Fluoxetine and desipramine selectively attenuate 2'-NH2-MPTP-induced depletions in serotonin and norepinephrine

Resistance to MPTP-neurotoxicity in α-synuclein knockout mice is complemented by human α-synuclein and associated with increased β-synuclein and Akt activation

Genetic and biochemical abnormalities of α-synuclein are associated with the pathogenesis of Parkinson's disease. In the present study we investigated the in vivo interaction of mouse and human α-synuclein with the potent parkinsonian neurotoxin, MPTP. We find that while lack of mouse α-synuclein in mice is associated with reduced vulnerability to MPTP, increased levels of human α-synuclein expression is not associated with obvious changes in the vulnerability of dopaminergic neurons to MPTP. However, expressing human α-synuclein variants (human wild type or A53T) in the α-synuclein null mice completely restores the vulnerability of nigral dopaminergic neurons to MPTP. These results indicate that human α-synuclein can functionally replace mouse α-synuclein in regard to vulnerability of dopaminergic neurons to MPTP-toxicity. Significantly, α-synuclein null mice and wild type mice were equally sensitive to neurodegeneration induced by 2′NH₂-MPTP, a MPTP analog that is selective for serotoninergic and noradrenergic neurons. These results suggest that effects of α-synuclein on MPTP like compounds are selective for nigral dopaminergic neurons. Immunoblot analysis of β-synuclein and Akt levels in the mice reveals selective increases in β-synuclein and phosphorylated Akt levels in ventral midbrain, but not in other brain regions, of α-synuclein null mice, implicating the α-synuclein-level dependent regulation of β-synuclein expression in modulation of MPTP-toxicity by α-synuclein. Together these findings provide new mechanistic insights on the role α-synuclein in modulating neurodegenerative phenotypes by regulation of Akt-mediated cell survival signaling in vivo.

Involvement of leptin receptor long isoform (LepRb)-STAT3 signaling pathway in brain fat mass- and obesity-associated (FTO) downregulation during energy restriction

Obesity is an important risk factor for cardiovascular disease, diabetes and certain cancers. The fat mass- and obesity-associated (FTO) gene is tightly associated with the pathophysiology of obesity, whereas the exact role of FTO remains poorly understood. Here, we investigated the alternations of FTO mRNA and protein expression in the peripheral metabolic tissues and the brain upon energy restriction (ER) and explored the involvement of the leptin signaling pathway in FTO regulation under ER status. ER decreased the FTO mRNA and protein expression in hypothalamus and brainstem but not in periphery. Using double-immunofluorescence staining, FTO was found to be colocalized with the leptin receptor long isoform (LepRb) in arcuate nucleus of hypothalamus and the nucleus of the solitary tract. In LepRb mutant db/db mice, the FTO downregulation in brain and body weight reduction induced by ER were completely abolished. The enhanced phosphorylation of signal transducer and activator of transcription 3 (STAT3) induced by ER was also impaired in db/db mice. Moreover, leptin directly activated the STAT3 signaling pathway and downregulated FTO in in vitro arcuate nucleus of hypothalamus cultures and in vivo wild-type mice but not db/db mice. Thus, our results provide the first evidence that the LepRb-STAT3 signaling pathway is involved in the brain FTO downregulation during ER.

MPTP-induced neuroblast apoptosis in the subventricular zone is not regulated by dopamine or other monoamine transporters

For 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to exert neurotoxicity on dopaminergic neurons, 1-methyl-4-phenylpyridinium (MPP+), a metabolite of MPTP, must be taken up into the dopaminergic neuron via the dopamine transporter (DAT). Previous reports have shown that MPTP also causes neuroblast apoptosis in the subventricular zone (SVZ) of adult mice. The aim of this study is to elucidate the role of DAT and other monoamine transporters including vesicular monoamine transporter 2 (VMAT2), the serotonin transporter (SERT), and the norepinephrine transporter (NET) on the neuroblast apoptosis induced by MPTP administration. There were no DAT-positive neuroblasts in the SVZ, whereas some neuroblasts were immunopositive for VMAT2 and SERT. To examine whether these transporters are involved in MPTP-induced neuroblast apoptosis in the SVZ, terminal deoxynucleotidyl transferase-mediated dUTP endlabeling (TUNEL)-positive cells were semiquantitatively analyzed after the injection of GBR12909 (GBR), a DAT inhibitor; tetrabenazine (TBZ), a VMAT2 inhibitor; fluoxetine (FLU), a SERT inhibitor, or desipramine (DES), a NET inhibitor, prior to MPTP injection. However, the injection of these transporter inhibitors had no influence on the MPTP-induced neuroblast apoptosis in the SVZ. It is likely that neither DAT nor other monoamine transporters are involved in MPTP-induced neuroblast apoptosis. The present findings suggest that the neurotoxicity of MPTP to neuroblasts in the SVZ does not require DAT or other monoamine transporters, and the apoptosis it induces may be executed through other unknown pathways.

