Inhibiting subthalamic D5 receptor constitutive activity alleviates abnormal electrical activity and reverses motor impairment in a rat model of Parkinson's disease.

Burst firing has been reported as a pathological activity of subthalamic nucleus (STN) neurons in Parkinson's disease. However, the origin of bursts and their causal link with motor deficits remain unknown. Here we tested the hypothesis that dopamine D5 receptors (D5Rs), characterized by a high constitutive activity, may contribute to the emergence of burst firing in STN. We tested whether inhibiting D5R constitutive activity depresses burst firing and alleviates motor impairments in the 6-OHDA rat model of Parkinson's disease. Intrasubthalamic microinjections of either an inverse agonist of D5Rs, flupenthixol, or a D2R antagonist, raclopride, were applied. Behavioral experiments, in vivo and in vitro electrophysiological recordings, and ex vivo functional neuroanatomy studies were performed. Using [(5)S]GTPγ binding autoradiography, we show that application of flupenthixol inhibits D5R constitutive activity within the STN. Furthermore, flupenthixol reduced evoked burst in brain slices and converted pathological burst firing into physiological tonic, single-spike firing in 6-OHDA rats in vivo. This later action was mimicked by calciseptine, a Cav1 channel blocker. Moreover, the same treatment dramatically attenuated motor impairment in this model and normalized metabolic hyperactivity in both STN and substantia nigra pars reticulata, the main output structure of basal ganglia in rats. In contrast, raclopride as well as saline did not reverse burst firing and motor deficits, confirming the selective action of flupenthixol on D5Rs. These results are the first to demonstrate that subthalamic D5Rs are involved in the pathophysiology of Parkinson's disease and that administering an inverse agonist of these receptors may lessen motor symptoms.

Emerging dysfunctions consequent to combined monoaminergic depletions in Parkinsonism.

The loss of dopamine (DA) neurons has been the pathophysiological focus of the devastating conditions of Parkinson's disease, but depletion of DA alone in animal models has failed to simultaneously elicit both the motor and non-motor deficits of PD. The present study aimed to investigate, in rats, the respective role of dopamine (DA), noradrenaline (NA) and serotonin (5-HT) depletions on motor and non-motor behaviors and on subthalamic (STN) neuronal activity. We show that NA or DA depletion significantly decreased locomotor activity and enhanced the proportion of bursty and irregular STN neurons. Anxiety-like states required DA depletion plus the depletion of 5-HT or NA. Anhedonia and "depressive-like" behavior emerged only from the combined depletion of all three monoamines, an effect paralleled by an increase in the firing rate and the proportion of bursty and irregular STN neurons. Here, we provide evidence for the exacerbation of behavioral deficits when NA and/or 5-HT depletions are combined with DA depletion, bringing new insight into the combined roles of the three monoamines in PD.

Metabolic and electrophysiological changes in the basal ganglia of transgenic Huntington's disease rats.

Huntington's disease (HD) is characterized by neuronal loss in the striatum, ultimately leading to an 'imbalance' in the electrical activity of the basal ganglia-thalamocortical circuits. To restore this 'imbalance' in HD patients, which is held responsible for (some) of the motor symptoms, different basal ganglia nuclei have been targeted for surgical therapies, such as ablative surgery and deep brain stimulation. However, evidence to target brain nuclei for surgical therapies in HD is lacking. We reasoned that a neuronal and metabolic mapping of the basal ganglia nuclei could identify a functional substrate for therapeutic interventions. Therefore, the aim of the present study was to investigate the metabolic and neuronal activity of basal ganglia nuclei in a transgenic rat model of HD (tgHD). Subjects were 10-12 month old tgHD rats and wildtype littermates. We examined the striatum, globus pallidus, entopeduncular nucleus, the subthalamic nucleus and substantia nigra at different levels. First, we determined the overall neuronal activity at a supracellular level, by cytochrome oxidase histochemistry. Secondly, we determined the subcellular metabolic activity, by immunohistochemistry for peroxisome proliferator-activated receptor-γ transcription co-activator (PGC-1α), a key player in the mitochondrial machinery. Finally, we performed extracellular single unit recordings in the nuclei to determine the cellular activity. In tgHD rats, optical density analysis showed a significantly increased cytochrome oxidase levels in the globus pallidus and subthalamic nucleus when compared to controls. PGC-1α expression was only enhanced in the subthalamic nucleus and electrophysiological recordings revealed decreased firing frequency of the majority of the neurons in the globus pallidus and increased firing frequency of the majority of the neurons in the subthalamic nucleus. Altogether, our results suggest that the globus pallidus and subthalamic nucleus play a role in the neurobiology of HD and can be potential targets for therapeutic interventions.

D2 receptor stimulation, but not D1, restores striatal equilibrium in a rat model of Parkinsonism.

