Elsevier

Behavioural Brain Research

Volume 252, 1 September 2013, Pages 58-67
Behavioural Brain Research

Research report
GABAA-receptor activation in the subthalamic nucleus compensates behavioral asymmetries in the hemiparkinsonian rat

https://doi.org/10.1016/j.bbr.2013.05.044Get rights and content

Highlights

  • Pharmacological STN manipulation influences motor parameters in 6-OHDA-treated rats.

  • Compensation of motor asymmetries by activation of GABAA-receptors in the STN.

  • Dose-dependent effect of muscimol on turning behavior and locomotion.

  • No acute effects of NMDA-antagonism in the STN on motoric behavior.

  • GABAergic mechanisms contribute to therapeutical effects of STN manipulations.

Abstract

The subthalamic nucleus (STN) has a pivotal role in the pathophysiology of Parkinson's disease (PD). Modulation of STN activity (by lesions, pharmacological or electrical stimulation) has been shown to improve motor parameters in PD patients and in animal models of PD. In an attempt to characterize the neurochemical bases for such antiparkinsonian action, we address specific neurotransmitter systems via local pharmacological manipulation of the STN in hemiparkinsonian rats. Here, we have focused on the GABAergic and glutamatergic receptors in the STN. In animals with unilateral 6-hydroxydopamine lesions of the nigro-striatal tract, we administered either the selective GABAA-agonist muscimol (0.5 μg and 1.0 μg), the non-competitive N-methyl-d-aspartate (NMDA)-antagonist MK-801 (dizocilpine; 2.5 μg), or vehicle (0.25 μl) into the STN. The effects of GABAergic and glutamatergic modulation of the STN on motor parameters were assessed by gauging rotational behavior and locomotion. Application of muscimol ipsilateral to the side of dopamine-depletion influenced turning behavior in a dose-dependent fashion, with the low dose re-adjusting turning behavior to a non-biased distribution, and the high dose evoking contraversive turning. The administration of MK-801 did not have such effects. These findings give evidence for the involvement of GABAergic activation in the STN in the compensation of motor asymmetries in the hemiparkinsonian rat, whereas N-methyl-d-aspartate (NMDA)-antagonism was ineffective in this model of PD.

Introduction

The subthalamic nucleus (STN) has a crucial role in the development of motor impairments and abnormal neural activity following degeneration of midbain dopamine (DA) neurons. Several manipulations of the STN (lesions, pharmacological or electrical stimulation) compensate for the deficits associated with parkinsonism. The neurochemical bases of such manipulations remain poorly understood. The present experiment aimed at characterizing the involvement of GABAergic and glutamatergic STN receptors in movement control in hemiparkinsonian rats.

The unilateral administration of the neurotoxin 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle (MFB) is a widely applied rodent model of experimental parkinsonism and is known to alter GABAergic [1], [2] and glutamatergic [3] neurotransmission within different nuclei of the basal ganglia (BG), including the STN. The DAergic degeneration of nigro-striatal projections has also been shown to alter the firing pattern [4] and oscillatory activity [5] of several nuclei of the BG. Excessive rhythmic burst-activity and an increase in local field potential oscillations in power and coherence in the beta range (∼15–30 Hz) of the STN have been discussed to contribute to the manifestation of motor impairments in animal models of PD [5], [6], [7] as well as in PD patients [8], [9], [10], [11], [12]. Several manipulations of the STN have been demonstrated to compensate behavioral deficits associated with DA-denervation, both, in PD patients and pre-clinical rodent models, including lesion procedures [13], [14], [15], electrical [16] and pharmacological [17], [18], as well as optogenetical modulations [19]. Lesions of the STN counteract experimental parkinsonism in monkeys [20]. Electrical high-frequency stimulation of the STN (STN-DBS) alleviates PD motor impairments in monkeys and rats [17], [21], [22], [23], [24]. GABAergic activation in the STN has antiparkinsonian effects in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys [25], [26] and in PD patients [18] and has beneficial effects on oscillatory activity in the DA-depleted BG [27]. Such manipulations of the STN lead to focal and remote changes in various neurotransmitter systems in diverse BG nuclei [28], [29], [30], [31], [32].

