title: Ancient origin of somatic and visceral neurons.
abstract: BACKGROUND
  A key to understanding the evolution of the nervous system on a large phylogenetic scale is the identification of homologous neuronal types. Here, we focus this search on the sensory and motor neurons of bilaterians, exploiting their well-defined molecular signatures in vertebrates. Sensorimotor circuits in vertebrates are of two types: somatic (that sense the environment and respond by shaping bodily motions) and visceral (that sense the interior milieu and respond by regulating vital functions). These circuits differ by a small set of largely dedicated transcriptional determinants: Brn3 is expressed in many somatic sensory neurons, first and second order (among which mechanoreceptors are uniquely marked by the Brn3+/Islet1+/Drgx+ signature), somatic motoneurons uniquely co-express Lhx3/4 and Mnx1, while the vast majority of neurons, sensory and motor, involved in respiration, blood circulation or digestion are molecularly defined by their expression and dependence on the pan-visceral determinant Phox2b.
  
  
  RESULTS
  We explore the status of the sensorimotor transcriptional code of vertebrates in mollusks, a lophotrochozoa clade that provides a rich repertoire of physiologically identified neurons. In the gastropods Lymnaea stagnalis and Aplysia californica, we show that homologues of Brn3, Drgx, Islet1, Mnx1, Lhx3/4 and Phox2b differentially mark neurons with mechanoreceptive, locomotory and cardiorespiratory functions. Moreover, in the cephalopod Sepia officinalis, we show that Phox2 marks the stellate ganglion (in line with the respiratory--that is, visceral--ancestral role of the mantle, its target organ), while the anterior pedal ganglion, which controls the prehensile and locomotory arms, expresses Mnx.
  
  
  CONCLUSIONS
  Despite considerable divergence in overall neural architecture, a molecular underpinning for the functional allocation of neurons to interactions with the environment or to homeostasis was inherited from the urbilaterian ancestor by contemporary protostomes and deuterostomes.
authors: Institut de Biologie de l'École Normale Supérieure-IBENS, CNRS UMR8197, INSERM U1024, Paris, France.
authors: Marc
authors: M
authors: Nomaksteinsky
authors: Stefan
authors: S
authors: Kassabov
authors: Zoubida
authors: Z
authors: Chettouh
authors: Henri-Corto
authors: HC
authors: Stoeklé
authors: Laure
authors: L
authors: Bonnaud
authors: Gilles
authors: G
authors: Fortin
authors: Eric R
authors: ER
authors: Kandel
authors: Jean-François
authors: JF
authors: Brunet
mesh_terms: D000818:Animals; D005075:Biological Evolution; D017952:Ganglia, Invertebrate / Q000166:cytology / Q000378:metabolism; D008195:Lymnaea / Q000166:cytology / Q000378:metabolism; D008465:Mechanoreceptors / Q000378:metabolism; D051379:Mice; D009474:Neurons / Q000166:cytology* / Q000378:metabolism; D051381:Rats; D011984:Sensory Receptor Cells / Q000166:cytology / Q000378:metabolism; D049833:Sepia / Q000166:cytology / Q000378:metabolism; D014158:Transcription, Genetic; D014714:Vertebrates / Q000378:metabolism*; D014781:Viscera / Q000294:innervation*
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