RNA editing produces glycine receptor alpha3(P185L), resulting in high agonist potency.

The function of supramedullary glycine receptors (GlyRs) is still unclear. Using Wistar rat collicular slices, we demonstrate GlyR-mediated inhibition of spike discharge elicited by low glycine (10 microM). Searching for the molecular basis of this phenomenon, we identified a new GlyR isoform. GlyR alpha3(P185L), a result of cytidine 554 deamination, confers high glycine sensitivity (EC50 approximately 5 microM) to neurons and thereby promotes the generation of sustained chloride conductances associated with tonic inhibition. The level of GlyR alpha3-C554U RNA editing is sensitive to experimentally induced brain lesion, inhibition of cytidine deamination by zebularine and inhibition of mRNA transcription by actinomycin D, but not to blockade of protein synthesis by cycloheximide. Conditional regulation of GlyR alpha3(P185L) is thus likely to be part of a post-transcriptional adaptive mechanism in neurons with enhanced excitability.

Cellular and subcellular localization of the inhibitory glycine receptor in hippocampal neurons.

Inhibitory glycine receptors are most abundant in spinal cord and brainstem, and glycinergic synapses have a well-established role in the regulation of locomotor behavior. Little is known about the function of glycine receptors in cortex and hippocampus, where GABA plays a dominant role in synaptic inhibition. Therefore, we have investigated tissue and cellular expression of glycine receptor alpha-subunits. Western blot and immunohistochemical analyses reveal the presence of glycine receptors in hippocampal tissue. Immunocytochemical experiments in hippocampal cultures show prominent cellular expression of glycine receptors in pyramidal neurons and GAD-positive interneurons similar to the calcium-binding protein VILIP-1 with widespread hippocampal distribution. On the subcellular level we found co-staining of GlyR and the presynaptic marker synapsin I. Furthermore, co-staining with GAD at synaptic terminals indicated partial co-localization of GABA- and glycine receptors.

Characterization of the inhibitory glycine receptor on entorhinal cortex neurons.

In addition to the well-established functional description of the glycine receptor (GlyR) in the spinal cord, GlyR expression has recently been found in higher brain regions, such as the striatum or hippocampus. In this study we have investigated the electrophysiological response of glycine in the rat entorhinal cortex slice. In all recorded cells we found significant current responses to glycine with an EC(50) value of about 100 micro m. Most importantly, we detected a cross-inhibition of glycine responses by GABA but not vice versa. These findings are in line with recent published data of cross-talks between GABA(A)R and GlyR but indicate a novel type of cross-inhibition of these receptors in the entorhinal cortex.

Inhibition of alpha-subunit glycine receptors by quinoxalines.

Quinoxalines are widely used compounds in electrophysiological studies to separate excitatory and inhibitory neurotransmission mediated by the strychnine-insensitive and strychnine-sensitive glycine receptor (NMDA, GlyR), respectively. We report here, that the quinoxaline NBQX is a potent antagonist of homo-oligomeric alpha1- and alpha2-subunit glycine receptors expressed in Xenopus laevis oocytes. NBQX elicited half-maximal inhibition of glycine evoked currents at a concentration of about 5 microM. DNQX and CNQX were found to be 5-fold and 20-fold less efficient than NBQX. At oocytes expressing alpha1beta2 subunit GABAA receptors the quinoxalines tested showed no significant inhibition of GABA responses up to a concentration of 100 microM. Our data indicates that these quinoxalines applied at concentrations sufficient to block NMDA receptor also attenuate GlyR responses.

Effects of taurine and glycine on epileptiform activity induced by removal of Mg2+ in combined rat entorhinal cortex-hippocampal slices.

PURPOSE: The imbalance between neuronal inhibition and excitation contributes to epileptogenesis. Inhibition in the central nervous system (CNS) is mediated by gamma-aminobutyric acid (GABA) and glycine. Recent studies indicate the expression of glycine receptor (GlyR) in hippocampus and neocortex. However, the function of GlyR in these regions is not clarified completely. The aim of this study was to investigate whether the GlyR agonists glycine and taurine promote an anticonvulsive effect. METHODS: We induced epileptiform discharges by reducing extracellular Mg2+ concentration in combined rat entorhinal cortex-hippocampal slices (400 micro m). Epileptiform discharges were detected by using extracellular recording techniques. RESULTS: Seizure-like events were suppressed by taurine, exhibiting a half-maximal inhibitory effect (IC50) of 0.9 mM. Suppression of late recurrent discharges in the medial entorhinal cortex and recurrent short discharges in the hippocampus was obtained at an IC50 value of 1.6 and 2.1 mM, respectively. Strychnine at concentrations <1 micro M abolished these effects. Likewise glycine, after an initial proconvulsant effect, suppressed epileptiform discharges. CONCLUSIONS: These findings show that GlyR agonists, in particular taurine, could serve as potential anticonvulsants and suggest an important role of GlyR in cortical function and dysfunction.