Increased subthalamic neuronal activity after nigral dopaminergic lesion independent of disinhibition via the globus pallidus

Effects of increasing doses of apomorphine during stereotaxic neurosurgery in Parkinson's disease: clinical score and internal globus pallidus activity. Short communication

We analysed the firing activity of internal globus pallidus cells in two Parkinson's disease patients undergoing stereotaxic surgery. Both patients showed an advanced rigid-akinetic syndrome with disabling levodopa induced dyskinesias. Apomorphine, intraoperatively administered at doses (1-2 mg) inducing a short but clear clinical improvement without involuntary movements, reduced the pallidal discharge rate by > 50% in both patients. An higher apomorphine dose (2.5 mg), tested in one hemisphere, blocked the firing activity with a time course independent from the occurrence of dyskinesias. These finding suggest that the reduction of internal pallidus excitability is one of the mechanisms underlying the efficacy of dopaminergic therapy, but also that changes in other basal ganglia stations are likely to be involved in dyskinesias.

Dystonic posturing and circling behaviors induced by dopaminergic agents in rats with unilateral globus pallidus lesions

The present study examined the behavioral effects of dopamine receptor agonists, antagonists, or N-methyl-D-aspartate (NMDA) glutamate receptor antagonist in rats with a unilateral excitotoxic lesion of the globus pallidus (GP). After the unilateral GP lesions were made by injections of the ibotenic acid, drugs were systemically given and the elicited behaviors were quantitatively assessed. Systemic administration of haloperidol, but not SCH23390, dose-dependently induced contraversive dystonic posturing in unilateral GP-lesioned rats. On the other hand, systemic administration of quinpirole, but not SKF38393, induced ipsiversive circling. MK-801, only when given at a high dose, unilateral GP-lesioned rat, the D2 receptor agonist and antagonist caused ipsiversive and contraversive posturing or circling, respectively. Since the rotational behavior is induced on the basis of asymmetry of the basal ganglia output activity, there must be a marked difference between the GP ablation and the administration of D2 receptor blockade on the basal ganglia output activity, supporting a speculation that overactivity of the basal ganglia under dopamine depletion is not solely a result of the disinhibition from the inhibitory GP efferents. The present unilateral GP-lesion model appears to be a useful one for the pharmacobehavioral investigation of D2-mediated mechanisms.

Movement disorders induced by gamma-aminobutyric agonist and antagonist injections into the internal globus pallidus and substantia nigra pars reticulata of the monkey

Injections of bicuculline into the medial segment of the globus pallidus (GPi) of the monkey induced dose-dependent hypokinesia with dystonic attitudes in contralateral limbs whereas muscimol injections elicited choreiform movements. Injections of the same drugs in substantia nigra pars reticulata (SNr) provoked severe axial postural anomalies with rotational behavior. Conversely, contralateral hypertonia after bicuculline and contralateral hypotonia after muscimol injections were observed. These data suggest that GABA inputs into GPi and SNr play different roles in terms of motor and postural control and add new insights into the pathophysiology of dystonias.

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

We investigated the effect of unilateral dorsal striatal dopamine depletion (by intrastriatal infusion of 6-OHDA), ibotenic acid lesions of the subthalamic nucleus (STN) and combined dopamine depletion and STN lesions on sensorimotor asymmetry using a test of somatosensory asymmetry [T. Schallert et al., Pharmacol. Biochem. Behav. 16 (1982) 455-462]. The unilateral striatal dopamine depletion resulted in a somatosensory asymmetry. This asymmetry was ameliorated in the rats with combined dopamine depletion and STN lesion. indicating the potential beneficial nature of STN inactivation in rats with striatal dopamine depletion.

Substantia nigra pars reticulata single unit activity in normal and 60HDA-lesioned rats: effects of intrastriatal apomorphine and subthalamic lesions

The spontaneous activity and the response to intrastriatal application of apomorphine of substantia nigra pars reticulata (SNpr) single units was studied in four experimental groups of rats: (1) normal rats; (2) subthalamic nucleus (STN) lesioned rats; (3) rats bearing a 6-hydroxydopamine (60HDA) lesion; and (4) 60HDA-lesioned animals with an additional STN lesion. Thirty-eight percent of units from 60HDA-lesioned rats showed a bursting pattern of spontaneous activity, which was never found in normal rats. STN lesions had no effect on the spontaneous activity of SNpr units from normal rats, but reduced the percentage of burst units in 60HDA-lesioned animals. Intrastriatal apomorphine produced responses in 62% of SNpr units from normal rats and 85% of units from 60HDA-lesioned animals (P < 0.05). In addition, the modifications in the firing rate and in the coefficient of variation of the interspike intervals induced by intrastriatal apomorphine were significantly greater for the units isolated from 60HDA-lesioned rats. In particular, it was noted that all the burst units responded to apomorphine, showing the highest changes in firing rate and coefficient of variation. However, intrastriatal apomorphine did not always turn the activity of burst units into a more physiological pattern. STN lesions reduced the percentage of units responding to intrastriatal apomorphine in normal rats. In 60HDA-lesioned rats, STN lesions reduced the number of responsive units, and their change in mean firing rate and coefficient of variation. Our results show that the STN participates in the genesis of the bursting pattern of activity of SNpr units in 60HDA-lesioned rats, and that STN lesions can partially revert the abnormal spontaneous and apomorphine-induced responses of SNpr units in these animals.

