Extrasynaptic NMDA Receptors on Rod Pathway Amacrine Cells: Molecular Composition, Activation, and Signaling

Abstract

In the rod pathway of the mammalian retina, axon terminals of glutamatergic rod bipolar cells are presynaptic to AII and A17 amacrine cells in the inner plexiform layer. Recent evidence suggests that both amacrines express NMDA receptors, raising questions concerning molecular composition, localization, activation, and function of these receptors. Using dual patch-clamp recording from synaptically connected rod bipolar and AII or A17 amacrine cells in retinal slices from female rats, we found no evidence that NMDA receptors contribute to postsynaptic currents evoked in either amacrine. Instead, NMDA receptors on both amacrine cells were activated by ambient glutamate, and blocking glutamate uptake increased their level of activation. NMDA receptor activation also increased the frequency of GABAergic postsynaptic currents in rod bipolar cells, suggesting that NMDA receptors can drive release of GABA from A17 amacrines. A striking dichotomy was revealed by pharmacological and immunolabeling experiments, which found GluN2B-containing NMDA receptors on AII amacrines and GluN2A-containing NMDA receptors on A17 amacrines. Immunolabeling also revealed a clustered organization of NMDA receptors on both amacrines and a close spatial association between GluN2B subunits and connexin 36 on AII amacrines, suggesting that NMDA receptor modulation of gap junction coupling between these cells involves the GluN2B subunit. Using multiphoton Ca 2ϩ imaging, we verified that activation of NMDA receptors evoked an increase of intracellular Ca 2ϩ in dendrites of both amacrines. Our results suggest that AII and A17 amacrines express clustered, extrasynaptic NMDA receptors, with different and complementary subunits that are likely to contribute differentially to signal processing and plasticity.

Presynaptic NMDA receptors cooperate with local action potentials to implement activity-dependent GABA release from the reciprocal olfactory bulb granule cell spine

“…These observations are of general importance in the context of spike-timing dependent plasticity and electrical compartmentalization of spines (e.g. Grunditz Presynaptic NMDARs in the cerebellum are also involved in enhancing spontaneous release of GABA (Glitsch & Marty 1999), with similar recent observations for extrasynaptic NMDA receptors on retinal A17 amacrine cells (Veruki et al 2019). However, to our knowledge direct triggering of release via presynaptic NMDARs during unitary transmission has not been observed so far, adding another pathway to the already highly diverse signalling downstream of NMDARs.…”

Alterations of STEP46 and STEP61 Expression in the Rat Retina with Age and AMD-Like Retinopathy Development

“…Besides, AII amacrine cells express functional NMDA receptors, which commonly mediate activity-dependent changes in postsynaptic neurons [ 42 ]. In the rat retina, NMDA and AMPA receptors are expressed also by ganglion, bipolar, and horizontal neurons as well as photoreceptors and Müller cells [ 43 , 44 ]. Nonetheless, we did not detect alterations in amounts of ganglion cells, photoreceptors, and Müller glia after TC-2153 treatment.…”

Cell Calcium Imaging as a Reliable Method to Study Neuron–Glial Circuits

“…Ca 2+ imaging studies have been used to detail network properties in ex vivo retinal preparations and revealed a variety of responses on bipolar cells (at least eight different clusters) in terms of Ca 2+ shifts in the axon terminals. On the other hand, activation of glutamatergic extrasynaptic NMDA receptors present in the rod pathway show increases in Ca 2+ dendritic levels in both AII and A17 amacrine cells, postsynaptically at rod bipolar dyad synapses ( Veruki et al, 2019 ). The depolarization of ganglion cells by voltage-dependent calcium channels (VDCC) contributes to soma and dendritic Ca 2+ increases ( Sargoy et al, 2014 ).…”

“…The dopaminergic system is one of the first phenotypes to be expressed in the developing retina ( Reis et al, 2007 ), and dopamine is a potent modulator of spontaneous neural activity and gap junction coupling. Markers for young (doublecortin, β III -tubulin, and PSA-NCAM) and mature (MAP-2) retinal neurons are associated with KCl depolarizing cells (due to the expression of VDCC) [reviewed by Catterall (2000) ]; retinal neurons are activated by ionotropic glutamatergic receptors, NMDA ( Veruki et al, 2019 ), AMPA, or kainate ( Okada et al, 1999 ; De Melo Reis et al, 2011 ; Passos et al, 2019 ). Some of these responses are modulated by cyclic AMP, which is induced by dopamine, adenosine, and PACAP receptors and functions as a differentiating factor for dopaminergic retinal cells in the chick ( Guimaraes et al, 2001 ) or rat retina ( Varella et al, 1999 ).…”