Bilateral 6-hydroxydopamine-induced lesion of the nigrostriatal dopamine pathway reproduces the effects of unilateral lesion on substance P but not on enkephalin expression in rat basal ganglia

What substance P might tell us about the prognosis and mechanism of Parkinson's disease?

The neuropeptide substance P (SP) plays an important role in neurodegenerative disorders, among which Parkinson's disease (PD). In the present work we have reviewed the involvement of SP and its preferred receptor (NK1-R) in motor and non-motor PD symptoms, in both PD animal models and patients. Despite PD is primarily a motor disorder, non-motor abnormalities, including olfactory deficits and gastrointestinal dysfunctions, can represent diagnostic PD predictors, according to the hypothesis that the olfactory and the enteric nervous system represent starting points of neurodegeneration, ascending to the brain via the sympathetic fibers and the vagus nerve. In PD patients, the α-synuclein aggregates in the olfactory bulb and the gastrointestinal tract, as well as in the dorsal motor nucleus of the vagus nerve often co-localize with SP, indicating SP-positive neurons as highly vulnerable sites of degeneration. Considering the involvement of the SP/NK1-R in both the periphery and specific brain areas, this system might represent a neuronal substrate for the symptom and disease progression, as well as a therapeutic target for PD.

The globus pallidus as a target for neuropeptides and endocannabinoids participating in central activities

The globus pallidus in the basal ganglia plays an important role in movement regulation. Neuropeptides and endocannabinoids are neuronal signalling molecules that influence the functions of the whole brain. Endocannabinoids, enkephalin, substance P, neurotensin, orexin, somatostatin and pituitary adenylate cyclase-activating polypeptides are richly concentrated in the globus pallidus. Neuropeptides and endocannabinoids exert excitatory or inhibitory effects in the globus pallidus mainly by modulating GABAergic, glutamatergic and dopaminergic neurotransmission, as well as many ionic mechanisms. Pallidal neuropeptides and endocannabinoids are associated with the pathophysiology of a number of neurological disorders, such as Parkinson's disease, Huntington's disease, schizophrenia, and depression. The levels of neuropeptides and endocannabinoids and their receptors in the globus pallidus change in neurological diseases. It has been demonstrated that spontaneous firing activity of globus pallidus neurons is closely related to the manifestations of Parkinson's disease. Therefore, the neuropeptides and endocannabinoids in the globus pallidus may function as potential targets for treatment in some neurological diseases. In this review, we highlight the morphology and function of neuropeptides and endocannabinoids in the globus pallidus and their involvement in neurological diseases.

Animal models of the non-motor features of Parkinson's disease

The non-motor symptoms (NMS) of Parkinson's disease (PD) occur in roughly 90% of patients, have a profound negative impact on their quality of life, and often go undiagnosed. NMS typically involve many functional systems, and include sleep disturbances, neuropsychiatric and cognitive deficits, and autonomic and sensory dysfunction. The development and use of animal models have provided valuable insight into the classical motor symptoms of PD over the past few decades. Toxin-induced models provide a suitable approach to study aspects of the disease that derive from the loss of nigrostriatal dopaminergic neurons, a cardinal feature of PD. This also includes some NMS, primarily cognitive dysfunction. However, several NMS poorly respond to dopaminergic treatments, suggesting that they may be due to other pathologies. Recently developed genetic models of PD are providing new ways to model these NMS and identify their mechanisms. This review summarizes the current available literature on the ability of both toxin-induced and genetically-based animal models to reproduce the NMS of PD.

Subtle cardiovascular dysfunction in the unilateral 6-hydroxydopamine-lesioned rat

The present study evaluated whether the unilateral 6-hydroxydopamine (6-OHDA) model of Parkinson's disease produces autonomic deficits. Autonomic parameters were assessed by implanting a small radiofrequency telemetry device which measured heart rate variability (HRV), diurnal rhythms of heart rate (HR), core body temperature (cBT) and locomotor activity (LA). Rats then received 6-OHDA lesion or sham surgery. 6-OHDA lesioned rats exhibited head and body axis biases, defective sensorimotor function ("disengage" test), and prominent apomorphine rotation (all P < .05 versus controls). Diurnal rhythm of HR was lower for 6-OHDA lesioned rats (n = 8) versus controls (n = 6; P < .05). Whilst HR decreased similarly in both groups during the day, there was a greater decrease in HR for the 6-OHDA lesioned rats at night (by 38 b.p.m. relative to 17 b.p.m. for controls). LA and cBT did not differ between surgery groups. This study indicates the unilateral 6-OHDA model of PD shows subtle signs of cardiovascular autonomic dysfunction.

