IMMUNOHISTOCHEMICAL ANALYSIS OF THE RELATION BETWEEN 5-HYDROXYTRYPTAMINE-IMMUNOREACTIVE AND NEUROPEPTIDE-IMMUNOREACTIVE ELEMENTS IN THE SPINAL-CORD OF AN AMPHIBIAN (XENOPUS-LAEVIS)

In mammals, a large proportion of the bulbospinal 5-hydroxytryptamine (5-HT) neurons also contain neuropeptides, such as substance P (SP) an d galanin (GAL). To examine whether a similar coexistence occurs in an amphibian, an immunofluorescence double-labelling technique was emplo yed on sections of the Xenopus laevis spinal cord. Antisera raised aga inst SP, GAL, enkephalin (ENK), corticotropin-releasing factor (CRF), calcitonin gene-related peptide (CGRP), and cholecystokinin (CCK) prod uced a labelling of fibers at all rostrocaudal levels of the spinal co rd, with the highest fiber densities for SP and ENK and intermediate d ensities for GAL, CCK, and CGRP, while CRF-immunoreactive fibers were barely detectable in intact animals. 5-HT-immunoreactive fibers were w idely distributed in the spinal cord, and they often occurred in the v icinity of different types of peptide-immunoreactive fibers. However, no coexistence between 5-HT and the different peptide immunoreactiviti es could be detected, although SP and GAL immunoreactivities were some times found to be colocalized in the same fiber. Similar negative resu lts were obtained when 5-HT + SP- and 5-HT + GAL-labelled sections wer e examined in single focal planes with a confocal microscope. After a spinal transection, (survival period 6 weeks to 4 months), almost all 5-HT-immunoreactive fibers below the lesion were lost, and a build-up of immunoreactive material occurred in fibers just rostral to the cut. In contrast, no significant loss of peptide-immunoreactive fibers occ urred, although some swollen SP-, GAL-, ENK-, CRF-, and CCK-immunoreac tive fibers were present rostral to the cut. The distribution of swoll en peptide-immunoreactive fibers did not overlap with that of the swol len 5-HT-immunoreactive fibers. Although negative immunohistochemical data must be interpreted with caution, in conjunction with previous st udies (Brodin et al. [1988] J. Comp. Neurol. 271:1-18; Sakamoto and At sumi [1991] Cell Tissue Res. 264:221-230), the present results indicat e that bulbospinal 5-HT neurons in nonmammalian vertebrates cocontain neuropeptides to a lesser extent than in mammals. (C) 1994 Wiley-Liss, Inc.