Scotopic multifocal visual evoked potentials

OBJECTIVE

To investigate the scope of scotopic multifocal visual evoked potentials (mfVEP_S) for the assessment of scotopic visual fields.

METHODS

Pattern-reversal mfVEP for photopic (mfVEP_P) and scotopic conditions (mfVEP_S; 0.003 cd/m²) were recorded from 36 visual field locations of a circular checkerboard pattern (25° radius) in 9 participants with normal vision. MfVEP_P were recorded with a conventional central fixation cross, mfVEP_S were recorded (i) with (mfVEP_S+) and (ii) without (mfVEP_S-) an additional fixation aid. Latency shifts were determined using cross-correlations, mfVEP magnitudes were analysed in an eccentricity dependent manner using signal-to-noise ratios (SNRs).

RESULTS

In comparison to mfVEP_P, mfVEP_S- and mfVEP_S+ were delayed by 101 ms and 97 ms, respectively, and had smaller signal-to-noise-ratios. Both mfVEP_S were reduced down to noise level in the center and also severely reduced for the most peripheral stimulus eccentricity used. The visual-field-coverage for the paracentral eccentricities of mfVEP_S+ and mfVEP_S- was 76% and 65% [4°-9°], respectively, and 79% and 66% [9°-16°].

CONCLUSIONS

MfVEP_S were delayed compared to mfVEP_P and demonstrated the expected central response drop-out typical for scotopic vision.

SIGNIFICANCE

MfVEP_S may hold promise of an objective, spatially resolved visual field test which motivates testing it in patients with diseases affecting scotopic vision.

Differential effects of optic media opacities on mfERGs and mfVEPs

OBJECTIVE

To assess different effects of image degradation that could result from optic media opacities on multifocal retinal (mfERG) and cortical responses (mfVEP).

METHODS

Monocular flash-mfERGs and pattern-reversal mfVEPs were recorded. MfERG-P1 amplitudes and implicit times and mfVEP root-mean-square values (RMS) and delays were compared for different filter conditions (none, 8% luminance, 50% luminance, 50% luminance plus blur) in a total of ten participants with normal vision.

RESULTS

Reducing stimulus luminance down to 50% and 8% reduced mfERG amplitudes to 86% and 42%, respectively, with no significant effect on mfVEP amplitude. Implicit times were increased for mfERGs by 0.9 ms and 6.0 ms, respectively, and for mfVEPs by 1.0 ms and 6.3 ms, respectively. For '50% luminance plus blur' mfERG amplitudes were significantly reduced centrally and enhanced peripherally and delayed by 1.3 ms. MfVEPs were reduced close to noise level independent of eccentricity.

CONCLUSIONS

Degradation of the retinal image is a potential source of discrepancies between mfERGs and mfVEPs. Image blur suppresses the mfVEP at all locations and changes mfERG topography, resulting in a selective loss of central responses.

SIGNIFICANCE

Considering optic media opacities is of importance for the correct interpretation of mfERG and mfVEP recordings, particularly in elderly patients.

Isolation, characterization and cDNA cloning of nicotianamine synthase from barley. A key enzyme for iron homeostasis in plants

Basic cellular processes such as electron transport in photosynthesis and respiration require the precise control of iron homeostasis. To mobilize iron, plants have evolved at least two different strategies. The nonproteinogenous amino acid nicotianamine which is synthesized from three molecules of S-adenosyl-L-methionine, is an essential component of both pathways. This compound is missing in the tomato mutant chloronerva, which exhibits severe defects in the regulation of iron metabolism. We report the purification and partial characterization of the nicotianamine synthase from barley roots as well as the cloning of two corresponding gene sequences. The function of the gene sequence has been verified by overexpression in Escherichia coli. Further confirmation comes from reduction of the nicotianamine content and the exhibition of a chloronerva-like phenotype due to the expression of heterologous antisense constructs in transgenic tobacco plants. The native enzyme with an apparent Mr of approximately 105 000 probably represents a trimer of S-adenosyl-L-methionine-binding subunits. A comparison with the recently cloned chloronerva gene of tomato reveals striking sequence homology, providing support for the suggestion that the destruction of the nicotianamine synthase encoding gene is the molecular basis of the tomato mutation.

A root-specific iron-regulated gene of tomato encodes a lysyl-tRNA-synthetase-like protein

The tomato mutant chloronerva exhibits a defect in iron-uptake regulation. Despite high apoplastic and symplastic iron concentrations, the mutant shows characteristic symptoms of iron deficiency. Using a subtractive-hybridisation approach, we have screened for cDNA clones specific for genes with altered expression in wild-type versus mutant root tissue. Based on this clone collection, we have isolated and characterised a 2075-bp full-length cDNA encoding a lysyl-tRNA-synthetase-like protein. The corresponding gene is localised as a single copy on chromosome 10. Its expression is strongly induced by changes in the iron status of the plant. This iron-dependent regulation is superimposed upon a strict root specificity of gene expression. Possible functions of the gene product other than in protein biosynthesis will be discussed.

Iron and copper nutrition-dependent changes in protein expression in a tomato wild type and the nicotianamine-free mutant chloronerva

The nicotianamine-deficient mutant chloronerva resembles phenotypically an Fe-deficient plant despite the high accumulation of Fe in the leaves, whereas if suffers from Cu deficiency in the shoot. Two-dimensional electrophoretic separation of proteins from root tips and leaves of wild-type Lycopersicon esculentum Mill. cv Bonner Beste and the mutant grown with and without Fe showed a number of consistent differences. In root tips of the Fe-deficient wild type and the Fe-sufficient as well as the Fe-deficient mutant, the expression of glyceraldehyde-3-phosphate dehydrogenase, formate dehydrogenase, and ascorbate peroxidase was increased. In leaves of the Fe-sufficient and -deficient mutant, Cu-containing chloroplastic and cytosolic superoxide dismutase (Cu-Zn) and plastocyanin (Cu) were nearly absent. This low plastocyanin content could be restored by supplying Cu via the xylem, but the superoxide dismutase levels could not be increased by this treatment. The differences in the protein patterns between wild type and mutant indicate that the apparent Fe deficiency of mutant plants led to an increase in enzymes involved in anaerobic metabolism as well as enzymes involved in stress defense. The biosynthesis of plastocyanin was diminished in mutant leaves, but it was differentially induced by increased Cu content.