Sex-related effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydro-pyridine treatment may be related to differences in monoamine oxidase B

Mfn2 Overexpression Attenuates MPTP Neurotoxicity In Vivo

Mitochondrial dysfunction represents a critical event in the pathogenesis of Parkinson’s disease (PD). Increasing evidence demonstrates that disturbed mitochondrial dynamics and quality control play an important role in mitochondrial dysfunction in PD. Our previous study demonstrated that MPP⁺ induces mitochondrial fragmentation in vitro. In this study, we aimed to assess whether blocking MPTP-induced mitochondrial fragmentation by overexpressing Mfn2 affords neuroprotection in vivo. We found that the significant loss of dopaminergic neurons in the substantia nigra (SN) induced by MPTP treatment, as seen in wild-type littermate control mice, was almost completely blocked in mice overexpressing Mfn2 (hMfn2 mice). The dramatic reduction in dopamine neuronal fibers and dopamine levels in the striatum caused by MPTP administration was also partially inhibited in hMfn2 mice. MPTP-induced oxidative stress and inflammatory response in the SN and striatum were significantly alleviated in hMfn2 mice. The impairment of motor function caused by MPTP was also blocked in hMfn2 mice. Overall, our work demonstrates that restoration of mitochondrial dynamics by Mfn2 overexpression protects against neuronal toxicity in an MPTP-based PD mouse model, which supports the modulation of mitochondrial dynamics as a potential therapeutic target for PD treatment.

The assessment of possible gender-related effect of endogenous striatal alpha-tocopherol level on MPTP neurotoxicity in mice

Several studies supported an increased vulnerability of males regarding Parkinson's disease (PD) and its animal models, the background of which has not been exactly revealed, yet. In addition to hormonal differences, another possible factor behind that may be a female-predominant increase in endogenous striatal alpha-tocopherol (αT) level with aging, even significant at 16 weeks of age, previously demonstrated by the authors. Accordingly, the aim of the current study was the assessment whether this difference in striatal αT concentration may contribute to the above-mentioned distinct vulnerability of genders to nigrostriatal injury. Female and male C57Bl/6 mice at the age of 16 weeks were injected with 12 mg/kg body weight 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) 5 times at 2 h intervals or with saline. The levels of some biogenic amines (striatum) and αT (striatum and plasma) were determined by validated high performance liquid chromatography methods. Although the results proved previous findings, i.e., striatal dopamine decrease was less pronounced in females following MPTP treatment, and striatal αT level was significantly higher in female mice, the correlation between these 2 variables was not significant. Surprisingly, MPTP treatment did not affect striatal αT concentrations, but significantly decreased plasma αT levels without differences between genders. The current study, examining the possible role of elevated αT in female C57Bl/6 mice behind their decreased sensitivity to MPTP intoxication for the first time, was unable to demonstrate any remarkable connection between these 2 variables. These findings may further confirm that αT does not play a major role against neurotoxicity induced by MPTP.

Plasma pharmacokinetic and metabolism of [18F]THK-5317 are dependent on sex

INTRODUCTION

Tau deposition is one of the hallmarks of Alzheimer's disease (AD) and can be visualized and quantified using [¹⁸F]THK-5317 together with kinetic modeling. To determine the feasibility of this approach, we measured blood/plasma pharmacokinetics and radiotracer metabolism in female and male rats.

METHODS

Female and male rats (n = 11-12) were cannulated via the femoral artery for continuous blood sampling. Blood sampling was performed at regular intervals after intravenous injection of [¹⁸F]THK-5317. After collection of the last blood sample, animals were sacrificed, and organs were excised. Blood from minute 5, 20 and 60 was centrifuged to obtain plasma. Radiolabeled metabolites in plasma, brain, liver and urine were analyzed by radio-thin-layer chromatography (radio-TLC).

RESULTS

Plasma pharmacokinetics and metabolism were significantly different between female and male rats. [¹⁸F]THK-5317 plasma clearance was faster in female (0.66 ± 0.08 mL/h/kg BW) than in male (0.52 ± 0.11 mL/h/kg BW) rats (p = .005). The percentage of unmetabolized parent was significantly different between both sexes at 20 min and 60 min p.i. In the liver, a 1.6-fold higher radioactivity concentration was found in male versus female animals and in addition also the percentage of unmetabolized parent was different.

CONCLUSION

Our results show pronounced sex differences in blood/plasma pharmacokinetics and metabolism of [¹⁸F]THK-5317 in rats. Female animals showed a faster plasma clearance compared to males. These results underline the importance of investigating both sexes and also support the notion that individual input functions or sex-specific population-based input functions are needed for kinetic modeling analyses.

