Histochemical studies on catecholaminergic cells in the carp retina

Histochemical studies on catecholaminergic cells were conducted with the carp (Cyprinus carpio) retina. Catecholamine (CA)-containing cell bodies appear sparsely distributed among amacrine cells in the innermost cellular row of the inner nuclear layer (INL) and occasionally in the outer half part of the inner plexiform layer (IPL); only exceptionally are they found among ganglion cells. The fluorescent cells interspersed with the amacrine cells and in the IPL send their fiber processes toward both the outer plexiform layer (OPL) and the IPL; the fine fibers form dense networks in the INL and IPL. Pretreatment of the fish with intramuscular injection of reserpine (20 hr prior to enucleation) completely depleted CA from the retina. The fluorescence of catecholaminergic cells was enhanced, and the number of fluorescent cells visible was increased, by intravitreous injection ofl-DOPA, DA, and NA (3 hr prior to enucleation). A combination of pretreatment with intramuscular reserpine and intravitreous NA was particularly effective. These results indicate that catecholamines may play an important role in the modulation of the membrane potential of horizontal cells.     

Cellular and tissue responses to heavy ions: Basic considerations

Summary Responses of the S/S variant of the L5178Y murine leukemic lymphoblast, the photoreceptor cell of the rabbit retina and the lenticular epithelium of the rabbit to heavy ions (²⁰Ne,²⁸Si,⁴⁰Ar and⁵⁶Fe) are described and discussed primarily from the standpoint of the need for a comprehensive theory of cellular radiosensitivity from which a general theory of tissue radiosensitivity can be constructed.The radiation responses of the very radiosensitive, repair-deficient S/S variant during the G₁- and early S phases of the cell cycle were found to be unlike those of normally radioresistant cells in culture: the relative biological effectiveness (RBE) did not increase with the linear energy transfer (LET) of the incident radiation. Such behavior could be anticipated for a cell which is lacking the repair system that operates in other (normal) cells when they are exposed to ionizing radiations in the G₁ phase of the cell cycle. The S/S variant does exhibit a peak of radioresistance to X-photons mid-G₁ + 8 h into the cell cycle, however, and as the LET was increased, the repair capacity responsible for that radioresistance was reduced progressively.Sensory cells (photoreceptors) in the retina of the New Zealand white (NZW) rabbit are very radioresistant to ionizing radiations, and several years elapsed after localized exposure (e.g., 5–10 Gy) to heavy ions (²⁰Ne,⁴⁰Ar) before photoreceptor cells were lost from the retina. During the first few weeks after such irradiations, damage to DNA in the photoreceptor cells was repaired to a point where it could not be demonstrated by reorienting gradient sedimentation under alkaline conditions, a technique that can detect DNA damage produced by <0.1 Gy of X-photons. Restitution of DNA structure was not permanent, however, and months or years later, butbefore loss of photoreceptor cells from the retina could be detected, progressive deterioration of the DNA structure began.     

The connectivity of cones and cone horizontal cells in a mosaic-type teleost retina

Summary A total of 20 Golgi-impregnated cone horizontal cells of Nannacara anomala (Cichlidae) were studied in alternating semi- and ultrathin sections in order to examine their connections with the overlying square mosaic of equal double and central single cones. Cone horizontal cells exhibit three types of processes: (a) the long horizontal axon, (b) short horizontal dendrites with a terminal swelling, and (c) cone contacting processes ascending towards the outer plexiform layer. As seen in tangential sections, the latter processes are arranged in the form of two concentric circles including a central spot. The processes of the inner circle contact the eight double cone pedicles of one square unit: processes of the outer circle contact eight more double cone pedicles which are directly adjacent to the square unit. The central spot represents a process which contacts the central single cone. Processes of the inner circle most often terminate in a dichotomous branching which represents the lateral elements to one ribbon synapse, whereas in the outer circle only a single terminal swelling is observed. Because of the mosaic of the cones and the constancy of this pattern of connectivity a model can be constructed where the dendritic fields of the cone horizontal cells overlap to a considerable extent. From this model, it follows that each double cone pedicle is contacted by four different horizontal cells. The functional significance of these findings for color vision is discussed in the light of recent work with the microspectrophotometer characterizing the cone system of this species as bichromatic. The mosaiclike arrangement of the horizontal cell dendrites supports the conclusion that the parallels between the patterns of receptor and horizontal cells are no coincidence but play an important role in lateral inhibition and neural adaptation of the retina.A preliminary report of this study was given at the international symposium Neural principles in vision held at the University of Munich in September 1975Supported by grant Wa 348/1 of the Deutsche Forschungsgemeinschaft     

