[Evolution and development of dopaminergic neurotransmitter systems in vertebrates]

Dopamine is a widespread neurotransmitter which exerts numerous neuromodulatory actions in the vertebrate central nervous system. This pleiotropic activity relies on the organisation of dopamine-synthesizing neuronal pathways and on a multiplicity of specific membrane receptors. A comparative approach has been undertaken to gain clues on the genetic events which took place during evolution to devise the dopamine systems of modern vertebrates. The localisation and phenotype of dopamine-synthesizing neurones is determined by different gene networks in each of the dopaminergic nuclei. However, despite this amazing diversity, the overall organisation of the dopaminergic nuclei is strinkingly conserved in the main vertebrates groups. In sharp contrast, the number of dopamine receptors subtypes has been multiplied by two major steps of gene duplications during vertebrates evolution. The first one occurred in the lineage leading to agnathans, whereas the second was concomitant to the emergence of cartilaginous fish. Accordingly, three subtypes exist in D1 receptor class (D1A, D1B, D1C) in all the jawed vertebrates, with two exceptions: eutherian mammals where only two D1 subtypes are found (D1A, D1B) and archosaurs where a fourth subtype is present (D1D). Comparisons of the pharmacological and biochemical characteristics of the dopamine receptors in vertebrate groups revealed homologous features that define each of the receptor subtypes and that have been fixed after gene duplications. The comparison of the distribution of the D1 receptor transcripts in the brain of teleosts and mammals points to significant conserved or derived expression territories, revealing previously neglected aspects of dopamine physiology in vertebrates.