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RAPID REPORT |
1 State Key Laboratory of Brain and Cognitive Sciences, Institute of Biophysics, Chinese Academy of Sciences, 15 Da-tun Road, Beijing 100101, China
2 Institute of Neuroscience, Shanghai Institutes of Biological Sciences, Chinese Academy of Sciences, 320 Yue-Yang Road, Shanghai, China
3 Department of Psychology, Australian National University, Canberra 2601, Australia
| Abstract |
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(Received 23 June 2006;
accepted after revision 7 August 2006;
first published online 10 August 2006)
Corresponding author S. He: State Key Laboratory of Brain and Cognitive Sciences, Institute of Biophysics, Chinese Academy of Sciences, 15 Da-tun Road, Beijing 100101, China. Email: shiganghe{at}moon.ibp.ac.cn
| Introduction |
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In contrast to the well-studied ONOFF DSGCs, the ON DSGCs are much less studied even in the rabbit retina, most probably due to the low density and therefore the low encounter rate. A few papers have described the dendritic morphology, light responses, pharmacology, axonal projection, and spike synchronization of the ON DSGCs of the rabbit retina (Caldwell et al. 1978; Ariel & Daw, 1982; Buhl & Peichl, 1986; Amthor et al. 1989; He & Masland, 1998; Dong et al. 2004; Ackert et al. 2006).
We chose to explore the mouse retina for responses of the ON DS type to settle the issue of whether there exists a physiological type of ON DSGCs in that species. The mouse model is important because of the burgeoning opportunities for applying modern genetic tools for resolving structurefunction problems. In an earlier morphological survey (Sun et al. 2002), we have already identified a subtype of mouse RGC, RGC1, exhibiting a dendritic branching pattern and level of stratification similar to that of rabbit ON DSGCs. This morphological type was also revealed using intracellular injection and a transgenic approach (Badea & Nathans, 2004; Kong et al. 2005).
| Methods |
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C57BL/6N mice were used in this study. The use and handling of animals were strictly in accordance with the institutional guidelines and the Society for Neuroscience's policies on the use of animals and human subjects in neuroscience research. All experimental procedures have been previously described (Weng et al. 2005). Mice were dark adapted for at least 1 h before experiments, deeply anaesthetized with an I.P. injection of a mixture of ketamine (50 mg kg1) and xylazine (10 mg kg1), decapitated, and the eyes immediately enucleated. A small cut was made in the sclera close to the cornea and the eyeball was submerged in Ames' medium equilibrated with 95% O2 and 5% CO2. The front parts were discarded and the retina carefully dissected from the pigment epithelium, and attached, ganglion cell side up, to a piece of black Millipore filter paper (AABP02500) with a 2 mm diameter hole in the centre for adequate visual stimulation and infrared illumination during the electrophysiological recording. The whole-mount retinal preparation was then transferred into a recording chamber (0.5 ml in volume) on the fixed stage of an upright microscope (Leica DMLFSA) equipped with a x40 water-immersion objective (NA 0.80). The preparation was continuously superfused with oxygenated bicarbonate-buffered Ames' medium at 35°C.
Electrophysiology
Micropipettes were manufactured from thick-walled borosilicate filament glass tubing. Under infrared illumination and visual control through a cooled CCD camera (CoolSNAP HQ, Photometrics, Atlanta, GA), the inner limiting membrane was dissected with a pipette to expose somas of several RGCs. RGCs with medium to large somas were examined with a pipette filled with Ames' medium (24 M
). Gentle suction was applied to establish loose-patch configuration and spike activities were recorded. Using a flashing spot and a moving bar, the ON DS responses could be identified, the preferrednull axis determined, and receptive fields mapped. For whole-cell voltage-clamp recording, the extracellular pipette was replaced with a patch pipette with 47 M
tip resistance filled with intracellular solution (120 mM caesium methanesulphonate, 0.5 mM CaCl2, 5 mM EGTA, 10 mM Hepes, 4 mM ATP, 0.5 mM GTP, and 5 QX-314 in mM, adjusted to pH 7.2 with 1 M CsOH). Neurobiotin (Molecular Probes, Eugene, OR, USA) was added to the intracellular solution (0.5%) for revealing dendritic morphology of recorded cells. Whole-cell configuration was formed when seal resistance was > 1 G
. Capacitance was always compensated. Series resistance, in most cases < 20 M
, was not compensated. The liquid junction potential of 10 mV was corrected. All data acquired from the Axopatch 200B amplifier were low-pass filtered at 2 kHz, digitized simultaneously with an A/D converter (Digidata 1322A, Axon Instruments), and stored on a personal computer. Data were analysed offline using Clampfit (Axon Instruments), Mini Analysis (Synaptosoft Inc., Leonia, NJ, USA), and figures were plotted with OriginPro 7.0 (MicroCal Software Inc., Northampton, MA, USA).
