Marc Morissette

Sex and estrous cycle variations of rat striatal dopamine uptake sites.

The reuptake of dopamine into nerve terminals is the primary mechanism of inactivation of this neurotransmitter in the synaptic cleft. We report sex differences and estrous cycle variations of rat striatal dopamine uptake sites. During the estrous cycle, peak density of striatal dopamine uptake sites labelled with [3H]GBR-12935 occurred in the morning of proestrus in coincidence with peak dopamine, serotonin, dihydroxyphenylacetic acid and 5-hydroxytryptophan levels pointing to a presynaptic effect of gonadal hormones. Striatal homovanillic acid and 5-hydroxyindoleacetic acid levels as well as [3H]GBR-12935 binding affinity remained unchanged throughout the estrous cycle. The density of [3H]GBR-12935 striatal binding sites was lower in ovariectomized rats compared to intact female rats during the estrous cycle, whereas it was similar in gonadectomized male rats, intact male rats and ovariectomized rats. Binding affinity was in general similar for all the groups of rats examined. The affinity of dopamine for striatal [3H]GBR-12935 binding sites was similar in males and ovariectomized females, and did not change during the female estrous cycle. In summary, striatal dopamine uptake site density was lower in male compared to intact female rats and was shown to fluctuate during the female estrous cycle. These results suggest that gonadal hormones could influence the activity of psychoactive drugs acting on neuronal dopamine uptake sites.

Ovarian steroids and selective estrogen receptor modulators activity on rat brain NMDA and AMPA receptors

Glutamate and glutamate receptors are well known to play a major excitatory role in the brain. Recent findings on ovarian steroids and selective estrogen receptor modulators (SERMs) activity on rat brain AMPA and NMDA receptors are reviewed. Ovarian steroid withdrawal by ovariectomy is without effect on NMDA and AMPA receptors in most brain regions, except in hippocampus, where it decreases NMDA receptor specific binding, compared to intact rat values. Estradiol treatment increases hippocampal NMDA receptor specific binding of ovariectomized rats while it decreases this binding in frontal cortex and striatum. Estradiol treatment has no effect on AMPA receptor specific binding in hippocampus, but decreases binding in frontal cortex, striatum and nucleus accumbens. Progesterone and estradiol+progesterone treatments decrease NMDA, but not AMPA receptors specific binding in frontal cortex compared to ovariectomized rats. No effect was observed in other brain regions. Tamoxifen and raloxifene are SERMs with varying effects on estrogen responses in mammary, bone and uterine tissues. Tamoxifen and raloxifene have estrogenic activity upon modulation of brain NMDA and AMPA receptors. Using specific ligands for binding autoradiography of NMDA receptor subunits and specific probes for subunits measured by in situ hybridization, it was shown that estradiol and SERMs modulate NR1 and NR2B subunits whereas the NR1/2A subunit remains unchanged. In summary, regional agonist estrogenic activity on brain AMPA and NMDA receptors of tamoxifen and raloxifene, like that of estradiol, is observed, whereas progesterone has limited effects or opposes the estradiol effect.

Contribution of estrogen receptors alpha and beta to the effects of estradiol in the brain

Clinical and experimental studies show a modulatory role of estrogens in the brain and suggest their beneficial action in mental and neurodegenerative diseases. The estrogen receptors ERalpha and ERbeta are present in the brain and their targeting could bring selectivity and reduced risk of cancer. Implication of ERs in the effect of estradiol on dopamine, opiate and glutamate neurotransmission is reviewed. The ERalpha agonist, PPT, is shown as estradiol to modulate hippocampal NMDA receptors and AMPA receptors in cortex and striatum of ovariectomized rats whereas the ERbeta agonist DPN is inactive. Striatal DPN activity suggests implication of ERbeta in estradiol modulation of D2 receptors and transporters in ovariectomized rats and is supported by the lack of effect of estradiol in ERbeta knockout (ERKObeta) mice. Both ERalpha and ERbeta agonists modulate striatal preproenkephalin (PPE) gene expression in ovariectomized rats. In male mice PPT protects against MPTP toxicity to striatal dopamine; this implicates Akt/GSK3beta signaling and the apoptotic regulators Bcl2 and Bad. This suggests a role for ERalpha in striatal dopamine neuroprotection. ERKOalpha mice are more susceptible to MPTP toxicity and not protected by estradiol; differences in ERKObeta mice are subtler. These results suggest therapeutic potential for the brain of ER specific agonists.

