Potentiation of malaoxon-induced convulsions by lithium: early neuronal injury, phosphoinositide signaling, and calcium

Molecular targets of lithium action

Lithium is an effective drug for both the treatment and prophylaxis of bipolar disorder. However, the precise mechanism of lithium action is not yet well understood. Extensive research aiming to elucidate the molecular mechanisms underlying the therapeutic effects of lithium has revealed several possible targets. The behavioral and physiological manifestations of the illness are complex and are mediated by a network of interconnected neurotransmitter pathways. Thus, lithium's ability to modulate the release of serotonin at presynaptic sites and modulate receptor-mediated supersensitivity in the brain remains a relevant line of investigation. However, it is at the molecular level that some of the most exciting advances in the understanding of the long-term therapeutic action of lithium will continue in the coming years. The lithium cation possesses the selective ability, at clinically relevant concentrations, to alter the PI second-messenger system, potentially altering the activity and dynamic regulation of receptors that are coupled to this intracellular response. Subtypes of muscarinic receptors in the limbic system may represent particularly sensitive targets in this regard. Likewise, preclinical data have shown that lithium regulates arachidonic acid and the protein kinase C signaling cascades. It also indirectly regulates a number of factors involved in cell survival pathways, including cAMP response element binding protein, brain-derived neurotrophic factor, bcl-2 and mitogen-activated protein kinases, and may thus bring about delayed long-term beneficial effects via under-appreciated neurotrophic effects. Identification of the molecular targets for lithium in the brain could lead to the elucidation of the pathophysiology of bipolar disorder and the discovery of a new generation of mood stabilizers, which in turn may lead to improvements in the long-term outcome of this devastating illness (1).

Neuropharmacological mechanisms of nerve agent-induced seizure and neuropathology

This paper proposes a three phase "model" of the neuropharmacological processes responsible for the seizures and neuropathology produced by nerve agent intoxication. Initiation and early expression of the seizures are cholinergic phenomenon; anticholinergics readily terminate seizures at this stage and no neuropathology is evident. However, if not checked, a transition phase occurs during which the neuronal excitation of the seizure per se perturbs other neurotransmitter systems: excitatory amino acid (EAA) levels increase reinforcing the seizure activity; control with anticholinergics becomes less effective; mild neuropathology is occasionally observed. With prolonged epileptiform activity the seizure enters a predominantly non-cholinergic phase: it becomes refractory to some anticholinergics; benzodiazepines and N-methyl-D-aspartate (NMDA) antagonists remain effective as anticonvulsants, but require anticholinergic co-administration; mild neuropathology is evident in multiple brain regions. Excessive influx of calcium due to repeated seizure-induced depolarization and prolonged stimulation of NMDA receptors is proposed as the ultimate cause of neuropathology. The model and data indicate that rapid and aggressive management of seizures is essential to prevent neuropathology from nerve agent exposure.

Malaoxon-induced brain phosphoinositide turnover and changes in brain calcium levels by female gender in pregnant and non-pregnant convulsing and non-convulsing rats

Alterations in malaoxon-(MO)-induced brain regional phosphoinositide (PI) turnover and in brain calcium levels were studied in female non-pregnant and pregnant rats, and in their offspring. The adult rats were followed for 1 or 4 h after MO for tonic-clonic convulsions. A dose of 8.2 mg kg-1 of MO caused similar convulsions in 74% of the pregnant rats as we have reported in young male rats with a dose of 39.2 mg kg-1. However, convulsions did not occur in non-pregnant female rats. Inositol and inositol monophosphate levels were similar in all control rats. MO decreased brain inositol both in pregnant and non-pregnant female rats, and in the cerebellum of the offspring. In contrast to the findings in male rats, MO only randomly increased brain inositol-1-phosphate in female rats, or in their offspring. However, cerebral inositol-4-phosphate levels were similarly increased both in the non-pregnant and the pregnant rats irrespectively of convulsions. MO did not elevate cerebral Ca2+ in female rats or their offspring, in contrast to the male rats. The present results suggest that female rats are more sensitive than male rats to MO-induced PI signalling, and during pregnancy, also to MO-induced overt convulsions, but not to changes in cerebral Ca2+.

Carbachol- and KCl-induced changes in phosphoinositide metabolism and free calcium in guinea pig cerebral cortex synaptosomes

Phosphoinositide (PI) and calcium metabolism were studied in guinea pig cerebral cortex synaptosomes. Mass amounts of inositol and inositol monophosphates, and the levels of free intrasynaptosomal calcium ([Ca2+]i) were measured after KCl (60 mM), after a direct cholinergic agonist carbachol (CA, 1mM), and after their combination. Inositol, inositol-1-phosphate (Ins1P), inositol-4-phosphate (Ins4P) and [Ca2+]i were measured with and without 10 mM LiCl in the incubation medium. CA-induced cholinergic stimulation elevated synaptosomal Ins4P levels by 40% but did not affect Ins1P or [Ca2+]i. On the contrary, KCl elevated Ins1P by 50% and [Ca2+]i by 40% above the resting level, and decreased inositol by 20%, whereas no alterations in Ins4P occurred. CA did not modify the responses of KCl, but KCl abolished the elevation of Ins4P by CA. LiCl attenuated KCl-induced elevation of Ins1P but amplified the CA-induced elevation of Ins4P. The elevation of presynaptic [Ca2+]i was accompanied by accumulation of Ins1P but not that of Ins4P. Hence, the present results suggest that presynaptic cholinergic stimulation and KCl-induced depolarization may activate different degradation pathways of inositolphosphate metabolism.

