Claude Forni

Sleep disorders in Parkinson's disease: The contribution of the MPTP non-human primate model

To replicate the sleep-wake disorders of Parkinsons disease (PD) and to understand the temporal relationship between these sleep disturbances and the occurrence of parkinsonism, we performed long-term continuous electroencephalographic monitoring of vigilance states in unrestrained rhesus monkeys using an implanted miniaturized telemetry device and tested the effect of MPTP intoxication on their sleep-wake organization. MPTP injection yielded a dramatic disruption of sleep-wake architecture with reduced sleep efficacy that persisted years after MPTP administration. Primary deregulation of REM sleep and increased daytime sleepiness occurring before the emergence of motor symptoms were a striking feature of the MPTP effect. This was concomitant with a breakdown of dopaminergic homeostasis, as evidenced by decreased dopamine turnover measured after a single MPTP injection. In the long term, partial re-emergence of REM sleep paralleled the partial adaptation to parkinsonism, the latter being known to result from compensatory mechanisms within the dopaminergic system. Altogether, these findings highlight the suitability of the MPTP model of PD as a tool to model the sleep/wake disturbances of the human disease. Ultimately, this may help in deciphering the specific role of dopamine depletion in the occurrence of these disorders.

High frequency stimulation of the subthalamic nucleus has beneficial antiparkinsonian effects on motor functions in rats, but less efficiency in a choice reaction time task

Chronic subthalamic nucleus high frequency stimulation (STN HFS) improves motor function in Parkinsons disease. However, its efficacy on cognitive function and the mechanisms involved are less known. The aim of this study was to assess the effects of STN HFS in hemiparkinsonian awake rats performing different specific motor tests and a cognitive operant task. Unilateral STN HFS applied in unilaterally DA‐depleted rats decreased the apomorphine‐induced circling behaviour and reduced catalepsy induced by the neuroleptic haloperidol. DA‐depleted rats exhibited severe deficits in the operant task, among which the inability to perform the task was not alleviated by STN HFS. However, in a few animals showing less impairment, STN HFS significantly reduced the contralateral neglect induced by the lesion. These results are the first to demonstrate a beneficial effect of STN HFS applied in awake rats on basic motor functions. However, STN HFS appears to be less effective on impaired cognitive functions.

Characterization of striatal lesions produced by glutamate uptake alteration: Cell death, reactive gliosis, and changes in GLT1 and GADD45 mRNA expression

This study investigated the time course of the striatal lesions produced by continuous local injection of the glutamate uptake inhibitor, L‐trans‐pyrrolidine‐2,4‐dicarboxylate (PDC) at the rate of 25 nmol/h in rats. The extent of the neurodegeneration area (defined as the lesion area) did not significantly vary with the duration of the PDC treatment between 3 and 14 days, but was markedly reduced 3 months after cessation of the 14‐day treatment, probably reflecting striatal atrophy. After the 3‐day treatment, the lesion zone showed calcium precipitates and marked microglial reaction contrasting with the reduction of astroglial labeling and loss of the glutamate transporter GLT1 mRNA expression; however reactive astrocytes were observed around the lesion. After the 14‐day treatment, the lesion zone presented reactive astrocytes and microglia without calcification, and a partial recovery of GLT1 mRNA expression. Interestingly, the growth arrest DNA damage‐inducible GADD45 mRNA expression was induced around the lesion after 3 days but inside the lesion after 14 days of treatment. Three months after the 14‐day treatment, the astroglial reactivity persisted within the lesion whereas most of the other markers examined tended to normalize. These data suggest that defective glutamate transport induces primary death of neurons and dysfunction of astrocytes. They strongly implicate reactive astrocytes with GLT1 and GADD45 transcripts in preventing secondary neuronal death. GLIA 29:222–232, 2000.

