Effects of neonatal and prepubertal hormonal manipulations upon estrogen neuroprotection of the nigrostriatal dopaminergic system within female and male mice.

Estrogen (E) can function as a neuroprotectant of the nigrostriatal dopaminergic (NSDA) system against methamphetamine (MA) neurotoxicity in female, but not male, mice. In the present report we examined whether the organizational effects of gonadal steroid hormones, as exerted in the early postnatal period, or developmental effects, as exerted during the pubertal period, would contribute to this sexually dimorphic neuroprotectant action of E. Neonatal gonadectomy and treatment with testosterone of female mice, retained the ability to show an E neuroprotectant response when tested as adults. However, females not treated with gonadal steroids failed to show an E-dependent neuroprotectant response. Neonatal gonadectomy of male mice, failed to result in the display of an E neuroprotectant response when tested as adults. Prepubertal gonadectomy of female mice, with or without testosterone treatment, abolished the capacity for E to produce neuroprotection against MA-induced NSDA neurotoxicity. Nor did prepubertal gonadectomy enable male mice to show an E neuroprotectant response. Taken together these results demonstrate that none of the manipulations performed within male mice enabled them to show an E-dependent neuroprotective response against MA-induced neurotoxicity of the NSDA system when tested as adults. For the female, it appears that the presences of gonadal steroids at these two developmental periods are needed for the display of an E-dependent neuroprotectant response within the adult.

Age-related changes in nigrostriatal dopaminergic function are accentuated in +/- brain-derived neurotrophic factor mice.

The effects of a deletion for the brain derived neurotrophic factor (BDNF) allele (+/- BDNF) upon age-related changes in nigrostriatal dopaminergic (NSDA) function were assessed. Behavioral (beam crossing and spontaneous activity) and neurochemical (potassium-stimulated dopamine release from superfused striatum) measures were compared among Young (4-5 month), Middle (11-13 month) and Aged (19-21 month) +/- BDNF and their wild type littermate control (+/+ BDNF) mice. No statistically significant differences were obtained between +/+ and +/- BDNF mice at the Young age sampling period for any of the behavioral or neurochemical measures. Behavioral and neurochemical responses indices of NSDA function begin to diverge between +/+ and +/- Middle age BDNF mice and maximal differences were observed at the Aged period. For both movement and stereotypy times, scores obtained from +/+ mice were significantly decreased compared with +/- BDNF mice at the Aged period and center time scores of +/+ mice were decreased at both the Middle and Aged periods compared with +/- BDNF mice. Neurochemically, potassium-stimulated DA release of +/+ mice was significantly greater than +/- BDNF mice with maximal differences obtained at the Aged period. These results demonstrate marked differences in age-related changes of NSDA function between +/+ and +/- BDNF mice and suggest that the deletion of one allele for BDNF may make these mice more susceptible to age-related declines in NSDA function.

Acute effects of estrogen upon methamphetamine induced neurotoxicity of the nigrostriatal dopaminergic system.

Estrogen acts as a neuroprotectant of the nigrostriatal dopaminergic system when given chronically to female mice prior to Methamphetamine (MA) insult. In this report, we tested the acute effects of Estradiol Benzoate (EB-10 microg in Oil) in ovariectomized CD-1 mice to function as a neuroprotectant when administered prior to (Experiment 1) or after (Experiment 2) MA treatment. Striatal dopamine (DA) concentrations and DOPAC/DA ratios were measured to assess the neuroprotective effects of EB. In Experiment 1, we observed that EB exerted a neuroprotective effect upon striatal dopamine concentrations when administered at 24 and 12, but not at 0.5, hours prior to MA injection and upon DOPAC/DA ratios when administered at 24, 12 and 0.5 hours prior to MA. In Experiment 2, no evidence for estrogen to protect the striatum from MA insult was obtained when EB was administered at 15, 30, 60 or 120 minutes after MA. These results show that EB can act as a modulator of MA-induced nigrostriatal dopaminergic neurotoxicity suggestive of a neuroprotectant, when administered within 0.5 hour of MA insult as assessed by measures of DOPAC/DA, but fails to prevent depletion of DA when given after MA insult. The data suggest that estrogen may exert this rapid neuroprotective effect through a non-genomic mechanism.

