The Synaptic Architecture of AMPA Receptors at the Cone Pedicle of the Primate Retina

Abstract

Cone pedicles, the output synapses of cone photoreceptors, transfer the light signal onto the dendrites of bipolar and horizontal cells. Cone pedicles contain between 20 and 45 ribbon synapses (triads) which are the release sites for glutamate, the cone transmitter. Several hundred postsynaptic dendrites contact individual cone pedicles, and we studied the glutamate receptors expressed and clustered at these contacts, particularly the AMPA receptor subunits.Using immunocytochemistry and confocal imaging we were able to resolve individual triads within the cone pedicles by light microscopy. We studied their differences in L/M-and S-cones, and we counted the number of triads per pedicle across the retina. The presynaptic matrix protein bassoon, the synapseassociated membrane protein P84, and peanut agglutinin were used to specifically label synaptic ribbons, invaginating dendrites of horizontal cells and invaginating dendrites of ON-cone bipolar cells, respectively.Pre-and post-embedding immunocytochemistry and electron microscopy were used to localize the AMPA receptor subunits at the cone pedicle base. They were aggregated at three different postsynaptic sites: at horizontal cell invaginating contacts, at bipolar cell flat contacts, and at desmosome-like junctions underneath the cone pedicles.We also performed double-labeling experiments with the triad-specific markers and the antibodies against the AMPA receptor subunits. AMPA receptors were preferentially expressed by horizontal cells, and to a lesser extent by OFF-cone bipolar cells. We did not observe any cone-selective expression of AMPA receptor subunits postsynaptic to L/M-or S-cones, suggesting AMPA receptors are not the key to understanding trichromatic signaling in the primate retina.

Chromatic Bipolar Cell Pathways in the Mouse Retina

“…PNA clusters at M-cones occupied a mean area of 5.2 Ϯ 1.1 m 2 (n ϭ 149), whereas those at S-cone pedicles had only a mean area of 3.4 Ϯ 0.9 m 2 (n ϭ 30; p Ͻ 0.001). For both markers, the labeled area at S-cone pedicles was significantly smaller than that at M-cones, consistent with the findings from primate (Ahnelt et al, 1990;Haverkamp et al, 2001a). This strengthens the view that S-cones are not merely variations of cones (as only defined by opsin expression) but a distinct photoreceptor type.…”

“…1C): M-cone pedicles (26.5 Ϯ 4.9 m 2 ; n ϭ 288) were on average larger than S-cone pedicles (22.4 Ϯ 4.3 m 2 ; n ϭ 25; p Ͻ 0.001). In line with this result and data from primate retina (Haverkamp et al, 2001a;Lee et al, 2005), we expected also a denser clustering of PNA at S-cones compared with M-cones. Because PNA colocalizes with mGluR6 at cone pedicles but not at rod spherules (Mataruga et al, 2007), this lectin can be used to label cone pedicle invaginations.…”

Neuroligin 2 Controls the Maturation of GABAergic Synapses and Information Processing in the Retina

“…Furthermore, the number of photoreceptors as determined on semithin sections were comparable in WT and KO (WT: 191 Ϯ 17 per 100 m length, n ϭ 4; KO: 197 Ϯ 13 per 100 m length, n ϭ 5). The integrity of photoreceptor connectivity in the OPL was examined by labeling for bassoon [a marker of rod and cone ribbon synapses (Brandstätter et al, 1999)], peanut agglutinin [PNA; specific for cone pedicles (Haverkamp et al, 2001)], Go ␣ [specific for rod and ON-cone bipolar cells (Vardi, 1998)], and calbindin [a marker of horizontal cells (Peichl and González-Soriano, 1994)] (Fig. 3A).…”

Rod and Cone Contributions to Horizontal Cell Light Responses in the Mouse Retina

“…However, retinal neurons in transgenic mice with photoreceptor deficiencies have been shown to reorganize their synaptic contacts (Strettoi et al, 2002;Dick et al, 2003). To rule out the possibility that axon terminals in Cx36-deficient mice contact cone pedicles, we incubated retinas containing injected axon terminals with peanut agglutinin, which labels the base of the cone pedicles (Hack and Peichl, 1999;Haverkamp et al, 2001). Rotations of single scans revealed no overlap between axon terminal and cone pedicle labeling (Fig.…”