Compartmentalized dendritic signaling in a multitasking retinal interneuron

The rich diversity of cell types and synaptic motifs in the vertebrate retina provides an excellent platform for uncovering fundamental principles of neural circuit function at the cellular, synaptic, and network levels. In PNAS, Chen et al. (1) report on a remarkable example of compartmentalized dendritic signaling in a retinal neuron despite its small dendritic field. Using two-photon calcium imaging, the authors find that the dendrites of VGluT3-expressing glutamatergic amacrine cells (GACs), which diffusely ramify in the inner plexiform layer (IPL) of the mouse retina, exhibit depth-dependent activation at light onset and offset in dendritic subregions that matches the general scheme of ON/OFF segregation in the IPL. Distinct patterns of local dendritic activation occur within a mere depth of ∼30 μm in the middle of the IPL, arising from highly localized excitatory inputs that are compartmentalized by synaptic inhibition. Chen et al.’s findings extend our mechanistic understanding of the functional segregation of ON/OFF visual channels at the level of dendritic subdomains.

Early in the visual pathway, bright and dark contrasts in the visual scene are separately processed by ON and OFF channels. This segregation first occurs in the IPL (2–4), the second synaptic layer of the retina that harbors neuronal processes of over 70 types of bipolar cells, amacrine cells, and retinal ganglion cells (5, 6). While the plethora of cell types are intricately connected to form multiple circuits that extract a variety of visual features in parallel, they are anatomically partitioned into the inner ON sublayer and the outer OFF sublayer of the IPL. ON bipolar cells terminate their axons in the ON sublayer and release more glutamate at light onset, while OFF bipolar cell axons occupy the OFF sublayer and release more glutamate at light offset (Fig. 1). The amacrine and retinal ganglion cells postsynaptic to bipolar cells extend their dendrites to various depths of the IPL, and acquire ON, OFF, or ON/OFF responses from their presynaptic bipolar cell inputs.

Fig. 1.

Schematic shows ON/OFF segregation of GAC dendrites in the IPL. ON and OFF bipolar cell (BP) axons terminate in the ON and OFF sublayers of the IPL, respectively, and provide local excitation to GAC dendritic segments. Small red dots on the GAC represent varicosities with predominant ON responses; small blue dots represent varicosities with predominant OFF responses; small dots with both colors at the ON/OFF sublayer boundary represent varicosities with symmetric ON/OFF responses. The black dendritic branch of the GAC spanning both ON and OFF sublayers illustrates segregation of ON- and OFF-dominated responses within a single dendritic branch.

How are GACs integrated into this general scheme of ON/OFF segregation? Like most retinal amacrine cells, GACs are axonless and release neurotransmitters from specialized structures, called varicosities, distributed across their dendritic tree. Since the diffusely ramified GAC dendrites overlap with axon terminals of both ON and OFF bipolar cells (7–9), GACs exhibit both ON and OFF responses during visual stimulation (7, 10, 11). However, an unusual property of GACs is that they release glutamate in addition to glycine, and therefore serve as an additional source of excitation for multiple types of ON and OFF retinal ganglion cells (10, 12). Glutamate release from GACs at both light onset and offset poses a challenge to segregated processing in the ON and OFF channels because the ganglion cell dendrites postsynaptic to GACs would indiscriminately receive both ON and OFF excitation regardless of their stratification depth of the IPL. The study by Chen et al. (1) shows that this problem is circumvented because the responses of GAC varicosities are not uniform throughout the dendritic tree for a given visual stimulus. Instead, the varicosities located in the ON sublayer of the IPL are preferentially activated at light onset, while those in the OFF sublayer prefer light offset (Fig. 1). Therefore, the general scheme of ON/OFF segregation is preserved in GACs through compartmentalized dendritic signaling.

To determine the patterns of light-evoked GAC dendritic activation at different depths of the IPL, Chen et al. (1) expressed the genetically encoded calcium indicator GCaMP6 in a sparse population of GACs, and measured GCaMP6 fluorescence at different locations of the dendritic tree evoked by flashing light spots. A particularly compelling observation of signal compartmentalization in GAC dendrites is made on varicosities along continuous dendritic segments that span across the ON and OFF sublayers of the IPL (1). Layer-specific ON/OFF response asymmetry persists within the same dendritic segment, indicating that compartmentalization of dendritic signals does not rely on dendritic branching. As a result of this compartmentalization along the vertical axis, the ON and OFF responses of GACs are differentially shaped by layer-specific inhibitory circuitry in the IPL. This is reflected by the different receptive field sizes for the ON and OFF responses even though the GAC dendritic field sizes in the ON and OFF sublayers are similar. Furthermore, electrotonic isolation of GAC dendritic signals also occurs laterally within each IPL sublayer. This is illustrated by the limited lateral spread of local calcium transients evoked by a small spot stimulus (1).

