The possible underlying neuroprotective mechanisms of agmatine in rats injected with streptozotocin

Abstract

Alzheimer''s disease (AD) is a representative neurodegenerative disease which is characterized by the progressive cognitive disability and memory loss commonly observed in the elderly people. Of importance, several studies demonstrate that NMDA receptor antagonists and antioxidant agents are able to inhibit these pathologies of AD. Agmatine is a polycationic and endogenous amine which is synthesized by arginine decarboxylase (ADC). Recent investigations delineate the possible functions of agmatine as a NMDA receptor antagonist or an antioxidant substance, respectively. Therefore, the present study was undertaken to explore the beneficial effects of agmatine in the brain of rats exposed to streptozotocin. In particular, it was examined whether and how agmatine relieves amyloid beta (Aβ) and oxidative stress-mediated toxicity in animal model. In the first part, rats were stereotaxically injected with streptozotocin via intra-cerebral route (STZ: 3mg/5㎕, 1st and 3rd day after surgery) and then, intra-peritoneally received agmatine (100mg/kg) for 14 days. Agmatine proved to recuperate the cognitive and memory deficit in rats exposed to STZ as evidenced by behavioral tests such as morris water maze and radial arm maze. As well, it suppressed the induction of amyloid precursor protein (APP) and amyloid beta (Aβ) peptide and reduced the immunoreactivity against caspase-3. To further investigate the underlying protective effect of agmatine, primary hippocampal neurons were treated with Aβ (20µM), MK801 (NMDA receptor antagonist), or combination of these agents, respectively. Agamtine inhibited the reduction of formazan (cell viability indicator) formation and the increase of Hoechst/PI positivity (cell death indicator) in primary hippocampal neurons treated with STZ. This effect of agmatine was comparable to it of MK801. Therefore, these findings demonstrate that agmatine might exert its protective effect through NMDA receptor antagonism. In the second part, rats were stereotaxically injected with streptozotocin via intracerebroventricular route (1.5mg/kg, 5µl, 1st and 3rd day after surgery) and then, intra-peritoneally received agmatine (100mg/kg) for 14 days. Like what was observed in rats intra-cerebrally injected with STZ, agmatine improved the cognitive and memory impairment and inhibited caspase 3 as demonstrated by behavioral tests or immunohistochemistry, respectively. Then, to more unravel the protective effect of agmatine, Bax, Bcl2, PI3K, AKT, Nrf2, and r-GCS were checked with western blotting. As expected, agmatine reduced Bax and raised the rest proteins mentioned above. It is interesting that agmatine affects protein levels of Nrf2 and r-GCS. These proteins are known to play the pivotal roles in neutralizing oxidative damage. Correspondingly, agmatine declined 8-oxo immunofluorescence in hippocampal neurons of rats injected with STZ, potentiating that it detoxifies oxidative condition within the cells. Therefore, the current data suggest that agmatine stimulates both cell survival and antioxidant related protein involving pathway to support cognitive function. Taken together, our data propose the idea that agmatine may ameliorate cognitive dysfunction in neurodegenerative diseases such as Alzheimer`s disease (AD), Parkinson`s disease (PD), Huntington`s disease (HD), and Amyotrophic lateral sclerosis (ALS) through the pro-survival and anti-oxidant action.