The contralateral cortex contributes to the effects of hemidecortication on neuropeptide Y immunoreactivity in the rat striatum

The interhemispheric connections of the striatum: Implications for Parkinson's disease and drug-induced dyskinesias

Parkinson's disease (PD) is characterized by loss of nigrostriatal neurons and depletion of dopamine. This pathological feature leads to alterations to basal ganglia circuitry and subsequent motor disability. Pharmacological dopamine replacement therapy with medications such as levodopa ameliorates the symptoms of PD but can lead to motor complications known as drug-induced dyskinesias. We have recently shown that clinically hemiparkinsonian rhesus monkeys do not develop levodopa-induced dyskinesias despite chronic intermittent exposure and significant unilateral loss of nigrostriatal neurons and dopamine. It is currently unclear what mechanisms prevent the onset of dyskinesias in these animals. Based on our study and results from previous lesioning studies in both the rat and monkey models of PD, we hypothesize that one potential mechanism that may prevent the genesis of dyskinesias in these animals is interhemispheric neuromodulation. Two potential interhemispheric connections that may modulate dyskinesias are the interhemispheric nigrostriatal and corticostriatal pathways. Few investigators have examined the interhemispheric nigrostriatal and corticostriatal connections and the functional role they may play in drug-induced dyskinesias in PD. Therefore, in the following review, we assess the neuroanatomical, electrophysiological and behavioral properties of these interhemispheric connections. Future studies evaluating these interhemispheric striatal pathways and the pathophysiological changes that occur to these pathways in the dyskinetic state are warranted to further develop treatments that prevent or mitigate drug-induced dyskinesias in PD.

A 'single toxin-double lesion' rat model of striatonigral degeneration by intrastriatal 1-methyl-4-phenylpyridinium ion injection: a motor behavioural analysis

Previous attempts to reproduce striatonigral degeneration, the core pathology underlying Parkinsonism in multiple system atrophy, have been impeded by interactions in the neurotoxins used to replicate striatal and nigral degeneration in rodents. To overcome these interactions, we have developed a new model of striatonigral degeneration which uses a single unilateral administration of 1-methyl-4-phenylpyridinium ion (MPP(+)) into the rat striatum. Spontaneous and drug-induced rotational behaviour, thigmotactic scanning, stepping adjusting steps and paw reaching deficits were compared in four groups of animals: group 1 (control), group 2 (20 microg quinolinic acid), group 3 (20 microg 6-hydroxydopamine), and group 4 (90 nmol MPP(+)). MPP(+) administration resulted in the absence of the amphetamine-induced ipsilateral bias observed in the 6-hydroxydopamine group and of the apomorphine-induced ipsilateral bias observed in the quinolinic acid group. There was no thigmotactic scanning asymmetry in the MPP(+)-injected rats compared to the quinolinic acid- and the 6-hydroxydopamine-injected rats. MPP(+) elicited a bilateral stepping adjustment deficit similar to that found in the quinolinic acid group when compared to controls. MPP(+) also elicited a more severe and significant contralateral deficit in paw reaching compared to controls, 6-hydroxydopamine and quinolinic acid groups. Histopathology revealed a significant reduction of the lesioned striatal surface (-47.53%) with neuronal loss and increased astrogliosis in the MPP(+) group grossly similar to that found in the quinolinic acid group. Contrary to the latter group, however, loss of intrastriatal and striatal-crossing fibre bundles was observed in the MPP(+) group as there was also some retrograde degeneration in the ipsilateral thalamic parafascicular nucleus. The mean loss of dopaminergic cells in the ipsilateral substantia nigra pars compacta in MPP(+) rats was less marked (-48.8%) than in the 6-hydroxydopamine rats (-63.6%) and was not significant in quinolinic acid rats (-5.2%). This study shows that a single unilateral intrastriatal administration of MPP(+) induces a unique motor behaviour resulting from both nigral and striatal degeneration, but also from possible extrastriatal damage. This 'single toxin-double lesion' paradigm may thus serve as a rat model of striatonigral degeneration.

