Different glutamate sources and endogenous co‐agonists activate extrasynaptic NMDA receptors on amacrine cells of the rod pathway microcircuit

Abstract

The NMDA receptors (NMDARs) expressed by AII and A17 amacrine cells, the two main inhibitory interneurons of the rod pathway microcircuit in the mammalian retina, are exclusively extrasynaptic, activated by ambient levels of glutamate, and molecularly distinct, with AII and A17 amacrines expressing GluN2B-and GluN2A-containing receptors, respectively. This important sensory microcircuit thus provides a unique model to study the activation and function of extrasynaptic NMDARs. Here, we investigated the sources of glutamate and the endogenous coagonists (d-serine or glycine) that activate these distinct populations of NMDARs.With acute slices from rat retina, we used whole-cell voltage-clamp recording and measurement of current noise to monitor levels of NMDAR activity. Pre-incubation of retina with bafilomycin A1 (an inhibitor of neurotransmitter uptake into synaptic vesicles) abolished NMDAR-mediated noise in AII, but not A17 amacrines, suggesting a vesicular source of glutamate activates AII NMDARs, whereas a non-vesicular source activates A17 NMDARs. Pre-incubation of retina with l-methionine sulfoximine (an inhibitor of glutamine synthetase) also abolished NMDAR-mediated noise in AII, but not A17 amacrines, suggesting a neuronal source of glutamate activates AII NMDARs, whereas a glial source activates A17 NMDARs. Enzymatic breakdown of d-serine reduced NMDAR-mediated noise in AII, but not A17 amacrines, suggesting d-serine is the endogenous co-agonist at AII, but not A17 NMDARs.Our results reveal unique characteristics of these two populations of extrasynaptic NMDARs. The differential and independent activation of these receptors is likely to provide specific contributions to the signal processing and plasticity of the cellular components of the rod pathway microcircuit.

Origin of Retinal Oscillatory Potentials in the Mouse, a Tool to Specifically Locate Retinal Damage

“…The effect observed by the intraocular injection of Glutamate or its antagonist DNQX on ionotropic receptors is also easily explained based on Figure 1 and Figure 4 . Both the glycinergic AII and GABAergic A17 amacrine cells postsynaptic to rod bipolar cells [ 30 , 31 , 32 ] would be continuously stimulated (due to the effect of Glutamate) or inhibited (due to the effect of DNQX), producing, in both cases, the abolition of Glycine or GABA release and, therefore, the oscillatory responses. The effect of GABA on the oscillatory response is also easily justified, since rod bipolar cells express GABA C receptors at their axon terminal [ 33 , 34 ], as shown in Figure 4 .…”

Early Microvascular and Oscillatory Potentials Changes in Human Diabetic Retina: Amacrine Cells and the Intraretinal Neurovascular Crosstalk

“…Amacrine cells are characterized by a great variety of shapes, sizes, and stratification patterns, which are still under research [23,24]. Previous studies found that different cellular morphological subtypes present specific stimuli and responses even into the same microcircuit, with various synaptic and extrasynaptic receptors with differential sensitivity to neurotransmitters [25]. Recent studies in animal models on the amacrine cells AII and A17 demonstrated that diabetes causes different changes in the pharmacological properties and single-channel conductance of the synaptic receptors of these two cell populations, variably modifying their Ca2 permeability.…”

Inhibitory inputs to an inhibitory interneuron: Spontaneous postsynaptic currents and GABAA receptors of A17 amacrine cells in the rat retina

“…The cell bodies of A17 amacrine cells can be readily targeted in the retinal slice based on their characteristic dome‐like shape with a flat base at the border between the inner nuclear layer and the inner plexiform layer (Figure 1a; e.g. Beltrán‐Matas et al, 2021; Castilho et al, 2015; Eggers & Lukasiewicz, 2006; Elgueta et al, 2018; Grimes et al, 2009, 2010, 2014; Menger & Wässle, 2000; Veruki et al, 2019; Zhou et al, 2016). To verify that a given recording (whole‐cell, outside‐out and nucleated patch) was from an A17 amacrine cell, the full morphology of the recorded cell and its dendritic tree was examined with fluorescence microscopy after diffusion of dye (Alexa 594) from the recording pipette.…”