Secretagogin immunoreactivity in the forebrain, midbrain and pituitary of the three spot gourami, Trichopodus trichopterus (Pallas, 1770).

Secretagogin (SCGN), a calcium-binding protein (CBP) implicated in neuroendocrine signaling, was investigated for its spatial distribution in the forebrain, midbrain, and pituitary of the three spot gourami, Trichopodus trichopterus, a representative labyrinthine teleost. Using immunohistochemistry, SCGN-immunoreactive (SCGN-ir) neurons and fibers were comprehensively mapped across forebrain and midbrain regions. In the telencephalon, strong SCGN expression was observed in the olfactory bulb, particularly within the granule cell layer, along with notable labeling in subdivisions of the pallium and subpallium. Distinct SCGN-ir cell populations were identified in the magnocellular preoptic nucleus and the nucleus preopticus periventricularis. In the hypothalamus, SCGN-positive cells were distributed throughout the dorsal, ventral, and caudal regions, with extension into adjacent thalamic structures such as the paraventricular organ. Additionally, dense SCGN-ir fibers were evident in the optic tectum. In the pituitary, intense SCGN immunoreactivity was localized to the rostral and proximal pars distalis, whereas the pars intermedia showed no labeling. This study represents the first detailed neuroanatomical mapping of SCGN in a labyrinthine teleost and reveals a widespread but region-specific pattern of SCGN localization within sensory and neuroendocrine-associated brain regions.