Localization and function of dopamine in the adult vertebrate retina.

Dopamine (DA) has satisfied many of the criteria for being a major neurochemical in vertebrate retinae. It is synthesized in amacrine and/or interplexiform cells (depending on species) and released upon membrane depolarization in a calcium-dependent way. Strong evidence suggests that it is normally released within the retina during light adaptation, although flickering and not so much steady light stimuli have been found to be most effective in inducing endogenous dopamine release. DA action is not restricted to those neurones which appear to be in "direct" contact with pre-synaptic dopaminergic terminals. Neurones that are several microns away from such terminals can also be affected, presumably by short diffusion of the chemical. DA thus affects the activity of many cell types in the retina. In photoreceptors, it induces retinomotor movements, but inhibits disc shedding acting via D2 receptors, without significantly altering their electrophysiological responses. DA has two main effects upon horizontal cells: it uncouples their gap junctions and, independently, enhances the efficacy of their photoreceptor inputs, both effects involving D1 receptors. In the amphibian retina, where horizontal cells receive mixed rod and cone inputs, DA alters their balance in favour of the cone input, thus mimicking light adaptation. Light-evoked DA release also appears to be responsible for potentiating the horizontal cell-->cone negative feed-back pathway responsible for generation of multi-phasic, chromatic S-potentials. However, there is little information concerning action of DA upon bipolar and amacrine cells. DA effects upon ganglion cells have been investigated in mammalian (cat and rabbit) retinae. The results suggest that there are both synaptic and non-synaptic D1 and D2 receptors on all physiological types of ganglion cell tested. Although the available data cannot readily be integrated, the balance of evidence suggests that dopaminergic neurones are involved in the light/dark adaptation process in the mammalian retina. Studies of the DA system in vertebrate retinae have contributed greatly to our understanding of its role in vision as well as DA neurobiology generally in the central nervous system. For example, the effect of DA in uncoupling horizontal cells is one of the earliest demonstrations of the uncoupling of electrotonic junctions by a neurally released chemical. The many other, diverse actions of DA in the retina reviewed here are also likely to become model modes of neurochemical action in the nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)     