Bilateral high-frequency stimulation of the subthalamic nucleus on attentional performance: transient deleterious effects and enhanced motivation in both intact and parkinsonian rats

It is now well established that subthalamic nucleus high‐frequency stimulation (STN HFS) alleviates motor problems in Parkinsons disease. However, its efficacy for cognitive function remains a matter of debate. The aim of this study was to assess the effects of STN HFS in rats performing a visual attentional task. Bilateral STN HFS was applied in intact and in bilaterally dopamine (DA)‐depleted rats. In all animals, STN HFS had a transient debilitating effect on all the variables measured in the task. In DA‐depleted rats, STN HFS did not alleviate the deficits induced by the DA lesion such as omissions and latency to make correct responses, but induced perseverative approaches to the food magazine, an indicator of enhanced motivation. In sham‐operated controls, STN HFS significantly reduced accuracy and induced perseverative behaviour, mimicking partially the effects of bilateral STN lesions in the same task. These results are in line with the hypothesis that STN HFS only partially mimics inactivation of STN produced by lesioning and confirm the motivational exacerbation induced by STN inactivation.

Electrochemical detection of dopamine release in the striatum of freely moving hamsters

It was proposed to characterize the electrochemical signal recorded with a multifibre carbon electrode chronically implanted in the striatum of freely moving hamsters when the electrode potential was increased from-175 mV to +325 mV. Both in vitro calibration in standard solutions of oxidative molecules and in vivo pharmacological studies were used for this purpose. Results show that after an appropriate electrochemical treatment of the electrode in vitro the oxidation of dopamine (DA) produces a characteristic signal, whereas standard solutions of DOPAC and ascorbic acid produce no response. The electrochemical response recorded in vivo from the striatum of freely moving hamsters when the potential of the electrode is raised from-175 mV to +325 mV seems to correspond to the in vitro response to DA. This in vivo response diminished considerably following the destruction of the nigro-striatal dopaminergic neurones by means of an intranigral injection of 6-hydroxydopamine, while the striatal levels of ascorbic acid are not affected by the lesion. The administration of both amphetamine (3 mg/kg, i.p.) and the inhibitor of monoamine oxidases, pargyline (90 mg/kg i.p.), enhances the amplitude of the electrochemical signal. These results suggest that the electrochemical response recorded with our device in the striatum of the freely moving hamster corresponds to the oxidation of DA and not that of its metabolite DOPAC. Ascorbic acid is also very unlikely to contribute to the signal since the dopaminergic lesion does not alter the striatal level of this acid and since pargyline increases the amplitude of the signal.

Down-regulation of striatin, a neuronal calmodulin-binding protein, impairs rat locomotor activity.

Striatin, an intraneuronal, calmodulin-binding protein addressed to dendrites and spines, is expressed in the motor system, particularly the striatum and motoneurons. Striatin contains a high number of domains mediating protein-protein interactions, suggesting a role within a dendritic Ca(2+)-signaling pathway. Here, we explored the hypothesis of a direct role of striatin in the motor control of behaving rats, by using an antisense strategy based on oligodeoxynucleotides (ODN). Rats were treated by intracerebroventricular infusion of a striatin antisense ODN (A-ODN) or mismatch ODN (M-ODN) delivered by osmotic pumps over 6 days. A significant decrease in the nocturnal locomotor activity of A-ODN-treated rats was observed after 5 days of treatment. Hypomotricity was correlated with a 60% decrease in striatin content of the striata of A-ODN-treated rats sacrificed on day 6. Striatin thus plays a role in the control of motor function. To approach the cellular mechanisms in which striatin is involved, striatin down-regulation was studied in a comparatively simpler model: purified rat spinal motoneurons which retain their polarity in culture. Treatment of cells by the striatin A-ODN resulted in the impairement of the growth of dendrites but not axon. The decrease in dendritic growth paralleled the loss of striatin. This model allows analysis of the molecular basis of striatin function in the dynamic changes occurring in growing dendrites, and offers clues to unravel its function within spines.

