Characterization of the Spinal Cord of the Annual Fish Garcialebias charrua: Morphology, Cell Proliferation, and NADPH-Diaphorase Activity.

The spinal cord plays a central role in sensorimotor integration and exhibits substantial diversity across vertebrates in relation to ecological and behavioral demands. In teleost fish, however, detailed morphological and cellular analyses of the adult spinal cord remain scarce. Here, we provide the first comprehensive characterization of the adult spinal cord of the annual fish Garcialebias charrua, a species displaying pronounced environmental adaptation and sexual dimorphism. Using adult males and females, the spinal cord was systematically partitioned into five equally sized rostrocaudal regions (SI-SV) to evaluate regional variation in morphology, neurochemical organization, and cell proliferation. We analyzed gross morphology and cross-sectional features, the distribution and morphology of NADPH-diaphorase-positive (NADP-d+) neurons as indicators of nitric oxide-related signaling, and proliferative activity using 5-bromo-2'-deoxyuridine (BrdU) and 5-ethynyl-2'-deoxyuridine (EdU) incorporation. Our results reveal marked rostrocaudal heterogeneity in spinal cord morphometry, with dimorphic variation in the segment SIII associated with the dorsal fin, region-specific patterns of NADPH-d+ neuronal populations, and sustained cell proliferation throughout the entire spinal cord in both gray and white matter. Focused analysis of segment SIII, which exhibits distinctive anatomical features, demonstrated higher proliferative activity in the central canal and dorsal regions compared to ventral areas in both sexes, with males showing significantly increased proliferation across all analyzed regions. Finally, the combination of BrdU labeling with a neuronal lineage marker provides the first evidence of adult spinal cord neurogenesis in G. charrua. These findings highlight the spinal cord as a dynamic and sexually dimorphic substrate underlying neuroplasticity in annual fishes.