Cocaine conditioned place preference (CPP) is intact in dopamine transporter (DAT) knockout (KO) mice and enhanced in serotonin transporter (SERT) KO mice. However, cocaine CPP is eliminated in double-KO mice with no DAT and either no or one SERT gene copy. To help determine mechanisms underlying these effects, we now report examination of baselines and drug-induced changes of extracellular dopamine (DA ex ) and serotonin (5-HT ex ) levels in microdialysates from nucleus accumbens (NAc), caudate putamen (CPu), and prefrontal cortex (PFc) of wild-type, homozygous DAT-or SERT-KO and heterozygous or homozygous DAT/SERT double-KO mice, which are differentially rewarded by cocaine. Cocaine fails to increase DA ex in NAc of DAT-KO mice. By contrast, systemic cocaine enhances DA ex in both CPu and PFc of DAT-KO mice though local cocaine fails to affect DA ex in CPu. Adding SERT to DAT deletion attenuates the cocaine-induced DA ex increases found in CPu, but not those found in PFc. The selective SERT blocker fluoxetine increases DA ex in CPu of DAT-KO mice, while cocaine and the selective DAT blocker GBR12909 increase 5-HT ex in CPu of SERT-KO mice. These data provide evidence that (a) cocaine increases DA ex in PFc independently of DAT and that (b), in the absence of SERT, CPu levels of 5-HT ex can be increased by blocking DAT. Cocaine-induced alterations in CPu DA levels in DAT-, SERT-, and DAT/SERT double-KO mice appear to provide better correlations with cocaine CPP than cocaine-induced DA level alterations in NAc or PFc.

These results suggest that LAT1 plays a significant role in cell proliferation, differentiation, and invasion in esophageal squamous cell carcinoma.

Dopamine transporter knockout (DAT KO) mice display deficits in sensorimotor gating that are manifested by reduced prepulse inhibition (PPI) of the acoustic startle reflex. Since PPI deficits may model some of the cognitive dysfunctions identified in certain neuropsychiatric patients, we have studied the effects of transporter blockers on PPI in wild-type and DAT KO mice. Treatments with High dose psychostimulants that block DAT as well as the norepinephrine (NET) and serotonin (SERT) transporters (60 mg/kg cocaine or methylphenidate) significantly impaired PPI in wild-type mice. By contrast, these treatments significantly ameliorated the PPI deficits observed in untreated DAT KO mice. In studies with more selective transport inhibitors, the selective NET inhibitor nisoxetine (10 or 30 mg/kg) also significantly reversed PPI deficits in DAT KO mice. By contrast, while the SERT inhibitor fluoxetine (30 mg/kg) normalized these PPI deficits in DAT KO mice, citalopram (30 or 100 mg/kg) failed to do so. The 'paradoxical' effects of cocaine and methylphenidate in DAT KO mice are thus likely to be mediated, at least in part by the ability of these drugs to block NET, although serotonin systems may also have some role. Together with recent microdialysis data, these results support the hypothesis that prefrontal cortical NET blockade and consequent enhancement of prefrontal cortical extracellular dopamine mediates the reversal of PPI deficits in DAT KO mice.

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These results suggest that DAT-KO mice exhibit impulsive CAR behavior that correlates with their PPI deficits. Blockade of monoamine transporters, especially the norepinephrine transporter (NET) in the prefrontal cortex (PFC), may contribute to pharmacological improvement of impulsivity in these mice.