A neurocomputational account of catalepsy sensitization induced by D2 receptor blockade in rats: context dependency, extinction, and renewal

RATIONALE

Repeated haloperidol treatment in rodents results in a day-to-day intensification of catalepsy (i.e., sensitization). Prior experiments suggest that this sensitization is context-dependent and resistant to extinction training.

OBJECTIVES

The aim of this study was to provide a neurobiological mechanistic explanation for these findings.

MATERIALS AND METHODS

We use a neurocomputational model of the basal ganglia and simulate two alternative models based on the reward prediction error and novelty hypotheses of dopamine function. We also conducted a behavioral rat experiment to adjudicate between these models. Twenty male Sprague-Dawley rats were challenged with 0.25 mg/kg haloperidol across multiple days and were subsequently tested in either a familiar or novel context.

RESULTS

Simulation results show that catalepsy sensitization, and its context dependency, can be explained by "NoGo" learning via simulated D2 receptor antagonism in striatopallidal neurons, leading to increasingly slowed response latencies. The model further exhibits a non-extinguishable component of catalepsy sensitization due to latent NoGo representations that are prevented from being expressed, and therefore from being unlearned, during extinction. In the rat experiment, context dependency effects were not dependent on the novelty of the context, ruling out the novelty model's account of context dependency.

CONCLUSIONS

Simulations lend insight into potential complex mechanisms leading to context-dependent catalepsy sensitization, extinction, and renewal.

Chronic 3,4-dihydroxyphenylalanine treatment induces dyskinesia in aphakia mice, a novel genetic model of Parkinson's disease

L-DOPA-induced dyskinesia (LID) is one of the main limitations of long term L-DOPA use in Parkinson's disease (PD) patients. We show that chronic L-DOPA treatment induces novel dyskinetic behaviors in aphakia mouse with selective nigrostriatal deficit mimicking PD. The stereotypical abnormal involuntary movements were induced by dopamine receptor agonists and attenuated by antidyskinetic agents. The development of LID was accompanied by preprodynorphin and preproenkephalin expression changes in the denervated dorsal striatum. Increased FosB-expression was also noted in the dorsal striatum. In addition, FosB expression was noted in the pedunculopontine nucleus and the zona incerta, structures previously not examined in the setting of LID. The aphakia mouse is a novel genetic model with behavioral and biochemical characteristics consistent with those of PD dyskinesia and provides a more consistent, convenient, and physiologic model than toxic lesion models to study the mechanism of LID and to test therapeutic approaches for LID.

Hold your horses: A dynamic computational role for the subthalamic nucleus in decision making

The basal ganglia (BG) coordinate decision making processes by facilitating adaptive frontal motor commands while suppressing others. In previous work, neural network simulations accounted for response selection deficits associated with BG dopamine depletion in Parkinson's disease. Novel predictions from this model have been subsequently confirmed in Parkinson patients and in healthy participants under pharmacological challenge. Nevertheless, one clear limitation of that model is in its omission of the subthalamic nucleus (STN), a key BG structure that participates in both motor and cognitive processes. The present model incorporates the STN and shows that by modulating when a response is executed, the STN reduces premature responding and therefore has substantial effects on which response is ultimately selected, particularly when there are multiple competing responses. Increased cortical response conflict leads to dynamic adjustments in response thresholds via cortico-subthalamic-pallidal pathways. The model accurately captures the dynamics of activity in various BG areas during response selection. Simulated dopamine depletion results in emergent oscillatory activity in BG structures, which has been linked with Parkinson's tremor. Finally, the model accounts for the beneficial effects of STN lesions on these oscillations, but suggests that this benefit may come at the expense of impaired decision making.

