Modulation of Glycinergic Neurotransmission may Contribute to the Analgesic Effects of Propacetamol.

Treating neuropathic pain remains challenging, and therefore new pharmacological strategies are urgently required. Here, the enhancement of glycinergic neurotransmission by either facilitating glycine receptors (GlyR) or inhibiting glycine transporter (GlyT) function to increase extracellular glycine concentration appears promising. Propacetamol is a N,N-diethylester of acetaminophen, a non-opioid analgesic used to treat mild pain conditions. In vivo, it is hydrolysed into N,N-diethylglycine (DEG) and acetaminophen. DEG has structural similarities to known alternative GlyT1 substrates. In this study, we analyzed possible effects of propacetamol, or its metabolite N,N-diethylglycine (DEG), on GlyRs or GlyTs function by using a two-electrode voltage clamp approach in Xenopus laevis oocytes. Our data demonstrate that, although propacetamol or acetaminophen had no effect on the function of the analysed glycine-responsive proteins, the propacetamol metabolite DEG acted as a low-affine substrate for both GlyT1 (EC50 > 7.6 mM) and GlyT2 (EC50 > 5.2 mM). It also acted as a mild positive allosteric modulator of GlyRα1 function at intermediate concentrations. Taken together, our data show that DEG influences both glycine transporter and receptor function, and therefore could facilitate glycinergic neurotransmission in a multimodal manner.

Therapeutic potential and underlying mechanism of sarcosine (N-methylglycine) in N-methyl-D-aspartate (NMDA) receptor hypofunction models of schizophrenia.

BACKGROUND: Compelling animal and clinical studies support the N-methyl-D-aspartate receptor (NMDAR) hypofunction hypothesis of schizophrenia and suggest promising pharmacological agents to ameliorate negative and cognitive symptoms of schizophrenia, including sarcosine, a glycine transporter-1 inhibitor. AIMS AND METHODS: It is imperative to evaluate the therapeutic potential of sarcosine in animal models, which provide indispensable tools for testing drug effects in detail and elucidating the underlying mechanisms. In this study, a series of seven experiments was conducted to investigate the effect of sarcosine in ameliorating behavioral deficits and the underlying mechanism in pharmacological (i.e., MK-801-induced) and genetic (i.e., serine racemase-null mutant (SR-/-) mice) NMDAR hypofunction models. RESULTS: In Experiment 1, the acute administration of 500/1000 mg/kg sarcosine (i.p.) had no adverse effects on motor function and serum biochemical responses. In Experiments 2-4, sarcosine significantly alleviated MK-801-induced (0.2 mg/kg) brain abnormalities and behavioral deficits in MK-801-induced and SR-/- mouse models. In Experiment 5, the injection of sarcosine enhanced CSF levels of glycine and serine in rat brain. In Experiments 6-7, we show for the first time that sarcosine facilitated NMDAR-mediated hippocampal field excitatory postsynaptic potentials and influenced the movement of surface NMDARs at extrasynaptic sites. CONCLUSIONS: Sarcosine effectively regulated the surface trafficking of NMDARs, NMDAR-evoked electrophysiological activity, brain glycine levels and MK-801-induced abnormalities in the brain, which contributed to the amelioration of behavioral deficits in mouse models of NMDAR hypofunction.

Preovulatory suppression of mouse oocyte cell volume-regulatory mechanisms is via signalling that is distinct from meiotic arrest.

GLYT1-mediated glycine transport is the main cell volume-homeostatic mechanism in mouse eggs and early preimplantation embryos. It is unique to these developmental stages and key to their healthy development. GLYT1 first becomes activated in oocytes only after ovulation is triggered, when meiotic arrest of the oocyte is released, but how this occurs was unknown. Here we show that GLYT1 activity is suppressed in oocytes in the preovulatory antral follicle and that its suppression is mediated by a mechanism distinct from the gap junction-dependent Natriuretic Peptide Precursor C (NPPC) pathway that controls meiotic arrest. GLYT1 remained suppressed in isolated antral follicles but not isolated cumulus-oocyte complexes (COCs) or isolated oocytes. Moreover, activating the NPPC signalling pathway could not prevent GLYT1 activation in oocytes within COCs despite maintaining meiotic arrest. Furthermore, blocking gap junctions in isolated follicles failed to induce GLYT1 activity in enclosed oocytes for an extended period after meiosis had resumed. Finally, isolated mural granulosa cells from preovulatory antral follicles were sufficient to suppress GLYT1 in oocytes within co-cultured COCs. Together, these results suggest that suppression of GLYT1 activity before ovulation is mediated by a novel signalling pathway likely originating from preovulatory mural granulosa cells.

Glycine transporter-1 controls nonsynaptic inhibitory actions of glycine receptors in the neonatal rat hippocampus.

Although functional glycinergic synapses have not been identified in the hippocampus, neurons in this area express Cl(-) permeable extrasynaptic glycine receptors (GlyRs). In experiments on CA3 pyramidal neurons on postnatal day 0-6 rat hippocampal slices, we detected robust GlyR activity as a tonic current and as single-channel events. Glycine release was independent of neuronal activity or extracellular Ca(2+). The endogenous GlyR activity was strongly enhanced by inhibition of the glycine-transporter-1 (GlyT1). Blockade of GlyT1 also caused a profound increase in the baseline current induced by exogenous glycine. Inhibition of GlyT1 reduced the frequency of spontaneous network events known as field giant depolarizing potentials (fGDPs) and of the unit activity in the absence of synaptic transmission. This inhibitory action on fGDPs was mimicked by applying 2 µm glycine or 0.1 µm isoguvacine, a GABAA-receptor agonist. Furthermore, 2 µm glycine suppressed unit spiking in the absence of synaptic transmission. Hence, despite the well known depolarizing Cl(-) equilibrium potential of neonatal hippocampal neurons, physiologically relevant extracellular glycine concentrations can exert an inhibitory action. The present data show that, akin to GABA uptake, GlyT1 exerts a powerful modulatory action on network events in the newborn hippocampus.

The glycine transporter GLYT1 in human intestine: expression and function.

Glycine is a well-documented cytoprotective agent and protects mammalian intestine against ischemia-reperfusion injury, irradiation and experimentally induced colitis. The specific glycine transporter GLYT1 is found throughout the human intestine where it is responsible for some 30-50% of glycine uptake into intestinal epithelial cells across the basolateral membrane and appears to function to maintain glycine supply to enterocytes and colonocytes. This paper reviews current knowledge of GLYT1 and presents recent evidence supporting its essential role in glycine mediated cytoprotection in intestinal absorptive cells. Regulatory mechanisms involved in intestinal expression of GLYT1 are discussed and the potential of glycine for use as an anti-inflammatory, protective agent in the management of inflammatory bowel disease examined.

Functional expression of the glycine transporter 1 on bullfrog retinal cones.

Using patch clamp techniques, we characterized glycine-induced currents from cones in bullfrog retinal slices. Application of glycine to cone terminals induced an inward current, which was in part suppressed by strychnine. The remaining strychnine-resistant current component, which did not show polarity reversion in a range of -120 mV to +40 mV, was blocked by N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl] sarcosine, an antagonist of glycine transporter 1 (GlyT1), but not affected by amoxapine, an inhibitor of glycine transporter 2. Application of sarcosine, an agonist of GlyT1, to cone terminals induced an inward current that was completely suppressed by N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl] sarcosine or when external Na in Ringer's was replaced by choline. All these results show for the first time the functional expression of GlyT1 on bullfrog cones.