Dopaminergic Modulation of Neurosecretory Cells in the Crayfish

The main aims of this paper are (a) to locate possible dopaminergic neurons in the eyestalk with anti-tyrosine hydroxylase antibodies, (b) to search for the presence of dopamine (DA) in the nervous structures of the eyestalk, (c) to explore its release, and (d) to test the effect of DA on neurosecretory cells in the eyestalk.Experiments were performed in adult crayfishes Procambarus clarkii, in isolated optic peduncle. Immunocytochemistry was made with the antibody against its precursor synthesizing enzyme tyrosine-hydroxylase. The content and release studies of DA were made using high performance liquid chromatography (HPLC). Extracellular and intracellular recordings were conducted with conventional recording techniques.A large number (2000) of immunopositive somata of different sizes and shapes were identified in various regions of the eyestalk. The majority of somata are of the smallest size (5–25 m diameter). DA content in the eyestalk was 5.6 ± 0.1 pmol per structure; the greatest content is in the MT (over 60%). A basal level release of DA was observed. Incubation of eyestalks in solution containing a high K⁺ concentration increased the DA release (79%). Two effects of DA on the excitability of X-organ neurons were observed; an excitatory effect on neurons of 25 m somata diameter and another inhibitory effect in the group of 35-m somata diameter neurons. The excitation occurs with a depolarization and decrement of membrane conductance in the cell soma while the inhibition occurs with a hyperpolarization and increment of membrane conductance in soma.We concluded the following: (1) Dopamine is present in each optic ganglia of the crayfish eyestalk. (2) There is a basal release of DA from the isolated eyestalk. (3) DA release is enhanced threefold by eyestalk incubation in 40 mM [K⁺] solution. (4) DA selectively excites a population of neurons with low-speed conduction axons, and small somata in the X-organ–sinus gland system, while inhibiting another population characterized by higher axonal conduction speed and large somata. (5) These observations support a role for DA as a neurotransmitter or neuromodulator in the X-organ neurons of the crayfish eyestalk.Dr. Hugo Aréchiga died on September 15th of 2003     

Changes in retinal fine structure induced in the crab Libinia by light and dark adaptation

Summary The cytological influence of light and dark adaptation (LA and DA) on the retinular cells of the spider crab Libinia emarginata has been studied by light and electron microscopy in four adaptive states: 17 hours darkness, 5 hours darkness, 5 hours diffuse light and 17 hours diffuse light. The rhabdom's fine structure is typical of decapods but its dual overall form and position mingle certain features of both apposition and superposition compound eye types. Distal and proximal retinal pigments both showed adaptive migration, but the distal pigment cells moved over a restricted range, and DA separated the retinular cell pigment granules into two groups, perinuclear and basilar.In the rhabdom no changes in its position, dimensions or microvillus fine structure were observed with LA or DA. But at the base of the rhabdom microvilli the rate of pinocytosis was strongly affected by the eye's adaptive state, being lowest after 17 hours DA and greatest after 17 hours LA; the wall of the 0.1 microvesicles so formed, looked like the membrane of the rhabdom microvillus and they were the same size as the vesicles in multivesicular bodies and in vesicular lamellar bodies.Three categories of complex cytoplasmic particles about 1 in diameter (multivesicular bodies, vesicular lamellar bodies and purely lamellar bodies) were all increased in number by decreased DA and by increased LA; similar quantitative effects occurred in the endoplasmic reticulum and in the ribosomes.The pinocytotic vesicles and the complex cytoplasmic bodies may represent part of an intracellular system to dispose of rhabdom metabolites whose production was initiated or increased by light absorption.Cytoplasmic and perirhabdomal vacuoles mainly distal in location, were also affected by light, but inversely; their maximal extent occurred after 17 hours DA; less DA or any LA significantly decreased their presence and aggregation.The data reported are of interest not only because they correlate retinal fine structure with the metabolism of vision but also because they provide a new and specific tool for distinguishing active from inactive neurosensory cells in the optic pathway.This research was initiated with the aid of U.S. Public Health Service Grant NB-03076 and has been continued with the support of U.S. Air Force Grant AFOSR-1064. The authors wish to thank Dr. Joseph G. Gall and Dr. William R. Adams for generously sharing their electron microscopic facilities; they are also grateful to Mrs. Mabelita Campbell for her collaboration on the light microscopy.     

Changes of lysosomal enzymes during hereditary degeneration and histogenesis of retina in mice

Summary During the post-natal development of the retina in mice, macrophages which are selectively stained for N-Acetyl--glucosaminidase enter the retina through the vascular route. Most of these cells finally occupy the outer and the inner levels of the inner nuclear layer adjoining the plexiform layers and are transformed into very small cells which persist in the adult retina without further change.In mice with hereditary retinal degeneration (rd rd) these -glucosaminidase positive macrophages enter the outer nuclear layer of the retina, soon after the onset of degeneration undergo extensive hypertrophy and rapidly phagocytize the degenerating photoreceptor cells. After the digestion of the ingested materials the enzyme activity is very much reduced and the cells become smaller in size. They eventually acquire the morphological features seen in the normal retina.     