Compartmentalized dendritic signaling is a prominent feature of many central neurons throughout the brain (13). In the retina, it has been exemplified in amacrine cells with extensive dendritic arbors. For example, each radial dendritic sector of a starburst amacrine cell (dendritic field diameter of ∼220 μm) is preferentially activated by a different direction of visual motion (14). In another example, individual varicosities of A17 amacrine cell dendrites (dendritic field diameter of ∼400 μm) independently mediate feedback inhibition to distinct rod bipolar cells (15). By contrast, small-field amacrine cells are generally considered to be electrotonically compact due to their small dendrites and to exhibit global dendritic signaling. Consistent with this idea, uniform dendritic activation is observed in the well-studied small-field AII amacrine cell, which has multistratified dendrites spanning the IPL with a lateral dendritic field size of 40–80 μm (1). Compared with AII amacrine cells, GAC dendrites are only slightly larger in the lateral dimension spanning ∼90 μm but are more narrowly stratified on the vertical axis covering a depth of ∼30 μm in the IPL. However, the prominent signal compartmentalization in GAC dendrites implies that global dendritic signaling is not a general rule for small-field amacrine cells and suggests that local dendritic computation needs to be considered a possibility when formulating hypotheses about the functions of other less-explored small-field amacrine cells.

How is signal compartmentalization implemented in the short dendritic segments of GACs? Using a pharmacological approach, Chen et al. (1) discover a critical role of synaptic inhibition in restricting signal propagation along GAC dendrites along both vertical and lateral axes. This finding will inspire future studies on GAC dendritic function to focus on the inhibitory synapses onto GACs. Identification of the presynaptic cell types, the subcellular distribution of inhibitory inputs onto GAC dendrites, and the mode of inhibitory signaling required for dendritic compartmentalization will further our understanding of the underlying mechanisms of GAC dendritic computation. Furthermore, synaptic inhibition likely works in concert with other well-documented dendritic mechanisms, such as dendritic morphology, expression and modulation of active membrane conductances, and localized synaptic excitation (15–20). How these factors collectively impact dendritic processing of GACs awaits future exploration.

The functional significance of compartmentalized dendritic signaling needs to be interpreted together with other notable features of GACs discovered in previous studies. In both ON and OFF sublayers of the IPL, the receptive field of GAC dendrites has a strong inhibitory surround (1, 10), making them preferentially activated by local contrast, such as boundaries and local motion of small objects (11). Dendritic activation leads to the release of glutamate and glycine from the varicosities. Importantly, glutamate and glycine are released onto separate groups of postsynaptic

Chen et al.'s findings extend our mechanistic understanding of the functional segregation of ON/OFF visual channels at the level of dendritic subdomains.

neurons (12). The preferential activation of GAC dendrites by local contrast triggers glutamate release to several retinal ganglion cell types, including ON/OFF direction-selective ganglion cells, ON direction-selective ganglion cells, OFF α-ganglion cells, and W3 cells, contributing to their contrast sensitivity (10–12). As indicated by the study by Chen et al. (1), the segregation of ON and OFF dendritic subdomains in GACs ensures that each postsynaptic ganglion cell type receives GAC-mediated glutamatergic excitation at their “designated” contrast polarity based on the stratification pattern of the ganglion cell dendrites. On the other hand, strong activation of GAC dendrites by local contrast also provides strong glycinergic inhibition to a different ganglion cell type called “uniformity detectors,” indicating a role of GACs in the suppressed-by-contrast response of these uniformity detectors (12). The role of compartmentalized dendritic signaling in visual processing is yet unclear for GAC’s glycinergic outputs.

As highlighted in the study by Chen et al. (1), this small retinal interneuron maximizes the functionality of its dendrites by dividing the dendritic tree into distinct computational subunits with considerable autonomy. This strategy is coupled with the diversification of neurotransmitter types and postsynaptic targets of the GAC, which enables the cell to participate in multiple retinal circuits. The experimental accessibility and the increasing repertoire of circuit analysis tools for the mouse retina offer exciting future opportunities to address the outstanding questions of GAC function, and to incorporate this multitasking neuron into comprehensive network models of parallel visual processing in the retina.