ADVANCES IN KNOWLEDGE

First preclinical study in rats showing pronounced sex differences in blood/plasma pharmacokinetics and metabolism of [¹⁸F]THK-5317.

IMPLICATIONS FOR PATIENT CARE

Sex-specific differences might also be present in humans and thus clinical trials should have adequate sample size to account for effects in men and women separately.

Differences in reserpine-induced striatal dopamine output and content between female and male mice: implications for sex differences in vesicular monoamine transporter 2 function

In this report a series of six in vitro experiments in which reserpine-evoked dopamine output and two in vivo experiments in which the effects of reserpine injections upon dopamine content from striatal tissue of female and male mice were performed as a means to assess possible sex differences in vesicular monoamine transporter 2 (VMAT2) function. Significantly greater amounts of dopamine were obtained from striatal tissue of female mice in response to either a brief (experiment 1) or continuous (experiment 2) infusion of reserpine. Similarly, reserpine-evoked dopamine output from striatal tissue of gonadectomized females was significantly greater that that of gonadectomized males (experiment 3). When reserpine-evoked dopamine responses were compared directly between intact versus gonadectomized females (experiment 4) or males (experiment 5) no statistically significant differences were obtained. Finally, comparisons of gonadectomized females treated or not with estrogen revealed no statistically significant differences in reserpine-evoked dopamine output (experiment 6). Injections of reserpine produced significantly greater depletions of striatal dopamine content within intact female versus male mice (experiment 7). Dopamine contents of gonadectomized females treated or not with estrogen did not differ following treatment with reserpine, but were significantly greater than that of gonadectomized males (experiment 8). Taken together, these results show that female striatal tissue is more responsive to reserpine-evoked dopamine output, and this sex difference appears to be estrogen independent. Similarly, the dopamine depleting effects of reserpine are greater in intact female mice, however, gonadectomy reverses this effect in an estrogen independent manner. The data suggest that female mice may have a greater amount/activity of VMAT2 function as revealed by the increased responsiveness to the VMAT2 blocking drug, reserpine. Such differences in VMAT2 function may be related to the gender differences observed in conditions like Parkinson's disease and drug addiction.

Dopamine transporter function differences between male and female CD-1 mice

It has been reported that male mice are more susceptible to the neurotoxic effects of methamphetamine (MA) upon the nigrostriatal dopaminergic (NSDA) system. Since MA utilizes the dopamine transporter (DAT) to exert its effects, in the present study, we tested for differences in the dynamics of DAT function between male and female mice as an approach to understand some of the bases for this sex difference in MA-induced NSDA neurotoxicity. To accomplish this goal, in Experiment 1, the amount of dopamine (DA) obtained following DA infusion into the superfused striatal tissue fragments of male and female mice was measured while in Experiment 2 responses to the DA uptake blocker, nomifensine (NMF), were assessed in these preparations. The differences obtained to these treatments demonstrate that marked differences in DA transporter activity exist between male and female mice. When combining the DA and DOPAC measures from these two experiments, the data suggest that the female mice show a more active and efficient recovery and vesicular packaging of extracellular DA. These findings have important implications for sex differences in NSDA functions and responses to neurotoxins which enter the neurons via the DAT.

Gender differences in methamphetamine-induced mRNA associated with neurodegeneration in the mouse nigrostriatal dopaminergic system

In this report female and male CD-1 mice were treated with a neurotoxic regimen of methamphetamine (MA) to compare gender differences in striatal dopamine depletion and concordant changes in mRNA markers of the transforming growth factor-beta injury response associated with neurodegeneration. Striatal dopamine concentrations of MA-treated female mice were less depleted and significantly greater than that of identically treated males. Associated with this gender difference in striatal dopamine depletion were significantly decreased mRNA levels of plasminogen activator inhibitor-1 and a trend for increased (p = 0.06) mRNA levels of glial fibrillary acidic protein within females. No statistically significant differences between MA-treated female and male mice were obtained in mRNA levels for transforming growth factor-beta, transforming growth factor-beta type 2 receptor, activin-like kinase-5 or fibronectin. These data demonstrate the presence of changes in two specific molecular markers of the transforming growth factor-beta injury response which are in accordance with gender differences in MA-induced striatal dopamine depletion. The results suggest that the neuroprotective advantage displayed by females may in part be related to reductions in the transforming growth factor-beta injury response as indicated by decreased mRNA plasminogen activator inhibitor-1 and an increased response of reactive astrocytes which promote neuronal survival as indicated by augmented glial fibrillary acidic protein mRNA levels.