Mitochondrial toxins in Basal Ganglia disorders: from animal models to therapeutic strategies

Current knowledge of the pathogenesis of basal ganglia disorders, such as Huntington's disease (HD) and Parkinson's disease (PD) appoints a central role to a dysfunction in mitochondrial metabolism. The development of animal models, based upon the use of mitochondrial toxins has been successfully introduced to reproduce human disease, leading to important acquisitions. Most notably, experimental evidence supports the existence, within basal ganglia, of a peculiar regional vulnerability to distinct mitochondrial toxins. MPTP and rotenone, both selective inhibitors of mitochondrial complex I have been extensively used to mimic PD. Accordingly, in human PD, a specific dysfunction of complex I activity was found in vulnerable dopaminergic neurons of the substantia nigra. Conversely, in HD a selective impairment of mitochondrial succinate dehydrogenase, key enzyme in complex II activity was found in medium spiny neurons of the caudate-putamen. The relevance of such finding is further demonstrated by the evidence that toxins able to primarily target mitochondrial complex II, such as malonic acid and 3-nitropropionic acid (3-NP), strikingly reproduce the main phenotypic and pathological features of HD.Despite the advances obtained from these experimental models, a deeper understanding of the molecular and cellular mechanisms underlying such neuronal vulnerability is lacking.The present review provides a brief survey of currently utilized animal models of mitochondrial intoxication, in attempt to address the cellular mechanisms triggered by energy metabolism failure and to identify potential therapeutic targets.

Dynamic Dopamine Modulation in the Basal Ganglia: A Neurocomputational Account of Cognitive Deficits in Medicated and Nonmedicated Parkinsonism

Dopamine (DA) depletion in the basal ganglia (BG) of Parkinson's patients gives rise to both frontal-like and implicit learning impairments. Dopaminergic medication alleviates some cognitive deficits but impairs those that depend on intact areas of the BG, apparently due to DA “overdose.” These findings are difficult to accommodate with verbal theories of BG/DA function, owing to complexity of system dynamics: DA dynamically modulates function in the BG, which is itself a modulatory system. This article presents a neural network model that instantiates key biological properties and provides insight into the underlying role of DA in the BG during learning and execution of cognitive tasks. Specifically, the BG modulates the execution of “actions” (e.g., motor responses and working memory updating) being considered in different parts of the frontal cortex. Phasic changes in DA, which occur during error feedback, dynamically modulate the BG threshold for facilitating/suppressing a cortical command in response to particular stimuli. Reduced dynamic range of DA explains Parkinson and DA overdose deficits with a single underlying dysfunction, despite overall differences in raw DA levels. Simulated Parkinsonism and medication effects provide a theoretical basis for behavioral data in probabilistic classification and reversal tasks. The model also provides novel testable predictions for neuropsychological and pharmacological studies, and motivates further investigation of BG/DA interactions with the prefrontal cortex in working memory.

Differential effects of prolonged high frequency stimulation and of excitotoxic lesion of the subthalamic nucleus on dopamine denervation-induced cellular defects in the rat striatum and globus pallidus

This study examined the effects of prolonged (4 days) high frequency stimulation (HFS) of the subthalamic nucleus (STN), in comparison with those of STN lesion, on the dopamine denervation-mediated cellular changes in the basal ganglia in a Wistar rat model of Parkinson's disease. STN HFS counteracted the dopamine lesion-induced increase in GAD67 mRNA expression in the output structures of the basal ganglia, as shown previously after STN lesion, providing cellular support for the similar antiparkinsonian benefits produced by the two surgical procedures. The dopamine denervation-induced increase in GAD67 mRNA levels in the globus pallidus was partially antagonized after HFS and totally reversed after ibotenate-induced STN lesion. The overexpression of striatal enkephalin mRNA tended to be further increased by HFS but was antagonized by STN lesion. The decrease in striatal substance P mRNA levels was affected neither by STN HFS nor lesion. As STN HFS for two hours was previously found not to interfere with the effects of dopamine lesion in the globus pallidus and striatum, the present data provide strong evidence that the effects of STN surgery in these structures involve long-term adaptive processes and that the rearrangements mediated by HFS and lesion are, at least in part, different.

