Comparative distribution of the mRNAs encoding urotensin I and urotensin II in zebrafish

The neural neurosecretory system of fishes produces two biologically active neuropeptides, i.e. the corticotropin-releasing hormone paralog urotensin I (UI) and the somatostatin-related peptide urotensin II (UII). In zebrafish, we have recently characterized two UII variants termed UIIα and UIIβ. In the present study, we have investigated the distribution of UI, UIIα and UIIβ mRNAs in different organs by quantitative RT-PCR analysis and the cellular localization of the three mRNAs in the spinal cord by in situ hybridization (ISH) histochemistry. The data show that the UI gene is mainly expressed in the caudal portion of the spinal cord and, to a lesser extent, in the brain, while the UIIα and the UIIβ genes are exclusively expressed throughout the spinal cord. Single-ISH labeling revealed that UI, UIIα and UIIβ mRNAs occur in large cells, called Dahlgren cells, located in the ventral part of the caudal spinal cord. Double-ISH staining showed that UI, UIIα and UIIβ mRNAs occur mainly in distinct cells, even though a few cells were found to co-express the UI and UII genes. The differential expression of UI, UIIα and UIIβ genes may contribute to the adaptation of Dahlgren cell activity during development and/or in various physiological conditions.