Enkephalins modulate [³H]dopamine release from rabbit retina in vitro

The aim of the present investigation was to examine the effect of morphine and synthetic opiate peptides on the calcium-dependent release of [³H]dopamine from the rabbit retina. The endogenous stores of dopamine in the rabbit retina were labeled in vitro with 0.1 μM [³H]dopamine release was elicited by electrical stimulation at 3 Hz (1 min, 20 mA, 2 msec). Two periods of electrical stimulation (S₁ and S₂) were applied in each experiment. The percentage of total tissue radioactivity released after the first period of electrical stimulation (S₁) was: 2.28 ± 0.19% (n = 9). The ratio obtained between the percentage of tritium released in the second (S₂) and first (S₁) periods of stimulation was S₂/S₁: 1.07 ± 0.08. Morphine (0.001-3 μM) and the opiate peptides Tyr-D-Ala-Gly-Phe-D-Leu-enkephalin (0.001-1 μM), Tyr-D-Ala-Gly-Phe-Leu-enkephalin (0.001-1 μM) and Tyr-D-Ala-Gly-Phe-Met-enkephalinamide (0.001-1 μM), when added before S₂, inhibited in a concentration-dependent manner the stimulation-evoked release of [³H]dopamine from pieces of rabbit retina. The concentrations of the opiate agonists which inhibited by 50% the release of [³H]dopamine (IC₅₀) were: 0.275 μM for morphine, 0.063 μM for Tyr-D-Ala-Gly-Phe-D-Leu-enkephalin, 0.046 μM for Tyr-D-Ala-Gly-Phe-Leu-enkephalin and 0.030 μM for Tyr-D-Ala-Gly-Phe-Met-enkephalinamide. Low concentrations of either (+)-naloxone (1 μM) or (-)-naloxone (0.01-1 μM) did not significantly affect the evoked release of [³H]dopamine. However, the active isomer (-)-naloxone completely antagonized the inhibition of release elicited by 0.1 μM Tyr-D-Ala-Gly-Phe-Leu-enkephalin, 0.1 μM Tyr-D-Ala-Gly-Phe-Met-enkephalinamide and 1 μM morphine. The (+)-isomer of naloxone failed to antagonize the actions of the opiate agonists. These results suggest that the opiate agonists inhibit the stimulation-evoked release of [³H]dopamine from the rabbit retina through activation of stereoselective presynaptic opiate receptors. These opiate receptors are different from the inhibitory dopamine autoreceptor of the D-2 subtype, as the inhibitory potency of morphine and the opiate peptides remained unchanged when the autoreceptor was blocked by 1 μM S-sulpiride.