Spontaneous recovery from motor asymmetry in adult rats with 6-hydroxydopamine-induced partial lesions of the substantia nigra

Characterisation of functional deficits induced by AAV overexpression of alpha-synuclein in rats

Background

In the last decades different preclinical animal models of Parkinson's disease (PD) have been generated, aiming to mimic the progressive neuronal loss of midbrain dopaminergic (DA) cells as well as motor and non-motor impairment. Among all the available models, AAV-based models of human alpha-synuclein (h-aSYN) overexpression are promising tools for investigation of disease progression and therapeutic interventions.

Objectives

The goal with this work was to characterise the impairment in motor and non-motor domains following nigrostriatal overexpression of h-aSYN and correlate the behavioural deficits with histological assessment of associated pathology.

Methods

Intranigral injection of an AAV9 expressing h-aSYN was compared with untreated animals, 6-OHDA and AAV9 expressing either no transgene or GFP. The animals were assessed on a series of simple and complex behavioural tasks probing motor and non-motor domains. Post-mortem neuropathology was analysed using immunohistochemical methods.

Results

Overexpression of h-aSYN led to progressive degeneration of DA neurons of the SN and axonal terminals in the striatum (STR). We observed extensive nigral and striatal pathology, resembling that of human PD brain, as well as the development of stable progressive deficit in simple motor tasks and in non-motor domains such as deficits in motivation and lateralised neglect.

Conclusions

In the present work we characterized a rat model of PD that closely resembles human PD pathology at the histological and behavioural level. The correlation of cell loss with behavioural performance enables the selection of rats which can be used in neuroprotective or neurorestorative therapies.

Characterizing the protracted neurobiological and neuroanatomical effects of paraquat in a murine model of Parkinson's disease

The primary motor symptoms of Parkinson's disease (PD) result from the degeneration of dopamine-producing neurons of the substantia nigra pars compacta (SNc), and often, the loss is asymmetrical, resulting in unilateral tremor presentation. Notably, age is the primary risk factor for PD, and it is likely that the disease ultimately stems from the impact of environmental factors, which interact with the aging process. Recent research has focused on the role of microglia and pro-oxidative responses in dopaminergic neuronal death. In this study, we sought to examine the neurodegenerative, inflammatory, and stress effects of exposure to the etiologically relevant pesticide, paraquat, over time (up to 6 months after injections). We also were interested in whether a high-resolution, 7-Tesla animal magnetic resonance imaging would be sensitive enough to detect the degenerative impact of paraquat. We found that paraquat induced a loss of dopaminergic SNc neurons and activation of microglia that surprisingly did not change over 6 months after the last injection. A long-lasting reduction was evident for body weight, and alterations in organ (lung and heart) weight were evident, which reflect the peripheral impact of the toxicant. The microglial proinflammatory actin-remodeling factor, WAVE2, along with the inflammatory transcription factor, nuclear factor kappa B were also elevated within the brain. Remarkably, the stress hormone, corticosterone, was still significantly elevated 1 month after paraquat, whereas the inflammasome factor, caspase-1, and antigen presentation factor, MFG-E8, both displayed delayed rises after the 6-month time. Using high-resolution magnetic resonance imaging, we detected no striatal changes but modest hemispheric differences in the SNc and time-dependent volumetric enlargement of the ventricles in paraquat-treated mice. These data suggest that paraquat induces long-term nigrostriatal pathology (possibly asymmetric) and inflammatory changes and stress and trophic/apoptotic effects that appear to either increase with the passage of time or are evident for at least 1 month. In brief, paraquat may be a useful nonspecific means to model widespread stress and inflammatory changes related to PD or age-related disease in general, but not the progressive nature of such diseases.

Increased P2X7 Receptor Binding Is Associated With Neuroinflammation in Acute but Not Chronic Rodent Models for Parkinson's Disease

The purinergic P2X7 receptor is a key mediator in (neuro)inflammation, a process that is associated with neurodegeneration and excitotoxicity in Parkinson’s disease (PD). Recently, P2X7 imaging has become possible with [¹¹C]JNJ-(54173)717. We investigated P2X7 availability, in comparison with availability of the translocator protein (TSPO), in two well-characterized rat models of PD using in vitro autoradiography at multiple time points throughout the disease progression. Rats received either a unilateral injection with 6-hydroxydopamine (6-OHDA) in the striatum, or with recombinant adeno-associated viral vector overexpressing human A53T alpha-synuclein (α-SYN) in the substantia nigra. Transverse cryosections were incubated with [¹¹C]JNJ-717 for P2X7 or [¹⁸F]DPA-714 for TSPO. [¹¹C]JNJ-717 binding ratios were transiently elevated in the striatum of 6-OHDA rats at day 14–28 post-injection, with peak P2X7 binding at day 14. This largely coincided with the time course of striatal [¹⁸F]DPA-714 binding which was elevated at day 7–21, with peak TSPO binding at day 7. Increased P2X7 availability co-localized with microglial, but not astrocyte or neuronal markers. In the chronic α-SYN model, no significant differences were found in P2X7 binding, although in vitro TSPO overexpression was reported previously. This first study showed an increased P2X7 availability in the acute PD model in a time window corresponding with elevated TSPO binding and motor behavior changes. In contrast, the dynamics of TSPO and P2X7 were divergent in the chronic α-SYN model where no P2X7 changes were detectable. Overall, extended P2X7 phenotyping is warranted prior to implementation of P2X7 imaging for monitoring of neuroinflammation.

Neuroprotection and neurorestoration as experimental therapeutics for Parkinson's disease

Disease-modifying treatments remain an unmet medical need in Parkinson's disease (PD). Such treatments can be operationally defined as interventions that slow down the clinical evolution to advanced disease milestones. A treatment may achieve this outcome by either inhibiting primary neurodegenerative events ("neuroprotection") or boosting compensatory and regenerative mechanisms in the brain ("neurorestoration"). Here we review experimental paradigms that are currently used to assess the neuroprotective and neurorestorative potential of candidate treatments in animal models of PD. We review some key molecular mediators of neuroprotection and neurorestoration in the nigrostriatal dopamine pathway that are likely to exert beneficial effects on multiple neural systems affected in PD. We further review past and current strategies to therapeutically stimulate these mediators, and discuss the preclinical evidence that exercise training can have neuroprotective and neurorestorative effects. A future translational task will be to combine behavioral and pharmacological interventions to exploit endogenous mechanisms of neuroprotection and neurorestoration for therapeutic purposes. This type of approach is likely to provide benefit to many PD patients, despite the clinical, etiological, and genetic heterogeneity of the disease.

Lateralized Basal Ganglia Vulnerability to Pesticide Exposure in Asymptomatic Agricultural Workers

Pesticide exposure is linked to Parkinson's disease, a neurodegenerative disorder marked by dopamine cell loss in the substantia nigra of the basal ganglia (BG) that often presents asymmetrically. We previously reported that pesticide-exposed agricultural workers (AW) have nigral diffusion tensor imaging (DTI) changes. The current study sought to confirm this finding, and explore its hemisphere and regional specificity within BG structures using an independent sample population. Pesticide exposure history, standard neurological exam, high-resolution magnetic resonance imaging (T1/T2-weighted and DTI), and [123I]ioflupane SPECT images (to quantify striatal dopamine transporters) were obtained from 20 AW with chronic pesticide exposure and 11 controls. Based on median cumulative days of pesticide exposure, AW were subdivided into high (AWHi, n = 10) and low (AWLo, n = 10) exposure groups. BG (nigra, putamen, caudate, and globus pallidus [GP]) fractional anisotropy (FA), mean diffusivity (MD), and striatal [123I]ioflupane binding in each hemisphere were quantified, and compared across exposure groups using analysis of variance. Left, but not right, nigral and GP FA were significantly lower in AW compared with controls (p's < .029). None of the striatal (putamen and caudate) DTI or [123I]ioflupane binding measurements differed between AW and controls. Subgroup analyses indicated that significant left nigral and GP DTI changes were present only in the AWHi (p ≤ .037) but not the AWLo subgroup. AW, especially those with higher pesticide exposure history, demonstrate lateralized microstructural changes in the nigra and GP, whereas striatal areas appear relatively unaffected. Future studies should elucidate how environmental toxicants cause differential lateralized- and regionally specific brain vulnerability.

