Restoration of brain myo-inositol levels in rats increases latency to lithium-pilocarpine seizures

Assessing the causal association between human blood metabolites and the risk of epilepsy

Background

Metabolic disturbance has been reported in patients with epilepsy. Still, the evidence about the causal role of metabolites in facilitating or preventing epilepsy is lacking. Systematically investigating the causality between blood metabolites and epilepsy would help provide novel targets for epilepsy screening and prevention.

Methods

We conducted two-sample Mendelian randomization (MR) analysis. Data for 486 human blood metabolites came from a genome-wide association study (GWAS) comprising 7824 participants. GWAS data for epilepsy were obtained from the International League Against Epilepsy (ILAE) consortium for primary analysis and the FinnGen consortium for replication and meta-analysis. Sensitivity analyses were conducted to evaluate heterogeneity and pleiotropy.

Results

482 out of 486 metabolites were included for MR analysis following rigorous genetic variants selection. After IVW and sensitivity analysis filtration, six metabolites with causal effects on epilepsy were identified from the ILAE consortium. Only four metabolites remained significant associations with epilepsy when combined with the FinnGen consortium [uridine: odds ratio (OR) = 2.34, 95% confidence interval (CI) = 1.48–3.71, P = 0.0003; 2-hydroxystearate: OR = 1.61, 95% CI = 1.19–2.18, P = 0.002; decanoylcarnitine: OR = 0.82, 95% CI = 0.72–0.94, P = 0.004; myo-inositol: OR = 0.77, 95% CI = 0.62–0.96, P = 0.02].

Conclusion

The evidence that the four metabolites mentioned above are associated with epilepsy in a causal way provides a novel insight into the underlying mechanisms of epilepsy by integrating genomics with metabolism, and has an implication for epilepsy screening and prevention.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-022-03648-5.

What is the Role of Lithium in Epilepsy?

Lithium is a well-known FDA-approved treatment for bipolar and mood disorders. Lithium has been an enigmatic drug with multifaceted actions involving various neurotransmitters and intricate cell signalling cascades. Recent studies highlight the neuroprotective and neurotrophic actions of lithium in amyotrophic lateral sclerosis, Alzheimer's disease, intracerebral hemorrhage, and epilepsy. Of note, lithium holds a significant interest in epilepsy, where the past reports expose its non-specific proconvulsant action, followed lately by numerous studies for anti-convulsant action. However, the exact mechanism of action of lithium for any of its effects is still largely unknown. The present review integrates findings from several reports and provides detailed possible mechanisms of how a single molecule exhibits marked pro-epileptogenic as well as anti-convulsant action. This review also provides clarity regarding the safety of lithium therapy in epileptic patients.

Effects of the putative lithium mimetic ebselen on pilocarpine-induced neural activity

Lithium, commonly used to treat bipolar disorder, potentiates the ability of the muscarinic agonist pilocarpine to induce seizures in rodents. As this potentiation by lithium is reversed by the administration of myo-inositol, the potentiation may be mediated by inhibition of inositol monophosphatase (IMPase), a known target of lithium. Recently, we demonstrated that ebselen is a 'lithium mimetic' in regard to behaviours in both mice and man. Ebselen inhibits IMPase in vitro and lowers myo-inositol in vivo in the brains of mice and men, making ebselen the only known inhibitor of IMPase, other than lithium, that penetrates the blood-brain barrier. Our objective was to determine the effects of ebselen on sensitization to pilocarpine-induced seizures and neural activity. We administered ebselen at different doses and time intervals to mice, followed by injection of a sub-seizure dose of pilocarpine. We assessed seizure and neural activity by a subjective seizure rating scale, by monitoring tremors, and by induction of the immediate early gene c-fos. In contrast to lithium, ebselen did not potentiate the ability of pilocarpine to induce seizures. Unexpectedly, ebselen inhibited pilocarpine-induced tremor as well as pilocarpine-induced increases in c-fos mRNA levels. Both lithium and ebselen inhibit a common target, IMPase, but only lithium potentiates pilocarpine-induced seizures, consistent with their polypharmacology at diverse molecular targets. We conclude that ebselen does not potentiate pilocarpine-induced seizures and instead, reduces pilocarpine-mediated neural activation. This lack of potentiation of muscarinic sensitization may be one reason for the lack of side-effects observed with ebselen treatment clinically.

Concentration- and time-dependent effects of myo-inositol on evoked epileptic afterdischarge in the hippocampus in vivo

Epilepsy is one of the most widespread neurological diseases characterized by spontaneous recurrent seizures. There is no cure for epilepsy, and available pharmacological treatments with anti-seizure drugs are only symptomatic. Moreover, about third of epilepsy patients are resistant to the anti-seizure drugs. Thus, it is essential to discover new anti-epilepsy drugs. Recently, myo-inositol has been identified as a promising antiepileptic compound. In the present study, using electrophysiological method, we examined for the first time, the effect of myo-inositol on the generation of epileptic afterdischarges in the hippocampus evoked by a local electrical stimulation. This was achieved by implanting two electrodes with a cannula into the same dorsal hippocampus, which allowed for simultaneous local injection of myo-inositol or saline and afterdischarges induction and recording from the same hippocampus. We found that myo-inositol has time- and concentration-dependent effects on the evoked afterdischarges. Specifically, 5 minutes after 1 M myo-inositol infusion, the afterdischarges duration was significantly decreased as compared to preinjection durations in the same animals and also as compared to preinjection level durations in saline injected or contralateral hippocampus myo-inositol infused animals. Further, 0.055 M myo-inositol significantly decreased afterdischarges duration at 5 minutes as compared to 40 minutes post-injection. At both concentrations, the afterdischarges duration recovered to the pre-injection value at 40 minutes after the myo-inositol injection. The present data, taken together with our previous results, strongly suggest that myo-inositol has significant local seizure-suppressant effect.

