Zebrafish (Danio rerio) is a teleost fish widely accepted as a model organism for neuroscientific studies. The adults show common basic vertebrate brain structures, together with similar key neuroanatomical and neurochemical pathways of relevance to human diseases. However, the brain of adult zebrafish possesses, differently from mammals, intense neurogenic activity, which can be correlated with high regenerative properties. Brain derived neurotrophic factor (BDNF), a member of the neurotrophin family, has multiple roles in the brain, due also to the existence of several biologically active isoforms, that interact with different types of receptors. BDNF is well conserved in the vertebrate evolution, with the primary amino acid sequences of zebrafish and human BDNF being 91% identical. Here, we review the available literature regarding BDNF in the vertebrate brain and the potential involvement of BDNF in telencephalic regeneration after injury, with particular emphasis to the zebrafish. Finally, we highlight the potential of the zebrafish brain as a valuable model to add new insights on future BDNF studies.

Several mammalian animal models of traumatic brain injury have been used, mostly rodents. However, reparative mechanisms in mammalian brain are very limited, and newly formed neurons do not survive for long time. The brain of adult zebrafish, a teleost fish widely used as vertebrate model, possesses high regenerative properties after injury due to the presence of numerous stem cells niches. The ventricular lining of the zebrafish dorsal telencephalon is the most studied neuronal stem cell niche because its dorso-lateral zone is considered the equivalent to the hippocampus of mammals which contains one of the two constitutive neurogenic niches of mammals. To mimic TBI, stab wound in the dorso-lateral telencephalon of zebrafish was used in studies devoted to fish regenerative properties. Brain-derived neurotrophic factor, which is known to play key roles in the repair process after traumatic brain lesions, persists around the lesioned area of injured telencephalon of adult zebrafish. These results are extensively compared to reparative processes in rodent brain. Considering the complete repair of the damaged area in fish, it could be tempting to consider brain-derived neurotrophic factor as a factor contributing to create a permissive environment that enables the establishment of new neuronal population in damaged brain.

Among the most-used fish species in aquaculture is the Nile tilapia, due to its rapid growth rate and its adaptation to a wide range of farming conditions. A careful description of the morphology of the digestive tract, particularly the esophagus and stomach, allows a better understanding of the relationship between structure and function. Combining scanning and light microscopy we highlighted the presence of five different zones in the stomach (1: esophagus-gastric lumen passage; 2: descending glandular portion; 3: fundic portion; 4: ascending glandular portion; 5: gastric-pyloric transition portion). Histochemical investigation showed a secretion of carboxylates mucopolysaccharides along the esophagus and sulphated complex carbohydrates in the stomach. These results suggest that mucins play a protective role of the epithelial lining, which is essential for a correct digestive process. Finally, the characterization of the main cellular structures may be inspiring for more advanced studies aiming to decipher the role of specific molecules, such as neuropeptides, involved in the physiological digestive process.

Nerve growth factor (NGF) receptors are evolutionary conserved molecules, and in mammals are considered necessary for ensuring the survival of cholinergic neurons. The age-dependent regulation of NTRK1/NTRKA and p75/NGFR in mammalian brain results in a reduced response of the cholinergic neurons to neurotrophic factors and is thought to play a role in the pathogenesis of neurodegenerative diseases. Here, we study the age-dependent expression of NGF receptors (NTRK1/NTRKA and p75/NGFR) in the brain of the short-lived teleost fish Nothobranchius furzeri. We observed that NTRK1/NTRKA is more expressed than p75/NGFR in young and old animals, although both receptors do not show a significant age-dependent change. We then study the neuroanatomical organization of the cholinergic system, observing that cholinergic fibers project over the entire neuroaxis while cholinergic neurons appear restricted to few nuclei situated in the equivalent of mammalian subpallium, preoptic area and rostral reticular formation. Finally, our experiments do not confirm that NTRK1/NTRKA and p75/NGFR are expressed in cholinergic neuronal populations in the adult brain of N. furzeri. To our knowledge, this is the first study where NGF receptors have been analyzed in relation to the cholinergic system in a fish species along with their age-dependent modulation. We observed differences between mammals and fish, which make the African turquoise killifish an attractive model to further investigate the fish specific NGF receptors regulation.