Neuroprotective effect of the antiparkinsonian drug pramipexole against nigrostriatal dopaminergic degeneration in rotenone-treated mice

Pramipexole, an agonist for dopamine (DA) D2/D3-receptors, has been used to treat both early and advanced Parkinson's disease (PD). In this study, we examined the effect of pramipexole on DA neurons in a PD model of C57BL/6 mice, which were treated with rotenone (30 mg/kg, p.o.) daily for 28 days. Pramipexole (1 mg/kg, i.p.) was injected daily 30 min before each oral administration of rotenone. Chronic oral administration of rotenone caused a loss of DA neurons in the substantia nigra pars compacta (SNpc), motor deficits and the up-regulation of α-synuclein immunoreactivity in some surviving DA neurons. Pramipexole inhibited rotenone-induced DA neuronal death and motor deficits, and reduced immunoreactivity for α-synuclein. In addition, pramipexole inhibited the in vitro oligomerization of human wild-type α-synuclein by H₂O₂ plus cytochrome c. To examine the neuroprotective effect of pramipexole against oxidative stress, we used a DJ-1-knockdown SH-SY5Y cell line and electron spin resonance (ESR) spectrometry. Simultaneous treatment with H₂O₂ and pramipexole resulted in the significant protection of DJ-1-knockdown cells against cell death in a concentration-dependent manner. A high concentration of pramipexole directly scavenged hydroxyl radical (OH) generated from H₂O₂ and Fe²⁺. Furthermore, pramipexole increased Bcl-2 immunoreactivity in DA neurons in the SNpc. These results suggest that pramipexole may protect DA neurons against exposure to rotenone by chronic oral administration, and this effect is mediated by multiple functions including scavenging of OH and induction of Bcl-2 protein.     

Immunocytochemical markers revealing retinal and pineal but not hypothalamic photoreceptor systems in the Japanese quail

Summary The retinal proteins opsin,-transducin, S-antigen and interstitial retinol-binding protein (IRBP) are essential for the processes of vision. By use of immunocyto-chemistry we have employed antibodies directed against these photoreceptor proteins in an attempt to identify the photoreceptor systems (retina, pineal and deep brain) of the Japanese quail. Opsin immunostaining was identified within many outer (basal portion) and inner segments of retinal photoreceptor cells and limited numbers of photoreceptor perikarya. Opsin immunostaining was also demonstrated in limited numbers of pinealocytes with all parts of these cells being immunoreactive. These results differ from previous observations. In contrast to the results obtained with the antibody against opsin, S-antigen and-transducin immunostaining was seen throughout the entire outer segments and many photoreceptor perikarya of the retina. In the pineal organ immunostaining was seen in numerous pinealocytes in all follicles. These results conform to previous findings in birds. In addition, IRBP has been demonstrated for the first time in the avian retina and pineal organ. These findings underline the structural and functional similarities between the retina and pineal organ and provide additional support for a photoreceptive role of the avian pineal. No specific staining was detected in any other region of the brain in the Japanese quail; the hypothalamic photoreceptors of birds remain unidentified.     

Different Subcellular Localization of Theiler's Murine Encephalomyelitis Virus Leader Proteins of GDVII and DA Strains in BHK-21 Cells

