Immunocytochemical localization of APUD cells in the hypothalamus of the grass parakeet (Melopsittacus undulatus) and the lizard (Varanus exanthematicus)

We investigated the localization of APUD (amine content and amine precursor uptake and decarboxylation) cells and tyrosine hydroxylase (TH)-negative, aromatic L-amino acid decarboxylase (AADC)-positive neurons (D neurons) in the hypothalamus of the bird (Melopsittacus undulatus) and the lizard (Varanus exanthematicus), and compared the differences between the two species, L-DOPA was demonstrated in the paraventricular organ (PVO) which is a characteristic region in the hypothalamus of the bird and the lizard. In the PVO of the grass parakeet, there were a small number of dopamine neurons in addition to APUD cells. However, in the PVO of the lizard, there were APUD cells but no dopamine neurons. Furthermore, TH-negative, AADC-positive neurons (D neurons) were found in the medial part of nucleus arcuatus of the bird, but not of the lizard.