Effect of D-Ala²GIP, a stable GIP receptor agonist on MPTP-induced neuronal impairments in mice

The aim of the present study was to evaluate the ability of D-Ala²GIP, a gastric inhibitory polypeptide (GIP) receptor agonist, to attenuate the behavioral phenotype of Parkinson's disease caused by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration in mice. In the behavioral studies, MPTP administration led to spontaneous locomotor activity deficits, impaired rotarod performance, akinesia, muscular rigidity and increased tremor amplitude, which was attenuated by pretreatment with D-Ala²GIP (50–100 nmol/kg, i.p.). This acute neuroprotective response by D-Ala²GIP was found to be blocked by a selective GIP receptor antagonist, (Pro³)GIP (50 nmol/kg, i.p.), indicating that the observed effects are mediated through GIP receptor mediated signaling pathway. Biochemical studies revealed that D-Ala²GIP reduced the brain malondialdehyde levels and enhanced the brain glutathione levels, thereby mitigating the MPTP-induced oxidative stress. MPTP administration resulted in reduction of the striatal concentration of dopamine and its metabolites, homovanillic acid (HVA) and 3, 4-Dihydroxyphenylacetic acid (DOPAC). Pretreatment with D-Ala²GIP attenuated the loss of striatal dopamine levels without affecting the normal dopamine catabolism. Thus, the observed effects in the MPTP-induced Parkinsonism model could be in part attributable to the antioxidant properties of D-Ala²GIP and enhanced turnover of dopamine in the nigrostriatal pathways in mouse brain. These findings together suggest that GIP receptor could be a therapeutic target in the management of symptoms of Parkinson's disease.