Ovarian steroids and raloxifene prevent MPTP-induced dopamine depletion in mice.

The activity of steroids was studied in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesioned retired breeder C57BL/6 male mice as a model of Parkinsons disease. Steroids were injected daily for 5 days before MPTP (4 injections, 15 mg/kg i.p., at 2 h intervals) and hormonal treatment continued for 5 more days. Mice that received 17β-estradiol or progesterone or raloxifene (a selective estrogen receptor modulator) and MPTP had striatal concentrations of dopamine (DA) and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) similar to those in control animals, whereas mice that received MPTP alone or with 17β-estradiol (the isomer with weak estrogenic activity) had an extensive decrease of DA and its metabolites. These results suggest stereospecific prevention of MPTP-induced dopamine loss by 17β-estradiol, which is also observed with progesterone and raloxifene.

Effect of Chronic Estradiol and Progesterone Treatments of Ovariectomized Rats on Brain Dopamine Uptake Sites

Abstract: Dopamine released from brain nerve terminals is mainly removed from the synaptic cleft by an uptake mechanism. Despite their functional importance, modulation of the dopamine uptake sites is still not well known. Steroid hormones were shown to modulate brain dopamine transmission. The aim of this study was thus to investigate in ovariectomized rats the effects of 17β‐estradiol and progesterone treatments on brain dopamine uptake sites. Treatments consisted of 17β‐estradiol (10 μg/0.2 ml), progesterone (0.72 mg/0.2 ml). 17β‐estradiol + progesterone, or the vehicle (0.3% gelatin in saline solution) twice daily for 2 weeks. The steroid treatments left the affinity of [3H]GBR 12935 binding to striatal homogenates unchanged (ovariectomized rats, 0.823 ± 0.028 nM), whereas the density was increased by these steroids alone or in combination to a similar extent of 16‐23%. Chronic treatment of ovariectomized rats with 17β‐estradiol progesterone, or their combination increased to the same extent and uniformly [3H]‐GBR 12935 binding in the striatum as measured by autoradiography; the increase was similar in the substantia nigra pars compacta, whereas no steroid effect was observed in the nucleus accumbens and in the substantia nigra pars reticulata. In summary, chronic exposure to 17β‐estradiol and/ or progesterone increased dopamine uptake site density in the nigrostriatal dopaminergic system, whereas the nucleus accumbens and the substantia nigra pars reticulata were unaffected.

Resveratrol, a red wine polyphenol, protects dopaminergic neurons in MPTP-treated mice

Phytoestrogens, and particularly resveratrol, a red wine polyphenol, are currently under study for their therapeutic antioxidant properties. Administration of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to C57BL/6 mice targets nigrostriatal dopaminergic neurons, leading to cell death and striatal dopamine (DA) depletion. The aim of the present study was to analyze the protective effect of a diet rich in resveratrol against MPTP-induced neuronal death. Male mice were kept on a phytoestrogen-free diet, supplemented or not with 50 or 100 mg/kg/day of resveratrol for 1 or 2 weeks, after which MPTP was injected intraperitoneally. We observed that daily administration of resveratrol prevented MPTP-induced depletion of striatal DA, and maintained striatal tyrosine hydroxylase (TH) protein levels. Our results also demonstrated that mice treated with resveratrol prior to MPTP administration showed more abundant TH-immunopositive neurons than mice given only MPTP, indicating that resveratrol protects nigral neurons from MPTP insults. Altogether, these data revealed that resveratrol can counteract the toxic effects of the neurotoxin MPTP and, as such, it may be regarded as a powerful molecule for complementary neuroprotective therapy.