Estrogen, but not testosterone, attenuates methamphetamine-evoked dopamine output from superfused striatal tissue of female and male mice.

The gonadal steroid hormone, estrogen, has the capacity to function as a neuroprotectant against methamphetamine (MA)-induced neurotoxicity of the nigrostriatal dopaminergic system within female, but not male, mice. In an attempt to understand some of the bases for this effect of estrogen, the incipient effects of MA upon evoked dopamine output from superfused striatal tissue fragments of gonadectomized female and gonadectomized as well as intact male mice were evaluated under conditions where estrogen (or testosterone) was present in the medium. The amount of dopamine evoked by MA was significantly reduced when estrogen was co-infused with MA. This attenuation was obtained with striatal tissue fragments of gonadectomized female and gonadectomized and intact male mice. In contrast to estrogen, co-infusion of testosterone failed to produce an overall statistically significant change in MA-evoked dopamine output within superfused striatal tissue fragments of gonadectomized female and male mice. In this way, the gonadal steroid hormones, estrogen and testosterone, exert differential modulatory effects upon MA-evoked dopamine output from superfused striatal tissue fragments. However, similar effects to these gonadal steroid hormones were observed between gonadectomized female and gonadectomized or intact male mice. These data reveal an absence of a sexual dimorphism in striatal responsiveness with regard to estrogen's ability to alter MA-evoked DA output. Accordingly, the sexually dimorphic capacity for estrogen to function as a neuroprotectant may involve a composite of actions upon the nigrostriatal dopaminergic system involving events/sites other than the initial stimulation of dopamine output.

Tamoxifen diminishes methamphetamine-induced striatal dopamine depletion in intact female and male mice.

It has been demonstrated that the nigrostriatal dopaminergic system of male mice is more sensitive to the neurotoxic effects of methamphetamine (MA). The basis for this difference can be related to oestrogen, which has the capacity to function as a neuroprotectant against neurotoxins that target the nigrostriatal dopaminergic system. We examined the effects of the anti-oestrogen, tamoxifen (TMX), upon MA-induced neurotoxicity of the nigrostriatal dopaminergic system in intact female and male CD-1 mice. Striatal dopamine concentrations of TMX-treated female and male mice receiving MA were significantly greater than mice receiving MA alone. In female, but not male, mice, oestrogen treatment also resulted in greater striatal dopamine concentrations compared to mice receiving MA alone. Interestingly, male mice treated with oestrogen were particularly sensitive to the acute toxic effects of MA and displayed no evidence of nigrostriatal neuroprotection. The dihydroxyphenylacetic acid/dopamine ratios following MA for female and male mice treated with TMX or females treated with oestrogen were significantly reduced compared to MA-treated mice and oestrogen + MA-treated male mice. No differences among the treatment groups were obtained for dopamine in the hypothalamus or olfactory bulb. These data demonstrate that TMX treatment of intact female and male mice diminishes striatal dopamine depletions to the nigrostriatal dopaminergic neurotoxin, MA. Oestrogen also displayed this capacity when administered to female, but accentuated acute toxicity in male mice. These effects are relatively specific for the nigrostriatal dopaminergic system. Such data suggest that TMX can function as a nigrostriatal dopaminergic neuroprotectant against MA-induced neurotoxicity in intact female and male mice.

Evaluation of nigrostriatal dopaminergic function in adult +/+ and +/- BDNF mutant mice.