Perinatal 6-Hydroxydopamine to Produce a Lifelong Model of Severe Parkinson's Disease

The classic rodent model of Parkinson's disease (PD) is produced by unilateral lesioning of pars compacta substantia nigra (SNpc) in adult rats, producing unilateral motor deficits which can be assessed by dopamine (DA) D₂ receptor (D₂-R) agonist induction of measurable unilateral rotations. Bilateral SNpc lesions in adult rats produce life-threatening aphagia, adipsia, and severe motor disability resembling paralysis-a PD model that is so compromised that it is seldom used. Described in this paper is a PD rodent model in which there is bilateral 99 % loss of striatal dopaminergic innervation, produced by bilateral intracerebroventricular or intracisternal 6-hydroxydopamine (6-OHDA) administration to perinatal rats. This procedure produces no lethality and does not shorten the life span, while rat pups continue to suckle through the pre-weaning period; and eat without impairment post-weaning. There is no obvious motor deficit during or after weaning, except with special testing, so that parkinsonian rats are indistinguishable from control and thus allow for behavioral assessments to be conducted in a blinded manner. L-DOPA (L-3,4-dihydroxyphenylalanine) treatment increases DA content in striatal tissue, also evokes a rise in extraneuronal (i.e., in vivo microdialysate) DA, and is able to evoke dyskinesias. D₂-R agonists produce effects similar to those of L-DOPA. In addition, effects of both D₁- and D₂-R agonist effects on overt or latent receptor supersensitization are amenable to study. Elevated basal levels of reactive oxygen species (ROS), namely hydroxyl radical, occurring in dopaminergic denervated striatum are suppressed by L-DOPA treatment. Striatal serotoninergic hyperinnervation ensuing after perinatal dopaminergic denervation does not appear to interfere with assessments of the dopaminergic system by L-DOPA or D₁- or D₂-R agonist challenge. Partial lesioning of serotonin fibers with a selective neurotoxin either at birth or in adulthood is able to eliminate serotoninergic hyperinnervation and restore the normal level of serotoninergic innervation. Of all the animal models of PD, that produced by perinatal 6-OHDA lesioning provides the most pronounced destruction of nigrostriatal neurons, thus representing a model of severe PD, as the neurochemical outcome resembles the status of severe PD in humans but without obvious motor deficits.

Long-term overexpression of human wild-type and T240R mutant Parkin in rat substantia nigra induces progressive dopaminergic neurodegeneration

Mutations in the parkin gene are the most common cause of early-onset autosomal recessive Parkinson disease (PD). The pathogenic mechanisms of how parkin mutations lead to the development of PD are not fully understood. Studies of cell cultures and of Drosophila have suggested a dominant negative effect for the clinical parkin mutant T240R. Conversely, the neuroprotective capacity of parkin has been widely reported; this suggests that the parkin protein may have a potential therapeutic role in PD. Here, we aimed to develop a novel genetic rodent model of PD by overexpression of T240R-parkin and human wild-type parkin as a control in the dopaminergic neurons of adult rats using adeno-associated viral vectors (rAAV2/8). Surprisingly, we found that overexpression not only of T240R-parkin but also of human wild-type parkin induced progressive and dose-dependent dopaminergic cell death in rats, starting from 8 weeks after injection. This degeneration was specific for parkin because similar overexpressionof enhanced green fluorescent protein did not lead to nigral degeneration. Our results warrant caution to the development of therapeutic strategies for PD based on overexpression of parkin or enhancing parkin activity because this might be deleterious for dopaminergic neurons in the long-term.

Glycine transporter-1 inhibition promotes striatal axon sprouting via NMDA receptors in dopamine neurons

NMDA receptor activity is involved in shaping synaptic connections throughout development and adulthood. We recently reported that brief activation of NMDA receptors on cultured ventral midbrain dopamine neurons enhanced their axon growth rate and induced axonal branching. To test whether this mechanism was relevant to axon regrowth in adult animals, we examined the reinnervation of dorsal striatum following nigral dopamine neuron loss induced by unilateral intrastriatal injections of the toxin 6-hydroxydopamine. We used a pharmacological approach to enhance NMDA receptor-dependent signaling by treatment with an inhibitor of glycine transporter-1 that elevates levels of extracellular glycine, a coagonist required for NMDA receptor activation. All mice displayed sprouting of dopaminergic axons from spared fibers in the ventral striatum to the denervated dorsal striatum at 7 weeks post-lesion, but the reinnervation in mice treated for 4 weeks with glycine uptake inhibitor was approximately twice as dense as in untreated mice. The treated mice also displayed higher levels of striatal dopamine and a complete recovery from lateralization in a test of sensorimotor behavior. We confirmed that the actions of glycine uptake inhibition on reinnervation and behavioral recovery required NMDA receptors in dopamine neurons using targeted deletion of the NR1 NMDA receptor subunit in dopamine neurons. Glycine transport inhibitors promote functionally relevant sprouting of surviving dopamine axons and could provide clinical treatment for disorders such as Parkinson's disease.

A phenotypic model recapitulating the neuropathology of Parkinson's disease

This study was undertaken to develop a phenotypic model recapitulating the neuropathology of Parkinson's disease (PD). Such a model would show loss of dopamine in the basal ganglia, appearance of Lewy bodies, and the early stages of motor dysfunction. The model was developed by subcutaneously injecting biodegradable microspheres of rotenone, a complex I inhibitor in 8-9 month old, ovariectomized Long-Evans rats. Animals were observed for changes in body weight and motor activity. At the end of 11-12 weeks animals were euthanized and the brains examined for histopathological changes. Rotenone treated animals gain weight and appear normal and healthy as compared to controls but showed modest hypokinesia around 5-6 weeks posttreatment. Animals showed loss of dopaminergic (DA) neurons and the appearance of putative Lewy bodies in the substantia nigra. Neuroinflammation and oxidative stress were evidenced by the appearance of activated microglia, iron precipitates, and 8-oxo-2'-deoxyguanosine a major product of DNA oxidation. The dorsal striatum, the projection site of midbrain DA neurons, showed a significant reduction in tyrosine hydroxylase immunostaining, together with an increase in reactive astrocytes, an early sign of DA nerve terminal damage. Levels of vesicular monoamine transporter 2 (VMAT2) were significantly reduced in the dorsal striatum; however, there was an unexpected increase in dopamine transporter (DAT) levels. Old, ovariectomized females treated with rotenone microspheres present with normal weight gain and good health but a modest hypokinesia. Accompanying this behavioral phenotype are a constellation of neuropathologies characteristic of PD that include loss of DA neurons, microglia activation, oxidative damage to nuclear DNA, iron deposition, and appearance of putative Lewy bodies. This phenotypic model recapitulating the neuropathology of Parkinson's disease could provide insight into early mechanisms of pathogenesis and could aid in the identification of biomarkers to identify patients in early stage, PD.

Delayed increase of tyrosine hydroxylase expression in rat nigrostriatal system after traumatic brain injury

Tyrosine hydroxylase (TH) is the key enzyme for synthesizing dopamine (DA) in dopaminergic neurons and its terminals. Emerging experimental and clinical evidence support the hypothesis of a disturbance in dopamine neurotransmission following traumatic brain injury (TBI). However, the effect of controlled cortical impact (CCI) injury on TH in the nigrostriatal system is currently unknown. To determine if there is an alteration in TH after CCI injury, we examined TH levels at 1 day, 7 days, and 28 days post-injury by utilizing a commercially available antibody specific to TH. Rats were anesthetized and surgically prepared for CCI injury (4 m/s, 3.2 mm) or sham surgery. Injured (N=6) and sham animals (N=6) were sacrificed and coronally sectioned (35 microm thick) through the striatum and substantia nigra (SN) for immunohistochemistry. Additionally, semiquantitative measurements of TH protein in striatal and SN homogenates from injured (N=6) and sham (N=6) rats sacrificed at the appropriate time post-surgery were assessed using Western blot analysis. TH protein is bilaterally increased at 28 days post-injury in nigrostriatal system revealed by immunohistochemistry in injured rats compared to sham controls. Western blot analysis confirms the findings of immunohistochemistry in both striatum and SN. We speculate that the increase in TH in the nigrostriatal system may reflect a compensatory response of dopaminergic neurons to upregulate their synthesizing capacity and a delayed increase in the efficiency of DA neurotransmission after TBI.

