Antioxidant capacity in rat brain after intracerebroventricular treatment with streptozotocin and alloxan--a preliminary study

Mitochondrial protective and anti-apoptotic effects of Rhodiola crenulata extract on hippocampal neurons in a rat model of Alzheimer's disease

In our previous study, we found that the edible alcohol extract of the root of the medicinal plant Rhodiola crenulata (RCE) improved spatial cognition in a rat model of Alzheimer's disease. Another study from our laboratory showed that RCE enhanced neural cell proliferation in the dentate gyrus of the hippocampus and prevented damage to hippocampal neurons in a rat model of chronic stress-induced depression. However, the mechanisms underlying the neuroprotective effects of RCE are unclear. In the present study, we investigated the anti-apoptotic effect of RCE and its neuroprotective mechanism of action in a rat model of Alzheimer's disease established by intracerebroventricular injection of streptozotocin. The rats were pre-administered RCE at doses of 1.5, 3.0 or 6.0 g/kg for 21 days before model establishment. ATP and cytochrome c oxidase levels were significantly decreased in rats with Alzheimer's disease. Furthermore, neuronal injury was obvious in the hippocampus, with the presence of a large number of apoptotic neurons. In comparison, in rats given RCE pretreatment, ATP and cytochrome c oxidase levels were markedly increased, the number of apoptotic neurons was reduced, and mitochondrial injury was mitigated. The 3.0 g/kg dose of RCE had the optimal effect. These findings suggest that pretreatment with RCE prevents mitochondrial dysfunction and protects hippocampal neurons from apoptosis in rats with Alzheimer's disease.

The rise and fall of insulin signaling in Alzheimer's disease

The prevalence of both diabetes and Alzheimer's disease (AD) are reaching epidemic proportions worldwide. Alarmingly, diabetes is also a risk factor for Alzheimer's disease. The AD brain is characterised by the accumulation of peptides called Aβ as plaques in the neuropil and hyperphosphorylated tau protein in the form of neurofibrillary tangles within neurons. How diabetes confers risk is unknown but a simple linear relationship has been proposed whereby the hyperinsulinemia associated with type 2 diabetes leads to decreased insulin signaling in the brain, with downregulation of the PI3K/AKT signalling pathway and its inhibition of the major tau kinase, glycogen synthase kinase 3β. The earliest studies of post mortem AD brain tissue largely confirmed this cascade of events but subsequent studies have generally found either an upregulation of AKT activity, or that the relationship between insulin signaling and AD is independent of glycogen synthase kinase 3β altogether. Given the lack of success of beta-amyloid-reducing therapies in clinical trials, there is intense interest in finding alternative or adjunctive therapeutic targets for AD. Insulin signaling is a neuroprotective pathway and represents an attractive therapeutic option. However, this incredibly complex signaling pathway is not fully understood in the human brain and particularly in the context of AD. Here, we review the ups and downs of the research efforts aimed at understanding how diabetes modifies AD risk.

Intracerebroventricular Streptozotocin Injections as a Model of Alzheimer's Disease: in Search of a Relevant Mechanism

Streptozotocin (STZ), a glucosamine-nitrosourea compound derived from soil bacteria and originally developed as an anticancer agent, in 1963 has been found to induce diabetes in experimental animals. Since then, systemic application of STZ became the most frequently studied experimental model of insulin-dependent (type 1) diabetes. The compound is selectively toxic toward insulin-producing pancreatic beta cells, which is explained as the result of its cellular uptake by the low-affinity glucose transporter 2 (GLUT2) protein located in their cell membranes. STZ cytotoxicity is mainly due to DNA alkylation which results in cellular necrosis. Besides pancreatic beta cells, STZ applied systemically damages also other organs expressing GLUT2, such as kidney and liver, whereas brain is not affected directly because blood-brain barrier lacks this transporter protein. However, single or double intracerebroventricular (icv) STZ injection(s) chronically decrease cerebral glucose uptake and produce multiple other effects that resemble molecular, pathological, and behavioral features of Alzheimer's disease (AD). Taking into consideration that glucose hypometabolism is an early and persistent sign of AD and that Alzheimer's brains present features of impaired insulin signaling, icv STZ injections are exploited by some investigators as a non-transgenic model of this disease and used for preclinical testing of pharmacological therapies for AD. While it has been assumed that icv STZ produces cerebral glucose hypometabolism and other effects directly through desensitizing brain insulin receptors, the evidence for such mechanism is poor. On the other hand, early data on insulin immunoreactivity showed intense insulin expression in the rodent brain, and the possibility of local production of insulin in the mammalian brain has never been conclusively excluded. Also, there are GLUT2-expressing cells in the brain, in particular in the circumventricular organs and hypothalamus; some of these cells may be involved in glucose sensing. Thus, icv STZ may damage brain glucose insulin producing cells and/or brain glucose sensors. Mechanistic explanation of the mode of action of icv STZ, which is currently lacking, would provide a valuable contribution to the field of animal models of Alzheimer's disease.

