Excitotoxic lesions of the subthalamic nucleus ameliorate asymmetry induced by striatal dopamine depletion in the rat

Basal ganglia, movement disorders and deep brain stimulation: advances made through non-human primate research

Studies in non-human primates (NHPs) have led to major advances in our understanding of the function of the basal ganglia and of the pathophysiologic mechanisms of hypokinetic movement disorders such as Parkinson's disease and hyperkinetic disorders such as chorea and dystonia. Since the brains of NHPs are anatomically very close to those of humans, disease states and the effects of medical and surgical approaches, such as deep brain stimulation (DBS), can be more faithfully modeled in NHPs than in other species. According to the current model of the basal ganglia circuitry, which was strongly influenced by studies in NHPs, the basal ganglia are viewed as components of segregated networks that emanate from specific cortical areas, traverse the basal ganglia, and ventral thalamus, and return to the frontal cortex. Based on the presumed functional domains of the different cortical areas involved, these networks are designated as 'motor', 'oculomotor', 'associative' and 'limbic' circuits. The functions of these networks are strongly modulated by the release of dopamine in the striatum. Striatal dopamine release alters the activity of striatal projection neurons which, in turn, influences the (inhibitory) basal ganglia output. In parkinsonism, the loss of striatal dopamine results in the emergence of oscillatory burst patterns of firing of basal ganglia output neurons, increased synchrony of the discharge of neighboring basal ganglia neurons, and an overall increase in basal ganglia output. The relevance of these findings is supported by the demonstration, in NHP models of parkinsonism, of the antiparkinsonian effects of inactivation of the motor circuit at the level of the subthalamic nucleus, one of the major components of the basal ganglia. This finding also contributed strongly to the revival of the use of surgical interventions to treat patients with Parkinson's disease. While ablative procedures were first used for this purpose, they have now been largely replaced by DBS of the subthalamic nucleus or internal pallidal segment. These procedures are not only effective in the treatment of parkinsonism, but also in the treatment of hyperkinetic conditions (such as chorea or dystonia) which result from pathophysiologic changes different from those underlying Parkinson's disease. Thus, these interventions probably do not counteract specific aspects of the pathophysiology of movement disorders, but non-specifically remove the influence of the different types of disruptive basal ganglia output from the relatively intact portions of the motor circuitry downstream from the basal ganglia. Knowledge gained from studies in NHPs remains critical for our understanding of the pathophysiology of movement disorders, of the effects of DBS on brain network activity, and the development of better treatments for patients with movement disorders and other neurologic or psychiatric conditions.

Human neural progenitor cell transplants into the subthalamic nucleus lead to functional recovery in a rat model of Parkinson’s disease

Despite the success of foetal nigral transplantation for the treatment of Parkinson's disease, supply limitations of tissue means that alternative sources must be found. Transplantation of human neural progenitor cells (HNPCs) may offer a solution, however few studies have shown functional recovery in animal models of PD without cell modification. Here we show that unmodified HNPC grafted into the subthalamic nucleus (STN) show excellent survival of up to 5 months and induce significant functional recovery following amphetamine-induced rotations within 4 weeks. For the first time we also show that HNPCs, which remain in an immature nestin-positive state, produce VEGF in vivo allowing further modification of the host brain. This suggests that even in the absence of cell replacement strategies utilising immature progenitor cells could be of real therapeutic value.

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.

Deficits of glutamate transmission in the striatum of experimental hemiballism

Hemiballism (HB) is a quite rare disorder, generally secondary to stroke, neoplasms or demyelinating plaques, classically considered as almost pathognomonic of a lesion in the subthalamic nucleus (STN). This alteration causes involuntary movements in the chorea-ballism spectrum. One theory is that the output nuclei of the basal ganglia are overinhibited in HB, while little is known about the physiological state of the striatum, the major input structure of the basal ganglia. In the present study, we recorded spontaneous and miniature excitatory and inhibitory postsynaptic currents (sEPSCs, mEPSCs, sIPSCs, mIPSCs) from projection neurons of the striatum of experimental HB. We found a selective reduction of striatal sEPSC and mEPSC frequency following chemical lesion of the STN of the rat, suggesting that reduced synaptic excitation of the input structure of the basal ganglia represents a physiological correlate of HB.

Lesions to the subthalamic nucleus decrease impulsive choice but impair autoshaping in rats: the importance of the basal ganglia in Pavlovian conditioning and impulse control

Although the subthalamic nucleus (STN) is involved in regulating motor function, and inactivation of this structure relieves the motor symptoms in Parkinsonian patients, recent data indicate that corticosubthalamic connections are involved in both the regulation of attention and the ability to withhold from responding. Considerable evidence suggests that the neural circuitry underlying such behavioural disinhibition or impulsive action can be at least partially dissociated from that implicated in impulsive decision-making and it has been suggested that the tendency to choose impulsively is related to the ability to form and use Pavlovian associations. To explore these hypotheses further, STN-lesioned rats were tested on the delay-discounting model of impulsive choice, where impulsivity is defined as the selection of a small immediate over a larger delayed reward, as well as in a rodent autoshaping paradigm. In contrast to previous reports of increased impulsive action, STN lesions decreased impulsive choice but dramatically impaired the acquisition of the autoshaping response. When the STN was lesioned after the establishment of autoshaping behaviour, lesioned subjects were more sensitive to the omission of reward, indicative of a reduction in the use of Pavlovian associations to control autoshaping performance. These results emphasize the importance of the STN in permitting conditioned stimulus-unconditioned stimulus associations to regulate goal-seeking, a function which may relate to the alterations in impulsive choice observed in the delay-discounting task. These data bear a striking similarity to those observed after lesions of the orbitofrontal cortex and are suggestive of an important role for corticosubthalamic connections in complex cognitive behaviour.