Substantia nigra compacta neurons that innervate the reticular thalamic nucleus in the rat also project to striatum or globus pallidus: Implications for abnormal motor behavior

The projections of the substantia nigra pars compacta (SNc) to the reticular thalamic nucleus (RTn) were assessed by measuring dopamine content and counting tyrosine hydroxylase positive (TH (+)) cells in rats with unilateral lesions induced by 6-hydroxydopamine (6-OHDA), and by using a fluorescent tract-tracing technique in rats without lesions. Injection of 6-OHDA in the RTn reduced dopamine content and the number of TH (+) cells in the SNc by about 50%. Branching of SNc was suggested by the finding that 6-OHDA deposited in the RTn significantly reduced dopamine in the striatum and globus pallidus. Moreover, injections of 6-OHDA into either the striatum or the globus pallidus significantly reduced dopamine content in the RTn. Fluorescent tracers injected into the RTn labeled TH (+) cells in the SNc. A high proportion of these TH (+) cells was double labeled when tracers were also injected into either the globus pallidus or striatum. Other experiments showed that systemic injection of apomorphine or methamphetamine induced turning behavior in rats with local deposits of 6-OHDA in either the RTn or the studied basal ganglia nuclei. The extensive dopaminergic branching suggests that the abnormal motor behavior of rats with 6-OHDA deposits in the RTn may be caused by dopaminergic denervation of more than one structure. The fact that lesion of a single dopaminergic neuron can reduce dopamine transmission in more than one structure is probably important in generating the manifestations of Parkinson's disease.

Timecourse of striatal re-innervation following lesions of dopaminergic SNpc neurons of the rat

Previously we described the extent of sprouting that axons of the rat substantia nigra pars compacta (SNpc) undergo to grow new synapses and re-innervate the dorsal striatum 16 weeks after partial lesions. Here we provide insights into the timing of events related to the re-innervation of the dorsal striatum by regenerating dopaminergic nigrostriatal axons over a 104-week period after partial SNpc lesioning. Density of dopamine transporter and tyrosine hydroxylase immunoreactive axonal varicosities (terminals) decreased up to 80% 4 weeks after lesioning but returned to normal by 16 weeks, unless SNpc lesions were greater than 75%. Neuronal tracer injections into the SNpc revealed a 119% increase in axon fibres (4 mm rostral to the SNpc) along the medial forebrain bundle 4 weeks after lesioning. SNpc cells underwent phenotypic changes. Four weeks after lesioning the proportion of SNpc neurons that expressed tyrosine hydroxylase fell from 90% to 38% but returned to 78% by 32 weeks. We discuss these phenotype changes in the context of neurogenesis. Significant reductions in dopamine levels in rats with medium (30-75%) lesions returned to normal by 16 weeks whereas recovery was not observed if lesions were larger than 75%. Finally, rotational behaviour of animals in response to amphetamine was examined. The clear rightward turning bias observed after 2 weeks recovered by 16 weeks in animals with medium (30-75%) lesions but was still present when lesions were larger. These studies provide insights into the processes that regulate sprouting responses in the central nervous system following injury.

Changes in function and ultrastructure of striatal dopaminergic terminals that regenerate following partial lesions of the SNpc

Following partial substantia nigra lesions, remaining dopaminergic neurones sprout, returning terminal density in the dorsal striatum to normal by 16 weeks. This suggests regeneration and maintenance of terminal density is regulated to release appropriate levels of dopamine. This study examined the structure and function of these reinnervated terminals, defining characteristics of dopamine uptake and release, density and affinity of the dopamine transporter (DAT) and ultrastructural morphology of dopamine terminals in the reinnervated dorsal striatum. Finally, rotational behaviour of animals in response to amphetamine was examined 4 and 16 weeks after substantia nigra pars compacta (SNpc) lesions. Dopamine transport was markedly reduced 16 weeks after lesioning along with reduced density and affinity of DAT. Rate of dopamine release and peak concentration, measured electrochemically, was similar in lesioned and control animals, while clearance was prolonged after lesioning. Ultrastructurally, terminals after lesioning were morphologically distinct, having increased bouton size, vesicle number and mitochondria, and more proximal contacts on post-synaptic cells. After 4 weeks, tendency to rotate in response to amphetamine was proportional to lesion size. By 16 weeks, rotational behaviour returned to near normal in animals where lesions were less than 70%, although some animals demonstrated unusual rotational patterns at the beginning and end of the amphetamine effect. Together, these changes indicate that sprouted terminals are well compensated for dopamine release but that transport mechanisms are functionally impaired. We discuss these results in terms of implications for dyskinesia and other behavioural states.

Protection of dopaminergic nigrostriatal afferents by GDNF delivered by microspheres in a rodent model of Parkinson's disease

The use of glial cell line-derived neurotrophic factor (GDNF) appears to be a promising strategy to promote survival and function of the nigrostriatal dopaminergic pathway damaged in Parkinson's disease (PD). However, effective intracerebral administration is required for optimal therapeutic benefit and tools to evaluate such therapies must be developed. A rodent model of PD was therefore developed using striatal injection of 6-hydroxydopamine (6-OHDA) with simultaneous implantation of GDNF-delivering microspheres. The effects of GDNF released from microspheres were assessed by classical methods such as amphetamine-induced rotating behavior and tyrosine hydroxylase (TH) immunoreactivity, as well as by quantitative autoradiography using PE2I, a dopamine transporter (DAT) radiotracer, which is also suitable for SPET imaging in humans. 6-OHDA-lesioned animals that received microspheres without GDNF were used as controls. During the first 3 weeks after simultaneous lesion and implantation, the amphetamine-induced rotating behavior of GDNF-treated rats was improved compared to controls and an increase in TH expression (+26%) was measured in the striatum 6 weeks after lesion. In accordance with these results, an increase in striatal PE2I-labeled DAT density was obtained (+17%) after 3 and 6 weeks of treatment. In conclusion, this study demonstrates the neuroprotective action of GDNF delivered by microspheres and suggests that PE2I may be an appropriate radiotracer for use in SPET scintigraphy to follow up treatment of PD in humans.

6-Hydroxydopamine-lesioning of the nigrostriatal pathway in rats alters basal ganglia mRNA for copper, zinc- and manganese-superoxide dismutase, but not glutathione peroxidase

The effects of nigrostriatal pathway destruction on the mRNA levels of copper, zinc-dependent superoxide dismutase (Cu,Zn-SOD), manganese-dependent superoxide dismutase (Mn-SOD), and glutathione peroxidase in basal ganglia of adult rat were investigated using in situ hybridization histochemistry and oligodeoxynucleotide (single-stranded complementary DNA) probes. The 6-hydroxydopamine (6-OHDA)-induced destruction of the nigrostriatal pathway resulted in contralateral rotation to apomorphine and a marked loss of specific [(3)H]mazindol binding in the striatum (93%; P<0.05) and of tyrosine hydroxylase mRNA in substantia nigra pars compacta (SC) (93%; P<0.05) compared with control rats. Levels of Cu,Zn-SOD mRNA were decreased in the striatum, globus pallidus, and SC on the lesioned side of 6-OHDA-lesioned rats compared with sham-lesioned rats (P<0.05). Levels of Mn-SOD mRNA were increased in the nucleus accumbens (P<0.05), but decreased in the SC (P<0.05) on the lesioned side of 6-OHDA-treated rats compared with sham-lesioned rats. Lesioning with 6-OHDA had no effect on glutathione peroxidase mRNA levels in any region of basal ganglia examined. The significant changes in Cu,Zn-SOD and Mn-SOD mRNA indicate that SOD is primarily expressed by dopaminergic neurons of the nigrostriatal pathway, and that the Mn-SOD gene appears to be inducible in rat basal ganglia in response to both physical and chemical damage 5 weeks after 6-OHDA-lesioning. These findings may clarify the status of antioxidant enzymes, particularly Mn-SOD, in patients with Parkinson's disease and their relevance to disease pathogenesis.