Light stimulation
Stimuli were generated using a program written in VC++ and Directx8 SDK, displayed on a monitor (Sony E230) and focused onto the retina through a microscope condenser. The brightness of the stimuli was about 0.35 x 1011 photons cm2 s1. Two types of light stimuli were generated. A spot of 251000 µm diameter, flashed for 0.510 s, was used to determine the size of the receptive field and response polarity. A rectangular bar of 100 µm x 500 µm moving parallel to its long axis at 375 µm s1 and in 12 directions at 30 deg intervals was used to determine the directionality.
Immunohistochemical staining and confocal imaging
Retinas were fixed with 4% paraformaldehyde for 1 h, washed 3 times and incubated in rabbit-anti-VAChT antibody (1: 100, Sigma) for 7 days at room temperature. The preparations were then incubated in donkey-anti-rabbit-TRITC (1: 100, Jackson Laboratory) for 1 day to label VAChT and streptavidin-FITC (1: 1000) to visualize Neurobiotin. Preparations were coverslipped with Vectorshield (Vector Laboratories), and sealed with nail polish. Images were collected using a Leica SP2 confocal microscope equipped with a x63 PlanApo objective (NA 1.4). Contrast and brightness of images were adjusted using Photoshop 8.0 (Adobe). The method for calculating the cofasiculation index was described in a previous study (Dong et al. 2004).
| Results |
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To investigate synaptic inputs of the ON DSGCs, we performed whole-cell voltage-clamp experiments and held the membrane potential at 65 mV and 0 mV to examine the excitatory and inhibitory inputs, respectively. Figure 3B shows that the EPSC is larger for motion in the preferred direction than in the null, and the IPSC is larger for motion in the null than in the preferred. This synaptic input pattern is similar to that observed in the ONOFF DSGCs (Weng et al. 2005). Data averaged from 11 cells confirmed the asymmetry (Fig. 3B), showing significantly larger EPSCs in the preferred direction (P < 0.05) and much larger IPSCs in the null direction (P < 0.01). Surprisingly, we observed a clear late input, containing both excitatory and inhibitory components, probably when the trailing edge of the rectangle swept across the receptive field (arrows in Fig. 3B). Neither the EPSCs nor IPSCs of the late input were directional (Fig. 3C). It is possible that a few branches straying into the OFF sublamina (Fig. 2C) picked up some OFF signals. However, even in strictly monostratified cells, the OFF signals can still be detected, so it is also possible that the OFF signals reach the ON layer via a specific pathway. We have reported a possible pathway to mediate the OFF signals to the ON layer (He et al. 2005) and we will address the detailed pharmacology of the OFF signals in a separate study.
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| Discussion |
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The synaptic mechanisms of the ONOFF DSGCs have been studied in detail for spatial and pharmacological characteristics. The ONOFF DSGCs receive an inhibition spatially offset to the null side and involving a number of transmitters (Fried et al. 2002, 2005); however, nothing has ever been reported about the synaptic input pattern of the ON DSGCs.
