Anticholinesterase Activity of Selected Medicinal Plants from Navarra Region of Spain and a Detailed Phytochemical Investigation of Origanum vulgare L. ssp. vulgare

Alzheimer’s disease is a neurodegenerative disease characterized by progressive memory loss and cognitive impairment due to a severe loss of cholinergic neurons in specific brain areas. It is the most common type of dementia in the aging population. Although many anti-acetylcholinesterase (AChE) drugs are already available on the market, their performance sometimes yields unexpected results. For this reason, research works are ongoing to find potential anti-AChE agents both from natural and synthetic sources. In this study, 90 extracts from 30 native and naturalized medicinal plants are tested by TLC and Ellman’s colorimetric assay at 250, 125 and 62.5 μg/mL in order to determine the inhibitory effect on AChE. In total, 21 out of 90 extracts show high anti-AChE activity (75–100% inhibition) in a dose-dependent manner. Among them, ethanolic extract from aerial parts of O. vulgare ssp. vulgare shows an IC50 value 7.7 times lower than galantamine. This research also establishes the chemical profile of oregano extract by TLC, HPLC-DAD and LC-MS, and twenty-three compounds are identified and quantified. Dihydroxycinnamic acids and flavonoids are the most abundant ones (56.90 and 25.94%, respectively). Finally, total phenolic compounds and antioxidant properties are quantified by colorimetric methods. The total phenolic content is 207.64 ± 0.69 µg/mg of extract. The antioxidant activity is measured against two radicals, DPPH and ABTS. In both assays, the oregano extract shows high activity. The Pearson correlation matrix shows the relationship between syringic acids, a type of dihydroxybenzoic acid, and anti-AChE (r(2) = −0.9864) and antioxidant activity (r(2) = 0.9409 and 0.9976). In conclusion, the results of this study demonstrate promising potential new uses of these medicinal herbs for the treatment of Alzheimer’s. Origanum vulgare ssp. vulgare and syringic acids, which have anti-AChE activity and beneficial antioxidant capacity, can be highlighted as potential candidates for the development of drugs for the treatment of Alzheimer’s disease and other diseases characterized by a cholinergic deficit.