Spontaneous long-term compensatory dopaminergic sprouting in MPTP-treated mice

The present study sought to determine whether severe 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication elicits spontaneous long-term compensatory sprouting in mice. Animals, once treated, were kept without further treatment for 0.5, 1, 5, or 7 months. The stability of the nigral degeneration was checked by evaluation of the number of tyrosine hydroxylase immunoreactive (TH-IR) neurons, whereas sprouting was assessed using both [(3)H]-dopamine (DA) uptake by striatal synaptosomes and optical density of TH-immunolabeled fibers in the striatum as markers. At 0.5 month after MPTP intoxication (80 mg/kg, i.p.), we observed comparable decreases of 83% in DA uptake, 83.3% in TH fiber density, and 74% in the number of TH-IR neurons compared to age-matched saline-treated animals. From 5 months onwards, both DA uptake and striatal TH fiber density increased significantly (50% and 34.9% at 5 months, 65% and 67.4% at 7 months, respectively) in comparison with age-matched saline-treated animals, although the number of TH-IR neurons remained stable (73% of degeneration at 7 months). These results indicate clearly that spontaneous long-term compensatory dopaminergic sprouting is a phenomenon that is not restricted to situations of partial nigral degeneration but can, on the contrary, constitute a response even to severe stable MPTP-induced nigral degeneration.

Axonal sprouting following lesions of the rat substantia nigra

Parkinson's disease is characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta. Symptoms do not appear until most nigral neurons are lost, implying that compensatory mechanisms are present. Sprouting has been proposed as one of these mechanisms. This study quantified the extent of compensatory axonal sprouting following injury of dopaminergic neurons within the substantia nigra pars compacta. Specifically, the extent of the axonal arbour and axonal varicosity morphology was measured after partial destruction (with 6-hydroxydopamine) of the substantia nigra of the adult male rat. Four months later, the substantia nigra was injected with the anterograde neuronal tracer dextran-biotin to trace the full extent of individual axons. An unbiased estimate of neuron number was performed in each animal. This demonstrated nigral neuronal loss ranging from 10 to 90% on the side that received the injection whilst a 7% reduction was observed in the side contralateral to the lesion. Coincident with this loss, some nigral neurons lose tyrosine hydroxylase expression. Vigorous axonal sprouting was observed in the terminal arbours of lesioned animals and was associated with an increased axonal varicosity size. Axonal varicosities and branching points were primarily confined to the dorsal 1.5mm of the caudate-putamen, an area predominantly innervated by nigral neurons. It appears that dopaminergic neurons were responsible for this sprouting because the density of dopamine transporter immunoreactive varicosities in the caudate-putamen was maintained until about a 70% loss of neurons. It was concluded that substantial compensation in the form of sprouting and new dopaminergic synapse formation occurs following lesions of the substantia nigra pars compacta.

Dopamine-deficient mice are hypersensitive to dopamine receptor agonists

Dopamine-deficient (DA-/-) mice were created by targeted inactivation of the tyrosine hydroxylase gene in dopaminergic neurons. The locomotor activity response of these mutants to dopamine D1 or D2 receptor agonists and l-3,4-dihydroxyphenylalanine (l-DOPA) was 3- to 13-fold greater than the response elicited from wild-type mice. The enhanced sensitivity of DA-/- mice to agonists was independent of changes in steady-state levels of dopamine receptors and the presynaptic dopamine transporter as measured by ligand binding. The acute behavioral response of DA-/- mice to a dopamine D1 receptor agonist was correlated with c-fos induction in the striatum, a brain nucleus that receives dense dopaminergic input. Chronic replacement of dopamine to DA-/- mice by repeated l-DOPA administration over 4 d relieved the hypersensitivity of DA-/- mutants in terms of induction of both locomotion and striatal c-fos expression. The results suggest that the chronic presence of dopaminergic neurotransmission is required to dampen the intracellular signaling response of striatal neurons.

Compensatory mechanisms in experimental and human parkinsonism: towards a dynamic approach

This paper provides an overview of the compensatory mechanisms which come into action during experimental and human parkinsonism. The intrinsic properties of the dopaminergic neurones of the substantia nigra pars compacta (SNc) which degenerate during Parkinson's disease are described in detail. It is generally considered that the nigrostriatal pathway is principally responsible for the compensatory preservation of dopaminergic function. It is also becoming clear that the morphological characteristics of dopaminergic neurones and the dual character, synaptic and asynaptic, of striatal dopaminergic innervation engender two modes of transmission, wiring and volume, and that both these modes play a role in the preservation of dopaminergic function. The plasticity of the dopamine neurones, extrinsic or intrinsic to the striatum, can thus be regarded as another compensatory mechanism. Recent anatomical and electrophysiological studies have shown that the SNc receives both glutamatergic and cholinergic inputs. The dynamic role this innervation plays in compensatory mechanisms in the course of the disease is explained and discussed. Recent developments in the field of compensatory mechanisms speak for the urgence to develop a valid chronic model of Parkinson's disease, integrating all the clinical features, even resting tremor, and illustrating the gradual evolution of nigral degeneration observed in human Parkinson's disease. Only a dynamic approach to the physiopathological study of compensatory mechanisms in the basal ganglia will be capable of elucidating these complex questions.

Pathophysiological mechanisms of genetic absence epilepsy in the rat

Generalized non-convulsive absence seizures are characterized by the occurrence of synchronous and bilateral spike and wave discharges (SWDs) on the electroencephalogram, that are concomitant with a behavioral arrest. Many similarities between rodent and human absence seizures support the use of genetic rodent models, in which spontaneous SWDs occur. This review summarizes data obtained on the neurophysiological and neurochemical mechanisms of absence seizures with special emphasis on the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). EEG recordings from various brain regions and lesion experiments showed that the cortex, the reticular nucleus and the relay nuclei of the thalamus play a predominant role in the development of SWDs. Neither the cortex, nor the thalamus alone can sustain SWDs, indicating that both structures are intimely involved in the genesis of SWDs. Pharmacological data confirmed that both inhibitory and excitatory neurotransmissions are involved in the genesis and control of absence seizures. Whether the generation of SWDs is the result of an excessive cortical excitability, due to an unbalance between inhibition and excitation, or excessive thalamic oscillations, due to abnormal intrinsic neuronal properties under the control of inhibitory GABAergic mechanisms, remains controversial. The thalamo-cortical activity is regulated by several monoaminergic and cholinergic projections. An alteration of the activity of these different ascending inputs may induce a temporary inadequation of the functional state between the cortex and the thalamus and thus promote SWDs. The experimental data are discussed in view of these possible pathophysiological mechanisms.

Behavioural recovery after unilateral lesion of the dopaminergic mesotelencephalic pathway: effect of repeated testing

Functional recovery following a complete unilateral lesion of the nigrostriatal pathway in adult rats was studied. We examined the effect of training on the spontaneous or induced postural bias following the lesion. Two tasks measuring lateralization were used to assess the lesion-induced postural bias: spontaneous asymmetry was evaluated in the Y-maze, whereas induced body bias was measured by hanging the rat by its tail. Recovery was assessed at three different times following the lesion. The effects of lesion in adult rats in the short, medium and long term were evaluated and compared with the effects of dopaminergic transplants. In adult lesioned rats, destruction of dopaminergic innervation of the neostriatum induced initially an ipsilateral bias as measured in the "tail hang test" and the Y-maze. Recovery of function was observed in the tail hang test as ipsilateral bias declined on repeated testing. Apart from this effect, there was a post-lesion interval effect, since the postural bias disappeared more rapidly on repeated testing in the long-term lesioned rats. This spontaneous recovery was impaired by intrastriatal dopaminergic grafts. Furthermore, no spontaneous recovery was observed in the Y-maze test. These observations show that repeated testing can influence the long-term effects of damage to the nigrostriatal dopamine system.

