Neuroscience Letters 250 (1998) 103–106
Lamprey ganglion cells contact photoreceptor cells Jean Paul Rio a ,*, Nicolas P. Vesselkin a , b, Jacques Repe´rant a, Natalia B. Kenigfest a , b, Claudine Versaux-Botteri c a
INSERM, U 106, Hoˆpital de la Salpeˆtrie`re, Paris, France Sechenov Institute of Evolutionary Physiology and Biochemistry, St. Petersburg, Russia c Laboratoire d’Anatomie Compare´e, MNHN, Paris, France
b
Received 17 March 1998; received in revised form 22 May 1998; accepted 25 May 1998
Abstract Lamprey retinal ganglion cells are localized in two separate layers: those close to the vitreous and those at the junction between the inner nuclear and inner plexiform layers, including some others in the inner nuclear layer, close to the photoreceptor cell layer. Whereas most ganglion cell dendrites arborize in the inner plexiform layer and contact amacrine, bipolar and retinopetal cell profiles, some of them, located in the inner nuclear layer, ascend radially through the outer plexiform layer and establish contacts with photoreceptor cells. This ganglion cell type might correspond to the biplexiform ganglion cells already described in gnathostome vertebrate species and could provide a fastforward signal from photoreceptors to ganglion cells, bypassing the usual bipolar cell interneuron. 1998 Elsevier Science Ireland Ltd. All rights reserved
Keywords: Lamprey; Ganglion cells; Photoreceptor cells; Ultrastructure
Among the five cell types characterized in the vertebrate retina [3], the ganglion cells (GCs) represent the pivotal element. Light is transduced into biological signals through a bisynaptic intraretinal pathway and GCs transmit the visual information to the different CNS primary visual centers. The laminated structure of the retina is composed of alternate plexiform and cellular layers. In the agnathan retina, most GCs are displaced and located at the boundary between the inner nuclear (INL) and inner plexiform (IPL) layers [21], and others lie close to the vitreous. In Lampetra fluviatilis retina, four types of GCs have been reported [7], two in the displaced layer ( ≈ 75%) and two at their orthotopic location, close to the vitreous ( ≈ 25%), and similar estimates have been obtained in another lamprey species (Ichthyomyzon unicuspis) [11]. Lampetra fluviatilis GC dendrites have been mainly observed in the IPL [21]. In the retina of the larval lamprey Petromyzon marinus, contacts
* Corresponding author. INSERM U 106, Neuromorphologie: De´veloppement, Evolution, Hoˆpital de la Salpeˆtrie`re, 47, Bd de l’Hoˆpital, 75651 Paris Cedex 13, France. Tel.: +33 1 42162677; fax: +33 1 45709990; e-mail:
[email protected]
between GC dendrites and photoreceptors have been reported in the outer plexiform layer (OPL), close to the photoreceptor cell layer [8]. In addition, biplexiform GCs establish contacts upon photoreceptors in the frog [19] and the macaque [15]. In this latter species, intracellular recordings from biplexiform GCs showed them to contact photoreceptor cell processes [22]. In contrast to other GCs, these biplexiform GCs possess dendrites which arborize both in the IPL and OPL [15]. The goal of the present study is to search for the presence of synaptic contacts between GC dendrites and photoreceptors in adult forms of Lampetra fluviatilis. Four adult lampreys (Lampetra fluviatilis), measuring 30–40 cm were used. Following anesthesia in MS 222 (0.1%), the optic nerve was carefully dissected, isolated from the oculomotor nerves and HRP (4% in 0.1 M phosphate buffer, pH 8.0) was iontophoretically applied with a glass micropipette (20–30 mm in diameter) connected to an electrode (positive current 10 mA for 20 min). Following survival times ranging from 8 to 14 days, the animals were transcardially perfused with 1% paraformaldehyde and 1% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4). The retinas were reacted in toto for HRP with the Adams tech-
0304-3940/98/$19.00 1998 Elsevier Science Ireland Ltd. All rights reserved PII S0304- 3940(98) 00440- 6