Preproenkephalin mRNA expression in the caudate-putamen of MPTP monkeys after chronic treatment with the D2 agonist U91356A in continuous or intermittent mode of administration: comparison with L-DOPA therapy.

The effect of chronic treatment with the D2 dopamine agonist U91356A or L-DOPA therapy on the regulation of preproenkephalin (PPE) mRNA was investigated in the caudate-putamen of previously drug-naive cynomolgus monkeys Macaca fascicularis rendered parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In MPTP monkeys, pulsatile treatment with either L-DOPA or U91356A relieved parkinsonian symptoms but caused progressive sensitization to treatment and, as expected, induced choreic dyskinesias. In contrast, U91356A given in a continuous mode led to partial behavioral tolerance without appearance of dyskinesias. Using in situ hybridization histochemistry, lesioning was shown to produce elevation of PPE mRNA levels in the lateral and medial parts of the putamen and in the lateral part of the caudate nucleus compared to control animals at the three rostrocaudal regions analyzed. In general, no change of PPE mRNA levels were observed in the medial caudate after MPTP lesioning with or without L-DOPA or U91356A treatments in the three rostrocaudal regions measured except for an increase in the caudal part of L-DOPA-treated MPTP monkeys. In the putamen and lateral caudate nucleus, elevated PPE mRNA expression by MPTP generally was not corrected (or only partially corrected) by chronic L-DOPA treatment except for the rostral medial putamen where correction to control values was observed. In general, pulsatile administration of U91356A partially corrected the lesion-induced elevation of PPE mRNA levels in the putamen and lateral caudate nucleus whereas the correction was more pronounced and widespread when MPTP monkeys received the continuous administration of this drug. These results indicate that the mode of administration of a D2 dopamine receptor agonist, such as U91356A, although at a roughly equivalent dosage influences the extent of inhibition of the expression of PPE in the denervated striatum of monkeys. In addition, the general lack of correction of the MPTP-induced increase of PPE mRNA in the striatum of L-DOPA-treated monkeys compared to the decreases observed with the D2 agonist treatments suggest that the D1 agonist component of L-DOPA therapy opposes the D2 agonist activity. Hence, D1 receptor agonist activity would stimulate PPE mRNA expression whereas D2 receptor agonists inhibit the expression of this peptide. Increases in PPE expression in the striatum may be implicated in the induction of dyskinesias since both groups of treated MPTP monkeys displaying dyskinesias had elevated striatal PPE mRNA levels whereas the MPTP monkeys with the lowest striatal PPE mRNA levels developed tolerance without dyskinesias.

Stereospecific prevention by 17?-estradiol of MPTP-induced dopamine depletion in mice

Neuroprotective activity of estrogens is reported in Alzheimer disease and recently has also been suggested for Parkinson disease, a disease affecting more men than women. To characterize this estrogenic activity, we studied the effects of 17β‐ and 17α‐estradiol treatment (1 μg twice daily 5 days before, during the day of four MPTP (15 mg/kg) injections, and for the following 5 days) on dopamine striatal toxicity induced by the neurotoxin MPTP in retired breeder male C57BL/6 mice. Striatal dopamine concentrations and its metabolites dihydroxyphenylacetic acid and homovanillic acid measured by HPLC in MPTP mice that received 17β‐estradiol were comparable to control animals, whereas MPTP mice treated with saline or 17α‐estradiol showed important decreases of dopamine and its metabolites. Striatal serotonin and its metabolite 5‐hydroxyindoleacetic acid concentrations remained unchanged after MPTP and treatments with steroids. Striatal [3H]GBR 12935 binding autoradiography to the dopamine transporter was as extensively decreased and correlated with dopamine depletion in MPTP mice, whereas this transporter mRNA decrease in the substantia nigra pars compacta was less pronounced. Treatment with steroids did not significantly change [3H]GBR 12935 binding, whereas dopamine transporter mRNA levels were not significantly different from controls. Under the present paradigm in retired breeder male mice, our results show dopaminergic and stereospecificity of estradiol to augment dopamine levels in MPTP‐lesioned mice without protecting against the extensive loss of dopamine terminals and moderate cell body loss. Synapse 37:245–251, 2000.