Consequences of nigrostriatal denervation on the functioning of the basal ganglia in human and nonhuman primates: an in situ hybridization study of cytochrome oxidase subunit I mRNA

To examine the consequences of nigrostriatal denervation and chronic levodopa (L-DOPA) treatment on functional activity of the basal ganglia, we analyzed, using in situ hybridization, the cellular expression of the mRNA encoding for cytochrome oxidase subunit I (COI mRNA), a molecular marker for functional neuronal activity, in the basal ganglia. This analysis was performed in monkeys rendered parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) Intoxication, some of which had been receiving L-DOPA, and in patients with Parkinson's disease (PD). In MPTP-intoxicated monkeys compared with control animals, COI mRNA expression was increased in the subthalamic nucleus (STN) and in the output nuclei of the basal ganglia, i.e., the internal segment of the globus pallidus and the substantia nigra pars reticulata. This increase was partially reversed by L-DOPA treatment. COI mRNA expression remained unchanged in the external segment of the globus pallidus (GPe). In PD patients, all of whom had been treated chronically by L-DOPA, COI mRNA expression in the analyzed basal ganglia structures was similar to that in control subjects. These results are in agreement with the accepted model of basal ganglia organization, to the extent that the output nuclei of the basal ganglia are considered to be overactive after nigrostriatal denervation, partly because of increased activity of excitatory afferents from the STN. Yet, our results would also seem to contradict this model, because the overactivity of the STN does not seem to be attributable to a hypoactivation of the GPe.

Electrophysiological and Fos immunohistochemical evidence for the excitatory nature of the parafascicular projection to the globus pallidus

Extracellular recordings and immunohistological detection of c-Fos-like immunoreactive proteins were used to determine the synaptic effect of the parafascicular projection to the globus pallidus. Electrical stimulation of the parafascicular neurons induced a single-spike excitatory response with a stable latency of 2.3 ms, suggesting a monosynaptically driven effect. Pharmacological stimulation of the parafascicular nucleus with carbachol increased tonically the pallidal discharge rate by 142%. The discharge rate of the pallidal neurons was described by 37% in parafascicular-lesioned rats. These results demonstrate the excitatory nature and the tonic action of the parafasciculopallidal projection. Carbachol activation of parafascicular neurons also induced the synthesis of c-Fos-like immunoreactive proteins in the pallidal neurons. Control experiments in subthalamic-lesioned rats showed that the parafascicular excitation of the pallidal neurons remained, but both electrophysiological and expression of c-Fos-like immunoreactive proteins were attenuated. This suggests that the direct parafascicular excitation of the pallidal neurons is indirectly reinforced by the previously described parafascicular excitatory input to the subthalamic nucleus. Conversely, the effect of this last input to the subthalamic nucleus is dramatically enhanced in rats with pallidal lesion. Our results demonstrate the complex role of the parafascicular nucleus in activating both the globus pallidus and the subthalamic nucleus, two closely related structures. These results illustrate the integrative capacities of the globus pallidus, whose activity is modulated by multiple afferents.

The response of subthalamic nucleus neurons to dopamine receptor stimulation in a rodent model of Parkinson's disease

Overactivity in the subthalamic nucleus (STN) is believed to contribute to the pathophysiology of Parkinson's disease. It is hypothesized that dopamine receptor agonists reduce neuronal output from the STN. The present study tests this hypothesis by using in vivo extracellular single unit recording techniques to measure neuronal activity in the STN of rats with 6-hydroxydopamine-induced lesions of the nigrostriatal pathway (a model of Parkinson's disease). As predicted, firing rates of STN neurons in lesioned rats were tonically elevated under basal conditions and were decreased by the nonselective dopamine receptor agonists apomorphine and L-3, 4-dihydroxyphenylalanine (L-DOPA). STN firing rates were also decreased by the D2 receptor agonist quinpirole when administered after the D1 receptor agonist (+/-)- 1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol (SKF 38393). Results of the present study challenge the prediction that dopaminergic agonists reduce STN activity predominantly through actions at striatal dopamine D2 receptors. Firing rates of STN neurons were not altered by selective stimulation of D2 receptors and were increased by selective stimulation of D1 receptors. Moreover, there was a striking difference between the responses of the STN to D1/D2 receptor stimulation in the lesioned and intact rat; apomorphine inhibited STN firing in the lesioned rat and increased STN firing in the intact rat. These findings support the premise that therapeutic efficacy in the treatment of Parkinson's disease is associated with a decrease in the activity of the STN, but challenge assumptions about the roles of D1 and D2 receptors in the regulation of neuronal activity of the STN in both the intact and dopamine-depleted states.

