The Journal of Neuroscience, October 9, 2013 • 33(41):16045–16059 • 16045 Systems/Circuits NaV1.1 Channels in Axon Initial Segments of Bipolar Cells Augment Input to Magnocellular Visual Pathways in the Primate Retina Theresa Puthussery,1 Sowmya Venkataramani,1 Jacqueline Gayet-Primo,1 Robert G. Smith,2 and W. Rowland Taylor1 1Casey Eye Institute, Department of Ophthalmology, Oregon Health & Science University, Portland, Oregon 97239, and 2Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania 19104 In the primate visual system, the ganglion cells of the magnocellular pathway underlie motion and flicker detection and are relatively transient,whilethemoresustainedganglioncellsoftheparvocellularpathwayhavecomparativelylowertemporalresolution,butencode higher spatial frequencies. Although it is presumed that functional differences in bipolar cells contribute to the tuning of the two pathways, the properties of the relevant bipolar cells have not yet been examined in detail. Here, by making patch-clamp recordings in acuteslicesofmacaqueretina,weshowthatthebipolarcellswithinthemagnocellularpathway,butnottheparvocellularpathway,exhibit voltage-gated sodium (NaV ), T-type calcium (CaV ), and hyperpolarization-activated, cyclic nucleotide-gated (HCN) currents, and can generate action potentials. Using immunohistochemistry in macaque and human retinae, we show that NaV1.1 is concentrated in an axon initial segment (AIS)-like region of magnocellular pathway bipolar cells, a specialization not seen in transient bipolar cells of other vertebrates. In contrast, CaV3.1 channels were localized to the somatodendritic compartment and proximal axon, but were excluded from the AIS, while HCN1 channels were concentrated in the axon terminal boutons. Simulations using a compartmental model reproduced physiological results and indicate that magnocellular pathway bipolar cells initiate spikes in the AIS. Finally, we demonstrate that NaV channels in bipolar cells augment excitatory input to parasol ganglion cells of the magnocellular pathway. Overall, the results demon- strate that selective expression of voltage-gated channels contributes to the establishment of parallel processing in the major visual pathways of the primate retina. Introduction ganglion cells exhibit relatively sustained light responses, and A central goal for understanding visual function is to determine show comparatively low temporal resolution, but are optimized how parallel retinal circuits produce the characteristic outputs of for form and color vision (De Monasterio and Gouras, 1975; for different retinal ganglion cell types. In primates, the midget and review, see Dacey, 2004; Lee et al., 2010). The neural mechanisms parasol ganglion cells are the most abundant and well character- that underlie the differential tuning of parasol and midget gan- ized retinal output neurons, providing the neural substrate for glion cells are not well understood, but the differences are pre- the parvocellular and magnocellular visual pathways, respectively sumed to arise at the level of the bipolar cells (for review, see (Perry et al., 1984; Watanabe and Rodieck, 1989). Parasol gan- Masland, 2012). glion cells exhibit transient light responses and respond to high- There are at least 10 morphologically distinct cone bipolar cell temporal frequency stimuli, making them fundamental for the types in the macaque and human retina (Boycott and Wa¨ssle, perception of motion and flicker. On the other hand, midget 1991; Haverkamp et al., 2003); these can be divided into OFF and ON types, which respond to decrements and increments in light intensity. The flat midget bipolar (FMB) and invaginating midget Received March 22, 2013; revised Aug. 20, 2013; accepted Aug. 24, 2013. bipolar (IMB) cells provide input to the OFF and ON midget Author contributions: T.P. and W.R.T. designed research; T.P., S.V., and J.G.-P. performed research; R.G.S. con- tributed unpublished reagents/analytic tools; T.P., S.V., R.G.S., and W.R.T. analyzed data; T.P. and W.R.T. wrote the ganglion cells, respectively (Polyak, 1941; Kolb and Dekorver, paper. 