Infusions of tyrosine hydroxylase antisense oligodeoxynucleotide into substantia nigra of the rat: effects on tyrosine hydroxylase mRNA and protein content, striatal dopamine release and behaviour

Modulation of the transcriptional message of tyrosine hydroxylase was investigated in vivo in the rat nigrostriatal dopamine system with unmodified antisense oligodeoxynucleotide, mismatch oligodeoxynucleotide or vehicle controls. Oligodeoxynucleotide was infused (0.5 microgram/0.5 microliter/h) unilaterally into the substantia nigra by an osmotic minipump system over 14 days. The presence of oligodeoxynucleotide in the brain was verified by in situ hybridization and fluorescence labelling. Animals treated with unmodified antisense oligodeoxynucleotide showed ipsilateral turning behaviour when challenged systemically with the indirect dopamine agonist amphetamine, whereas mismatch- and vehicle-infused rats showed no such behavioural asymmetries. In the substantia nigra, antisense treatment had no effects on tyrosine hydroxylase mRNA, but it led to a reduction in tyrosine hydroxylase protein content. Tissue levels of dopamine, measured in postmortem tissue punches of the neostriatum and substantia nigra, were reduced in the oligodeoxynucleotide-treated hemisphere. Furthermore, basal extracellular levels of dopamine, monitored by in vivo microdialysis, were also lower in the neostriatum ipsilateral to antisense infusion and showed a weaker response to an amphetamine challenge when compared with the contralateral side. These effects were not observed after infusion of mismatch oligodeoxynucleotide or vehicle into the substantia nigra. Finally, the GABAergic enzyme glutamate decarboxylase was not affected in the antisense-treated substantia nigra, indicating that non-specific damage in this area was not caused by this treatment. Our results indicate that antisense oligodeoxynucleotide treatment against tyrosine hydroxylase in the substantia nigra has behavioural and neurochemical effects that are comparable with known actions of dopamine neurotoxins, which are conventional pharmacological tools for the depletion of dopamine. Furthermore, our data show the potential of antisense targetting to reveal new relationships between neurotransmitter-related enzymes and behavioural parameters, because the possibility of selectively and discretely manipulating tyrosine hydroxylase function is likely to produce new insights into the physiological and behavioural functions of the dopaminergic nigrostriatal system.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

Dopaminergic sprouting in the rat striatum after partial lesion of the substantia nigra

The capacity of the dopaminergic nerve system to reinnervate the denervated adult striatum was analyzed in a model of partial 6-hydroxydopamine-induced unilateral lesion of rat substantia nigra pars compacta. Sprouting of dopaminergic fibers entering the ventrolateral part of the striatum from a narrow zone of the external capsule was detected on the lesioned side 4 and 7 months, but not 10 days, after lesioning. Ultrastructural examination of the zone of sprouting revealed hypertrophic dopaminergic fibers and growth-cone-like structures, confirming the existence of an ongoing process of spontaneous regrowth of dopaminergic fibers. The identification of the factors involved in the regrowth of dopaminergic fibers may help to orientate molecular research into new treatments for Parkinson's disease.

Functional and molecular differentiation of the dopamine system induced by neonatal denervation

The administration of the neurotoxin 6-hydroxydopamine (6-OHDA) to damage the mesostriatal dopamine (DA) system in the neonate results in different neurochemical and behavioral consequences as compared to lesions made in adulthood. There have been few direct data to support the conclusion that the behavioral changes following neonatal 6-OHDA lesions reflect plasticity of the DA system. It is our hypothesis that the plasticity of the developing DA system is fundamentally different from that of the adult. Responses to 6-OHDA lesions can only be understood within the context of the status of the mesostriatal DA system at the time of the lesion. There are stages of development in the early postnatal period when certain components of the mesostriatal DA system are differentially sensitive to 6-OHDA lesions. These "windows" of vulnerability can be predicted from an analysis of the developmental expression of DA receptors and the maturation of the subpopulation of the mesostriatal DA system that innervates them. We review the differences in the behavioral plasticity of the adult and neonate sustaining 6-OHDA lesions to the mesostriatal DA system, the mechanisms responsible for the behavioral plasticity in the adult, and our conceptualization of which mechanisms are affected in the neonate.

Centrally active differentiation factors in the nervous system

The adult mammalian brain is a remarkably heterogeneous structure comprised of more than 50 biochemically distinct types of neurons. This phenotypic diversity is established during development, not only as the result of genetic but also epigenetic influences. It is believed that extracellular proteins, called differentiation factors, both instruct neurons in their original choice of neurotransmitter substance and, in certain situations, revise those biochemical decisions. The first candidate differentiation factor in the brain has only recently been proposed. This muscle-derived substance has the unique ability, in culture, to initiate expression of genes associated with catecholamine transmitter synthesis in non-catecholamine neurons of the brain. Because it also amplifies expression in cultured catecholamine-producing neurons in vitro and in vivo, it may prove to be an important therapeutic agent in diseases involving catecholamine shortages.

Behavioral analysis of asymmetries induced by unilateral 6-OHDA injections into the substantia nigra

The intent of this study was to perform a detailed analysis of behavioral asymmetries (turning behavior) exhibited by animals which had sustained a unilateral lesion of the substantia nigra (SN). Rats were tested for behavior asymmetries over 16 days, once before, and 7 times after 6-OHDA had been injected into one SN. On the basis of the number of narrow diameter ipsiversive half turns produced during testing, they were then assigned to one of three groups: (a) those which showed an initial asymmetry from which they recovered, (b) those which were asymmetrical throughout testing, and (c) those in which the asymmetry only emerged during the testing period. We examined several different aspects of turning behavior. Recovery from asymmetry was associated with an increase in the number of contraversive, as well as a decrease in the number of ipsiversive narrow-angle turns. The average diameter of these ipsiversive turns also increased. Animals which recovered showed a contraversive asymmetry for wide-diameter turns which increased during testing. Narrow-diameter contraversive turns decreased during testing in the two nonrecovery groups. Both nonrecovery groups showed a contraversive asymmetry for wide-diameter turns. Asymmetry was attributable to a tendency to circle and move ipsiversively in the two nonrecovery groups, whereas it was due to circling behavior in animals which showed recovery. Additionally, more cells, labeled by HRP injected into the ipsilateral caudate putamen, were found in the damaged SN of animals which recovered.

Amphetamine-induced rotational behavior in rats: relationship to hypothalamic and striatal degeneration

When lesions are placed unilaterally in the nigrostriatal system of experimental animals, rotational behavior occurs in response to peripheral administration of dopamine (DA) agonists. In spite of considerable evidence to the contrary, it is assumed that in order for this rotation to occur, an almost complete depletion of striatal DA must be achieved. To test this hypothesis further, 20 male Sprague-Dawley rats were injected unilaterally with 2 microL of 8 micrograms/microL of 6-hydroxydopamine (6-OHDA) via acute injection needles or chronically indwelling cannulae. Acute injection of 6-OHDA resulted in a rotation rate of 7.2 to 18.9 revolutions per minute in response to peripheral amphetamine injection (5 mg/Kg) while injection of 6-OHDA through chronically indwelling cannulae produced rotation ranging from 1.4 to 9.9 rotations per minute. Under the conditions of either method of injection, the animals displaying the most severe rotation still showed partial denervation of striatal DA as revealed by catecholamine fluorescence histochemistry. Conversely, numerous animals demonstrating very low rates of amphetamine-induced rotation often displayed a complete loss of striatal, accumbens, and olfactory tubercle catecholamine fluorescence. Moreover, large quantities of lateral hypothalamic amine accumulation were observed in rotating rats indicating that this neurochemical change may be of functional significance for rotational responses. The present results, when taken into consideration with previous work, indicate that the routine selection of rotating animals for pharmacological testing for potential antiParkinsonian medication or intracerebral grafting purely on the basis of their rotational behavior does not necessarily imply that complete striatal denervation has occurred. Moreover, these findings demonstrate that amine accumulation in the lateral hypothalamus of rotating animals with DA depleting lesions is an important phenomenon implicated in the expression of rotational behavior in animals and possibly in the pathophysiology of Parkinson's disease.

Grafts in the treatment of Parkinson's disease: animal models

Long-term treatment of parkinsonian patients with L-DOPA leads to a loss of efficacy over time and the appearance of important side effects such as dyskinesias. Grafts of chromaffin cells of the adrenal medulla or fetal ventral mesencephalic neurons bring behavioral improvement in animal models of Parkinson's disease. These improvements are likely to be related to the secretion of dopamine by the grafted cells and/or to the reinnervation of the host tissue. In addition, a leak in the blood-brain barrier may allow peripheral catecholamines to gain access to the brain. Lack of clear effects of grafts in parkinsonian patients may be due to their poor survival in the human brain. Improvement of grafting techniques as well as the addition of neurotrophic factors to grafts may help increase their survival and improve behavioral effects. Recently, genetic techniques have allowed the creation of genetically modified cell lines which can produce L-DOPA and these cells may be grafted in the brain. Interestingly, these cell lines may be encapsulated in permselective membranes which can protect them from immunological rejection and avoid the uncontrolled cell growth of these mitotically active cells. Grafting techniques seem to be an interesting alternative to treat parkinsonian patients. Improvement of grafting procedures may help increase survival of grafts and thus enhance behavioral improvements. Moreover, genetic modification of well-known tumor cell lines or patient's own cells such as astrocytes may help avoid the low availability as well as ethical and immunological problems linked to the use of fetal human tissue.

