Inositol 1,4,5-trisphosphate accumulation in brain of lithium-treated rats

Pharmacogenetics in model systems: defining a common mechanism of action for mood stabilisers

Defining the underlying causes of psychiatric disorders has provided an ongoing and intractable problem. The analysis of the genetic basis of manic depression, in particular, has been impeded by the absence of a suitable model system and by the lack of candidate causative genes. One recent approach to overcome these problems has involved identifying those genes which control the sensitivity to anti-manic drugs in a model organism. Characterisation of the role of these genes and their encoded proteins in this model has allowed the analysis of their mammalian homologues to elucidate the therapeutic role of these drugs and the possible aetiology of manic depression. This approach has been used successfully with the cellular slime mould, Dictyostelium discoideum. This article introduces the use of model systems for pharmacogenetics research. It describes the identification of prolyl oligopeptidase in D. discoideum as a modulator of inositol phosphate signalling, and the subsequent identification of a common mechanism of action of three anti-manic drugs in mammalian neurons. The use of pharmacogenetics in model systems will provide a powerful tool for the ongoing analysis of both the treatment and cause of psychiatric disorders.

Increased inositol-monophosphatase activity by lithium treatment in bipolar patients

Inhibition of inositol monophosphatase (IMPase) enzyme is the target mechanism of action of lithium. However, increased activity of mRNA levels by lithium has been reported. These two diverse effects were shown after relatively short periods of lithium administration. The aim of this study was to observe the effect of prolonged use of lithium on IMPase activity. The authors investigated IMPase activities in fresh erythrocytes and leukocytes in 63 bipolar patients (42 euthymic, 8 depressive, 13 manic episodes) and 16 control subjects. We found that erythrocyte IMPase activity was higher in lithium treated euthymic patients than non-lithium treated patients. The duration of lithium use was positively correlated with leukocyte IMPase activity. Increased IMPase activity by chronic lithium use suggests an up-regulation of the enzyme activity.

Concurrent measures of protein kinase C and phosphoinositides in lithium-treated bipolar patients and healthy individuals: a preliminary study

This study examined the hypothesis that lithium inhibits the PI signaling pathway in humans during in vivo administration by concurrently measuring PKC isozymes and platelet membrane phosphoinositides in lithium-treated patients and healthy individuals. The platelet membrane and cytosolic levels of PKC alpha, beta I, beta II, delta, and epsilon were measured using Western blotting. The relative platelet membrane contents of phosphatidylinositol (PI), phosphatidylinositol-4-phosphate (PIP), and phosphatidylinositol-4,5-bisphosphate (PIP(2)) were measured with two-dimensional thin-layer chromatography. Nine euthymic lithium-treated bipolar subjects and 11 healthy control subjects were studied. Compared to control subjects, lithium-treated bipolar patients had significantly lower levels of cytosolic PKC alpha isozyme (t-test=-3.24, d.f.=17, P=0.01) and PIP(2) platelet membrane levels (t-test=-2.51, d.f.=18, P=0.02), and a trend toward reduced levels of cytosolic PKC beta II isozyme (t=-2.17, d.f.=17, P=0.05). There was no significant correlation between PIP(2) and any of the PKC isozymes. These preliminary findings suggest that chronic lithium treatment may decrease the levels of both cytosolic PKC alpha isozyme and membrane PIP(2) in platelets of bipolar disorder patients.

Li+ and muscarine cooperatively enhance the cationic tail current in rat cortical pyramidal cells

Li+ is known to facilitate the onset of status epilepticus induced by cholinergic stimulation, although the underlying mechanisms are not clear. Under whole-cell current clamp conditions with a CsCl-based internal solution, cortical pyramidal cells display a single plateau-spike followed by a slow depolarizing afterpotential (DAP) in response to injection of a short current pulse. However, the same current pulse generated a burst of plateau-spikes after application of Li+ (2 mM) and muscarine (10 microM). As similar bursts of plateau-spikes were generated through an enhancement of the slow DAP when [K+]o was raised (Kang et al. 1998), we have investigated the effects of Li+ and muscarine on the Ca2+-dependent cationic current underlying the slow DAP, measured as the slow tail current evoked after the offset of depolarizing voltage pulses. Muscarine enhanced the amplitudes of both early and late components of the slow tail current. This effect of muscarine was markedly potentiated by Li+, while Li+ by itself affected the slow tail current only slightly. Intracellular application of heparin (0.5-1 mg/mL) suppressed the effect of muscarine in the presence of Li+. These results suggest that inositol-trisphosphate-induced Ca2+ release is involved in the cooperative enhancement of the slow tail current, and this cooperation may be one of the mechanisms underlying facilitation of the onset of epilepsy induced by these agents.

