TRANSVERSAL INVESTIGATION OF APATHIC BEHAVIORS IN A PARKINSON’ S DISEASE MOUSE MODEL WITH 6-HYDROXYDOPAMINE LESIONING IN THE BILATERAL DORSAL STRIATUM
Authors: Okitsu *Masato , Fujita Masayo , Takahashi Kazushi , Ikeda Kazutaka 
Published: February 2025

Abstract

Background
Parkinson's disease (PD) is associated with various non-motor symptoms. Among these, apathy consists of many symptoms related to emotional, self-care, and social interaction behaviors. Apathy has great impact on the lives of PD patients; however, its detailed pathology remains unknown.

Aims & Objectives
We transversally investigated each domain of apathic behaviors in PD mouse model with 6-hydroxydopamine (6-OHDA), which induces dopamine neuronal death, an intrinsic pathological change of PD, lesioned in the bilateral dorsal striatum.

Methods
We allocated 8-week-old male C57/BL6J wild-type mice (n=21) to either the 6-OHDA (n=13) or sham (n=8) group. The 6-OHDA group received bilateral administration of 4 &micro;g/1 &micro;L 6- OHDA dissolved in 0.25% ascorbic acid (AA) in saline, while the sham group received 1 &micro;L of 0.25% AA in saline alone, both into the dorsal striatum. We conducted rotarod, pole, and balance beam tests before 6-OHDA lesioning and 3 weeks later. Behavioral experiments related to apathy were subsequently conducted. The sucrose preference test (SPT) and novelty-suppressed feeding test (NSFT) were used to assess anhedonia; the nest-building test (NBT) and splash test were used to assess self-grooming; the open-field test (OFT) and hole-board test (HBT) were used to assess the anxiety and novelty-seeking behavioral aspects; and the three-chamber social test was used to assess the social interaction behavior. Finally, pathological evaluation was performed using tyrosine hydroxylase (TH) immunostaining for two different slice levels: 1. Caudoputamen (CPu) and nucleus accumbens (NAcc) at the cranial level, and 2. Substantia nigra (SN) and the ventral tegmental area (VTA) at the caudal level. These histological images were analyzed using ImageJ software. The number of TH-positive (TH+) pixels extracted from the contrast differences in each pixel was counted.

Results
One mouse in the 6-OHDA group died postoperatively. Three weeks after 6-OHDA lesioning, the 6-OHDA group exhibited significant impairments compared to the sham group in all the motor performance tests, confirming the use of these mice as PD models. In the apathy-related behavioral tests, a significant difference observed in the exploration time for the novel object in the sociability session of the three-chamber social test (6-OHDA group:90.0±24.5 s vs. sham group:112.0±15.5 s; P&lt;0.05), head dip counts in the HBT (10.1±5.3 vs. 16.6±5.5; P&lt;0.05), and latency time to eat food in the NSFT (255.6±97.7 s vs. 159.1±91.7 s; P&lt;0.05) was observed. Upon pathological evaluation, the number of TH+ pixels in CPu (6-OHDA group:115.6±18.8 vs. sham group: 140.7±15.6; P&lt;0.001) and SN (10.3± 1.9 vs. 13.2±3.2; P&lt;0.05) were significantly reduced in the 6-OHDA group compared to the sham group. However, the number of TH+ pixels in NAcc (6-OHDA group: 25.9±3.3 vs. sham group:27.2±3.1; P=0.4) and VTA (11.3±1.9 vs. 13.3±2.3; P=0.060) did not significantly differ between the two groups. One mouse in the 6-OHDA group showed no neuronal loss in the CPu, suggesting that lesioning with 6-OHDA failed. Therefore, that individual was excluded from all the evaluations.

Discussion & Conclusion
In the present study, a PD mouse model with relatively localized dopaminergic neuronal loss in the dorsal striatum was generated and exhibited impaired novelty seeking.