Local regeneration in the retina of the goldfish

Continued search for the cellular signals that regulate regeneration of dopaminergic neurons in goldfish retina

Intraocular injections of low doses (0.7-1.4 mM estimated intraocular concentration) of 6-hydroxydopamine (6OHDA) selectively destroy dopaminergic neurons in the inner nuclear layer (INL) of goldfish retina, and they never regenerate. However, injection of a higher dose of 6OHDA (2.9 mM) destroys > 30% (but not all) of the cells in both the INL and the outer nuclear layer (ONL), but within 3 weeks, neurons in both the INL (including dopaminergic neurons) and the ONL regenerate. We hypothesize that the regenerated neurons derive from mitotic rod precursors in the ONL and that damage to the surrounding micro-environment (i.e. destruction of photoreceptors) triggers the regenerative response. To directly test this hypothesis, we selectively ablated > 99% of dopaminergic neurons (with low doses of 6OHDA) and up to 55% of rod photoreceptors (with tunicamycin), and asked whether the dopaminergic neurons regenerated, as evidenced by double immunolabeling with anti-tyrosine hydroxylase and anti-bromodeoxyuridine. After 38 days, the number of bromodeoxyuridine-immunoreactive rod nuclei was increased 2.4-fold compared to controls, but no regenerated dopaminergic neurons were found. These data suggest that although the rate of rod production increases, rod precursors do not alter their normal pathway of development to replace dopaminergic neurons in the INL when damage to the ONL is limited to destruction of rods.

Homeobox genes are expressed in the retina and brain of adult goldfish

The goldfish (Carassius auratus) visual pathway displays continuous growth and plasticity throughout life. Since homeobox genes are important transcriptional regulators in development, we searched for homeobox genes in the adult goldfish retina and brain. Using the PCR, we discovered a repertoire of homeobox sequences expressed in these tissues. In addition to isolating homeodomain sequences found in the vertebrate Hox gene clusters, a sequence identical to the chicken CHox7 homeodomain was characterized. Furthermore, a sequence with significant homologies to the Xenopus XIHbox8 and leech Htr-A2 homeodomains was identified, and these sequences may define an additional class of homeodomain. Finally, a sequence belonging to the paired class (prd) of homeodomains is reported. Homeobox gene expression in the adult goldfish retina and brain may be associated with the persistent developmental features of these tissues.