Protein phosphorylation is involved in the regeneration of the nervous system. Taurine modulates the phosphorylation of specific proteins in the retina, and also increases outgrowth from ganglion cells. In order to test the possible role of protein phosphorylation on the outgrowth from the retina and on the trophic effect of taurine, in vitro studies were performed in the presence of phorbol and nonphorbol protein kinase C activators, the protein kinase C inhibitor tamoxifen, and phosphatase inhibitors. After crush of the optic nerve, explants of the goldfish retina were cultured and the outgrowth was evaluated by measuring the length and the density of neurites. The activation of protein kinase C decreased the outgrowth from the explants and impaired the stimulatory effect of taurine. Phosphatase inhibitors produced a similar effect on basal and taurine-modulated outgrowth. In certain concentrations, some of these drugs did not affect the emission of neurites in the absence of taurine, but decreased the effect of the amino acid. Tamoxifen also reduced the outgrowth, probably acting at other cellular levels or indicating that the regulation of outgrowth by phosphorylation is a complex and dual process. The response to the drugs, evaluated by length or density of fibers, was not the same, since rate of outgrowth was more affected than density, which suggests that both parameters are modulated at differential stages or sensitivities to the tested agents.
The sulfur amino acid taurine and the indoleamine serotonin increases and decreases, respectively, the outgrowth from goldfish retinal explants. Taurine seems to be acting, at least partially, through an increase in calcium fluxes, and the serotonin-inhibiting effect appears to be mediated by serotonin1A receptors and cAMP. Isolated cells of postcrush goldfish retina and of retina from 5-day-old rats were cultured in the presence of taurine or serotonin. In the goldfish, the classical morphology of postcrush ganglion cells was observed. An antibody against the glycoprotein Thy-1 labelled three types of cells in the cultures of goldfish retina. The number of cells outgrowing and the length of the main neurite was measured at 5 days in culture in both species. The number of cells presenting neurites was increased in the goldfish retina by the addition of taurine, and decreased by serotonin. However, the length of the neurites was unaffected by the addition of the modulators. In the rat, only a slight decrease in the number of cells outgrowing was observed in the presence of serotonin. The incorporation of [3H]thymidine was not modified after 5 days in culture in the presence of taurine or serotonin, either in the goldfish or in the rat retina. The antibody Thy 1.1 can label retinal cells of the goldfish in vitro, one of them being ganglion cells. The trophic effect exerted by taurine in the postcrush goldfish retina needs the integrity of the tissue favoring the interaction of cells and factors, because outgrowth increases in retinal explants, but not in isolated cells.
The interaction between innervated tissues, targets and nerves is crucial in the maintenance of physiological conditions, and the disturbance of this harmony causes the production of morphological and biochemical changes. After lesion of the optic nerve, several modifications take place in the retina, the optic tectum and the optic nerve. The influence of the tectum on the outgrowth from the goldfish retina and the possible role of taurine was studied. Ganglion retinal cells were identified by retrolabeling with Dil. Crushing the optic nerve 10 days prior to plating retinal cells, as compared with optic axotomy, did not affect the survival of cultured retinal cells, as well as the length of the neurites. However, the number of neurites per cell and the branching of the longest fiber were higher after axotomy than after crushing. The addition of taurine to the medium did not modify this response at 5 days in culture. At early periods in culture, the stimulatory effect on isolated ganglion cell outgrowth produced by taurine was enhanced after axotomy respecting crushing of the optic nerve, but was not affected in retinal explants. The addition of medium from cultured optic tectum several days post‐crush of the optic nerve to retinal explants from intact retinas or coming from post‐crush retina modified the outgrowth, being inhibitory or stimulatory in a time‐dependent manner. The co‐culture of optic tectum and retina also affected the outgrowth from the retina with a byphasic shape. The results support the differential response of the retina facing partial or complete interruption with the target and limit the effect of taurine to early periods in culture. In addition, the production of inhibitory factors from the tectum, plus the stimulatory ones, are strongly supported by this work.
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