Differential effects of corticostriatal and thalamostriatal deafferentation on expression of the glutamate transporter GLT1 in the rat striatum

This study compared the effects of the disruption of the two main presumably glutamatergic striatal inputs, the corticostriatal and thalamostriatal pathways, on GLT1 expression in the rat striatum, using in situ hybridization and immunohistochemistry. Unilateral ibotenate-induced thalamic lesion produced no significant changes in striatal GLT1 mRNA labeling and immunostaining as assessed at 5 and 12 days postlesion. In contrast, significant increases in both parameters were measured after bilateral cortical lesion by superficial thermocoagulation. GLT1 mRNA levels increased predominantly in the dorsolateral part of the striatum; there, the increases were significant at 5 (+84%), 12 (+101%), and 21 (+45%) but not at 35 days postlesion. GLT1 immunostaining increased significantly and homogeneously by 17-26% at 12 and 21 days postlesion. The increase in GLT1 expression at 12 days postlesion was further confirmed by western blot analysis; in contrast, a 36% decrease in glutamate uptake activity was measured at the same time point. These data indicate that striatal GLT1 expression depends on corticostriatal but not thalamostriatal innervation. Comparison of our results with previous data showing that cortical lesion by aspiration downregulates striatal GLT1 expression further suggests that differential changes in GLT1 expression, and thus presumably in glial cell function, may occur in the target striatum depending on the way the cortical neurons degenerate.

Ultrastructural and metabolic changes in the neuropeptide Y-containing striatal neuronal network after thermocoagulatory cortical lesion in adult rat

This study examined the effects of unilateral thermocoagulatory cortical lesion on the pattern of neuropeptide Y immunostaining in the rat ipsilateral striatum at 4 and 21 days post-lesion. Light microscopic analysis showed a significant increase in the number of neuropeptide Y-positive neurons vs. control at both time points; paradoxically, the intraneuronal level of labelling significantly decreased at 4 days post-lesion but increased at 21 days post-lesion. Ultrastructural analysis in control condition showed a higher proportion of dendritic versus axonal labelled processes (3.5 ratio); all the neuropeptide Y synaptic terminals formed symmetrical contacts, mostly onto unlabelled dendrites. At 4 days post-lesion, the neuropeptide Y-positive axon density dramatically increased (+576%) without significant change in the labelled dendrite density, vs. control values; the density of neuropeptide Y synaptic terminals increased in parallel by 233%. In addition, a significant proportion of large neuropeptide Y boutons forming asymmetrical synapses onto unlabelled spines were observed. At 21 days post-lesion, densities of neuropeptide Y dendrites, axons, and synaptic terminals increased by 68, 246 and 125%, respectively, vs. control. But, the morphological features of the neuropeptide Y axonal processes and synaptic specializations of the boutons were similar to those observed in control condition. These data (1) raise an important issue regarding the origin of the terminals forming asymmetrical synapses in the striatum, (2) suggest that adaptative changes in the neuropeptide Y neuronal network may be a main component of striatal remodelling resulting from the progressive loss of cortical inputs, and (3) reinforce the view that neuropeptide Y and excitatory amino acid functions may be tightly linked in the striatum.

Changes in striatal neuropeptides and GAD67 expression following a minimal cortical lesion

We have analyzed the effects of a small cortical infarct which is known to induce dramatic changes in gene expression in the entire cerebral cortex, on the gene expression in the striatum, a target structure of cortical neurons. Striatal glutamic acid decarboxylase (GAD67) and enkephalin expressions were increased in the striatum ipsilateral to the lesion. Conversely, neuropeptide Y- and somatostatin-like immunoreactivity were decreased in the ipsilateral striatum and this decrease was only related to a decrease in the labeling of processes with no changes in the number of labeled neurons. A minimal cortical lesion may therefore induce changes in gene expression in a subcortical structure through hyperactivity of glutamatergic synaptic inputs. One should therefore remember these extensive and long-lasting effects when surgical manipulations are performed on rat brain for stereotaxic surgery and placement of electrodes or probes.