Long-Term Effects of Myoinositol on Behavioural Seizures and Biochemical Changes Evoked by Kainic Acid Induced Epileptogenesis

Epilepsy is one of the most devastating neurological diseases and despite significant efforts there is no cure available. Occurrence of spontaneous seizures in epilepsy is preceded by numerous functional and structural pathophysiological reorganizations in the brain—a process called epileptogenesis. Treatment strategies targeting this process may be efficient for preventing spontaneous recurrent seizures (SRS) in epilepsy, or for modification of disease progression. We have previously shown that (i) myoinositol (MI) pretreatment significantly decreases severity of acute seizures (status epilepticus: SE) induced by kainic acid (KA) in experimental animals and (ii) that daily post-SE administration of MI for 4 weeks prevents certain biochemical changes triggered by SE. However it was not established whether such MI treatment also exerts long-term effects on the frequency of SRS. In the present study we have shown that, in KA-induced post-SE epilepsy model in rats, MI treatment for 28 days reduces frequency and duration of behavioural SRS not only during the treatment, but also after its termination for the following 4 weeks. Moreover, MI has significant effects on molecular changes in the hippocampus, including mi-RNA expression spectrum, as well as mRNA levels of sodium-MI transporter and LRRC8A subunit of the volume regulated anionic channel. Taken together, these data suggest that molecular changes induced by MI treatment may counteract epileptogenesis. Thus, here we provide data indicating antiepileptogenic properties of MI, which further supports the idea of developing new antiepileptogenic and disease modifying drug that targets MI system.

Myoinositol Attenuates the Cell Loss and Biochemical Changes Induced by Kainic Acid Status Epilepticus

Identification of compounds preventing or modifying the biochemical changes that underlie the epileptogenesis process and understanding the mechanism of their action are of great importance. We have previously shown that myoinositol (MI) daily treatment for 28 days prevents certain biochemical changes that are triggered by kainic acid (KA) induced status epilepticus (SE). However in these studies we have not detected any effects of MI on the first day after SE. In the present study we broadened our research and focused on other molecular and morphological changes at the early stages of SE induced by KA and effects of MI treatment on these changes. The increase in the amount of voltage-dependent anionic channel-1 (VDAC-1), cofilin, and caspase-3 activity was observed in the hippocampus of KA treated rats. Administration of MI 4 hours later after KA treatment abolishes these changes, whereas diazepam treatment by the same time schedule has no significant influence. The number of neuronal cells in CA1 and CA3 subfields of hippocampus is decreased after KA induced SE and MI posttreatment significantly attenuates this reduction. No significant changes are observed in the neocortex. Obtained results indicate that MI posttreatment after KA induced SE could successfully target the biochemical processes involved in apoptosis, reduces cell loss, and can be successfully used in the future for translational research.

The Inositol-3-Phosphate Synthase Biosynthetic Enzyme Has Distinct Catalytic and Metabolic Roles

Inositol levels, maintained by the biosynthetic enzyme inositol-3-phosphate synthase (Ino1), are altered in a range of disorders, including bipolar disorder and Alzheimer's disease. To date, most inositol studies have focused on the molecular and cellular effects of inositol depletion without considering Ino1 levels. Here we employ a simple eukaryote, Dictyostelium discoideum, to demonstrate distinct effects of loss of Ino1 and inositol depletion. We show that loss of Ino1 results in an inositol auxotrophy that can be rescued only partially by exogenous inositol. Removal of inositol supplementation from the ino1(-) mutant resulted in a rapid 56% reduction in inositol levels, triggering the induction of autophagy, reduced cytokinesis, and substrate adhesion. Inositol depletion also caused a dramatic generalized decrease in phosphoinositide levels that was rescued by inositol supplementation. However, loss of Ino1 triggered broad metabolic changes consistent with the induction of a catabolic state that was not rescued by inositol supplementation. These data suggest a metabolic role for Ino1 that is independent of inositol biosynthesis. To characterize this role, an Ino1 binding partner containing SEL1L1 domains (Q54IX5) and having homology to mammalian macromolecular complex adaptor proteins was identified. Our findings therefore identify a new role for Ino1, independent of inositol biosynthesis, with broad effects on cell metabolism.

The "Other" Inositols and Their Phosphates: Synthesis, Biology, and Medicine (with Recent Advances in myo-Inositol Chemistry)

Cell signaling via inositol phosphates, in particular via the second messenger myo-inositol 1,4,5-trisphosphate, and phosphoinositides comprises a huge field of biology. Of the nine 1,2,3,4,5,6-cyclohexanehexol isomers, myo-inositol is pre-eminent, with "other" inositols (cis-, epi-, allo-, muco-, neo-, L-chiro-, D-chiro-, and scyllo-) and derivatives rarer or thought not to exist in nature. However, neo- and d-chiro-inositol hexakisphosphates were recently revealed in both terrestrial and aquatic ecosystems, thus highlighting the paucity of knowledge of the origins and potential biological functions of such stereoisomers, a prevalent group of environmental organic phosphates, and their parent inositols. Some "other" inositols are medically relevant, for example, scyllo-inositol (neurodegenerative diseases) and d-chiro-inositol (diabetes). It is timely to consider exploration of the roles and applications of the "other" isomers and their derivatives, likely by exploiting techniques now well developed for the myo series.

