Effects of systematically administered lithium on tryptophan transport and exchange in plasma-membrane vesicles isolated from rat brain.

The effect of lithium on the sodium-dependent high-affinity system for tryptophan uptake was examined in plasma membrane vesicles derived from rat brain. We demonstrated that Na+ could be replaced by lithium in the external medium and the presence of lithium produced an increase in the Vmax of the tryptophan transport whereas it had no significant effect on the Km for the substrate. Plasma membrane vesicles derived from synaptosomes obtained from long-term lithium-treated rats are able to accumulate tryptophan to a greater extent than normal rats and maintain a more negative membrane potential than controls. Our data support the idea that the stimulation by lithium of the high-affinity uptake system for tryptophan by maintaining adequate membrane potentials across the membrane, could lead to the stabilization of serotonin production, as has been demonstrated in long term-lithium treatment.