In our previous study, the presence and distribution of nerve growth factor (NGF) were described in adult zebrafish brain [1]. In mammals, the signaling of mature NGF (mNGF) is transduced by the tropomyosin receptor kinase A (TrkA) and acts as mediator in many essential functions in the central nervous system. Since TrkA expression was described in developing zebrafish [2], the aim of the present study is to describe the pattern of distribution of TrkA mRNA in the brain of adult zebrafish. The investigation was conducted on brain fixed in paraformaldehyde. Sections were firstly incubated overnight with digoxigenin-labeled riboprobes for TrkA [2] and then with anti-digoxigenin Fab fragments conjugated with alkaline phosphatase. The chromogenic reaction was carried out by using Fast Red substrate. After counterstaining with DAPI, sections were visualized in fluorescence microscopy. In the telencephalon, TrkA mRNA was found in the olfactory bulbs, specifically in the glomerular layer and in the external cellular layer. In addition, some positive round small neurons were seen in the regions of the dorsal area and rare positive neurons in the ventral area. In the diencephalon, TrkA mRNA was seen in some neurons of the hypothalamus, in few neurons of mammillary body and synencephalon. In the mesencephalon, TrkA mRNA was detected in neurons of the optic tectum, specifically in the periventricular grey zone and superficial grey and white zone, and in neurons of the semicircular tori. Concerning rhombencephalon, TrkA mRNA was highly present in neurons of all regions of the cerebellum, and in neurons of cerebellar crest. Our study highlighted the wide expression of TrkA receptor throughout all brain regions of adult zebrafish. In mammals, this receptor mediates the action of mNGF [3]. However, in zebrafish brain extracts, only the immature form of NGF (pro NGF) was found. These findings suggest a different NGF signaling in zebrafish or the presence of low and fleeting undetectable quantities of mNGF. Cacialli P et al. Nerve growth factor is expressed and stored in central neurons of adult zebrafish. J Anat. Apr 4. doi: 10.1111/joa.12986, 2019. [2] Nittoli V. et al. A comprehensive analysis of neurotrophins and neurotrophin tyrosine kinase receptors expression during development of zebrafish. J Comp Neurol, 526:1057–1073, 2018. [3] Bucci C et al. The Role of Rab Proteins in Neuronal Cells and in the Trafficking of Neurotrophin Receptors. Membranes 4(4), 642-677; 2014.