Slow changes of tyrosine hydroxylase gene expression in dopaminergic brain neurons after neurotoxin lesioning: a model for neuron aging

Slow neuron regression develops during the adult phase of life in select brain systems of mammals. We describe a model in adult rats that resolves several phases in a slow atrophic process that differentially influences levels of mRNA and protein for tyrosine hydroxylase (TH). Responses of striatal dopaminergic markers to 6-hydroxydopamine (6-OHDA) lesions in rats indicated that the striatal terminals maintained TH protein, despite greater than 3-fold loss of TH mRNA in the substantia nigra pars compacta (SNC) cell bodies whose axons project to the striatum. The loss of TH mRNA/cell was progressive up to 9 months, whereas SNC cell body shrinkage stabilized by 3 months post-lesioning. Consideration of possible mechanisms in protein turnover motivated a search for PEST motifs in the TH of rats and other vertebrates that could be a point of regulation by altering the rate of TH protein turnover.

Changes in the nigrostriatal projection associated with recovery from lesion-induced behavioral asymmetry

Possible neuronanatomical changes correlated with recovery from lesion-induced behavioral asymmetry were examined. Rats, with 6-OHDA injected into the substantia nigra (SN) on one side, were either assigned to a group with a 48-hour survival period, or one with a 15-day recovery period. Control groups, without a lesion, were also included. All animals were tested for behavioral asymmetry and, at the end of the survival period, had horseradish peroxidase (HRP) deposited in the caudate-putamen (CPU) ipsilateral to the lesion (right or left CPU in the controls). Both substantia nigrae of all animals were examined for HRP-labeled cells. Animals given a 15-day recovery period had more HRP-labeled cells in the SN ipsilateral to the HRP deposition site than those given a 1-day recovery period. Also animals which showed behavioral recovery had more HRP-labeled cells in their ipsilateral SN than either those which showed no recovery or those which were not given time to recover. Thus, our results suggested that behavioral recovery from lesion-induced asymmetry was associated with an increase in HRP uptake and retrograde transport by the remaining nigrostriatal terminals. Animals which recovered showed the expected decrease in ipsiversive, and increase in contraversive turning with time. Unexpectedly, animals which did not recover exhibited an increase in ipsiversive, and a decrease in contraversive turning with time.

Relationships between indices of behavioral asymmetries and neurochemical changes following mesencephalic 6-hydroxydopamine injections

Behavioral and neurochemical changes were investigated in rats that had received one of 3 doses of 6-hydroxydopamine (6-OHDA), injected unilaterally into the ventral mesencephalon. The behavioral analysis comprised that of tight turns (diameter less than 30 cm), wide turns (diameter greater than 55 cm), and locomotor activity. 6-OHDA-injected animals were assigned to 3 different groups according to their degree of asymmetry in tight turns, both in spontaneous behavior and after the dopamine receptor agonist apomorphine (0.05 mg/kg). Thus, 6-OHDA-injected animals showed either (i) no spontaneous ipsiversive asymmetry (group 1), or (ii) an ipsiversive asymmetry, from which they did not recover during the three postoperative weeks of testing and which could not be reversed by apomorphine (group 2), or (iii) an even stronger ipsiversive asymmetry from which they did not recover but which could be reversed by apomorphine (group 3). The analysis of wide turns, which might reflect exploratory behavior of the environment, namely thigmotactic scanning, provided further information, as it indicated an asymmetry even in group 1; however, in contrast to the other groups a contraversive asymmetry was observed. Neurochemically, the three experimental groups were clearly different from each other with respect to the degree of neostriatal dopamine depletion, and the increase in dopamine metabolism in the damaged hemisphere as indicated by increased metabolite/transmitter ratios. The ipsiversive asymmetry in tight turns was negatively correlated with dopamine levels in the damaged neostriatum and positively correlated with the increase in metabolism. Furthermore, indications of changes in neostriatal serotonin activity were found. These results are discussed with respect to the necessity of differential measures of behavioral asymmetry, the role of dopaminergic mechanisms of 6-OHDA-induced deficits and mechanisms of recovery. The asymmetries in tight versus wide turns are suggested to reflect the preponderance of a motor deficit in the former case versus that of sensory neglect in the latter. Thus, the analysis of tight versus wide turns may provide distinctive and sensitive indices related to different functional deficits in animal models of hemiparkinsonism.

Involvement of the nigral output pathways in the inhibitory control of the substantia nigra over generalized non-convulsive seizures in the rat

Activation of GABAergic transmission within the substantia nigra has been shown to suppress several forms of generalized seizures in experimental models of epilepsy. More especially, such pharmacological manipulations suppress spontaneous and chemically-induced generalized non-convulsive seizures in the rat. The aim of the present study was to examine the role of the dopaminergic and GABAergic thalamic and collicular nigral outputs in this antiepileptic effect. For this purpose, we examined the effects of output destruction on the antiepileptic effect of intranigral injections of a GABA agonist or pharmacological blockade of the neurotransmission at the nerve terminal level in rats with spontaneous absence seizures. After selective destruction of dopaminergic neurons within the substantia nigra with 6-hydroxydopamine (5 micrograms/side) or hemisection of the ascending nigral output, bilateral intranigral injection of muscimol (2 ng/side) still significantly suppressed generalized non-convulsive seizures. Bilateral lesioning of the ventromedial nucleus of the thalamus did not abolish the antiepileptic effects of intranigral muscimol (2 ng/side) and the GABA antagonist, picrotoxin, when given into this thalamic nucleus (10 ng/side) also failed to induce suppression of spike and wave discharges. The antiepileptic effects of intranigral injection of muscimol (2 ng/side) was reversed by bilateral electrolytic lesions of the superior colliculus. Blockade of the GABAergic transmission at this level with picrotoxin (40 ng/side) significantly suppressed generalized non-convulsive seizures. Finally, excitation of collicular cell bodies with low doses of kainic acid (4 and 8 ng/side) also resulted in a suppression of spike and wave discharges. These results demonstrate that the GABAergic nigrocollicular pathway is critical for the inhibitory control of the substantia nigra over generalized non-convulsive seizures. The data further suggest that antiepileptic effects observed following potentiation of GABAergic transmission in the substantia nigra result from a disinhibition of collicular cell bodies.

A 6-hydroxydopamine-induced selective parkinsonian rat model

Previous parkinsonian rat models have generally been characterized by unilateral destruction of both the nigrostriatal pathway and the mesolimbic pathway using the neurotoxin 6-hydroxydopamine (6-OHDA). We created a hemiparkinsonian model in which there is 6-OHDA-induced destruction of the dopaminergic nigrostriatal pathway but sparing of the dopaminergic mesolimbic pathway. This resulted in reproducible, quantifiable rotational behavior in response to either amphetamine or apomorphine and a near total depletion of dopamine in the striatum ipsilateral to the lesion with a dorsolateral distribution of supersensitive dopaminergic D2 receptors. This model parallels the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced hemiparkinsonian model in primates and more closely approximates the extent of neurodegeneration seen in human idiopathic Parkinson's disease than previous parkinsonian rat models. It may therefore prove a convenient model for studying the recently reported phenomenon of sprouting from host dopaminergic neurons following tissue implantation.

Chronic lesions differentially decrease tyrosine hydroxylase messenger RNA in dopaminergic neurons of the substantia nigra

Long-term effects of lesions were analyzed in terms of gene expression. Nine months after unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (s. nigra), the remaining dopaminergic (DAergic) neurons (tyrosine hydroxylase (TH) cells determined by immunocytochemistry (ICC] on the lesioned side were atrophic with smaller nucleoli. By in situ hybridization, the DAergic neurons on the lesioned side had a 50% smaller TH-mRNA concentration than on the contralateral non-lesioned side. However, beta-tubulin mRNA concentration in DAergic neurons was unaffected by the lesion. The lesions did not alter TH-mRNA concentration in the contralateral non-lesioned side by comparison with unoperated controls. We propose that chronic lesions have long-term effects on gene expression because of damage sustained during compensatory hyperactivity after the lesion, or because of decreased trophic support from other neurons.