Deletion of a single copy of the BDNF gene has been shown to affect the nigrostriatal dopaminergic system of young adult BDNF mice. In the present report we evaluated various indices of nigrostriatal dopaminergic function between 9-month-old wild-type (+/+) and heterozygous (+/-) BDNF mutant mice. Performance in a sensorimotor beam walking task was significantly decreased in +/- mice as indicated by increased times required to traverse both a wide (21 mm) and narrow (6 mm) beam. No differences in spontaneous locomotor behavior were observed between the +/+ and +/- mice. Amphetamine-stimulated (5 mg/kg) locomotor behavior was increased to a greater degree in the +/- mice, with the number of movements performed by these mice being significantly greater than their +/+ controls. Corpus striatal dopamine concentrations were significantly greater in the +/- BDNF mice. The absence of any significant differences for dopamine concentrations within the hypothalamus and olfactory bulb of these mice, as well as an absence of any difference in striatal norepinephrine concentrations, suggested a relative specificity of these effects to the corpus striatum. Both the +/- and +/+ mice showed similar reductions in striatal dopamine concentrations in response to a neurotoxic regimen of methamphetamine (20 mg/kg). Collectively these data show increased levels of striatal dopamine concentrations associated with altered behavioral responses involving the nigrostriatal dopaminergic system within the heterozygous BDNF mutant mice.

Tamoxifen eliminates estrogen's neuroprotective effect upon MPTP-induced neurotoxicity of the nigrostriatal dopaminergic system.

The capacity for 17-alpha and 17-Beta estradiol to modulate MPTP-induced neurotoxicity of the nigrostriatal dopaminergic system and potential antagonism of this modulation by the anti-estrogen, tamoxifen, were evaluated. Treatment of retired breeder ovariectomized C57/Bl mice with 17-Beta estradiol diminished the amount of striatal dopamine reduction resulting from MPTP treatment with striatal dopamine concentrations of these 17-Beta estradiol treated mice failing to differ significantly from vehicle treated controls. A combined administration of 17-Beta estradiol with tamoxifen abolished this neuroprotective effect of estrogen as striatal dopamine concentrations of this group were significantly lower than vehicle treated controls. Results to 17-alpha estradiol were less effective since striatal dopamine concentrations of these mice following MPTP treatment were significantly decreased as compared with vehicle controls. In contrast to the nigrostriatal dopaminergic system, no statistically significant effects of these treatments were observed upon olfactory bulb dopamine concentrations. Taken together, these results show that 17-Beta, but not an equivalent concentration of 17-alpha estradiol was effective in decreasing striatal dopamine neurotoxicity to MPTP. This effect of 17-Beta estradiol was abolished by tamoxifen. These data have important implications regarding the mechanisms of estrogen-tamoxifen interactions within the nigrostriatal dopaminergic system as well as for clinical applications of tamoxifen within pre-menopausal women.

Interactive effects of tamoxifen and oestrogen upon the nigrostriatal dopaminergic system: long-term treatments.

In the present report adult female rats were ovariectomized (OVX) and assigned to one of four treatment conditions. Treatments consisted of administering pellets containing 17beta-oestradiol (E), tamoxifen (TMX), a combination of TMX and E or no further treatment (OVX). Animals received these treatments immediately following OVX and were maintained in these conditions for a 40-day period. Subsequently, the corpus striatum (CS) was dissected from each animal and prepared for determinations of basal and amphetamine stimulated DA output using in-vitro superfusion. No statistically significant differences among the four treatment groups were obtained for basal dopamine output. The highest levels of amphetamine-stimulated dopamine responses were obtained from E treated rats. These values were significantly greater than that obtained from OVX rats and rats treated with a combination of TMX+E. The significance of these findings is that they indicate both a non-traditional central nervous system site and mechanism of action through which tamoxifen-oestrogen interactions can function. Such data may have important implications for administration of tamoxifen to premenopausal women as this anti-oestrogen may compromise nigrostriatal dopaminergic function under conditions where oestrogenic modulation is present.

Tamoxifen alters dopamine output through direct actions upon superfused corpus striatal tissue fragments.

Tamoxifen (10 pg/ml) was infused directly into superfused striatal tissue fragments of ovariectomized rats for a 50 min period. Immediately following the termination of tamoxifen there was a significant increase in dopamine output compared with non-infused controls. No such significant increase was observed with use of a 100 pg/ml tamoxifen dose. Although dopamine output was again increased upon termination of a 2 h infusion of tamoxifen, these levels failed to differ significantly from that of non-infused controls. Similarly, a shorter 10 min duration infusion of tamoxifen failed to alter dopamine output. Finally, we examined whether the tamoxifen-induced, post-infusion increase in dopamine output, as observed following a 50 min infusion of 10 pg/ml, involved a calcium dependent process. To achieve this goal, superfusions were performed with Calcium/Tamoxifen, No Calcium/Tamoxifen, No Calcium/No Tamoxifen and Calcium/No Tamoxifen. A significant increase in dopamine output post-tamoxifen infusion was obtained for the Calcium/Tamoxifen condition compared with the remaining three groups which failed to differ from one another. Taken together these results show that tamoxifen can alter dopamine output through direct, non-genomic effects upon striatal neurons. Responses to this anti-estrogen are intriguing since they are apparent following removal, but not during tamoxifen infusion and represent a calcium-dependent process. These data suggest that tamoxifen may represent an important modulator of nigrostriatal dopaminergic function.

