Blockade of dopamine D2, but not of D1 receptors in the rat globus pallidus induced Fos-like immunoreactivity in the caudate-putamen, substantia nigra and entopeduncular nucleus

Chronic MPTP administration regimen in monkeys: a model of dopaminergic and non-dopaminergic cell loss in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder clinically characterized by cardinal motor deficits including bradykinesia, tremor, rigidity and postural instability. Over the past decades, it has become clear that PD symptoms extend far beyond motor signs to include cognitive, autonomic and psychiatric impairments, most likely resulting from cortical and subcortical lesions of non-dopaminergic systems. In addition to nigrostriatal dopaminergic degeneration, pathological examination of PD brains, indeed, reveals widespread distribution of intracytoplasmic inclusions (Lewy bodies) and death of non-dopaminergic neurons in the brainstem and thalamus. For that past three decades, the MPTP-treated monkey has been recognized as the gold standard PD model because it displays some of the key behavioral and pathophysiological changes seen in PD patients. However, a common criticism raised by some authors about this model, and other neurotoxin-based models of PD, is the lack of neuronal loss beyond the nigrostriatal dopaminergic system. In this review, we argue that this assumption is largely incorrect and solely based on data from monkeys intoxicated with acute administration of MPTP. Work achieved in our laboratory and others strongly suggest that long-term chronic administration of MPTP leads to brain pathology beyond the dopaminergic system that displays close similarities to that seen in PD patients. This review critically examines these data and suggests that the chronically MPTP-treated nonhuman primate model may be suitable to study the pathophysiology and therapeutics of some non-motor features of PD.

Intra-nucleus-accumbens SKF38393 improved the impaired acquisition of morphine-conditioned place preference in depression-like rats

Dopaminergic activity in the nucleus accumbens (NAc) and the globus pallidus (GP) is important for the interaction between depression and addiction, with D1- and D2-like receptors playing different roles. Here, we address the effect of depression on morphine reward and its underlying D1- and D2-like effects in the NAc and/or the GP. Novelty-seeking behaviors and the forced open-space swimming test were used to assess a depression-like state in rats that had undergone chronic mild restraint. Depression-like rats were then trained with morphine-induced conditioned place preference (CPP, 3 mg/kg, 4 days), and showed impaired acquisition of the CPP compared with controls. To examine the receptor-specific dopaminergic mechanism underlying this phenomenon, we microinjected the D1-like agonist SKF38393 (1 μg/side) or the D2-like agonist quinpirole (1 μg/side) into the NAc or the GP. The impairment in acquisition of CPP was reversed only by injecting the D1- but not the D2-like agonist in the NAc. These results suggest that enhancement of dopaminergic transmission in the NAc (via D1-like receptors) may be effective in recovering impaired reward learning during a depression-like state.

Intrapallidal injection of 6-hydroxydopamine induced changes in dopamine innervation and neuronal activity of globus pallidus

The globus pallidus (GP) plays an important role in basal ganglia circuitry. In contrast to the well-characterized actions of dopamine on striatal neurons, the functional role of the dopamine innervation of GP is still not clearly determined. The present study aimed to investigate the effects of intrapallidal injection of 6-hydroxydopamine (6-OHDA) on rotational behavior induced by apomorphine, on the loss of dopamine cell bodies in the substantia nigra pars compacta (SNc) and fibers in the GP and striatum and on in vivo extracellularly-recorded GP neurons in the rat. Injection of 6-OHDA into GP induced severe loss of tyrosine hydroxylase-immunoreactive (TH-IR) fibers in GP (-85%) with a reduction in the number of TH-IR cell bodies in the SNc (-52%) and fibers in the striatum (-50%). S.c. injection of apomorphine in these rats induced a moderate number of turns (26+/-6 turns/5 min). Electrophysiological recordings show that 6-OHDA injection in GP induced a significant decrease of the firing rate of GP neurons (16.02+/-1.11 versus 24.14+/-1.58 spikes/sec in control animals and 22.83+/-1.28 in sham animals, one-way ANOVA, P<0.0001) without any change in the firing pattern (chi(2)=1.03, df=4, P=0.90). Our results support the premise of the existence of collaterals of SNc dopaminergic axons projecting to the striatum and GP and that dopamine plays a role in the modulation of the firing rate but not the firing pattern of GP neurons. Our data provide important insights into the functional role of the SNc-GP dopaminergic pathway suggesting that dopamine depletion in GP may participate in the development of motor disabilities.

Effects of physiological and pharmacological stimuli on dopamine release in the rat globus pallidus

A major aspect of understanding functions of the globus pallidus (GP) within the basal ganglia is the significance of its dopamine innervation. Here, we used in vivo-microdialysis in rats to characterize pallidal dopamine release in response to a number of physiological and pharmacological stimuli known to activate dopamine neurons. Results reveal that an aversive stimulus, i.e. handling for 20 min, significantly increased dialysate dopamine in the globus pallidus to about 130% of baseline levels. Likewise, a novel and appetitive stimulus, i.e. presentation of unfamiliar, palatable food, significantly elevated pallidal dopamine to about 150% of baseline levels both in rats which did and did not consume the food reward. These findings provide evidence that increases of dopamine (DA) efflux may largely reflect stimulus saliency implicating an involvement of pallidal dopamine signalling in control of behaviour governed by salient stimuli. Results further showed that reverse microdialysis of D-amphetamine and cocaine in augmenting concentrations of 0.1-100 microM elevated dialysate dopamine in a concentration-dependent manner suggesting a role of pallidal dopamine in mediating behavioural effects of psychostimulant drugs.

