Mutation of a conserved Asp (D98) in the rat serotonin (5HT) transporter (rSERT) to Glu (D98E) led to decreased 5HT transport capacity, diminished coupling to extracellular Na+ and Cl-, and a selective loss of antagonist potencies (cocaine, imipramine, and citalopram but not paroxetine or mazindol) with no change in 5HT Km value. D98E, which extends the acidic side chain by one carbon, affected the rank-order potency of substrate analogs for inhibition of 5HT transport, selectively increasing the potency of two analogs with shorter alkylamine side chains, gramine, and dihydroxybenzylamine. D98E also increased the efficacy of gramine relative to 5HT for inducing substrate-activated currents in Xenopus laevis oocytes, but these currents were noticeably dependent on extracellular medium acidification. I-V profiles for substrate-independent and -dependent currents indicated that the mutation selectively impacts ion permeation coupled to 5HT occupancy. The ability of the D98E mutant to modulate selective aspects of substrate recognition, to perturb ion dependence as well as modify substrate-induced currents, suggests that transmembrane domain I plays a critical role in defining the permeation pathway of biogenic amine transporters.

Using degenerate oligonucleotide probes encoding conserved regions of the γ-aminobutyric acid/norepinephrine transporter gene family, we have cloned a rat brain cDNA encoding a creatine transporter (rCREAT). rCREAT encodes a highly hydrophobic, 635-amino-acid protein possessing 12 potential transmembrane domains and canonical sites for N-linked glycosylation and protein phosphorylation. Transfection of rCREAT cDNA into mammalian cells results in the expression of [¹⁴C]creatine uptake, which is blocked by low micromolar concentrations of recognized creatine uptake inhibitors. Two rCREAT mRNAs are expressed in the rat brain, retina, kidney and heart. Whole-brain rCREAT mRNAs demonstrate a marked postnatal rise to steady-state adult levels. In situ hybridization studies indicate a widespread, differential rCREAT mRNA expression in adult rat brain, with high expression noted over myelinated fiber tracts, cerebellar granule cells, hippocampal pyramidal cells, brainstem nuclei and endothelial cells of the choroid plexus. These studies will allow the development of new molecular probes useful for defining the creatine transporter's cellular expression pattern, function in ATP homeostasis and association with disorders of cellular energy metabolism.

Chemical signaling by dopamine (DA) and L-norepinephrine (L-NE) at synapses is terminated by uptake via specialized presynaptic transport proteins encoded by the DA transporter (DAT) and L-NE transporter (NET) genes, respectively. In some vertebrate neurons, particularly the sympathetic neurons of amphibians, L-NE is converted to L-epinephrine (L-Epi, adrenaline) and released as the primary neurotransmitter. Although evidence exists for a molecularly distinct L-Epi transporter (ET) in the vertebrate brain and peripheral nervous system, a transporter specialized for extracellular L-Epi clearance has yet to be identified. To pursue this issue, we cloned transporter cDNAs from bullfrog (Rana catesbiana) paravertebral sympathetic ganglia and characterized functional properties via heterologous expression in non-neuronal cells. A cDNA of 2514 bp (f ET) was identified for which the cognate 3.1 kb mRNA is highly enriched in frog sympathetic ganglia. Sequence analysis of the f ET cDNA reveals an open reading frame coding for a protein of 630 amino acids. Inferred f ET protein sequence bears 75, 66, and 48% amino acid identity with human NET, DAT, and the 5-hydroxytryptamine transporter (SERT), respectively. Transfection of f ET confers Na ϩ -and Cl Ϫ -dependent catecholamine uptake in HeLa cells. Uptake of [ 3 H]-L-NE by f ET is inhibited by catecholamines in a stereospecific manner. L-Epi and DA inhibit f ET-mediated [3 H]-L-NE uptake more potently than they inhibit [ 3 H]-L-NE uptake by human NET (hNET), whereas L-NE exhibits equivalent potency between the two carriers. Moreover, f ET exhibits a greater maximal velocity (V max ) for the terminal products of catecholamine biosynthesis (L-Epi Ͼ L-NE Ͼ Ͼ DA), unlike hNET, in which a V max rank order of L-NE Ͼ DA Ͼ L-Epi is observed. f ET-mediated transport of catecholamines is sensitive to cocaine and tricyclic antidepressants, with antagonist potencies significantly correlated with hNET inhibitor sensitivity. Amino acid conservation and divergence of f ET with mammalian catecholamine transporters help define residues likely to be involved in catecholamine recognition and translocation as well as blockade by selective reuptake inhibitors.

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Transport of norepinephrine (NE+) by cocaine- and antidepressant-sensitive transporters in presynaptic terminals is predicted to involve the cotransport of Na+ and Cl-, resulting in a net movement of charge per transport cycle. To explore the relationship between catecholamine transport and ion permeation through the NE transporter, we established a human norepinephrine transporter (hNET) cell line suitable for biochemical analysis and patch-clamp recording. Stable transfection of hNET cDNA into HEK-293 (human embryonic kidney) cells results in lines exhibiting (1) a high number of transporter copies per cell (10(6)), as detected by radioligand binding and hNET-specific antibodies, (2) high-affinity, Na(+)-dependent transport of NE, and (3) inhibitor sensitivities similar to those of native membranes. Whole-cell voltage-clamp of hNET-293 cells reveals NE-induced, Na(+)-dependent currents blocked by antidepressants and cocaine that are absent in parental cells. In addition to NE-dependent currents, transfected cells posses an NE-independent mode of charge movement mediated by hNET. hNET antagonists without effect in non-transfected cells abolish both NE-dependent and NE-independent modes of charge movement in transfected cells. The magnitude of NE-dependent currents in these cells exceeds the expectations of simple carrier models using previous estimates of transport rates. To explain our observations, we propose that hNETs function as ion-gated ligand channels with an indefinite stoichiometry relating ion flux to NE transport. In this view, external Na+ and NE bind to the transporter with finite affinities in a cooperative fashion. However, coupled transport may not predict the magnitude or the kinetics of the total current through the transporter. We propose instead that Na+ gates NE transport and also the parallel inward flux of an indeterminate number of ions through a channel-like pore.