Localization, distribution and expression of growth hormone in the brain of Asian Catfish, Clarias batrachus.

Intense immunoreactivity was observed in several neurons of the nucleus preopticus (NPO) located in the preoptic area (POA), in addition to several GH cells in the proximal pars distalis (PPD) of the pituitary gland of Clarias batrachus. The immunoreactive cells were located in the paraventricular as well as supraoptic subdivisions of the NPO. GH immunoreactive fibers projecting from the neurons were traced caudally to the pituitary gland via the conspicuous preoptico-hypophysial tract (PHT) in the ventral tuberal area to the neurohypophysis of the pituitary. Apart from these immunoreactive fibers in the preoptico-hypophysial tract, some fine caliber fiber probably arising from the neurons located dorsally in the NPO also showed GH immunoreactivity, and these fibers constituted an independent tract. Bilaterally, it extended caudally through the dorsal hypothalamus almost as far as the saccus vasculosus where it curved sharply to descend into the caudal tuberal region and finally entered into the pituitary gland. The fibers of this tract were mainly distributed in the rostral pars distalis (RPD). This tract is quite distinct from the preoptico-hypophysial tract and herein called as the accessory preoptico-hypophysial tract (APHT). Expression of GH mRNA in the NPO was found 65-fold more than that of the control region, rostral cerebellum. These results altogether suggest that GH secreted by NPO neurons might serve as a neuro-modulatory role in the brain of C. batrachus. Transportation of GH to the pituitary via two independent tracts may represent the duality of neuroendocrine function. The present study underscores the possibility that GH in the brain of vertebrates may be a phylogenetically conserved phenomenon and provide clues to our understanding of the evolutionary course taken by the hormone.