The uptake of low concentrations of lithium ions into rat cerebral cortex slices and its dependence on cations

Astrocytic alkalinization by therapeutically relevant lithium concentrations: implications for myo-inositol depletion

RATIONALE

One theory for therapeutic effects of the lithium ion (Li+) in bipolar disorder is that myo-inositol, needed for phospholipase C-mediated signaling, is depleted by Li(+)-induced inhibition of inositolphosphate hydrolysis or of myo-inositol uptake, an effect demonstrated in cultured mouse astrocytes at high myo-inositol concentrations. In contrast, myo-inositol uptake is inhibited at low concentrations, reflecting that it occurs both by the high-affinity Na(+)-dependent myo-inositol transporter (SMIT) and the lower-affinity H(+)-dependent inositol transporter (HMIT). Increased intracellular pH (pHi) stimulates SMIT but inhibits HMIT, suggesting that the effect of Li+ could be caused by intracellular alkalinization. In this study, we therefore investigated Li+ effects on intracellular pH in astrocytes, measured by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) fluorescence.

RESULTS

Chronic treatment with the therapeutically relevant Li+ concentration of 1 mM for 2 or 3 weeks increased pHi by approximately 0.10, whereas 0.5 mM was ineffective, and 2 mM caused a larger increase. The alkalinization resulted from acute stimulation of the Na+/H+ exchanger (NHE) by extracellular Li+, demonstrated after acid load with NH4Cl. In response to continuous stimulation, NHE1 mRNA was down-regulated, but protein was not.

CONCLUSIONS

Chronic treatment with pharmacologically relevant Li+ concentrations increases pHi in astrocytes, creating conditions for decreased uptake of high myo-inositol concentrations and increased uptake of low concentrations. The pharmacological relevance of this effect is supported by literature data suggesting brain acidosis in bipolar patients and by preliminary observations that carbamazepine and valproate also increase pHi in astrocytes. Stimulation of NHE1-stimulated sodium ion uptake might also trigger uptake of chloride ions and osmotically obliged water.

Depolarization of brain cortex slices and synaptosomes by lithium. Determination of K+-equilibrium potential in cortex slices

K+-equilibrium potential was determined in brain cortex slices of rat by measuring 86Rb+ distribution between the extra- and intracellular space. The ratio of internal to external Rb+ concentration was 39 +/- 1.8, corresponding to a resting membrane potential of 93.8 mV. Li+ (1-126 mM) decreased the membrane potential in both cortex slices and synaptosomes in a concentration-dependent manner. The presence of 1 mM Li+ was enough to cause a slight but distinct depolarization. During incubation in Li+-containing medium slices took up K+; however, for depolarization the presence of extracellular Li+ seemed to be necessary.

Effects of lithium on electrical activity and potassium ion distribution in the vertebrate central nervous system

Three different regions of the vertebrate central nervous system maintained in vitro (frog spinal cord, guinea pig olfactory cortex and hippocampus) have been used to investigate how Li+ influences membrane potential, membrane resistance, action potentials, synaptic potentials and the transmembrane K+-distribution of neurons and glial cells. In view of the therapeutic action of Li+ in manic-depressive disease, a special effort was made to determine the threshold concentration for the actions of Li+ on the parameters described above. It was observed that Li+ induced a membrane depolarization of both neurons and glial cells, a decrease of action potential amplitudes, a facilitation of monosynaptic excitatory postsynaptic potentials and a depression of polysynaptic reflexes. The membrane resistance of neurons was not altered. Li+ also induced an elevation of the free extracellular potassium concentration and a decrease of the free intracellular potassium concentration. Furthermore, in the presence of Li+ a slowing of the recovery of the membrane potential of neurons and glial cells, and of the extracellular potassium concentration after repetitive synaptic stimulation was observed. The threshold concentrations for the effects of Li+ were below 5 mmol/l in the frog spinal cord and below 2 mmol/l in the guinea pig olfactory cortex and hippocampus. The basic mechanism underlying the action of Li+ may be an interaction with the transport-function of the Na+/K+ pump.

Lithium attenuates dopamine depleting effects of reserpine and tetrabenazine but not that of alpha methyl-p-tyrosine

Concurrent administration of lithium (Li) significantly attenuates the dopamine (DA) depleting effects of reserpine and tetrabenazine, but does not change alpha-methyl-p-tyrosine (AMPT) induced DA depletion in rat brain. This effect of Li is probably mediated, in part, by inhibiting the magnesium-dependent binding of both reserpine and tetrabenazine to their specific receptor sites. Such interaction between these drugs may attenuate the beneficial effects of tetrabenazine and reserpine on patients with tardive dyskinesia or tardive dystonia who are treated concurrently with lithium for their psychiatric disorder.

