Abnormal involuntary movement (AIM) expression following D2 dopamine agonist challenge is determined by the nature of prior dopamine receptor stimulation (priming) in 6-hydroxydopamine lesioned rats

D2R signaling in striatal spiny neurons modulates L-DOPA induced dyskinesia

Degeneration of dopaminergic neurons leads to Parkinson's disease (PD), characterized by reduced levels of striatal dopamine (DA) and impaired voluntary movements. DA replacement is achieved by levodopa treatment which in long-term causes involuntary movements or dyskinesia. Dyskinesia is linked to the pulsatile activation of D1 receptors of the striatal medium spiny neurons (MSNs) forming the direct output pathway (dMSNs). The contribution of DA stimulation of D2R in MSNs of the indirect pathway (iMSNs) is less clear. Using the 6-hydroxydopamine model of PD, here we show that loss of DA-mediated inhibition of these neurons intensifies levodopa-induced dyskinesia (LID) leading to reprogramming of striatal gene expression. We propose that the motor impairments characteristic of PD and of its therapy are critically dependent on D2R-mediated iMSNs activity. D2R signaling not only filters inputs to the striatum but also indirectly regulates dMSNs mediated responses.

Effects of L-dopa priming on cortical high beta and high gamma oscillatory activity in a rodent model of Parkinson's disease

Prolonged L-dopa treatment in Parkinson's disease (PD) often leads to the expression of abnormal involuntary movements known as L-dopa-induced dyskinesia. Recently, dramatic 80 Hz oscillatory local field potential (LFP) activity within the primary motor cortex has been linked to dyskinetic symptoms in a rodent model of PD and attributed to stimulation of cortical dopamine D1 receptors. To characterize the relationship between high gamma (70-110 Hz) cortical activity and the development of L-dopa-induced dyskinesia, cortical LFP and spike signals were recorded in hemiparkinsonian rats treated with L-dopa for 7 days, and dyskinesia was quantified using the abnormal involuntary movements (AIMs) scale. The relationship between high gamma and dyskinesia was further probed by assessment of the effects of pharmacological agents known to induce or modulate dyskinesia expression. Findings demonstrate that AIMs and high gamma LFP power increase between days 1 and 7 of L-dopa priming. Notably, high beta (25-35 Hz) power associated with parkinsonian bradykinesia decreased as AIMs and high gamma LFP power increased during priming. After priming, rats were treated with the D1 agonist SKF81297 and the D2 agonist quinpirole. Both dopamine agonists independently induced AIMs and high gamma cortical activity that were similar to that induced by L-dopa, showing that this LFP activity is neither D1 nor D2 receptor specific. The serotonin 1A receptor agonist 8-OH-DPAT reduced L-dopa- and DA agonist-induced AIMs and high gamma power to varying degrees, while the serotonin 1A antagonist WAY100635 reversed these effects. Unexpectedly, as cortical high gamma power increased, phase locking of cortical pyramidal spiking to high gamma oscillations decreased, raising questions regarding the neural substrate(s) responsible for high gamma generation and the functional correlation between high gamma and dyskinesia.

Unilateral Lesion of Dopamine Neurons Induces Grooming Asymmetry in the Mouse

Grooming behaviour is the most common innate behaviour in animals. In rodents, it consists of sequences of movements organized in four phases, executed symmetrically on both sides of the animal and creating a syntactic chain of behavioural events. The grooming syntax can be altered by stress and novelty, as well as by several mutations and brain lesions. Grooming behaviour is known to be affected by alterations of the dopamine system, including dopamine receptor modulation, dopamine alteration in genetically modified animals, and after brain lesion. While a lot is known about the initiation and syntactic modifications of this refined sequence of movements, effects of unilateral lesion of dopamine neurons are unclear particularly regarding the symmetry of syntactic chains. In the present work we studied grooming in mice unilaterally lesioned in the medial forebrain bundle by 6-hydroxydopamine. We found a reduction in completion of grooming bouts, associated with reduction in number of transitions between grooming phases. The data also revealed the development of asymmetry in grooming behaviour, with reduced tendency to groom the contralateral side to the lesion. Symmetry was recovered following treatment with L-DOPA. Thus, the present work shows that unilateral lesion of dopamine neurons reduces self-grooming behaviour by affecting duration and numbers of events. It produces premature discontinuation of grooming chains but the sequence syntax remains correct. This deficient grooming could be considered as an intrinsic symptom of Parkinson’s disease in animal models and could present some similarities with abnormalities of motor movement sequencing seen in patients. Our study also suggests grooming analysis as an additional method to screen parkinsonism in animal models.