title: Glycine tranporter-1 blockade potentiates NMDA-mediated responses in rat prefrontal cortical neurons in vitro and in vivo.
abstract: The N-methyl-D-aspartate (NMDA) receptor (NMDA-R) has pivotal roles in neural development, learning, memory, and synaptic plasticity. Functional impairment of NMDA-R has been implicated in schizophrenia. NMDA-R activation requires glycine to act on the glycine-B (GlyB) site of the NMDA-R as an obligatory co-agonist with glutamate. Extracellular glycine near NMDA-R is regulated effectively by a glial glycine transporter (GlyT1). Using whole-cell voltage-clamp recordings in prefrontal cortex (PFC) slices, we have shown that exogenous GlyB site agonists glycine and D-serine, or a specific GlyT1 inhibitor N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS) in the presence of exogenous glycine (10 microM), potentiated synaptically evoked NMDA excitatory postsynaptic currents (EPSCs) in vitro. Furthermore, in urethan-anesthetized rats, microiontophoretic NMDA pulses excite single PFC neurons. When these responses were blocked by approximately 50% to approximately 90% on continuous iontophoretic application of the GlyB site, antagonist (+)HA-966, intravenous NFPS (5 mg/kg), or a GlyB site agonist D-serine (50 mg/kg iv) reversed this (+)HA-966 block. NFPS may elevate endogenous glycine levels sufficiently to displace (+)HA-966 from the GlyB sites of the NMDA-R, thus enabling reactivation of the NMDA-Rs by iontophoretic NMDA applications. D-Serine (50-100 mg/kg iv) or NFPS (1-2 mg/kg iv) alone also augmented NMDA-evoked excitatory responses. These data suggest that direct GlyB site stimulation by D-serine, or blockade of GLYT1 to elevate endogenous glycine to act on unsaturated GlyB sites on NMDA-Rs, potentiated NMDA-R-mediated firing responses in rat PFC. Hence, blockade of GlyT1 to elevate glycine near the NMDA-R may activate hypofunctional NMDA-R, which has been implicated to play a critical role in the pathophysiology of schizophrenia.
authors.affiliation: Neuroscience Discovery, Eli Lilly and Co., Lilly Corporate Center, Indianapolis, Indiana 46220, USA.
authors.forename: Long
authors.initials: L
authors.lastname: Chen
authors.forename: Mark
authors.initials: M
authors.lastname: Muhlhauser
authors.forename: Charles R
authors.initials: CR
authors.lastname: Yang
mesh_terms: D026921:Amino Acid Transport Systems, Neutral / Q000037:antagonists & inhibitors* / Q000378:metabolism; D000818:Animals; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology; D005998:Glycine / Q000493:pharmacokinetics; D050487:Glycine Plasma Membrane Transport Proteins; D007478:Iontophoresis; D008297:Male; D016202:N-Methylaspartate / Q000494:pharmacology; D009474:Neurons / Q000502:physiology*; D009924:Organ Culture Techniques; D018408:Patch-Clamp Techniques; D017397:Prefrontal Cortex / Q000166:cytology / Q000502:physiology*; D011760:Pyrrolidinones / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism*; D012521:Sarcosine / Q000031:analogs & derivatives* / Q000494:pharmacology; D012694:Serine / Q000494:pharmacology
chemical_list: D026921:Amino Acid Transport Systems, Neutral; D018690:Excitatory Amino Acid Agonists; D050487:Glycine Plasma Membrane Transport Proteins; C435025:N-(3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)-3-(4'-phenylphenoxy)propyl)sarcosine; D011760:Pyrrolidinones; D016194:Receptors, N-Methyl-D-Aspartate; C493266:Slc6a9 protein, rat; D012694:Serine; D016202:N-Methylaspartate; C001369:1-hydroxy-3-amino-2-pyrrolidone; D005998:Glycine; D012521:Sarcosine