Detection of anxiety and depression-like behavior and intra-brain pathological markers in Parkinsonian cynomolgus monkeys.

OBJECTIVE

Anxiety and depression are common non-motor symptoms of Parkinson's disease, but its underlying mechanism and treatment are still unclear. The key factor to solve this problem is to systematically establish an objective evaluation system of anxiety and depression-like behavior on a suitable PD animal model and reveal its pathological changes in the brain.

METHODS

Ten male cynomolgus monkeys were chosen, with five undergoing construction of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced left-sided symptoms-predominant PD model, following individualization principles. The PD model's reliability was validated via the Kurlan score and L-dopa efficacy test. Five healthy cynomolgus monkeys served as the control group with no interventions. Initially, a modified and simplified device was established to detect anxiety-like and depression-like behavior. Subsequently, ten cynomolgus monkeys were assessed for anxiety-like behavior using the human Intruder test (HIT) and novel fruit test (NFT). Depression-like behavior was evaluated using the progressive ratio test (PRT) and the apathy feeding test (AFT). Upon completion of behavioral tests, brain tissue from three cynomolgus monkeys that underwent PD modeling and three normal monkeys was utilized from the existing tissue bank. Initially, tyrosine hydroxylase (TH) staining was conducted in the substantia nigra region using the immunohistochemical method to verify dopamine neuron loss extent. Subsequently, TH, tryptophan hydroxylase 2 (TPH2), dopamine-β hydroxylase (DβH), and c-Fos protein, associated with neuronal activity, were stained in the three anxiety and depression-related brain regions: hippocampus, locus coeruleus, and amygdala, respectively, to explore changes in biomarkers related to anxiety and depression in the brain.

RESULTS

In this study, all PD cynomolgus monkeys selected exhibited consistent and stable PD symptoms, primarily characterized by bradykinesia, rigidity, tremor, and postural flexion. Following MPTP modeling, there was no spontaneous recovery observed over 3 months. A modified and simplified device for screening anxiety and depression-like behavior was developed, and behavioral tests were conducted. The HIT assessed four behavioral phenotypes: anxious behavior, aggressive behavior, pacing, and sitting posture. The PD group displayed evident anxious behavior in a natural state, exhibited defensive behavior tending to inhibit actions after stress, and showed reduced aggression toward intruders. In the NFT, the PD group demonstrated lower grab rate of novel and familiar fruits and took longer to reach for fruit compared to the control group, indicating anxiety-like behavior. The PRT revealed a lower breakpoint for food intake in the PD group, suggesting reduced motivation for food, although no significant difference was noted between the groups. In the AFT, the PD group exhibited lower grab rate and prolonged fruit-grabbing time, indicative of reduced motivation and depression-like behavior. Immunohistochemical analysis of brain tissue revealed substantial loss of dopamine neurons in the left substantia nigra in the PD group, confirming the PD model from a pathological standpoint. Moreover, TPH2 expression was notably elevated in the amygdala region of the PD group, with a significant increase in c-Fos protein expression observed in the same region. TH expression levels were diminished in the hippocampus region, while DβH expression levels in the locus coeruleus and amygdala region were significantly higher than those in the control group, indicating pathological changes associated with anxiety and depression in PD cynomolgus monkeys.

CONCLUSIONS

This study was conducted in order to test anxiety and depressive symptoms in a typical PD monkey model, as well as anxiety and depression-related pathological changes in their brains. Behavioral tests revealed that PD monkeys exhibited anxiety-like behavior characterized by behavioral inhibition and depression-like behavior characterized by lack of motivation. Immunohistochemical staining revealed impairment of dopaminergic, serotonergic, and noradrenergic systems and hyperactivation of c-Fos protein in the brains of PD monkeys. These results provide technical support and scientific basis for evaluating non-motor behaviors in the psychopathology of Parkinson's disease, preclinical efficacy assessment, and further research on the pathogenesis of Parkinson's disease.