Tricyclic antidepressants exert their pharmacological effect -inhibiting the reuptake of serotonin, norepinephrine and dopamine -by directly blocking neurotransmitter transporters (SERT, NET and DAT, respectively) in the presynaptic membrane. The drug-binding site and the mechanism of this inhibition are poorly understood. We determined the crystal structure at 2.9 Å of the bacterial leucine transporter (LeuT), a homolog of SERT, NET and DAT, in complex with leucine and the antidepressant desipramine. Desipramine binds at the inner end of the extracellular cavity of the transporter and is held in place by a hairpin loop and by a salt bridge. This binding site is separated from the leucine-binding site by the extracellular gate of the transporter. By directly locking the gate, desipramine prevents conformational changes and blocks substrate transport. Mutagenesis experiments on human SERT and DAT indicate that both the desipramine-binding site and its inhibition mechanism are probably conserved in the human neurotransmitter transporters. Na + /Cl − -dependent neurotransmitter transporters for serotonin (SERT), norepinephrine (NET) and dopamine (DAT) in the presynaptic plasma membrane terminate neuronal signal transmission in the central nervous system through a reuptake mechanism (1-6). These systems have been shown to modulate mood, emotion, sleep and appetite (7). Depression, arguably the most prevalent psychiatric disorder, is directly associated with perturbation of serotonergic neurotransmission (8, 9), and drugs blocking serotonin reuptake have been used successfully for its treatment. One class of these drugs, tricyclic antidepressants (TCAs) such as desipramine and imipramine, binds to serotonin and norepinephrine transporters with affinities of nanomolar to tens of nanomolar concentrations and blocks transport activity (10). The response rate of patients to TCAs is typically 60-70% (11). More recently, highly selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine (Prozac) have also been developed and are increasingly prescribed to treat depression (12). The molecular The human SERT, DAT and NET proteins all belong to a family of transporters for amino acids and their derivatives, the Neurotransmitter:Sodium Symporter (NSS) family (2)(3)(4)(5)14). Whilst the dopamine transporters from human, bovine or rat are inhibited by TCAs at a K i of micromolar concentrations, the DAT proteins from C. elegans (15) and D. melanogaster (16) are inhibited by TCAs at a K i of nanomolar and sub-micromolar concentrations, respectively (17). As bacterial NSS proteins share up to 30 % sequence identity with human SERT and NET as well as worm and fly DATs, we hypothesized that bacterial NSS proteins also possess high binding affinity to TCAs and could provide opportunities for studying proteindrug interactions. We therefore chose a bacterial NSS protein, the leucine transporter (LeuT) from Aquifex aeolicus, to study the molecular mechanism of neurotransmitter transporter binding to TCAs (18). LeuT shares 2...

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Insulin activates insulin receptors (InsRs) in the hypothalamus to signal satiety after a meal. However, the rising incidence of obesity, which results in chronically elevated insulin levels, implies that insulin may also act in brain centres that regulate motivation and reward. We report here that insulin can amplify action potential-dependent dopamine (DA) release in the nucleus accumbens (NAc) and caudate–putamen through an indirect mechanism that involves striatal cholinergic interneurons that express InsRs. Furthermore, two different chronic diet manipulations in rats, food restriction (FR) and an obesogenic (OB) diet, oppositely alter the sensitivity of striatal DA release to insulin, with enhanced responsiveness in FR, but loss of responsiveness in OB. Behavioural studies show that intact insulin levels in the NAc shell are necessary for acquisition of preference for the flavour of a paired glucose solution. Together, these data imply that striatal insulin signalling enhances DA release to influence food choices.

The SLC6 family is a diverse set of transporters that mediate solute translocation across cell plasma membranes by coupling solute transport to the cotransport of sodium and chloride down their electrochemical gradients. These transporters probably have 12 transmembrane domains, with cytoplasmic N- and C-terminal tails, and at least some may function as homo-oligomers. Family members include the transporters for the inhibitory neurotransmitters GABA and glycine, the aminergic transmitters norepinephrine, serotonin, and dopamine, the osmolytes betaine and taurine, the amino acid proline, and the metabolic compound creatine. In addition, this family includes a system B(0+) cationic and neutral amino acid transporter, and two transporters for which the solutes are unknown. In general, SLC6 transporters act to regulate the level of extracellular solute concentrations. In the central and the peripheral nervous system, these transporters can regulate signaling among neurons, are the sites of action of various drugs of abuse, and naturally occurring mutations in several of these proteins are associated with a variety of neurological disorders. For example, transgenic animals lacking specific aminergic transporters show profoundly disturbed behavioral phenotypes and probably represent excellent systems for investigating psychiatric disease. SLC6 transporters are also found in many non-neural tissues, including kidney, intestine, and testis, consistent with their diverse physiological roles. Transporters in this family represent attractive therapeutic targets because they are subject to multiple forms of regulation by many different signaling cascades, and because a number of pharmacological agents have been identified that act specifically on these proteins.