Markers of dopamine depletion and compensatory response in striatum and cerebrospinal fluid

Summary Though depletion of CSF homovanillic acid (HVA) concentration has often been regarded as a direct indicator of dopamine (DA) deficiency in Parkinson's Disease (PD), CSF HVA is normal in mildly affected patients. To explore why, we measured DA and its metabolites in striatum and CSF in rabbits receiving reserpine for 5 days. Reserpine, which depletes striatal DA by disrupting vesicular storage of the neurotransmitter, results in a compensatory increase of DA turnover. In response to a 96% depletion of striatal DA, its catabolic intermediates 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxytyramine (3-MT) decreased 64% and 92% in striatum, although the endproduct, HVA, was unchanged. In contrast, CSF concentrations of HVA and DOPAC increased significantly, though 3-MT and levodopa (LD) were unaltered. A 5-fold rise in striatal LD concentration after reserpine-induced DA depletion provided evidence for enhanced DA synthesis. As in PD, the compensatory increase of DA synthesis after reserpine administration confounds the ability of CSF HVA to reflect DA depletion.     

Changes in the cochlear dopaminergic system of the aged rat

The levels of dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) have been quantified in cochleae of male and female rats aged 3, 6, 9, 12, 19 and 24 months. Animals were exposed for 1 h, under general anesthesia, to: (1) silence (basal conditions) or (2) white noise at 90 dB SPL. Afterwards, the concentrations of DA, DOPAC and HVA were determined by HPLC with electrochemical detection in homogenates of individual cochleae. In basal conditions, the cochlear concentrations of DA, DOPAC and HVA in aged females were higher than in adult ones. The concentrations of DA and DOPAC were also higher in aged males with respect to adult ones. A decrease in DA and an increase in DOPAC and HVA concentrations, with respect to silence, were detected when adult animals were exposed to noise. Meanwhile, aged animals showed either a noise-induced increase or no modification of DA and DOPAC with respect to basal levels. Present results suggest age-related failures in DA release and metabolizing mechanisms within the cochlea, together with a compensatory DA synthesis increase. However, the possibility of an initial damage in the primary auditory neurons which could also stimulate the synthesis of DA must not be excluded. Present age-related changes could indicate that the cochlear dopaminergic innervation is affected during the aging process. Since this innervation plays an important role in both the modulation and the protection of the primary auditory neurons, its metabolic alteration could profoundly modify the auditory process.     

Effects of morphine treatment and withdrawal on striatal and limbic monoaminergic activity and ascorbic acid oxidation in the rat

Since ascorbic acid (AA) reportedly suppresses tolerance to and dependence on morphine in humans and rodents, levels of dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 3-methoxytyramine (3-MT), 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), AA, dehydroascorbic acid (DHAA), uric acid, xanthine, hypoxanthine, glutamate and γ-amino-butyric acid (GABA) were determined by high-pressure liquid chromatography (HPLC) in the striatum and in the limbic forebrain of the rat following morphine treatment (single or repeated) and withdrawal. Single morphine administration (20 mg/kg s.c.) increased DOPAC + HVA/DA, 5-HIAA/5-HT and DHAA/AA ratios, uric acid levels, and decreased xanthine, hypoxanthine, glutamate and GABA levels in both regions. 3-MT levels were decreased in the striatum and increased in the limbic forebrain. After 7 days of morphine treatment, striatal DOPAC + HVA/DA and DHAA/AA ratios and uric acid levels were still higher and striatal and limbic xanthine levels still lower than in controls, while all other parameters were in the range of control values in both regions. Morphine treatment also increased the glutamate/GABA ratio in the striatum. In all morphine-treated rats, individual striatal DOPAC + HVA/DA and DHAA/AA ratio values were directly correlated. After a 48 h withdrawal period, both striatal AA oxidation and glutamate/GABA ratio further increased; limbic 3-MT levels further decreased, while all other parameters did not differ from control values. We conclude that: (i) tolerance to morphine-induced increase in hypoxanthine, xanthine and AA oxidation develops in the limbic forebrain faster than in the striatum; (ii) the morphine-induced increase in striatal and limbic AA oxidation may be considered a consequence of increased formation of reactive oxygen species due to increased DA, hypoxanthine and xanthine oxidative metabolism; (iii) a striatal excitotoxic imbalance characterizes the withdrawal state and may be taken into account to explain the further increase in striatal AA oxidation.     