The effects of nicotine on dopamine and DOPAC output from rat striatal tissue.

The effects of varying doses of nicotine infusion upon spontaneous (basal) and subsequent potassium chloride-stimulated dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) output from superfused corpus striatal tissue fragments of male rats were tested. Spontaneous dopamine and DOPAC outputs were increased in response to 1, 5 and 10, but not to 0.1 and 0 (control) microM concentrations of nicotine. Interestingly, the subsequent K+-stimulated (30 mM) dopamine output was completely abolished in preparations infused with the 5 and 10 microM nicotine, but not with the 1 or 0.1 microM nicotine. No overall significant differences in K+-stimulated DOPAC were obtained among the five doses. In experiment 2, the effects of an initial infusion of amphetamine (10 microM), potassium chloride (30 mM), nicotine (10 microM) or normal superfusion medium (control) were compared upon subsequent K+-evoked dopamine release. The amount of dopamine released in response to the second (subsequent) infusion of K+ was significantly greater in the potassium chloride and control conditions versus the nicotine and amphetamine stimulated groups. No overall differences in DOPAC output were observed among the four conditions of experiment 2. These results demonstrate that nicotine can exert differential modulatory effects upon striatal dopaminergic activity as a function of the dose. The augmented levels of DOPAC output along with the abolition of the K+-stimulated dopamine release in response to the 5 and 10 microM nicotine doses suggest that these doses may simultaneously produce an activation of intraneuronal metabolism of dopamine to DOPAC along with an activation of release and inhibition of uptake to diminish stores available for subsequent responses to K+ stimulation.

Interactive effects of tamoxifen and estrogen upon the nigrostriatal dopaminergic system.

Adult female rats were ovariectomized and received a 21-day release pellet containing either 17 beta-estradiol (0.1 mg), tamoxifen (5.0 mg), a combination of estradiol and tamoxifen or no further treatment. At 14 days following ovariectomy +/- hormone treatments rats were sacrificed, the corpus striatum removed and prepared for assessment of dopamine release using in vitro superfusion. Maximal potassium-stimulated dopamine release rates were obtained with the estradiol+tamoxifen-treated rats and these levels were significantly greater than those from animals receiving only tamoxifen. Similarly, maximally amphetamine-stimulated responses were obtained from estradiol+tamoxifen-treated rats, however, in contrast to potassium, these values were significantly greater than both animals receiving either estradiol or tamoxifen alone. These data demonstrate that the nigrostriatal dopaminergic system appears particularly sensitive to the modulatory effects of a combined treatment with estradiol+tamoxifen. Moreover, some of the potential mechanisms of these responses are indicated by the differential dopamine outputs as evoked by potassium or amphetamine. The significance of these synergistic actions is their potential to mimic changes that may occur under conditions of tamoxifen treatment of premenopausal women as has been suggested for women a risk for breast cancer.

Estrogen differentially modulates nicotine-evoked dopamine release from the striatum of male and female rats.

In the present experiment we examined the effects of an in vitro infusion of nicotine (10 microM) upon dopamine release from superfused striatum of castrated male and female rats treated or not treated with estrogen. Estrogen exerted bidirectional effects on nicotine-evoked dopamine release as a function of the sex of the animal. Nicotine-evoked dopamine release was increased in estrogen treated females and decreased in estrogen treated males. Peak nicotine-evoked dopamine output from estrogen treated females was significantly greater than that of estrogen treated males. These results may be related to the gender differences in response to nicotine and smoking behavior.