Synaptic release at mammalian bipolar cell terminals

Abstract

Bipolar cells play a vital role in the transfer of visual information across the vertebrate retina. The synaptic output of these neurons is regulated by factors that are extrinsic and intrinsic. Relatively little is known about the intrinsic factors that regulate neurotransmitter exocytosis. Much of what we know about intrinsic presynaptic mechanisms that regulate glutamate release has come from the study of the unusually large and accessible synaptic terminal of the goldfish rod-dominant bipolar cell, the Mb1 bipolar cell. However, over the past several years, examination of presynaptic mechanisms governing neurotransmitter release has been extended to the mammalian rod bipolar cell. In this review, we discuss the recent advances in our understanding of synaptic vesicle dynamics and neurotransmitter release in rodent rod bipolar cells and consider how these properties help to shape the synaptic output of the mammalian retina.

Figure 1. Rod bipolar cells of the mammalian retina

A) A vertical section of a mouse retina tripled-labeled for PKC-α (a rod bipolar cell marker; green), SV2 (a synaptic vesicle protein; red), and VGluT1 (a vesicular glutamate transporter; blue). A single optical section is shown. VGluT1 and SV2 are selectively found in the outer and inner plexiform layers (OPL and IPL), as expected for proteins located on synaptic vesicles. Photoreceptor terminals, which label for SV2 and VGLuT1 but not PKC-α, are seen as magenta. Rod bipolar cell dendrites, somata and axons, which express PKC-α, are seen as green. Rod bipolar cell terminals, located in the distal IPL, express all three markers and are seen as white. In the IPL, not all SV2 puncta co-localize with VGluT1 in the IPL, consistent with the presence of both glutamatergic and non-glutamatergic synapses. Scale bar: 20 μm. B) A paired recording from a synaptically-coupled rod bipolar and AII amacrine cell in a retinal slice from the mouse retina. A 100 ms depolarization evokes a sustained Ca²⁺ current in the presynaptic rod bipolar cell. The postsynaptic AII amacrine cell EPSC exhibits both transient and sustained components. Journal of Physiology 587/11 by Snellman et al., 2009 by John Wiley and Sons reproduced with permission from John Wiley and Sons via Copyright Clearance Center. C) A simple two-step simulation (Innocenti & Heidelberger, 2008) depicting the occupancy state of the rapid vesicle pool, which underlies the transient component of the EPSC. With a maximal fusion rate of 1000 s⁻¹, the rapid pool is quickly depleted (red line), consistent with a prominent EPSC transient component. With a fusion rate of 100 s⁻¹, the depletion of the rapid pool is slower (black line), suggestive of a smaller, broader EPSC transient. Increasing the refilling rate, as might occur when increasing the temperature from ambient room temperature to physiological temperature (Dinkelacker et al., 2000; Pyott & Rosenmund, 2002; Kushmerick et al., 2006), reduces the speed with which the rapid pool is depleted. Dashed line shows a simulation with a release rate of 100 s⁻¹ and a three-times faster refilling rate than that of the solid black line.

Figure 2. Recovery from depression and the role of presynaptic calcium

(A) Recovery of the releasable pool from depression was measured in the isolated mouse rod bipolar cell using a paired-pulse paradigm. The second capacitance response as a percentage of the first is plotted against the interpulse interval. The time course of refilling is described by a single exponential function (curve) given by the equation Y=Y₀+A₁e^(−x/τ) where Y₀ = 97.3 ± 5.0, A₁=90.2 ± 6.3, τ = 6.8 ± 1.5 s. The non-zero intercept is suggests the presence of a fast component of refilling. Reproduced with permission from Wan et al. (2008), copyright Cambridge University Press. (B) Elevated presynaptic Ca²⁺ enhances the rate of refilling of the rapid pool in the mouse rod bipolar cell. The cumulative exocytosis, ΔC_m, evoked by a brief 4Hz stimulus train (Wan et al., 2010), is plotted as a function of time for each pulse in the train. Each pulse in the train was sufficient to deplete the rapid pool. Filled circles represent data from control cells loaded with standard internal solution (containing 0.5 mM EGTA and no added Ca²⁺). Open circles represent data from cells loaded with internal solution containing 0.2 mM EGTA and 0.15 mM Ca²⁺. With the latter solutions, cells had an average resting Ca²⁺ of ≈ 115 nM compared with ≈ 35 nM in controls and higher stimulus-evoked rises in Ca²⁺ (Figure 7A, Wan et al., 2010). Data replotted from Wan et al., 2010.

Figure 3. Dynasore inhibits compensatory endocytosis in the mouse rod bipolar cell

(A) Representative example of exocytosis followed by endocytosis evoked by a 500 ms depolarization in a mouse rod bipolar cell. (B) Representative example of the response to a 500 ms depolarization in a mouse rod bipolar cell treated with 160 μM dynasore. Note the slow time course of membrane recovery. Inset shows a small rise in membrane capacitance that occurs after the closure of calcium channels. (C) The extent of membrane retrieval 10 s after calcium channel closure is significantly reduced in rod bipolar cells treated with dynasore (p <0.05). Ten seconds after calcium channel closure, control cells retrieved ≈ 76% of membrane added via stimulus-evoked exocytosis. By contrast, the extent of membrane retrieval in this same interval was reduced to ≈ 40% in cells exposed to dynasore. In 2/14 cells, dynasore fully blocked endocytosis, whereas all control cells exhibited membrane recovery. Error bars represent SEM. (Wan and Heidelberger, unpublished data)