Synaptic Elements for GABAergic Feed-Forward Signaling between HII Horizontal Cells and Blue Cone Bipolar Cells Are Enriched beneath Primate S-Cones

Abstract

The functional roles and synaptic features of horizontal cells in the mammalian retina are still controversial. Evidence exists for feedback signaling from horizontal cells to cones and feed-forward signaling from horizontal cells to bipolar cells, but the details of the latter remain elusive. Here, immunohistochemistry and confocal microscopy were used to analyze the expression patterns of the SNARE protein syntaxin-4, the GABA receptor subunits α1 and ρ, and the cation-chloride cotransporters NKCC and KCC2 in the outer plexiform layer of primate retina. In macaque retina, as observed previously in other species, syntaxin-4 was expressed on dendrites and axon terminals of horizontal cells at cone pedicles and rod spherules. At cones, syntaxin-4 appeared densely clustered in two bands, at horizontal cell dendritic tips and at the level of desmosome-like junctions. Interestingly, in the lower band where horizontal cells may synapse directly onto bipolar cells, syntaxin-4 was highly enriched beneath short-wavelength sensitive (S) cones and colocalized with calbindin, a marker for HII horizontal cells. The enrichment at S-cones was not observed in either mouse or ground squirrel. Furthermore, high amounts of both GABA receptor and cation-chloride cotransporter subunits were found beneath primate S-cones. Finally, while syntaxin-4 was expressed by both HI and HII horizontal cell types, the intense clustering and colocalization with calbindin at S-cones indicated an enhanced expression in HII cells. Taken together, GABA receptors beneath cone pedicles, chloride transporters, and syntaxin-4 are putative constituents of a synaptic set of proteins which would be required for a GABA-mediated feed-forward pathway via horizontal cells carrying signals directly from cones to bipolar cells.

Functional Circuitry of the Retina

“…There are typically one or two classes of horizontal cell: The so-called A-type (or HI in primates) has an axon, and the B-type (HII in primates) does not. Both are GABAergic neurons (Guo et al 2010, Puller et al 2014). The dendrites of both A- and B-type horizontal cells are found postsynaptic to cones; the axon terminals of B-type cells are postsynaptic to rods.…”

“…Horizontal cells are coupled electrically, thereby increasing the area over which they can exert this surround inhibition (Thoreson & Mangel 2012, Weiler et al 2000). Additionally, GABA released from horizontal cells could hyperpolarize bipolar cells directly, thereby potentiating surround inhibition (Herrmann et al 2011, Puller et al 2014). …”

“…Two mechanisms might allow a small bistratified cell in primate retina to be S versus L/M opponent: S cones themselves are color-opponent because they receive horizontal cell feedback mediated by L/M cones (Packer et al 2010, Dacey et al 2014, Puller et al 2014), and the bistratified cell integrates inputs from L/M-cone-driven OFF bipolar cells and the S-cone-driven ON bipolar cells (Crook et al 2009). …”

Evolution of the circuitry for conscious color vision in primates

“…53 In subsequent experiments, as predicted by this hypothesis, the existence of other components of a GABA-mediated pathway was verified, including GABA receptors and the concomitant enrichment of the Na-K-Cl co-transporter with syntaxin-4. 54 The enrichment beneath S-cones reveals synapses for feedforward signaling between HII horizontal cells and S-cone bipolar cells. However, colocalization with the HII horizontal cell marker indicates a general elaboration of HII horizontal-to-bipolar cell feedforward synapses providing a synaptic pathway for S-cone signals to be introduced into a subset of L/M cone midget bipolar cells.…”

“…The two types of neurons, so produced, have chromatic responses aligned along roughly orthogonal axes. Now, as a solution to the problem of the origins of the unique hues, it is clear that feedforward from S-cones, via HII horizontal cells to L/M opponent midget bipolar cells 53,54 is capable of producing a subset of midget ganglion cells with responses that can account for human hue perception. The combinations of S-cone inputs to L/M opponent midget ganglion cells produces exactly the hue axes corresponding to phenomenological hues.…”