Myrtenal mitigates streptozotocin-induced spatial memory deficit via improving oxido inflammatory, cholinergic and neurotransmitter functions in mice

Abstract

The occurrence of chronic neurodegenerative disorders is on the rise, but with no effective treatment due to the paucity of information on the pathological mechanism underlying these disorders. Thus, this study investigated the role of oral administration of myrtenal in mitigating memory deficits and neuro-biochemical alterations in streptozotocin-demented mice model. Mice (n ​= ​35) were randomly allocated into five cohorts consisting of 7 mice each; Group I: Control mice received vehicle alone; Group II: streptozotocin; Group III: streptozotocin + 100 ​mg/kg myrtenal; Group IV: streptozotocin +200 ​mg/kg myrtenal; and Group V: streptozotocin ​+ ​donepezil 0.5 ​mg/kg. Data from this study demonstrated that the administration of streptozotocin (STZ) impaired spatial memory and induced alterations in markers of oxido-inflammatory response, cholinergic function, cytoarchitecture, and neurotransmitter levels in mice hippocampus. Notably, administration of myrtenal enhanced spatial memory performance in STZ-demented mice by improving the activities of endogenous antioxidant enzymes to protect the brain from oxido-inflammatory stress. Treatment with myrtenal also restored cholinergic function and stabilized the homeostasis of neurotransmitters in STZ-demented mice. The authors infer that fruits rich in myrtenal may be beneficial for treating patients living with dementia associated with Alzheimer's disease.

Keywords: Antioxidant; Dementia; Memory; Mice; Myrtenal; Streptozotocin.

Graphical abstract

Fig. 1

Chemical structure and IUPAC name of myrtenal.

Fig. 2

Diagrammatic illustration of the experimental design in the current study. The numbers on the central bar indicate the days when behavioural or treatment interventions were conducted.

Fig. 3

Effect of myrtenal on memory and open-field performance in Streptozotocin (STZ)-induced demented mice (Mean ± SEM, n = 7). Behavioural assessment of mice showing effect of myrtenal on A) memory performance, B) locomotion, and C) frequency of rearing. Memory performance was assessed using the Y-maze Test. Alternation performance was recorded as consecutive entries into all three arms. The percentage alternation was evaluated as total complete alternations/total entries – 2) x 100. Open-field performance was used to assess horizontal locomotor activity and vertical frequency of rearing (vertical counts). Assessing locomotor activity involved placing each mouse in the central point of the box and allowing it to walk freely for 3 ​min and familiarize with the environment. Thereafter, and for the next 2 ​min, the number of squares crossed with all the paws by each mouse was recorded to assess locomotion while rearing was determined by measuring the vertical counts. The significance of the alteration in performance of mice determined using ANOVA, followed by Tukey's multiple comparison test is indicated by asterisks ∗ P ​< ​0.05, ∗∗P ​< ​0.01, ∗∗∗P ​< ​0.001, compared to the STZ group.

Fig. 4

Effect of myrtenal on brain oxidative stress markers in Streptozotocin (STZ)-induced demented mice (Mean ​± ​SEM, n ​= ​7). Following the completion of behavioural experiments, all the mice were anesthetized and sacrificed. Thereafter, brain tissues were instantly removed and dissected to isolate about 1 ​mg of hippocampal tissue which was homogenized in 10 ​mL of ice-cold phosphate-buffered solution (pH 7.0), centrifuged at 4 ​°C and 3000g for 5min. The supernatant was retrieved and used to determine NO and MDA concentrations. The concentration of MDA was assayed spectrophotometrically and the change in absorbance was read at 532 ​nm. The levels of MDA in hippocampal samples were estimated using a molar extinction coefficient of 1.56 ​× ​105 ​M⁻¹ ​cm⁻¹ and expressed as μmoles of MDA/tissue weight in grams. The concentration of NO was assayed calorimetrically using the Griess reaction and thereafter, the absorbance was read at 548 ​nm. The assayed nitrite levels in each sample were extrapolated from the standard curve of sodium nitrite (0–100 ​μM). The significance of the change in concentration determined using ANOVA, followed by Tukey's post-hoc test is indicated by asterisks ∗ p ​< ​0.05, ∗∗p ​< ​0.01, compared to the Streptozotocin group.

Fig. 5

Effect of myrtenal on brain antioxidant enzymes activity in STZ-induced demented mice (Mean ± SEM, n = 7). Activities of SOD, GPx, and CAT were estimated using hippocampal tissues and significance of the change in concentration was determined using ANOVA, followed by Tukey's posthoc test is indicated by asterisks ∗ p ​< ​0.05, ∗∗p ​< ​0.01, ∗∗∗p ​< ​0.001, compared to the STZ group.

Fig. 6

Effect of myrtenal on brain acetylcholinesterase activity and TNF-α in STZ-induced demented mice (Mean ± SEM, n = 7/group). Activities of acetylcholinesterase and TNF-α was estimated in hippocampal tissues and the significance of the change in concentration determined using ANOVA, followed by Tukey's posthoc test is indicated by asterisks ∗ p ​< ​0.05, ∗∗p ​< ​0.01, ∗∗∗p ​< ​0.001, compared to the STZ group.

Fig. 7

Photomicrograph showing the mitigating effect of myrtenal on the hippocampus of STZ-demented mice stained with H & E. Stars indicate neuronal degeneration and loss of neuronal cells in STZ treated mice (X20).

Fig. 8

Correlation of MDA concentration and memory performance in STZ-demented mice (Mean ± SEM, n = 7/group). r ​= ​Pearson's correlation coefficient, p ​< ​0.05 was considered significant.

Fig. 9

Schematic representation of the possible underlying mechanism of memory enhancement by myrtenal.