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 (60HDA) selectively destroy dopami nergic neurons in the inner nuclear layer (INL) of goldfish retina, an d they never regenerate. However, injection of a higher dose of 60HDA (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 th e INL (including dopaminergic neurons) and the ONL regenerate. We hypo thesize that the regenerated neurons derive from mitotic rod precursor s in the ONL and that damage to the surrounding micro-environment (i.e . destruction of photoreceptors) triggers the regenerative response. T o directly test this hypothesis, we selectively ablated > 99% of dopam inergic neurons (with low doses of 60HDA) and up to 55% of rod photore ceptors (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 com pared 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 r eplace dopaminergic neurons in the INL when damage to the ONL is limit ed to destruction of rods.