PF 9601N [N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine], a new MAO-B inhibitor, attenuates MPTP-induced depletion of striatal dopamine levels in C57/BL6 mice

Monoamine oxidase isoform B (MAO-B) is involved in Parkinson's disease (PD) induced by the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxin (MPTP) in human and non-human-primate. MAO-B inhibitors, such as L-deprenyl have shown to prevent against MPTP-toxicity in different species, and it has been used in Parkinson therapy, however, the fact that it is metabolized to (-)-methamphetamine and (-)-amphetamine highlights the need to find out new MAO-B inhibitors without a structural amphetaminic moiety. In this context we herein report, for the first time, anywhere a novel non-amphetamine-like MAO-B inhibitor, PF 9601N, N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine. This attenuates the MPTP-induced striatal dopamine depletion in young-adult and adult-old C57/BL mice, using different schedules of administration, and which behave "ex vivo" as a slightly more potent and selective MAO-B inhibitor than L-deprenyl, assayed for comparative purposes in the same experimental conditions. The MAO-B ID(50) values were calculated from the total MAO-B activity measured against [14C] phenylethylamine (22 microM) as substrate, at each inhibitor concentration. The MAO-B ID(50) values resulted to be 381 and 577 nmol/kg for PF 9601N and L-deprenyl, respectively. The intraperitoneally (i.p.) co-administration to young-adult C57/BL6 mice of MPTP (30 mg/kg), with different concentrations of PF 9601N or L-deprenyl (29.5-0.357 micromol/kg) showed a dose-dependent protective effect against striatal dopamine depletion, measuring the dopamine contents and its metabolites by HPLC. The ED(50) value proved to be 3.07 micromol/kg without any significant differences between either MAO-B inhibitor. Nevertheless, lower doses of PF 9601N (1.5 micromol/kg) were necessary to get almost total protection, without any change in the DOPAC and HVA content, when administered 2 h before MPTP (30 mg/kg), whereas partial protection (45%) against dopamine depletion was observed in the case of L-deprenyl. In both cases, MAO-B inhibition was a necessary condition in order to observe the protective effect. When adult-old (8-10 months) C57/BL6 mice were used, MPTP (25 mg/kg) administration induced 25 days later, an irreversible dopamine depletion. In these conditions, chronic administration with 0.15 micromol/kg of PF 9601N, before the toxin, every 24 h for 10 days, rendered almost total protection of dopamine depletion, whereas L-deprenyl yielded only 50% protection of the dopamine content, assayed in the same conditions. It is worth remarking, that in both cases MAO-B was not affected. From these results, it can be concluded that PF 9601N attenuates MPTP neurotoxicity "in vivo" better than L-deprenyl through different mechanisms, with special relevance to the protective effect, independent of MAO-B inhibition, observed in the irreversibly MPTP-lesioned adult-old mice. Therefore, this novel non-amphetamine MAO-B inhibitor could be potentially effective in PD therapy.

Neuroprotective role of estrogen upon methamphetamine and related neurotoxins within the nigrostriatal dopaminergic system

In this report we describe some of the data on the capacity for estrogen to function as a neuroprotectant of the nigrostriatal dopaminergic (NSDA) system. The data show that estrogen (E) can alter two different response characteristics to NSDA neurotoxins. The first being that striatal DA concentrations of ovariectomized rodents treated with E are consistently greater than non-E-treated animals in response to neurotoxins which produce degeneration of the NSDA system. The second being that E significantly reduces the amount of DA output upon initial exposure to the NSDA neurotoxin, 1-methyl-4-phenylpyridium ion (MPP+). At present, it is not known whether these two response characteristics are related. An intriguing possibility is that the E-dependent changes in initial DA output are related to the resultant neurotoxicity (attenuations in DA concentration reductions). So far our incipient findings do not seem to support this eventuality. However, additional testing on this topic is required. The present data suggest that one of the mechanisms by which E can exert these effects is through inhibition of DAT activity. This conclusion results from data which show that E produces: 1) an inhibition of [3H]DA uptake, 2) a reduction in DA clearance rates, and 3) an effect upon DA recovery that is similar to that observed to the putative DA uptake blocker, nomifensine. The capacity and significance for steroid hormones to modulate neurotransmitter transporters has been recently reviewed.

