Mood disorders cause much suffering and are the single greatest cause of lost productivity worldwide. Although multiple medications, along with behavioral therapies, have proven effective for some individuals, millions of people lack an effective therapeutic option. A common serotonin (5-HT) transporter (5-HTT/SERT, SLC6A4) polymorphism is believed to confer lower 5-HTT expression in vivo and elevates risk for multiple mood disorders including anxiety, alcoholism, and major depression. Importantly, this variant is also associated with reduced responsiveness to selective 5-HT reuptake inhibitor antidepressants. We hypothesized that a reduced antidepressant response in individuals with a constitutive reduction in 5-HTT expression could arise because of the compensatory expression of other genes that inactivate 5-HT in the brain. A functionally upregulated alternate transporter for 5-HT may prevent extracellular 5-HT from rising to levels sufficiently high enough to trigger the adaptive neurochemical events necessary for therapeutic benefit. Here we demonstrate that expression of the organic cation transporter type 3 (OCT3, SLC22A3), which also transports 5-HT, is upregulated in the brains of mice with constitutively reduced 5-HTT expression. Moreover, the OCT blocker decynium-22 diminishes 5-HT clearance and exerts antidepressantlike effects in these mice but not in WT animals. OCT3 may be an important transporter mediating serotonergic signaling when 5-HTT expression or function is compromised.5HTTLPR ͉ antidepressant ͉ polymorphism ͉ hippocampus ͉ chronamperometry

The inactivation of synaptic serotonin (5-hydroxytryptamine, 5-HT) is largely established through the actions of the presynaptic, antidepressant-sensitive 5-HT transporter (SERT, SLC6A4). Recent studies have demonstrated post-translational regulation of SERT mediated by multiple Ser/Thr kinases, including protein kinases C and G (PKC and PKG) and p38 mitogen-activated protein kinase (MAPK), as well as the Ser/ Thr phosphatase PP2A. Less well studied are specific surface receptors that target these signaling pathways to control SERT surface expression and/or catalytic rates. Using rat basophilic leukemia 2H3 cell line (RBL-2H3), we previously established that activation of A 3 adenosine receptors (A 3 AR) stimulates SERT activity via both PKG and p38 MAPK (Zhu et al., 2004a). Whether A 3 ARs regulate SERT in the central nervous system (CNS) is unknown. Here we report that the A 3 AR agonist N 6 -(3-iodobenzyl)-N-methyl-5Јcarbamoyladenosine(IB-MECA) rapidly (10 min) and selectively stimulates 5-HT transport in mouse midbrain, hippocampal, and cortical synaptosomes. IB-MECA-induced stimulation of 5-HT uptake is blocked by the selective A 3 AR antagonist 3-ethyl-5-benzyl-2-methyl-phenylethynyl-6-phenyl-1,4(Ϯ)dihydropyridine-3,5-dicarboxylate (MRS1191) and is absent from synaptosomes prepared from Serotonin (5-hydroxytryptamine, 5-HT) signaling is critical to thermoregulation, appetite, sexual drive, and mood. Multiple mechanisms contribute to the presynaptic control of serotonergic signaling, including the synthesis, storage, release, and inactivation of 5-HT. 5-HT inactivation after release is accomplished via the antidepressant-sensitive 5-HT transporter (SERT), a presynaptic membrane protein. SERT is a member of the Na ϩ /Cl Ϫ -dependent solute transporter family (SLC6A4) (Blakely et al., 1991;Ramamoorthy et al., 1993) and is related most closely to dopamine and norepinephrine transporters. Targeted disruption of the murine SERT gene leads to disruption of presynaptic 5-HT homeostasis (Murphy et al., 2004) and is accompanied by anxiety-related behavioral changes (Jennings et al., 2006). Human SERT gene variants have been linked to mental disorders, including autism, depression, anxiety, and obses- Article, publication date, and citation information can be found at

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In the present study, we used high-speed chronoamperometry to examine serotonin (5-HT) transporter (5-HTT) function in vivo in 2-, 5-, and 10-month-old brain-derived neurotrophic factor (BDNF)+/) mice. The rate of clearance of exogenously applied 5-HT was measured in CA3 region of hippocampus. In 2-month-old mice, the rate of 5-HT clearance did not differ between BDNF+/+ and BDNF+/) mice. In BDNF+/+ mice, 5-HT clearance rate (T c ) increased markedly with age. In contrast, T c remained relatively static in BDNF+/) mice across 2-, 5-, and 10-month age groups. At 5 months of age, female BDNF+/+ mice had a lower maximal velocity (V max ) for 5-HT clearance than male BDNF+/+ mice. There was a similar trend in 5-month-old BDNF+/) mice, but this did not reach statistical significance. There was an age-dependent increase in K T value for 5-HT clearance (i.e., decreased in vivo affinity of 5-HTT), but no significant effect of genotype or gender. 5-HTT density, as measured by [3 H]cyanoimipramine binding, was not different between BDNF+/+ and BDNF+/) mice, although there was a significant increase in 5-HTT binding with age.The selective 5-HT reuptake inhibitor fluvoxamine (50 and 100 pmol) significantly decreased 5-HT clearance in BDNF+/+ mice, but not in BDNF+/) mice. Our data suggest that the profoundly reduced ability of 5-and 10-month-old BDNF+/) mice to clear 5-HT is not because of a decrease in the total number of 5-HTTs, but may be due to functional deficits in the 5-HTT, e.g., in the machinery/signaling required for insertion of 5-HTTs into the plasma membrane and/or activation of the 5-HTT once it is positioned to take up 5-HT from extracellular fluid.

The serotonin transporter (SERT) controls the strength and duration of serotonergic neurotransmission by the high-affinity uptake of ser otonin (5-HT) from extracellular fluid. SERT is a key target for many psychotherapeutic and abused drugs, therefore understanding how SERT activity and expression are regulated is of fundamental importance. A growing literature suggests that SERT activity is under regulatory control of the 5-HT1B autoreceptor. The present studies made use of mice with a constitutive reduction (5-HT1B+/−) or knockout of 5-HT1B receptors (5-HT1B−/−), as well as mice with a constitutive knockout of SERT (SERT−/−) to further explore the relationship between SERT activity and 5-HT1B receptor expression. High-speed chronoamperometry was used to measure clearance of 5-HT from CA3 region of hippocampus in vivo. Serotonin clearance rate, over a range of 5-HT concentrations, did not differ among 5-HT1B receptor genotypes, nor did [3H]cyanoimipramine binding to SERT in this brain region, suggesting that SERT activity is not affected by constitutive reduction or loss of 5-HT1B receptors; alternatively, it might be that other transport mechanisms for 5-HT compensate for loss of 5-HT1B receptors. Consistent with previous reports, we found that the 5-HT1B receptor antagonist, cyanopindolol, inhibited 5-HT clearance in wild-type mice. However, this effect of cyanopindolol was lost in 5-HT1B−/− mice and diminished in 5-HT1B+/− mice, indicating that the 5-HT1B receptor is necessary for cyanopindolol to inhibit 5-HT clearance. Likewise, cyanopindolol was without effect on 5-HT clearance in SERT−/− mice, demonstrating a requirement for the presence of both SERT and 5-HT1B receptors in order for cyanopindolol to inhibit 5-HT clearance in CA3 region of hippocampus. Our findings are consistent with SERT being under the regulatory control of 5-HT1B autoreceptors. Future studies to identify signaling pathways involved may help elucidate novel therapeutic targets for the treatment of psychiatric disorders, particularly those linked to gene variants of the 5-HT1B receptor.

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