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nique [1], and prepared for conventional electron microscopy. Examination of semi-thin transverse sections of the retina under the light microscope revealed different profiles labeled with HRP. Retrogradely-labeled GCs were mainly observed at the junction between the INL and IPL, and some near the inner limiting membrane, close to the vitreous. These were of several shapes, either oval, rounded or polygonal with a diameter ranging from 10 to 30 mm. The photoreceptor cells were of two types, either long or short described, respectively, by several authors [7,13,14] as cones and rods, depending on the size of the inner segment and the position of the outer segment. The optic nerve fiber layer did not consist of a separate layer, and fibers were mostly organized in fine fascicles. A minute analysis of the OPL showed some HRP-labeled GC dendrites running radially into this layer, up to the level of the photoreceptor cell layer, and some were observed to ascend from the displaced GCs. Under the electron microscope, the same pattern of labeling was observed in the different layers. Orthotopicallylocated GCs lay close to the vitreous and were sparsely distributed. They were bipolar with the long axis parallel to the inner limiting membrane. Occasionally, the primary dendrites were seen to emerge and were oriented toward the IPL, giving off poorly-ramifying dendritic profiles of smooth appearance. In this layer they were contacted by amacrine and bipolar cell processes, as well as by anterogradely-labeled retinopetal axon terminals [17]. The main body of labeled GCs were found at the boundary between the IPL and INL, among the optic nerve fiber layer. Moreover, some large polygonal GCs occupied the entire width of the INL, and rare medium-sized GCs were observed in the outer part of the INL (Fig. 1). These cells had a more rounded shape and most dendritic processes were oriented toward the OPL. Some of these dendrites were observed in the vicinity of the photoreceptor cells (Fig. 2). In some cases, both membranes of the photoreceptor cell and GC dendrite were in close apposition (Fig. 3). The labeled GC dendrites were also observed to be apposed to photoreceptor cell processes which contained synaptic ribbons. In addition, rare densely HRP-labeled terminal-like profiles, filled with rounded synaptic vesicles, were also observed in the OPL close to the photoreceptor cells (Fig. 4). Our results confirm previous data obtained in different lamprey species with regard to the localization of GCs
[7,10,11,20,21], mostly situated at the boundary between the INL and IPL, and some in the innermost part of the IPL, adjacent to the inner limiting membrane. Moreover, a few GCs and dendrites are present in the INL, close to the OPL, and these have been also described in the silver lamprey [11]. These latter cells are very reminiscent of the biplexiform GCs reported in the retina of teleosts [5], frog [19], mouse [9] and primate [15,22]. Contacts between biplexiform GC dendrites and photoreceptor cells have been described in the frog [19] and primate [15,22]. We have also observed the same synaptic contacts between photoreceptor cell processes and GC dendrites, but, as in larval lampreys [8], the postsynaptic GC dendrites could not be clearly assumed to belong to biplexiform GCs, whose existence in agnathans still remains to be demonstrated, in contrast to gnathostomes. However, as stressed in the macaque [15], these biplexiform cells are also postsynaptic at non-ribbon synapses, which has been also observed in the present study. The question arises as to the nature of the HRP-labeled terminal-like profiles observed in the OPL, close to the photoreceptor cell processes. Do they correspond to centrifugal visual axon terminals or to GC axon collaterals? The first observations of Tretjakoff [20] in Lampetra fluviatilis have reported the presence of retinopetal fibers in the OPL, and these have not been confirmed more recently, both in this species [17,21] and in larval forms of Petromyzon marinus [2]. Moreover, our previous results