The highly virulent GDVII strain of Theiler''s murine encephalomyelitis virus causes acute and fatal encephalomyelitis, whereas the DA strain causes mild encephalomyelitis followed by a chronic inflammatory demyelinating disease with virus persistence. The differences in the amino acid sequences of the leader protein (L) of the DA and GDVII strains are greater than those for any other viral protein. We examined the subcellular distribution of DA L and GDVII L tagged with the FLAG epitope in BHK-21 cells. Wild-type GDVII L was localized predominantly in the cytoplasm, whereas wild-type DA L showed a nucleocytoplasmic distribution. A series of the L mutant experiments demonstrated that the zinc finger domain, acidic domain, and C-terminal region of L were necessary for the nuclear accumulation of DA L. A GDVII L mutant with a deletion of the serine/threonine (S/T)-rich domain showed a nucleocytoplasmic distribution, in contrast to the predominant cytoplasmic distribution of wild-type GDVII L. A chimeric DA/GDVII L, D/G, which encodes the N region of DA L including the zinc finger domain and acidic domain, followed by the GDVII L sequence including the S/T-rich domain, was distributed exclusively throughout the cytoplasm but not in the nucleus, as observed with wild-type GDVII L. Another chimeric L, G/D (which is the converse of the D/G construct), accumulated in the nucleus as well as the cytoplasm, as was observed for wild-type DA L. The findings suggest that the differential distribution of DA L and GDVII L is determined primarily by the S/T-rich domain. The S/T-rich domain may be important for the viral activity through the regulation of the subcellular distribution of L.Theiler''s murine encephalomyelitis virus (TMEV) belongs to the genus Cardiovirus of the family Picornaviridae, and its strains are divided into two subgroups on the basis of their different biological activities. The neurovirulent strains, such as GDVII and FA, produce acute and fatal encephalomyelitis in mice. The persistent strains, such as TO, DA, BeAn, etc., induce mild and nonfatal encephalomyelitis, followed by a chronic demyelinating disease with virus persistence in the spinal cords of mice. This late demyelinating disease is thought to be an excellent experimental model for the human demyelinating disease multiple sclerosis (MS) (5, 17, 20).The TMEV genome is a single-stranded RNA molecule and translated as a long precursor polyprotein to yield 12 viral proteins by autoproteolytic cleavage (23). Two subgroup strains of TMEV have a sequence identity of approximately 95% at the amino acid level. The amino acid sequences of the proteins encoded by the P1, P2, and P3 regions of both strains are highly conserved and show 94, 96, and 98% identity, respectively. The genome has another coding region, designated the leader (L), at the most amino-terminal location of the precursor polyprotein. The L coding region encodes 76 amino acids (aa) and shows a low sequence identity of only 85% to the above-described three regions (16, 19, 22). Therefore, L has the greatest difference in amino acid sequence among any of the viral proteins and may play an important role in subgroup-specific biological activities of TMEV. In this study, we have investigated the subcellular localization of the L proteins of GDVII and DA strains and characterized the functional domains involved in the differential distribution between DA L and GDVII L in BHK-21 cells by a series of deletion mutant and chimeric construct experiments.     

Depolarization elicits,while hyperpolarization blocks uptake of endogenous glutamate by retinal horizontal cells of the turtle

We have employed an immunoreaction against glutamate to qualitatively demonstrate varying levels of glutamate in retinal horizontal cells of the turtle. Glutamate-like immunoreactivity (GLI) in horizontal cells could be demonstrated after glutamate decarboxylase was inhibited by aminooxy acetic acid (AOAA) and its degradation to GABA was blocked. Depolarization of horizontal cells by kainic acid (KA) induces strong glutamate immunoreactivity in these cells, whereas hyperpolarization by 2,3-cis piperidine dicarboxylate (PDA) abolishes glutamate-like immunoreactivity in horizontal cells. When glutamate release from cones and bipolar cells is blocked in the absence of calcium, or when glutamate uptake is blocked by DL-threo -hydroxy aspartate, KA/AOAA treatment of the retina does not induce GLI in horizontal cells. Our data show that horizontal cells are capable of taking up glutamate from the endogenous retinal pool in an activity dependent way. Our interpretation of these findings is that retinal horizontal cells are capable of regulating glutamate levels in the extracellular space of the cone pedicle complex by an activity-dependent uptake system. We suggest that inhibition of glutamate uptake upon hyperpolarization rather than inhibition of GABA release may evoke the antagonistic surround response of retinal bipolar cells.     

