In the mouse, the light-sensitive po of cAMP can be eliminated in the dark by application of the dopamine hydrochloride]. The rank-order affinity of the ability of the D2-like antagonists to block the action of LY 171555 matched that of the rat D4 receptor. Reverse transcription of retina mRNA followed by DNA amplification using D4-specifIc nucleotides demonstrates the presence of D4 mRNA in retina. In situ hybridization studies using D4-speciflic digoxygenin-labeled oligonucleotides or MS-labeled UTP RNA probes demonstrate the presence of D4 mRNA in the photoreceptor cell layer and in the inner nuclear and ganglion cell layers. The modulation by D4 ligands of the dark level of light-sensitive cAMP in photoreceptors demonstrates the physiological coupling of the D4 receptor subtype.The identification and characterization of dopamine receptors represents an issue of wide interest in neurobiology, particularly in light of the recent work (43) demonstrating the multiplicity of types of dopamine receptor. When cloned and expressed in transfected cell lines, these subtypes reveal distinct pharmacological differences when compared in the same cell background (1-8). Thus, in mouse fibroblasts, the human D4 receptor has a 10-to 15-fold higher affinity for quinpirole (LY 171555), a 2-fold lower affinity for sulpiride, and a 5-to 15-fold higher affinity for clozapine than does the D2 receptor (7). Similar results were found for the rat D4 receptor (9). The D3 receptor has a distinct D2-like binding profile in Chinese hamster ovary cells but does not appear to be physiologically coupled to a guanine nucleotide binding protein (6). The functional significance of binding differences in transfected heterologous cell types is unclear.The retina, a model central nervous system tissue, is well suited for studies of dopaminergic pathways, and previous studies on the retina have implicated potential roles for dopamine in a number of processes. This central nervous system division contains biosynthetic enzymes for dopamine production, and classically defined D1 and D2 dopamine receptors that modulate levels of cAMP. For example, the photoreceptor pool of cAMP is sensitive to either light or dopamine (10) and can be studied selectively by using incubation medium containing 10 mM glutamate or aspartate. Such media eliminate the light-evoked responses of neurons postsynaptic to photoreceptors (11)(12)(13)(14)(15)(16)(17)(18), allowing the identification of transmitters that in the dark can alter the size of the light-sensitive pool of cAMP through their action on membrane receptors on these cells. Recent pharmacological experiments (10) examining regulation of the size of this cAMP pool found evidence for its modulation by a receptor distinct from the classic D2 receptor.Previous autoradiographic studies suggested that Drlike receptors were present throughout the outer nuclear layer (19)(20)(21)(22). However, studies using D2-specific mRNA probes failed to observe )2 mRNA within the photoreceptor layer of the rat (23), while i...

The exposure to bright light of dark-adapted (DKA) mouse retinas incubated in the dark (DI) in IBMX-containing medium causes a marked loss of cyclic AMP. This light response also occurs if the medium contains 10 mM aspartate or cobaltous ion, agents believed to confine the effects of light to photoreceptors. Thus, the action of light in the presence of either of these agents defines a light-sensitive pool of cyclic AMP in photoreceptors. This pool could also be reduced or eliminated in DKA-DI retinas by nanomolar to micromolar levels of dopamine (if the medium contained SCH23390, a potent antagonist of Dl receptors), thus indicating an agonistic action of dopamine at D2 receptors. The D2 agonists LY171555 (EC 50 10 nM) or (+)-3-PPP also reduced the cyclic AMP level in the dark. Of the D2 antagonists tested, the butyrophenone spiperone (in the presence of the 5HT-2 blocker ketanserin) countered the action of the D2 agonists but substituted benzamides were ineffective. Consistently, the D2 agonists had no effect on cyclic AMP levels of mutant retinas lacking photoreceptors (rd/rd), but reduced cyclic AMP in DKA-DI glutamate-modified retinas which exhibit a major loss of inner retinal neurons without apparent loss of photoreceptors. The Dl antagonist SCH23390 only reduced cyclic AMP levels of DKA-DI retinas when cyclic AMP levels had been elevated by adding dopamine to the incubation medium.

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