Comparative developmental neurobiology of molluscs and zebrafish

The developmental and evolutionary origins of the nervous system are generally unclear. When, for example, do the first neurons develop in the lives of different animals? What are the functions of these cells? Can individual neurons or neural circuits be recognized between different groups of animals? If so, which features of the cells and circuits are consistent between animals and which ones vary to permit evolutionary change? We proposed to answer such questions by focusing on 3 different neural circuits that can be recognized on the bases of conserved neurotransmitter contents and details of development. 1) Larval neurons related to sensory functions in molluscs. Some of the very first cells to develop in molluscan larvae contain the neurotransmitter serotonin. These cells appear to be sensory and to control cilia involved in feeding and locomotion. Furthermore, similar cells can be recognized in other marine invertebrate larvae, thus suggesting common evolutionary origins (i.e., homology). We propose to expand the catalogue of larval cells containing other known transmitters by examining the distributions of histamine, -amino butyric acid (GABA), acetylcholine and possibly glutamate which all appear to play roles in chemosensation and the sense of balance in adult molluscs but have not yet been studied in developing molluscs or other marine larvae. We hypothesize, however, that transmitter-specific labels will permit the identification of homologous neurons that are widespread in the animal kingdom, and identification of these homologues will provide new insights into the early evolution of nervous systems. 2) Olfactory pathways in zebrafish. We have previously studied the development of how axons of sensory neurons are organized as they enter the olfactory bulb. We will next concentrate on the development of interneurons within the olfactory bulb which contain on GABA and dopamine and are found throughout the vertebrates. 3) Intracardiac nervous system in zebrafish. The anatomy of autonomic pathways controlling the heart through the release of acetylcholine and norepinephrine are now well described in fish and are strikingly similar with those controlling the hearts of mammals. We will next seek to examine the ontogeny of those pathways and gain insights into when they first become functional.