Environmental enrichment enhances synaptic plasticity by internalization of striatal dopamine transporters.

Environmental enrichment (EE) with a complex combination of physical, cognitive and social stimulations enhances synaptic plasticity and behavioral function. However, the mechanism remains to be elucidated in detail. We aimed to investigate dopamine-related synaptic plasticity underlying functional improvement after EE. For this, six-week-old CD-1 mice were randomly allocated to EE or standard conditions for two months. EE significantly enhanced behavioral functions such as rotarod and ladder walking tests. In a [18F]FPCIT positron emission tomography scan, binding values of striatal DAT were significantly decreased approximately 18% in the EE mice relative to the control mice. DAT inhibitor administrated to establish the relationship of the DAT down-regulation to the treatment effects also improved rotarod performances, suggesting that DAT inhibition recapitulated EE-mediated treatment benefits. Next, EE-induced internalization of DAT was confirmed using a surface biotinylation assay. In situ proximity ligation assay and immunoprecipitation demonstrated that EE significantly increased the phosphorylation of striatal DAT as well as the levels of DAT bound with protein kinase C (PKC). In conclusion, we suggest that EE enables phosphorylation of striatal DAT via a PKC-mediated pathway and causes DAT internalization. This is the first report to suggest an EE-mediated mechanism of synaptic plasticity by internalization of striatal DAT.

Dopaminergic neuron destruction reduces hippocampal serotonin 1A receptor uptake of trans-[(18)F]Mefway.

The purpose of the present study is to investigate the relationship between dopaminergic neuron destruction and 5-HT system changes in a hemiparkinsonian rat model. We performed PET imaging studies with trans-[(18)F]Mefway in a hemiparkinsonian model of unilateral 6-hydroxydopamine (6-OHDA) rats. Region-of-interests (ROIs) were drawn in the hippocampus (HP) and cerebellum (CB). HP uptake, the ratios of specific binding to non-specific binding in the HP, and non-displaceable binding potential (BPND) in the HP were compared between 6-OHDA and control rats. As a result, unilateral 6-OHDA-lesioned rats exhibited significant bilateral reduction of HP uptake and trans-[(18)F]Mefway BPND compared to the intact control group. Therefore, the results demonstrate that destruction of the dopaminergic system causes the reduction of the serotonergic system.

Acute physical stress induces the alteration of the serotonin 1A receptor density in the hippocampus.

Stress affects the serotonergic system, which is associated with depression. Previous research has showed that chronic stress causes the deactivation of the limbic system. However, the influence of the acute physical stress on the serotonergic system in vivo was primarily unclear. The purpose of this research is to elucidate the effects of the acute physical stress in vivo using PET. For quantification of the 5-HT1A receptors in the brain, we measured [(18)F]Mefway uptake in the two experiment groups (control and despair rats). The despair group was subjected to the external stressful situation (i.e., forced swimming) and total duration time of immobility, refers to the despair severity, and was analyzed. In the intercomparison experiment, the resulting PET images of [(18)F]Mefway in the despair rat displayed a significant reduction of radioactivity in the hippocampus (HP) compared with the control. The nondisplaceable binding potential (BPND ) refers to the ratio of the concentration of radioligand in the receptor-rich region (i.e., HP) to the concentration of that in the receptor-free region (i.e., cerebellum). The hippocampal uptake and the BPND in the despair group were respectively about 25 and 18% lower than those of the control group. The ratio of specific binding to nonspecific binding in the despair group was 18% lower than that of the control. In the intracomparison experiments, the BPND and immobility in the despair group showed a strong negative correlation. Taken together, the data illustrates that an acute physical stress induces the change in the serotonergic system that correlates with the behavioral despair.

Evaluation of dopamine transporters and D2 receptors in hemiparkinsonian rat brains in vivo using consecutive PET scans of [18F]FPCIT and [18F]fallypride.

The aim of this study was to investigate dopaminergic function in unilaterally lesioned 6-OHDA rats by dual PET radioligands: [(18)F]FPCIT (a dopamine transporter imaging radioligand) and [(18)F]fallypride (a dopamine D2 receptors imaging radioligand). As a result, the brain uptake of [(18)F]FPCIT was significantly reduced and that of [(18)F]fallypride was increased in the ipsilateral striatum (lesion side) of the 6-OHDA rats. These findings implicated that dopamine transporter is down-regulated and dopamine D2 receptor is up-regulated in this hemiparkinsonian rat model.

Effective microPET imaging of brain 5-HT(1A) receptors in rats with [(18) F]MeFWAY by suppression of radioligand defluorination.

INTRODUCTION: [(18) F]MeFWAY has been developed for imaging the serotonin 1A receptors in the brain. The purpose of this study were to verify the metabolic stability of [(18) F]MeFWAY, to measure the degree of defluorination of [(18) F]MeFWAY in vivo, to investigate methods of inhibition of defluorination of [(18) F]MeFWAY, and to assess the efficacy of [(18) F]MeFWAY in rat brains in vivo. METHODS: MicroPET experiments in rats were conducted to confirm the distribution of radioactivity in the brain. Nondisplaceable binding potential (BP(ND) ) in the hippocampus and frontal cortex were also analyzed. Miconazole and fluconazole were tested for the ability to suppress defluorination of [(18) F]MeFWAY. We conducted a blockade and displacement experiment by treating with WAY-100635. RESULTS: In vitro stability tests showed that MeFWAY was very stable in serum for 6 h, but PET revealed that authentic [(18) F]MeFWAY underwent significant defluorination in vivo. In vitro inhibition study against decreasing parent activity in liver microsomes, miconazole and fluconazole suppressed metabolic elimination of MeFWAY. However, in the PET study, fluconazole showed more potent inhibitory activity than miconazole. In the suppression of metabolizing enzymes using fluconazole, radioactivity in skull was dramatically decreased by 81% (compared with 69% with miconazole) and it was coupled with an increase in brain uptake. Moreover, BP(ND) in hippocampus was 5.53 and 2.66 in frontal cortex. The blockade and displacement study showed the specificity of [(18) F]MeFWAY to 5-HT(1A) receptors. CONCLUSION: In the rat brain, [(18) F]MeFWAY microPET showed skull uptake due to defluorination in vivo. We can effectively overcome this drawback with fluconazole.