Plants as a Source of Useful Therapeutic Agents in Cognitive Diseases

Numerous plants are reputed in traditional practices of medicine to alleviate the cognitive decline that can be associated with general ageing, but they may also be relevant in the treatment of specific cognitive disorders such as Alzheimer’s disease and other dementias. Thus, plants reputed to have ‘anti-ageing’ or ‘memory-enhancing’ effects could also be considered for their potential efficacy in disorders now recognised to be associated with cognitive dysfunction, including those conditions in which dementia occurs.

A mixture of plants is commonly prescribed in some practices of traditional medicine including Ayurveda and traditional Chinese medicine (TCM). The plant constituents may not only act synergistically with other constituents from the same plant, but they may also enhance the activity of compounds from other plants in a particular remedy or herbal formula. For example, the interaction of a compound at a target receptor may affect the activity of another compound at that receptor, possibly due to allosteric effects, which can occur at some types of receptor. An ethnopharmacological approach can assist with the search for plants and, eventually, potential new drugs that could be relevant for the treatment of cognitive disorders, including Alzheimer’s disease.

There are numerous examples of plants used in various traditional practices of medicine which have a reputation for influencing cognitive functions. However, relatively few of these plants have been investigated to establish any scientific basis for their reputed effects. The plants described below are mainly those which have stimulated an interest in establishing a pharmacological basis for the reputed effects. It should also be considered that there are many other traditional medicines which have yet to be investigated for a scientific basis to explain their traditional uses and many that have shown interesting biological activity in some studies but which have not been investigated extensively. Although some of these remedies have been promoted as ‘alternative’ or ‘complementary’ therapies, in many instances, there is a lack of substantial evidence for their efficacy and safety or their potential interactions with other medicines. Some of these remedies may contain compounds which are more active when combined in a mixture than when isolated and used alone. Thus, the use of a plant extract may be preferred to single isolated constituents. In addition, in a mixture such as an extract, there may be a variety of compounds with polyvalency, i.e. the different compounds present act in a number of different but relevant ways, and at different molecular targets to produce the overall pharmacological effects for treating a particular condition.