Postural changes after lesions of the substantia nigra pars reticulata in hemiparkinsonian monkeys

Current neurosurgical strategies target overactive brain regions including the subthalamic nucleus, globus pallidus and thalamus to control various symptoms of Parkinson's disease. Subthalamotomy improves akinesia and can induce postural deficits in both parkinsonian humans and animals, pallidotomy improves limb dyskinesia and more variably, distal bradykinesia whilst thalamotomy improves tremor. Because the SNr also becomes overactive in PD and there are few surgical studies in parkinsonian primates, we therefore evaluated the effects of lesioning the SNr in hemiparkinsonian marmosets to establish the effects on symptomatology. Nine monkeys received unilateral 6-hydroxydopamine (6-OHDA) lesions. Seven weeks later, four received kainic acid lesions of the SNr. Behavioural tests were performed prior to 6-OHDA surgery and then fortnightly for 14 weeks. Unilateral 6-OHDA lesions induced ipsilateral postural bias, ipsilateral rotation after amphetamine injection and bradykinesia. Whilst, SNr lesions significantly altered the direction of head position and amphetamine-induced rotation relative to 6-OHDA lesions, there was no improvement in 6-OHDA-induced reaching deficits or sensorimotor neglect. Unbiased quantitation of the nigral lesions showed on average 88% loss of dopaminergic neurons after 6-OHDA lesions and 77% loss of non-dopaminergic neurons after SNr lesions. Our results demonstrate that the SNr is important in body orientation changes in parkinsonism.

Death of dopaminergic neurones in the rat substantia nigra can be induced by damage to globus pallidus

Parkinson's disease is a debilitating disorder that results from the death of dopaminergic neurones in the substantia nigra. Subthalamic nucleus neurones use glutamate as their neurotransmitter and send excitatory projections to the substantia nigra. Changes in both the mean firing rate and firing pattern of neurones of the subthalamic nucleus have been found in patients with this disease. This has led to the suggestion that hyperactivity of the subthalamic nucleus may be involved in the pathology of the dopaminergic neurones. Subthalamic nucleus lesions or treatment with glutamatergic antagonists can be neuroprotective in animal models of Parkinson's disease but until now there has been no direct evidence that hyperactivity of subthalamic nucleus neurones can lead to downstream cell death. Here we show that lesions of the rat globus pallidus (a treatment that has been shown to increase subthalamic nucleus neuronal activity) result in a significant reduction of the number of dopaminergic neurones in the substantia nigra.

Enhanced food-related motivation after bilateral lesions of the subthalamic nucleus

Although inactivation of the subthalamic nucleus (STN) has beneficial effects on motor symptoms of parkinsonism, little is known of possible actions on nonmotor symptoms of cognition or mood. Here, we used several forms of converging evidence to show that STN lesions can enhance behavioral motivation. Thus, bilateral fiber-sparing lesions of the STN in rats reduced the time required to eat a standard number of food reward pellets, without affecting food intake, and altered performance on a number of behavioral measures consistent with enhanced motivation for food. Thus, STN-lesioned rats showed greater levels of locomotor activity conditioned to food presentation, enhanced control over responding by food-related conditioned reinforcers, and a higher breaking point associated with elevated rate of lever press under a progressive ratio schedule of reinforcement. These results reveal a new functional role schedule for STN, possibly because of its involvement in ventral, as well as dorsal, striatal circuitry and are relevant to the therapeutic effects of STN stimulation in Parkinson's disease.

Subthalamic nucleus lesions induce deficits as well as benefits in the hemiparkinsonian rat

Lesions of the subthalamic nucleus can restore some imbalances in motor output of the basal ganglia induced by nigrostriatal dopamine depletion, and have been proposed as a potential therapy for Parkinson's disease. Although there is substantial supporting evidence from experimental studies in both rats and primates, there is less information on the effects of subthalamic lesions alone. In order to characterize potential side effects, the present study evaluates the behavioural effects of unilateral excitotoxic lesions of the subthalamic nucleus in rats that have previously received either unilateral saline or 6-hydroxydopamine injections into the nigrostriatal bundle on the same side. The 6-hydroxydopamine lesions induced ipsilateral orientation asymmetries in head position and body axis bias, rotational asymmetries following injections of direct or indirect dopamine agonists, neglect of contralateral stimuli, and a reduction in the numbers of pellets retrieved with the contralateral paw in a skilled reaching task. Subsequent excitotoxic lesions of the subthalamic nucleus reduced (but did not abolish) rotational asymmetries, had no effects on the measures of neglect and skilled paw-reaching, and produced contralateral orientation biases in head turning and body axis curling. Rats that received subthalamic lesions alone exhibited de novo impairments comprising contralateral biases in the orientation tests. These results support a neuromodulatory role of the subthalamic nucleus in regulating motor outputs of the basal ganglia, and caution that there may be distinct side effects of the lesion by itself. Whereas some impairments attributable to dopamine depletion may be alleviated by subthalamic manipulations, other symptoms are not, or may even be aggravated.