Biomedical subjects

U Markstahler

Publications and source records attributed to U Markstahler.

3 recordsLinked to original sources

PubMed1998

Transient molecular visualization of ocular dominance columns (ODCs) in normal adult marmosets despite the desegregated termination of the retino-geniculo-cortical pathways.

The activity-dependent immediate early gene protein Krox-24, expressed in the neocortex at high basal levels, decreases rapidly upon neuronal deactivation (Chaudhuri et al. [1995] Vis. Neurosci. 12:35-50). In infant marmosets, as in most primates, the geniculo-cortical terminations segregate into eye-specific anatomical ocular dominance columns (ODCs), which disappear, however, during adolescence (Spatz [1989] Brain Res. 488:376-380), resulting in balanced inputs from the two eyes (Sengpiel et al. [1996] Vis. Neurosci. 13:145-160). Nevertheless, we found, in adult marmosets, 24 hours after monocular retinal activity blockade by tetrodotoxin, distinct alternating compartments of potentiated and depressed Krox-24-like immunoreactivity (Krox-IR) in layer IV of area 17. This pattern of Krox-IR disappeared at 10 days of retinal silencing, but was still present at this survival time in stains for cytochrome oxidase or NADPH-diaphorase. After 20 days of retinal silencing, the pattern was not demonstrable with any of the three stains. We term these compartments physiological ODCs, in contrast to the anatomical ODCs of most primates. If the anatomical ODCs of infant marmosets disappear by collateral sprouting into the inappropriate ODCs, then the collateral geniculo-cortical synapses might differ slightly in their properties from the original ones. We silenced the sets of original and collateral synapses of the one ocularity. This apparently transiently initiated, at the synapses driven by the intact eye, two different complex processes leading to molecular potentiation at the original synapses and to molecular depression at the collateral synapses.

Age Factors↗

PubMed1993

Cation specificity and pharmacological properties of the Ca(2+)-dependent K+ channel of rat cortical collecting ducts.

The luminal membrane of principal cells of rat cortical collecting duct (CCD) is dominated by a K+ conductance. Two different K+ channels are described for this membrane. K+ secretion probably occurs via a small-conductance Ca(2+)-independent channel. The function of the second, large-conductance Ca(2+)-dependent channel is unclear. This study examines properties of this channel to allow a comparison of this K+ channel with the macroscopic K+ conductance of the CCD and with similar K+ channels from other preparations. The channel is poorly active on the cell. It has a conductance of 263 +/- 11 pS (n = 36, symmetrical K+ concentrations) and of 139 +/- 3 pS (n = 91) with 145 mmol/l K+ on one side and 3.6 mmol/l K+ on the other side of the membrane. Its open probability is high after excision (0.71 +/- 0.03, n = 85). The channel flickers rapidly between open and closed states. Its permeability in the cell-free configuration was 7.0 +/- 0.2 x 10(-13) cm3/s (n = 85). It is inhibited by several typical blockers of K+ channels such as Ba2+, tetraethylammonium, quinine, and quinidine and high concentrations of Mg2+. The Ca2+ antagonist verapamil and diltiazem also inhibit this K+ channel. As is typical for the maxi K+ channel, it is inhibited by charybdotoxin but not by apamin. The selectivity of this large-conductance K+ channel demonstrates significant differences between the permeability sequence (pK > pRb > pNH4 > pCs = pLi = pNa = pcholine = 0) and the conductance sequence (gK > gNH4 > gRb > gLi = gcholine > gCs = gNa = 0). The only other cations that are significantly conducted by this channel besides K+ (gK at Vc = infinity is 279 +/- 8 pS, n = 88) re NH+4 (gNH4 = 127 +/- 22 pS, n = 10) and Rb+ (gRb = 36 +/- 5 pS, n = 6). The K+ currents through this channel are reduced by high concentrations of choline+, Cs+, Rb+, and NH+4. These properties and the dependence of this channel on Ca2+ and voltage classify it as a "maxi" K+ channel. A possible physiological function of this channel is discussed in the accompanying paper.

Animals↗

PubMed1993

Temporal and spatial summation in the human rod visual system.

1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the fovea with 520 nm targets of varying size and duration. Measurements were made under rod-isolation conditions in two normal observers and in a typical, complete achromat observer who has no cone-mediated vision. The purpose of these experiments was to determine how the temporal and spatial summation of rod-mediated vision changes with light adaptation. 2. The absolute threshold and the rise in increment threshold with background intensity depend upon target size and duration, but the psychophysically estimated dark light of the eye (the hypothetical light assumed to be equivalent to photoreceptor noise) does not. 3. The rise in increment threshold for tiny (10 min of arc), brief (10 ms) targets approaches the de Vries-Rose square-root law, varying according to the quantal fluctuations of the background light. The slope of the rod increment threshold versus background intensity (TVI) curves in logarithmic co-ordinates is about 0.56 +/- 0.04 (when cones are not influencing rod field adaptation). For large (6 deg) and long (200 ms) targets, a maximum slope of about 0.77 +/- 0.03 is attained. 4. The steeper slopes of the rod-detected TVI curves for large, long targets implies some reduction in temporal or spatial summation. In fact, the change in summation area is much more critical: under conditions where only the rod system is active the TVI curve slope is independent of target duration, suggesting that temporal summation is practically independent of background intensity. 5. The rise in threshold also depends on the wavelength of the background field in the normal observer but not in the achromat, confirming reports that the field adaptation of the rods is not independent of the quantal absorptions in the cones. The cone influence is most conspicuous on long-wavelength backgrounds and is found for all target sizes and durations, but is greater for large and long targets than for the other conditions.

Adult↗