Developing neurons are guided to their appropriate targets by specific guidance substances that have neurotrophic actions. The aim of the present study was to elucidate the mechanism by which hypothalamic neurons reach the pars intermedia (PI) by correlating the development of dopaminergic (DA) neurons arising in the periventricular nucleus (PeV) of fetal rats with the expression of brain-derived neurotrophic factor (BDNF) in the rat pituitary. The differentiation of DA neurons was observed by immunohistochemistry using an antibody against tyrosine hydroxylase (TH), whereas the ontogenesis of BDNF mRNA in the PI was examined by in situ hybridization and RT-PCR. Immunoreactive TH-neurons were first observed in the PeV at embryonic day (E) 16.5, following which time their axons elongated toward the pituitary. TH-positive reactions were observed in the connective tissue between the PI and the pars nervosa at E20.5. Innervation of the PI by TH-positive neurons was determined at postnatal day (P) 1.5; however, BDNF mRNA was first detected in the PI cells at E17.5, with an increase in its expression clearly visible at E21.5 and continuing high expression levels in the PI thereafter. These results suggest that BDNF is a specific guidance cue for DA neurons elongating from the PeV to the PI.
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