Persistent increase in olfactory type G-protein alpha subunit levels may underlie D1 receptor functional hypersensitivity in Parkinson disease

Although L-dopa remains the most effective treatment of Parkinson disease, its long-term administration is hampered by the appearance of dyskinesia. Hypersensitivity of dopamine D1 receptors in the striatum has been suggested to contribute to the genesis of these delayed adverse effects. However, D1 receptor amounts are unchanged in Parkinson disease, suggesting alterations of downstream effectors. In rodents, striatal D1 receptors activate adenylyl cyclase through olfactory type G-protein alpha subunit (Galphaolf) and G-protein gamma 7 subunit (Ggamma7). We found that Galphaolf was enriched in human basal ganglia and was markedly diminished in the putamen of patients with Huntington disease, in relation with the degeneration of medium spiny neurons. In contrast, in the putamen of patients with Parkinson disease, Galphaolf and Ggamma7 levels were both significantly increased. In the rat, the degeneration of dopamine neurons augmented Galphaolf levels in the striatal neurons, specifically at the plasma membrane, an effect accounting for the increase of D1 response on cAMP production in dopamine-depleted striatum. In lesioned rats, Galphaolf levels were normalized by a 3 week treatment with l-dopa or a D1 agonist but not with aD2-D3 agonist, supporting a Galphaolf regulation by D1 receptor usage. In contrast, the increases of Galphaolf levels in patients were not affected by the duration of l-dopa treatment but correlated with duration of disease. In conclusion, our results revealed in the parkinsonian putamen a prolonged elevation of Galphaolf levels that may lead to a persistent D1 receptor hypersensitivity and contribute to the genesis of long-term complications of L-dopa.

Classic toxin-induced animal models of Parkinson's disease: 6-OHDA and MPTP

Neurological disorders in humans can be modeled in animals using standardized procedures that recreate specific pathogenic events and their behavioral outcomes. The development of animal models of Parkinson's disease (PD) is important to test new neuroprotective agents and strategies. Such animal models of PD have to mimic, at least partially, a Parkinson-like pathology and should reproduce specific features of the human disease. PD is characterized by massive degeneration of dopaminergic neurons in the substantia nigra, the loss of striatal dopaminergic fibers and a dramatic reduction of the striatal dopamine levels. The formation of cytoplasmic inclusion bodies (Lewy bodies) in surviving dopaminergic neurons represents the most important neuropathological feature of PD. Furthermore, the massive striatal dopamine deficiency causes easily detectable motor deficits in PD patients, including bradykinesia, rigidity, and resting tremor, which are the cardinal symptoms of PD. Over the years, a broad variety of experimental models of PD were developed and applied in diverse species. This review focuses on the two most common "classical" toxin-induced PD models, the 6-hydroxy-dopamine (6-OHDA model) and the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model. Both neurotoxins selectively and rapidly destroy catecholaminergic neurons, whereas in humans the PD pathogenesis follows a progressive course over decades. This discrepancy reflects one important and principal point of weakness related to most animal models. This review discusses the most important properties of 6-OHDA and MPTP, their modes of administration, and critically examines advantages and limitations of selected animal models. The new genetic and environmental toxin models of PD (e.show $132#g. rotenone, paraquat, maneb) are discussed elsewhere in this "special issue."

