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R S Stephenson

Publications and source records attributed to R S Stephenson.

12 recordsLinked to original sources
PubMed1999

Identification and distribution of dietary precursors of the Drosophila visual pigment chromophore: analysis of carotenoids in wild type and ninaD mutants by HPLC.

A dietary source of retinoid or carotenoid has been shown to be necessary for the biosynthesis of functional visual pigment in flies. In the present study, the larvae or adults of Drosophila melanogaster were administered specific carotenoid-containing diets and high performance liquid chromatography was used to identify and quantify the carotenoids in extracts of wild type and ninaD visual mutant flies. When beta-carotene was fed to larvae, wild type flies were shown to hydroxylate this molecule and to accumulate zeaxanthin and a small amount of beta-cryptoxanthin. Zeaxanthin content was found to increase throughout development and was a major carotenoid peak detected in the adult fly. Carotenoids were twice as effective at mediating zeaxanthin accumulation when provided to larvae versus adults. In the ninaD mutant, zeaxanthin content was shown to be specifically and significantly altered compared to wild type, and was ineffective at mediating visual pigment synthesis when provided to both larval and adult mutant flies. It is proposed that zeaxanthin is the larval storage form for subsequent visual pigment chromophore biosynthesis during pupation, that zeaxanthin or beta-crytoxanthin is the immediate precursor for light-independent chromophore synthesis in the adult, and that the ninaD mutant is defective in this pathway.

Animals↗
PubMed1998

Effect of alpha2-adrenoceptor stimulation on isolated canine Purkinje fiber contraction.

We have recently identified the presence of postjunctional alpha2-adrenoceptors in canine Purkinje fibers. In this study, we examined the effects of alpha2-adrenoceptor stimulation on the contraction strength of isolated Purkinje fibers. Exposure to the alpha2-adrenoceptor specific agonist and antagonist, UK 14,304 (5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) and yohimbine (17-hydroxyyohimban-16-carboxylic acid methyl ester hydrochloride) alone at 0.1 microM respectively, did not produce any significant effect on Purkinje contraction strength. Purkinje contraction strength was augmented by isoproterenol (0.1 microM), forskolin (0.1 microM), or 8-bromo-adenosine cyclic 2',3'-monophosphate (8-bromo-cAMP, 10 microM). UK 14,304 significantly reversed the effects of isoproterenol and forskolin but not those of 8-bromo-cAMP on Purkinje contraction strength. After incubation with pertussis toxin, the positive inotropic effect of forskolin on Purkinje contraction strength remained intact, but the forskolin effect could no longer be reversed by UK 14,304. These results suggest that the postjunctional alpha2-adrenoceptors in canine Purkinje fibers are coupled to a pertussis toxin-sensitive G protein, probably Gi. Stimulation of the alpha2-adrenoceptor antagonizes the effect of beta-adrenoceptor stimulation on Purkinje contraction strength in an accentuated antagonism manner.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1995

Dissociation of photoreceptors from whole heads of the fruit fly, Drosophila melanogaster.

Photoreceptor cells that were mostly free of extracellular material and suitable for most electrophysiological study procedures were dissociated from whole heads of the fruit fly, Drosophila melanogaster, by a simple "smash" technique employing gentle chopping by a razor blade through Parafilm sheets. A variety of commonly available proteolytic and glycolytic digestion enzymes were tested as additions to the basic dissociation procedure described. With the aid of Nomarski interference contrast optics, periodic acid-Schiff staining, and fluorescent labeling and microscopy methods, it was determined that proteolytic enzymatic digestion does little to enhance the dissociation procedure, and instead, often damages the cells that one is attempting to recover. Unexpectedly, certain glycolytic enzymes, when added to the basic procedure, appear to enhance the recovery of intact viable Drosophila photoreceptors that are stripped of most extracellular material. Based on these results, a hypothesis concerning the biochemical nature of the extracellular matrix of the Drosophila retina is proposed. Drosophila photoreceptors are an interesting model system for the study of invertebrate phototransduction and photoreceptor cell biology because of their many well-characterized mutant strains. The technique described here should produce clean viable photoreceptors or ommatidia that respond to light, and that are suitable for patch clamping or cell culture.

Animals↗
PubMed1988

On the interpretation of voltage noise in small cells.

Noise analysis is a powerful technique for studying membrane conductance mechanisms, but it can be applied straightforwardly only to current noise recorded under voltage clamp. Many small cells, however, such as some sensory receptors and interneurons, are not suited for voltage clamping. In such cases it is of great interest to be able to interpret recordings of voltage noise. Voltage noise must be corrected for the effects of: (1) changes in membrane potential, (2) the cells' input impedance, and (3) noise contributed by conductances other than the one of interest. This paper describes in detail the factors--including in particular voltage-dependent K+-conductances--that affect voltage-noise recordings, and describes how these factors may be measured and corrected for in practice. A novel method allows membrane impedance to be measured at the same time as voltage noise. An example is given, showing how this approach may be applied to voltage noise from an insect photoreceptor.

