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Dopamine (DA) is an important molecule that plays a role in the nervous and immune systems. DA is produced by a wide variety of animals and it is considered one of the oldest neurotransmitters. However, its specific function in immune cells has not been completely revealed. In a group of chordate animals, the ascidians, DA is reported to be produced by cells in the central nervous system (CNS); however, no dopaminergic receptor in their genomes has been described until now. Because this is an integrating characteristic of the ascidian dopamine system, here it was investigated the pharmacology, function, and phylogeny of DA and dopaminergic receptors (DRs) in the modulation of nitric oxide (NO) in the Phallusia nigra immune cells. The data disclosed, for the first time, that DA modulates NO production by immune cells. Its modulation probably occurs though adrenergic receptors, which display a special characteristic, in that they are capable of binding to noradrenaline (NA) and DA. A pharmacological analysis revealed that receptors present on the ascidian immune cells showed a high affinity to butaclamol, a non-selective D2-class receptor, increasing NO production. In addition, calcium intracellular mobilization was observed when DA was added to immune cells. In conclusion, the data revealed novel insights about the presence of catecholaminergic receptors (CRs) on the P. nigra immune cells, indicating that ascidian CRs have special pharmacological characteristics that are worth highlighting from an evolutionary point of view. ∙ Ascidians central nervous system produce dopamine (DA), however no dopaminergic receptor has been described ∙Addition of DA decreased nitric oxide (NO) production in the ascidian immune cells ∙The modulation of NO production occurs through downstream calcium signaling pathway ∙Ascidians genomes lack orthologs of vertebrates dopaminergic receptors; DA modulates immune cells through adrenergic receptors present in their surface. [ABSTRACT FROM AUTHOR]