Inositol-deficient food augments a behavioral effect of long-term lithium treatment mediated by inositol monophosphatase inhibition: an animal model with relevance for bipolar disorder

Lithium treatment in rodents markedly enhances cholinergic agonists such as pilocarpine. This effect can be reversed in a stereospecific manner by administration of inositol, suggesting that the effect of lithium is caused by inositol monophosphatase inhibition and consequent inositol depletion. If so, inositol-deficient food would be expected to enhance lithium effects. Inositol-deficient food was prepared from inositol-free ingredients. Mice with a homozygote knockout of the inositol monophosphatase 1 gene unable to synthesize inositol endogenously and mimicking lithium-treated animals were fed this diet or a control diet. Lithium-treated wild-type animals were also treated with the inositol-deficient diet or control diet. Pilocarpine was administered after 1 week of treatment, and behavior including seizures was assessed using rating scale. Inositol-deficient food-treated animals, both lithium treated and with inositol monophosphatase 1 knockout, had significantly elevated cholinergic behavior rating and significantly increased or earlier seizures compared with the controls. The effect of inositol-deficient food supports the role of inositol depletion in the effects of lithium on pilocarpine-induced behavior. However, the relevance of this behavior to other more mood-related effects of lithium is not clear.

Acute intracerebroventricular inositol does not reverse the effect of chronic lithium treatment in the forced swim test

BACKGROUND

Lithium has numerous biochemical effects but it is difficult to dissect which of these is responsible for its therapeutic action in bipolar disorder. In the current study we aimed to address one of the major hypotheses, the inositol depletion hypothesis. This hypothesis postulates that lithium's mood-stabilizing effect is mediated by the depletion of brain inositol levels and the subsequent effect on cellular signaling.

METHODS

We studied whether acute intracerebroventricular (ICV) administration of myo-inositol could reverse the antidepressant-like effect of chronic lithium treatment in the forced swim test (FST).

RESULTS

In contrast with our prediction, acute myo-inositol administration did not reverse the effect of chronic lithium to decrease immobility in the FST.

CONCLUSIONS

The results of the present study are limited due to the following: (1) inositol was given acutely while possible events downstream of inositol depletion might require a longer period and (2) ICV inositol may not have reached those areas of the brain involved in the FST.

KCNQ1, KCNE2, and Na+-coupled solute transporters form reciprocally regulating complexes that affect neuronal excitability

Na(+)-coupled solute transport is crucial for the uptake of nutrients and metabolic precursors, such as myo-inositol, an important osmolyte and precursor for various cell signaling molecules. We found that various solute transporters and potassium channel subunits formed complexes and reciprocally regulated each other in vitro and in vivo. Global metabolite profiling revealed that mice lacking KCNE2, a K(+) channel β subunit, showed a reduction in myo-inositol concentration in cerebrospinal fluid (CSF) but not in serum. Increased behavioral responsiveness to stress and seizure susceptibility in Kcne2(-/-) mice were alleviated by injections of myo-inositol. Suspecting a defect in myo-inositol transport, we found that KCNE2 and KCNQ1, a voltage-gated potassium channel α subunit, colocalized and coimmunoprecipitated with SMIT1, a Na(+)-coupled myo-inositol transporter, in the choroid plexus epithelium. Heterologous coexpression demonstrated that myo-inositol transport by SMIT1 was augmented by coexpression of KCNQ1 but was inhibited by coexpression of both KCNQ1 and KCNE2, which form a constitutively active, heteromeric K(+) channel. SMIT1 and the related transporter SMIT2 were also inhibited by a constitutively active mutant form of KCNQ1. The activities of KCNQ1 and KCNQ1-KCNE2 were augmented by SMIT1 and the glucose transporter SGLT1 but were suppressed by SMIT2. Channel-transporter signaling complexes may be a widespread mechanism to facilitate solute transport and electrochemical crosstalk.

Knockout mice in understanding the mechanism of action of lithium

Lithium inhibits IMPase (inositol monophosphatase) activity, as well as inositol transporter function. To determine whether one or more of these mechanisms might underlie lithium's behavioural effects, we studied Impa1 (encoding IMPase) and Smit1 (sodium-myo-inositol transporter 1)-knockout mice. In brains of adult homozygous Impa1-knockout mice, IMPase activity was found to be decreased; however, inositol levels were not found to be altered. Behavioural analysis indicated decreased immobility in the forced-swim test as well as a strongly increased sensitivity to pilocarpine-induced seizures. These are behaviours robustly induced by lithium. In homozygous Smit1-knockout mice, free inositol levels were decreased in the frontal cortex and hippocampus. These animals behave like lithium-treated animals in the model of pilocarpine seizures and in the Porsolt forced-swim test model of depression. In contrast with O'Brien et al. [O'Brien, Harper, Jove, Woodgett, Maretto, Piccolo and Klein (2004) J. Neurosci. 24, 6791-6798], we could not confirm that heterozygous Gsk3b (glycogen synthase kinase 3beta)-knockout mice exhibit decreased immobility in the Porsolt forced-swim test or decreased amphetamine-induced hyperactivity in a manner mimicking lithium's behavioural effects. These data support the role of inositol-related processes rather than GSK3beta in the mechanism of the therapeutic action of lithium.

