Frogs maintained on a diurnal light-dark cycle (14 hours light and 10 hours darkness) shed their rod photoreceptor outer segment tips shortly after the onset of light. Shedding is synchronous and occurs in about 25 percent of the rod photoreceptors each day. Prolonged exposure to total darkness decreases the amount of shedding, after which exposure to light results in a large burst of synchronous shedding. Thus in the frog retina, the synchronous shedding of rod outer segment tips is shown to be directly related to light stimulation.
Isolated frog retinas were incubated with radiolabeled glycoprotein precursors in the presence or absence of tunicamycin (TM), a selective inhibitor of protein N-glycosylation. In dual-label incubations, TM inhibited the incorporation of [3H~mannose into total retina Cl3CCOOH-precipitable material by 85% relative to controls, whereas incorporation of [ The vertebrate retinal rod photoreceptor is a polarized cell that exhibits an extraordinary degree of morphological and metabolic compartmentation (see Fig. 1). The rod outer segmernt (ROS) is composed of a highly ordered stack of discrete, disc-shaped double membranes which are surrounded by the plasma membrane of the cell. The first several disc membranes at the base of the ROS are actually continuous infoldings of the plasma membrane, from which the discrete disc membranes of the ROS are derived. The ROS disc membranes are composed almost entirely of proteins and lipids, in approximately equal proportions by weight (1). Rhodopsin, the rod visual pigment, accounts for >95% of the total ROS membrane protein, making it the major structural as well as functional ROS membrane component (2, 3). It is composed of a glycoprotein (opsin) to which an 11-cis-retinaldehyde chromophore is covalently linked. Opsin consists of a single polypeptide to which two asparagine-linked oligosaccharides are attached near the amino terminus (2, 4-6).The oligosaccharides are unusually short hybrids of the complex type of N-linked oligosaccharides (7), primarily having the structure GlcNAc(Man)3(GlcNAc)2, with lesser amounts of GlcNAc(Man)4(GLcNAc)2 and GlcNAc(Man)5(GlcNAc)2 (5, 6). drates have been suggested (8-11), the biological significance of the oligosaccharide moieties of opsin remains to be elucidated.The ROS is joined to its adjaceM l ompartment, the rod inner segment (RIS), by a connecting cilium which serves as the sole link by which intercompartmental exchange between the ROS and RIS occurs. The RIS is subdivided into two relatively distinct compartments: the myoid (containing much of the endoplasmic reticulum and the Golgi apparatus), and the ellipsoid (characterized by a dense cluster of mitochondria). A substantial proportion of the total proteins synthesized in the rough endoplastnic reticulum of the rod cell are transported sequentially from the myoid through the ellipsoid to the periciliary plasma membrane of the RIS before being incorporated into the basal disc membranes of the ROS (for a comprehensive review, see refs. 2, 12, 13). Opsin is initially glycosylated cotranslationally with mannose and N-acetylglucosamine in the endoplasmic reticulum, with subsequent posttranslational addition of N-acetylglucosamine in the Golgi apparatus (2,12,13). Recent studies (14,15) have shown that passage of opsin through the Golgi apparatus is obligatory for its incorporation into ROS disc membranes.Biochemical studies have demonstrated both the formation of various intermediates and the presence of endogenous acceptors for those intermediates implicated in the lipid interme...
