Immunocytochemical localization of monoaminergic neurons in the brain of grass parakeet (Melopsittacus undulatus)

We have elucidated the distribution of tyrosine hydroxylase (TH)-positive but aromatic L-amino acid decarboxylase (AADC)-negative neurons [distinctive TH-immunoreactive (IR) neurons], and AADC-positive but TH- or serotonin (5HT)-negative neurons (distinctive AADC-IR neurons) of the grass parakeet brain, using the immunocytochemical technique with antibodies to TH, AADC, and 5HT. The distinctive TH-IR neurons were found in the substantia grisea centralis of the ventral part of posterior commissure at the level of corpus mammilare. The distinctive AADC-IR neurons were found in the spinal cord, nucleus of the solitary tract, nucleus arcuatus, and around the anterior commissure. By electron microscopy, TH- and AADC-positive reaction products were distributed throughout the cytoplasmic matrix, but not in the nucleus, mitochondria, and Golgi complex. AADC-IR products of the nucleus arcuatus were also observed in the granules of the perikaria and axon terminals. The number of the distinctive AADC-IR neuron groups in the grass parakeet brain was smaller than those in the rat brain. This result suggests the specious difference between mammals and birds.