Late degeneration of nigro-striatal neurons in ATM-/- mice

The generation of an Atm -/- mouse model of the human ataxia-telangiectasia (AT) opened new avenues toward a better understanding of the molecular and cellular basis of AT. We have recently reported that 5-month-old Atm-/- mice exhibit severe loss of tyrosine hydroxylase-positive, dopaminergic nigro-striatal neurons, down to 26% of age-matched controls. In the present study we analyzed development of the dopaminergic cell loss in the context of the nigro-striatal system. We found that dopaminergic neurons are formed normally in the Atm-/- mouse, and degenerate during the first few months of life; there was no difference between 1-month-old Atm-/- and control mice in the number of dopaminergic cells that were retrogradely labeled by an injection of fluorescent tracer into the striatum. On the other hand, a dramatic reduction in the number of labeled cells was found in 5-month-old Atm-/- mice. This cell loss was significant in areas A9 and A10 but not in area A9-I. These findings indicate that midbrain dopaminergic neurons in Atm-/- mice initially send normal axons to the striatum, only to degenerate later in life. In addition, an age-dependent as well as topographic, medial-to-lateral loss of GAD, met-enkephaline and substance-P immunopositive cells was found in the striatum of the Atm-/- mice. This phenomenon was significant only in the 5-month-old Atm-/- mice (3 months after the beginning of detectable dopaminergic cell loss). In both the striatum and the substantia nigra, the apparent cell loss was accompanied by gliosis. In addition, alpha-synuclein immunopositive bodies were observed in the cortex, striatum and substantia nigra of these mice. The present data indicate that Atm-/- mice exhibit a progressive, age-dependent, reduction in dopaminergic cells of the substantia nigra, followed by a reduction in projection neurons of the striatum. Thus, the Atm-/- mouse may model the extrapyramidal motor deficits seen in AT patients.

Animal models of Parkinson's disease in rodents induced by toxins: an update

The development of animal models of Parkinson's disease is of great importance in order to test substitutive or neuroprotective strategies for Parkinson's disease. Such models should reproduce the main characteristics of the disease, such as a selective lesion of dopaminergic neurons that evolves over time and the presence of neuronal inclusions known as Lewy bodies. Optimally, such models should also reproduce the lesion of non-dopaminergic neurons observed in a great majority of patients with Parkinson's disease. From a behavioral point of view, a parkinsonian syndrome should be observed, ideally with akinesia, rigidity and rest tremor. These symptoms should be alleviated by dopamine replacement therapy, which may in turn lead to side effects such as dyskinesia. In this review, we analyze the main characteristics of experimental models of Parkinson's disease induced by neurotoxic compounds such as 6-hydroxydopamine, MPTP and rotenone. We show that, whereas MPTP and 6-hydroxydopamine induce a selective loss of catecholaminergic neurons that in most cases evolves over a short period of time, rotenone infusion by osmotic pumps can induce a chronically progressive degeneration of dopaminergic neurons and also of non-dopaminergic neurons in both the basal ganglia and the brainstem.

Metabotropic glutamate 5 receptor blockade alleviates akinesia by normalizing activity of selective basal-ganglia structures in parkinsonian rats

Glutamate overactivity within the basal ganglia has been shown to be central to the expression of motor symptoms in advanced stages of Parkinson's disease, and metabotropic glutamate receptors (mGluRs) represent promising targets for new therapeutic strategies in this pathology. Little is known, however, about the cellular and behavioral changes occurring in the early stages of the disease when dopamine depletion is moderate. Here, we report that rats with partial bilateral dopamine lesions exhibit akinetic deficits associated with dramatically increased neuronal metabolic activity in selective structures of the basal ganglia such as the subthalamic nucleus and the substantia nigra pars reticulata, but not in the entopeduncular nucleus. Furthermore, chronic treatment with the mGluR5 antagonist 2-methyl-6-(phenylethylnyl)-pyridine alleviated the akinesia and was associated with a normalization of the activity of these two overactive structures. These data stress the therapeutic potential of mGluR5 antagonists in the treatment of parkinsonian patients in the early stages of the disease.

Acute and subchronic MPTP administration differentially affects striatal glutamate synaptic function

We previously reported that 1 month following unilateral loss (>95%) of striatal dopamine, there is an increase in striatal glutamate function as measured by in vivo microdialysis and quantitative immuno-gold electron microscopy, Neuroscience 88, 1-16). The goal of this study was to determine the effect of bilateral loss of striatal dopamine on striatal glutamate function following acute or subchronic administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to C57/B6J mice. Animals were administered either single injections (ip) of 30 mg/kg/day for 7 days (subchronically treated group) or 20 mg/kg x 4 doses every 2 h (acutely treated group) of the toxin or saline. One month following the first injection, there was a 44 and 65% loss in the relative density of tyrosine hydroxylase (TH) immunolabeling within the dorsolateral striatum in the subchronically and acutely MPTP-treated groups compared to the saline group, respectively. There was a decrease in the basal level of extracellular glutamate within the striatum in the subchronically MPTP-treated animals compared to an increase in the acutely treated group in relationship to the saline group. Ultrastructurally, only in the acutely MPTP-treated group was there a decrease in the density of glutamate immunolabeling within nerve terminals associated with an asymmetrical synaptic contact in the dorsolateral striatum compared to either the subchronic or saline groups. In addition, there was a decrease in the relative density of GluR-2/3 subunit immunolabeling within the dorsolateral striatum in the acute MPTP compared to the saline group. These data indicate that differences in striatal glutamate function appear to be associated with the dosing interval of MPTP administration and the variable loss of striatal TH immunolabeling.

