Localization of rat glycine receptor alpha1 and alpha2 subunit transcripts in the developing auditory brainstem.

Glycine receptors (GlyRs), aside from GABA(A) receptors, mediate fast postsynaptic inhibition in the mammalian nervous system. Spatial and temporal expression of the genes encoding ligand-binding alpha subunits (alpha1-alpha4) and the structural beta subunit leads to the formation of various GlyR isoforms. Currently, the idea of the GlyRs containing the "adult" alpha1 subunit replacing those with the "neonatal" alpha2 subunit during early postnatal development predominates. Here, we describe the patterns of expression of the GlyR alpha1 and alpha2 subunit genes in the rat auditory brainstem between postnatal day (P) 0 and P20, by using both nonradioactive and radioactive in situ hybridization. We show that the alpha1 subunit mRNA appears throughout the auditory brainstem during the first 8 postnatal days, which resembles the time of onset described within the spinal cord. In the rostral auditory nuclei (nuclei of the lateral lemniscus and inferior colliculus), the alpha1 subunit transcript appears later (P8) than in the caudal nuclei (cochlear nuclear complex and superior olivary complex; P0). Surprisingly, we found that low levels of the alpha2 subunit transcript are present in the auditory brainstem at birth and persist throughout the period analyzed. However, alpha2 subunit mRNA is present at high levels in other neonatal brainstem structures, such as cranial motor nuclei. Therefore, we conclude that the changes in GlyR composition in the auditory brainstem deviate from the classic alpha2 to alpha1 subunit switch observed in spinal cord. Our data suggest that genes for other GlyR subunits (e.g., alpha3 and alpha4) may be expressed during early development in the auditory brainstem.

Reading of concentration gradients by axonal growth cones.

Wiring up the nervous system occurs as a self-organizing process during animal development. It has long been proposed that directional growth of axons towards their targets is achieved by gradients of guiding molecules and the conceptual framework of gradient guidance was introduced more than a decade ago. Novel experimental results now allow the formulation of models incorporating more mechanistic detail. We first summarize some crucial in vitro and in vivo results concerning the development of the chick retinotectal projection. We then review two recent theoretical models based on these findings (the models of Nakamoto and colleagues, and of Honda). Neither model considers the latest observation that putative guidance ligands, in addition to their tectal expression, are expressed in a similar pattern on the retina and that a disturbance of this expression affects topography These findings suggest that retinal axons might grow into the tectum until they have reached a ligand concentration matching that of their site of origin. We call this the imprint-matching concept of retinotectal guidance. As a framework for pinpointing logical difficulties of the mechanistic description of the guidance process and to stimulate further experiments we finally suggest two extended versions of Honda's model implementing imprint matching, which we call 'the variable set-point' and 'the gradient-sensitive adaptation' model. Strengths and weaknesses of both mechanisms are discussed.

Modulation of EphA receptor function by coexpressed ephrinA ligands on retinal ganglion cell axons.

The Eph family is thought to exert its function through the complementary expression of receptors and ligands. Here, we show that EphA receptors colocalize on retinal ganglion cell (RGC) axons with EphA ligands, which are expressed in a high-nasal-to-low-temporal pattern. In the stripe assay, only temporal axons are normally sensitive for repellent axon guidance cues of the caudal tectum. However, overexpression of ephrinA ligands on temporal axons abolishes this sensitivity, whereas treatment with PI-PLC both removes ephrinA ligands from retinal axons and induces a striped outgrowth of formerly insensitive nasal axons. In vivo, retinal overexpression of ephrinA2 leads to topographic targeting errors of temporal axons. These data suggest that differential ligand expression on retinal axons is a major determinant of topographic targeting in the retinotectal projection.

Olfaction in zebrafish: what does a tiny teleost tell us?

Zebrafish, Danio rerio, possess a well-developed sense of smell which governs a variety of behaviors. Both the number of odorant receptor genes and the number of modules in the olfactory bulb (glomeruli) are about an order of magnitude smaller than those of mammals. Nevertheless, the spatial organization of functional properties within the sensory surface and the olfactory bulb are comparable to those of mammals. The quantitatively reduced olfactory system of zebrafish, together with the suitability of this species for developmental and genetic studies, make zebrafish an interesting model system to study olfactory differentiation and neuronal representation of olfactory information.

Nested expression domains for odorant receptors in zebrafish olfactory epithelium.

The mapping of high-dimensional olfactory stimuli onto the two-dimensional surface of the nasal sensory epithelium constitutes the first step in the neuronal encoding of olfactory input. We have used zebrafish as a model system to analyze the spatial distribution of odorant receptor molecules in the olfactory epithelium by quantitative in situ hybridization. To this end, we have cloned 10 very divergent zebrafish odorant receptor molecules by PCR. Individual genes are expressed in sparse olfactory receptor neurons. Analysis of the position of labeled cells in a simplified coordinate system revealed three concentric, albeit overlapping, expression domains for the four odorant receptors analyzed in detail. Such regionalized expression should result in a corresponding segregation of functional response properties. This might represent the first step of spatial encoding of olfactory input or be essential for the development of the olfactory system.

Determination of lysozyme content in eggs and egg products using SDS-gel electrophoresis.

Lysozyme can be separated commercially from other egg components and is used as a natural, antibiotically active protein. A method for the detection of de-lysozymed egg products using SDS-polyacrylamide gel electrophoresis (PAGE) is described. The method can also be used for lysozyme quantification. Lysozyme concentrations in egg products are tentatively assigned to three classes, based on the lysozyme contents found in intact eggs. The results obtained from a series of egg products on the market allow two groups to be distinguished. The first group has values for average lysozyme content, standard deviation and total protein content very close to the values found for intact eggs, and the second group clearly shows a lower lysozyme content but a total protein content comparable with intact eggs.