Specific alterations of tyrosine hydroxylase immunopositive cells in the retina of NT-4 knock out mice

Dopaminergic amacrine cell number, plexus density, and dopamine content in the mouse retina: Strain differences and effects of Bax gene disruption

Many types of retinal neuron modulate the distribution of their processes to ensure a uniform coverage of the retinal surface. Dendritic field area, for instance, is inversely related to the variation in cellular density for many cell types, observed either across retinal eccentricity or between different strains of mice that differ in cell number. Dopaminergic amacrine (DA) cells, by contrast, have dendritic arbors that bear no spatial relationship to the presence of their immediate homotypic neighbors, yet it remains to be determined whether their coverage upon the retina, as a population, is conserved across variation in their total number. The present study assessed the overall density of the dopaminergic plexus in the inner plexiform layer in the presence of large variation in the total number of DA cells, as well as their retinal dopamine content, to determine whether either of these features is conserved. We first compared these traits between two strains of mice (C57BL/6J and A/J) that exhibit a two-fold difference in DA cell number. We subsequently examined these same traits in littermate mice for which the pro-apoptotic Bax gene was either intact or knocked out, yielding a five-fold difference in DA cell number. In both comparisons, we found greater plexus density and DA content in the strain or condition with the greater number of DA cells. The population of DA cells, therefore, does not appear to self-regulate its process coverage to achieve a constant density as the DA mosaic is established during development, nor its functional dopamine content in maturity.

Strategies to improve quantitative assessment of immunohistochemical and immunofluorescent labelling

Binary image thresholding is the most commonly used technique to quantitatively examine changes in immunolabelled material. In this article we demonstrate that if implicit assumptions predicating this technique are not met then the resulting analysis and data interpretation can be incorrect. We then propose a transparent approach to image quantification that is straightforward to execute using currently available software and therefore can be readily and cost-effectively implemented.

Overexpression of neurotrophin-3 stimulates a second wave of dopaminergic amacrine cell genesis after birth in the mouse retina

Dopaminergic amacrine (DA) cells play multiple and important roles in retinal function. Neurotrophins are known to modulate the number and morphology of DA cells, but the underlying regulatory mechanisms are unclear. Here, we investigate how neurotrophin-3 (NT-3) regulates DA cell density in the mouse retina. We demonstrate that overexpression of NT-3 upregulates DA cell number and leads to a consequent increase in the density of DA cell dendrites. To examine the mechanisms of DA cell density increase, we further investigate the effect of NT-3 overexpression on retinal apoptosis and mitosis during development. We find that NT-3 does not affect the well known wave of retinal cell apoptosis that normally occurs during the first 2 weeks after birth. Instead, overexpression of NT-3 promotes additional mitosis of DA cells at postnatal day 4, but does not affect cell mitosis before birth, the peak period of amacrine cell genesis in wild-type retinas. We next show that retinal explants cultured from birth to day 7 without extra NT-3 produced by lens exhibit similar number of DA cells as in wild type, further supporting the notion that postnatal overexpression of lens-derived NT-3 affects DA cell number. Moreover, the additional mitosis after birth in NT-3-overexpressing mice does not occur in calretinin-positive amacrine cells or PKC-positive rod ON bipolar cells. Thus, the NT-3-triggered wave of cell mitosis after birth is specific for the retinal DA cells.