Estrogen modulates responses of striatal dopamine neurons to MPP(+): evaluations using in vitro and in vivo techniques

In vitro superfusion and in vivo electrochemistry were used to investigate the role of estrogen in modulating MPP(+)-induced dopamine output in the corpus striatum and nucleus accumbens of ovariectomized female rats. For in vitro superfusion experiments, dopamine and dihydroxyphenylacetic acid release were determined using HPLC with electrochemical detection from superfusion of corpus striatum fragments with Kreb's ringer phosphate buffer pulsed with MPP(+) alone or MPP(+) with estrogen. The in vivo electrochemistry experiments recorded the dopamine signal from carbon fiber microelectrodes stereotaxically passed through the corpus striatum and nucleus accumbens. Dopamine release was stimulated by pressure ejection of MPP(+) alone or in combination with estrogen through glass micropipettes fastened to the electrodes. Dopamine output from superfusion chambers which received infusion of MPP(+) with estrogen showed significantly lower output of dopamine compared with chambers which received MPP(+) alone. Outputs of dihydroxyphenylacetic acid did not increase following MPP(+) infusions. Data from the electrochemistry experiments demonstrated that estrogen significantly reduced both the amplitude and clearance rates of the MPP(+)-evoked dopamine signal in both the corpus striatum and nucleus accumbens. Results of this study demonstrate that: (1) MPP(+) evokes striatal dopamine release and this effect is significantly reduced in the presence of estrogen as determined by both in vivo electrochemistry and in vitro superfusion: (2) similar, albeit attenuated effects are observed in the nucleus accumbens as determined with in vivo electrochemistry; (3) estrogen acts to inhibit the clearance of dopamine in both the striatum and nucleus accumbens; and (4) estrogen may function as a neuroprotectant by reducing the uptake of neurotoxin into dopaminergic neurons.

Estrogen reduces acute striatal dopamine responses in vivo to the neurotoxin MPP+ in female, but not male rats

The effects of in vivo estrogen treatment upon MPP(+)-induced dopamine (DA) release were determined using in vivo microdialysis in female and male rats. Ovariectomized female rats were implanted or not with an estrogen pellet (0.1 mg, 17beta estradiol) and subjected to microdialysis 6 days later. After baseline DA release was determined, 5 mM MPP(+) was infused through the microdialysis probe for one 20-min interval. Perfusion resumed with normal medium for the duration of the experiment. A significant attenuation of MPP(+)-induced DA release was obtained in estrogen-treated females. One week later, striatal DA and dihydroxyphenylacetic acid (DOPAC) concentrations were determined for the lesioned and non-lesioned striata of each animal. MPP(+) infusion significantly decreased striatal DA concentrations, however, there was no effect of estrogen treatment on striatal DA depletion. This experiment was repeated using orchidectomized male rats treated with 0, 0.1, or 5 mg estradiol. In contrast to the females, no differences in MPP(+)-induced DA release were seen among these males, and there was no significant effect of the varying estrogen treatments on striatal DA or DOPAC concentrations. These results demonstrate that in vivo estrogen treatment attenuates MPP(+)-induced striatal DA release in gonadectomized female, but not male, rats.

Strain-dependent recovery of open-field behavior and striatal dopamine deficiency in the mouse MPTP model of Parkinson's disease

The neurotoxin MPTP can damage dopamine systems in the brains of rodents, cats, or monkeys, and is therefore widely used to model degenerative processes that underlie human Parkinson's disease. Here, we investigated the relationships between behavioral and neurochemical effects of systemic MPTP treatment in C57Bl/6 and Balb/c mice. Initially, different doses of MPTP were used to determine which of them might be useful to establish severe striatal dopamine depletions. These data showed that four injections of 20mg/kg at two hour intervals, were more efficient than 10 or 15mg/kg per injection. However, this dose was not usable due to its severe lethality in females. In contrast, 4x 15mg/kg had a low risk of lethality and led to substantial dopamine depletions, which were more severe in the neostriatum than the ventral striatum, and more severe in C57 than in Balb mice. In the first open field test, which was performed two hours after the last injection, this treatment led to severe behavioral inactivation in all parameters taken (distance and speed of locomotion, peripheral activity, frequency and duration of rearing). This effect was seen in both strains and gender. Thereafter, recovery differed between strains, since Balb mice, which had sustained the smaller lesions, had completely recovered on the subsequent day, whereas similar recovery took longer in C57 mice. On the fourth day, all groups appeared largely normal; however, the measure of rearing behavior still showed a deficit in C57 mice. This deficit on day 4 was correlated with neostriatal dopamine depletion; that is, the larger the lesion, the less the number and duration of rearings. Interestingly, these relationships were also observed with respect to ventral striatal dopamine damage, which was correlated with the rearing deficit not only on day 4, but also on day 1. These data will be discussed with respect to mechanisms of toxicity, functional recovery, and the function of striatal dopamine systems.

