A confocal study of spinal interneurons in living larval zebrafish

Abstract

We used confocal microscopy to examine the morphology of spinal interneurons in living larval zebrafish with the aim of providing a morphological foundation for generating functional hypotheses. Interneurons were retrogradely labeled by injections of fluorescent dextrans into the spinal cord, and the three-dimensional morphology of living cells was reconstructed from confocal optical sections through the transparent fish. At least eight types of interneurons are present in the spinal cord of larval zebrafish; four of these are described here for the first time. The newly discovered cell types include classes of commissural neurons with axons that ascend, descend, and bifurcate in the contralateral spinal cord. Our reexamination of previously described cell types revealed functionally relevant features of their morphology, such as undescribed commissural axons, as well as the relationships between the trajectories of the axons of interneurons and the descending Mauthner axons. In addition to describing neurons, we surveyed their morphology at multiple positions along the spinal cord and found longitudinal changes in their distribution and sizes. For example, some cell types increase in size from rostral to caudal, whereas others decrease. Our observations lead to predictions of the roles of some of these interneurons in motor circuits. These predictions can be tested with the combination of functional imaging, single-cell ablation, and genetic approaches that make zebrafish a powerful model system for studying neuronal circuits.

Nicotinic Receptors Mediate Changes in Spinal Motoneuron Development and Axonal Pathfinding in Embryonic Zebrafish Exposed to Nicotine

“…Nicotine exposure also results in a delay in this programmed downregulation of the znS5 marker, because somatic labeling persists in 66 hpf embryos. This provides a third line of evidence that nicotine delays motoneuron differentiation; it also indicates that nicotine delays differentiation of other dorsal spinal neurons (likely interneurons) (Kuwada et al, 1990;Hale et al, 2001). Clearly, the effects of nicotine are not restricted to secondary motoneuron development in embryonic/larval zebrafish.…”

“…It is known that GFP expression is confined to a subclass of secondary motoneurons (Higashijima et al, 2000) in embryos older than 3 d postfertilization. During early developmental stages, some spinal cord interneurons express GFP; however, they are easily distinguished from secondary motoneurons because they lie dorsal to the primary and secondary motoneurons (Kuwada et al, 1990;Hale et al, 2001). Secondary motoneurons that express GFP in the islet-1 transgenic embryo are identified according to their ventral positions in spinal cord and by the fact that their axons can be easily detected innervating either the ventral or dorsal musculature of the embryo.…”

Generation of Multiple Classes of V0 Neurons in Zebrafish Spinal Cord: Progenitor Heterogeneity and Temporal Control of Neuronal Diversity

“…3 E, G). Unipolar V0-eD neurons fulfill all the hallmarks of the unipolar commissural descending (UCoD) neurons that were previously identified by backfilling studies (Hale et al, 2001;Higashijima et al, 2004b). The features include: (1) unipolar, mediolaterally elongate cell bodies, (2) medially located somata midway along the dorso-ventral axis (soma position ϭ 0.49 Ϯ 0.06, n ϭ 16, Fig.…”

Six cadm/synCAM genes are expressed in the nervous system of developing zebrafish

“…Finally, the ventral domain comprises floor plate cells and motoneurons. The morphology, localization, and axon projections of zebrafish spinal cord neurons have been well described (Hale et al, 2001;Lewis and Eisen, 2003).…”