AMPA receptor is involved in transmission of cone signal to ON bipolar cells in carp retina

An Ultrastructural and Immunohistochemical Analysis of the Outer Plexiform Layer of the Retina of the European Silver Eel (Anguilla anguilla L)

Here we studied the ultrastructural organization of the outer retina of the European silver eel, a highly valued commercial fish species. The retina of the European eel has an organization very similar to most vertebrates. It contains both rod and cone photoreceptors. Rods are abundantly present and immunoreactive for rhodopsin. Cones are sparsely present and only show immunoreactivity for M-opsin and not for L-, S- or UV-cone opsins. As in all other vertebrate retinas, Müller cells span the width of the retina. OFF-bipolar cells express the ionotropic glutamate receptor GluR4 and ON-bipolar cells, as identified by their PKCα immunoreactivity, express the metabotropic receptor mGluR6. Both the ON- and the OFF-bipolar cell dendrites innervate the cone pedicle and rod spherule. Horizontal cells are surrounded by punctate Cx53.8 immunoreactivity indicating that the horizontal cells are strongly electrically coupled by gap-junctions. Connexin-hemichannels were found at the tips of the horizontal cell dendrites invaginating the photoreceptor synapse. Such hemichannels are implicated in the feedback pathway from horizontal cells to cones. Finally, horizontal cells are surrounded by tyrosine hydroxylase immunoreactivity, illustrating a strong dopaminergic input from interplexiform cells.

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

L-Glutamate, the photoreceptor neurotransmitter, depolarizes horizontal cells and OFF-bipolar cells by ionotropic receptors and hyperpolarizes ON-bipolar cells by metabotropic receptors. Despite extensive light microscopy on the distribution of glutamate receptors in zebrafish retina, there are little ultrastructural data. Given the importance of zebrafish in studies on the genetic manipulation of retinal development and function, precise data on the synaptic neurochemical organization of the zebrafish retina is needed. Immunohistochemical techniques were used to determine the ultrastructural localization of glutamate receptor subunits GluR2, GluR4, NMDA2B (NR2B) and mGluR1alpha in zebrafish outer plexiform layer (OPL). These antibodies were chosen because of an apparent conservation of localization of GluR2, GluR4 and mGluR1alpha in the vertebrate OPL, while there is some support for NMDA receptors in the OPL. GluR2-immunoreactivity (IR) was in all horizontal cell dendrites that invaginated cone pedicles and rod spherules. Three arrangements of dendrites contained GluR-IR in rod spherules: classical-type with GluR2-IR on lateral horizontal cell dendrites, a butterfly-shaped horizontal cell dendrite, and a goblet-shaped dendrite, likely of bipolar cell origin. GluR4-IR was restricted to dendrites of OFF-bipolar cells that innervated rod and cone terminals. NR2B-IR was restricted to a subtype of cone ON-bipolar cell. mGluR1alpha-IR was restricted to ON mixed rod/cone (Mb) bipolar cells whose dendrites innervated rod and cone synaptic terminals. The presence of mGluR1alpha on Mb bipolar cell dendrites is consistent with a role in retrograde endocannabinoid suppression. The subunit composition of glutamate receptors should affect the kinetics and pharmacology of these cells to glutamate receptor activation.

Localization of ionotropic glutamate receptors to invaginating dendrites at the cone synapse in primate retina

The separation of OFF pathways that signal light decrements from ON pathways that signal light increments occurs at the first retinal synapse. The dendrites of OFF bipolar cells abut the cone pedicle at basal positions distal to the site of glutamate release and express ligand-gated or ionotropic glutamate receptors (GluR). The dendrites of ON bipolar cells penetrate narrow invaginations of the cone pedicle proximal to the site of release and express the G-protein-coupled, metabotropic glutamate receptor, mGluR6. However, recent studies demonstrating the expression of GluR subunits in the rodent rod bipolar cell, known to yield an ON response to light, call this basic segregation of receptors into question. The light-microscopic distribution of many glutamate receptors in the primate retina is now well established. We reexamined their ultrastructural localization in the outer retina of Macaca fascicularis to test systematically whether invaginating dendrites at the cone synapse, presumably from ON bipolar cells, also express one or more ionotropic subunits. Using preembedding immunocytochemistry for electron microscopy, we quantified the distribution of the AMPA-sensitive subunits GluR2/3 and GluR4 and of the kainate-sensitive subunits GluR6/7 across 207 labeled dendrites occupying specific morphological loci at the cone pedicle. We report, in agreement with published investigations, that the majority of labeled processes for GluR2/3 (70%) and GluR4 (67%) either occupy basal positions or arise from horizontal cells. For GluR6/7, we find a significantly lower fraction of labeled processes at these positions (47%). We also find a considerable number of labeled dendrites for GluR2/3 (10%), GluR4 (21%), and GluR6/7 (18%) at invaginating positions. Surprisingly, for each subunit, the remainder of labeled processes corresponds to "fingers" of presynaptic cytoplasm within the cone invagination.

