Injury to neuronal tissues in the central nervous system (CNS) of mammals results in neural degeneration and sometime leads to loss of function, whereas fish retain a remarkable potential for neuro-regeneration throughout life. Thus, understanding the mechanism of neuro-regeneration in fish CNS would be useful to improve the poor neuro-regenerative capability in mammals. In the present study, we characterized a neuro-regenerative process in the brain of a cichlid, tilapia, Oreochromis niloticus. Morphological observations showed that the damaged brain region (habenula) successfully regrew and reinnervated axonal projections by 60 days post-damage. A fluorescent carbocyanine tracer, DiI tracing revealed a recovery of the major neuronal projection from the regenerated habenula to the interpenduncular nucleus by 60 days post-damage. TUNEL assay showed a significant increase of apoptotic cells (~234%, P<0.01) at one day post-damage, while the number of bromodeoxyuridine (BrdU)-positive proliferative cells were significantly increased (~92%, P<0.05) at 7 days post-damage compared with sham-control fish. To demonstrate a potential role of apoptotic activity in the neuro-regeneration, effects of degenerative neural tissue on cell proliferation were examined in vivo. Implantation of detached neural but not non-neural tissues into the cranial cavity significantly (P<0.01) increased the number of BrdU-positive cells nearby the implantation regions at 3 days after the implantation. Furthermore, local injection of the protein extract and cerebrospinal fluid collected from injured fish brain significantly induced cell proliferation in the brain. These results suggest that factor(s) derived from apoptotic neural cells may play a critical role in the neuro-regeneration in teleost brain.
Dietary organic acids are increasingly being investigated as a potential means of improving growth and nutrient utilization in aquatic animals. A 9-week study was performed to compare equal amounts (2%) of different organic acids (sodium butyrate, acetate, propionate, or formate) on the growth, muscle proximate composition, fatty acid composition, cholesterol and lipid peroxidation, differential cell counts, plasma biochemistry, intestinal short-chain fatty acid (SCFA) level, and liver histopathology to red hybrid tilapia (Oreochromis sp.) (initial mean weight of 2.87 g). A second experiment was performed to determine their effects on lipid peroxidation and trimethylamine (TMA) when added at 1% to tilapia meat and left out for 24 h. The results of the first experiment showed no treatment effect to growth, feeding efficiencies, or muscle fatty acid composition, but all dietary organic acids significantly decreased intestinal SCFA. Dietary butyrate and propionate significantly decreased muscle lipid peroxidation compared to the control group, but the dietary formate treatment had the lowest lipid peroxidation compared to all treatments. Muscle crude protein and lipid in tilapia fed the formate diet were significantly lower and higher, respectively, and showed evidence of stress based on the differential cell counts, significantly higher plasma glucose and liver glycogen, as well as inflammatory responses in the liver. Although a potential benefit of dietary organic acids was a reduction to lipid peroxidation, this could be accomplished post-harvest by direct additions to the meat. In addition, inclusions of butyrate and propionate to tilapia meat significantly decreased TMA, which might be a more cost-effective option to improve the shelf life of tilapia products.
Stress impairs the hypothalamic-pituitary-gonadal (HPG) axis, probably through its influence on the hypothalamic-pituitary-adrenal (= interrenals in the teleost, HPI) axis leading to reproductive failures. In this study, we investigated the response of hypothalamic neuropeptides, gonadotropin-inhibitory hormone (GnIH), a component of the HPG axis, and corticotropin-releasing hormone (CRH) a component of the HPI axis, to acute social defeat stress in the socially hierarchical male Nile tilapia (Oreochromis niloticus). Localization of GnIH cell bodies, GnIH neuronal processes, and numbers of GnIH cells in the brain during acute social defeat stress was studied using immunohistochemistry. Furthermore, mRNA levels of GnIH and CRH in the brain together with GnIH receptor, gpr147, and adrenocorticotropic hormone (ACTH) in the pituitary were quantified in control and socially defeated fish. Our results show, the number of GnIH-immunoreactive cell bodies and GnIH mRNA levels in the brain and the levels of gpr147 mRNA in the pituitary significantly increased in socially defeated fish. However, CRH and ACTH mRNA levels did not change during social defeat stress. Further, we found glucocorticoid type 2b receptor mRNA in laser captured immunostained GnIH cells. These results show that acute social defeat stress activates GnIH biosynthesis through glucocorticoid receptors type 2b signalling but does not change the CRH and ACTH mRNA expression in the tilapia, which could lead to temporary reproductive dysfunction.