetitive inhibition of inositol monophosphatase (IMPase) by lithium [17], [36] leads to an accumulation of inositol monophosphates and a corresponding depletion of myo-inositol, particularly in overactive cells. Regarding lithium’s effects on the PI-cycle, animal studies have provided support for this hypothesis [1], [19], [28], [41]. A number of in vitro cell studies have also found changes consistent with the inositol depletion hypothesis following stimulation of the PI cycle via receptor linked activation by a suitable agonist [15], [18], [42], [46]. In contrast to lithium, valproate does not appear to inhibit IMPase [47] thus leading to the conclusion that this drug should not affect concentration levels of myo-inositol or inositol monophosphates.

As observed in Fig. 1, the generation of two important intracellular second messengers, inositol 1,4,5-triphosphate (Ins[1,4,5]P3) and sn-1,2-diacylglycerol (DAG), occurs within the PI-cycle. Their formation is thus dependent on the efficient breakdown of inositol phosphates and subsequent formation of inositol. A dampening of the PI-cycle by lithium has a number of implications for cell function based on the roles of Ins[1,4,5]P3 and DAG in mediating Ca2+ release from the endoplasmic reticulum and protein kinase C (PKC) activity, respectively.

In vivo magnetic resonance spectroscopy (MRS) is increasingly being used to examine the neurochemical basis of psychiatric illness, and the effects of psychiatric medications. MRS has gained popularity in the study of bipolar disorder because myo-inositol concentrations can be measured using 1H MRS. Quantitative in vivo 1H MRS studies of bipolar patients treated with or without lithium have revealed that myo-inositol decreases following lithium treatment [34], [35] and that bipolar patients may have elevated pre-treatment levels of myo-inositol [51].

Human 31P MRS has also been used extensively in studies of bipolar disorder, since the inositol monophosphates are contained in the phosphomonoester (PME) peak of 31P spectra. In one study yielding results consistent with the inositol depletion hypothesis, an increase in the PME peak was found in lithium-treated healthy controls as compared to placebo-treated controls following amphetamine stimulation of the PI-cycle [44]. A number of other studies have found increased PME peak ratios in manic and depressed bipolar patients [21], [23], [24] and decreased PME peak ratios during the euthymic phase of the illness compared to healthy controls [21], [24]. However, these effects have been observed in both medicated and unmedicated bipolar patients [9], [10], [21] and thus it is difficult to isolate drug effects from the effects of the illness.

In the present study, we examined the effects of chronic lithium and sodium valproate administration on the concentrations of myo-inositol and inositol monophosphates in whole rat brain. Acute d-amphetamine was administered and tested as a possible in vivo stimulant of the PI-cycle. Also examined were the effects of these two drugs on glycine, the other brain neurochemical contributing to the human in vivo 1H myo-inositol resonances, and glucose-6-phosphate (G6P) and phosphocholine (PC), two compounds which contribute to the human in vivo 31P PME. High resolution in vitro NMR was used to resolve these peaks into their component parts, which cannot yet be accomplished in humans in vivo. Finally, we wished to examine the effects of lithium and valproate on the commonly used in vivo 1H reference peaks of creatine+phosphocreatine (Cr+PCr) and N-acetyl aspartate (NAA) which are often used in the expression of ratio data where absolute quantification methods are not employed.

Section snippets

Materials and methods

This study was approved by the local ethics committee and all procedures were carried out in accordance with the guidelines of the Canadian Council on Animal Care. Adult male Sprague–Dawley rats (Ellerslie Biosciences), weighing 250–350 g were housed in Plexiglas cages. The rats were given free access to food and water and were maintained on an alternating 12-h light/12-h dark cycle. Injections of lithium, sodium valproate, and saline were started 4 days after the rats arrived giving them an

1H NMR spectroscopy

Myo-inositol has been previously shown to give multiplet signals at 3.28, 3.54, 3.62, and 4.06 ppm in 1H NMR spectra of rat brain extracts [2]. Because the signals at 3.54 and 3.62 ppm are well resolved and relatively free of overlap from other metabolite signals, an average area of these two multiplets was used to quantify myo-inositol concentrations in the brain extracts. Fig. 2 shows a cropped 500 MHz 1H NMR spectrum of the brain extracts focused on the region where these two peaks are

Effects of lithium and sodium valproate on the PI-cycle

The inositol depletion hypothesis was originally proposed to explain lithium’s clinical effectiveness following evidence of IMPase inhibition by lithium. For the first time we have demonstrated that therapeutic doses of either lithium or sodium valproate increase the concentration of inositol monophosphates and decrease the concentration of myo-inositol following chronic administration in rats. However, it is unlikely that both of these drugs act directly via IMPase, since only lithium has been

Acknowledgements

Grant support provided by the Medical Research Council of Canada, Canadian Psychiatric Research Foundation, Alberta Heritage Foundation for Medical Research. We also wish to thank Dr Glen Baker and Mrs Gail Rauw of the Neurochemical Research Unit, Department of Psychiatry, University of Alberta, for exceptional technical support and advice on the assay development, and Mr Richard Strel for technical assistance.

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