Time-course of recovery of dopamine neuron activity during reinnervation of the denervated striatum by fetal mesencephalic grafts as assessed by in vivo voltammetry.

SummaryIn vivo voltammetry was used to monitor dopamine (DA) neuron activity during the course of reinnervation of the initially denervated caudateputamen by grafted mesencephalic neurons. Fetal DA neurons were implanted as a cell suspension into the depth of the caudate-putamen in adult 6-hydroxydopamine-lesioned recipient rats. Recordings were performed over a period of 2.5–4 months, starting within a week after transplantation, using chronically implanted surface-treated multifiber carbon electrodes. The voltammetric method used in this study has generated considerable discussion centred on the ability of the multifiber electrodes to measure DA alone in vivo, but the results of previous studies have led to the conclusion that changes in the voltammetric signal most probably reflect dopaminergic terminal activity. It seems therefore possible to follow the time-course of changes in the voltammetric signal amplitude during the process of dopaminergic reinnervation of the host striatum from the grafts. A 6-hydroxydopamine lesion of the mesostriatal dopamine pathway caused a substantial (> 80%) reduction of the voltammetric signal within 8–10 days, and the low residual signal remained essentially unchanged for time periods up to at least 5 months in the non-grafted control rats. In 7 of 11 rats with DA-rich grafts there was a recovery of the signal amplitude to levels within, or close to, the range recorded from the striatum of normal intact rats. The increase was observed 6–8 weeks after grafting in the rats which had received the largest transplants, and at about 13–14 weeks after grafting in the rats which had received the smallest ones. The recovery of the signal amplitude, from baseline to maximal response, was quite rapid and typically developed between two or three recording sessions, i.e. over a period of one to two weeks. In contrast to the intact striatum, the recovered signal in the graft-reinnervated striata showed a progressive decline within one hour of sampling time at high sampling frequencies (1 per min to 1 per 3 min). Grafted striata also showed a larger response to systemically administered amphetamine than did intact striata. Since the changes in the voltammetric signal recorded with the multifiber electrode mainly reflect dopaminergic terminal activity, the results provide evidence that the intrastriatal DA-rich grafts are spontaneously active, and that the grafted DA neurons can restore DA neuro-transmission in the reinnervated part of the host caudate-putamen to levels which are within the normal range. From the time-course of changes in the voltammetric signal it can be estimated that the outgrowing DA fibers, after an initial maturation period, expand from the graft into the host striatum at a maximum rate of about 0.1 mm per week, and that the advancing front of graft-derived fibers may be capable of saturating the area around the electrode tip with new terminals within a time period of about 1–2 weeks. The characteristics of the signal seem compatible with the view that the activity of the individual grafted DA neurons is greater than that of the mesostriatal DA neurons in situ.

In vitro and in vivo characterization of the properties of a multifiber carbon electrode allowing long-term electrochemical detection of dopamine in freely moving animals

