Synaptic connections of amacrine cells containing vesicular glutamate transporter 3 in baboon retinas

Abstract

The goals of these experiments were to describe the morphology and synaptic connections of amacrine cells in the baboon retina that contain immunoreactive vesicular glutamate transporter 3 (vGluT3). These amacrine cells had the morphology characteristic of knotty bistratified type 1 cells, and their dendrites formed two plexuses on either side of the center of the inner plexiform layer. The primary dendrites received large synapses from amacrine cells, and the higher-order dendrites were both pre- and postsynaptic to other amacrine cells. Based on light microscopic immunolabeling results, these include AII cells and starburst cells, but not the polyaxonal amacrine cells tracer-coupled to ON parasol ganglion cells. The vGluT3 cells received input from ON bipolar cells at ribbon synapses and made synapses onto OFF bipolar cells, including the diffuse DB3a type. Many synapses from vGluT3 cells onto retinal ganglion cells were observed in both plexuses. At synapses where vGluT3 cells were presynaptic, two types of postsynaptic densities were observed; there were relatively thin ones characteristic of inhibitory synapses and relatively thick ones characteristic of excitatory synapses. In the light microscopic experiments with Neurobiotin-injected ganglion cells, vGluT3 cells made contacts with midget and parasol ganglion cells, including both ON and OFF types. Puncta containing immunoreactive gephyrin, an inhibitory synapse marker, were found at appositions between vGluT3 cells and each of the four types of labeled ganglion cells. The vGluT3 cells did not have detectable levels of immunoreactive γ-aminobutyric acid (GABA) or immunoreactive glycine transporter 1. Thus, the vGluT3 cells would be expected to have ON responses to light and make synapses onto neurons in both the ON and the OFF pathways. Taken with previous results, these findings suggest that vGluT3 cells release glycine at some of their output synapses and glutamate at others.

Segregated Glycine-Glutamate Co-transmission from vGluT3 Amacrine Cells to Contrast-Suppressed and Contrast-Enhanced Retinal Circuits

“…Both glycine and glutamate are released in a Ca 2+ -dependent manner, indicating an underlying vesicular release mechanism, even though vIAAT was previously not immunolocalized to GACs (Haverkamp and Wassle, 2004; Johnson et al, 2004). Interestingly, the two co-released transmitters target separate postsynaptic GC circuits, suggesting most parsimoniously that glycine and glutamate are released from two separate populations of vesicles at different synapses, as also supported by a recent EM study which found both symmetric and asymmetric synapses on GAC processes in the primate retina (Marshak et al, 2015). …”

“…Recently, it was discovered that a special subclass of amacrine cell, which is immunoreactive to vesicular glutamate transporter 3 (vGluT3) (Fremeau et al, 2002; Gong et al, 2006; Haverkamp and Wassle, 2004; Johnson et al, 2004; Stella et al, 2008), releases glutamate and makes excitatory synapses onto specific GC types (Krishnaswamy et al, 2015; Lee et al, 2014). These glutamatergic amacrine cells (GACs) have also been found to be immunoreactive to glycine and glycine transporter GlyT1 (but not vesicular inhibitory amino acid transporter, vIAAT) (Haverkamp and Wassle, 2004; Johnson et al, 2004), raising an intriguing possibility that GACs may release glycine in addition to glutamate (Marshak et al, 2015), even though functional glycine-glutamate co-release has not been reported in the mature nervous system. In order to test this possibility and gain a functional insight into a potential glycine-glutamate co-transmission system, one needs to determine (1) whether GACs release glycine under physiological conditions, and if they do, (2) whether the release is through a vesicular mechanism, (3) whether GACs release glycine and glutamate onto the same or different postsynaptic targets, and (4) what specific synaptic circuits and functional advantages may be subserved by such inhibitory-excitatory co-transmission.…”

Cellular and Circuit Mechanisms Shaping the Perceptual Properties of the Primate Fovea

“…Amacrine synapses were identified in electron microscopy studies based on the absence of ribbons (Calkins et al, 1994). MGCs could receive non-ribbon excitatory contacts, e.g., from glutamatergic amacrine cells (Lee et al, 2014; Marshak et al, 2015), and bipolar cells could make some conventional synapses on the dendrites of MGCs (Kolb and Dekorver, 1991). The MGC is contacted by a single midget bipolar cell with a claw-like synaptic terminal containing 25–50 ribbons (Calkins et al, 1994) packed into a small volume (~70 μm 3 ).…”

Wavy multistratified amacrine cells in the monkey retina contain immunoreactive secretoneurin

“…The unlabeled profiles were identified by their characteristic ultrastructure [22]. The criteria for evaluating labeled synapses were described in detail recently [23]. The sample included 436 synapses, including approximately equal numbers from each of the strata of the IPL in which labeled dendrites were found (Table 2).…”

“…These include: diffuse cells [39], knotty type 2 cells [40], A8 cells [11] and several other knotty amacrine cell types that are known only from retinas labeled with the Golgi method [19]. Amacrine cells that contain vesicular glutamate transporter 3 are also glycinergic, but because they are not labeled by antisera to GlyT-1 [23], they did not account for any of the profiles observed in this study.…”