It was not expected that the pattern of excitatory and inhibitory synaptic currents during motion in preferred and null directions for ON DSGCs would so closely resemble the pattern of ONOFF DSGCs, given the differences in response dynamics, velocity range and destination of central projection. Previous studies reported that GABA antagonist picrotoxin, and AChE inhibitor physostigmine, abolish direction selectivity of both types of cells (Wyatt & Daw, 1976; Ariel & Daw, 1982). Taken together with the current findings that the synaptic input pattern is very similar for both types of DSGCs, and dendrites of both DSGCs tightly cofasciculate with cholinergic plexus, it leads to the strong suggestion that ON SAs provide directional input to both types of DSGCs. The important question then arises is this: are there sufficient SAs present to satisfy the different directional signals required by both ON and ONOFF DSGCs? It has been shown that in the rabbit retina, there are sufficient displaced SAs to provide directional input to ON and ONOFF DSGCs (Dong et al. 2004). More information such as distribution of preferred directions of the DSGCs and the coverage factor of the ON SAs is needed before the issue can be pursued in the mouse. The main difference between ON and ONOFF DSGCs is that ON DSGCs respond better to more slowly moving objects. If SAs can supply sustained inhibition, then it is probably not necessary to designate a special type of SA for ON DSGCs exclusively. In a recent study on the cholinergic amacrine cells, GABA release from them has been shown to be quite sustained (Zheng et al. 2004), supporting the proposal that the SAs may provide directional input to both the ON and the ONOFF DSGCs.
There is a recent report that the ON DSGCs in the rabbit retina receive non-directional excitatory inputs and directional inhibitory inputs (Vaney et al. 2005), indicating that the synaptic mechanism of the ON DSGCs is somewhat different from that of ONOFF DSGCs. We cannot provide any explanation for this difference. However, in another series of studies examining possible functions of the ON DSGCs in the rabbit retina, we also performed voltage-clamp recording on three ON DSGCs, and the results were almost identical to those reported here for mouse ON DSGCs. A most recent report confirmed our previous report for an OFF component in the ON DSGCs, but found the OFF component to be directional, exhibiting a preferred direction opposite to that of the ON preferred direction (Ackert & Bloomfield, 2006).
On the other hand, the extensive dendritic cofasciculation between ON DSGCs and SAs warrants discussion of the candidate bipolar classes that could provide inputs to the ON DSGCs. In several recent studies, the morphological classification of bipolar cells in the mouse retina has been carried out (Badea & Nathans, 2004; Ghosh et al. 2004a; Pignatelli & Strettoi, 2004). A key characteristic for present purposes is the stratification range of the axonal arbors. The axonal terminal of type 6 bipolar cells of Ghosh et al. 2004a, later revised to type 7 (Ghosh et al. 2004b), is in the position of supplying input to the ON DSGCs. A similar type has been identified as CB4b (Pignatelli & Strettoi, 2004), another type CB34 exhibits diffused axonal terminals and is also possibly providing inputs to the ON DSGCs. According to Lin & Masland (2005), this type is comparable to the specific ON cone bipolar expressing green fluorescent protein (GFP) in the transgenic mouse (line 357 of GUS8.4GFP) produced by Huang et al. (2003). The latter group clearly showed this bipolar stratification to accurately abut the ON cholinergic band on its inner side (their Fig. 1G). Limited types of bipolar cells stratifying at the level of the ON DSGC dendritic arbor suggest that two types of DSGCs receive bipolar inputs from the same type of bipolar cells. In turn, Lin & Masland (2005) showed that the stratification colocalizes with the dendritic tree of a large monostratified RGC. The present paper brings this chain of evidence closer to completion by likening the monostratified RGC to type RGC1 of Sun et al. (2002) and identifying it with the physiological class of ON DSGCs.
Conclusion
We identified ON DSGCs in the mouse retina, and revealed the physiology and dendritic morphology. We also showed that the dendrites of the ON DSGCs tightly cofasciculate with the cholinergic plexus. The synaptic input pattern of the ON DSGCs is quite similar to that of the ONOFF DSGCs. All the evidence suggests that these two different types of DSGCs may share a common mechanism for coding motion directions.
| Footnotes |
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| Acknowledgements |
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