We previously reported that lithium stimulated extracellular glutamate accumulation in monkey and mouse cerebrocortical slices. We report here that this is caused by lithium-induced inhibition of glutamate uptake into the slice. Glutamate release was amplified 5-fold over inhibition of uptake. When the effects of lithium and the specific glutamate transporter inhibitors, L-trans-pyrrolidine-2,4-dicarboxylic acid and dihydrokainic acid, were plotted as glutamate accumulation vs. inhibition of glutamate uptake, the plots were superimposable. This finding strongly indicates that lithium-induced glutamate accumulation is caused entirely by inhibition of uptake. With cerebrocortical synaptosomes, inhibition of glutamate uptake was greater than in slices, suggesting that presynaptic nerve endings are the primary site of inhibition of uptake by lithium. Inhibition of uptake was caused by a progressive lowering of V max , as the lithium concentration was increased, whereas the K m remained constant, indicating that lithium inhibited the capacity of the transporter but not its affinity. Chronic treatment of mice with lithium, achieving a blood level of 0.7 mM, which is on the low side of therapeutic, up-regulated synaptosomal uptake of glutamate. This would be expected to exert an antimanic effect. Lithium is a mood stabilizer, dampening both the manic and depressive phases of bipolar disorder. Interestingly, although the uptake of glutamate varied widely in individual control mice, uptake in lithium-treated mice was stabilized over a narrow range (variance in controls, 0.423; in lithium treated, 0.184).We previously observed that the antibipolar drug, lithium, stimulated the release of the excitatory neurotransmitter, glutamate, from cerebral cortex slices of rhesus monkey and mouse (1, 2). This release was accompanied by an increase in inositol 1,4,5-trisphosphate [Ins(1,4,5)P 3 ] accumulation. The increase in Ins(1,4,5)P 3 accumulation was caused by the selective activation of the N-methyl-D-aspartate (NMDA) receptor͞channel by glutamate. Activation of the NMDA receptor is known to cause increased Ins(1,4,5)P 3 accumulation (3).At that stage of the investigation, the mechanism of Liinduced glutamate release was not known. There were several possibilities. Lithium could increase the discharge of glutamate from presynaptic nerve endings and͞or glial cells. Lithium could inhibit the reuptake of glutamate by the glutamate transporter(s) in presynaptic nerve endings and͞or in glial cells.In the present study, we have investigated the mechanism of lithium-induced glutamate release and the chronic consequences of this inhibition. Evidence is presented that shows that glutamate release is caused by lithium-inhibited uptake of glutamate in presynaptic nerve endings (synaptosomes).The clinical benefit of lithium requires 1-2 weeks of therapy. We therefore have investigated the effect of chronic lithium ingestion in mice on synaptosomal uptake of glutamate. Synaptosomal uptake was up-regulated. Because this would remove ...

Beginning at therapeutic concentrations (1-1.5 mM), the anti-manic-depressive drug lithium stimulated the release of glutamate, a major excitatory neurotransmitter in the brain, in monkey cerebral cortex slices in a time-and concentration-dependent manner, and this was associated with increased inositol 1,4,5-trisphosphate [Ins(1,4,5) monkey (2, 3). In the case of guinea pig and rabbit, inositol 1,3,4,5-tetrakisphosphate was also increased. These effects are in contrast to cholinergically stimulated cerebral cortex slices of rat and mouse, where lithium decreased accumulation ofIns(1,4,5)P3 and inositol 1,3,4,5-tetrakisphosphate (2, 4, 5). The explanation for the species differences lies in the fact that rat and mouse cerebral cortex slices are uniquely deficient in inositol (2, 6, 7), and when the incubation medium was supplemented with inositol, cerebral cortex slices of these two species also showed lithium-stimulated increases in Ins(1,4,5)P3 and inositol 1,3,4,5-tetrakisphosphate (2).In the case of rhesus monkey, neither inositol nor an agonist was required to demonstrate lithium-stimulated accumulation of Ins(1,4,5)P3 (3). This suggested that an endogenous neurotransmitter was involved in the lithium effect. We show here that, beginning at therapeutic concentrations, lithium stimulated the release of glutamate in rhesus monkey and mouse cerebral cortex slices, and this in turn increased accumulation of Ins(1,4,5)P3 via activation of the N-methyl-D-aspartate (NMDA) receptor.

Valproic acid and lithium are effective antibipolar drugs. We recently showed that lithium stimulated the release of glutamate in monkey and mouse cerebral cortex slices, which, through activation of the N-methyl-D-aspartate receptor, increased accumulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P 3 ]. We show here that valproate behaves similarly to lithium in that at therapeutic concentrations it stimulates glutamate release and Ins(1,4,5)P 3 accumulation in mouse cerebral cortex slices. The fact that these two effects are a common denominator for two structurally unrelated antibipolar drugs suggests that these effects are important in their antibipolar action. The effects of maximal concentrations of lithium and valproate on glutamate release are additive, suggesting different mechanisms for release, which are discussed. The additivity of the two drugs on glutamate release is consistent with the clinical benefit of combining the two drugs in the treatment of subsets of bipolar patients, e.g., in rapid cycling manic-depression. Unlike lithium, valproate does not increase accumulation of inositol monophosphates, inositol bisphosphates, or inositol 1,3,4-trisphosphate. This is additional evidence against the ''inositol depletion'' hypothesis, which states that, by trapping inositol in the form of inositol monophosphates and certain inositol polyphosphates, lithium exerts its antimanic action by inhibiting resynthesis of phosphoinositides with resultant blunting of Ins(1,4,5)P 3 signaling.

Li+, beginning at a concentration as low as 1 mm, produced a time-and dose-dependent increase in accumulation of and Ins(1,3,4,5)P4 accumulation in these species, but we could reverse this inhibition by adding 10-30 mM-inositol; we then observed a Li'-induced increase in Ins(1,4,5)P3 and Ins(1,3,4,5)P4. The species differences observed in the absence of supplemented inositol were explained by the fact that a much higher concentration of inositol was required to bring the Li+-elevated levels of CDP-diacylglycerol (CDPDG) down to baseline in the rat and mouse. These data suggest that inositol is more rate-limiting for phosphatidylinositol synthesis in the presence of Li+ in rat and mouse, which can account for the previous reports of inhibition of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulation by this ion in these species. Thus, in all species examined, Li+ could be shown to increase accumulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in cholinergically stimulated brain cortex slices if the slices were supplemented with sufficient inositol to bring the Li'-elevated level of CDPDG down to near baseline, as seen in the absence of Li'. In guinea-pig brain cortex slices, increases in Ins(1,4,5)P3and Ins(1,3,4,5)P4 accumulation could then be seen at Li+ concentrations as low as 1 mm, which falls within the therapeutic range of plasma concentrations in the treatment of manic-depressive disorders. These observations may have therapeutic implications. INTRODUCTIONReceptor activation of phosphodiesteratic cleavage of phosphoinositides generates intracellular second-messenger

No abstract