Subtype-specific effects of lithium on glutamate receptor function.

Abstract

We report that substitution of sodium with lithium (Li+) in the extracellular solution causes subtype-specific changes in the inward and outward currents of glutamate receptors (GluRs), without a shift in reversal potential. Li+ produces an increase of inward and outward currents of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors and decreases in the currents of kainate (KA) and N-methyl-D-aspartate receptors. The greatest effect of Li+ was observed with GluR3. A concentration-response curve for GluR3 reveals that the potentiation caused by Li+ is greatest at saturating agonist concentrations. GluR1, which shows no potentiation by Li+ at 100 microM KA, shows a small but significant potentiation at saturating KA and glutamate concentrations. The effects of Li+ on outward current, where Li+ is not the primary charge carrier, and the lack of reversal potential shift argue for a mechanism of potentiation not associated with Li+ permeation. This potentiation of current is specific for Li+ because rubidium, although causing an increase of inward current, shifted the reversal potential and did not increase outward current. The effects of Li+ are different for KA, a weak desensitizing agonist, and glutamate, a strong desensitizing agonist, suggesting that Li+ might interact with a mechanism of desensitization. By using cyclothiazide (CTZ) to reduce desensitization of GluR3, we find that for low concentrations of KA and glutamate potentiation of the response by a combination of CTZ and Li+ is no greater than by CTZ or Li+ alone. However, at high concentrations of agonist, the potentiation of the response by a combination of CTZ and Li+ is significantly greater than by CTZ or Li+ alone. This potentiation was additive for glutamate but not for KA. At high agonist concentration in the presence of CTZ, the intrinsically lower desensitization produced with KA-evoked responses may preclude Li+ from potentiating the current to the same degree as it can potentiate glutamate-evoked responses. The additive effects of CTZ and Li+ were unique to the flop variant of GluR3.