In vivo microdialysis studies of age-related alterations in potassium-evoked overflow of dopamine in the dorsal striatum of Fischer 344 rats

Intracerebral microdialysis was used to measure basal levels and potassium (K(+))-stimulated overflow of dopamine (DA), homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC), in the dorsal striatum of young (6 months) and aged (24 months) Fischer 344 (F344) rats. Basal levels of HVA were lower in aged rats whereas basal DA and DOPAC did not differ significantly between the two groups. The administration of three low to moderate doses of K(+) (10, 25, and 50 mM) through the microdialysis probe for one collection period revealed differences between the two age groups of F344 rats. DA overflow increased in a dose-dependent manner in the young but not aged rats. Extracellular levels of DOPAC and HVA decreased during the K(+) stimulation and there was a significant difference in the changes in HVA produced by K(+) stimulation in the young vs aged animals. These data support the hypothesis that low to moderate doses of K(+) may be necessary to demonstrate age-related differences in K(+)-evoked DA overflow, since previous microdialysis studies using higher doses have not reported age-related differences in DA overflow.     

Increased striatal dopamine turnover following acute administration of rotenone to mice

Because of the potential role of mitochondrial dysfunction in nigrostriatal degeneration in Parkinson's disease, the effects of rotenone (an inhibitor of mitochondrial NADH dehydrogenase and a naturally occurring toxicant) on the levels of striatal dopamine (DA) and DA metabolites were evaluated after acute and subchronic administration to mice. Systemic acute treatment with relatively high doses of rotenone did not affect DA concentration, but caused a significant increase in both DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). DOPAC and HVA changes were measured at 1 day and were reversed within 1 week, paralleling the time course of rotenone-induced increase in striatal lactate levels. Subchronic administration with a relatively mild dose of rotenone did not significantly alter the striatal levels of DA and DOPAC, while it slightly reduced HVA concentration. No neurochemical signs of dopaminergic damage were seen when mice were co-exposed to rotenone and diethyldithiocarbamate, a compound known to enhance nigrostriatal injury caused by the neurotoxicant 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP). Also, rotenone did not cause additional injury to animals previously lesioned by MPTP. Taken together, data indicate that rotenone is not capable of causing overt dopaminergic toxicity under the testing paradigms used in this study. Rather, an increase in DA turnover, as indicated by a higher (DOPAC+HVA)/DA ratio, seems to be associated to rotenone-induced striatal energy impairment.     

Dehydroepiandrosterone (DHEA) such as 17beta-estradiol prevents MPTP-induced dopamine depletion in mice

Previous work from our laboratory has shown prevention of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced striatal dopamine (DA) depletion in mice by 17beta-estradiol, progesterone, and raloxifene. Dehydroepiandrosterone (DHEA), a neurosteroid, was shown to have neuroprotective activities in various paradigms of neuronal death but its effect in vivo in mice on MPTP toxicity has not been reported. We investigated the effects of 17beta-estradiol (2 microg/day) and DHEA (3 mg/day) for 5 days before and after an acute treatment of four MPTP (10 mg/kg) injections in male C57Bl/6 mice. Striatal DA concentrations and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured by HPLC. MPTP mice that received 17beta-estradiol or DHEA had striatal DA, DOPAC, and HVA concentrations comparable to intact animals and higher than striatal DA, DOPAC, and HVA levels in saline-MPTP-treated mice. MPTP treatment led to an increase of striatal DA turnover (assessed with the HVA/DA ratio); DHEA and 17beta-estradiol prevented this increase. 17beta-Estradiol did not affect striatal DA and metabolites concentrations in intact mice in this paradigm. Furthermore, in the substantia nigra DHEA and 17beta-estradiol prevented the MPTP-induced dopamine transporter and tyrosine hydroxylase mRNA decreases measured by in situ hybridization. Therefore, DHEA such as 17beta-estradiol is active in preventing the catecholamine-depleting effect of MPTP and our results suggest that this involves neuroprotection of DA neurons.     

