[Evolution of monoamine receptors and the origin of motivational and emotional systems in vertebrates].

The evolving vertebrate nervous system was accompanied by major gene duplication events generating novel organs and a sympathetic system. Vertebrate neural pathways synthesizing catecholamine neurotransmitters (dopamine and noradrenaline), were subsequently recruited to process increased information demands by mediating psychomotor functions such as selective attention/predictive reward and emotional drive via the activation of multiple G-protein linked catecholamine receptor subtypes. Here we show that the evolution of these receptor-mediated events were similarly driven by forces of gene duplication, at the cephalochordate/vertebrate transition. In the cephalochordate Amphioxus, a sister group to vertebrates, a single catecholamine receptor gene was found, which based on molecular phylogeny and functional analysis formed a monophyletic group with both vertebrate dopamine D1 and beta adrenergic receptor classes. In addition, the presence of dopamine but not of noradrenaline was assayed in Amphioxus. In contrast, two distinct genes homologous to jawed vertebrate dopamine D1 and beta adrenergic receptor genes were extant in representatives of the earliest craniates, lamprey and hagfish, paralleling high dopamine and noradrenaline content throughout the brain. These data suggest that a D1/beta receptor gene duplication was required to elaborate novel catecholamine psychomotor adaptive responses and that a noradrenergic system specifically emerged at the origin of vertebrate evolution.