TYROSINE AUGMENTS DOPAMINE RELEASE IN STIMULATED RAT RETINA

Endogenous dopamine (DA) release was measured in perfused rat retinae. Perfusion with elevated potassium (40 mM K) resulted in a 5-6-fold increase in DA release over baseline or 11.6 +/- 0.9% of final tissue DA content. When the selective DA D2 receptor agonist quinpirole was added to the perfusion medium (at 1 and 10 muM), K-stimulated DA release was significantly decreased compared to controls (to 7.0 +/- 1.6 and 6.1 +/- 1.4%, respectively). Addition of the D2 antagonist (+/-)-sulpiride (10 muM) significantly increased DA release to 19.1 +/- 1.3%. DA could be released with successive pulses of K; an initial 10 min pulse resulted in a 4-5-fold increase in endogenous DA release over basal levels or 11.4% of the final retinal tissue DA content and a 3-fold increase (a 9.3% fractional release) upon a second K stimulation given 50 min later. The ratio of stimulated DA release during the two K pulses was 0.82 +/- 0.04. When L-tyrosine (100 muM) was included in the medium throughout the perfusion, K2/K1 was increased to 1.14 +/- 0.13. Both tissue DA level and release were decreased by the tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine (AMPT). At 10 muM AMPT K-stimulated DA release was reduced by 50% during the first pulse and completely abolished during the second K pulse. At 100 muM both basal and K-stimulated release were significantly reduced. Exposure of dark-adapted retinae to light in L-tyrosine-supplemented perfusion medium resulted in an increased release of DA compared to retinae perfused with tyrosine-free medium. Thus, both basal and stimulated release of DA in the perfused rat retina are dependent on stores of newly synthesized DA and in situations where DA synthesis is stimulated (during K depolarization, exposure to light) increasing precursor level can increase the rate at which DA is synthesized and released.