Cerebral cortical blood flow maps are reorganized in MAOB-deficient mice

Cerebral cortical blood flow (CBF) was measured autoradiographically in conscious mice without the monoamine oxidase B (MAOB) gene (KO, n=11) and the corresponding wild-type animals (WILD, n=11). Subgroups of animals of each genotype received a continuous intravenous infusion over 30 min of phenylethylamine (PEA), an endogenous substrate of MAOB, (8 nmol g-1 min-1 in normal saline at a volume rate of 0.11 microl g-1 min-1) or saline at the same volume rate. Maps of relative CBF distribution showed predominance of midline motor and sensory area CBF in KO mice over WILD mice that received saline. PEA enhanced CBF in lateral frontal and piriform cortex in both KO and WILD mice. These changes may reflect a differential activation due to chronic and acute PEA elevations on motor and olfactory function, as well as on the anxiogenic effects of this amine. In addition to its effects on regional CBF distribution, PEA decreased CBF globally in KO mice (range -31% to -41% decrease from control levels) with a lesser effect in WILD mice. It is concluded that MAOB may normally regulate CBF distribution and its response to blood PEA.

Tissue-specific effects of estrogen on monoamine oxidase A and B in the rat

Estrogen replacement therapy is widely used in postmenopausal women. The current study examines the effect of varying concentrations of estrogen on the levels of activity of monoamine oxidase A and -B in brain and in other tissues. Adult female rats were ovariectomized and randomized to receive a subcutaneous, slow-release preparation of either placebo or one of three doses of 17-beta-estradiol (0.05, 0.5, or 5.0 mg/pellet, estimated serum levels of 20-25 pg/ml, 100-600 pg/ml, and 1-2 ng/ml, respectively). Animals were sacrificed at 3 weeks and MAO-A and -B activity was assessed in homogenates of heart, liver, lung, uterus, kidney, adrenal and small intestine using 5-hydroxytryptamine and phenylethylamine as substrates. Cortex, amygdala and hypothalamus were microdissected from frozen sections of the brain and were also assayed for MAO-A and -B activity. High dose estrogen (5 mg/pellet) significantly decreased MAO-B activity and resulted in lesser or insignificant changes in MAO-A activity, respectively in liver (-30%, +1%), kidney (-22%, -11%), and uterus (-57%, -35%) (p < 0.05). No significant changes in enzyme activity were observed in heart, adrenal, lung and small intestine. In brain, estrogen (5 mg/pellet) decreased MAO-A activity in the hypothalamus (-28%) and amygdala (-21%), with no significant change seen in MAO-B. Our results suggest that estrogen exerts a tissue-specific, differential regulation of MAO-A and -B activity.

Estrogen as a neuromodulator of MPTP-induced neurotoxicity: effects upon striatal dopamine release

The effects of estrogen upon MPTP-induced neurotoxicity were examined using in vitro superfusion. In Experiment 1, striatal tissue from ovariectomized rats was infused with MPP+ (10 microM), a combination of MPP+ and 17beta-estradiol (300 nM), the same dose of estradiol preceding MPP+, or no treatment infusion. The effects of these treatments on dopamine release rates during the infusion periods were determined. Infusion of MPP+ resulted in a significant increase in dopamine release as compared to the control. Estradiol added to the MPP+ infusion significantly attenuated this dopamine (DA) release, while estradiol treatment preceding the MPP+ had no effect. In Experiment 2, three different doses of estradiol (0.3, 3, or 300 nM) were infused simultaneously with the MPP+. Doses of estradiol below 300 nM did not attenuate the DA release. In Experiment 3, estradiol alone (300 nM) was infused, to determine dopamine release rate effects of the hormone itself. There was no difference between estradiol treated and non-infused control groups. These results demonstrate that the gonadal steroid hormone estradiol can modulate responses of striatal dopamine neurons to MPP+ by altering the immediate increase in dopamine release which occurs in response to this neurotoxin. These modulating effects of estradiol are dose-dependent, and represent a direct effect upon striatal neurons, most likely involving a non-genomic mechanism of action. These results implicate that hormonal modulation of nigrostriatal dopaminergic neurotoxicity may represent an important variable responsible for the sex differences which are reported in Parkinson's disease.

