IMMUNOCYTOCHEMICAL LOCALIZATION OF THE NMDA-R2A RECEPTOR SUBUNIT IN THE CAT RETINA

Immunocytochemical studies were performed to determine the distributio n and cellular localization of the NMDA-R2A receptor subunit (R2A) in the cat retina. R2A-immunoreactivity (R2A-IR) was noted in all layers of the retina, with specific localizations in the outer segments of re d/green and blue cone photoreceptors, B-type horizontal cells, several types of amacrine cells, Muller cells and the majority of cells in th e ganglion cell layer. In the inner nuclear layer, 48% of all cells re siding in the amacrine cell layer were R2A-IR including a cell resembl ing the GABAergic A17 amacrine cell. Interestingly, the AII rod amacri ne cell was devoid of R2A-IR. Although the localization of the R2A sub unit was anticipated in ganglion cells, amacrines and Muller cells, th e presence of this receptor subunit to the cells in the outer retina w as not expected. Here, both the R2A and the R2B subunits were found to be present in the outer segments of cone photoreceptors and to the ti ps of rod outer segments. Although the function of these receptor subu nits in rod and cone photoreceptors remains to be determined, the fact that both R2A and R2B receptor subunits are localized to cone outer s egments suggests a possible alternative pathway for calcium entry into a region where this cation plays such a crucial role in the process o f phototransduction. To further classify the cells that display NR2A-I R, we performed dual labeling experiments showing the relationship bet ween R2A-labeled cells with GABA. Results showed that all GABAergic-am acrines and displaced amacrines express the R2A-subunit protein. In ad dition, approximately 11% of the NR2A-labeled amacrines, did not stain for GABA. These findings support pharmacological data showing that NM DA directly facilitates GABA release in retina and retinal cultures [I .L. Ferreira, C.B. Duarte, P.F. Santos, C.M. Carvalho, A.P. Carvalho, Release of [H-3]GABA evoked by glutamate receptor agonist in cultured chick retinal cells: effect of Ca2+, Brain Res. 664 (1994) 252-256; G, D. Zeevalk, W.J. Nicklas, Action of the anti-ischemic agent ifenprodil on N-methyl-D-aspartate and kainate-mediated excitotoxicity, Brain Re s. 522 (1990) 135-139; R. Huba, H.D. Hofmann, Transmitter-gated curren ts of GABAergic amacrine-like cells in chick retinal cultures, Vis. Ne urosci. 6 (1991) 303-314; M. Yamashita, R. Huba, H.D. Hofmann, Early i n vitro development of voltage- and transmitter-gated currents in GABA ergic amacrine cells, Dev. Brain Res. 82 (1994) 95-102; R. lentile, S. Pedale, V. Picciurro, V. Macaione, C. Fabiano, S. Macaione, Nitric ox ide mediates NMDA-evoked [3H]GABA release from chick retina cells, FEE S Lett. 417 (1997) 345-348; R.C. Kubrusly, M.C. deMello, F.G. deMello, Aspartate as a selective NMDA agonist in cultured cells from the avia n retina, Neurochem. Intl. 32 (1998) 47-52] or reduction of GABA in vi vo [N.N. Osborn, A.J. Herrera, The effect of experimental ischaemia an d excitatory amino acid agonist on the GABA and serotonin immunoreacti vities in the rabbit retina, Neurosci. 59 (1994) 1071-1081]. Since the majority of GABAergic synapses in the inner retina are onto both rod and cone bipolar axon terminals [R.G. Pourcho, M.T. Owzcarzak, Distrib ution of GABA immunoreactivity in the cat retina: A light and electron -microscopic study, Vis. Neurosci. 2 (1989) 425-435], we hypothesize t hat the NMDA-receptor plays a crucial role in providing feedback inhib ition onto rod and cone bipolar cells. (C) 1998 Elsevier Science B.V. All rights reserved.