Dopaminergic Regulation of Cone Retinomotor Movement in Isolated Teleost Retinas: II. Modulation by γ-Aminobutyric Acid and Serotonin

In the accompanying paper we reported that 3,4-dihydroxyphenylethylamine (dopamine) induced light-adaptive retinomotor movements in teleost photoreceptors and that this effect was mediated by D2 dopamine receptors located on the photoreceptors themselves. In this study, we investigated the effects on cone retinomotor movement of three agents that have been reported by others to modulate retinal dopamine release: gamma-aminobutyric acid (GABA), 5-hydroxytryptamine (5-HT, serotonin), and melatonin. We report here that the GABA antagonists bicuculline and picrotoxin induced light-adaptive cone contraction in dark-adapted green sunfish retinas cultured in constant darkness; thus they mimic the effect of light or exogenously applied dopamine. Since their effects were blocked by either the D2 dopamine antagonist sulpiride or by Co2+, it seems likely that these agents act by enhancing retinal dopamine release. The GABA agonist muscimol produced effects opposite to those of GABA antagonists. Muscimol inhibited light-induced cone contraction in previously dark-adapted retinas and induced dark-adaptive cone elongation in light-adapted retinas. These results suggest that in green sunfish retinas, as has been reported for other retinas, GABA inhibits dopamine release. 5-HT induced light-adaptive cone contraction in dark-adapted retinas; thus 5-HT also mimics the effect of light or exogenously applied dopamine. The effect of 5-HT was blocked by sulpiride, Co2+, or the 5-HT antagonist mianserin. These results suggest that 5-HT induces cone contraction by stimulating dopamine release. Melatonin neither inhibited dopamine-induced cone contraction in retinas cultured in the dark nor induced cone elongation in retinas cultured in the light. Our results suggest that both GABA and 5-HT (but not melatonin) affect cone retinomotor movements in green sunfish by modulating dopamine release: GABA by inhibiting and 5-HT by stimulating dopamine release. We report in the companion paper that dopamine induced contraction in isolated cone fragments. Together these observations strongly suggest that dopamine serves as the final extracellular messenger directly inducing light-adaptive cone retinomotor movement, and that GABA and 5-HT affect these movements by modulating dopamine release.     

Effects of low-chloride solutions on action potentials of sheep cadiac purkinje fibers

The rapid repolarization during phase 1 of the action potential of sheep cardiac purkinje fibers has been attributed to a time- and voltage-dependent chloride current. In part, this conclusion was based on experiments that showed a substantial slowing of phase 1 when larger, presumably impermeant, anions were substituted for chloride in tyrode’s solution. We have re- examined the electrical effects of low-chloride solutions. We recorded action potentials of sheep cardiac purkinje fibers in normal tyrode’s solution and in low-chloride solutions made by substituting sodium propionate, acetylglycinate, methylsulfate, or methanesulfonate for the NaCl of Tyrode’s solution. Total calcium was adjusted to keep calcium ion activity of test solutions equal to that of control solutions. Propionate gave qualitatively variable results in preliminary experiments; it was not tested further. Low-chloride solutions made with the other anions gave much more consistent results: phase 1 and the notch that often occurs between phases 1 and 2 were usually unaffected, and the action potential duration usually increased. The only apparent change in the resting potential was a transient 3-6 mV depolarization when low-chloride solution was first admitted to the chamber, and a symmetrical transient hyperpolarization when chloride was returned to normal. If a time- and voltage-dependent chloride current exists in sheep cardiac purkinje fibers, our results suggest that it plays little role in generating phase 1 of the action potential.     

Thin (actin) and thick (myosinlike) filaments in cone contraction in the teleost retina

The long slender retinal cones of fishes shorten in the light and elongate in the dark. Light-induced cone shortening provides a useful model for stuying nonmuscle contraction because it is linear, slow, and repetitive. Cone cells contain both thin (actin) and thick (myosinlike) filaments oriented parallel to the axis of contraction. This study examines the polarities of the cone's thin filaments and the changes in filament distribution which accompany light-induced contraction, in an attempt to elucidate the structural basis for the cone's contractile process. The proximal half of the cone is fixed to its cellular neighbors in the outer nuclear layer while the distal half is free. Thus, all shortening takes place in a necklike region (the myoid) in the distal half of the cone which extends into the space between the neural retina and the pigmented retinal epithelium. Thin filaments are found throughout the length of the cone, whereas thick filaments occur predominantly in the proximal (axon) regions of both light- and dark-adapted cones. Thus, thick filaments are primarily localized outside the region where shortening takes place. Observations from myosin subfragment-1 binding studies suggest that the cone's thin filaments are organized into two opposing sets. In the distal half of the cone (including the myoid), virtually all filaments have proximally directed arrowheads. In the more proximal regions of the axon, many thin filaments have opposite polarity, their arrowheads being distally directed. Near the synaptic proximal end of the light-adapted (contracted) cone, filaments of opposite polarities occur in approximately equal numbers. Thus, in the cone axon there appear to be two overlapping sets of actin filaments whose opposite polarities correspond to the two actin halves of a muscle sarcomere. In elongated, dark-adapted cones, thick filaments are localized throughout the axon region of the cone. In light, thick filaments accumulate towards the proximal end of the cone. These observations are consistent with a "sliding hypothesis" for cone contraction, in which thick myosinlike filaments produce sliding interdigitation of the two sets of oppositely directed actin filaments in the proximal axon region. Thus, the myoid thin filaments would be essentially reeled into the axon region to produce shortening. The mechanism of re-elongation depends on microtubules, as discussed in the companion paper.     

