A Na+/Cl--dependent transporter for catecholamines, identified as a norepinephrine transporter, is expressed in the brain of the teleost fish medaka (Oryzias latipes)

We report the isolation, functional characterization, and localization of a Na+/Cl--dependent catecholamine transporter (meNET) present in the brain o f the teleost fish medaka. This carrier is very similar to the human neuron al norepinephrine transporter (NET) and the human neuronal dopamine transpo rter (DAT), showing 70 and 64% amino acid identity, respectively. When expr essed in COS-7 cells, this transporter mediates the high-affinity uptake of dopamine (K-M = 290 nM) and norepinephrine (K-M = 640 nM). Its pharmacolog ical profile reveals more similarities with NET, including a high affinity for the tricyclic antidepressants desipramine (IC50 = 0.92 nM) and nortript yline (IC50 = 16 nM). In situ hybridization on the medaka brain shows that meNET mRNA is present only in a subset of tyrosine hydroxylase-positive neu rons found in the noradrenergic areas of the hindbrain, such as the locus c eruleus and area postrema. None of the dopaminergic areas anterior to the i sthmus contains any labeled neurons. Neither reverse transcriptase-polymera se chain reaction with degenerate primers specific for gamma -aminobutyric acid transporter/NET nor autoradiographic experiments with [I-125]3b-(4-iod ophenyl)-tropane-2b-carboxylic acid methyl ester revealed an additional cat echolamine transporter in the medaka brain. Uptake experiments with medaka. brain synaptosomes show an endogenous transport with a pharmacological pro file identical to that of the recombinant meNET. Thus, meNET is probably th e predominant-if not the only-catecholamine transporter in the medaka fish brain. In view of the highly conserved primary structures and pharmacologic al properties of meNET, it is tempting to speculate that a specific dopamin e transport developed later in vertebrate evolution and probably accompanie d the tremendous enlargement of the meso-telence-phalic dopaminergic pathwa ys in amniotes.