In vivo and in vitro characterization of gastrointestinal dysmotility in the 6-OHDA rat model of Parkinson's disease

Abstract

The aetiology of Parkinson's disease (PD), an aging-related disease characterized by progressive degeneration of dopaminergic neurons of the substantia nigra is not fully clarified yet, and moreover, there is an ever-increasing understanding that this disease is more than a motor disorder. Research into the non motor symptoms of PD is the focus of intense investigations, and there is hope of developing treatments that not only arrest the progress of the disease but stop it in its tracks. Among the non motor autonomic disorders, complications in the gastrointestinal (GI) system are the most peculiar, which degrade the quality of life of the patient and may interfere, as in the case of changes in the normal gastrointestinal transit, with the proper absorption of drugs used in the treatment of the disease itself. Moreover they can appear in very early stage of the disease, or even before the onset of motor symptoms, until they have been mentioned as possible pre-clinical signs. The purpose of the present thesis work was to characterize and study the effects of a central dopaminergic parkinsonian deficit, reproduced in Sprague Dawley rats bearing nigrostriatal 6-hydroxydopamine (6-OHDA) lesion, on the motor functions of the gastrointestinal tract. It has been adopted different in vivo methods to evaluate macroscopically the gastric motility and intestinal transit, and subsequently in vitro studies to verify the presence of any neuronal or receptor changes or any disruption caused by oxidative stress, and also to check the functionality of the pacemaker cells, Interstitial Cells of Cajal (ICC). In addition, it has been evaluated the evolution of gastrointestinal disorders following an oral subchronic four weeks treatment with L-DOPA/Benserazide, still considered a "gold standard" in the treatment of Parkinson's disease.

The in vivo results showed alterations of the gastrointestinal motility which develop in different time points following the whole course of the experiments. It has been demonstrated the presence of a delayed emptying of the stomach and a slowed intestinal transit after 8 weeks from 6-OHDA intracerebral injection, which can reflect the symptoms of gastroparesis and constipation detected in PD patients. The in vitro investigation provides evidence that 6-OHDA rats present a loss of responsiveness to exogenous contractile agents in gastric circular muscles and an impairment of pyloric nitrergic control. To test the validity of the hypothesis regarding the role and involvement of ICC in gastroparesis, never investigated before in the experimental model adopted here, it has been performed an immunohistochemical study detecting a disruption of ICC plasticity consequently to the dopaminergic central deficit in the gastric regions of fundus, corpus and antrum.

As regards the oral subchronic treatment with L-DOPA/Benserazide, it shows a partial benefit after 8 weeks in terms of gastric and colonic dysmotility, bringing it to normal values. While it decreases the state of oxidative stress, detected by the index of lipoperoxidation MDA, in stomach and terminal colon. In conclusion, this research has provided significant results and methodological knowledge. Comparing different types of investigations, it has been possible to identify the modalities that, in this specific experimental model endowed with translational validity, with higher reproducibility and limited variability, allow to monitor in vivo the gastrointestinal motor activity.