Fig. 46. Examples of GABAergic natural products from flowering plants.
GABAergic presynaptic inhibition of excitatory transmission of primary afferent neurones of the spinal cord resulting in epileptiform convulsions, myosis, and dyspnea with more or less prolonged apnea.
One of the most spectacular applications in this field is the development of a GABAa receptor agonist: the benzodiaxepine. About 15 types of benzodiazepine derivatives are currently available in the United States and marketed as sedatives, anxioly-tics, muscle relaxants, intravenous anesthetics, and anticonvulsants. Anxiolytics known as benzodiazepines, which bind to a very specific region of GABAA receptors. However, benzodiazepines are sedatives that induce serious memory impairment, drowsiness, and dependence, and there is a need for agents with lighter side effects. Experimental observations have led to the suggestion that the etiology of Huntington's chorea, epilepsy, and Alzheimer's disease could be linked to the GABAergic system. The search for GABAA agonists appears, therefore, as a very exiting quest, and one looking for such agent might look into the medicinal flora of the Asia-Pacific region.
A classic example of a medicinal plant of Asia and the Pacific with GABAergic properties is Piper methysticum Forst. (Kava, British Pharmaceutical Codex, 1934), or kava-kava, the rhizomes of which have been used since a very early period of time by Polynesians to allay anxiety and reduce fatigue. Kava has been marketed in Europe to treat sleep disorders and anxiety. The beverage normally induces a form of euphoria, described as a happy state of complete comfort and peace, with ease of conversation and increased perceptivity, followed by muscular fatigue and restful sleep. Klohs et al. identified the active constituents as series of a-pyrone including kawaine, which mediates anxiolytic effects through GABAa receptor binding (34,35).
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