High-frequency stimulation of the subthalamic nucleus selectively reverses dopamine denervation-induced cellular defects in the output structures of the basal ganglia in the rat

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) is now recognized as an effective treatment for advanced Parkinson's disease, but the molecular basis of its effects remains unknown. This study examined the effects of unilateral STN HFS (2 hr of continuous stimulation) in intact and hemiparkinsonian awake rats on STN neuron metabolic activity and on neurotransmitter-related gene expression in the basal ganglia, by means of in situ hybridization histochemistry and immunocytochemistry. In both intact and hemiparkinsonian rats, this stimulation was found to induce c-fos protein expression but to decrease cytochrome oxidase subunit I mRNA levels in STN neurons. STN HFS did not affect the dopamine lesion-mediated overexpression of enkephalin mRNA or the decrease in substance P in the ipsilateral striatum. The lesion-induced increases in intraneuronal glutamate decarboxylase 67 kDa isoform (GAD67) mRNA levels on the lesion side were reversed by STN HFS in the substantia nigra, partially antagonized in the entopeduncular nucleus but unaffected in the globus pallidus. The stimulation did not affect neuropeptide or GAD67 mRNA levels in the side contralateral to the dopamine lesion or in intact animals. These data furnish the first evidence that STN HFS decreases the metabolic activity of STN neurons and antagonizes dopamine lesion-mediated cellular defects in the basal ganglia output structures. They provide molecular substrate to the therapeutic effects of this stimulation consistent with the current hypothesis that HFS blocks STN neuron activity. However, the differential impact of STN HFS on the effects of dopamine lesion among structures receiving direct STN inputs suggests that this stimulation may not cause simply interruption of STN outflow.

Cellular mRNA expression of the transcription factor NGFI-B suggests a gene regulatory role in striatal opiate-peptide neurons

Previous studies have shown that NGFI-B mRNA is highly expressed in the adult striatum. In the present study we analyzed the anatomical distribution of NGFI-B mRNA within this brain region as well as the degree of co-existence of NGFI-B with different striatal markers in the adult brain. NGFI-B mRNA levels were found to be significantly higher within the dorsomedial portion of the striatum as compared to the ventrolateral striatum. This distribution pattern was maintained throughout the rostro--caudal axis of the striatum. Double in situ hybridization studies showed that striatal NGFI-B mRNA colocalized with a subset of preproenkephalin and prodynorphin positive spiny neurons within the dorsomedial striatum; 22--28% of all opiate-peptide positive cells co-expressed NGFI-B mRNA. NGFI-B did not colocalize with striatal aspiny interneurons expressing choline acetyl transferase mRNA or those containing the calcium-binding protein parvalbumin. The pattern of NGFI-B mRNA expression within different striatal spiny projecting neurons suggests that this transcription factor may have a direct effect on the function of different striatal efferent pathways.

Ipsilateral and contralateral subthalamic activity after unilateral dopaminergic lesion

Unilateral lesions of the dopaminergic nigral neurons in rats are currently used as a model of Parkinson's disease. However, several neurochemical studies have questioned the possible influence of the lesioned side on the contralateral non-lesioned side. To address this question, electrophysiological recordings in the ipsilateral and contralateral subthalamic nucleus was performed on anaesthetized rats, 3, 7 and 14 days after induction of a unilateral dopaminergic lesion. At these three times, the mean discharge rate of the subthalamic neurons recorded ipsilateral to the lesion was increased by 85, 176 and 127%, respectively, whereas this rate was decreased by 16, 27 and 43%, respectively, in the opposite subthalamic nucleus. This result emphasizes the importance of interhemispheric regulation of this structure, contrasting with the unilateralized current model of the functional organization of the basal ganglia.

