Dopaminergic Receptors in Bovine Retina and Their Interaction with Thyrotropin-Releasing Hormone

A superfusion technique was employed to study the release of 3H]dopamine from isolated bovine retina. Only K+-stimulated release was observed from both light- and dark-adapted retina; release by other stimuli was from dark-adapted retina only. Light-evoked release of 3H]dopamine from dark-adapted retina was blocked by thyrotropin-releasing hormone (TRH), which has previously been identified as a retinal neuropeptide. TRH itself released small amounts of 3H]dopamine from dark-adapted retina. These results are interpreted as indicating that TRH acts as a modulator of dopaminergic activity in retina through the agency of presynaptic autoreceptors. Evidence of the existence of a feedback inhibition system, probably mediated by dopaminergic autoreceptors, was found by the inclusion of sulpiride, a dopaminergic D2 receptor antagonist in the perfusate, which, in a stereoselective manner, enhanced spontaneous and light-evoked release of 3H]dopamine. On the other hand, dopamine (1 microM) reduced these effects. TRH did not affect the high-affinity uptake system for dopamine in retina; this, then, could not account for the effects on release. Radioligand binding showed a specific, saturable high-affinity binding system for 3H]TRH, with an apparent KD of 2.2 nM and a Bmax of 23 fmol/mg protein in bovine retinal membranes. Displacement experiments showed that specific 3H]TRH binding was displaced in the nanomolar range by spiperone and in the micromolar range by dopamine, whereas L-(--)-sulpiride was virtually inactive in displacing 3H]TRH.(ABSTRACT TRUNCATED AT 250 WORDS)