Functional N-Methyl-<i>D</i>-Aspartate Receptors Are Expressed in Cone-Driven Horizontal Cells in Carp Retina

Abstract

Glutamate works as a major excitatory neurotransmitter in the vertebrate retina. Whole-cell recordings made from isolated carp cone horizontal cells (H1 cells) showed that N-methyl-D-aspartate (NMDA), co-applied with glycine, induced inward currents that were blocked by the NMDA receptor competitive antagonist D-2-amino-5-phosphonopentanoate (D-AP5) and 5,7-dichlorokynurenic acid (DCKA), a selective NMDA receptor antagonist acting at the glycine site on the NMDA receptor complex. Moreover, calcium imaging showed that NMDA caused a significant elevation of intracellular calcium levels ([Ca²⁺]_i) of H1 cells, which was also blocked by D-AP5. In contrast, neither inward currents nor changes in [Ca²⁺]_i could be induced by NMDA in rod horizontal cells (H4 cells). Intracellular recordings made from H1 cells in the isolated retina, superfused with Ringer’s containing 1 mM Mg²⁺, in the dark demonstrated that NMDA reduced the light-off overshoot of H1 cells. We therefore conclude that the functional NMDA receptor is expressed in carp H1 cells, from which this receptor has been thought to be absent, and this receptor may play a role in modulating cone-driven signal of horizontal cells in the dark.

Ultrastructural analysis of the glutamatergic system in the outer plexiform layer of zebrafish retina

“…The GluR2 subunit tends to be found on horizontal cell dendrites, the GluR4 subunit on OFF-bipolar cell dendrites and the mGluR1a along with the mGluR6 subunit on rod bipolar cells (Yang, 2004, for review). Evidence for NMDA receptors in the OPL is controversial with immunohistochemical evidence for NR1 or NR2 subunits in the OPL of fish and rat (Wenzel et al, 1997;Fletcher et al, 2000;Kalloniatis et al, 2004;Shen et al, 2006). In the present study we have continued the systematic investigation of the zebrafish retina with focus on the synaptic organization of glutamate receptor subunits GluR2, GluR4, mGlur1a and NR2B in the OPL by electron microscopical immunohistochemistry (EM-IHC).…”

“…Although there is physiological evidence for NMDA-mediated events in catfish, goldfish, carp and turtle horizontal cells (Yazulla, 1983;Christensen, 1986, 1989;Perlman et al, 1987;Shen et al, 2006), at least in goldfish (Vandenbranden et al, 2000), they do not label for NR2B-IR and thus may not require NR2B subunits. Shen et al (2006) however, reported that cone horizontal cells, but not rod horizontal cells, of carp label for NR1-IR. Connaughton et al (2004) described six types of ON-bipolar cell in zebrafish with axon terminals in the proximal sublamina b of the IPL.…”

Intracellular calcium concentration changes initiated by N-methyl-D-aspartic acid receptors in retinal horizontal cells

“…Apart from the NMDA-triggered CICR from ER described in this work, role of ER in AMPA-triggered transient increase of [Ca 2 + ] i in retinal HCs has also been reported [3]. In contrast, NMDA receptors were generally believed to be absent in retinal HCs until a recent study [7]. NMDA receptors are much more permeable to Ca 2 + and are known to be involved in a variety of physiological and pathological events because of its high Ca 2 + permeability [11].…”

“…2. The application of 100 mM NMDA in the presence of 50 mM glycine, which is a coagonist of NMDA receptors [7], induced a swift increase in [Ca 2 + ] i . The ratio value of F340/F380 increased from a base level of 0.45 to a peak level of 0.95 within 10 s. This increase was transient and the ratio value then gradually decreased to a steady level of 0.72 over a period of about 30 s. Consistent results were observed in seven H1 cells.…”

“…In the vertebrate retina, fast excitatory synaptic activation in the HCs is generally believed to be mediated by AMPA-type glutamate receptors [6]. In contrast, it was recently reported that functional NMDA receptors are expressed in cone-driven HCs (H1 cells) of carp retina [7]. In this study, changes in cytoplasmic [Ca 2 + ] i induced by exogenous application of NMDA were investigated in freshly dissociated H1-type HCs of carp retina using a fura-2-based Ca 2 + imaging technique.…”