Urotensin I-like immunoreactivity in the central nervous system of Aplysia californica

In the present study the occurrence and localization of urotensin I (UI, a corticotropin releasing factor-like peptide) in the CNS of Aplysia californica were investigated by immunocytochemistry and radioimmunoassay. The RIA cross-reactivity pattern indicated that the UI antiserum used recognized an epitope in the C-terminal region of the UI, but it did not cross-react with mammalian corticotropin-releasing factor (CRF) and partially recognized sauvagine (SVG, a frog CRF-like peptide). The use of CRF-specific and sauvagine-specific antisera failed to give positive immunostaining. The application of UI antiserum (which does not cross-react with CRF in RIA) gave a positive staining, which was blocked by synthetic sucker ( Catostomus commersoni) UI, but not by rat/human CRF (10 μM). On the basis of immunostaining and RIA parallel to fish UI displacement curves of cerebral ganglia extracts, the unknown UI/CRF-like substance in the Aplysia ganglia is likely to have greater homology with sucker UI than with the known CRF peptides. Urotensin I-immunoreactive (UI-ir) neurons were seen mainly in the F neuron clusters, located in the midline and rostrodorsal portion of the cerebral ganglia. Few UI-ir neurons were also found in the C and D neuron clusters of the cerebral ganglia, as well as in the left pleural and abdominal ganglia. In addition, numerous fine and coarse, and beaded UI-ir fibers were found in the cerebral commissure. UI-ir fibers were also seen in the neuropile of the buccal, pedal and pleural ganglia, and abdominal ganglion. A cuff-like arrangement of UI-ir fibers was seen in the supralabial nerves. Fewer fibers were seen in the anterior tentacular nerves. No UI-ir fibers were seen in other nerves. Coarse and fine UI-ir fibers in the connectives from the circumesophageal ganglia suggest that a putative UI-like peptide may affect the function of these ganglia.