A substance with somatostatin-like immunoreactivity (SLI) was found in extracts of goldfish, frog, and cow retina. Dilutions of retinal'SLI parallel the standard curve for radioimmunoassay obtained with synthetic somatostatin. Chromatography of goldfish retinal extract on Sephadex' G-50 revealed two peaks of SLI, one that coeluted with synthetic somatostatin and one that eluted as a larger molecule. Incubation in 8 M urea did not alter the chromatographic pattern of the extract. SLI was present'in extracts of frog optic nerve and tectum in concentrations higher than those found in the retina. In goldfish retina, SLI was localized by immunofluorescence to four types of processes in the inner plexiform layer; their origins could be traced to three classes of SLI-containing cell bodies in the proximal row of the inner nuclear layer and one class in the ganglion cell layer. Localization of SLI to cells of the retina and characterization of the molecular forms of retinal SLI suggest that the retina is a promising model system for studies on the potential neurotransmitter function of somatostatin.'Somatostatin is a tetradecapeptide that was initially isolated from ovine hypothalamus and identified by its action as an inhibitor of growth hormone release (1). In addition to its actions as a modulator of the release of a wide variety of hormones and physiological secretions (2), somatostatin is thought to function as a neurotransmitter, primarily because a substance with somatostatin-like immunoreactivity (SLI) has been demonstrated in axons and nerve cell bodies of primary sensory neurons (3), sympathetic neurons (4), and intrinsic neurons of the intestine (5, 6) as well as in neurons in the central nervous system (7). Furthermore, SLI has been localized to secretory granules in the median eminence (8) and terminal synaptosomes of neurons of the hypothalamus (9, 10). As with neurotransmitters, release of SLI from nerve tissues by depolarization is calcium dependent (11). Electrophysiological studies have demonstrated that low concentrations of somatostatin affect the discharge rate of neurons in various regions of the brain (12-14), dorsal horn of the spinal cord (15), and myenteric plexus of the intestine (16).The retina provides an excellent system for further studies of the functional role of somatostatin in the central nervous system for the following reasons: (i) the anatomy, physiology, and pharmacology of the retina have been studied extensively and each of the major cell types has been characterized by intracellular recording (17), (ii) the retina is easily isolated without contamination from other tissues and can be maintained in organ culture for prolonged periods, and (iii) the ability to stimulate discrete populations of cells within the retina with light or with pharmacologic agents permits parallel electrophysiological and biochemical studies. Nevertheless, the retina had been neglected in the search for neuroactive peptides until Schaeffer et al. (18) METHODSPreparation of Tissue Extracts. All fr...
A robust amount of rOx is produced in the ellipsoid when photoreceptors are exposed to blue light. This light-induced effect is antioxidant sensitive and strongly coupled to mitochondrial electron transport. The cumulative effect of light on rOx generation over time may implicate a role for mitochondria in light-induced oxidative damage of photoreceptors.
Antibodies directed against somatostatin have been used to label a population of interplexiform cells (IPCs) in the Xenopus laevis retina. These cells have spherical soma which lie in the inner nuclear layer (INL), adjacent to or one cell distal to the inner plexiform layer (IPL). Processes from these cells project throughout the IPL, with a fairly dense accumulation of labeled dendrites in the upper two-fifths of the IPL and a dense, narrow band of labeled dendrites adjacent to the ganglion cell layer. These cells also have finer processes, originating at the cell body, that traverse the INL and ramify in the outer plexiform layer (OPL). Double label experiments show that all of the cells that contain somatostatin-like immunoreactivity (SOM-LI) in the INL are also labeled by high-affinity uptake with 3H-glycine. Immunocytochemistry of retinal whole mounts shows that these cells are evenly distributed across the retina at a density of 542 +/- 65 cells/mm2. On the basis of the colocalization experiments and the morphological homogeneity of these cells, we suggest that they represent a single cell type. Interplexiform cell processes were further characterized by electron microscopy after immunocytochemistry or 3H-glycine autoradiography. In the IPL, IPC processes are seen to be postsynaptic at both ribbon and conventional synapses. This input is found almost entirely in the distal two-fifths of the IPL. Interplexiform cell processes are presynaptic to unlabeled processes in both the distal and proximal IPL. In the OPL, labeled processes are found near or contiguous with photoreceptor bases, and are often presynaptic to small-diameter processes. The postsynaptic processes have been identified as bipolar cell dendrites in six cases. Interplexiform cell processes may also contact horizontal cell processes in the OPL.
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