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

Lithium pretreatment in rats potentiates the epileptogenic effects of pilocarpine and other cholinergic agonists. In order to determine if this effect of lithium could be reversed by myo-inositol, rats were pretreated with intracerebroventricular (ICV) injections of myoinositol, artificial CSF or L-chiro-inositol. Lithium chloride, 3 meq/kg was administered intraperitoneally 20-24 h prior to the subcutaneous injection of pilocarpine, 20 or 30 mg/kg. In both experiments, myo-inositol significantly prolonged the latency to the appearance of clonic seizures and lowered the pilocarpine seizure score. myo-Inositol prevented the development of clonic seizures in 50% of the rats receiving pilocarpine, 20 mg/kg. The levels of cortical myo-inositol in rats injected with myo-inositol were approximately double those of the CSF and L-chiro-inositol groups.

Lithium-induced decrease of brain inositol and increase of brain inositol-1-phosphate is transient

The effects of a single dose of LiCl (2.5 or 10 mEq/kg) on brain inositol and inositol-1-phosphate (Ins1P), intermediates of brain phosphoinositide (PI) turnover, were determined in male Han: Wistar rats. There was a remarkable, 36-58 fold elevation of brain Li+ as the single dose of LiCl was increased 4-fold. Moreover, the accumulation of brain lithium was slow during repeated administration of LiCl. Brain lithium did not correlate with changes in brain PI turnover either after a single or repeated doses. Thus, after a single dose of LiCl the increases in brain Ins1P were much less than the decreases in brain inositol. Also, brain inositol was significantly decreased only with the high dose of LiCl whereas brain Ins1P accumulation was more prominent with the lower dose. Moreover, repeated daily doses of LiCl only transiently increased brain Ins1P at 1 and 7 d whereas inositol remained at control levels throughout the 14 d observation period. Lithium probably caused the transient decrease in brain inositol by inhibiting several enzymes, in addition to the inhibition of myo-inositol mono-phosphates, in the PI cycle. Moreover, a slow dampening down of PI turnover by lithium, possible via an inhibitory action on G-protein-coupling, may also explain the present findings.

Cerebral lithium, inositol and inositol monophosphates

Cerebral regional inositol, inositol-1-phosphate (Ins1P), and inositol-4-phosphate (Ins4P), intermediates in phosphoinositide (PI) cycle, and brain lithium levels were studied in male Han:Wistar rats 24 hr after an intraperitoneal injection of a single dose (2.5-18 mEq./kg) of LiCl. A dose of LiCl higher than 5 mEq/kg caused a remarkable accumulation of Li+ in the brain. Basal brain regional inositol levels (17-22 mmol/kg) were reduced by 6-8 mmol/kg dry brain tissue at doses exceeding 5 mEq/kg of LiCl in all brain regions except the piriform cortex. However, higher doses of LiCl did not cause any further decrease in brain inositol. LiCl increased basal brain regional Ins1P levels (170-240 mumol/kg) by 0.8 mmol/kg dry brain tissue at most, and there were no consistent additional increases of Ins1P at LiCl doses exceeding 5 mEq./kg. Moreover, lithium slightly decreased regional cerebral concentrations of Ins4P. Thus, lithium-induced accumulation of Ins1P or changes of Ins4P levels do not explain lithium-induced decrease in cerebral inositol. Effects of lithium on brain P1 turnover are likely to be multifocal and to differ markedly at different concentrations of Li+ in the brain.

Lithium modifies convulsions and brain phosphoinositide turnover induced by organophosphates

Inositol-1-phosphate (Ins1P), an index of phosphoinositide (PI) turnover, was measured in frontal and piriform cortices, caudate, thalamus, hippocampus and cerebellum in saline or LiCl (5 m Eq./kg) pretreated rats 60 min. after graded doses of DFP, paraoxon, or soman. DFP only produced bursts of convulsive activity whereas both paraoxon and soman produced prolonged tonic-clonic convulsions. All three organophosphates (OP) produced convulsions at a lower dose in LiCl than in saline pretreated rats. Regional Ins1P correlated better with the presence or absence of convulsions than with the dose of paraoxon or soman. This was true both in saline and LiCl pretreated rats. In saline pretreated non-convulsing rats, there was a cholinergic increase (1.5-2.0 X) in Ins1P in all brain regions except cerebellum after OP injection. In saline pretreated convulsing rats, there was a marked seizurogenic further increase in Ins1P; highest in caudate (8 X) and cortex (6 X). In LiCl pretreated nonconvulsing rats, the OP-induced cholinergic increase in Ins1P was significant only in caudate, thalamus and hippocampus. In LiCl pretreated convulsing rats, the further seizurogenic increase in Ins1P was less than in saline pretreated rats except in thalamus and hippocampus. Thus, OP produce both a cholinergic and a seizurogenic increase in PI turnover. These data suggest that increased PI turnover in the hippocampus may indicate a lithium-induced lowering of the seizure threshold for OP in limbic regions.