Homozygote inositol transporter knockout mice show a lithium-like phenotype

OBJECTIVE

Lithium inhibits inositol monophosphatase and also reduces inositol transporter function. To determine if one or more of these mechanisms might underlie the behavioral effects of lithium, we studied inositol transporter knockout mice. We previously reported that heterozygous knockout mice with reduction of 15-37% in brain inositol had no abnormalities of pilocarpine sensitivity or antidepressant-like behavior in the Porsolt forced swim test. We now report on studies of homozygous inositol transporter knockout mice.

METHODS

Homozygote knockout mice were rescued by 2% inositol supplementation to the drinking water of the dam mice through pregnancy and lactation. Genotyping was carried out by polymerase chain reaction followed by agarose electrophoresis. Brain free myo-inositol levels were determined gas-chromatographically. Motor activity and coordination were assessed by the rotarod test. Behavior of the mice was studied in lithium-pilocarpine seizure models for lithium action and in the Porsolt forced swim test model for depression.

RESULTS

In homozygote knockout mice, free inositol levels were reduced by 55% in the frontal cortex and by 60% in the hippocampus. There were no differences in weight or motor coordination by the rotarod test. They behaved similarly to lithium-treated animals in the model of pilocarpine seizures and in the Porsolt forced swimming test model of depression.

CONCLUSIONS

Reduction of brain inositol more than 15-37% may be required to elicit lithium-like neurobehavioral effects.

IMPA1 is essential for embryonic development and lithium-like pilocarpine sensitivity

Lithium has been the standard pharmacological treatment for bipolar disorder over the last 50 years; however, the molecular targets through which lithium exerts its therapeutic effects are still not defined. We characterized the phenotype of mice with a dysfunctional IMPA1 gene (IMPA1-/-) to study the in vivo physiological functions of IMPA1, in general, and more specifically its potential role as a molecular target in mediating lithium-dependent physiological effects. Homozygote IMPA1-/- mice died in utero between days 9.5 and 10.5 post coitum (p.c.) demonstrating the importance of IMPA1 in early embryonic development. Intriguingly, the embryonic lethality could be reversed by myo-inositol supplementation via the pregnant mothers. In brains of adult IMPA1-/- mice, IMPase activity levels were found to be reduced (up to 65% in hippocampus); however, inositol levels were not found to be altered. Behavioral analysis of the IMPA1-/- mice indicated an increased motor activity in both the open-field test and the forced-swim test as well as a strongly increased sensitivity to pilocarpine-induced seizures, the latter supporting the idea that IMPA1 represents a physiologically relevant target for lithium. In conclusion the IMPA1-/- mouse represents a novel model to study inositol homeostasis, and indicates that genetic inactivation of IMPA1 can mimic some actions of lithium.

Myo-inositol-1-phosphate (MIP) synthase inhibition: in-vivo study in rats

Lithium and valproate are the prototypic mood stabilizers and have diverse structures and targets. Both drugs influence inositol metabolism. Lithium inhibits IMPase and valproate inhibits MIP synthase. This study shows that MIP synthase inhibition does not replicate or augment the effects of lithium in the inositol sensitive pilocarpine-induced seizures model. This lack of effects may stem from the low contribution of de-novo synthesis to cellular inositol supply or to the inhibition of the de-novo synthesis by lithium itself.

The behavioral actions of lithium in rodent models: leads to develop novel therapeutics

For nearly as long as lithium has been in clinical use for the treatment of bipolar disorder, depression, and other conditions, investigators have attempted to characterize its effects on behaviors in rodents. Lithium consistently decreases exploratory activity, rearing, aggression, and amphetamine-induced hyperlocomotion; and it increases the sensitivity to pilocarpine-induced seizures, decreases immobility time in the forced swim test, and attenuates reserpine-induced hypolocomotion. Lithium also predictably induces conditioned taste aversion and alterations in circadian rhythms. The modulation of stereotypy, sensitization, and reward behavior are less consistent actions of the drug. These behavioral models may be relevant to human symptoms and to clinical endophenotypes. It is likely that the actions of lithium in a subset of these animal models are related to the therapeutic efficacy, as well the side effects, of the drug. We conclude with a brief discussion of various molecular mechanisms by which these lithium-sensitive behaviors may be mediated, and comment on the ways in which rat and mouse models can be used more effectively in the future to address persistent questions about the therapeutically relevant molecular actions of lithium.

Lack of lithium-like behavioral and molecular effects in IMPA2 knockout mice

Lithium is a potent mood-stabilizing medication in bipolar disorder. Despite 50 years of clinical use, the mechanism of action is unknown. Multiple effects have been attributed to lithium including the uncompetitive inhibition of inositol monophosphatase (IMPase). IMPA2, one of the genes that encode IMPase, is located in a region with linkage to bipolar disorder. Owing to the role of IMPase in cell signaling and the possibility that this enzyme is a target for mood-stabilizing drugs, we generated IMPA2(-/-) mice. Possible involvement of IMPase in complex behaviors related to affective disorders was assessed by monitoring the behavior of the IMPA2(-/-) mice in the forced swim test, the tail suspension test (TST), the elevated zero-maze and open field test. It has been described that chronically lithium-treated mice exhibit reduced immobility time in the forced swim test and decreased exploratory behavior. We found increased rearing of IMPA2(-/-) mice in the open field, suggesting an increased exploratory behavior. Although immobility time of IMPA2(-/-) female but not male mice in the forced swim test was reduced, no difference was found between male and female IMPA2(-/-) and IMPA2(+/+) mice in the TST and overall there was no clear effect of the deletion of IMPA2 on depression-like behavior. Frontal cortex IMPase activity and inositol levels in the IMPA2(-/-) mice did not differ from IMPA2(+/+) mice, but kidney inositol levels were reduced. In conclusion, phenotypic characterization of the IMPA2(-/-) mouse indicates that deleting IMPA2 does not mimic the effects of lithium treatment.