have not mentioned the presence of any GC axon collaterals in the retina of Lampetra fluviatilis [21], whereas these have been described in another lamprey species (Ichthyomyzon unicuspis) [11]. As emphasized by Fritzsch and Collin [11], these collaterals might have escaped notice in Lampetra [21], due to the much denser ganglion cell axonal network present in this species in comparison to that in Ichthyomyzon [11]. Nevertheless, these collaterals have been also observed in the INL in the developing retina of various vertebrate species (lizards [4], birds [12], mammals [16]) and only described in adult mammals [6]. These HRPlabeled terminal-like profiles that we have observed in the OPL have morphologies similar to the optic terminals (particularly in the size and shape of the synaptic vesicles) seen in the primary visual centers in this species [18]. Therefore, it is possible that the terminal-like profiles belong to these GC axon collaterals. The direct contacts between photoreceptor cell processes
Fig. 1. Electron micrograph of an HRP-labeled GC close to the photoreceptor cell layer. On the upper right (asterisk), note part of a photoreceptor cell process. Scale bar, 1 mm. Fig. 2. An HRP-labeled GC dendrite comes in close apposition with a photoreceptor cell (asterisk). The dendrite is postsynaptic at non-ribbon synapse (arrow). Scale bar, 0.5 mm. Fig. 3. Example of a non-ribbon synapse between a GC dendrite and a photoreceptor cell process (asterisk). Scale bar, 0.25 mm. Fig. 4. This micrograph illustrates an HRP-labeled axon terminal (empty star) in close apposition with an unidentified profile (filled star) at the level of the photoreceptor cell layer. Scale bar: 2.5 mm.
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and GC dendrites appear to be highly conserved during phylogenesis, since these are present in both larval and adult lampreys and some gnathostome species, including mammals. Whether these contacts are established on rods or cones still remains to be determined. Functionally, these contacts might act as an additional and quick access from photoreceptor cells to ganglion cells probably transmitting fastforward scotopic signals, bypassing the usual interneuron (bipolar cell) circuitry in the retina [15,19,22]. Authors are indebted to Denis Le Cren for his skillful photographic assistance. This work was financially supported by INSERM (Action Spe´cifique INSERM/CEI) and MNHN (JE 336). Thanks are due to Dr. L. Upton for revising the English. [1] Adams, J.C., Heavy metal intensification of DAB-based HRP reaction product, J. Histochem. Cytochem., 29 (1977) 775. [2] Anado´n, R., Mele´ndez-Ferro, M., Pe´rez-Costas, E., Pombal, M.A. and Rodicio, M.A., Centrifugal fibers are the only GABAergic structures of the retina of larval sea lamprey: an immunocytochemical study, Brain Res., 782 (1998) 297–302. [3] Boycott, B.B. and Dowling, J.E., Organization of the primate retina: light microscopy, Phil. Trans. R. Soc. Lond. B, 255 (1969) 109–184. [4] Cajal, S., La re´tine des Verte´bre´s, La Cellule, 9 (1893) 119– 225. [5] Cook, J.E., Kondrashev, S.L. and Podugolnikova, T.A., Biplexiform ganglion cells, characterized by dendrites in both outer and inner plexiform layers, are regular, mosaic-forming elements of teleost fish retinae, Vis. Neurosci., 13 (1996) 517– 528. [6] Dacey, D.M., Wide-spreading terminal axons in the inner plexiform layer of the cat’s retina: evidence for intrinsic axon collaterals of ganglion cells, J. Comp. Neurol., 242 (1990) 247–262. [7] Dalil-Thiney, N., Etude Histologique et Morphofonctionnelle de la Re´tine de Lamproie Lampetra fluviatilis: Approches Ultrastructurales et Immunohistochimiques. The`se, Paris, 1995. [8] de Miguel, E., Rodicio, M.C. and Anado´n, R., Ganglion cells and retinopetal fibers of the larval lamprey retina: an HRP ultrastructural study, Neurosci. Lett., 106 (1989) 1–6. [9] Doi, M., Uji, Y. and Yamamura, H., Morphological classification of retinal ganglion cells in mice, J. Comp. Neurol., 356 (1995) 368–386.
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