Effects of lithium deficiency in some insulin-sensitive tissues of diabetic Chinese hamsters

In this work, we report the effect of low-dose lithium carbonate on blood glucose levels and tissue lithium content in hereditary spontaneous diabetic Chinese hamsters (HSDCHs). Hepatic lithium levels are significantly lower in diabetic hamsters when compared to healthy controls: 2.05 +/- 0.26 and 3.04 +/- 0.11 micrograms/g, respectively. The same trend was observed in kidney and muscle: 18.26 +/- 0.24 vs 20.23 +/- 1.10 micrograms/g and 4.66 +/- 0.17 vs 5.95 +/- 0.67 micrograms/g, respectively. The significance level was p < 0.05 in all cases. Supplementation with lithium carbonate eliminated tissue lithium deficiency, and had a normalizing effect on blood glucose and glycosylated serum protein levels. The insulin sensitivity index (ISI) increased, thus reducing insulin resistance. Our results suggest that lithium deficiency in certain insulin-sensitive tissues may be associated with blood glucose imbalance resulting from insulin resistance.

Chronic lithium does not alter human myo-inositol or phosphomonoester concentrations as measured by 1H and 31P MRS

Lithium may act by decreasing intracellular concentrations of myo-inositol. The present study measured the effects of chronic lithium on myo-inositol concentrations in volunteers. Eleven subjects received either lithium (n = 7) or placebo (n = 4) for 7 days in a double-blind study. Myo-inositol concentrations at baseline and day 8 were measured in vivo using 1H magnetic resonance spectroscopy (MRS). The results showed that lithium did not alter brain myo-inositol concentrations compared to placebo. In 5 other subjects we used 1H MRS and 31P MRS to measure changes in both myo-inositol and phosphomonoester concentrations. This second study showed that lithium did not alter myo-inositol or phosphomonoester concentrations. Thus, the present studies do not support the hypothesis that lithium significantly affects the brain concentrations of myo-inositol or phosphomonoesters; however, it is possible these findings represent an inability to detect the changes in myo-inositol and phosphomonoester concentrations that may have occurred following lithium administration.

Li+ stimulates ATP-regulated dopamine uptake in PC12 cells

Increased levels of dopamine have been associated with schizophrenia and mania; conversely, decreased levels of dopamine are associated with depression. Since the main mechanism for the termination of dopamine's pharmacological action is by re-uptake into the presynaptic cell, the speed of dopamine transport dramatically influences the concentration of dopamine present in the synaptic cleft, which in turn could determine brain disorders. Our preliminary studies have found that ATP can stimulate dopamine transport in rat synaptosomal preparation. We have also observed this ATP-regulated moiety of the dopamine uptake system when tested in PC12 cells. The large magnitude of ATP stimulation suggests that this dopamine uptake pathway may be important in the etiology and treatment of brain disorders. In order to test the relevance of ATP-stimulated dopamine uptake, we tested the effect of lithium salts on this system. Lithium chloride, one of the first drugs used in the treatment of mania, has become one of the most important agents utilized for the treatment of manic-depression and many schizoeffective disorders. Unfortunately, despite all efforts that have been made to explain lithium's mode of action, a clear cut biochemical mechanism has not been defined. We have found that lithium chloride, at therapeutic levels, is able to stimulate the ATP-regulated component of the dopamine uptake system by 49%. The further enhancement of dopamine re-uptake by lithium ions is consistent with its therapeutic effect. It is suggested that any substance that facilitates dopamine re-uptake could be of great importance in defining a useful treatment for mania and schizophrenia, as well as depression.