To investigate the neuroprotective effect of L-serine and its underlying mechanisms, focal cerebral ischemia was induced in rats by occlusion of middle cerebral artery (MCAO) with a suture, and reperfusion was given by filament withdrawal 2 hr later. Meanwhile, rat hippocampal neurons were primarily cultured, and incubated in serum-free medium in an incubator containing 1% O(2) for hypoxic exposure of 5 hr, or incubated in serum-free medium containing 1 mM glutamate for glutamate exposure of 2 hr. Brain tissue injury and cell damage were then measured. L-serine dose-dependently decreased the neurology deficit score and infarct volume, elevated the cell viability and inhibited the leakage of lactate dehydrogenase. These effects were blocked by strychnine in both MCAO rats and cultured hippocampal neurons. Furthermore, L-serine (168 mg.kg(-1)) reduced the brain water content, permeability of blood-brain barrier, neuronal loss and the expression of activated caspase-3 in the cortex. In addition, L-serine effectively protected the brain from damage when it was administered within 6 hr after the end of MCAO. It is suggested that L-serine could exert a neuroprotective effect on the ischemic-reperfused brain and on the hypoxia- or glutamate-exposed hippocampal neurons, which may be mediated by activating glycine receptors.

In the presence of glutamate and co-agonists, e.g., glycine, the N-methyl-D-aspartate receptor (NMDAR) plays an important role in physiological and pathophysiological brain processes. Previous studies indicate glycine could inhibit NMDAR responses induced by high concentration of NMDA in hippocampal neurons. The mechanism underlying this inhibitory impact, however, has been unclear. In this study, the whole-cell patch-clamp recording and Ca 2+ imaging with Fluo-3/AM under laser scanning confocal microscope were used to analyze the possible involvement of NMDAR subunits in this effect. We found that the peak current of NMDARs and Ca 2+ influx induced by high concentration of NMDA were reduced by treatment of glycine (0.03-10 mol L 1 ) in a dose-dependent manner, and that the glycine-dependent inhibition of NMDAR responses, which were induced at 300 mol L 1 NMDA, was reversed by ZnCl 2 through the blocking of the NR2A subunit of NMDARs, but was less influenced by ifenprodil, a NR2B inhibitor. Our results suggest that the glycine-dependent inactivation of NMDARs is potentially modulated by the regulatory subunit NR2A. N-methyl-D-aspartate (NMDA), NMDARs (NMDARs), glycine, zinc, inactivation, hippocampal neurons Citation:Li X, Chen Z Q, Jiang Z L, et al. Zinc reverses glycine-dependent inactivation of NMDARs in cultured rat hippocampal neurons. Sci China Life Sci, 2012Sci, , 55: 1075Sci, -1081Sci, , doi: 10.1007 N-methyl-D-aspartate receptor (NMDAR) ion channels are generally considered as postsynaptic targets of glutamate-mediated synaptic transmission in the central nervous system. NMDARs play critical roles in physiological processes such as synaptic plasticity and memory by allowing glutamate-gated calcium influx into neuronal cells [1]. A property of the NMDARs is its voltage-dependent activation, a result of ion channel block by extracellular Mg 2+ ions, which allows the flow of Na + and small amounts of Ca 2+ ions into the cell and K + out of the cell [2]. Besides, the NMDAR requires co-activation of the glycine site located on its NR1 subunit by D-serine or glycine in addition to the synaptically released neurotransmitter glutamate [3,4].However, the mechanistic connection between glutamate and glycine binding in the modulation of NMDAR activation and inactivation is unclear. Many mechanisms have been proposed to account for the mediation of the inactivation of NMDARs, including glycine-dependent desensitization, glycine-independent desensitization [5,6], Ca 2+ -dependent inactivation [7][8][9] and surface membrane receptors internalization [10]. Activation of NMDARs that have been pre-equilibrated with glycine produces a peak current, which decays or desensitizes to a steady-state level at high concentrations of NMDA. According to the desensitization of currents in the presence of saturation concentrations of glutamate and glycine agonists, two types of desensitization, glycine-independent or glycine-dependent, have been characterized for NMDARs.

D-serine can also induce inactivation of NMDARs, the NR2A subunit is required for the induction of this effect, and this inactivation is Ca(2+)-dependent in nature. This action of D-serine is hypothesized to play a neuroprotective role upon a sustained large glutamate insult to the brain.