The roles of dopamine and related compounds in reward-seeking behavior across animal phyla

Abstract

Motile animals actively seek out and gather resources they find rewarding, and this is an extremely powerful organizer and motivator of animal behavior. Mammalian studies have revealed interconnected neurobiological systems for reward learning, reward assessment, reinforcement and reward-seeking; all involving the biogenic amine dopamine. The neurobiology of reward-seeking behavioral systems is less well understood in invertebrates, but in many diverse invertebrate groups, reward learning and responses to food rewards also involve dopamine. The obvious exceptions are the arthropods in which the chemically related biogenic amine octopamine has a greater effect on reward learning and reinforcement than dopamine. Here we review the functions of these biogenic amines in behavioral responses to rewards in different animal groups, and discuss these findings in an evolutionary context.

Keywords: biogenic amine; catecholamine; dopamine; motivation; nucleus accumbens; octopamine; reward.

Figure 1

Chemical relationships and biosynthetic pathways linking dopamine, tyramine, octopamine, and norepinephrine (enzymes in italics). Dopamine, tyramine, octopamine, and norepinephrine are all derived from tyrosine. In order to synthesize dopamine, tyrosine is first converted to DOPA by tyrosine-hydroxylase, which is then decarboxylated by DOPA-decarboxylase to yield dopamine. Tyramine is either produced directly from tyrosine by tyrosine-decarboxylase, or (more rarely) dehydroxylated from dopamine by dopamine-dehydroxylase. This figure summarizes the most common synthesis pathways, but there are variations among the phyla. In some groups, octopamine is a trace amine and synthesized from tyramine by dopamine-β-hydroxylase, while in other phyla, norepinephrine is physiologically irrelevant and not present at any biologically meaningful level.

Figure 2

Phylogeny of groups discussed in this paper, based on nearly complete ribosomal RNA gene analyses (Mallatt et al., 2010). Discussed phyla represent examples of protostome, deuterostome, and diploblastic groups, yet dopamine is a modulator in the motor systems of all these diverse phyla, and affects responses to rewarding or punishing stimuli in mollusks, platyhelminths, arthropods, nematodes, and chordates.