Biomedical subjects

Josef Ammermüller

Publications and source records attributed to Josef Ammermüller.

6 recordsLinked to original sources

PubMed2006

Complex spike-event pattern of transient ON-OFF retinal ganglion cells.

ON-OFF transient ganglion cells of the turtle retina show distinct spike-event patterns in response to abrupt intensity changes, such as during saccadic eye movements. These patterns consist of two main spike events, with the latency of each event showing a systematic dependency on stimulus contrast. Whereas the latency of the first event decreases monotonically with increasing contrast, as expected, the second event shows the shortest latency for intermediate contrasts and a longer latency for high and low contrasts. These spike-event patterns improve the discrimination of different light-intensity transitions based on ensemble responses of the ON-OFF transient ganglion cell subpopulation. Although the discrimination results are far better than chance using either spike counts or latencies of the first spikes, they are further improved by using properties of the second spike event. The best classification results are obtained when spike rates and latencies of both events are considered in combination. Thus spike counts and temporal structure of retinal ganglion cells carry complementary information about the stimulus condition, and thus spike-event patterns could be an important aspect of retinal coding. To investigate the origin of the spike-event patterns in retinal ganglion cells, two computational models of retinal processing are compared. A linear-nonlinear model consisting of separate filters for on and off response components fails to reproduce the spike-event patterns. A more complex cascade filter model, however, accurately predicts the timing of the spike events by using a combination of gain control loop and spike rate adaptation.

Algorithms↗

PubMed2006

The temporal structure of transient ON/OFF ganglion cell responses and its relation to intra-retinal processing.

A subpopulation of transient ON/OFF ganglion cells in the turtle retina transmits changes in stimulus intensity as series of distinct spike events. The temporal structure of these event sequences depends systematically on the stimulus and thus carries information about the preceding intensity change. To study the spike events' intra-retinal origins, we performed extracellular ganglion cell recordings and simultaneous intracellular recordings from horizontal and amacrine cells. Based on these data, we developed a computational retina model, reproducing spike event patterns with realistic intensity dependence under various experimental conditions. The model's main features are negative feedback from sustained amacrine onto bipolar cells, and a two-step cascade of ganglion cell suppression via a slow and a fast transient amacrine cell. Pharmacologically blocking glycinergic transmission results in disappearance of the spike event sequence, an effect predicted by the model if a single connection, namely suppression of the fast by the slow transient amacrine cell, is weakened. We suggest that the slow transient amacrine cell is glycinergic, whereas the other types release GABA. Thus, the interplay of amacrine cell mediated inhibition is likely to induce distinct temporal structure in ganglion cell responses, forming the basis for a temporal code.

Action Potentials↗

PubMed2005

Deletion of connexin45 in mouse retinal neurons disrupts the rod/cone signaling pathway between AII amacrine and ON cone bipolar cells and leads to impaired visual transmission.

Connexin45 (Cx45) is known to be expressed in the retina, but its functional analysis was problematic because general deletion of Cx45 coding DNA resulted in cardiovascular defects and embryonic lethality at embryonic day 10.5. We generated mice with neuron-directed deletion of Cx45 and concomitant activation of the enhanced green fluorescent protein (EGFP). EGFP labeling was observed in bipolar, amacrine, and ganglion cell populations. Intracellular microinjection of fluorescent dyes in EGFP-labeled somata combined with immunohistological markers revealed Cx45 expression in both ON and OFF cone bipolar cells. The scotopic electroretinogram of mutant mice revealed a normal a-wave but a 40% reduction in the b-wave amplitude, similar to that found in Cx36-deficient animals, suggesting a possible defect in the rod pathway of visual transmission. Indeed, neurotransmitter coupling between AII amacrine cells and Cx45-expressing cone bipolar cells was disrupted in Cx45-deficient mice. These data suggest that both Cx45 and Cx36 participate in the formation of functional heterotypic electrical synapses between these two types of retinal neurons that make up the major rod pathway.

Amacrine Cells↗

PubMed2004

Expression of connexin36 in cone pedicles and OFF-cone bipolar cells of the mouse retina.

Transgenic technology, immunocytochemistry, electrophysiology, intracellular injection techniques, and reverse transcription PCR were combined to study the expression of neuronal connexin36 (Cx36) in the outer plexiform layer of the mouse retina. Transgenic animals expressed either a fusion protein of full-length Cx36 with enhanced green fluorescent protein (EGFP) attached at the C terminus or exon 2 of Cx36 was replaced bybeta-galactosidase (beta-gal). In the outer nuclear layer,beta-gal-positive cell bodies, which were confined to the most distal region close to the outer limiting membrane, displayed immunoreactivity against S-cone opsin. Cx36-EGFP puncta colocalized with cone pedicles, which were visualized by intracellular injection. In reverse transcriptase PCR experiments, Cx36 mRNA was never detected in samples of rods harvested from the outer nuclear layer. These results strongly suggest expression of Cx36 in cones but not in rods. In vertical sections, Cx36 expression in the vitreal part of the outer plexiform layer was characterized by a patchy distribution. Immunocytochemistry with antibodies against the neurokinin-3 receptor and the potassium channel HCN4 (hyperpolarization-activated cyclic nucleotide-gated potassium channel) displayed clusters of the Cx36 label on the dendrites of OFF-cone bipolar cells. In horizontal sections, these clusters of Cx36 appeared as round or oval-shaped groups of individual puncta, and they were always aligned with the base of cone pedicles. Double-labeling experiments and single-cell reverse transcriptase PCR ruled out expression of Cx36 in horizontal cells and rod bipolar cells. At light microscopic resolution, we found close association of Cx36-EGFP with the AMPA-type glutamate receptor subunit GluR1 but not with GluR2-GluR4, the kainate receptor subunit GluR5, or the metabotropic glutamate receptor mGluR6.

Animals↗

PubMed2003

The presynaptic active zone protein bassoon is essential for photoreceptor ribbon synapse formation in the retina.

The photoreceptor ribbon synapse is a highly specialized glutamatergic synapse designed for the continuous flow of synaptic vesicles to the neurotransmitter release site. The molecular mechanisms underlying ribbon synapse formation are poorly understood. We have investigated the role of the presynaptic cytomatrix protein Bassoon, a major component of the photoreceptor ribbon, in a mouse retina deficient of functional Bassoon protein. Photoreceptor ribbons lacking Bassoon are not anchored to the presynaptic active zones. This results in an impaired photoreceptor synaptic transmission, an abnormal dendritic branching of neurons postsynaptic to photoreceptors, and the formation of ectopic synapses. These findings suggest a critical role of Bassoon in the formation and the function of photoreceptor ribbon synapses of the mammalian retina.

Animals↗

PubMed2002

Retinal ganglion cell synchronization by fixational eye movements improves feature estimation.

Image movements relative to the retina are essential for the visual perception of stationary objects during fixation. Here we have measured fixational eye and head movements of the turtle, and determined their effects on the activity of retinal ganglion cells by simulating the movements on the isolated retina. We show that ganglion cells respond mainly to components of periodic eye movement that have amplitudes of roughly the diameter of a photoreceptor. Drift or small head movements have little effect. Driven cells that are located along contrast borders are synchronized, which reliably signals a preceding movement. In an artificial neural network, the estimation of spatial frequencies for various square wave gratings improves when timelocked to this synchronization. This could potentially improve stimulus feature estimation by the brain.

Animals↗