Dopamine receptor responsible for neuronal signaling in the mesolimbic system of the brain, an area of the brain that regulates emotion and complex behavior. Activated by dopamine, but also by epinephrine and norepinephrine, and by numerous synthetic agonists and drugs (PubMed:16423344, PubMed:27659709, PubMed:29051383, PubMed:9003072). Agonist binding triggers signaling via G proteins that inhibit adenylyl cyclase (PubMed:16423344, PubMed:27659709, PubMed:29051383, PubMed:7512953, PubMed:7643093). Modulates the circadian rhythm of contrast sensitivity by regulating the rhythmic expression of NPAS2 in the retinal ganglion cells (By similarity). {ECO:0000250|UniProtKB:P51436, ECO:0000269|PubMed:16423344, ECO:0000269|PubMed:1840645, ECO:0000269|PubMed:27659709, ECO:0000269|PubMed:29051383, ECO:0000269|PubMed:7512953, ECO:0000269|PubMed:7643093, ECO:0000269|PubMed:8078498, ECO:0000269|PubMed:9003072}. This is the function of Ensembl gene identifier ENSG00000069696 (DRD4, dopamine receptor D4).