PLoS One. 2015 Sep 23;10(9):e0137185. doi: 10.1371/journal.pone.0137185.

Unilateral Lesion of Dopamine Neurons Induces Grooming Asymmetry in the Mouse.

Pelosi A1, Girault JA1, Hervé D1.
  • 1Inserm UMR-S 839, 75005, Paris, France; Institut du Fer à Moulin, 75005, Paris, France; Sorbonne Universités, Université Pierre et Marie Curie (UPMC, Paris-6), Paris, France.



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.

PMID: 26397369



Grooming is the most common behavior found in mammals, birds and insects considered as a behavior taking care of the body and fur [1]. However, it can also contribute to thermo-regulation and social relationships, when it is directed at conspecifics (1). In rodents grooming is an innate set of stereotyped movements affecting all parts of the body and forming a syntax chain (2, 3), characterized by four different phases with rostro-caudal progression. It mainly consists of a series of forelimb movements and licks first targeting the nose (phase 1), then the head (phase 2), and the body flanks (phase 3) until reaching the genitals and tail (phase 4 and 5), shown in Figure 1. In normal conditions, the motor sequences are symmetrical, affecting alternatively the left and right side of the animals (4-6).



fig1­­Figure 1: Grooming in mice. From left to right the five successive phases characterizing the grooming syntax chain are represented.


Basal ganglia circuits and in particular the striatal function is critical for coordination of grooming sequences. The syntactic disruption of grooming chain occurs particularly when striatal damage extends to a specific region in the dorsolateral striatum (6). Grooming is regulated by dopamine signaling through activation of D1 and D2 receptors. D1 receptor activation plays a particular role in the grooming accomplishment. D1 receptor agonist (SKF38393) increases the grooming duration and alters the rostro-caudal sequence, by reducing the face grooming and increasing the flank grooming (7). Moreover, mutant mice lacking D1 receptor are less likely to complete grooming chains than normal mice (8). Reduced activation of dopamine receptors by bilateral lesion of dopamine neurons produces alterations in grooming behavior in rats, particularly by disrupting the completion of syntactic grooming chains (9).

This line of evidence indicates that diseases affecting the striatum, such as Parkinson’s disease (PD) can affect grooming. PD is a neurodegenerative disorder characterized by loss of nigrostriatal dopamine neurons leading to emergence of tremor, rigidity, akinesia, and bradykinesia. PD is often an asymmetrical disorder, with symptoms being more pronounced in one side of the body throughout the disease (10).

We hypothesized that grooming analysis in parkinsonian animal models could show alterations, especially in the performance of symmetry.

To test this hypothesis, we produced hemiparkinsonian mice by unilateral microinjection of the neurotoxin 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle (MFB), the ascending dopaminergic axons to the striatum (11). This approach leads to severe unilateral dopamine neurons depletion (>80%), and results in alterations in behavioral symmetry characterized by turning on the ipsilateral side of lesion and preferential ipsilateral scanning (12). However, the consequences of unilateral 6-OHDA-induced lesion and L-DOPA treatment on grooming symmetry are unknown.

We first analyzed the effect of the dopamine loss on the grooming behavior and found a decrease in the lesioned mice as compared to sham-operated animals. The decrease mainly affected the frequency of grooming events (bouts), while the general syntax was preserved.

In addition, we observed in 6-OHDA-lesioned mice significant behavioral asymmetries that are not detected in sham animals. They were characterized by a rotation behavior towards the side of lesion (ipsilateral side, right in our experiments), as reported by Ungerstedt and colleagues (13) (Figure 2). Moreover, in accordance to our hypothesis, we observed asymmetries in self-grooming, with the lesioned animals displaying significantly more grooming bouts on the ipsilateral side to the lesion compared to the total grooming events (directed both to the ipsilateral and contralateral sides)(Figure 2). The grooming symmetry was not affected in sham animals.



fig2-3Figure 2: Unilateral 6-OHDA lesion induces behavioral asymmetries. A) Increased ipsilateral turns in unilaterally lesioned animals compared to sham-operated animals (t = 5.42, p < 0.0001). (B) Increased ipsilateral grooming (t = 2.60, p = 0.014). Unpaired two-tailed t test with Welch’s correction, *p< 0.05, **** p< 0.0001.


L-DOPA is the most commonly used pharmaceutical treatment in PD. In this study we wonder whether low dose of L-DOPA (2.5 mg/kg) could restore the grooming symmetry, as it is known for turning behavior. We decided to use this low dose of L-DOPA to avoid the appearance of abnormal movements characterizing the L-DOPA-induced dyskinesia, which could affect per se the grooming behavior. In our conditions, L-DOPA was still able to induce some abnormal movements but their duration was short, allowing the grooming analysis afterwards. One injection of L-DOPA at this low dose was sufficient to restore the symmetry in turning and grooming to the levels observed in the same animals before the unilateral lesion (Figure 3).


fig3-4Figure 3: Low dose of L-DOPA restores the behavioral symmetry. In this experiment, animals were analyzed three times: before the microinjection of 6-OHDA (Before surgery), after the lesion (No Trt, After surgery) and after the lesion and L-DOPA administration (L-DOPA, After surgery). A) L-DOPA treatment was able to bring the levels of ipsilateral turns to the one seen in basal conditions, F(2,21) = 10.9, p < 0.001. B) One administration of low dose of L-DOPA in lesioned mice was enough to rescue the levels of ispilateral grooming to that seen before surgery, F(2,14) = 4.27, p < 0.035. Statistical analysis, one-way ANOVA followed by Sidak’s post-hoc tests, *p < 0.05, ***p < 0.001.


L-DOPA was not only restoring the symmetries but was also increasing other grooming parameters, such as the total time the animals spent in grooming by most probably, increasing the general activity of the animals. However, it is interesting to note that even though grooming behavior was increased and grooming symmetry was restored, L-DOPA was unable to increase the number of grooming bouts in which all the phases were correctly executed. This suggests that the correct execution of grooming behavior requires a complex coordination in the activation of dopamine neurons.

Thus, collectively these data show for the first time that unilateral lesion of the dopamine ascending neurons affects the symmetric execution of grooming syntax, like the dopamine neuron degeneration in PD patients is generating asymmetries in gait control. They also demonstrate that L-DOPA treatment restores symmetric balance of grooming movements in rodents as it does in patients, at least in the first years of treatment.

Importance of the study: our data suggest that defects in grooming behavior are indications of parkinsonism in animal models of Parkinson’s disease. These defects can be evaluated, together with the turning and scanning behaviors, to have a more complete analysis of the severity of symptoms and to measure the efficiency of new antiparkinsonian drugs. Moreover, our results suggest that grooming analysis can be also used to better understand the mechanisms underlying the deficits in control of complex motor sequences found in Parkinsonian patients. In particular, defects in striatal functions seem to be involved in speech disorders (14), particularly dysarthria and syllable repetitions. These speech disorders are very difficult, even impossible, to model in animals and we propose that the pathophysiology of these symptoms could be indirectly study by the analysis of syntactic chains of grooming.



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