Concurrent locomotor stimulation and decrease in dopamine release in rats and mice after treatment with the competitive NMDA receptor antagonists D-CPPene and CGS 19755

Summary The present study was aimed at investigating the effects of the competitive N-methyl-D-aspartate (NMDA) receptor antagonists D-CPPene (3-(2-carboxypiperazine-4-yl)-propenyl-1-phosphonic acid) and CGS 19755 (cis-4-(phosphonomethyl)piperidine-2-carboxylic acid) on dopamine (DA) transmission and motor activity in mice and rats. As measures of DA release we used mouse brain 3-methoxytyramine (3-MT) levels, an indirect estimate of DA release, and striatal dialysate measures of DA in conscious and freely moving rats by means of microdialysis. To obtain additional information about monoaminergic neurotransmission, brain tissue levels of DA, DOPAC, HVA, 5-HT and 5-HIAA were measured in both mice and rats. The animals were sacrificed at the time when NMDA antagonist-induced locomotor stimulation was maximal. In mice, D-CPPene and CGS 19755 decreased striatal 3-MT levels, whereas, in general, 3-MT levels in the limbic forebrain were not significantly altered. Treatment with CGS 19755 decreased rat striatal dialysate levels of DA but increased 5-HIAA at time points when locomotor activity was increased. D-CPPene and CGS 19755 have been observed to produce psychotic symptoms in man. The present study suggests that these symptoms are not a result of an increase in central dopamine release.     

Isoflurane anesthesia induces biphasic effect on dopamine release in the rat striatum

The effect of isoflurane anesthesia on changes in the extracellular concentrations of dopamine (DA) and its metabolites (3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA)) modulated by pargyline, monoamine oxidase inhibitor, was studied using in vivo microdialysis techniques. A microdialysis probe was implanted into the right striatum of male SD rats. Each rat (n=5-6) was given saline or the same volume of 30 or 75 mg kg(-1) pargyline intraperitoneally with or without 1 h isoflurane anesthesia (1 or 3%). Isoflurane anesthesia increased the extracellular concentration of DA in high dose (3%) and increased the metabolite concentrations in a dose-dependent manner. Pargyline administration increased the extracellular concentration of DA and 3-MT, and decreased that of other metabolites. After 30 mg kg(-1) pargyline treatment, 1% isoflurane-induced DA release and increasing of 3-MT were preserved, whereas high dose isoflurane (3%) decreased the concentration of metabolites (DOPAC and HVA), despite of the increase by low dose isoflurane (DOPAC). When 75 mg kg(-1) pargyline was administered, isoflurane anesthesia decreased the concentration of DA and DOPAC. The isoflurane-induced 3-MT increase was preserved in all experiments. Our results suggest that isoflurane anesthesia induced biphasic effect on DA regulation probably by the potentiation of DA release and the inhibition of DA synthesis. Isoflurane might modulate DA homeostasis presynaptically.     

Long-term decreases in striatal dopamine, 3,4-dihydroxyphenylacetic acid, and homovanillic acid after a single injection of amphetamine in iprindole-treated rats: Time course and time-dependent interactions with amfonelic acid

