Histopathological Evaluation of The Neuroprotective Effect of Andrographolide–Piperine Combination in Streptozotocin-Induced Alzheimer’s Disease

Abstract

Streptozotocin (STZ) is a glucosamine-nitrosourea derivative that can interfere with insulin-mediated signalling in the brain and disturb neuronal glucose utilisation. These alterations are associated with progressive cognitive dysfunction and several pathological features resembling Alzheimer’s disease (AD). When administered intracerebroventricularly (ICV), STZ produces central metabolic and neuronal disturbances, including increased amyloid-β (Aβ) deposition and impairment of learning and memory. Andrographolide (ANDRO), a naturally occurring diterpenoid obtained from Andrographis paniculata, has attracted attention because of its multiple pharmacological effects, particularly its antioxidant, anti-inflammatory, anti-cholinesterase, and neuroprotective activities. Therefore, the present investigation was designed to determine whether ANDRO could protect against the behavioural and neurochemical abnormalities produced by ICV-STZ in Wistar rats. For induction of the experimental AD-like condition, the animals were administered STZ (3 mg/kg) twice through the ICV route. Following model induction, ANDRO was administered orally at doses of 15, 30, and 60 mg/kg, whereas donepezil (5 mg/kg, orally) was used as the standard treatment for 14 consecutive days. Behavioural performance was monitored at predetermined weekly intervals to assess the progression of cognitive and functional deficits. At the end of the experimental period, i.e., on day 21, the animals were sacrificed and hippocampal tissue was collected for biochemical and neurochemical analyses. The investigated parameters included malondialdehyde (MDA), nitrite, reduced glutathione (GSH), superoxide dismutase (SOD), catalase, and acetylcholinesterase (AChE). The hippocampal concentrations of the inflammatory mediators interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumour necrosis factor-alpha (TNF-α), together with the neurotransmitters glutamate and γ-aminobutyric acid (GABA), were also determined. In addition, amyloid-β1-42 (Aβ1-42) and phosphorylated tau (p-tau) were quantified to assess AD-associated pathological alterations. The experimental findings demonstrated that exposure to ICV-STZ resulted in pronounced behavioural impairment accompanied by disruption of hippocampal oxidative balance, cholinergic function, inflammatory status, and neurotransmitter homeostasis. Administration of ANDRO ameliorated these STZ-associated alterations, indicating a protective influence on both behavioural and biochemical outcomes. ANDRO treatment also produced a reduction in the hippocampal burden of Aβ1-42 and p-tau, suggesting that its protective activity may extend beyond symptomatic improvement to modulation of important pathological features associated with AD-like neurodegeneration. Overall, the histopathological findings indicate that ANDRO exerts a protective effect against ICV-STZ-induced hippocampal neuronal degeneration and loss. The improved preservation of hippocampal neuronal architecture, particularly following higher-dose ANDRO treatment and ANDRO–piperine combination therapy, supports its potential neuroprotective role in experimental AD-like neurodegeneration. These findings warrant further investigation of ANDRO, alone and in combination with piperine, as a potential phytotherapeutic approach for attenuating structural neuronal damage associated with AD-like pathology.