In Parkinson's disease dopamine depletion imbalances the two major output pathways of the striatum. L-DOPA replacement therapy is believed to correct this imbalance by providing effective D1 and D2 receptor stimulation to striatonigral and striatopallidal neurons, respectively. Here we tested this assumption in the rat model of Parkinsonism by monitoring the spike response of identified striatal neurons to cortical stimulation. As predicted, in 6-hydroxydopamine lesioned rats we observed that L-DOPA (6 mg/kg+benserazide), apomorphine and the D2 agonist quinpirole (0.5 mg/kg i.p.) counteract the enhanced responsiveness of striatopallidal neurons. Unexpectedly, the depressed responsiveness of striatonigral neurons was corrected by quinpirole whereas D1 stimulation exerted no (apomorphine, cPB) or worsening effects (L-DOPA, SKF38393 10 mg/kg). Therefore, quinpirole, but not D1 stimulation, restores functional equilibrium between the two striatal output pathways. Our results might explain the therapeutic effect of D2-based medications in Parkinson's disease.

Mouse DRG Cell Line with Properties of Nociceptors.

In vitro cell lines from DRG neurons aid drug discovery because they can be used for early stage, high-throughput screens for drugs targeting pain pathways, with minimal dependence on animals. We have established a conditionally immortal DRG cell line from the Immortomouse. Using immunocytochemistry, RT-PCR and calcium microfluorimetry, we demonstrate that the cell line MED17.11 expresses markers of cells committed to the sensory neuron lineage. Within a few hours under differentiating conditions, MED17.11 cells extend processes and following seven days of differentiation, express markers of more mature DRG neurons, such as NaV1.7 and Piezo2. However, at least at this time-point, the nociceptive marker NaV1.8 is not expressed, but the cells respond to compounds known to excite nociceptors, including the TRPV1 agonist capsaicin, the purinergic receptor agonist ATP and the voltage gated sodium channel agonist, veratridine. Robust calcium transients are observed in the presence of the inflammatory mediators bradykinin, histamine and norepinephrine. MED17.11 cells have the potential to replace or reduce the use of primary DRG culture in sensory, pain and developmental research by providing a simple model to study acute nociception, neurite outgrowth and the developmental specification of DRG neurons.

Involvement of dopamine loss in extrastriatal basal ganglia nuclei in the pathophysiology of Parkinson's disease.

Parkinson's disease (PD) is a neurological disorder characterized by the manifestation of motor symptoms, such as akinesia, muscle rigidity and tremor at rest. These symptoms are classically attributed to the degeneration of dopamine neurons in the pars compacta of substantia nigra (SNc), which results in a marked dopamine depletion in the striatum. It is well established that dopamine neurons in the SNc innervate not only the striatum, which is the main target, but also other basal ganglia nuclei including the two segments of globus pallidus and the subthalamic nucleus (STN). The role of dopamine and its depletion in the striatum is well known, however, the role of dopamine depletion in the pallidal complex and the STN in the genesis of their abnormal neuronal activity and in parkinsonian motor deficits is still not clearly determined. Based on recent experimental data from animal models of Parkinson's disease in rodents and non-human primates and also from parkinsonian patients, this review summarizes current knowledge on the role of dopamine in the modulation of basal ganglia neuronal activity and also the role of dopamine depletion in these nuclei in the pathophysiology of Parkinson's disease.

Involvement of Basal Ganglia network in motor disabilities induced by typical antipsychotics.

BACKGROUND: Clinical treatments with typical antipsychotic drugs (APDs) are accompanied by extrapyramidal motor side-effects (EPS) such as hypokinesia and catalepsy. As little is known about electrophysiological substrates of such motor disturbances, we investigated the effects of a typical APD, alpha-flupentixol, on the motor behavior and the neuronal activity of the whole basal ganglia nuclei in the rat. METHODS AND FINDINGS: The motor behavior was examined by the open field actimeter and the neuronal activity of basal ganglia nuclei was investigated using extracellular single unit recordings on urethane anesthetized rats. We show that alpha-flupentixol induced EPS paralleled by a decrease in the firing rate and a disorganization of the firing pattern in both substantia nigra pars reticulata (SNr) and subthalamic nucleus (STN). Furthermore, alpha-flupentixol induced an increase in the firing rate of globus pallidus (GP) neurons. In the striatum, we recorded two populations of medium spiny neurons (MSNs) after their antidromic identification. At basal level, both striato-pallidal and striato-nigral MSNs were found to be unaffected by alpha-flupentixol. However, during electrical cortico-striatal activation only striato-pallidal, but not striato-nigral, MSNs were found to be inhibited by alpha-flupentixol. Together, our results suggest that the changes in STN and SNr neuronal activity are a consequence of increased neuronal activity of globus pallidus (GP). Indeed, after selective GP lesion, alpha-flupentixol failed to induce EPS and to alter STN neuronal activity. CONCLUSION: Our study reports strong evidence to show that hypokinesia and catalepsy induced by alpha-flupentixol are triggered by dramatic changes occurring in basal ganglia network. We provide new insight into the key role of GP in the pathophysiology of APD-induced EPS suggesting that the GP can be considered as a potential target for the treatment of EPS.