mGluR5 metabotropic glutamate receptors and dyskinesias in MPTP monkeys

Modulation of excessive glutamatergic transmission within the basal ganglia is considered as an alternative approach to reduce l-Dopa-induced dyskinesias (LIDs) in Parkinsons disease (PD). In this study receptor binding autoradiography of [3H]MPEP, a metabotropic glutamate receptor 5 (mGluR5) selective radioligand, was used to investigate possible changes in mGluR5 in the basal ganglia of l-Dopa-treated 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys having developed LIDs compared to animals in which LIDs was prevented by adjunct treatments. LIDs were associated with an increase of mGluR5 specific binding in the posterior putamen and pallidum (+41% and +56%) compared to controls. By contrast, prevention of dyskinesias was associated with an important decrease of mGluR5 specific binding in these areas (-37% and -48%) compared with dyskinetic animals. Moreover, an upregulation (+34%) of mGluR5 receptor binding was seen in the anterior caudate nucleus of saline treated MPTP monkeys. This study is the first to provide evidence that enhanced mGluR5 specific binding in the posterior putamen and pallidum may contribute to the pathogenesis of LIDs in PD.

Differential Regulation of Striatal Preproenkephalin and Preprotachykinin mRNA Levels in MPTP-Lesioned Monkeys Chronically Treated with Dopamine D1 or D2 Receptor Agonists

Abstract : Studies in 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐lesioned monkeys and in parkinsonian patients show elevated preproenkephalin (PPE) mRNA levels, unaltered by chronic l‐DOPA therapy, whereas preprotachykinin (PPT) mRNA levels are decreased by the lesion and corrected by l‐DOPA. The relative contributions of the dopamine D1 and D2 receptors for PPE mRNA regulation were investigated in the present study and compared with those for PPT mRNA. In situ hybridization was used to measure peptide mRNA levels in the striatum of MPTP cynomolgus monkeys after chronic 1‐month treatment with the D1 agonist SKF‐82958, administered subcutaneously in pulsatile or continuous mode, compared with the long‐acting D2 agonist cabergoline. Normal as well as untreated MPTP animals were also studied. PPE mRNA levels were elevated in the caudate nucleus and putamen of untreated MPTP monkeys compared with control animals with a more pronounced increase in the lateral as compared with the medial part of both structures. PPT mRNA levels showed a rostrocaudal gradient, with higher values in the middle of the caudate‐putamen and more so in the medial versus the lateral parts. PPT mRNA levels were decreased in the caudate and putamen of untreated MPTP monkeys compared with control animals, and this was observed in the middle and posterior parts of these brain areas. Elevated PPE and decreased PPT mRNA levels observed after MPTP exposure were corrected after treatment with cabergoline (0.25 mg/kg, every other day), a dose that had antiparkinsonian effects and did not give sustained dyskinesia. In contrast, elevated PPE mRNA levels observed in untreated MPTP monkeys were markedly increased by pulsatile administration of SKF‐82958 (1 mg/kg, three times daily) in two monkeys in which the parkinsonian symptoms were improved and dyskinesias developed, whereas it remained close to control values in a third one that did not display dyskinesias despite a sustained improvement in disability ; a shorter duration of motor benefit (wearing off) over time was observed in these three animals. By contrast, pulsatile administration of SKF‐82958 corrected the decreased PPT level observed in untreated MPTP monkeys. Continuous treatment with SKF‐82958 (equivalent daily dose) produced no clear antiparkinsonian and dyskinetic responses and did not alter the denervation‐induced elevation of PPE or decrease of PPT mRNA levels. The present data suggest an opposite contribution of the dopamine D1 receptors (stimulatory) as compared with the dopamine D2 receptors (inhibitory) on PPE mRNA, whereas a similar stimulatory contribution of D1 or D2 receptors is observed for PPT mRNA. An increase in PPE expression could be involved in the induction of dyskinesias and wearing off, whereas our data do not support this link for PPT. The antiparkinsonian response was associated with a correction of the lesion‐induced decrease of PPT.