Protection against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonism by the catecholamine uptake inhibitor nomifensine: behavioral analysis in monkeys with partial striatal dopamine depletions

The neurotoxic effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on dopamine neurons in monkeys were found to be reduced when the catecholamine uptake inhibitor nomifensine was administered during several weeks after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The obtained protection was partial, leading to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced changes in dopamine levels to 8, 16, 52 and 59% of control values in the caudate nucleus and to 10, 16, 101 and 99% in the putamen of four animals, respectively. At the same doses, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine alone is known to deplete striatal dopamine levels to 0.5-7% of control values. Extra-nigrostriatal monoamine neurons were generally well protected by nomifensine. Neurological examinations revealed modest hypokinesia for a maximum of 10 days after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the two more severely affected animals. Reaction times of arm and eye movements were measured in a formal task in two of the monkeys having a moderate and a more important depletion of striatal dopamine, respectively. Only moderate impairments were seen during the initial 2 weeks after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in both animals. All parameters recovered to control levels thereafter. At 3.5 and 5.5 months after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, task performance was significantly better than control. The speed of arm movement remained largely unaffected during all periods of experimentation. Spontaneous eye movements were reduced in frequency and amplitude during the initial 1-2 weeks after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, and recovered completely thereafter. These data suggest a substantial reduction of neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine by inhibition of catecholamine uptake. Particularly striking was the absence of major and permanent impairments in behavioral tests in which monkeys treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine alone were severely impaired. These results may warrant the development of new catecholamine uptake inhibitors for protecting nigrostriatal dopamine neurons against potential environmental toxins.

Normalization of extracellular dopamine in striatum following recovery from a partial unilateral 6-OHDA lesion of the substantia nigra: a microdialysis study in freely moving rats

It has been hypothesized that striatal dopamine (DA) terminals undergo compensatory changes in response to partial damage of the mesostriatal DA system, which results in higher concentrations of DA in the extracellular space than would be predicted by DA concentrations in post-mortem tissue. But, this hypothesis has never been tested directly in vivo, and therefore, the present study was designed signed to do so. Microdialysis was used in freely moving rats to estimate the concentration of DA, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) in striatal extracellular fluid; simultaneously from the hemisphere with unilateral 6-hydroxydopamine (6-OHDA) lesion of the substantia nigra and from the intact hemisphere. It was found that following recovery from a 6-OHDA lesion, and during the resting state, the extracellular concentrations of DA were normal on the lesion side, even after that side was depleted of up to 99.0% of the DA measured in post-mortem tissue. Furthermore, the extracellular concentrations of DA were elevated in the intact hemisphere of animals with a greater than 95% DA depletion. In rats with a less than 95% DA depletion amphetamine (1.5 mg/kg) caused a large increase in the extracellular concentration of DA in both the lesion and intact hemispheres (intact greater than lesion), but in rats with a greater than 95% tissue DA depletion amphetamine only enhanced extracellular DA on the intact side; on the lesion side amphetamine produced a progressive decrease in extracellular DA to nondetectable levels. Animals rotated towards the lesion side. Unlike DA, the extracellular concentrations of DOPAC and HVA were greatly reduced on the lesion side, and the extent of the depletion was highly correlated with lesion size. It is concluded that following partial unilateral damage to mesostriatal DA projections there are massive changes in the remaining DA terminals that are sufficient to normalize the extracellular (and presumably synaptic) concentrations of DA. The normalization of extracellular DA concentrations seen after extensive (but incomplete) damage to the mesostriatal system must play a major role in the sparing and recovery of behavioral function that is so characteristics of this system. After extensive damage the capacity of the remaining DA neurons to respond to increased demand is limited, however, and this may explain why behavioral deficits can be reinstated by stimuli that challenge the system.

Lateralized wall-facing versus turning as measures of behavioral asymmetries and recovery of function after injection of 6-hydroxydopamine into the substantia nigra

The tendency of a rat to approach the wall of an open-field and to travel along the perimeter of the field (termed as "peritaxis" or "wall-facing") is affected by unilateral removal of the vibrissae. Peritaxis is lateralized by hemivibrissotomy. The finding that the dopamine agonist apomorphine reversed the direction of wall-facing asymmetry after 10 days of vibrissae removal suggested a link between this sensorimotor asymmetry and dopamine transmission. The present experiment examined the influence of a unilateral injection of 6-hydroxydopamine into the substantia nigra on peritaxis and compared this behavioral measure with turning behavior as an index of lesion-induced sensorimotor asymmetries and of recovery of function. The lesion of the substantia nigra reduced wall-facing with the side contralateral to the lesion to near-zero values. During the first week after the injection animals with incomplete dopamine depletion recovered from this asymmetry. The changes in wall-facing behavior were paralleled by turning asymmetries. Wall-facing was at least as sensitive to application of amphetamine and apomorphine as turning behavior. We concluded that lateralized wall-facing, or peritaxis, can serve as a useful index of dysfunction in the nigrostriatal dopamine system and the influence of catecholaminergic drugs. Wall-facing can also serve as a measure of recovery of function.

A two-population model of rat rotational behavior: effects of unilateral nigrostriatal 6-hydroxydopamine on striatal neurochemistry and amphetamine-induced rotation

Rats received intrastriatal or intranigral injections of 6-hydroxydopamine (6-OHDA) on the same side towards which they made most of their turns during a previous test of amphetamine-induced rotational behavior. One week later they were retested for amphetamine-induced rotational behavior and it was found that only approximately half of them increased their rotational behavior towards the lesioned side more than non-lesioned controls. In fact, compared to their pre-operative behavior numerous rats decreased or actually reversed their net turning towards the lesioned side. While the post-lesion rotational behavior of the two groups of rats was clearly different, pre-operative turning was not. Furthermore, the neurochemical effects of the intracerebral 6-OHDA injections were not different in the two groups of rats, either with respect to the magnitude of the resulting dopamine (DA) depletion, or with respect to the compensatory increase in the turnover of DA by surviving DA neurons on the lesioned side. The data are discussed in terms of their lack of support for current notions about the role of nigrostriatal DA in turning, and in terms of their support for a two-population model we have previously proposed. An additional, unrelated, finding from the present work was that bilateral striatal serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels decreased bilaterally one week following unilateral intrastriatal administration of 6-OHDA.

The crossed nigrostriatal projection decussates in the ventral tegmental decussation

Horseradish peroxidase (HRP) tract-tracing techniques were used in 44 rats in order to establish the site of decussation of the crossed nigrostriatal projection. Somata in both the ipsilateral and the contralateral ventromedial mesencephalon were labelled after injection of HRP into the caudate nucleus. In agreement with previous studies, contralateral labelling constituted about 3% of the ipsilateral labelling. Midsagittal transection of the mesodiencephalic junction did not prevent the contralateral labelling. However, mid-sagittal transection of the ventral mesencephalon, or selective 6-hydroxydopamine (6-OHDA) lesions of the ventral tegmental decussation did prevent the contralateral labelling. Moreover, 6-OHDA lesions of the substantia nigra ipsilateral to the horseradish peroxidase injection also prevented contralateral labelling. We conclude that the crossed nigrostriatal projection decussates in the ventral tegmental decussation, and that this projection is susceptible to damage by standard 6-OHDA lesions located on the opposite side to the origin of the crossed pathway.

