Open Access

Olivia Nicola Dumitrescu

• The mammalian retina contains around 30 morphological varieties of amacrine cell types. These interneurons receive excitatory glutamatergic input from bipolar cells and provide GABA- and glycinergic inhibition to other cells in the retina. Amacrine cells exhibit widely varying light evoked responses, in large part defined by their presynaptic partners. We wondered whether amacrine functional diversity is based on a differential expression of glutamate receptors among cell populations and types. In whole cell patch-clamp experiments on mouse retinal slices, we used selective agonists and antagonists to discriminate responses mediated by NMDA/ non-NMDA (NBQX) and AMPA/ KA receptors (cyclothiazide, GYKI 52466, GYKI 53655, SYM 2081). We sampled a large variety of individual cell types, which were classified by their dendritic field size into either narrow-field or wide-field cells after filling with Lucifer yellow or neurobiotin. In addition, we used transgenic GlyT2-EGFP mice, whose glycinergic neurons express EGFP. This allowed us to classify amacrines on basis of their neurotransmitter into either glycinergic or GABAergic cells. All cells (n = 300) had good responses to non-NMDA agonists. Specific AMPA receptor responses could be obtained from almost all cells recorded: 94% of the AII (n = 17), 87% of the narrow-field (n = 45), 81% of the wide-field (n = 21), 85% of the glycinergic (n = 20) and 78% of the GABAergic cells (n = 9). KA receptor selective drugs were also effective on the majority of the AII (79%, n = 14), narrow-field (93%, n = 43), wide-field (85%, n = 26), glycinergic (94%, n = 16) and GABAergic amacrine cells (100%, n = 6). Among the cells tested for the two receptors (n = 65), we encountered both exclusive expression of AMPA or KA receptors and co-expression of the two types. Most narrow-field (70%, n = 27), glycinergic (81%, n = 16) and GABAergic cells (67%, n = 6) were found to have both AMPA and KA receptors. In contrast, only less than half of the wide-field cells (43%, n = 14) were found to co-express AMPA and KA receptors, most of them expressing exclusively AMPA (36%) or KA receptors (21%). We could elicit small NMDA responses from most of the wide-field (75%, n = 13) and GABAergic cells (67%, n = 3), whereas only 47% of the narrow-field (n = 15), 14% of the AII (n = 22) and no glycinergic cell (n = 2) reacted to NMDA. Abstract 83 Our data suggest that AMPA, KA and NMDA receptors are differentially expressed among different types of amacrine cells rather than among populations with different neurotransmitters or different dendritic coverage of the retina. Selective expression of kinetically different glutamate receptors among amacrine types may be involved in generating transient and sustained inhibitory pathways in the retina. Since AMPA and KA receptors are not generally clustered at the same postsynaptic sites, a single amacrine cell expressing both AMPA and KA receptors may provide inhibition with different temporal characteristics to individual synaptic partners.
• Amakrinzellen sind, morphologisch wie physiologisch, die heterogenste Klasse aller retinalen Neurone. Sie erhalten glutamaterge Synapsen von Bipolarzellen, mit denen sie eine Rückkoppelungsschleife bilden (reziproke Synapsen). Weitere Ausgangssynapsen bilden sie mit Ganglienzellen, die die Lichtsignale der Retina aufsummieren und im optischen Nerv an die höheren visuellen Zentren weiterleiten. Die vorliegende Studie sollte klären, ob die funktionelle Vielfalt der verschiedenen Amakrinzelltypen durch eine spezifische Expression unterschiedlicher Glutamatrezeptoren zustande kommt. Die drei Hauptgruppen der ionotropen Glutamatrezeptoren werden nach ihren Liganden AMPA, KA und NMDA genannt. In einem vertikalen Schnittpräparat ( slice ) der Mausretina wurden elektrophysiologische Ableitungen durchgeführt und die Expression von AMPA, KA und NMDA Rezeptoren wurde pharmakologisch untersucht. Hierfür wurden selektive Agonisten und Antagonisten verschiedener Rezeptortypen, wie etwa Cyclothiazide (CTZ), GYKI 52466, GYKI 53655, SYM 2081 und NBQX, verwendet und deren Auswirkungen auf die Applikation der Glutamatrezeptor-Agonisten AMPA, KA und NMDA untersucht.