Modulation of [3H]dopamine release from rabbit retina.

The calcium-dependent release of [3H]dopamine ([3H]DA) elicited by field stimulation or potassium is modulated through activation of stereoselective inhibitory DA autoreceptors of the D-2 subtype that are pharmacologically different from the D-1 DA receptor subtype linked to the stimulation of adenylate cyclase (EC 4.6.1.1). The D-2 DA autoreceptors appear to be endogenously activated by DA because DA receptor antagonists such as S-sulpiride increased the stimulation-evoked release of [3H]DA. Nanomolar concentrations of norepinephrine (NE) and epinephrine (E) inhibited in a concentration-dependent manner the electrical stimulation-evoked release of [3H]DA. The inhibitory effect of these catecholamines was not modified by S-sulpiride, which, on the contrary, selectively antagonized the inhibition of [3H]DA release elicited by exogenous DA. Phentolamine or (+/-)-propranolol did not affect the release of [3H]DA from rabbit retina. The alpha antagonist phentolamine competitively antagonized the inhibitory effect of both NE and E, which suggests that these catecholamines activate alpha receptors in retina. The decrease by catecholamines of the calcium-dependent release of [3H]DA appears not to involve beta adrenoceptors because their inhibitory effect was not modified by propranolol. Under identical experimental conditions (i.e., nomifensine, 30 microM), serotonin did not modify the stimulated release of [3H]DA. In conclusion, in the rabbit retina, DA autoreceptors of the D-2 subtype appear to modulate endogenously released DA whereas inhibitory presynaptic alpha receptors might be of pharmacological importance as sites of action for retinal or blood-borne catecholamines.