Interneurons constitute most of the neurons in the vertebrate CNS and they function in almost all neuronal circuits and behaviors but we still know very little about how interneurons develop. The spinal cord is a particularly good system for studying interneuron specification because compared to the brain it is relatively simple. Zebrafish embryos are also a powerful model system for studying interneuron development as they have a relatively small number of different interneurons, all of which can be identified by their unique morphologies, we can observe the development of identified neurons in live embryos and both single genes and combinations of genes can easily be "knocked-down" in the same embryos.

We are interested in 3 distinct but related questions:

1. How do different classes of spinal interneurons form in their correct numbers and positions?
2. What are the functions of specific post-mitotically expressed transcription factors in regulating the distinct functional characteristics of spinal interneurons (e.g. their soma positions, morphologies, neurotransmitter phenotypes, synaptic connections and functions in particular behaviours)?
3. To what extent is spinal cord development conserved between zebrafish and other vertebrates

Recently, we have demonstrated that Pax2a, Pax2b and Pax8 transcription factors are redundantly required to specify the glycinergic and GABAergic fates of Circumferential Ascending (CiA) and other Pax2/8-expressing spinal interneurons. This function of Pax2⁄8 is very specific: in triple knock-down embryos CiAs lose expression of glycinergic and GABAergic (inhibitory) neurotransmitters, but they do not become excitatory (glutamatergic or cholinergic) and their morphologies and axon trajectories are unchanged (Batista and Lewis, 2008).

We have also been analysing V2 interneuron development. V2 cells are initially molecularly-identical. However, as they start to become postmitotic and differentiate they subdivide into two intermingled, molecularly-distinct populations, V2a and V2b cells. We have shown that these cells are conserved in zebrafish and amniotes. In zebrafish, V2a cells develop into excitatory Circumferential Descending (CiD) interneurons (Kimura et al., 2006) and we have now shown that V2b cells develop into inhibitory Ventral Lateral Descending (VeLD) interneurons (Batista et al., 2008; see also Kimura et al., 2008). We have also shown that Notch signalling is required for V2 cells to subdivide in this manner: in the absence of Notch signalling all V2 cells develop into CiDs.