1991; Calkins et al., 1994), whereas the diffuse bipolar (DB) cell This research was supported by National Eye Institute Grants EY014888 (W.R.T.) and EY016607 (R.G.S.); a Re- type DB3 provides the major input to OFF parasol ganglion cells search to Prevent Blindness (RPB) Lew R. Wasserman Merit Award (W.R.T.); a Collins Medical Trust Grant (T.P.); an (Jacoby et al., 2000; Calkins and Sterling, 2007), and DB4 cells unrestricted RPB grant to the Department of Ophthalmology, Oregon Health & Science University (OHSU), and the likely provide input to ON parasol cells (Boycott and Wassle, Ophthalmology (P30-EY010572) and Advanced Imaging core (P30-NS061800) facilities at OHSU. We thank Dr. Francoise Haeseleer for providing the CaBP5 antibody; Drs. Paul Martin, Ulrike Gru¨nert, and Ilya Buldyrev for com- 1991). The functional properties of these bipolar cells have not ments on earlier versions of the manuscript; and Drs. David Wilson and Alison Skalet for providing human tissue been examined in detail, but work in other mammals suggests samples. that functional diversity could arise, as follows: (1) at the den- The authors declare no competing financial interests. dritic input, through differences in glutamate receptors (Awatra- Correspondence should be addressed to W. Rowland Taylor, Department of Ophthalmology, Oregon Health & Science University, 3375 SW Terwilliger Boulevard, Portland, OR 97239. E-mail: [email protected]. mani and Slaughter, 2000; DeVries, 2000); (2) at the axon DOI:10.1523/JNEUROSCI.1249-13.2013 terminal output, through differences in calcium dynamics Copyright © 2013 the authors 0270-6474/13/3316045-15$15.00/0 (Baden et al., 2013a) and amacrine cell connectivity (Eggers and 16046 • J. Neurosci., October 9, 2013 • 33(41):16045–16059 Puthussery et al. • Voltage-Gated Channels in Primate Bipolar Cells Lukasiewicz, 2011); and (3) intrinsically, through differences in staining as the monoclonal Ankyrin G antibody from the UC Davis/NIH expression of voltage-gated channels (Ma et al., 2003; Mu¨ller et NeuroMab Facility. al., 2003; Cui and Pan, 2008). Here, we exploit the well charac- Immunohistochemistry. For immunohistochemistry on sections, Ͼ terized circuits of the macaque retina to determine how voltage- pieces of peripheral retina ( 4 mm) were fixed in 4% paraformaldehyde gated channels in bipolar cells contribute to the physiological (PFA) at 25°C from 5 to 30 min. Short fixation times (5 min) were critical to preserve the antigenicity of the NaV1.1 subunit. After fixation, retinae properties of the major ganglion cell types. were cryoprotected in sucrose solutions (10%, 20%, and 30%), embed- There is mounting evidence that not all bipolar cells signal ded in cryosectioning medium, and cut transversely at 14 ␮m on a cryo- exclusively through graded voltage signals; some exhibit voltage- stat. Cryostat sections were blocked for1hinanincubation buffer (IB) gated sodium (NaV) and calcium (CaV) currents and can produce containing 3% normal horse serum, 0.3–1% Triton X-100 (TX-100), and spikes (Cui and Pan, 2008; Saszik and DeVries, 2012; Baden et al., 0.025% NaN3. Primary antibodies were diluted in IB and applied over- 2013a,b). Such bipolar cells have not been identified in primate night at 25°C or 4°C. Secondary antibodies, raised in donkey, and retina (Han et al., 2000), and it is not clear in any species which coupled to Alexa Fluor 488, 594, and/or 647 (Invitrogen), were di- channel subunits drive spiking, where the channels are located, luted in IB without TX-100 (1:800) and applied for1hat25°C. Confocal imaging was performed on an Olympus FV1000 confocal whether the channels are functionally significant, or which reti- ϫ nal circuits these bipolar cells are part of. Here, we provide evi- microscope with an Olympus Plan Apo (60 /1.42 oil) objective at a resolution of 7.7 pixels/␮m. Image channels were pseudo-colored dence that voltage-gated channels in bipolar cells contribute to into RGB (red, green, blue) color space using Adobe Photoshop CS. In functional differences in the magnocellular and parvocellular vi- some cases, linear alterations to image contrast were made to the sual pathways. entire image using Adobe Photoshop CS. Immunohistochemistry and imaging after patch-clamp recordings. Bipo- lar and ganglion cells were anatomically classified by visual inspection of Materials and Methods the fluorescence fill at the conclusion of the recordings. In some cases, the Tissue preparation. Eyes were obtained from adult rhesus (Macaca mu- patch pipette was removed, and the vibratome slice or whole mount was latta) or cynomolgus macaques (Macaca fascicularis) of either sex. The fixed in 4% PFA for 15–30 min and imaged on a confocal microscope tissue was recovered immediately postmortem from animals that were (Figs. 1A–C, 3A,B). For immunohistochemistry, slices and whole exsanguinated in the course of unrelated experiments at the Oregon mounts were incubated in IB for 5–7 d at 25°C. Secondary antibodies National Primate Research Center (ONPRC). All animal procedures were diluted in IB without TX-100 (1:800) and applied overnight at 25°C. were performed in accordance with the ONPRC Tissue Distribution Pro- For illustration of bipolar cell morphology (Fig. 1A), confocal z-stacks gram. The anterior eye and vitreous were removed under room lighting, were combined into a single image plane and overlayed on a transmitted and the posterior