Axonal migration of taurine in the goldfish visual system

Taurine is an ubiquitous amino sulfonic acid present in high concentrations in neural and muscle tissue. Except for its role in the conjugation of bile acids, its biological significance is uncertain 5,7. However, there is considerable recent evidence which suggests that it may function as a neurotransmitter or neuromodulator in nervous tissue and in retina 1, and that it might be involved in brain development 1~-16. The present experiments were performed in order to test the possibility that taurine might be axonally transported in the optic nerve of the goldfish, a system which has been used extensively to study the axonal transport of macromolecules 4,6,1°. We also have used this system to investigate the axonal migration of cysteine, a precursor of taurine, and of?-aminobutyric acid (GABA), an amino acid analog of taurine and also a putative neurotransmitter. [85S]Taurine (64 mCi/mmole, obtained from Amersham/Searle Corp.), [~5S] cysteine (22 mCi/mmole, obtained from Amersham/Searle Corp.), or [14C]GABA (49 mCi/mmole, obtained from New England Nuclear Corp.) were dissolved in 0.9 saline. Four microliters of each solution (containing 0.5-1.0/~Ci) were injected in separate experiments into the vitreous of the right eye of goldfish (4-5 in. in length, obtained from Ozark Fisheries, Stoutland, Mo.). Fish were killed at various times after injection and their right retinas and both optic tecta were removed and homogenized in deionized H20. Trichloroacetic acid (TCA), 30 ~o w/v, was added to the homogenate to give a final concentration of 1 0 ~ TCA. Determination of TCAinsoluble and TCA-soluble radioactivity was performed as described previously 6. Since visual fibers of the goldfish cross completely at the optic chiasm, radioactivity in the tectum ipsilateral to the injected eye (right tectum in this case) represents material which has arrived by general circulation, whereas radioactivity in the left tectum represents material which has arrived by axonal transport or axonal diffusion