Lithium and central monoamine neurotransmitter systems

Magnesium valproate attenuates hyperactivity induced by dexamphetamine-chlordiazepoxide mixture in rodents

A mixture of dexamphetamine and chlordiazepoxide induces hyperactivity in both mice and rats. This type of hyperactivity has been proposed as an animal model of mania. Magnesium valproate itself had little influence on the activity of normal mice and rats. Acute pretreatment of mice with magnesium valproate (75-300 mg/kg p.o., 37.5-150 mg/kg i.v.) attenuated the mixture-induced hyperactivity. Pretreatment of rats with a single dose of magnesium valproate (75-300 mg/kg, p.o. or i.p.) also counteracted the locomotor hyperactivity caused by the mixture. The effects of magnesium valproate in the mixture-treated mice and rats could be abolished by bicuculline and picrotoxin, but not isoniazid. These findings suggest that the antimanic effect of valproate may be related to its ability to enhance the postsynaptic effects of gamma-aminobutyric acid (GABA).

Lithium transport in the mouse brain

Using the stable isotopes of lithium 6Li and 7Li, and the nuclear reaction 6Li(n,alpha)3H for detection, we have studied the isotopic exchange of lithium in various areas of the mouse brain and in the mouse plasma, under conditions of constant concentration of total lithium. The neutron irradiations were performed using 'cold' neutrons, at the European Institute Von Laue-Langevin. The nuclear reaction track densities were determined using an automatic image analyser. In the plasma, the isotopic ratios, 6Li/7Li, were measured using 'Secondary Ion Mass Spectrometry'. The concentration of total lithium in the plasma was kept close to 0.28 mM. The brain concentration of total lithium (referred to the tissue water content) ranged from more than 2 mM in the thalamus to less than 0.65 mM in the white matter of the cerebellum. The Nernst potential of lithium thus ranged from approx. -50 to approx. -20 mV, which means that lithium is probably not far from electrochemical equilibrium between brain cells and plasma. At any moment, the isotopic abundance of 6Li (ratio of 6Li to total lithium) in the different brain areas, were not significantly different from one another. The time-course of the isotopic abundance of 6Li in the brain was fitted by the composition of two exponential terms. The time-course of the isotopic abundance of 6Li in the plasma was also fitted by the composition of two exponential terms. These analytic curves (for the brain and for the plasma) were not significantly different from each other, at the precision of the measurements. This means that the isotopic equilibration of lithium between brain and plasma is almost instantaneous (i.e. accomplished in a few min at the most).

Mechanism of lithium action: in vivo and in vitro effects of alkali metals on brain superoxide dismutase

Intraperitoneal administration of lithium (2 mEq/kg/day) was found to increase the superoxide dismutase (SOD) activity in certain brain regions after 24 hours (2 injections) and 3 days (once a day) of exposure. In vitro addition of wide range of lithium (0.1 to 8 mEq) to enzyme preparation as well activated cortical SOD activity; however, at 10 mEq concentrations an inhibition was observed. The increase in SOD activity did not appear to be region specific as under both in vivo and in vitro conditions lithium enhanced enzyme activity in all the tested brain regions. The effects of intraperitoneal administration of 2 mEq/kg rubidium and cesium for 24 hr (2 injections) and 6 days (once a day) were also studied on central SOD. Both the alkali metals were not found to produce any significant alteration in the cortical enzymic activity. When the in vitro effects of these monovalent alkali metals were tested, only 2 mEq rubidium was found to increase cortical SOD; however, cesium and potassium at similar concentration did not produce any appreciable effects. It appears from the data that lithium-induced increase in brain SOD activity is not an unspecific effect of alkali metals. SOD enzyme disposes cytotoxic superoxide radicals which, if not removed, could impair the normal functioning of cellular membrane and produce a variety of psychedelic compounds as well. The activation of central SOD by lithium would enhance the disposal process of superoxide radicals whose pathological concentrations may be present in affective disorders. The mechanism of lithium-induced activation of SOD, at present, is not known.

Combined lithium and valproate treatment and subsequent withdrawal: serotonergic mechanism of their interaction in discrete brain regions

Rats were administered with lithium carbonate (1 mmole/kg), valproate (300 mg/kg) and their combination for 12 days. Rats in the withdrawal group were treated for 10 days, followed by saline for 2 days. Mid-brain tryptophan levels increased in all the groups except lithium-treated rats. The administration of lithium together with valproate decreased tryptophan hydroxylase activity and 5-hydroxytryptamine levels in certain brain regions compared to either treatment alone. Also, the magnitude of valproate-induced 5-hydroxyindoleacetic acid elevation was decreased after combined treatment. Most of the changes observed after combined exposure persisted even after 2 days of treatment withdrawal. It appears from the data that combined administration of the two drugs produces persistent decrease in 5-hydroxytryptamine synthesis and turnover in the brain.