A pharmacological analysis of mice with a targeted disruption of the serotonin transporter

RATIONALE

Partial or complete ablation of serotonin transporter (SERT) expression in mice leads to altered responses to serotonin receptor agonists and other classes of drugs.

OBJECTIVES

In the current report, we review and integrate many of the major behavioral, physiological, and neurochemical findings in the current literature regarding pharmacological assessments made in SERT mutant mice.

RESULTS

The absence of normal responses to serotonin reuptake inhibiting (SRI) antidepressants in SERT knockout (-/-) mice demonstrates that actions on SERT are a critical principle mechanism of action of members of this class of antidepressants. Drugs transported by SERT, (+)-3,4-methylenedioxymethamphetamine (MDMA) and 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH(2)-MPTP), are also inactive in SERT -/- mice. Temperature, locomotor, and electrophysiological responses to various serotonin receptor agonists, including 8-hydroxy-2-(di-n-propylamino)-tetraline (8-OH-DPAT), ipsapirone, and RU24969, are reduced in SERT -/- mice, despite comparatively lesser reductions in Htr1a and Htr1b binding sites, G-proteins, and other signaling molecules. SERT -/- mice exhibit an approximately 90% reduction in head twitches in response to the Htr2a/2c agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), associated with a profound reduction in arachidonic acid signaling, yet only modest changes in Htr2a and Htr2c binding sites. SERT -/- mice also exhibit altered behavioral responses to cocaine and ethanol, related to abnormal serotonin, and possibly dopamine and norepinephrine, homeostasis.

CONCLUSIONS

Together, these studies demonstrate a complex and varied array of modified drug responses after constitutive deletion of SERT and provide insight into the role of serotonin, and in particular, its transporter, in the modulation of complex behavior and in the pharmacological actions of therapeutic agents and drugs of abuse.

The neurotoxin 2′-NH2-MPTP degenerates serotonin axons and evokes increases in hippocampal BDNF

1-Methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH2-MPTP) causes long-term depletions in cortical and hippocampal serotonin (5-HT) and norepinephrine (NE) that are accompanied by acute elevations in glial fibrillary acidic protein (GFAP) and argyrophilia. To further investigate the hypothesis that these changes are reflective of serotonergic and noradrenergic axonal degeneration, 2'-NH2-MPTP was administered to mice and innervation densities were determined immunocytochemically. Regional responses of the neurotrophin, brain-derived neurotrophic factor (BDNF), to putative damage were also assessed. Three days after 2'-NH2-MPTP, 5-HT axons exhibited a beaded, tortuous appearance indicative of ongoing degeneration. At 21 days, numbers of serotonin axons were significantly decreased, with the greatest axonal losses occurring in cortex and hippocampus. Serotonin axons in the amygdala were contrastingly spared long-term damage, as were 5-HT and NE cell bodies in the brain stem. BDNF protein levels were selectively increased in the hippocampus 3 days post-dose and returned to normal 21 days later. These results, in conjunction with previous findings, demonstrate that 2'-NH2-MPTP causes degeneration of serotonergic axons innervating the cortex and hippocampus on par with depletions in neurotransmitter levels. Moreover, damage to the hippocampus, a brain region important for learning and memory, and the modulation of anxiety and stress responsiveness, results in a transitory increase in BDNF.