Properties of the membrane proteins of rat liver lysosomes. The majority of lysosomal membrane proteins are exposed to the cytoplasm.

Rat liver lysosomes were lysed and subfractionated by differential centrifugation through 0.2M-NaCl to yield a membranous pellet. This membrane fraction contains less than 20% of the lysosomal protein, adenosine triphosphatase activity of about 1.2mumol/min per mg of protein, 120nmol of thiol groups/mg of protein and at least 16 protein and glycoprotein bands on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. The gel patterns of membranes isolated from lysosomes after treatment with (1) [125I]iodidehydrogen peroxide-lactoperoxidase, (2) toluene 2,4-di-isocyanate-activated bovine serum albumin, (3) trypsin and (4) subtilisin indicate that most of the membrane proteins are exposed to the cytoplasm. These exposed proteins are candidates for intracellular receptors which recognize either substances that are to be degraded or vesicles containing those substances.     

A nuclear factor that enhances binding of thyroid hormone receptors to thyroid hormone response elements

Recent studies from this laboratory have demonstrated the presence of thyroid hormone response elements (TREs) in the 5'-flanking region of the rat alpha and TSH beta subunit genes. Using an avidin-biotin complex DNA binding assay, we have shown that these TREs bind the thyroid hormone (T3) receptor present in nuclear extracts of GH3 cells, as well as the in vitro synthesized Hc-erbA beta, which has been identified as a member of the family of T3 receptors. The binding of Hc-erbA beta to the alpha subunit TRE can be enhanced 3-4-fold by including GH3 nuclear extract in the binding assay. Binding to the TRE present in the TSH beta gene or the rat growth hormone gene was similarly enhanced, although to a lesser degree. The enhanced binding activity is trypsin-sensitive and heat labile, and is not reproduced by the addition of histones, bovine serum albumin, or cytosol instead of nuclear extract. Gel exclusion chromatography suggests a molecular size of approximately 65,000 Da. This protein, which is present in several different cell types, is also able to complement binding of the rat erbA alpha-1 and the pituitary-specific erbA beta-2 forms of the receptor. These data suggest that the binding of the T3 receptor to a TRE is augmented by another nuclear protein, which may be involved in the mechanism of action of thyroid hormone.     

Plasminogen activator and collagenase production by cultured capillary endothelial cells

Cultured bovine capillary endothelial (BCE) cells produce low levels of collagenolytic activity and significant amounts of the serine protease plasminogen activator (PA). When grown in the presence of nanomolar quantities of the tumor promoter 12-O-tetradecanoyl phorbol-13-acetate (TPA), BCE cells produced 5-15 times more collagenolytic activity and 2-10 times more PA than untreated cells. The enhanced production of these enzymes was dependent on the dose of TPA used, with maximal response at 10(-7) to 10(-8) M. Phorbol didecanoate (PDD), an analog of TPA which is an active tumor promoter, also increased protease production. 4-O-methyl-TPA and 4α-PDD, two analogs of TPA which are inactive as tumor promoters, had no effect on protease production. Increased PA and collagenase activities were detected within 7.5 and 19 h, respectively, after the addition of TPA. The TPA-stimulated BCE cells synthesized a urokinase-type PA and a typical vertebrate collagenase. BCE cells were compared with bovine aortic endothelial (BAE) cells and bovine embryonic skin (BES) fibroblasts with respect to their production of protease in response to TPA. Under normal growth conditions, low levels of collagenolyic activity were detected in the culture fluids from BCE, BAE, and BES cells. BCE cells produced 5-13 times the basal levels of collagenolytic activity in response to TPA, whereas BAE cells and BES fibroblasts showed a minimal response to TPA. Both BCE and BAE cells exhibited relatively high basal levels of PA, the production of which was stimulated approximately threefold by the addition of TPA. The observation that BCE cells and not BAE cells produced high levels of both PA and collagenase activities in response to TPA demonstrates a significant difference between these two types of endothelial cells and suggests that the enhanced detectable activities are a property unique to bovine capillary and microvessel and endothelial cells.