SUMMARY

Impaired brain insulin signaling contributes to the progress and development of Alzheimer’s disease (AD). AD is a debilitating neurodegenerative disorder identified by progressive loss of neurons, cognition decline, and memory deficits. Beta-amyloid (Aβ) accumulation in the brain is the most common pathological feature of AD. Many animal and human studies indicate that insulin enhances synaptic spine formation and influences the clearance Aβ plaques. Insulin is now considered a novel potential therapeutic strategy for the treatment of AD. Intranasal (IN) administration, transports insulin to the brain from the nasal cavity along the olfactory and trigeminal nerves without impacting peripheral blood glucose levels.

In the current study, two main experiments were carried out. First experiment aimed at comparing the effects of scopolamine (SCO), D-galactose (D-gala), and aluminum chloride (AlCl₃) on body weight, some blood parameters, biochemical parameters, memory performance, histological alterations of the rat hippocampus, as well as selecting the best model for inducing Alzheimer’s-like disease. Twenty-four male adult albino rats (250–300g) were used in this study, which were randomly assigned into four groups (6 rats in each) as the follows: the first group (control) rats were given normal saline; the second group (SCO) rats were given (2 mg/kg) of scopolamine; the third group (D-gala) rats were given of (125 mg/kg) of D-galactose; the fourth group (AlCl₃) rats were given (50 mg/kg) (AlCl₃). The chemicals were given intraperitoneally (I.P) for 30 consecutive days. At the end of the period of treatments, all rats were subjected to novel object recognition (NOR) and Barnes maze (BM) tests to assess memory performance and open field (OF) test for locomotor activity. After finishing behavioral tests, all the rats were anesthetized, blood samples were collected for hematological and biochemical examination, and the brain was swiftly removed to examine the hippocampal structure. In addition, morphometric analysis was performed by counting cells in the CA1, CA3, and DG regions of the hippocampus and measuring the thickness of layers. Neuronal damage in the SCO, D-gala, and AlCl₃ groups was compared with the control group.

The results demonstrated that SCO, D-gala, and AlCl₃ caused memory impairment by increasing latency time and time to find the escape box in the BM test and by significantly decreasing the discrimination index (DI) in the NOR test. However, in the OF test, there were no significant changes in general locomotor activity. A significant decrease in body weight gain was observed in group treated with AlCl₃ as compared with the control, SCO, and D-gala groups. Blood parameters such as RBC and HGB were shown a significant decrease and WBC significant increase in AlCl₃ group. Biochemical analysis showed a significant decrease in Ca²⁺ in the AlCl₃ treated group and a significant decrease in the D-gala group as compared with the control and SCO groups. Malondialdehyde (MDA) levels significantly increased in SCO, D-gala, and AlCl₃ groups. Moreover, histological analysis showed degenerative pyknotic cells in the hippocampal CA1, CA3, and DG regions with Aβ deposition. Regarding morphometric results, a significant decrease in the number of intact neurons in SCO, D-gala, and AlCl₃ groups were observed. These histopathological changes are signs of neurodegeneration. It was concluded that SCO (2 mg/kg) is the best model for mimicking AD-like disease among D-gala and AlCl₃ by enhancing histopathological alternations and declining memory in rats without significantly affecting blood and electrolyte parameters.

The second experiment: The purpose of this experiment was to evaluate the effects of short-acting insulin (Humalog) and long-acting insulin (Levemir) on spatial learning and memory and hippocampal structure in the SCO model of AD in rats. Twenty-four male adult rats (250–300g) were used in this study, which were randomly divided into four groups (6 rats in each) as follows: In the first group (control), rats were given normal saline I.P, and after that, the same rats were given normal saline intranasally (IN). Second group (SCO): Rats were given SCO (2 mg/kg) I.P and after that, the animals given normal saline IN. Third group (SCO+H): Rats were given SCO (2 mg/kg) I.P, and after that, the animals were given Humalog (H) (2 IU) IN.  Fourth group (SCO+L): Rats were given SCO (2 mg/kg) I.P and after that, the animals were given Levemir (L) (2 IU) IN. All treatment was carried out for 30 consecutive days. Memory performance was measured by NOR, BM, and OF tests for locomotor activity. After finishing behavioral tests, the rats were anesthetized, and blood was collected for oxidative stress analysis. Brains were rapidly removed for histological and histochemical studies.

The results showed that SCO caused memory impairment by decreasing DI and increasing escape latency in the behavior tests. Reduced glutathione (GSH), superoxide dismutase (SOD), significantly decreased and malondialdehyde (MDA) serum levels significantly increased. Histological analysis showed degenerative pyknotic cells in the hippocampus with Aβ plaque deposition and decreased number of healthy neurons in SCO-treated group. These histopathological changes are signs of neurodegeneration. Intranasal (IN) insulin administration of Humalog and Levemir prevented the memory decline and hippocampal structure damage as well as prevented decrease in the number of healthy neurons caused by SCO. IN insulin prevented aggregation of beta amyloid deposition. Furthermore, a significant increase in the levels of SOD and GSH and a significant decrease in serum MDA were observed in groups treated with IN insulin compared to SCO group. The results suggest that IN delivery prevents cognitive function decline, oxidative stress damage, and histopathological changes in hippocampal structure without changing peripheral blood glucose levels.