The administration of a single dose of (+)-amphetamine sulfate (9.2 mg/kg) to rats treated with iprindole hydrochloride (10 mg/kg) produced marked decreases in the striatal concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) one week after drug administration. Significant changes were not observed in striatal 5-hydroxyindoleacetic acid (5-HIAA) nor in norepinephrine, DA, DOPAC, HVA, and 5-HIAA concentrations in frontal cortex and a limbic forebrain sample containing primarily nucleus accumbens and olfactory tubercles. In time-course experiments, decreases in striatal DA were apparent by 12 h after amphetamine plus iprindole administration and persisted for at least 4 weeks. Decreases in striatal DOPAC and HVA followed a similar time course, except decreases in these parameters were observed at 6 h as well. The administration of amfonelic acid, a potent DA uptake inhibitor, up to 8 h but not at 12 h after amphetamine administration prevented the decreases in striatal DA, DOPAC and HVA at one week after the administration of the drug to iprindole-treated rats. These data indicate that the actions of amphetamine which are necessary and sufficient for the production of long-term decreases in striatal DA, DOPAC and HVA are dependent upon the integrity of the neuronal uptake mechanism for DA and occur within 12 h after the administration of amphetamine to iprindole-treated rats. Although amfonelic acid prevented the long-term effects of amphetamine on striatal DA neurons, it did not alter the decrease in DOPAC produced by amphetamine at 6 h after the administration of amphetamine plus iprindole. This finding suggests that the ability of amfonelic acid to prevent the long-term effects of amphetamine on striatal DA neurons in iprindole-treated rats is not due to a blockade of the entry of amphetamine into the neuron and, thus, suggests that the access of amphetamine to the inside of the neuron is not sufficient for the production of its long-term, possibly neurotoxic, effects on striatal DA neurons.     

Ischemia in the dorsal hippocampus is associated with acute extracellular release of dopamine and norepinephrine

Summary The cerebral dialysis technique was employed to monitor extracellular concentrations of dopamine (DA), norepinephrine (NE), dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) in the dorsal hippocampus of gerbils before and after cerebral ischemia induced by carotid artery occlusion. Extracellular concentrations of DA and NE in the dorsal hippocampus increased from baseline levels of <35 fmol/collection interval to 180 and 200 fmol/collection, respectively, within 36 minutes following carotid artery ligation (n=8 animals). Extracellular concentrations of the DA metabolites, DOPAC and HVA, did not change significantly following carotid artery ligation. These data demonstrate that ischemia in the dorsal hippocampus is associated with a mared release of DA and NE. This release may contribute to the selective vulnerability of the dorsal hippocampus to neuronal damage during ischemia.     

Mouse strain- and age-dependent effects of binge methamphetamine on dopaminergic signaling

We have shown that a single "binge" dose of methamphetamine (Meth) in mice has long-lasting effects on open-field behavior dependent on mouse strain and age. Here we further investigated the impact of genotype and age on tyrosine hydroxylase (TH) loss and dopamine (DA) metabolism due to a high binge dose of Meth (4 × 5 mg/kg × 2 h × 2 days). Administration of high dose Meth or saline (Sal) to adolescent (PND 40) and adult (PND 80) C57BL/6 (B6), DBA/2 (DBA), and 129S6SvEv/Tac (129) mice was followed by a 1mg/kg Meth or Sal (control) challenge 40 days later. Striatal and prefrontal cortex tissues were collected 1h following the challenge. Meth-pretreated adolescent B6 and DBA mice exhibited losses in striatal DA concentrations; DBA adolescents also showed losses in striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and increased DA turnover. Pre-exposed B6 and 129 adults demonstrated significant decreases in striatal DA, DOPAC, and increased DA turnover; DBA adults showed significant losses in striatal DA and increased DA turnover. 129 and DBA adults exhibited increases and decreases, respectively, in prefrontal cortex DA. Adult pretreated B6 mice produced significant losses in striatal TH. The results again show age and genotype dependent differences in Meth-induced DA alterations.     

Distribution of dopamine D1 receptors in rat cortical areas, neostriatum, olfactory bulb and hippocampus in relation to endogenous dopamine contents

The tritiated dopamine D1 antagonist SCH23390 was employed to determine the densities of D1 receptors in seven discrete and functionally identified cortical areas (cingulate, frontal, parietal, primary somatosensory, primary visual, retrosplenial and entorhinal-piriform) as well as in the neostriatum, hippocampus and olfactory bulbs. In addition, the tissue levels of the catecholamines NA, AD, DA, the indoleamine 5-HT and their main metabolites (MHPG, DOPAC, HVA, 3-MT, 5-HTP and 5-HIAA) were measured in the different regions by HPLC with electrochemical detection. The Scatchard analysis of saturation curves revealed the highest density of [3H]SCH23390 binding sites for the neostriatum, while the densities were 10-20 times lower for total cerebral cortex and hippocampus respectively. For the olfactory bulb and other cortical areas, D1 receptor densities were determined by equilibrium binding at a fixed radioligand concentration approaching saturation. The distribution of D1 receptors was heterogeneous with the greatest densities in entorhinal-piriform and cingulate cortices. The endogenous DA levels measured for all regions correlated with their metabolite (DOPAC, HVA and 3-MT) contents (r = 0.999; P less than 0.001). There was also a very good correlation (r = 0.981; P less than 0.001) between tissue DA and D1 receptor densities. This quantitative information reflects particular features of the organization of the DA systems and is discussed in relation to turnover and recently established aspects of the DA innervation.     