Lithium–pilocarpine seizures as a model for lithium action in mania

Lithium (Li) pre-treatment of rats or mice given low dose pilocarpine induces a unique limbic seizure syndrome. This syndrome is stereospecifically reversed by myo-inositol, which suggests that it is a behavioral model for Li depletion of brain inositol. However, this syndrome has little face validity because seizures are not a component of bipolar disorder. Moreover, other animal species that maintain higher brain inositol levels than mice or rats do not show Li-pilocarpine seizures and a study in humans suggests that humans do not show this syndrome as well. It could be suggested that Li-pilocarpine seizures are an in vivo bioassay for inositol depletion. Recent studies of knockout mice lacking inositol monophosphatase-1 or the sodium myo-inositol transporter-1 found that both these knockout mice given pilocarpine develop limbic seizures as if they had been pre-treated with Li. These mice in addition to such pilocarpine sensitivity have other behaviors such as decreased immobility in the Porsolt forced swim test that suggests that their inositol depletion has Li-like effects. Thus, the Li-pilocarpine seizure model may, despite its lack of face validity, be a biochemical marker for a model of mania treatment in animals.

Behavioural phenotyping of sodium-myo-inositol cotransporter heterozygous knockout mice with reduced brain inositol

Inositol plays a key role in dopamine, serotonin, noradrenaline and acetylcholine neurotransmission, and inositol treatment is reported to have beneficial effects in depression and anxiety. Therefore, a reduction in brain intracellular inositol levels could be a cause of some psychiatric disorders, such as depression or anxiety. To determine the behavioural consequences of inositol depletion, we studied the behaviour of sodium-dependent myo-inositol cotransporter-1 heterozygous knockout mice. In heterozygous mice, free inositol levels were reduced by 15% in the frontal cortex and by 25% in the hippocampus, but they did not differ from their wild-type littermates in cholinergic-mediated lithium-pilocarpine seizures, in the apomorphine-induced stereotypic climbing model of dopaminergic system function, in the Porsolt forced-swimming test model of depression, in amphetamine-induced hyperactivity, or in the elevated plus-maze model of anxiety. Reduction of brain inositol by more than 25% may be required to elicit neurobehavioural effects.

Cellular plasticity cascades: genes-to-behavior pathways in animal models of bipolar disorder

BACKGROUND

Despite extensive research, the molecular/cellular underpinnings of bipolar disorder (BD) remain to be fully elucidated. Recent data has demonstrated that mood stabilizers exert major effects on signaling that regulate cellular plasticity; however, a direct extrapolation to mechanisms of disease demands proof that manipulation of candidate genes, proteins, or pathways result in relevant behavioral changes.

METHODS

We critique and evaluate the behavioral changes induced by manipulation of cellular plasticity cascades implicated in BD.

RESULTS

Not surprisingly, the behavioral data suggest that several important signaling molecules might play important roles in mediating facets of the complex symptomatology of BD. Notably, the protein kinase C and extracellular signal-regulated kinase cascades might play important roles in the antimanic effects of mood stabilizers, whereas glycogen synthase kinase (GSK)-3 might mediate facets of lithium's antimanic/antidepressant actions. Glucocorticoid receptor (GR) modulation also seems to be capable to inducing affective-like changes observed in mood disorders. And Bcl-2, amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors, and inositol homeostasis represent important pharmacological targets for mood stabilizers, but additional behavioral research is needed to more fully delineate their behavioral effects.

CONCLUSIONS

Behavioral data support the notion that regulation of cellular plasticity is involved in affective-like behavioral changes observed in BD. These findings are leading to the development of novel therapeutics for this devastating illness.

Intracerebroventricular antisense to inositol monophosphatase-1 reduces enzyme activity but does not affect Li-sensitive behavior

Inositol monophosphatase (IMPase) inhibition is a hypothesized mechanism of action of lithium (Li). To test this hypothesis, the authors used the approach of antisense administration. Three days of an intracerebroventricular (icv) administration of 5 microg/20 microl 3'-phosphorothioated IMPA-1 antisense oligonucleotide sequence resulted in 20% reduction of rat periventricular IMPase activity. Li potentiates pilocarpine-induced seizures, because inhibition of IMPase leads to reduction in brain inositol levels. However, antisense-induced reduction in IMPase activity was not followed by seizures induced by subconvulsive pilocarpine doses.

An exploratory approach to the serotonergic hypothesis of depression: bridging the synaptic gap

In this exploratory review, we attempt to integrate pre and post synaptic theories of the biochemical basis of depression--in particular with regard to 5-HT. We will be providing evidence that in major depressive disorder, there is a continuity of dysfunction of neural function, i.e. pre and post synaptic serotonergic symptoms are affected. Furthermore, we will also be providing the implications of this approach for normal treatments for depressive disorder.