Dopaminergic neurons in the retina of Xenopus laevis: amacrine vs. interplexiform subtypes and relation to bipolar cells

Presumed dopaminergic neurons were visualized in the retina of the clawed frog, Xenopus laevis, by anti-tyrosine hydroxylase (TH) immunoreactivity. The studied cells constitute a uniform population with perikarya at the junction of inner nuclear (INL) and inner plexiform (IPL) layers. Each cell body gives rise to 4–6 relatively stout processes (0.5–2.0 m in diameter) which run for up to 1.2 mm in strata 4–5 of the IPL. These processes have a very asymmetric distribution in the horizontal plane of the retina. A dense plexus of TH fine fibers is distributed uniformly in stratum 1 of the IPL. TH cells are distributed evenly but sparsely (16–20 cells/mm²) across the retina. About 20% of the TH neurons emit 1–3 distally directed fine processes, the majority of which extend < 20 m, which barely suffices to reach the outer plexiform layer (OPL). Other longer processes are typically unbranched; some reach the OPL, others run tangentially in the INL. The axon terminals of Golgi-impregnated bipolar cells are characterized according to the strata of the IPL in which they arborize. About 80% are confined either to strata 1–2 or 3–5, conforming to the off and on zones defined by Famiglietti and Kolb (1976). The remainder appear to end in both zones, some extending across the entire width of the IPL. EM examination showed that TH processes receive bipolar synaptic input in both distal and proximal portions of the IPL.     

Endogenous homovanillic acid levels differ between rat and rabbit caudate,hippocampus, and cortical regions

Endogenous dopamine (DA) levels and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), 3-methoxytyramine (3MT) and homovanillic acid (HVA) were measured by high-performance liquid chromatography in the entorhinal-piriform (EnPi), cingulate (CIN), sensorimotor (SSM) and visual (VIS) cortices as well as is the caudate (CAU) and hippocampus (HIP) of Sprague-Dawley (SD) rats and New Zealand (NZ) rabbits. The DA, DOPAC and 3MT contents were similar in both species. The HVA levels however, although they followed DA distribution, were several-fold higher in NZ rabbits than in SD rats for all cortices, HIP and CAU. In addition, total metabolite contents and DA turnove (estimated from DA metabolite/DA ratios) were significantly higher in NZ rabbits than in SD rats, suggesting an increased release and/or metabolism in the former species. The HVA/DA ratios were much higher for NZ rabbit regions than for SD rats, indicating an increased DA release in the former species since the DOPAC/DA ratios (index of intraneuronal degradation) were similar.Herbert H. Jasper Postdoctoral Fellow, Centre de recherche en sciences neurologiques.     

Temporal tuning and nonlinearity of intraretinal pathways in turtle: Effects of temperature,stimulus intensity,and size

Flash responses, amplitude and phase transfer functions, and nonlinearities were measured in turtle retina for pathways with photoreceptor inputs and outputs from horizontal (HC), hyperpolarizing bipolar (HBC), sustained amacrine (AC), and on-off ganglion (GC) cells. Flash responses slowed and attenuated in all cells as temperature decreased. Whitenoise transfer properties of sustained-type cells (HC, HBC, AC) were of low- or bandpass type; highfrequency cut-off (f _c) and phase crossover frequency decreased with temperature. f _c increased as spot diameter was increased. Nonlinearity of these sustained-response pathways (distortion product frequencies in response to a sum-of-sinusoids input probe) increases with intensity and may depend on amplitude saturation limiting. On/off GC synaptic and spike activity increased as spot diameter decreased and intensity increased. Amplitude transfer functions had a low-frequency peak (PSP activity) and monotonically decreasing amplitude vs. frequency shape (spikes and transient PSP activity). Nonlinearity increased with stimulus intensity; it was maximal with 1 mm spot size, less with smaller (500 m) and larger (5 mm) spots. It may depend on the functional equivalent of full-wave rectification (on-off response).This work was supported by NEI grant R01 EY03383     

