Caratteirizzazione dell'evoluzione comportamentale di un modello animale emiparkinsoniano indotto da 6-OHDA

The establishment of a correlation between behavior and structure changes is essential to understand disorders diagnosed on the bases of “behavioral” disturbances. Among these Parkinson’s Disease (PD) has been traditionally considered as a "pure" movement disorder secondary to focal degeneration of dopaminergic (DAergic) neurons in the Substantia Nigra pars compacta (SNc). However, in recent years the clinical phenotype has been revised, showing that PD is a multisystem neurodegenerative disorder with motor and non-motor features showing different time evolutions. Several animal models have been employed in experimental studies concerning the pathogenetic mechanism of PD and therapeutic strategies. A commonly used animal model of PD is the unilateral injection of 6-hydroxydopamine (6-OHDA) into the SNc of rodents capable of inducing a stable nigrostriatal lesion within 3 weeks. The time-course of the anatomical, metabolic and behavioral changes that occur at the late-phase has been extensively studied. However, despite a large body of evidence has been provided about the 6-OHDA PD model, some crucial questions concerning the early-phase PD lesioned-rat behavior remain unanswered. Indeed, it is very interesting to study the early molecular and functional changes time-course observed in the emergence of the Parkinsonian lesion in the standard 6-OHDA-induced by high-DAergic depletion model and whether they might be predictive of the severity of the lesion. In this PhD thesis we investigated the early-phase time course of behavioral and structural changes in the nigrostriatal pathway during the degeneration of SNc neurons induced by intranigral injection of 6-OHDA (8 μg/4 μL) in the right hemisphere. In particular, we investigated if early (1 to 7 days post-lesion) motor behavioral deficits can be fully characterized and whether they may be predictive of the severity of the DAergic lesion at later time points (14, 21 days post lesion). To determine the time-course of the emergence of potential spontaneous motor deficits, we modified the Tail Suspension Test, here indicated as Tail Suspension Swing Test (TSST).We compared the TSST results to those obtained with the standard rotation test in the Open Field Test (OFT).We recorded spontaneous and apomorphine-induced behavioral asymmetries using a modified implementation of the TSST and the standard OFT, before and following 6-OHDA injection. The accuracy of the 6-OHDA lesion was evaluated by means of ex vivo whole-brain high-resolution Magnetic Resonance Imaging (MRI). The DAergic lesion was then evaluated by immunohistochemical staining for tyrosine hydroxylase (TH) to detect TH-immunoreactive (TH-ir) neurons and fibers in the SNc and the striatum(ST), respectively. Moreover, we studied the behavioral features of the animals after 21 days of 6-OHDA-induced lesion of the DAergic nigrostriatal system, providing a T-Pattern Analysis (TPA) of the sequential organization of the behavior, in terms of sequences detected and features. Our main experimental result is that TSST is more sensitive to reveal the spontaneous motor asymmetries. On the other hand, the results of the TSST and OFT tests after acute and repetitive apomorphine treatment, well correlated with immunohistochemical striatal DA staining and nigral DAergic neuronal loss seen at the different time-points. The TSST can therefore be useful for the selection of those animals to study early pathophysiological mechanisms, that occur from the first day after the 6-OHDA lesion and new neuroprotective strategies without the confounding apomorphine administration. Moreover, our TPA results at 21 days post-lesion show for the first time that the temporal structure and fine characteristics of behavioral sequencing are altered in the late phase of the PD rodent model obtained with a unilateral SNc injection of 6-OHDA.