Strong glutamate immunoreactivity was observed by both light and electron microscopy in bipolar cells of the turtle (Pseudemys scripta elegans) retina after postembedding immunohistochemistry. Virtually all bipolar cells showed strong labeling, on average 18 times that of the Muller (glial) cells. The data suggest that both on-and off-center bipolar cells are glutamatergic. Photoreceptors were also labeled, but with a labeling intensity about half that of the bipolar cells. Other types of retinal neurons showed less immunoreactivity, except for a small population of strongly labeled amacrine cells.Bipolar cells carry visual information from the outer to the inner retina and are the first cells along the visual pathway to be divided into separate on-and off-channels. Furthermore, they show a center-surround organization similar to that observed in retinal ganglion cells and other neurons in the visual system (1, 2). Surprisingly, little is known about the transmitters employed by the bipolar cells. Physiological evidence has indicated that bipolar cells are excitatory to ganglion cells (3-6), but firm evidence for the presence of an excitatory transmitter in bipolar cells has not been forthcoming.Both amacrine and ganglion cells possess receptors specific for the acidic amino acids, and therefore it has been proposed that L-glutamate or a similar excitatory amino acid is a neurotransmitter in bipolar cells (7-11). Attempts at localizing endogenous glutamate in the retina have been only partially successful, and at times contradictory, most likely because of the indirect nature of the methods available (12).Recently, a technique has been developed to localize glutamate immunohistochemically by applying a purified antibody to etched plastic sections and demonstrating the binding site of the first antibody with a second antibody tagged with small (15-nm) colloidal gold particles (13,14). The method gives a good signal-to-noise ratio, and since only a small fraction of the glutamate of the cell is available for detection by the primary antibody (i.e., that at the section surface), the gold particles do not obscure the cytological characteristics of the labeled neurons. We have used this technique to localize glutamate in the turtle retina. MATERIALS AND METHODSFor light microscopy, small pieces from the posterior pole of light-adapted eyes of the turtle Pseudemys scripta elegans were fixed in 2% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4) for 2 hr at room temperature, dehydrated, and embedded in Durcupan ACM (Fluka). For electron microscopy, small pieces of retina were fixed at room temperature for 90 min with 4% glutaraldehyde, 1% formaldehyde, and 0.2 mM CaCl2 in 0
Fluorescence microscopy has revealed a new type of amine-containing retinal neuron, the interplexiform cell, that extends processes in both plexiform layers. After intravitreal injection of 5,6-dihydroxytryptamine in goldfish and Cebus monkey, the processes of these cells can be identified by electron microscopy. In goldfish, the processes are pre- and postsynaptic to amacrine cells in the inner plexiform layer and presynaptic to bipolar and horizontal cells in the outer plexiform layer. Interplexiform cells thus provide an intraretinal centrifugal pathway from inner to outer plexiform layers.
Purpose: To examine immunohistochemical markers in straight, well-laminated retinal transplants with special attention paid to the interphotoreceptor matrix, the Müller cells and the ganglion cells as these three retinal components have been abnormal in transplants produced by previous methods. Methods: Nine rabbits underwent subretinal transplantation of a complete full-thickness embryonic neuroretina. After 31 or 49 days, the transplants were stained for light microscopy and processed for immunohistochemistry. Results: Six of 9 eyes contained transplants with straight, well-laminated regions with all light-microscopic characteristics of a normal retina. In the outer segment region, the expression of peanut agglutinin showed segmental labeling of cone domains in the interphotoreceptor matrix, and interphotoreceptor retinoid binding protein immunoreactivity was found. Glial fibrillary acidic protein and vimentin immunoreactivity revealed normal Müller cell morphology. In 3 transplants the AB5-antibody-labeled ganglion cells in the ganglion cell layer and all transplants contained nerve fibers in the nerve fiber layer labeled by an antibody against neurofilament of 160 kD. The latter also labeled fibers connecting the transplant with the host. Conclusions: Full-thickness embryonic retinal transplants develop the normal retinal appearance and display several of the retinal components necessary for normal function which are not found in transplants produced by previous methods.
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