Transient tyrosine hydroxylase-immunoreactive neurons in the region of the anterior olfactory nucleus of pre- and postnatal mice do not contain dopamine.

The transient appearance of tyrosine hydroxylase (TH)-immunoreactive (IR) neurons in the anterior olfactory nuclear region of mice was investigated using TH-immunocytochemistry. In this region, a new cell group composed of a small number of neurons immunoreactive to TH was demonstrated for the first time from the embryonic stages of E16-E18. These cells were not shown using antisera against aromatic L-amino acid decarboxylase, dopamine-beta-hydroxylase, phenylethanolamine-N-methyl-transferase, dopamine or serotonin. TH-IR cells progressively increased in number until birth. After birth the numbers reached maximum at postnatal days 9-12 and decreased until 4 weeks old, and then mostly disappeared at 6 weeks. Only single TH-IR cells were occasionally observed in this brain area of adult mice. Ultrastructurally some of these TH-IR neurons had immature Golgi apparatus, only a few mitochondria and deformed nuclei along with thin cytoplasma. Some other TH-IR cells, however, had mature Golgi apparatus, many mitochondria and a round nucleus more closely resembling mature cells. These neurons do not belong to the dopaminergic neuron system, because they lack dopamine production, and may be tentatively involved in early limbic circuits.