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Alteration of metabolism of retinal taurine following prolonged light and dark adaptation: A quantitative comparison with gamma‐aminobutyric acid (GABA)

Alteration of metabolism of taurine in prolonged light‐ and dark‐adapted frog retinae were studied in comparison with that of gamma‐aminobutyric acid (GABA) and the following results were obtained. (1) Statistically significant alterations in retinal taurine, an increase in dark‐adapted, and a decrease in light‐adapted states, respectively, occurred when frogs were adapted continuously to light or dark for more than 3 weeks. Under the same experimental conditions, no alteration in retinal GABA was noted. (2) At 3 weeks and thereafter, a significant increase of retinal cysteine sulfinic acid decarboxylase (CSD; EC 4.1.1.12) activity, an enzyme involved in the biosynthetic pathway of taurine, also occurred in the dark, whereas the activity in the light‐adapted retina was reduced. On the other hand, the retinal activity of L‐glutamate decarboxylase (GAD; EC 4.1.1.15), the rate‐limiting enzyme of GABA biosynthesis, was not altered in dark‐ as well as light‐adapted state. Similarly, retinal GABA‐transaminase (GABA‐T; EC 2.6.1.19)‐succinic semialdehyde dehydrogenase (SSADH; EC 1.2.1.16) was unaltered. (3) These alterations in retinal taurine were, however, unaccompanied by any changes in factors related to transmitter actions such as evoked release, high affinity uptake, and specific binding to synaptic membranes. The above results suggest that, different from GABA as a potent candidate for inhibitory neurotransmitter, retinal taurine may act as neuromodulator and/or may play an important role as a basic factor for maintaining cellular integrity under certain pathophysiological conditions.