**Identification and Expression of Neurotrophin-6 in the Brain of** *Nothobranchius furzeri***: One More Piece in Neurotrophin Research**

**Adele Leggieri 1,**†**, Chiara Attanasio 1,2,3,**†**, Antonio Palladino 2, Alessandro Cellerino 4,5, Carla Lucini 1, Marina Paolucci 6, Eva Terzibasi Tozzini 4, Paolo de Girolamo <sup>1</sup> and Livia D'Angelo 1,7,\***


Received: 13 March 2019; Accepted: 22 April 2019; Published: 30 April 2019

**Abstract:** Neurotrophins contribute to the complexity of vertebrate nervous system, being involved in cognition and memory. Abnormalities associated with neurotrophin synthesis may lead to neuropathies, neurodegenerative disorders and age-associated cognitive decline. The genome of teleost fishes contains homologs of some mammalian neurotrophins as well as a gene coding for an additional neurotrophin (NT-6). In this study, we characterized this specific neurotrophin in the short-lived fish *Nothobranchius furzeri*, a relatively new model for aging studies. Thus, we report herein for the first time the age-related expression of a neurotrophin in a non-mammalian vertebrate. Interestingly, we found comparable expression levels of NT-6 in the brain of both young and old animals. More in detail, we used a locked nucleic acid probe and a riboprobe to investigate the neuroanatomical distribution of NT-6 mRNA revealing a significant expression of the neurotrophin in neurons of the forebrain (olfactory bulbs, dorsal and ventral telencephalon, and several diencephalic nuclei), midbrain (optic tectum, longitudinal tori, and semicircular tori), and hindbrain (valvula and body of cerebellum, reticular formation and octavolateral area of medulla oblongata). By combining in situ hybridization and immunohistochemistry, we showed that NT-6 mRNA is synthesized in mature neurons. These results contribute to better understanding the evolutionary history of neurotrophins in vertebrates, and their role in the adult brain.

**Keywords:** neurotrophin-6; phylogeny; LNA probe; riboprobe; neuroanatomy; fish; aging
