title: Mammalian retinal horizontal cells are unconventional GABAergic neurons.
abstract: Lateral interactions at the first retinal synapse have been initially proposed to involve GABA by transporter-mediated release from horizontal cells, onto GABA(A) receptors expressed on cone photoreceptor terminals and/or bipolar cell dendrites. However, in the mammalian retina, horizontal cells do not seem to contain GABA systematically or to express membrane GABA transporters. We here report that mouse retinal horizontal cells express GAD65 and/or GAD67 mRNA, and were weakly but consistently immunostained for GAD65/67. While GABA was readily detected after intracardiac perfusion, it was lost during classical preparation for histology or electrophysiology. It could not be restored by incubation in a GABA-containing medium, confirming the absence of membrane GABA transporters in these cells. However, GABA was synthesized de novo from glutamate or glutamine, upon addition of pyridoxal 5'-phosphate, a cofactor of GAD65/67. Mouse horizontal cells are thus atypical GABAergic neurons, with no functional GABA uptake, but a glutamate and/or glutamine transport system allowing GABA synthesis, probably depending physiologically from glutamate released by photoreceptors. Our results suggest that the role of GABA in lateral inhibition may have been underestimated, at least in mammals, and that tissue pre-incubation with glutamine and pyridoxal 5'-phosphate should yield a more precise estimate of outer retinal processing.
authors.affiliation: Department of Neurobiology and Genetics, Institut de Génétique et de Biologie Moléculaire et Cellulaire, CNRS UMR_7104, Inserm U 964, Université de Strasbourg, Illkirch, France.
authors.forename: Sercan
authors.initials: S
authors.lastname: Deniz
authors.forename: Eric
authors.initials: E
authors.lastname: Wersinger
authors.forename: Yannick
authors.initials: Y
authors.lastname: Schwab
authors.forename: Carole
authors.initials: C
authors.lastname: Mura
authors.forename: Ferenc
authors.initials: F
authors.lastname: Erdelyi
authors.forename: Gábor
authors.initials: G
authors.lastname: Szabó
authors.forename: Alvaro
authors.initials: A
authors.lastname: Rendon
authors.forename: José-Alain
authors.initials: JA
authors.lastname: Sahel
authors.forename: Serge
authors.initials: S
authors.lastname: Picaud
authors.forename: Michel J
authors.initials: MJ
authors.lastname: Roux
mesh_terms: D000818:Animals; D005968:Glutamate Decarboxylase / Q000235:genetics / Q000378:metabolism; D018698:Glutamic Acid / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D057507:Mice, 129 Strain; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D009433:Neural Inhibition / Q000502:physiology; D009434:Neural Pathways / Q000166:cytology / Q000378:metabolism; D011732:Pyridoxal Phosphate / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D012160:Retina / Q000166:cytology / Q000201:enzymology / Q000378:metabolism*; D051248:Retinal Horizontal Cells / Q000166:cytology / Q000378:metabolism*; D009435:Synaptic Transmission / Q000502:physiology; D014785:Vision, Ocular / Q000502:physiology; D005680:gamma-Aminobutyric Acid / Q000096:biosynthesis / Q000502:physiology*
chemical_list: D012333:RNA, Messenger; D049452:Green Fluorescent Proteins; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D011732:Pyridoxal Phosphate; D005968:Glutamate Decarboxylase; C402565:glutamate decarboxylase 1; C401141:glutamate decarboxylase 2