Anticonvulsive activity of Albizzia lebbeck, Hibiscus rosa sinesis and Butea monosperma in experimental animals

The ethanolic extracts of leaves of Albizzia lebbeck and flowers of Hibiscus rosa sinesis and the petroleum ether extract of flowers of Butea monosperma exhibited anticonvulsant activity. The bioassay guided fractionation indicated that the anticonvulsant activity lies in the methanolic fraction of chloroform soluble part of ethanolic extract of the leaves of A. lebbeck, acetone soluble part of ethanolic extract of H. rosa sinesis flowers and acetone soluble part of petroleum ether extract of B. monosperma flowers. The fractions protected animals from maximum electro shock, electrical kindling and pentylenetetrazole-induced convulsions in mice. The fractions also inhibited convulsions induced by lithium-pilocarpine and electrical kindling. However, they failed to protect animals from strychnine-induced convulsions. The fractions antagonised the behavioral effects of D-amphetamine and potentiated the pentobarbitone-induced sleep. The fractions raised brain contents of gamma-aminobutyric acid (GABA) and serotonin. These fractions were found to be anxiogenic and general depressant of central nervous system.

A model of inositol compartmentation in astrocytes based upon efflux kinetics and slow inositol depletion after uptake inhibition

Intracellular compartmentation of inositol was demonstrated in primary cultures of mouse astrocytes, incubated in isotonic medium, by determination of efflux kinetics after "loading" with [3H]inositol. Three kinetically different compartments were delineated. The largest and most slowly exchanging compartment had a halflife of approximately 9 hr. This slow release leads to retention of a sizeable amount of pre-accumulated inositol in the tissue 24 hr after the onset of uptake inhibition, as confirmed by the observation that the inositol uptake inhibitor fucose caused a larger inhibition of unidirectional inositol uptake than of inositol pool size, measured as accumulated [3H]inositol after 24 hr of combined exposure to the inhibitor and the labeled isotope. Based upon the present observations and literature data, it is suggested that the large, slowly exchanging compartment is largely membrane-associated and participating in signaling via the phosphatidylinositide second messenger system, whereas inositol functioning as an osmolyte is distributed in the cytosol and located in one or both of the compartments showing a faster release.

Inhibition of inositol uptake in astrocytes by antisense oligonucleotides delivered by pH-sensitive liposomes

An oligonucleotide of 20 bases, complementary to a region of the sodium/myo-inositol cotransporter (SMIT) mRNA, was used to investigate the uptake efficiency and activity of transferred antisense oligonucleotides with regard to substrate uptake. We compared the efficiency of oligonucleotide delivery after application of either free or liposome-encapsulated material. Delivery of liposome-encapsulated material (marker or oligonucleotides) into astrocytoma cells and primary astrocyte cultures was more effective with pH-sensitive liposomes [dioleoylphosphatidylethanolamine (DOPE)/cholesteryl hemisuccinate (CHEMS)] than with non-pH-sensitive liposomes (soy lecithin) or free material in solution. Antisense activity was evaluated by determination of myo-inositol uptake and detection of SMIT transcripts by RT-PCR. Encapsulation of oligonucleotides in pH-sensitive liposomes increased the inhibition of inositol uptake at least 50-fold compared with application of free oligonucleotides in solution.

Nordidemnin potently inhibits inositol uptake in cultured astrocytes and dose-dependently augments lithium's proconvulsant effect in vivo

It has been suggested that inositol uptake across the cell membrane is of importance for maintenance of the inositol pool involved in lithium's therapeutic effect in bipolar disease and in the lithium-pilocarpine seizure test in freely moving rats (measuring the latency of a normally subconvulsive concentration of pilocarpine to seizure induction in the additional presence of lithium). We have tested this hypothesis by: 1) demonstrating an extremely high potency of nordidemnin as an inhibitor of myo-inositol uptake in primary cultures of mouse astrocytes; and 2) determining the dose-response correlation of a nordidemnin-induced decrease in the latency before appearance of seizures in the lithium-pilocarpine test after intracerebroventricular injection of minute samples (10 microl) of virtually isotonic saline solution.

Intrathecal lithium reduces neuropathic pain responses in a rat model of peripheral neuropathy

We tested the ability of lithium (Li(+)) to block heat hyperalgesia, cold allodynia, mechanical allodynia and mechanical hyperalgesia in rats experimentally subjected to painful peripheral neuropathy. Chronic constrictive injury (CCI) to the sciatic nerve induced persistent hyperalgesia and allodynia. Intrathecal injection of Li(+) (2.5-40 micromol) into the region of lumbar enlargement dose-dependently reduced heat hyperalgesia, cold allodynia and mechanical allodynia for 2-6 h after injection, but had no effect on mechanical hyperalgesia. Li(+) had no significant effect on responses from control and sham-operated animals. Intrathecal injection of myo-inositol (2.5 mg) significantly reversed both the anti-hyperalgesic and anti-allodynic effect of Li(+). These findings suggest that intrathecal Li(+) suppresses neuropathic pain response in CCI rats through the intracellular phosphatidylinositol (PI) second messenger system in spinal cord neurons. Lithium (Li(+)) has already found widespread clinical application; these results suggest that its therapeutic utility may be extended to include treatment of neuropathic pain syndromes resulting from peripheral nerve injury.

Interactions of lithium and drugs that affect signal transduction on behaviour in rats

The therapeutic mechanism of the action of lithium in the treatment of bipolar affective disorder is not known, in spite of a burgeoning number of biochemical studies linking lithium to signal transduction processes. This article reviews a decade of studies examining the behavioural manifestations of manipulating inositol, cyclic adenosine monophosphate (cAMP) and G proteins in rats. Inositol, forskolin, dibutyryl cAMP and pertussis toxin all interacted with lithium when rearing behavior was measured. Lithium potentiated the increase in locomotion induced by injections of cholera toxin into the nucleus accumbens, consistent with the hypothesis that it inactivates inhibitory G proteins. More specific interactions were found between lithium and inositol following cholinergic and serotonergic stimulation. Inositol, but not forskolin, attenuated lithium-pilocarpine seizures and the enhancement of the serotonin syndrome; however, inositol had no effect on lithium-induced attenuation of wet dog shakes following an injection of 5-hydroxytryptophan. Behavioural evidence supports biochemical findings suggesting that lithium's interactions with the phoshphatidyl inositol and cyclic AMP signal transduction systems may be relevant to its therapeutic effects in bipolar disorder. Further research on more specific behaviours may elucidate the relevant pharmacological mechanisms underlying the therapeutic effect of lithium.

