Modulation of N-methyl-d-aspartate receptor function by glycine transport

The recent discovery of glycine transporters in both the central nervous system and the periphery suggests that glycine transport may be critical to N-methyl-d-aspartate receptor (NMDAR) function by controlling glycine concentration at the NMDAR modulatory glycine site. Data obtained from whole-cell patch–clamp recordings of hippocampal pyramidal neurons, in vitro, demonstrated that exogenous glycine and glycine transporter type 1 (GLYT1) antagonist selectively enhanced the amplitude of the NMDA component of a glutamatergic excitatory postsynaptic current. The effect was blocked by 2-amino-5-phosphonovaleric acid and 7-chloro-kynurenic acid but not by strychnine. Thus, the glycine-binding site was not saturated under the control conditions. Furthermore, GLYT1 antagonist enhanced NMDAR function during perfusion with medium containing 10 μM glycine, a concentration similar to that in the cerebrospinal fluid in vivo, thereby supporting the hypothesis that the GLYT1 maintains subsaturating concentration of glycine at synaptically activated NMDAR. The enhancement of NMDAR function by specific GLYT1 antagonism may be a feasible target for therapeutic agents directed toward diseases related to hypofunction of NMDAR.

Dopamine inhibits mammalian photoreceptor Na+,K+-ATPase activity via a selective effect on the alpha3 isozyme.

The rat retina contains dopaminergic interplexiform cells that send processes to the outer plexiform layer where dopamine is released in a light-dependent manner. We report herein that physiologically relevant concentrations of dopamine inhibited ouabain-sensitive photoreceptor oxygen consumption in dark- and light-adapted rat retinas and inhibited Na+,K+-ATPase specific activity (EC 3.6.1.37) in a rat rod outer-inner segment preparation. Experiments with the selective D1 agonist fenoldopam or D2 agonist quinpirole and experiments with dopamine plus either the D1 antagonist SCH23390 or D2/D4 antagonist clozapine showed that the inhibition of oxygen consumption and enzyme activity were mediated by D2/D4-like receptors. The amphetamine-induced release of dopamine, monitored by the inhibition of oxygen consumption, was blocked by L-2-amino-4-phosphonobutyric acid and kynurenic acid. Pharmacological and biochemical experiments determined that the IC50 values of ouabain for the alpha1-low and alpha3-high ouabain affinity isozymes of photoreceptor Na+,K+-ATPase were approximately 10(-5) and approximately 10(-7) M, respectively, and that the D2/D4-like mediated inhibition of Na+,K+-ATPase was exclusively selective for the alpha3 isozyme. The dopamine-mediated inhibition of alpha3 first occurred at 5 nM, was maximal at 100 microM (-47%), had an IC50 value of 382 +/- 23 nM, and exhibited negative cooperativity (Hill coefficient, 0.27). Prior homogenization of the rod outer-inner segment completely prevented the long-lasting inhibition, suggesting that the effect was coupled to a second messenger. Although the physiological significance of our findings to photoreceptor function is unknown, we hypothesize that these results may have relevance for the temporal tuning properties of rods.