Striatal trophic activity is reduced in the aged rat brain

Our previous studies demonstrated that the survival of a mesencephalic graft was reduced in aged animals suggesting an age-related decline in target-derived neurotrophic activity. We tested this hypothesis by examining dopamine (DA) and trophic activities from the striatum of intact or unilateral 6-hydroxydopamine (6-OHDA) lesioned rats of increasing age. Fisher 344 rats were 4, 12, 18, and 23 months old (m.o.) at sacrifice. Half the animals had received unilateral 6-OHDA lesions of the mesostriatal DA pathway 8 weeks earlier. Striatal tissue punches were analyzed for DA, homovanillic acid (HVA), and DA activity (HVA/DA) using HPLC. The remainder of the striatal tissue was homogenized to generate tissue extracts which were added to E14.5 ventral mesencephalic cultures to test trophic activity. In the non-lesioned animals, striatal DA was reduced and striatal DA activity was increased in the 18 and 23 m.o. animals relative to the 4 and 12 m.o. animals. Striatal trophic activity was inversely related to age. In the lesioned animals, striatal DA ipsilateral to 6-OHDA infusion was below detection limits while the contralateral striatum exhibited age-related changes in DA similar to those seen in the non-lesioned animals. In 4 m.o. lesioned rats, striatal trophic activity ipsilateral to 6-OHDA infusion was elevated by 26% relative to the contralateral side. The ipsi/contra-lateral differences in striatal trophic activity were reduced in 12 m.o. animals and absent in the 18 and 23 m.o. groups. These data suggest that advancing age is associated with a reduction in striatal DA as well as trophic activity. Moreover, the aged striatum loses its ability to biochemically and trophically compensate for DA reduction and therefore may represent a more challenging environment for the survival, growth, and function of a fetal graft.     

Effect of aging on dopamine metabolism in the rat cerebral cortex: a regional analysis

Summary Age-related changes in the levels of dopamine (DA) and its metabolites were measured in seven cerebral cortical areas and in the striatum of 3, 10 and 27 month-old Sprague-Dawley rats. An age-related increase in DA levels was observed in the somatomotor (SM) cortex. In contrast, a decrease was observed in the temporal (T) cortex. Decreases in homovanillic acid (HVA) levels were observed in prelimbic (PL), pyriform (PY) and T cortex of aged rats, whereas significant increases in the levels of 3-methoxytyramine (3-MT) were observed in PL, prefrontal (PF), cingulate (C) as well as in T cortex. In the striatum, DA and HVA were decreased but the level of 3-MT remained unchanged. Norepinephrine (NE) levels increased in rats from 3 to 27 months in all the cortical areas. The increase in the levels of the DA extraneuronal metabolite, 3-MT, confirms our previous results showing that the release of DA might be increased with age in some cortical areas. The present results show that there is no general age-related decrease in the level of monoamines and of their metabolites in the rat cerebral cortex and that the changes display a complex, area-specific pattern.     

