Characterization of glycine transport in cultured Müller glial cells from the retina

✍ Scribed by Ana Gadea; Edith López; Ana María López-Colomé

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 108 KB
• Volume: 26
• Category: Article
• ISSN: 0894-1491
• DOI: 10.1002/(sici)1098-1136(199906)26:4<273::aid-glia1>3.0.co;2-z

✦ Synopsis

Rapid termination of the synaptic action of glutamate (Glu) and glycine (Gly) is achieved by uptake into the presynaptic terminal and glial cells. In the vertebrate CNS, Gly acts both as an inhibitory neurotransmitter and as a Glu modulator or coagonist at postsynaptic N-methyl-D-aspartate (NMDA) receptors. We have previously described NMDA receptors in Mu ¨ller cells of chick retina coupled to the phosphoinositide cascade, the entry of calcium, and the activation of protein kinase C (PKC; Lo ´pez-Colome ét al. Glia 9:127-135, 1993). A colocalization of Gly transporters and NMDA receptors has been reported in brain tissue (Smith et al. Neuron 8:927-936, 1992); since the concentration of Gly could participate in the modulation of Glu excitatory transmission in the vertical pathways of the retina, transport of Gly in monolayer cultures of Mu ¨ller cells was studied. Gly transport was found pH-sensitive with an optimum at pH 7.4. Kinetic analysis of the saturation curve for Gly within a concentration range of 0.01-2 mM, revealed two components of transport: a low-affinity system with K m ϭ 1.7 mM, V max ϭ 30 nmol/10 min/mg protein, and a high-affinity one with a K m ϭ 27 µM, V max ϭ 3 nmol/10 min/mg protein. Both systems were Na ϩ -dependent; the high-affinity system proved also dependent on external Cl -and was inhibited by sarcosine, characteristic of GLYT1 transporters. The inhibition of low-affinity uptake by 2-(methylamino)isobutyric acid (MeAIB) and 2-aminoisobutyric acid (AIB) suggests the presence of transport system A in Mu ¨ller cells. The process is energy-requiring, since Gly transport was decreased by metabolic inhibitors. Data obtained are in keeping with a modulatory role for Mu ¨ller glia on excitatory transmission in the retina.