Modulation of the stimulation-evoked release of [³H]dopamine in the rabbit retina

The rabbit retina was isolated and the endogenous stores of dopamine, presumably located in a subpopulation of amacrine neurons within the inner nuclear layer, were labeled with 0.1 μM [³H]dopamine. After 120 min superfusion with Krebs' solution, intact [³H]dopamine represents about 90% of the total tissue radioactivity. The spontaneous outflow of radioactivity from the rabbit retina consisted mainly of [³H]dopamine metabolites. Less than 10% of the tritium was present as unmetabolized ³H-transmitter. In contrast, the unmetabolized amine accounted for most of the increase in radioactivity released by electrical stimulation at 3 Hz for 2 min. The metabolite pattern of [³H]dopamine in the rabbit retina suggests that O-methylation plays an important role in the inactivation of the catecholaminergic neurotransmitter in this tissue. The effect of dopamine agonists and antagonists on the [³H]dopamine release from the rabbit retina was examined in experiments in which the release of [³H]dopamine was elicited by electrical stimulation at 3 Hz (1 min, 20 mA, 2 msec). The percentage of total tissue radio-activity released after the first period of electrical stimulation (S₁) was 2.26 ± 0.23 (n = 6). The percentage of tritium release in the second stimulation period (S₂) was 2.48 ± 0.37 and the S₂/S₁ ratio was: 1.09 ± 0.11. The release of [³H]dopamine elicited by electrical stimulation is totally calcium-dependent. The dopamine agonist, apomorphine, when added before S₂, significantly reduced the stimulation-evoked release of tritium (S₂/S₁: 0.92 ± 0.14, n = 7; 0.32 ± 0.06, n = 3, P < .001; and 0.18 ± 0.005, n = 6, P < .001 for 0.01, 0.1 and 1 μM apomorphine, respectively). In contrast, the dopamine antagonists, S-sulpiride (0.01-1 μM) and metoclopramide (0.01-1 μM) increased, in a concentration-dependent manner, the release of radioactivity when added before S₂. The R-isomer of sulpiride was about 100 times less active than S-sulpiride. The neuroleptic α-flupenthixol (0.01-10 μM) failed to modify the stimulation-evoked release of [³H]dopamine from the rabbit retina and a significant increase in [³H]dopamine release was elicited only with a concentration of fluphenazine of 0.1 μM. S-sulpiride (1 μM) completely antagonized the apomorphine-induced inhibition of [³H]dopamine release in the rabbit retina, whereas the same concentration of R-sulpiride was inactive. These results suggest that in the rabbit retina there are present stereoselective dopamine autoreceptors which may be involved in the modulation of dopamine release from the amacrine cells. Moreover, these dopamine receptors appear to possess the characteristics of a D₂ receptor, since the benzamides (metoclopramide and S-sulpiride) increased the stimulation-evoked release of [³H]dopamine, whereas α-flupenthixol and fluphenazine were relatively inactive in this respect.