Intraspinal serotonergic neurons consist of two, temporally distinct populations in developing zebrafish

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Zebrafish intraspinal serotonergic neuron (ISN) morphology and distribution have been examined in detail at different ages; however, some aspects of the development of these cells remain unclear. Although antibodies to serotonin (5-HT) have detected ISNs in the ventral spinal cord of embryos, larvae, and adults, the onlytryptophan hydroxylase(tph) transcript that has been described in the spinal cord istph1a. Paradoxically, spinaltph1ais only expressed transiently in embryos, which brings the source of 5-HT in the ISNs of larvae and adults into question. Because thepet1andtph2promoters drive transgene expression in the spinal cord, we hypothesized thattph2is expressed in spinal cords of zebrafish larvae. We confirmed this hypothesis throughin situhybridization. Next, we used 5-HT antibody labeling and transgenic markers oftph2-expressing neurons to identify a transient population of ISNs in embryos that was distinct from ISNs that appeared later in development. The existence of separate ISN populations may not have been recognized previously due to their shared location in the ventral spinal cord. Finally, we used transgenic markers and immunohistochemical labeling to identify the transient ISN population as GABAergic Kolmer-Agduhr double-prime (KA″) neurons. Altogether, this study revealed a novel developmental paradigm in which KA″ neurons are transiently serotonergic before the appearance of a stable population oftph2-expressing ISNs. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 673–687, 2016

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