Time-dependent striatal dopamine depletion after injection of 6-hydroxydopamine in the rat. Comparison of single bilateral and double bilateral lesions

For future investigation of possible perturbation of circadian rhythm in animal models of Parkinson's disease we needed an animal model providing lasting 80-100% striatal dopaminergic depletion in rats, but without induced mortality. We have thus compared the effects of a single hydroxydopamine bilateral striatal lesion (SB-hydroxydopamine) with those of a double hydroxydopamine bilateral lesion (DB-hydroxydopamine) at the same dose (16 microg/striatum) by HPLC determination of dopamine and 3,4-dihydrophenylacetic acid (dopac) levels in the striatum. Two weeks after neurosurgery, SB-hydroxydopamine and DB-hydroxydopamine induced dopaminergic depletion of at least 81% compared with control groups. After eight weeks striatal dopaminergic depletion was only 60.97% in SB-hydroxydopamine rats, suggesting a compensatory phenomenon, whereas in DB-hydroxydopamine rats dopaminergic loss was stable at 88%. For the DB-hydroxydopamine group the dopac/dopamine ratio was significantly increased at week 2 only, whereas no significant change was observed for other groups. This increase might be explained by increased dopamine turnover. We have demonstrated that striatal DB-hydroxydopamine injection induces lasting 80-100% neuronal loss, close to that observed in the disease in man, without induced mortality, and provides a tool which meets our experimental requirements.

Recovery after nigral grafting in 6-hydroxydopamine lesioned rats is due to graft function and not significantly influenced by the remaining ipsilateral or contralateral host dopaminergic system

The aim of this study was to evaluate whether the recovery observed after grafting of fetal nigral cells in 6-hydroxydopamine lesioned rats is due to the graft itself, and whether the participation of the remaining host dopaminergic system is necessary. The effects of unilateral 6-hydroxydopamine lesion on rotational behavior were not significantly affected by sham grafting or by sham grafting plus repeat ipsilateral lesion, but were suppressed by nigral grafting, and by contralateral lesion. Immunohistochemical and in situ hybridization study of right striata of rats subjected to right-side lesion then right-side sham-grafting, and of right and left striata from rats subjected to right-side lesion then right-side sham-grafting then repeat right-side lesion then left-side lesion, revealed (a) no significant amphetamine-induced Fos activation, (b) marked increases in preproenkephalin mRNA levels, and (c) decreases in preprotachykinin levels, with no significant differences in any of these variables among these three types of striata. After nigral grafting, however, intense Fos expression was observed in the striatum, and preproenkephalin and preproenkephalin mRNA levels returned to normal. This recovery was maintained after subsequent repeat ipsilateral 6-hydroxydopamine lesion followed by contralateral lesion. The results demonstrate that, after dopaminergic denervation, the nigral graft itself is able to induce recovery in the assessed parameters, and that these effects of grafting into striata with maximal unilateral 6-hydroxydopamine lesion are due to graft function, and are not significantly influenced by the remaining ipsilateral or contralateral host dopaminergic system. Additionally, it is interesting to note that bilateral denervation led to changes in striatal preproenkephalin and preproenkephalin mRNA levels similar to those observed after unilateral lesion.

Differential regulation of striatal preproenkephalin and preprotachykinin mRNA levels in MPTP-lesioned monkeys chronically treated with dopamine D1 or D2 receptor agonists

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.

