Tyrosine hydroxylase containing neurons lacking aromatic amino acid decarboxylase in the hamster brain

We have recently described populations of tyrosine hydroxylase-immunoreactive neurons in the hamster brain in regions not known to contain catecholamine cell bodies. In the present study, the nature of the tyrosine hydroxylase immunoreactivity in the hamster brain was determined. In addition, these tyrosine hydroxylase-immunoreactive cell groups were examined for their ability to express aromatic amino acid decarboxylase. Immunohistochemistry with two different antibodies to tyrosine hydroxylase identified immunoreactive cell bodies in regions known to contain catecholamine neurons, including the substantia nigra and locus ceruleus. In addition, tyrosine hydroxylase-immunoreactive neurons were observed in other regions, including the basal forebrain, inferior colliculus, lateral parabrachial nucleus, and dorsal motor nucleus of the vagus. Western blotting indicated that hamster brain contained only one immunoreactive molecule, very similar in size to rat tyrosine hydroxylase. Thus it is likely that the immunohistochemical studies stained authentic hamster tyrosine hydroxylase. Indeed, in situ hybridization studies using a synthetic oligonucleotide probe against tyrosine hydroxylase mRNA resulted in specific and heavy labelling of these novel tyrosine hydroxylase-immunoreactive neurons. When adjacent sections were stained with antibodies to aromatic amino acid decarboxylase, known catecholamine cell groups were stained. However, the novel tyrosine hydroxylase cell groups did not display any aromatic amino acid decarboxylase immunoreactivity. These results suggest that neurons are present in the hamster brain that are able to hydroxylate tyrosine to L-DOPA, but that lack the ability to decarboxylate aromatic amino acids to produce dopamine or other catecholamines.