Animals↗
PubMed1986

An improved microelectrode resistance meter.

A device is described for measuring the resistance of micropipette electrodes. The useful range of electrode resistances that it can measure is 100 k omega to 1 G omega. It is more convenient to use than previously described or commercially available meters, especially for very high-resistance electrodes. Resistance of even the finest microelectrodes can be measured accurately while their tips are inserted by hand into a test solution. This eliminates the need for special holders, speeding and simplifying the screening of large numbers of electrodes. Test solutions are stored in interchangeable reservoirs, making it easy to characterize the resistance of an electrode in solutions of different resistivity. Test solutions can also be capped and removed when not in use to prevent evaporation. To protect very high-resistance electrodes from damage during measurement, the measuring current is low (only 300 pA on the highest resistance range) and the test voltage across the electrode is limited to +/- 1 V.

Electric Conductivity↗
PubMed1981

Mutation that selectively affects rhodopsin concentration in the peripheral photoreceptors of Drosophila melanogaster.

A Drosophila mutant (ninaAP228) that is low in rhodopsin concentration but identical to the wild-type fly in photoreceptor morphology has been isolated. R1-6 photoreceptors of the mutant differ from those of wild type in that (a) the prolonged depolarizing afterpotential (PDA) is absent, (b) concentrations of rhodopsin and opsin are substantially reduced, and (c) intramembrane particle density in the membranes of the rhabdomeres is low. Each of these traits is mimicked by depriving wild-type flies of vitamin A. The ninaAP228 mutation differs from vitamin A deprivation in that in the mutant (a) the rhabdomeric membrane particle density is reduced only in the R1-6 photoreceptors and not in R7 or R8, (b) the PDA can be elicited from the R7 photoreceptors, and (c) photoconversion of R1-6 rhodopsin to metarhodopsin by ultraviolet (UV) light is considerably more efficient than in vitamin A-deprived flies. The absorption properties of the mutant rhodopsin in the R1-6 photoreceptors appear to be identical to those of wild type as judged from rhodopsin difference spectra. The results suggest that the mutation affects the opsin, rather than the chromophore, component of rhodopsin molecules in the R1-6 photoreceptors. The interaction between the chromophore and R1-6 opsin, however, appears to be normal.

Animals↗
PubMed1980

Heterogenic components of a fast electrical potential in Drosophila compound eye and their relation to visual pigment photoconversion.

The electroretinogram of the dipteran compound eye in response to an intense flash contains an early, diphasic potential that has been termed the M potential. Both phases of the M potential arise from the photostimulation of metarhodopsin. The early, corneal-negative component, the M1, can be recorded intracellularly in the photoreceptors and has properties similar to the classical early receptor potential (ERP). The M1 is resistant to cold, anaesthesia, and anoxia and has no detectable latency. It depends on flash intensity and metarhodopsin fraction in the manner predicted for a closed, two-state pigment system, and its saturation is shown to correspond to the establishment of a photoequilibrium in the visual pigment. On the other hand, the dominant, corneal-positive component, the M2, does not behave like an ERP. It arises, not in the photoreceptors, but deeper in the retina at the level of the lamina, and resembles the on-transient of the electroretinogram in its reversal depth and sensitivity to cooling or CO2. The on-transient, which is present over a much wider range of stimulus intensity than the M potential, has been shown to arise from neurons in the lamina ganglionaris. Visual mutants in which the on-transient is absent or late are also defective in the M2. It is proposed that the M2 and the on-transient arise from the same or similar groups of second-order neurons, and that the M2 is a fast laminar response to the depolarizing M1 in the photoreceptors, just as the on-transient is a fast laminar response to the depolarizing late receptor potential. Unlike the M1, the M2 is not generally proportional to the amount of metarhodopsin photoconverted, and the M2 amplitude is influenced by factors, such as a steady depolarization of the photoreceptor, which do not affect the M1.

Action Potentials↗
PubMed1978

A bipolar electrode for electromyography of limb muscles of small animals.

A bipolar electrode for recording from or stimulating the skinned surface of small, moving muscles is described. The electrode is so light that it clings to the moist surface of the muscle and moves with it, like a piece of confetti. It can be positioned or repositioned instantly and with no damage to the muscle. Because of its small size and bipolar nature, it records from or stimulates a very small volume. It is inexpensive, simple, and requires no special apparatus for its consturction. The electrode is especially suited for muscles in the limbs of small animals, in which movement and small size present special problems.

Action Potentials↗