Late onset loss of hippocampal 5-HT and NE is accompanied by increases in BDNF protein expression in mice co-expressing mutant APP and PS1

Transgenic mice expressing both mutant amyloid precursor protein (APPswe) and presenilin-1 (PS1DeltaE9) develop amyloid deposits as early as 4 months of age and preliminary evidence suggests that this may be associated with degenerative changes in serotonin axons innervating the dentate gyrus of the hippocampus. In the present investigation, which focused on further delineating the effects of amyloid deposition on hippocampal neurochemistry, decreases in serotonin neurotransmitter levels (-25%) were discovered to be present at 18 months in APP+/PS1+ mice, while norepinephrine was reduced in the hippocampus of 12- (-30%) and 18-month-old (-45%) APP+/PS1+ double mutants. In addition, brain-derived neurotrophic factor (BDNF) protein levels were investigated since changes in BDNF are reported to occur in AD, and BDNF has been shown to have trophic effects on serotonin and norepinephrine neurons. In doubly, but not singly mutant mice, hippocampal BDNF levels were increased at 12 (+70%) and 18 months (+170%). Furthermore, in a different model of serotonergic and noradrenergic degeneration, BDNF protein levels were similarly increased in response to depletions in hippocampal serotonin and norepinephrine caused by the chemical neurotoxin 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH2-MPTP). These findings show that early amyloid deposition in mice expressing mutant human APP and PS-1 is associated with a progressive loss of serotonin and norepinephrine neurotransmitter levels in the hippocampus later in life. Furthermore, BDNF protein levels are increased in APP+/PS1+ and 2'-NH2-MPTP-treated mice, possibly as a compensatory response to serotonergic and noradrenergic neurodegeneration in a brain region important for learning and memory.

2'-NH(2)-MPTP [1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine] depletes serotonin and norepinephrine in rats: a comparison with 2'-CH(3)-MPTP [1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine]

The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) analog, 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH(2)-MPTP), depletes brain serotonin and norepinephrine in mice without affecting striatal dopamine. The present study was conducted to determine whether 2'-NH(2)-MPTP would be similarly neurotoxic to rats. Four injections of 20 mg/kg 2'-NH(2)-MPTP caused 80 to 90% depletions in serotonin and norepinephrine in frontal cortex and hippocampus in rats 1 week post-treatment. A lower dose of 2'-NH(2)-MPTP (4 x 15 mg/kg) also produced large decrements in serotonin and norepinephrine levels and in serotonin transporter density measured 3 weeks after neurotoxin administration. Furthermore, this lower dose of 2'-NH(2)-MPTP altered functional serotonin neurotransmission as evidenced by a 2-fold potentiation of 1-(3-chlorophenyl)-piperazine.2HCl-induced hyperthermia, an index of serotonergic denervation supersensitivity. At both doses, 2'-NH(2)-MPTP was without effect on striatal dopamine. For comparison, additional rats were treated with a second 2'-substituted analog of MPTP, 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'-CH(3)-MPTP), at 2 x 20 mg/kg. This dosing regimen causes substantial striatal dopamine depletion in mice. 2'-CH(3)-MPTP had no effect on brain levels of serotonin, norepinephrine, or dopamine in rats. Together, these results demonstrate that rats are sensitive to the toxic effects of 2'-NH(2)-MPTP but not to 2'-CH(3)-MPTP at doses known to cause neurotoxicity in mice. Moreover, this study clearly shows that 2'-NH(2)-MPTP can be utilized in rats as a tool to study the serotonergic and noradrenergic neurotransmitter systems.

Genetic perspectives on the serotonin transporter

The serotonin transporter (5-HTT) is most well known as the site of action of the serotonin reuptake inhibitors, which were initially developed as antidepressants, but now are the most widely used agents in the treatment of many additional neuropsychiatric and related disorders. The discovery that the gene that expresses the 5-HTT possesses a functional promoter-region polymorphism, which is associated with temperament and personality traits such as anxiety and negative emotionality as well as some behaviors, led to many studies examining this polymorphism in individuals with different neuropsychiatric disorders. The subsequent development of mice with a targeted disruption of the 5-HTT in our laboratory has provided an experimental model to examine the many consequences of diminished (in +/-, heterozygote mice) or absent (in -/-, homozygote knockout mice) function of the 5-HTT. The 5-HTT-deficient mice were also crossed with other knockout mice, allowing the study of multiple neurobiologic dysfunctions. As multiple genes are probably involved in the expression of complex behaviors such as anxiety, as well as neuropsychiatric disorders, these more genetically complex mice may more closely model disorders with complex etiologies. Thus, the combination of these comparative human and mouse studies may extend the opportunities to examine genetic alterations from a novel "bottom-up" approach [gene knockout or partial gene knockout in a combinational gene x gene x (yet unknown) gene approach], which is complementary to the traditional "top-down" genetic approach based upon studies of individuals with diagnosed neuropsychiatric disorders and their family members.