Agmatine improves cognitive dysfunction and prevents cell death in a streptozotocin-induced Alzheimer rat model

Abstract

Purpose: Alzheimer's disease (AD) results in memory impairment and neuronal cell death in the brain. Previous studies demonstrated that intracerebroventricular administration of streptozotocin (STZ) induces pathological and behavioral alterations similar to those observed in AD. Agmatine (Agm) has been shown to exert neuroprotective effects in central nervous system disorders. In this study, we investigated whether Agm treatment could attenuate apoptosis and improve cognitive decline in a STZ-induced Alzheimer rat model.

Materials and methods: We studied the effect of Agm on AD pathology using a STZ-induced Alzheimer rat model. For each experiment, rats were given anesthesia (chloral hydrate 300 mg/kg, ip), followed by a single injection of STZ (1.5 mg/kg) bilaterally into each lateral ventricle (5 μL/ventricle). Rats were injected with Agm (100 mg/kg) daily up to two weeks from the surgery day.

Results: Agm suppressed the accumulation of amyloid beta and enhanced insulin signal transduction in STZ-induced Alzheimer rats [experimetal control (EC) group]. Upon evaluation of cognitive function by Morris water maze testing, significant improvement of learning and memory dysfunction in the STZ-Agm group was observed compared with the EC group. Western blot results revealed significant attenuation of the protein expressions of cleaved caspase-3 and Bax, as well as increases in the protein expressions of Bcl2, PI3K, Nrf2, and γ-glutamyl cysteine synthetase, in the STZ-Agm group.

Conclusion: Our results showed that Agm is involved in the activation of antioxidant signaling pathways and activation of insulin signal transduction. Accordingly, Agm may be a promising therapeutic agent for improving cognitive decline and attenuating apoptosis in AD.

Keywords: Agmatine; Alzheimer's disease; apoptosis; cognitive dysfunction; insulin signal transduction; streptozotocin.

Fig. 1

Agmatine attenuated Aβ accumulation and promoted phosphorylation of IRS-1 in the STZ-icv rat model. (A) The expression of Aβ accumulation in the sham group, EC group, and STZ-Agm group. The image was shown at the magnification of 400. Scale bar: 200 µM. (B) The expression of phosphorylated IRS-1 in the sham, EC, and STZ-Agm groups in hippocampus sections. The image was shown at the magnification of 200. Scale bar: 400 µM. (C) The expression of phosphorylated IRS-1 in the sham, EC, and STZ-Agm groups in cortex sections. The image was shown at the magnification of 200. Scale bar: 400 µM. (D) The latency time of Morris water maze was measured in the sham, EC, and STZ-Agm groups. The time required to reach the platform (escape latency) was measured on each day (1-5 days). Data were expressed as mean±SEM, and were analyzed statistically using one-way ANOVA, followed by Scheffe's post hoc (^*p<0.05, ^**p<0.001 compared to Sham group, ^†p<0.05, ^††p<0.01 compared to EC group with STZ-Agm group). DAPI, 4',6-diamidino-2-phenylindole; PI, propidium iodide; p-IRS-1, phosphorylated IRS-1; STZ, streptozotocin; EC, experimental control.

Fig. 2

Histological analysis of the hippocampus and cortex regions in STZ-icv rats. (A) Hippocampus sections from the sham group (a, b, c), EC group (d, e, f), and STZ-Agm group (g, h, i). CA1 (a, d, g), CA2 (b, e, h), CA3 (c, f, i). Scale bars were indicated. (B) Cortex sections from the sham group (a), EC group (b), and STZ-Agm group (c). All slides were stained by hematoxylin and eosin (H&E). STZ, streptozotocin; EC, experimental control.

Fig. 3

Agmatine treatment decreased the expression of apoptotic proteins in STZ-icv rats. (A) Representative blots showed the protein levels of cleaved caspase-3 from the total protein extracts prepared from the hippocampus regions of each group. Bars represented the relative protein quantification of active cleaved caspase-3 on the basis of β-actin, respectively. Representative blots showed the protein levels of Bcl2 (B) and Bax (C) from the total protein extracts prepared from the hippocampus regions of each group. (D) Representative blots showed the protein levels of PI3K from the total protein extracts prepared from the hippocampus regions of each group (^*p<0.05, ^**p<0.01 compared to Sham group, ^†p<0.05, ^††p<0.01 compared to EC group). STZ, streptozotocin; EC, experimental control.

Fig. 4

Agmatine treatment decreased the immunoreactivity of 8-OHdG and increased the expression of Nrf2 and γ-GCS in STZ-icv rats. (A) Immunohistochemistry images showed immunostaining of 8-OhdG (red) in the sham group, EC group and STZ-Agm group. The image was shown at the magnification of 400. Scale bar: 200 µM. (B) Western blot showed the amount of Nrf2 protein from the total protein extracts prepared from the hippocampus regions of each group. Bar graph showed the quantification of Nrf2 protein levels in all groups. (C) Western blot showed the expression levels of γ-GCS from the total protein extracts prepared from the hippocampus regions of each group. Bar graph showed the quantification of γ-GCS protein levels (^*p<0.05, ^**p<0.01 compared to the sham group, ^†p<0.05 compared to the EC group). DAPI, 4',6-diamidino-2-phenylindole; STZ, streptozotocin; GCS, glutamyl cysteine synthetase; EC, experimental control.