Peculiarities of L-DOPA treatment of Parkinson’s disease

Summary. L-Dihydroxyphenylalanine (L-DOPA), the anti-parkinsonian drug affording the greatest symptomatic relief of parkinsonian symptoms, is still misunderstood in terms of its neurotoxic potential and the mechanism by which generated dopamine (DA) is able to exert an effect despite the absence of DA innervation of target sites in basal ganglia. This review summaries important aspects and new developments on these themes. On the basis of L-DOPA therapy in animal models of Parkinsons disease, it appears that L-DOPA is actually neuroprotective, not neurotoxic, as indicated by L-DOPAs reducing striatal tissue content of the reactive oxygen species, hydroxyl radical (HO), and by leaving unaltered the extraneuronal in vivo microdialysate level of HO. In addition, the potential beneficial anti-parkinsonian effect of L-DOPA is actually increased because of the fact that the basal ganglia are largely DA-denervated. That is, from in vivo microdialysis studies it can be clearly demonstrated that extraneuronal in vivo microdialysate DA levels are actually higher in the DA-denervated vs. the intact striatum of rats – owing to the absence of DA transporter (i.e., uptake sites) on the absent DA nerve terminal fibers in parkinsonian brain. In essence, there are fewer pumps removing DA from the extraneuronal pool. Finally, the undesired motor dyskinesias that commonly accompany long-term L-DOPA therapy, can be viewed as an outcome of L-DOPAs sensitizing DA receptors (D₁–D₅), an effect easily replicated by repeated DA agonist treatments (especially agonist of the D₂ class) in animals, even if the brain is not DA-denervated. The newest findings demonstrate that L-DOPA induces BDNF release from corticostriatal fibers, which in-turn enhances the expression of D₃ receptors; and that this effect is associated with motor dyskinesias (and it is blocked by D₃ antagonists). The recent evidence on mechanisms and effects of L-DOPA increases our understanding of this benefical anti-parkinsonian drug, and can lead to improvements in L-DOPA effects while providing avenues for reducing or eliminating L-DOPAs deleterious effects.     

Über den Bau und die Hell-Dunkel-Adaptation der Augen des Polychäten Platynereis dumerilii

Summary The eye of Platynereis dumerilii consists of three components: a short optic nerve, a cup-shaped retina, and a vitreous body within the cup. The opening of the retinal cup is called pupil. The retina is composed of supporting cells and visual cells. The supporting cells are stuffed with dark blue violet pigment granules. The visual cells have orange pigment granules which are only found in the narrow middle piece of the cells. The supporting cell pigment may be lacking in abnormally pigmented eyes. The jellylike matter of the vitreous body apparently is produced by the supporting cells. It is of high protein contents and does not seem to be derived from the cuticle which consists of polysaccharides.The ultrastructure of the photoreceptor region shows club-shaped processes of visual cells. Each club is of low electron density and contains elongate membranous structures. It is surrounded by many microvilli. The clubs correspond to the rods in light microscopy.The eye of Platynereis dumerilii adapts to changes in light intensity by movements of the retina and the rods. The cup-shaped retina spreads towards its pupillar opening thus adapting the pupil area to light intensity. The length of the rods in darkfixed immature specimens is about 20, in light-fixed ones about 7 . In mature specimens (Heteronereis) the length is 46 or 19 respectively.During metamorphosis the eyes enlarge to about three times their original volume. This enlargement is due to an increase in volume of the retina and the vitreous body, not to cell divisions.

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Durchgeführt mit Unterstützung durch ein Stipendium aus Mitteln der Fritz-Thyssen-Stiftung, ferner mit Hilfe der Deutschen Forschungsgemeinschaft.