Endogenous dopamine modulates corticopallidal influences via GABA

Acute experiments on Sprague-Dawley rats were performed to study the effects of local application of D1 and D2 receptor antagonists (SCH 23390 and raclopride) on the responses of neurons in the globus pallidus induced by stimulation of the somatosensory cortex. SCH 23390 induced short-latency inhibition in response to stimulation of the cortex and blocked long-latency inhibition. Application of raclopride suppressed short-latency inhibition and induced a long-latency inhibitory response to stimulation of the cortex. It is suggested that these changes are based on modulation of GABA release from striopallidal terminals by endogenous dopamine.

The role of the globus pallidus D2 subfamily of dopamine receptors in pallidal immediate early gene expression

The globus pallidus plays an important role in basal ganglia circuitry, representing the first relay nucleus of the 'indirect pathway' of striatal efferents. In contrast to the well-characterized actions of dopamine on striatal neurons, the functional role of the dopamine innervation of globus pallidus is less well understood. Previous research showed that systemic administration of either a dopamine D2 receptor antagonist or combined dopamine D1 and D2 receptor agonists induces Fos, the protein product of the immediate early gene c-fos, in neurons of globus pallidus [Ruskin and Marshall (1997) Neuroscience 81, 79-92]. To determine whether the ability of the D2 receptor antagonist, sulpiride, to induce Fos in rat pallidal neurons is mediated by D2-like receptors in striatum or globus pallidus, intrastriatal or intrapallidal sulpiride infusions were conducted. The diffusion of intrastriatal sulpiride was estimated by measuring this antagonist's competition for N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)-induced D2 receptor inactivation. The phenotype of the striatal neurons expressing Fos after intrastriatal infusion was assessed by combining Fos immunocytochemistry with D2 receptor mRNA in situ hybridization. Intrastriatal infusions of (-)-sulpiride (10-200 ng) dose-dependently increased the number of striatal cells expressing Fos; and the Fos-immunoreactive striatal cells were D2 receptor mRNA-expressing, the same population in which systemic D2 receptor antagonists induce Fos. Intrastriatal infusions of high (5 microg), but not low (10-200 ng), (-)-sulpiride doses also induced Fos in globus pallidus cells but the sulpiride appeared to spread to the globus pallidus. Direct intrapallidal infusions of (-)-sulpiride (50-100 ng) dose-dependently induced Fos in globus pallidus with minimal influence on striatum or other basal ganglia structures. Using sensitive in situ hybridization conditions, prominent labeling of D2 receptor mRNA was evident in globus pallidus. D2 receptor mRNA was densest in a lateral 200 microm wide band that follows the curvature of the pallidal/striatal boundary. Cellular analysis revealed silver clusters associated with D2 receptor mRNA labeling over globus pallidus neurons that were immunoreactive for neuron-specific nuclear protein. These results strongly suggest that the dopaminergic innervation of globus pallidus, acting through D2-like receptors internal to this structure, can control gene expression in pallidal neurons.

Microdialysis in Parkinsonian patient basal ganglia: acute apomorphine-induced clinical and electrophysiological effects not paralleled by changes in the release of neuroactive amino acids

During stereotaxic neurosurgery for deep brain stimulation in Parkinson's disease (PD), we performed a microdialysis study of the extracellular amino acid (aspartate, glutamate, glycine, and GABA) concentrations. Their levels were measured in the GPe/GPi of five and in the STN of four different PD patients, after prolonged therapy washout. The results show stable values of basal release of the examined amino acids within 1 h. The basal levels of GABA in "OFF" state were significantly higher in the GPi than in the GPe. Acute apomorphine administration, while inducing clinical amelioration and electrophysiological changes in the examined nuclei, did not change amino acid concentrations. This result could be related to a limited microdialysis ability to detect subtle changes in amino acid spontaneous release. Alternatively, it could suggest that dopaminergic receptors located in the output nuclei, possibly present also in humans, might mediate the acute apomorphine clinical effects, not involving amino acid changes along the direct and/or indirect pathway.

Dopamine release in the rat globus pallidus characterised by in vivo microdialysis

Brain microdialysis has been used to examine the in vivo effects of potassium and calcium on dopamine release in the dorsal globus pallidus (GP) of rats. Furthermore, the effects of food presentation and consumption on dopamine release in the GP were investigated. Basal dopamine levels in the GP were below the detection limit, therefore nomifensine (30 microM) was added to the perfused artificial cerebrospinal fluid (aCSF). A prominent increase of dopamine release to 370% was observed after perfusion with elevated potassium (100 mM), while perfusion with calcium-free aCSF produced a significant decrease of dopamine efflux to 36% of control levels. Furthermore, presentation and consumption of food resulted in a rapid increase of extracellular dopamine to 130%. The present experiments demonstrate that in the GP extracellular dopamine can be measured by in vivo brain microdialysis. The data suggest that the dopamine release in the GP can be stimulated by a depolarising agent and involves a partially calcium-dependent release mechanism. The data further suggest that dopamine in basal ganglia structures downstream the striatum as the GP is involved in signalling of important stimuli in the environment, e.g. food.

Dopamine D1 or D2 receptor blockade in the globus pallidus produces akinesia in the rat

In the present study, the involvement of dopamine D1 and D2 receptors in the dorsal globus pallidus (GP) in motor control was investigated in rats. Results show that bilateral microinfusions of the dopamine D1 receptor antagonist SCH23390 or the dopamine D2 antagonist S( - )-sulpiride into the GP induced akinesia determined by means of the catalepsy test. These findings indicate that pallidal dopamine D1 and D2 receptors are critically involved in the control of motor behaviour. The findings further imply that defective dopaminergic transmission in the GP might contribute to akinesia due to lesion- or drug-induced dopamine hypofunction in experimental animals and in neurodegenerative diseases, e.g. Parkinson's disease, affecting the nigrostriatal dopamine system.