Lithium interferes with reserpine-induced dopamine depletion

Long-term lithium treatment attenuated the hypokinetic effect of reserpine in a patient with tardive dyskinesia. In rats, prophylactic lithium administration inhibits reserpine-induced dopamine depletion in the brain. These data indicate that lithium may limit the therapeutic efficacy of reserpine in tardive dyskinesia.

Lithium-potassium interaction in acutely treated cortical neurons and astrocytes

Pure mouse primary cultures of cortical astrocytes and of cortical neurons were exposed to 1 mM Li+, i.e., a therapeutically relevant concentration. The 42K uptake rates of neurons were not influenced, whereas those of astrocytes showed an 11% inhibition (P less than 0.01). Internal loading with Li+ did not change the K+ uptake rates in either cell type. Neurons, which had been exposed for 5 min to veratridine, a situation which mimics neuronal activity, showed also no change in K+ uptake rate when Li+ was present during this time. Na+-K+ ATPase activity from cell homogenates was not changed in neuronal preparations by exposure to Li+, but astrocytic preparations appeared to show a slight increase by 14%. These experiments point out that the Li+ effects on ion distribution of the brain which have been described in the literature, are due to partly impairment of astrocytic K+ uptake. The mechanism of action, underlying the Li+ effect is probably a competition with K+ for transport sites at the external site of the Na+-K+ ATPase. This leads to a decrease of K+ uptake, but an enhancement of ATPase activity in the presence of Li+.

Some fundamental theoretical and practical problems associated with neurochemical techniques in mammalian studies

The assumptions inherent in (i) a pharmacokinetic experiment in vivo, (ii) a subcellular fractionation, (iii) a chemical assay in an isolated neuron, are listed as three examples of well known neurochemical techniques. From these lists of assumptions a number of general hiatuses in knowledge of the effect of preparation of tissue on the results of experiment have been identified. The importance and different kinds of control experiments are discussed. Comment is made on how the different parameters to which measurements are referred may affect the result of the experiments. Optimal techniques are preferably non-disruptive and non-invasive. A few new techniques are proposed.

Transport pathways for lithium ions in neuroblastoma x glioma hybrid cells at 'therapeutic' concentrations of Li+

The pathways of Li+ transport in neuroblastoma X glioma hybrid cells were studied at 2 mM external Li+. Five components of Li+ transport were identified. (1) A Na+-dependent Li+ countertransport system mediating Li+ transport in both directions across the plasma membrane. This transport pathway is insensitive to ouabain or external K+. It shows trans-stimulation (i.e. acceleration of Li+ extrusion by external Na+ and stimulation of Li+ uptake by internal Na+) and cis-inhibition (i.e. reduction of Li+ uptake by external Na+). (2) The Na+-K+ pump mediates Li+ uptake but not Li+ release in cells with physiological Na+ and K+ content. Li+ uptake by the pump in choline media is inhibited by both external Na+ and K+. In Na+ media, external K+ exhibits a biphasic effect: in concentrations up to about 1 mM, K+ accelerates, and at higher concentrations, K+ inhibits Li+ uptake by the pump. (3) Li+ can enter the voltage-dependent Na+ channel. Li+ uptake through this pathway is stimulated by veratridine and scorpion toxin, the stimulation being blocked by tetrodotoxin. Residual pathways comprise (4) a saturable component, which is comparable to basal Na+ uptake, and (5) a ouabain-resistant component promoting Li+ extrusion against an electrochemical gradient in choline media. The mechanisms for Li+ extrusion described here possibly explain how neuronal cells maintain the steady-state ratio of internal to external Li+ below 1 during chronic exposure to 1-2 mM external Li+.

Acute and chronic effects of lithium in therapeutically relevant concentrations on potassium uptake into astrocytes

Potassium uptake into astrocytes in primary cultures was measured by the aid of 42K. Acute application of lithium in concentrations of 1 and 5 mM, but not 0.5 und 0.25 mM, exerted a significant inhibition of the potassium uptake rates. This effect is due to a partial impairment of the ouabain-sensitive part of the uptake into the cells caused by a lithium interaction with the extracellular K+-activated site of the Na+, K+-ATPase. After 14 days of exposure of the astrocytes to 1 mM lithium, the potassium uptake remained lower in the presence of lithium than in its absence. However, the cells had adjusted to the chronic presence of lithium by increasing their potassium uptake to such an extent that, during the exposure to 1 mM lithium, it was indistinguishable from that in cultures from the same batches grown in the absence of lithium and measured in the absence of this compound. The interference by lithium with potassium uptake into astrocytes may well be related to the inhibition of potassium clearance in the CNS described in the literature.