Brain catalase in the streptozotocin-rat model of sporadic Alzheimer's disease treated with the iron chelator-monoamine oxidase inhibitor, M30

Low intracerebroventricular (icv) doses of streptozotocin (STZ) produce regionally specific brain neurochemical changes in rats that are similar to those found in the brain of patients with sporadic Alzheimer's disease (sAD). Since oxidative stress is thought to be one of the major pathologic processes in sAD, catalase (CAT) activity was estimated in the regional brain tissue of animals treated intracerebroventricularly with STZ and the multitarget iron chelator, antioxidant and MAO-inhibitor M30 [5-(N-methyl-N-propargylaminomethyl)-8-hydroxyquinoline]. Five-day oral pre-treatment of adult male Wistar rats with 10 mg/kg/day M30 dose was followed by a single injection of STZ (1 mg/kg, icv). CAT activity was measured colorimetrically in the hippocampus (HPC), brain stem (BS) and cerebellum (CB) of the control, STZ-, M30- and STZ + M30-treated rats, respectively, 4 weeks after the STZ treatment. STZ-treated rats demonstrated significantly lower CAT activity in all three brain regions in comparison to the controls (p < 0.05 for BS and CB, p < 0.01 for HPC). M30 pre-treatment of the control rats did not influence the CAT activity in HPC and CB, but significantly increased it in BS (p < 0.05). M30 pre-treatment of STZ-treated rats significantly increased CAT activity in the HPC in comparison to the STZ treatment alone (p < 0.05) and normalized to the control values. These findings are in line with the assumption that reactive oxygen species contribute to the pathogenesis of STZ in a rat model of sAD and indicate that multifunctional iron chelators such as M30 might also have beneficial effects in this non-transgenic sAD model.

In vivo cross-sectional characterization of cerebral alterations induced by intracerebroventricular administration of streptozotocin

Cerebral aging is often associated with the occurrence of neurodegenerative diseases leading to dementia. Animal models are critical to elucidate mechanisms associated to dementia and to evaluate neuroprotective drugs. Rats that received intracerebroventricular injection of streptozotocin (icv-STZ) have been reported as a model of dementia. In these animals, this drug induces oxidative stress and brain glucose metabolism impairments associated to insulin signal transduction failure. These mechanisms are reported to be involved in the pathogenesis of Alzheimer's disease and other dementia. Icv-STZ rats also display memory impairments. However, little is known about the precise location of the lesions induced by STZ administration. In this context, the present study characterized the cerebral lesions induced by two-doses of icv-STZ by using high-field magnetic resonance imaging to easily and longitudinally detect cerebral abnormalities and by using immunohistochemistry to evaluate neuronal loss and neuroinflammation (astrocytosis and microgliosis). We showed that, at high doses, icv-STZ induces severe and acute neurodegenerative lesions in the septum and corpus callosum. The lesions are associated with an inflammation process. They are less severe and more progressive at low doses. The relevance of high and low doses of icv-STZ to mimic dementia and evaluate new drugs is discussed in the final part of this article.

Brain glucose transporter protein 2 and sporadic Alzheimer’s disease

Sporadic Alzheimer’s disease (sAD) is associated with decreased glucose/energy metabolism in the brain. The majority of glucose utilization in the brain appears to be mediated through glucose transporter protein 1 and 3 (GLUT1 and GLUT3). Deficiency of GLUT1 and GLUT3 in the brain has been found in sAD patients post mortem; however this is not unique to the disease as it is associated with different clinical syndromes as well. In line with recent findings that insulin resistant brain state precedes and may possibly cause sAD, an experimental sAD model based on the central application of the streptozotocin (STZ-icv rat model), which is a selective GLUT2 substrate, has drawn attention to the possible significance of the brain GLUT2 in sAD etiopathogenesis. Important steps in the GLUT2 and sAD interplay are reviewed and discussed. It is concluded that increased vulnerability of GLUT2 expressing neurons may be involved in development of sAD.