Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, plays an important role in neuronal network plasticity by promoting differentiation and survival of neurons during development and in adult brain. BDNF signals by activating its cognate receptor tyrosine kinase B, NTRK2 and has trophic effects on the serotonergic system. The exact role of NTRK2 in the behavioral effects of serotonin remains poorly understood. Alterations in the BDNF-NTRK2 signaling and/or in 5-HT neurons have been implicated in the pathophysiology of psychiatric or mood disorders. This study pursues to interrogate the role of NTRK2 in brain development and more specifically the effects on the serotonergic system. The aim of this study is to understand the developmental effects of TrkB in the zebrafish model. The TrkB receptor has two forms in zebrafish Ntrk2a and Ntrk2b. The Ntrk2a is not found in significant amounts therefore the focus has been mostly on Ntrk2b.
The mRNA expression of Ntrk2b was assessed by in-situ hybridization in the larval stages and adult brain of zebrafish. Wide distribution of the mRNA was observed in the CNS such as dorsal telencephalon, diencephalon, in the parvocellular pre-optic nucleus, hypothalamus, positive radial glial cells lining the mesencephalic ventricle, cerebellum and dispersed positive fibers in the medulla oblongata. The expression of mRNA for BDNF complemented with specific Ntrk2b expression pattern. The morpholino oligonucleotides against Ntrk2b was designed and validated by western blotting using Trk specific antibody. The Ntrk2b morphants had no major gross phenotype and the motor behavior did not change significantly as compared to the controls. There was a significant difference in the monoamines levels of dopamine and serotonin observed in the Ntrk2b morphants. Immunoreactivity using antibodies for serotonin, GABA1H and tyrosine hydroxylase was reduced in the morphants as compared to the controls. The expression level of serotonin transporter (serta), and tyrosine hydroxylase (th1) was reduced in the morphants. The transient loss could be rescued by over expression of full length Ntrk2b mRNA. No major change in cell death in the Ntrk2b morphants was observed by TUNEL staining method. These results reveal potential role of NTRK2 during development and its effect on the monoaminergic system. The results establish that zebrafish Ntrk2b has wide spread distribution throughout development and loss of Ntrk2b plays an important role in monoamines mainly the serotonergic and the dopaminergic system.