Chronic Administration of Hesperidin improves working, but not reference memory in Rat Model of Alzheimer’s disease

Technical Summary: Chronic Administration of Hesperidin in a Rat Model of Alzheimer's Disease

Problem Statement
Despite extensive research, there remains no appropriate or reliable treatment for Alzheimer's disease (AD). The disease is characterized by memory loss, extracellular amyloid β accumulation, intraneuronal hyperphosphorylated tau, neuroinflammation, and oxidative stress. Aging, a primary risk factor for AD, is associated with elevated reactive oxygen and nitrogen species, mitochondrial dysfunction, and cognitive decline. Consequently, there is a critical need for novel therapeutic interventions, particularly those utilizing natural antioxidant components like flavonoids to rebalance the oxidant-antioxidant status in the brain. This study aimed to evaluate the effectiveness of Hesperidin (Hsp), a bioflavonoid found in citrus fruits, on cognitive decline and Brain-Derived Neurotrophic Factor (BDNF) levels in the hippocampus within an AD model.

Methodology
The study utilized 32 male Wistar rats (3 months old, 200–250 g) divided into four groups (n=8 per group): Saline+Saline, Saline+Hsp, STZ+Saline, and STZ+Hsp.

• AD Induction: Alzheimer's disease was modeled via intracerebroventricular (i.c.v.) injection of Streptozotocin (STZ) at 3 mg/kg (1.5 mg/5 µl/side) on days 1 and 3 post-surgery.
• Treatment: The STZ+Hsp group received daily intraperitoneal (i.p.) injections of Hesperidin (50 mg/kg) for 21 consecutive days. Control groups received saline.
• Behavioral Assessment: Spatial memory was evaluated using the Morris Water Maze (MWM). The protocol included four blocks of 16 trials to assess working memory (acquisition phase) and a 90-second probe test to assess reference memory. Parameters measured included latency to the platform, total time spent in the target quadrant (TTS), distance traveled, and swimming speed. A visual test was conducted to rule out visual or locomotor deficits.
• Biochemical Analysis: Following behavioral testing, hippocampi were dissected.
  • BDNF: Levels were quantified via Western blotting, measuring the ratio of BDNF to β-actin.
  • Glutathione (GSH): Intracellular GSH levels were determined using the DTNB (Ellman's reagent) colorimetric method.
• Statistical Analysis: Data normality was assessed using the Kolmogorov–Smirnov test. One-way ANOVA with Tukey's post hoc tests and Two-way ANOVA were used for analysis, with significance set at P < 0.05.

Key Results

• STZ Effects: STZ injection significantly increased escape latency and reduced hippocampal BDNF levels (P < 0.001) compared to healthy controls. It also significantly reduced hippocampal GSH levels (P = 0.003).
• Hesperidin in Healthy Controls: Chronic Hsp administration significantly improved both working and reference memory in healthy rats. This was evidenced by reduced latency to the platform and increased TTS in the probe test (P < 0.05). Furthermore, Hsp significantly increased hippocampal BDNF levels in the healthy control group.
• Hesperidin in AD Model (STZ-treated):
  • Working Memory: Hsp treatment significantly reduced escape latency during the acquisition phase (blocks 2 and 4) compared to the STZ+Saline group (P = 0.001), indicating an improvement in working memory.
  • Reference Memory: In the probe test, Hsp treatment did not significantly restore the total time spent in the target quadrant (TTS) compared to the untreated STZ group. Thus, reference memory was not improved.
  • BDNF Levels: Unlike in healthy controls, Hsp administration failed to significantly increase hippocampal BDNF levels in the AD group (P = 0.67).
  • Antioxidant Status: Hsp treatment significantly restored intracellular GSH levels in STZ-treated animals (P = 0.002).

Significance and Conclusions
The study concludes that chronic administration of Hesperidin facilitates working memory but fails to improve reference memory in a sporadic AD rat model induced by STZ. While Hsp successfully restored antioxidant capacity (GSH) in the AD model, this was insufficient to normalize reference memory or elevate hippocampal BDNF levels in the diseased state.

The authors suggest that the differential effects on memory types may stem from distinct underlying mechanisms: working memory relies on short-term synaptic plasticity and prefrontal cortex interactions, which may benefit from Hsp's antioxidant and blood-flow enhancing properties. In contrast, reference memory relies heavily on hippocampal CA1 place cells, late-LTP, and BDNF elevation, which were not restored by the 21-day Hsp treatment in the AD model.

The paper posits that while restoring antioxidant capacity is beneficial, it is not sufficient to normalize complex cognitive deficits in multifactorial neurodegenerative contexts like AD. The authors note that the lack of BDNF elevation in the hippocampus of AD animals supports the insignificant effect on reference memory. They emphasize that the failure to improve reference memory in this model aligns with some previous studies but contrasts with others using genetic AD models, likely due to differences in AD induction methods (toxin vs. genetic) and treatment duration. The study highlights the need for future research involving longer treatment durations, dose-response studies, and region-specific molecular assays (e.g., prefrontal cortex BDNF, pro-BDNF vs. mature BDNF ratios) to fully elucidate the therapeutic potential of Hesperidin.