The STN holds a strategic position within the cortico-BG-thalamo-cortical loop and receives GABAergic inputs from the external part of the globus pallidus (GPe) and glutamatergic afferents from extensive cortical areas associated with motor control. It sends excitatory projections to the GPe, the internal segment of the globus pallidus (GPi) and to the substantia nigra pars reticulata (SNr), using glutamate as its major neurotransmitter. Given the unique role of GABA and glutamate in BG in- and output circuitry, these neurotransmitters have gained special attention in physiological and pathological BG functioning. Unilateral administration of the GABAA-receptor agonist muscimol into the STN evoked contraversive turning behavior (a widely applied behavioral measure in the assessment of unilateral nigro-striatal lesions) in rats with intact nigro-striatal projections [17] and in non-lesioned cats [33], although others have found no effects of subthalamic muscimol injection on turning behavior in non-parkinsonian rats [34]. There is also evidence for the participation of glutamatergic mechanisms in the control of motor behavior by the STN. The involvement of STN NMDA-receptors in turning behavior and other motor deficits incurred by nigro-striatal denervation has been shown with respect to prolonged intrasubthalamic NMDA antagonism. Prolonged (∼3 weeks) infusion of MK-801 into the STN immediately after administration of 6-OHDA prevented the development of motor impairments [35]. The acute unilateral injection of MK-801 into the STN led to contraversive dystonic postures when haloperidol was systemically administered afterwards [36]. Intranigral NMDA-antagonism evokes contralateral turning in intact [37] and 6-OHDA treated rats [38]. Furthermore, systemic administration of MK-801 was found to increase contraversive turning behavior evoked by systemically applied dopamine agonists in the unilaterally 6-OHDA lesioned rat [39].

In an attempt to characterize the involvement of subthalamic GABAergic and glutamatergic transmission on motor parameters in a unilateral rodent model of PD, we here administered the selective GABAA-agonist muscimol or the non-competitive NMDA-antagonist MK-801 focally into the STN. Given the role of GABA and glutamate in physiological and pathological movement control, we hypothesized that intrasubthalamic injections of muscimol and MK-801 would compensate for motoric deficits (including turning behavior and locomotion) in the hemiparkinsonian rat. We decided to determine the motoric effects of GABAA-agonism and NMDA-antagonism in the STN in rats experimentally rendered (hemi-)parkinsonian since, to our knowledge, this is the first study to behaviorally characterize the effects of focal pharmacological manipulations of the STN in rats bearing massive unilateral lesions of the nigrostriatal tract.

Turning behavior in the hemiparkinsonian rat is differentially modulated by different types of systemically applied dopamine agonists [40]. To characterize the distinct impact of GABAA-agonism and NMDA-antagonism on pre- vs. post-synaptic DAergic mechanisms, we also challenged animals with l-DOPA and d-amphetamine, respectively, in parallel with the intrasubthalamic administrations.

Section snippets

Experimental procedures

All experiments were performed in accordance with the European Communities Council Directive (86/609/EEC) and the effective German Law of animal protection of 1998 and have been approved by local authorities (Central Animal Facility of the University of Düsseldorf and the The North Rhine-Westphalia State Environment Agency). Efforts were made to minimize the number of animals used and to minimize suffering.

Neurochemistry

There was a significant reduction in content of DA in the lesioned striatum (t11 = 4.743, P = .001, mean 90.79%, ±3.00 SEM). There was also a significant decrease in DOPAC in the affected striatum (t11 = 5.295, P < .001) with a mean loss of 54.17% (±9.32 SEM). HVA content (mean 58.26%, ±16.16 SEM) did not differ significantly (t11 = .905, P = .05). See also Table 1 for an overview (the mean values presented display averages across individual animals).

Vehicle (control) group

There were differences between effects of sole STN

Discussion

The principal finding of the present experiment was that injections of a GABAA-receptor agonist into the STN ipsilateral to the side of the 6-OHDA lesion influenced turning behavior in a dose-dependent manner, while blocking NMDA-receptors had no effects on motor asymmetries. There was a distinct time-course effect displayed by the low-dose muscimol group in the asymmetry index showing a gradual shift from initial ipsiversive turning, over non-biased turning to contraversive turning. The

Conclusions

Taken together, the present findings indicate the participation of GABAergic STN receptors in the acute mediation of movement control in unilateral DA-depleted rats. The activation of GABAA-receptors in the STN alleviated motor impairments incurred by a unilateral 6-OHDA lesion. It seems that there is a therapeutic window with respect to stimulating GABAA-receptors in the STN and that an overshoot along this dimension evokes dyskinetic effects in 6-OHDA lesioned rats. Considering the behavioral

Conflict of interest

The authors declare no competing financial interests.

Acknowledgements

This project was supported by a grant provided by the Forschungskommision (FoKo) of the Medical Faculty, University of Düsseldorf and the Interdisciplinary Graduate School for Brain Research and Translational Neuroscience (iBrain) of the Neuroscience Network Düsseldorf (NND).

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