The influence of external sodium and potassium on lithium uptake by primary brain cell cultures at "therapeutic" lithium concentration

The ionic regulating of lithium homeostasis and steady-state intra:extracellular lithium distribution in the brain can be approached by experimental methods using intact nerve cells in vitro. Primary cultures prepared from chick embryonic brain were applied to study the effect of extracellular sodium and potassium on the lithium uptake of nerve cells at 'therapeutic' lithium concentration (1.5 mM). Lithium influx and the level of steady-state intracellular lithium were significantly reduced by increasing the external sodium concentration. At physiological extracellular sodium level, the steady-state content of lithium in the brain cells was about half of that observed in the presence of 10 mM sodium in the incubation media and the value of the intra:extracellular lithium distribution ratio was below 1. External potassium (0.5 - 3mM) strongly inhibited lithium uptake of the nerve cells. Ouabain (10(-4)M) had no effect on this potassium-sensitive lithium uptake in Tyrode media. Sodium influx studied by isotope tracer methodology was higher in cultures preloaded with lithium as compared to that of the controls. It can be concluded that sodium and potassium ions, at physiological concentrations, significantly influence lithium uptake as well as the intra:extracellular lithium distribution in brain cell cultures.

The pharmacokinetics of lithium in the brain, cerebrospinal fluid and serum of the rat

1 Addition of lithium carbonate (55 mmol/kg dry wt.) to the diet of rats for 4 days resulted in ratios between lithium in the brain and serum and between the cerebrospinal fluid (CSF) and serum of approx. 1 and 0.4, respectively. The relationships between the concentrations were linear. 2 After single intraperitoneal injections of lithium chloride (5 mmol/kg body wt.) the concentration of lithium in the CSF was greater than that of the brain for 2 h. 3 Repeated subcutaneous injections of lithium chloride (0.9 mmol/kg body wt.) resulted in steady state ratios corresponding to those observed when lithium was given in the diet. The rate of elimination from the CSF was intermediate between that of the serum and cerebral tissue until a new equilibrium was reached after approx. 24 h. At that time the ratios between lithium in the brain and serum, and in the CSF and serum were increased to approx. 5 and 0.8, respectively. 4 These results are consistent with passive transfer kinetics of lithium in the CSF and elimination of lithium from the cerebral tissue via the CSF. 5 The results may explain some of the phenomena observed in patients during intoxication with lithium.

Studies on the lithium transport across the red cell membrane. II. Characterization of ouabain-sensitive and ouabain-insensitive Li+ transport. Effects of bicarbonate and dipyridamole

In studies on Li+ net-transport across the human red cell membrane following results were obtained: 1. In K+- and Na+-free choline chloride media, Li+ is transported into the erythrocytes against an electrochemical gradient. This Li+ uphill transport as well as Li+ downhill transport into the cells is inhibited by ouabain, ATP-depletion, and by external K+ and Na+. The effects of K+ and Na+ are relieved at high Li+ concentrations. 2. Ouabain-sensitive Li+ uptake, determined at 10 mM external Na+, does not obey simple Michaelis-Menten kinetics and exhibits a maximum at about pH 7. 3. Ouabain-resistant Li+ downhill transport into erythrocytes increases with rising pH. It is comprised of a saturating component and a component linearly dependent on external Li+. The linear component is partly inhibited by dipyridamole and accelerated by bicarbonate. The bicarbonate effect can be completely blocked by dipyridamole, phlorizin and phenylbutazone. 4. Li+ release is not inhibited by ouabain, ATP-depletion and external K+. It increases with external Na+ concentration, tending to saturate at 150 mM Na+. Na+-independent Li+ release is stimulated by bicarbonate. It is concluded that ouabain-sensitive Li+ uptake is mediated at the K+-site(s) of the Na+-K+ pump. Li+, K+ and Na+ appear to compete for a common site (or sites). The stimulation of Li+ transfer by bicarbonate and the inhibition by dipyridamole suggest a participation of anionic species in ouabain-resistant Li+ transfer. The Na+-dependent Li+ release and the "saturating component" of Li+ uptake are ascribed to the Na+-dependent Li+ countertransport system.