Inositol has behavioral effects with adaptation after chronic administration

Inositol is a simple dietary polyol that serves as a precursor in important second messenger systems. Inositol in pharmacological doses has been reported recently to be therapeutic in depression, panic disorder and obsessive compulsive disorder. We hereby report effects of inositol on the elevated plus maze model of anxiety. These results should allow development of new inositol analogs that could expand psychoactive drug development possibilities via second messenger manipulation.

Lack of effect of ECS on rat brain inositol monophosphatase activity and inositol levels and of i.c.v. inositol on ictal and post-ictal length

Lithium (Li) has often been compared to ECT in therapeutic spectrum and mechanism. Inhibition of inositol monophosphatase and reduction of brain inositol are major mechanisms of Li action. Many Li effects in animals and humans are reversible by inositol. We therefore studied interactions of ECS and inositol. ECS in rats did not reduce brain inositol monophosphatase activity or brain inositol levels. Intracerebroventricular injection of 10 mg inositol, a dose that reverses Li effects, had no effect on seizure length or post-ictal length.

Epi-inositol is biochemically active in reversing lithium effects on cytidine monophosphorylphosphatidate (CMP-PA). Short communication

In CHOm3 cells and rat cerebral cortex slices, epi-inositol was less potent but as effective as myo-inositol in reversing carbachol/lithium-stimulated CMP-PA accumulation whereas L-chiro- and scyllo-inositol were less active or inactive. These results with the four inositol isomers in two tissues correlate exactly with their effects on lithium-pilocarpine induced seizures and suggest a common mechanism of action for biochemical and behavioural effects.

Inositol monophosphatase--a putative target for Li+ in the treatment of bipolar disorder

Attenuation of the phosphatidylinositol (PI) signal transduction pathway as a consequence of inhibition of inositol monophosphatase (IMPase) has been proposed as the mechanism for the efficacy of Li+ in the treatment of bipolar disorder. Nevertheless, Li+ also affects other aspects of PI signal transduction, and it is therefore not clear whether modulation of PI responses by Li+ can be attributed solely to inhibition of IMPase. However, inhibitors of IMPase mimic the effects of Li+ on some aspects of PI cell signalling, thus highlighting the potential of IMPase as a target for the treatment of bipolar disorder. The recent description of the three-dimensional structure of IMPase in conjunction with site-directed mutagenesis and kinetic studies has led to the elucidation of the enzyme mechanism. These structural and mechanistic data should prove useful in the development of novel inhibitors of IMPase that might ultimately prove useful clinically.

Modulation by inositol of cholinergic- and serotonergic-induced seizures in lithium-treated rats

Hippocampal and cortical EEG recordings in rats were used to monitor the in vivo modulation by lithium of responses to agonists for 5HT2/5HT1c serotonergic (DOI) and cholinergic (pilocarpine) receptors and the influence of inositol administration. Administration of DOI (8 mg/kg) or pilocarpine (30 mg/kg) to rats pretreated with lithium acutely (3 mmol/kg) or chronically (dietary, 4 weeks) resulted in seizures, whereas these doses did not cause seizures without lithium pretreatment. This indicated that lithium most likely affects a signal transduction process common to both systems, which is the phosphoinositide second messenger system. To examine the potential influence of altered inositol levels on these responses, we tested the effects of infusions (10 mg, i.c.v.) of myo-inositol, a precursor of phosphoinositide synthesis, and of epi-inositol, an isomer not used for phosphoinositide synthesis. Administration of myo-inositol (10 mg) slightly reduced the incidence of seizures induced by acute lithium plus DOI but almost completely blocked seizures induced by acute lithium plus pilocarpine. This was surprising since seizures induced by acute lithium plus DOI were less severe than those after acute lithium plus pilocarpine, but myo-inositol was more effective in blocking the latter. Epi-inositol also blocked seizures under both conditions but it was less effective than myo-inositol after treatment with acute lithium plus pilocarpine. The latencies to seizures and/or severity of seizures were potentiated more by chronic than acute lithium pretreatment with both DOI and pilocarpine, but attenuation by myo-inositol was less with each agonist after chronic lithium compared with acute lithium treatment. Peripheral administration of a high dose of myo-inositol blocked seizures induced by acute lithium plus pilocarpine, but the inositol treatment itself was toxic and caused seizures prior to pilocarpine administration, so the mechanism of action cannot simply be attributed to increased brain inositol levels. These results demonstrate that lithium modulates the in vivo responses to DOI and pilocarpine, most probably through an effect on the phosphoinositide signal transduction system. They also show that centrally administered myo-inositol modifies responses to these agents, but the effectiveness of epi-inositol and other results leave unclear the mechanistic basis of its actions.