Modulation by melatonin of glutamatergic synaptic transmission in the carp retina

Melatonin is involved in a variety of physiological functions through activating specific receptors coupled to GTP-binding protein. Melatonin and its receptors are abundant in the retina. Here we show for the first time that melatonin modulates glutamatergic synaptic transmission from cones to horizontal cells (HCs) in carp retina. Immunocytochemical data revealed the expression of the MT1 receptor on carp HCs. Whole-cell recordings further showed that melatonin of physiological concentrations potentiated glutamate-induced currents from isolated cone-driven HCs (H1 cells) in a dose-dependent manner, by increasing the efficacy and apparent affinity of the glutamate receptor. The effects of melatonin were reversed by luzindole, but not by K 185, indicating the involvement of the MT1 receptor. Like melatonin, methylene blue (MB), a guanylate cyclase inhibitor, also potentiated the glutamate currents, but internal infusion of cGMP suppressed them. The effects of melatonin were not observed in cGMP-filled and MB-incubated HCs. These results suggest that the melatonin effects may be mediated by decreasing the intracellular concentration of cGMP. Consistent with these observations, melatonin depolarized the membrane potential of H1 cells and reduced their light responses, which could also be blocked by luzindole. These effects of melatonin persisted in the presence of the antagonists of receptors for dopamine, GABA and glycine, indicating a direct action of melatonin on H1 cells. Such modulation by melatonin of glutamatergic transmission from cones to HCs is thought to be in part responsible for circadian changes in light responsiveness of cone HCs in teleost retina.

Interactions of allosteric modulators of AMPA/kainate receptors on spreading depression in the chicken retina

The functional role of AMPA and kainate receptors in spreading depression (SD) was investigated in the isolated chicken retina. Competitive (NBQX) and non-competitive (GYKI 52466, GYKI 53405 and GYKI 53655) antagonists of the AMPA receptor inhibited AMPA-induced SD in a concentration-dependent manner. Concentrations of drugs caused 50% inhibition (IC(50) values) are 0.2, 16.6, 7.0 and 1.4 microM, respectively. AMPA receptor positive modulator cyclothiazide was more effective in the potentiation of SD evoked by AMPA than by kainate. Slight potentiation of either AMPA- or kainate-induced SD was observed only at high concentration (1 mg/ml) by the kainate receptor modulator concanavalin A. Compounds that positively modulate AMPA receptor function (cyclothiazide, IDRA-21, S 18986, 1-BCP and aniracetam) caused a concentration-dependent potentiation in SD. Concentrations of drugs that caused 50% potentiation (estimated EC(50) values) are 9, 135, 142, 450 and 1383 microM, respectively. Interaction between cyclothiazide, aniracetam or S 18986 administered with each other, or with GYKI 52466, respectively, was also investigated. When cyclothiazide and S 18986 were co-applied, their effects seemed to be additive. However, lack of additivity was obtained when S 18986 was added together with aniracetam. Positive modulators applied at equiactive concentrations reduced the inhibitory action of GYKI 52466 and differently shifted its concentration-response curve. In this respect, S 18986 was the most effective (IC(50) of GYKI 52466 changed from 16.6 to 51.9 microM). Our findings indicate the contribution of AMPA rather than kainate receptors in the mediation of retinal spreading depression. Our data further support the idea that multiple positive modulatory sites are present on the AMPA receptor complex in addition to a negative modulatory site.

Characterization of receptors for glutamate and GABA in retinal neurons

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.