We recently developed a multifiber carbon electrode for voltammetric studies which shows an apparent selectivity for dopamine (DA) in vitro and which can be used over very long periods of time after implantation for in vivo recordings in the striatum of rodents. This series of experiments was undertaken to further characterize our voltammetric signal detected in the brain. Comparison of voltammetric signals obtained in vitro after electric pretreatment of the electrode in solutions of various oxidizable endogenous compounds at regularly increasing concentrations showed that the sensitivity of the electrode for DA is 10,000 times higher than for ascorbic acid (AA) and 1000 times higher than for 3,4-dihydroxyphenylacetic acid (DOPAC), two compounds which are detectable in vivo. Measurements of the DA signal in the presence of DOPAC or AA showed that interactions occurred between the oxidized forms of the various molecules. DOPAC decreased the DA sensitivity of the electrode, whereas a potentiation of the DA signal was observed with AA at high concentrations, showing the presence of an electro-catalytic effect. At lower AA concentrations a decreased DA sensitivity of the electrode was observed as in the case of DOPAC. The brain distribution of the in vivo voltammetric signal was studied in anesthetized hamsters, showing a regional specificity which was positively correlated to the dopaminergic innervation. In animals with chronically implanted electrodes, various pharmacological compounds known to interfere with DA metabolism or the activity of dopaminergic neurons were injected. alpha-Methyl-p-tyrosine and reserpine were shown to induce a decrease in signal amplitude. Similar data were obtained with gamma-hydroxybutyrate and gamma-butyrolactone, which have been previously shown to decrease the dopaminergic neuronal firing rate. An increase in the striatal response was on the contrary obtained in anesthetized rats following electrical stimulation of the medial forebrain bundle containing dopaminergic fibers. Drugs acting on dopaminergic receptors such as neuroleptics and apomorphine, were shown to increase and diminish the striatal signal, respectively. Finally, AA peripheral administration was found to reduce the striatal signal amplitude. These data reinforce the idea that DA mainly contributes to our in vivo brain voltammetric response, although extracellular DOPAC or AA levels may influence this response. Variations in the striatal voltammetric signal recorded in vivo in freely moving animals over very long periods of time after electrode implantation may thus reflect variations in the activity of the nigrostriatal dopaminergic neurons.

Continuous administration of the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate produces striatal lesion

This study examined the effects of chronic intrastriatal infusion of L-trans-pyrrolidine-2,4-dicarboxylate (PDC), a selective competitive inhibitor of high affinity glutamate transport systems, via osmotic minipumps in rats. Injection of PDC at the rate of 25 nmol/h for 14 days caused striatal lesion. Histological evaluation on frontal striatal sections showed that the lesion was circumscribed to a circular area showing a dramatic neuronal loss accompanied by gliosis and representing 30% of the whole striatal surface at the level of the injection site. A total loss of neurons expressing glutamate decarboxylase (GAD67), enkephalin or substance P mRNA was observed on a similar circular area, suggesting degeneration of the two populations of striatal efferent neurons. In the whole striatum outside the region devoided of hybridization signal, a selective 27% decrease in enkephalin mRNA expression occurred, suggesting a higher sensitivity of enkephalin neurons versus substance P neurons to glutamate uptake-mediated alterations. Injection of PDC at the rate of 25 nmol/h for 3 days produced striatal lesion of similar extent. In contrast, PDC at the rate of 5 nmol/h did not produce neuronal damage when administered over 14 days. This study provides new in vivo evidence that defective glutamate transport is one of the critical conditions that may give rise to toxicity of an endogenous transmitter system in the striatum, and may underlie neuronal death in neurodegenerative diseases.

Serotonin depletion produces long lasting increase in striatal glutamatergic transmission

The ability of serotonin (5‐HT) to influence striatal glutamatergic transmission was examined by determining changes over time in glutamate extracellular levels, transporter expression and synaptosomal uptake in rats with lesion of serotonergic neurones. By 8 days after intraraphe injections of 5,7‐dihydroxytryptamine, producing 80% decreases in striatal tissue 5‐HT levels, no changes were observed in the glutamatergic transmission. When 5‐HT depletion was almost complete (21 days post‐lesion), high affinity glutamate uptake in striatal synaptosomal preparations was significantly increased (156% of control), although no changes in striatal GLT1, GLAST and EAAC1mRNAs, and GLT1 protein were detected by in situ hybridization and immunohistochemistry. Meanwhile, the serotonin lesion produced large increases in basal extracellular levels of glutamate and glutamine (364% and 259%, respectively) determined in awake rats by in vivo microdialysis, whereas no change was observed in dopamine levels as compared with control rats. High potassium depolarization as well as ltrans‐pyrrolidine‐2,4‐dicarboxylate, also induced larger increases in extracellular levels of glutamate in lesioned rats than in controls. Finally, similar changes in glutamate transmission were observed by 3 months post‐lesion. These results suggest that 5‐HT has a long lasting and tonic inhibitory influence on the striatal glutamatergic input, without affecting the basal dopaminergic transmission.