The sea lamprey tyrosine hydroxylase: cDNA cloning and in situ hybridization study in the brain

Lampreys belong to the oldest group of extant vertebrates, the agnathans or cyclostomes. Thus, they occupy a key phylogenetic position near the root of the vertebrate tree, which makes them important to the study of nervous system evolution. Tyrosine hydroxylase is the rate-limiting enzyme of catecholamine biosynthesis and is considered a marker of catecholaminergic neurons. In the present study, we report partial cloning of the sea lamprey tyrosine hydroxylase (TH) cDNA and the pattern of TH transcript expression in the adult brain by means of in situ hybridization. Sea lamprey TH mRNA is characterized by the presence of a large untranslated sequence in the 3′ end that contains a typical polyadenylation signal (ATTAAA). The deduced partial TH protein sequence presents a conserved domain with two His residues coordinating Fe2+ binding and a conserved cofactor binding site. Neurons expressing the TH transcript were observed in the preoptic, postoptic commissure, dorsal hypothalamic, ventral hypothalamic, mammillary and paratubercular nuclei of the prosencephalon. In situ hybridization experiments also confirmed the existence of a catecholaminergic (dopaminergic) striatal population in the brain of the adult sea lamprey. A few granule-like cells in the olfactory bulbs also showed weak TH transcript expression. No cells showing TH transcript expression were observed in the rostral rhombencephalon, which suggests the absence of a locus coeruleus in the sea lamprey. Comparison of the pattern of TH mRNA expression in the prosencephalon between lampreys and teleost fishes revealed both similarities and differences. Our results suggest that the duplication of the TH gene might have occurred before the separation of agnathans and gnathostomes.