Subthalamic ablation reverses changes in basal ganglia oxidative metabolism and motor response to apomorphine induced by nigrostriatal lesion in rats

In Parkinson's disease, the functional architecture of the basal ganglia nuclei undergoes profound alterations, one of the most important of which is overactivity of the basal ganglia output nuclei. This phenomenon seems to be intimately related to pathological overactivity of the subthalamic nucleus, which directly modulates the basal ganglia output through its glutamatergic projections. In this study, we investigated the effects of unilateral subthalamic nucleus lesions on the activities of succinate dehydrogenase and cytochrome oxidase, two markers of neuronal activity, in rats with prior unilateral lesions of the nigrostriatal tract. We also explored the effect of subthalamic nucleus lesions on the rotational response to systemic apomorphine. Rats with unilateral lesions of the nigrostriatal tract showed ipsilateral increases in enzyme activity in the basal ganglia output nuclei, entopeduncular nucleus and substantia nigra pars reticulata. Selective subthalamic nucleus destruction completely reversed this phenomenon. In addition, subthalamic nucleus lesions abolished the rotational response to apomorphine. These results confirm that overactivity of the subthalamic nucleus plays a pivotal role in the functional alterations of basal ganglia associated with Parkinson's disease. They also shed further light on the neural mechanisms through which manipulations of subthalamic activity can ameliorate Parkinson's disease symptoms.

Re-evaluation of the functional anatomy of the basal ganglia in normal and Parkinsonian states

In the late 1980s, a functional and anatomical model of basal ganglia organization was proposed in order to explain the clinical syndrome of Parkinson's disease. According to this model, the pathological overactivity observed in the subthalamic nucleus and the output station of the basal ganglia plays a crucial role in the pathophysiology of the motor signs of Parkinson's disease. The hyperactivity of subthalamic neurons in Parkinsonism is viewed as a direct consequence of a pathological hypoactivity of the external segment of the pallidum. This article reviews recent data from different experimental approaches that challenge the established model of basal ganglia organization by reinterpreting the functional interaction between the external segment of the pallidum and the subthalamic nucleus in both the normal and pathological state. Indeed, recent neurobiochemical studies have rather unexpectedly shown that the GABAergic and metabolic activities of the external pallidum are not decreased in human and non-human primates with Parkinsonism. This absence of any decrease in activity might be explained by the functionally antagonistic influences of the striatal and subthalamic afferences within the external pallidum, as suggested by several anatomical studies. In addition, there are clues from electrophysiological studies to suggest that the hyperactivity found in the subthalamic neurons in Parkinsonism may not depend solely on the level of activity in the external pallidum. In such a framework, the hyperactivity of the subthalamic neurons would have to be explained, at least in part, by other sources of excitation or disinhibition. However, any explanation for the origin of the subthalamic overactivity in Parkinsonism remains speculative.

The regulation of motor control: an evaluation of the role of dopamine receptors in the substantia nigra

The importance of the nigrostriatal dopaminergic pathway in motor control is widely accepted and it is generally believed that the motor symptoms of Parkinson's disease result solely from reduced release of dopamine from terminals in the striatum. Over recent years there has been a growing body of evidence which suggests that dendritic dopamine release in the substantia nigra is of importance in the regulation of neuronal activity and behaviour. This evidence is reviewed together with a description of our recent findings that show nigral dopamine receptors are essential for the maintenance of normal muscle tone. It is concluded that current views of the basal ganglia circuitry involved in motor control need to be re-evaluated to take into account these recent reports. A scheme is suggested to explain how dopamine mechanisms in the substantia nigra regulate motor activity.

Effects of L-DOPA-therapy on dopamine D2 receptor mRNA expression in the striatum of MPTP-intoxicated parkinsonian monkeys

The cellular expression of dopamine D2 receptor mRNA was examined in striatal (caudate nucleus and putamen) neurones of 9 Macaca fascicularis monkeys rendered parkinsonian by systemic injection of MPTP. Messenger RNA abundance was determined by quantitative in situ hybridization using human-specific 35S-labelled oligonucleotides. Control monkeys were untreated and received neither MPTP nor L-DOPA while the rest were rendered parkinsonian and received chronic levodopa therapy to induce dyskinesia. In the control brains a strong dopamine D2 receptor hybridization signal was detected overlying medium-sized and some large neurons in both the caudate nucleus and putamen. Neurons from the lateral and medial regions of the caudate nucleus, and from the dorsal and ventral regions of the putamen were analysed separately. A significant increase in the cellular abundance of dopamine D2 receptor mRNA was seen in the striatum of MPTP-treated monkeys; this increase being restricted to the population of medium-sized striatal cells. No such increase in dopamine D2 receptor mRNA was observed in (dyskinetic) L-DOPA-treated monkeys suggesting that levodopa-therapy normalises D2 receptor expression in post-synaptic striatal cells. The cellular abundance of dopamine D2 receptor mRNA expressed by large striatal neurons (putative cholinergic cells) was unaffected by either MPTP treatment or levodopa therapy. The implications of these findings for the development of levodopa-induced dyskinesias is discussed.