Impact of Psidium Guajava Leaf Extract on Phosphorylated Tau Protein in the Hippocampus of Scopolamine-Induced Alzheimer's Disease Wistar Rats

The study investigated the effect of Psidium guajava leaf on tau protein accumulation in the CA3 region of the hippocampus of Alzheimer’s disease scopolamine-induced rats. P. guajava leaves were obtained from a botanical garden in Agada 1 Abua, Rivers State, Nigeria. Leaves were washed in running tap water, air dried in shady place for one week then chopped into pieces with the aid of a knife. The chopped leaves were ground into fine particles by means of a mortar and pestle, then extracted using methanol.  The study was done in two phases, phase one (acute) lasted for 14 days and phase two (chronic) 84 days. 80 male Wistar rats with weight 100 - 230 g were divided into 5 groups (n = 8). Group 1 served as normal control, group 2 received 1 mg/kg b.wt. i.p. of scopolamine plus distilled water, group 3 received 1 mg/kg scopolamine plus 100 mg/kg b.wt. of P. guajava leaf extract, group 4 received 1 mg/kg scopolamine plus 300 mg/kg b.wt. of P. guajava leaf extract and group 5 received 1 mg/kg scopolamine plus 5 mg/kg donepezil. The phytochemical screening and quantification of the bio-constituents of P. guajava leaf indicated the presence of flavonoids, alkaloid, tannins, saponin, glycoside, carbohydrate and anthraquinone; with flavonoid being the most abundant and anthraquinone the least. Also the methanol proved to be a better solvent for the extraction of the phytochemicals in P. guajava leaf.  At the end of the experiment in the two phases, the rats were euthanised with 100 mg/kg b.wt. of ketamine (i.p.), decapitated to remove the brain. Brain tissues for brain-derived neurotrophic factor (BDNF) ELISA assay were rinsed in normal saline then homogenised in phosphate buffer using ceramic mortar and pestle placed on ice then transferred to the lab in ice pack. Brain tissues for histological study were fixed in 10 % neutral buffered formalin. The hippocampus was excised out, sliced into 10 µm thick slides using a microtome and stained in tau antibody immunohistochemistry. Both doses of P. guajava leaf extract significantly reduced tau accumulation in the CA3 region of the hippocampus. However, the 100 mg/kg b.wt. dose had better efficacy as it demonstrated high ameliorative ability comparable to the normal control rats in group 1 in both phases of the study, while the 300 mg/kg b.wt. performed poorly in the acute phase but improved its attenuative effect in the chronic study indicating a time dependent benefit. The extracts also significantly (p < 0.05) increased the BDNF level. Donepezil also improved the level of BDNF and reduced phosphorylated tau but not as compared with the extract.