Thalamo-striatal deafferentation affects preproenkephalin but not preprotachykinin gene expression in the rat striatum

This study examined the effects of thalamo-striatal deafferentation on preprotachykinin and preproenkephalin mRNA expression in the rat neostriatum, using quantitative in situ hybridization histochemistry. Unilateral ibotenate-induced intralaminar thalamic lesion produced a significant decrease in preproenkephalin mRNA levels (-27%) restricted to the ipsilateral striatum at 5 days post-lesion. At 12 days post-lesion, significant decreases in striatal preproenkephalin mRNA expression were found on both brain sides. This post-lesional response was more pronounced in the ipsilateral (-32%) than contralateral (-18%) striatum. All these changes were homogeneously distributed between the dorsolateral and ventromedial parts of the striatum. In parallel, no significant change in preprotachykinin mRNA expression was found at either 5 or 12 days after thalamic lesion, when considering the striatum as a whole. However, at 5 days post-lesion, the regional analysis revealed a slight decrease (-17%) in preprotachykinin mRNA expression, confined to the dorsolateral part of the ipsilateral striatum. These results show that thalamic lesion preferentially affects preproenkephalin vs. preprotachykinin gene expression in the striatum, suggesting, at the first site, a predominant influence of thalamo-striatal inputs on the enkephalin-containing striato-pallidal pathway. However, given that the thalamo-striatal projection is strictly ipsilateral, the bilateralization of the down-regulation of preproenkephalin mRNA expression at 12 days post-lesion suggests an involvement of interhemispheric adaptive mechanisms via cortical networks.

Time course of striatal, pallidal and thalamic alpha 1, alpha 2 and beta 2/3 GABAA receptor subunit changes induced by unilateral 6-OHDA lesion of the nigrostriatal pathway

Immunocytochemical techniques were used to investigate the distribution and abundance of GABAA receptor subunits (alpha 1, alpha 2 and beta 2/3) in the brains of unilaterally 6-OHDA-lesioned rats. Three and 7 days after lesion, the alpha 2-subunit was significantly more abundant in the lesion-ipsilateral striatum than in the lesion-contralateral striatum; by 4 weeks after lesion, however, no significant between-side differences were observed. Three and 7 days after lesion, the alpha 1-subunit was significantly less abundant in the lesion-ipsilateral globus pallidus than in the lesion-contralateral side; again, this difference disappeared within 4 weeks of lesion. Similarly, alpha 1 was initially less abundant in several relay thalamic nuclei on the lesioned side while alpha 2 was initially more abundant in intralaminar thalamic nuclei on the lesioned side. There were no significant between-side changes for the beta 2/3-subunits. Comparison of non-lesioned and 6-OHDA-lesioned rats revealed significant differences in brain areas which also showed differences on comparison of the lesioned and non-lesioned sides of 6-OHDA-lesioned rats. These results suggest that there is an early adaptation to the lesion, achieved through changes in GABAA receptor abundance. That some of these changes are no longer apparent after 4 weeks is due not only to partial reversion of the changes in the lesioned side but also to compensatory changes in the non-lesioned side.

Differential regional effects of long-term L-DOPA treatment on preproenkephalin and preprotachykinin gene expression in the striatum of 6-hydroxydopamine-lesioned rat

The present study examined the effects of prolonged L-DOPA treatment (6 months) alone or in combination with unilateral 6-hydroxydopamine-induced lesion of the mesostriatal dopaminergic pathway on substance P and enkephalin mRNA expression in the rat neostriatum. This was done by means of quantitative in situ hybridization histochemistry. As reported previously, the unilateral dopaminergic lesion induced a significant and homogeneous decrease in striatal substance P mRNA expression and a marked increase in enkephalin mRNA expression in the ipsilateral neostriatum which was more pronounced in the dorsolateral than ventromedial part of the structure. Long-term L-DOPA treatment alone had no significant effects on the two striatal peptide mRNA levels. The chronic L-DOPA treatment in 6-hydroxydopamine-lesioned rats was found to partially reverse the lesion-induced down-regulation of substance P mRNA expression, without significantly affect the up-regulation of enkephalin when considering the neostriatum as a whole. Topographical analysis revealed that long-term L-DOPA treatment reversed, in fact, both post-lesional enkephalin and substance P responses to 6-hydroxydopamine lesion, in the ventromedial neostriatum, without significantly modified these peptide responses in the dorsolateral neostriatum. These findings provide new evidence that prolonged L-DOPA treatment differentially affects the post-lesional peptide responses in the ventromedial and dorsolateral parts of the neostriatum, suggesting regional cellular mechanisms in the neostriatum underlying the benefit and/or side-effects of L-DOPA treatment in parkinsonian patients.