Chronic treatment of Syrian hamsters with low-dose selegiline increases life span in females but not males

The only intervention conclusively shown to prolong life span in mammals is caloric restriction. Selegiline, a selective, irreversible inhibitor of monoamine oxidase B (MAO-B), is the first drug reported to reproducibly increase mean and maximum life span in animals, although this has only been demonstrated in male rats and mice. The effect on life span is commonly assumed to depend on MAO-B inhibition, but final experimental proof is missing. Therefore, we investigated the possible relationship between selegiline's effect on life span and MAO-B by monitoring survival data and MAO activity in Syrian hamsters of both sexes. Selegiline (0.05 mg/kg) significantly increased life span in female Syrian hamsters, but not in males. In contrast, MAO-B was inhibited equally in both sexes by about 40%, although females had a higher baseline MAO-B activity. No increase in MAO-B with age was observed. Female control hamsters had a shorter life span than male controls. Interestingly, this sex difference disappeared in the selegiline-treated animals. These findings suggest that the increase of life span by selegiline might be independent of MAO-B inhibition, but is possibly related to mechanisms determining sex differences of life span.

The dopamine-depleting effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in CD-1 mice are gender-dependent

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a selective dopaminergic neurotoxin affecting the nigrostriatal system in a variety of species including, rodents, nonhuman primates and humans. There exists, however, a great deal of variability in the sensitivity of different species to the effects of MPTP. The present study was designed to determine whether a significant difference in gender susceptibility to the toxin in CD-1 mice might also exist. A dosing regiment of 30 mg/kg MPTP once a day for 3 days (90 mg/kg total dose) in 4-month-old male and female CD-1 mice led to a significant depletion of striatal dopamine in both sexes. Two way ANOVA analysis of a time-course generated by measuring striatal dopamine at 4, 12 and 24 h after each dose of MPTP revealed that the initial dopamine reduction is significantly greater in male CD-1 mice (P < 0.001). Further, dopamine levels were reduced to a greater extent in male mice 5 days after the last dose (31% vs. 59% of control; P < 0.02). HPLC analysis using fluorescence detection revealed no difference in the striatal nor the cerebellar levels of MPP+ between the two sexes, however, accumulation of larger amounts of MPP+ was observed in the livers of the female mice. These findings suggest that, while female CD-1 mice are more resistant to the dopamine-depleting effects of MPTP, this gender difference is not due to decreased or accumulation of striatal MPP+.

Estrogen as a neuroprotectant against MPTP-induced neurotoxicity in C57/B1 mice

Castrated retired breeder male and female mice were treated or not with a 17 beta-estradiol pellet. At 10 days postcastration +/- estrogen treatment all animals were treated with MPTP. Five days later, concentrations of dopamine were determined from the corpus striatum and olfactory tubercle. Both castrated male and female mice treated with estrogen had significantly greater concentrations of dopamine within the corpus striatum compared with their respective gender controls, which did not receive estrogen. By contrast, no statistically significant differences in olfactory tubercle dopamine concentrations were obtained. Overall concentrations of dopamine within the corpus striatum, but not olfactory tubercle, were substantially greater in female vs. male mice. These data demonstrate that treatment with estrogen prevents reductions in corpus striatal dopamine concentrations in castrated mice treated with MPTP. Interestingly, this effect of estrogen was observed in both male and female mice. These results suggest that estrogen may serve as a neuroprotectant against an agent that is toxic to the nigrostriatal dopaminergic system in both male and female animal models of Parkinsonism.

Striatal concentrations of vesicular monoamine transporters are identical in MPTP-sensitive (C57BL/6) and -insensitive (CD-1) mouse strains

Sensitivity to the neurotoxic actions of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) varies greatly among strains of mice. The numbers of vesicular monoamine transporters was examined in various brain regions of MPTP-sensitive (C57BL/6) and MPTP-insensitive (CD-1) mouse strains. In vivo radioligand binding to the vesicular monoamine transporter was studied using [11C]methoxytetrabenazine, and in vitro Bmax values determined using [3H]dihydrotetrabenazine autoradiography. Using either technique, no significant differences between the two strains were seen in the striatal binding of these radioligands to the vesicular monoamine transporter. The in vivo binding of radioligands to this transporter in the striatum was also not gender dependent. The relative resistance of CD-1 mice to the neurotoxic effects of peripheral MPTP administration thus does not appear to be a result of enhanced protection by higher levels of vesicular storage in dopaminergic neurons of the striatum.