Acute changes in dopamine release and turnover in rat caudate nucleus following a single dose of methamphetamine

Summary. Acute changes in dopamine (DA) turnover were studied in the caudate nucleus (CN) of adult male rats between 0–24 h after a single injection of Methamphetamine (20 mg/kg, ip). A single dose of METH-induced an increase in DA turnover [(DOPAC + HVA)/DA] concomitant with an acute DA release followed by transient DA and DOPAC depletion in the rat CN. Received February 15, 2002; accepted May 1, 2002 Published online June 28, 2002     

Intracerebral dialysis monitoring of striatal dopamine release and metabolism in response to L-DOPA

Summary We have used intracerebral dialysis to monitor the striatal extracellular fluid (ECF) in rats with unilateral lesions of the nigrostriatal dopamine (DA) pathway. Dialysis samples were collected before and after L-dihydroxyphenylalanine (L-DOPA) administration both in the presence and absence of carbidopa, an extracerebral DOPA decarboxylase (DDC) inhibitor. The baseline ECF levels of DA, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were always higher in the intact than in the lesioned striata. In the normal striata, dopamine (DA) concentrations increased following L-DOPA administration. Pretreatment with carbidopa prolonged the duration of the DA increase. In the lesioned striata, DA levels increased following L-DOPA administration only in animals pretreated with carbidopa. Following L-DOPA administration, striatal HVA and DOPAC levels increased considerably more in animals not pretreated with carbidopa than they did in pretreated animals. This increase was particularly marked in the lesioned striata and leads us to conclude that extracerebrally produced HVA and DOPAC can enter the brain extracellular space.     

Effects of capsaicin on central monoaminergic mechanisms in the rat

Summary The acute and chronic effects of capsaicin (s.c.) on the monoamines in the preoptic region + hypothalamus (RPO-H), spinal cord, substantia nigra and striatum were studied. Levels of DOPA, DA, DOPAC, HVA, 3-MT, NA, Trp, 5-HTP, 5-HT and 5-HIAA were determined by means of liquid chromatography (HPLC-EC). In response to acute capsaicin treatment, the levels of DA, DOPAC and DA synthesis rate (DOPA formation) were increased in a dose-dependent manner in the RPO-H and spinal cord. The disappearance rate of NA was accelerated in both regions. In substantia nigra, increased DOPAC levels were found whereas the levels of 3-MT were decreased in striatum after acute capsaicin treatment. Only minor changes on the levels of 5-HT and 5-HIAA in the regions studied were noted. Neonatal or adult capsaicin treatment failed to affect the levels of NA, DA and 5-HT (measured two months or five weeks after injection, respectively) in the regions studied. A capsaicin injection to rats pretreated with the drug as adults did not affect either the monoamines in the RPO-H and spinal cord or the body temperature. In contrast, in rats pretreated with capsaicin as neonates, a second injection of the drug to adult animals elicited hypothermia and changes in monoamines similar to those observed in naive animals.     

Combined administration of direct dopamine receptor agonists and L-dopa does not interfere with utilization of exogenous L-dopa in rat corpus striatum

The effect of combined administration of L-dopa with apomorphine or with bromocriptine on rat striatal concentrations of dopamine (DA), dihydroxyphenylacetic (DOPAC) and dopa was investigated. Injections of apomorphine or bromocriptine alone decreased striatal DOPAC levels indicating suppression of DA turnover in nigrostriatal terminals. The dopa-induced elevations in striatal concentrations of DA, DOPAC and dopa were similar among animals injected with L-dopa alone or in combination with apomorphine or bromocriptine. These findings suggest that although direct DA receptor agonists decrease striatal DA turnover, they do not interfere with utilization of exogenous L-dopa in striatum.     

Increase of striatal dopamine turnover by stress in MPTP-treated mice

The characteristics of motor function and brain dopamine (DA) metabolism in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice after immersion immobilization stress were investigated. There was no significant difference in locomotor activities between MPTP-treated and saline-treated mice, but locomotor activities of MPTP-treated mice after stress decreased more remarkably than those of saline-treated mice. Immediately after stress, striatal DA concentrations of MPTP-treated mice were significantly lower than those of saline-treated mice. Striatal DA levels improved when 24 h passed after stress. The striatal and cortical (DOPAC + HVA)/DA ratios of MPTP-and stress-treated mice was significantly higher than that of saline-and stress-treated mice. It is due to the decreased DA level and the enhancement of DA turnover that MPTP-treated mice became remarkably akinetic after stress, and that L-DOPA therapy is not effective when the symptoms in patients with Parkinson's disease worsen due to stress.