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Die Anregung zu dieser Arbeit und das Platynereis-Zuchtmaterial verdanke ich Herrn Prof. Dr. C. Hauenschild. Für Hilfe bei der Herstellung des Bildmaterials danke ich Frl. U. Poltz (Freiburg).     

Ganglion cells of chicken retina possess nicotinic rather than muscarinic acetylcholine receptors

Chicken retinas were exposed to intravitreal kainic acid to destroy amacrine and bipolar cells at low concentrations, and horizontal cells at high concentrations in addition. Ganglion cells were destroyed by intravitreal injections of colchicine. Low doses of kainic acid reduced the number of binding sites for both [³H]quinuclidinyl benzilate (muscarinic acetylcholine receptors) and N-[propionyl ³H]-bungarotoxin (nicotinic acetylcholine receptors), with little additional loss at higher doses. In contrast, colchicine reduced the number of binding sites for N-[propionyl-³H]-bungarotoxin, but had little or no effect on the number of binding sites for [³H]quinuclidinyl benzilate. These results are consistent with the idea that, in chicken retina, cholinergic amacrine cells make contact with ganglion cell dendrites at sites which possess mainly nicotinic acetylcholine receptors, while both types of receptor are involved in interactions between amacrine cells and perhaps bipolar cells.     

Cytological and molecular observations on Solanum phureja-induced dihaploid potatoes

Summary Seventeen potato dihaploids, produced by pollinating the tetraploid (2n = 48) cv Pentland Crown with pollen from Solanum phureja (2n = 24) dihaploid inducer clones, were studied. Since dihaploids are thought to develop parthenogenetically from unfertilized ovules they were expected to be euploid (2n = 24), but somatic chromosome counts showed that 15 of the 17 dihaploids were aneusomatic. Ten of the clones were predominantly diploid (2n = 24) with a proportion of hyperploid cells that contained 25 or 26 chromosomes. Five of the dihaploids contained variable numbers of triploid cells (2n = 36). RFLP analysis was used to determine whether the additional chromosomes were from S. phureja or S. tuberosum. Unique hybridizing fragments present in S. phureja but not in Pentland Crown were identified. These S. phureja-specific restriction fragments were present in some of the dihaploid offspring of Pentland Crown. Of the 5 clones that contained triploid cells 4 had S. phureja type banding. Four of the 10 aneusomatic clones that contained hyperploid cells had the unique S. phureja hybridizing fragments. We propose that ovules of Pentland Crown were fertilized by pollen from S. phureja and that the aneusomatic clones were derived from triploid zygotes from which some of the S. phureja chromosomes were eliminated. We consider that this is an additional mechanism of dihaploid formation in potato.     

Identification of a Circadian Clock-Controlled Neural Pathway in the Rabbit Retina

Background

Although the circadian clock in the mammalian retina regulates many physiological processes in the retina, it is not known whether and how the clock controls the neuronal pathways involved in visual processing.

Methodology/Principal Findings

By recording the light responses of rabbit axonless (A-type) horizontal cells under dark-adapted conditions in both the day and night, we found that rod input to these cells was substantially increased at night under control conditions and following selective blockade of dopamine D₂, but not D₁, receptors during the day, so that the horizontal cells responded to very dim light at night but not in the day. Using neurobiotin tracer labeling, we also found that the extent of tracer coupling between rabbit rods and cones was more extensive during the night, compared to the day, and more extensive in the day following D₂ receptor blockade. Because A-type horizontal cells make synaptic contact exclusively with cones, these observations indicate that the circadian clock in the mammalian retina substantially increases rod input to A-type horizontal cells at night by enhancing rod-cone coupling. Moreover, the clock-induced increase in D₂ receptor activation during the day decreases rod-cone coupling so that rod input to A-type horizontal cells is minimal.

Conclusions/Significance

Considered together, these results identify the rod-cone gap junction as a key site in mammals through which the retinal clock, using dopamine activation of D₂ receptors, controls signal flow in the day and night from rods into the cone system.