Meccanismo d'azione del Pramipexolo nella terapia della malattia di Parkinson

Pramipexole (PPX) is a dopamine (DA) D3 and D2 receptors agonist widely used alone or in combination with levodopa as Dopamine Replacement Therapy in Parkinson’s disease. In clinical and preclinical studies, PPX improved motor deficits, this evidence led to lowering daily dose of levodopa, delaying the motor side effects associated with its use. Recently, PPX administration has been associated to the development of addictive-like behaviors related to the DA Dysregulation Syndrome, particularly in a subpopulation of treated patients, characterized by impulsive-compulsive personality traits as well as previous addiction’s experience. Based on these evidences, the aim of this study was twofold: first to investigate the pharmacological action of PPX, using a unilateral model of Parkinson’s disease in which 6-OHDA was injected in the medial forebrain bundle. After two weeks, we tested in primed and naive rats, the ability of three different doses of PPX (0,035; 0,1 and 0,35 mg/kg s.c.), to induce contralateral turning behavior as well c-fos expression after pretreatment of DA D1 antagonist SCH 39166. Next, we checked the ability of PPX to induce contralateral rotations after D2 (eticlopride) and D3 (S33084) DA antagonist pretreatment. In order to investigate the role of PPX (0,05 mg/kg s.c.) in behavioral sensitization, we tested its effect with S33084 pretreatment in levodopa sensitized rats. Second, we assessed the correlation between PPX treatment, Parkinson’s disease and the onset of DA Dysregulation Syndrome on Conditioned Place Preference (CPP) paradigm. To do this, 6-OHDA was injected bilaterally in DA striatal terminals, in three different strains of rat: the addiction prone Lewis (LEW), the addiction resistant Fisher 344 (F344) inbred strains, and the Sprague Dawley (SD) outbred strain. Furthermore, to test its rewarding properties, PPX was directly infused in the nucleus accumbens shell (NAc), a DA mesolimbic region known to be involved in the rewarding effects of drugs of abuse, in healthy rats belonging to the above mentioned strains. We discovered that in primed rats, PPX (0,35 mg/kg s.c.) induced turning behavior that was increased by SCH 39166 pretreatment (0,1 mg/kg s.c.). No effect was seen in naive rats both for turning behavior and c-fos expression. D2 receptors antagonist eticlopride (0,1 mg/kg s.c.) reduced PPX-induced turning behavior more than D3 receptors antagonist S33084 (0,5 mg/kg s.c.), also a previous levodopa sensitization increased PPX-induced turning behavior on its first administration. This suggests that PPX’s action could be related to D2 stimulation, and it seems to require a previous D1/D2 stimulation to observe a behavioral outcome. PPX (1 mg/kg s.c.) was able to induce a significant CPP in SD and LEW lesioned rats but not in F344 and control rats, and the persistence of preference was stronger in LEW than in SD rats. When injected into the NAc shell, PPX (5 μg/0.5 μl) induced CPP in all rat strains, but the persistence of its effect was more strong in LEW compared to SD and F344 rats. These results suggest that the parkinsonian state might be more sensitive to the rewarding properties of PPX, which do not seem entirely influenced by phenotype.