GABAergic neuronal ontogeny in the central nervous system of the viviparous teleost Poecilia sphenops.

The neurotransmitter gamma-aminobutyric acid (GABA) plays essential roles in vertebrate neurodevelopment and neuroendocrine regulation. However, the post-embryonic ontogeny of the GABAergic system remains relatively poorly characterized, particularly in viviparous teleosts. In this study, we describe postnatal GABAergic development across the central nervous system of Poecilia sphenops from birth to 120 days post-birth (DPB). Immunofluorescence labeling revealed that GABAergic neurons were present at birth predominantly in the spinal cord and hindbrain, indicating early maturation of circuits supporting sensorimotor, visceral, and autonomic functions. During juvenile development, GABA-immunoreactive (GABA-ir) neurons progressively extended into mesencephalic and diencephalic regions, with neuronal differentiation generally preceding the incorporation of GABA-ir fibre systems in midbrain integration centres and neuroendocrine forebrain nuclei. The telencephalon exhibited delayed maturation, with GABAergic neurons initially restricted to limited subdivisions at birth and broader neuronal and fibre differentiation occurring during later juvenile stages, whereas the olfactory bulb showed a complete absence of GABAergic immunoreactivity at birth, with GABAergic elements emerging only during juvenile development. Inferior lobe and gustatory hindbrain nuclei retained early and persistent GABA-ir populations, whereas hypophysiotropic and pituitary-projecting elements differentiated only during advanced juvenile phases. Quantitative morphometric analysis revealed prolonged, nucleus-specific enlargement of GABAergic neuronal somata, indicating extended postnatal maturation of GABAergic circuits. Together, these findings demonstrate a rostrocaudal and functional gradient in the emergence and maturation of GABAergic systems in P. sphenops, with GABAergic circuits maturing earliest in caudal sensorimotor regions and latest in forebrain and neuroendocrine areas.