Mammalian retinal horizontal cells are unconventional GABAergic neurons

Abstract

J. Neurochem. (2011) 116, 350–362. Abstract Lateral interactions at the first retinal synapse have been initially proposed to involve GABA by transporter‐mediated release from horizontal cells, onto GABAA 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.

A Transgenic Mouse Line Expressing the Red Fluorescent Protein tdTomato in GABAergic Neurons

“…Finally, horizontal cells as well as a subpopulation of amacrine cells in the retina are GABAergic interneurons [40,41]. Also expression of GAD65 has been demonstrated in a subset of these cells [41–43]. Consistent with this knowledge, neurons expressing tdTomato were observed in the inner nuclear layer of the retina of TgN(GAD65-tdTomato)-mice (Fig 6).…”

Specific Wiring of Distinct Amacrine Cells in the Directionally Selective Retinal Circuit Permits Independent Coding of Direction and Size

“…It is also possible that horizontal cell feedback is more important for shaping the ganglion cell responses in the temporal domain under high photopic conditions (Pandarinath et al, 2010), which we have not specifically tested here. Also, it has been shown that horizontal cell GABA release may be impaired in standard recording conditions (Deniz et al, 2011). Thus, while we have revealed a clear role for WACs in spatial processing in the DS circuit, the specific role of horizontal cells remains to be evaluated.…”

Rod Vision Is Controlled by Dopamine-Dependent Sensitization of Rod Bipolar Cells by GABA

“…The light-dependency of GABA staining in horizontal cells abolished in D1R −/− mice makes these cells a potential candidate. Horizontal cells have long been known to contain GABA (Deniz et al, 2011; Guo et al, 2010; Schwartz, 1987; Vardi et al, 1994; Wassle and Chun, 1989); Figure S4), but the role of GABA release from horizontal cells, at least for the rod circuit, remains poorly understood. For instance, the recently reported inhibitory feedback from these cells onto rod terminals does not appear to rely on GABA (Babai and Thoreson, 2009).…”