Autoregulation of dopamine release and metabolism by intrastriatal nigral grafts as revealed by intracerebral dialysis

The autoregulation of dopamine release and metabolism by intrastriatal grafts of mesencephalic dopamine neurons was examined in vivo using an intracerebral dialysis technique. Dopamine-rich cell suspension grafts were implanted into the head of the caudate putamen in rats with complete unilateral 6-hydroxydopamine lesion of the nigrostriatal dopamine pathway. Six months later behavioural tests indicated that the grafts had reversed the lesion-induced rotational behaviour. Extracellular levels of striatal dopamine and its metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid were monitored bilaterally in the halothane-anaesthetized grafted rat, both under basal conditions, and also following low (0.05 mg/kg) and high (0.5 mg/kg) doses of the dopamine receptor agonist apomorphine. The perfusate from the grafted striatum showed levels of dopamine which were not statistically different from those of the intact contralateral striatum, indicating that the baseline release of dopamine from the graft was close to normal. Similarly, 3-4-dihydroxyphenylacetic acid and homovanillic acid levels were well recovered on the grafted side (67% and 52%, respectively, of control values). Consistent with previous observations, levels of the serotonin metabolite 5-hydroxyindoleacetic acid measured in perfusate collected from the grafted side was elevated significantly above normal. Subsequent histological analysis revealed large grafts, rich in dopamine-containing neurons (mean +/- SEM number equalled 3138 +/- 630), giving rise to an approximately normal density of dopamine-containing fibres in the area of the host caudate putamen surrounding the probe. Treatment with 0.05 mg/kg (subcutaneous) apomorphine did not affect extracellular dopamine recovered from the grafted striatum, while extracellular DA decreased by a maximum of 30% on the intact side. However, a subsequent injection of 0.5 mg/kg apomorphine produced a large decrease of the dopamine recovered from both the grafted (maximum 40% decrease) and intact striata (maximum 80% decrease). Both the low and the high dose of apomorphine reduced extracellular dopamine metabolite levels, a response which was essentially similar for both the intact and grafted sides. Finally, the dopamine reuptake blocker nomifensine (10(-5) M) added to the perfusion medium produced similar large increases in dopamine in perfusates collected from both grafted and intact striata, while 3,4-dihydroxyphenylacetic acid and homovanillic acid did not change.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous and graft-induced behavioral recovery after 6-hydroxydopamine lesion of the nucleus accumbens in the rat

In the present study the long-term evolution of behavioral deficits following a local lesion of the dopaminergic innervation of the nucleus accumbens with 6-hydroxydopamine (6-OHDA) was compared in two groups of rats: lesioned animals and animals bearing a dopaminergic implant in the nucleus accumbens. Lesioned animals gradually recovered on various behavioral tests (amphetamine-induced locomotion, exploration, hoarding) and were indistinguishable from the control group on most parameters by 10 months postlesion. The deficits were, however, reinstated by a second intra-accumbens 6-OHDA lesion, a finding which suggests a role for dopaminergic reinnervation in the observed recovery. Conversely, grafted animals still displayed marked deficits even 10 months after grafting, although the lesioned areas were well reinnervated by the graft. These results indicate that the graft, while being unable on its own to compensate for part of the deficits, can nevertheless impair and compete with endogenous processes leading to behavioral recovery following a local lesion.

Exposure to amphetamine after substantia nigra lesion interferes with the process of behavioral recovery

Recovery from unilateral substantia nigra lesion may be indicated by re-emergence of circling in the pre-lesion preferred direction. Following 6-OHDA-induced lesion of the dominant SN, we examined: (a) The effect of the delay from lesioning on amphetamine-induced rotation asymmetry, and (b) The effect of early post-lesion exposure to amphetamine on later rotation asymmetry. d-Amphetamine was initially injected either 7, 14, 21, or 30 days after lesioning. Transient circling in pre-lesion preferred direction (contralateral to lesioned side) was more frequently encountered on days 7 and 30 after lesioning, as compared to days 14 and 21. The contralateral rotation observed on day 7 is attributed to degeneration-induced DA release, whereas contralateral rotation noted on day 30 is believed to reflect the operation of post-lesion compensatory processes within the spared DA neurons. In response to subsequent amphetamine administration 30 days after lesioning, rats with previous exposure to the drug circled ipsilaterally, whereas most rats given amphetamine for the first time in that session rotated contralaterally to the lesion. These findings suggest that post-lesion administration of amphetamine interferes with the process of recovery.

Transient contralateral rotation following unilateral substantia nigra lesion reflects susceptibility of the nigrostriatal system to exhaustion by amphetamine

Following unilateral 6-OHDA induced SN lesion, a transient period of contralateral rotation has been reported to precede the predominant ipsilateral circling. In order to clarify the nature of this initial contralateral rotation we examined the effect of the duration of recovery period after the lesion, on amphetamine-induced rotational behavior. Three days post lesion, most rats circled predominantly contralaterally to the lesion. Such contralateral rotation may result from either degeneration-induced breakdown of the DA pool, or lesion-induced increase of DA turnover in the spared neurons. A substantial degree of contralateral preference was still evident when amphetamine was administered for the first time 24 days after lesioning, indicating involvement of spared cells in the contralateral rotation. However, regardless of the duration of recovery (and irrespective of either lesion volume, amphetamine dose, or post-lesion motor exercise), amphetamine-induced rotation tended to become gradually more ipsilateral as the observation session progressed, and all rats circled ipsilaterally to the lesion in response to further amphetamine injections. These findings suggest that amphetamine has an irreversible effect on the post-lesion DA pool contributing to contralateral rotation.

Relationship of changes in spontaneous motor activity to spontaneous circling in rats with unilateral 6-hydroxydopamine lesions of the substantia nigra

Two unilateral injections of 6-hydroxydopamine into the substantia nigra and ventral tegmental area of adult rats pretreated with desmethylimipramine produced severe unilateral depletions of limbic and striatal dopamine. These animals sustained global deficits in spontaneous motor activity with no recovery evident from 1 to 14 weeks postoperative. The frequency of spontaneous circling increased during the 1st few weeks postoperative and then remained stable during the last 2 months of observation. The changes in circling were independent of changes in global spontaneous motor behaviors. Biochemical determinations indicated an increased dopamine turnover in residual limbic and striatal dopamine neurons but with substantially changed dihydroxyphenylacetic acid:homovanillic acid ratios. The absence of functional recovery of both normal and asymmetric motoric behavior indicates that severe uncompensated deficits in motoric function produced by unilateral 6-hydroxydopamine lesions can provide a useful animal model for the study of dopaminergic contributions to movement disorders.

Homologous cholinergic efferents spared by partial fimbrial lesions contribute to the recovery of hippocampal cholinergic enzymes in adult rats

Cholinergic fibers spared by partial lesions of the fimbrial bundle contribute to the recovery of enzyme markers for hippocampal cholinergic terminals. This recovery, most likely representing structural restoration of cholinergic terminals lost to lesion-induced degeneration, is amplified by a 'conditioning' lesion. This model of functionally relevant postlesion reconstruction, by and of structures within the CNS, is suitable to search for means to enhance homotypic collateral sprouting.

Denervation-induced collateral sprouting: no case for tracing methods

Sprouting of axonal collaterals is assumed to represent a major feature of the regenerative capacity of the CNS. Following the denervation of a brain region, synaptic contact sites become free and are replaced by sprouted collaterals of intact afferents of this area. During the last three years, numerous studies have been published which have used anatomical tracing methods to demonstrate this morphological consequence of lesions. This paper criticizes the use of tracing methods in this research field because of the striking problems in quantifying the altered connectivity of a denervated structure. This critique is illustrated by summarizing the studies on the lesion-induced morphological effects within the nigro-striatal system which has become a paradigmatic neuronal circuit for the study of neuronal and functional reorganization. It is concluded that anatomical tracing methods seem to be inappropriate in studying denervation-induced collateral sprouting.

Acute ipsilateral hyperalgesia and chronic contralateral hypoalgesia after unilateral 6-hydroxydopamine lesions of the substantia nigra

The effect of unilateral 6-hydroxydopamine lesions on flinch-jump thresholds to footshock was investigated during 3 months postoperative in rats. Preoperatively, the ipsilateral-contralateral thresholds were closely matched but postoperatively the flinch-jump thresholds to footshock applied to the body surface contralateral to the lesion hemisphere increased, whereas ipsilateral thresholds decreased. The ipsilateral decrease in threshold was maximal at postoperative day 1 but the contralateral increase persisted throughout testing. In control rats no ipsilateral-contralateral differences in the threshold were obtained. In both the control and 6-hydroxydopamine-treated groups there was an additional bilateral decline in jump threshold during the course of the experiment. These results suggest a role for dopaminergic and nondopaminergic participation in an animal's response to nociceptive stimulation.