Neuronal labeling patterns in the spinal cord of adult transgenic Zebrafish.

We describe neuronal patterns in the spinal cord of adult zebrafish. We studied the distribution of cells and processes in the three spinal regions reported in the literature: the 8th vertebra used as a transection injury site, the 15th vertebra mainly used for motor cell recordings and also for crush injury, and the 24th vertebra used to record motor nerve activity. We used well-known transgenic lines in which expression of green fluorescent protein (GFP) is driven by promoters to hb9 and isl1 in motoneurons, alx/chx10 and evx1 interneurons, ngn1 in sensory neurons and olig2 in oligodendrocytes, as well as antibodies for neurons (HuC/D, NF and SV2) and glia (GFAP). In isl1:GFP fish, GFP-positive processes are retained in the upper part of ventral horns and two subsets of cell bodies are observed. The pattern of the transgene in hb9:GFP adults is more diffuse and fibers are present broadly through the adult spinal cord. In alx/chx10 and evx1 lines we respectively observed two and three different GFP-positive populations. Finally, the ngn1:GFP transgene identifies dorsal root ganglion and some cells in dorsal horns. Interestingly some GFP positive fibers in ngn1:GFP fish are located around Mauthner axons and their density seems to be related to a rostrocaudal gradient. Many other cell types have been described in embryos and need to be studied in adults. Our findings provide a reference for further studies on spinal cytoarchitecture. Combined with physiological, histological and pathological/traumatic approaches, these studies will help clarify the operation of spinal locomotor circuits of adult zebrafish.