Myo-inositol attenuates the enhancement of the serotonin syndrome by lithium

Lithium elicits opposite effects on two behavioural syndromes in rats: enhancement of the 5-HT1A-linked serotonin syndrome and attenuation of the 5-HT2-linked wet dog shakes. The ability of intracerebroventricular (ICV) myo-inositol or forskolin to reverse the enhancement of the serotonin syndrome by lithium was tested in rats that were fed chronic dietary lithium or control diet and injected with the serotonin agonist 5-MeODMT (5-methoxy-N, N-dimethyltryptamine). Lithium enhanced the total serotonin syndrome score and particularly flat posture and tremor. Inositol, but not forskolin, mitigated the effects of lithium. Inositol was also injected in the lateral ventricle of rats pretreated with chronic dietary lithium or regular rat chow for 3 weeks and injected with carbidopa and L-5-hydroxytryptophan (5-HTP). Lithium attenuated wet dog shakes, but inositol had no significant effect on lithium-treated or control rats. These findings suggest that the enhancement of the serotonin syndrome by lithium may be related to lithium-induced inositol depletion.

A new conotoxin affecting sodium current inactivation interacts with the delta-conotoxin receptor site

We describe a new peptide conotoxin affecting sodium current inactivation, that competes on binding with delta-conotoxin TxVIA (delta TxVIA). The amino acid sequence of the new toxin, designated conotoxin NgVIA (NgVIA), is SKCFSOGTFCGIKOGLCCSVRCFSLFCISFE (where O is trans-4-hydroxyproline). The primary structure of NgVIA has an identical cysteine framework and similar hydrophobicity as delta TxVIA but differs in its net charge. NgVIA competes with delta TxVIA on binding to rat brain synaptosomes and molluscan central nervous system and strongly inhibits sodium current inactivation in snail neurons, as does delta TxVIA. In contrast to delta TxVIA, NgVIA is a potent paralytic toxin in vertebrate systems, its binding appears to be voltage-dependent, and it synergically increases veratridine-induced sodium influx to rat brain synaptosomes. delta TxVIA acts as a partial antagonist to NgVIA in rat brain in vivo. NgVIA appears to act via a receptor site distinct from that of delta TxVIA but similar to that of Conus striatus toxin. This new toxin provides a lead for structure-function relationship studies in the delta-conotoxins and will enable analysis of the functional significance of this complex of receptor sites in gating mechanisms of sodium channels.

Behavioral evidence for the existence of two pools of cellular inositol

Lithium reduces brain inositol levels by inhibiting inositol monophosphatase. In a previous study it was found that administration of pilocarpine to Li-treated rats causes limbic seizure behavior which can be reversed by i.c.v. myo-inositol but not chiro-inositol, suggesting that this behavior is related to inositol depletion in the PI cycle. Hyponatremia can lower brain inositol and hypernatremia can raise brain inositol. We now report that induction of low brain inositol by hyponatremia followed by pilocarpine did not cause limbic seizures. Induction of high brain inositol using hypernatremia followed by Li-pilocarpine administration did not reverse limbic seizures. These data support the concept that inositol available for P1 synthesis and inositol for osmotic function are sequestered in different cellular pools.

High-dose peripheral inositol raises brain inositol levels and reverses behavioral effects of inositol depletion by lithium

Lithium (Li) reduces brain inositol levels. Berridge has suggested that this effect is related to Li's mechanism of action. It had previously been shown that pilocarpine causes a limbic seizure syndrome in lithium treated rats, and that these lithium-pilocarpine seizures are reversible by intracerebroventricular inositol administration to rats. We now show that although inositol passes the blood-brain barrier poorly, large doses of intraperitoneal (IP) inositol can also reverse Li-pilocarpine seizures. Using gas chromatography, IP inositol can raise brain inositol levels. Demonstration that inositol enters brain after peripheral administration provides a basis for possible pharmacological intervention in psychiatric disorders at the level of second messengers linked to the phosphatidylinositol cycle.

Ziskind-Somerfeld Research Award 1993. Biochemical, behavioral, and clinical studies of the role of inositol in lithium treatment and depression

Lithium (Li) reduces brain inositol levels by inhibiting the enzyme inositol monophosphatase. The enzyme inositol-1-phosphatase was measured in human red blood cells of controls, Li-free bipolar patients, and Li-treated bipolar patients and was found to be reduced by 80% in Li-treated bipolars, thus supporting the concept that chronic Li at therapeutic concentrations inhibits this enzyme. Two behaviors in rats caused by Li, reduction of rearing, and Li-pilocarpine seizures, are reversed by intracerebroventricular replenishment of inositol. The reversal is stereospecific to the naturally occurring myo-inositol; whereas the stereoisomer L-chiro-inositol is ineffective. The reversal is dose-dependent, requiring a dose consistent with known quantities of brain inositol depletion; and is time-dependent, as inositol must be given 1-8 h before stimulation. High-dose peripheral inositol also reverses the limbic seizures induced by Li-pilocarpine, and using gas chromatography was shown to increase brain inositol levels that had been reduced by Li treatment. Low-dose inositol could be shown to reverse a peripheral Li-induced side effect, polyuria/polydipsia, in rats and in patients treated with Li. A higher dose of inositol markedly reduced Hamilton Depression Ratings in 9 of 11 unipolar major depressive disorder patients previously unresponsive to tricyclics, in an open design, but had no effect on chronic schizophrenics in a controlled double-blind randomized crossover trial. A new inositol monophosphatase inhibitor, a fungal product originally discovered as a complement inhibitor, was found to act like Li and lower the seizure threshold for subconvulsant doses of pilocarpine. These data suggest that inositol monophosphatase inhibition is a key mechanism of Li's therapeutic action and that design of new inositol monophosphatase inhibitors may be a practical strategy to create new compounds with Li-like therapeutic effects.