Neuropharmacology Activity of Alkaloids from South American Medicinal Plants

Abstract

Higher plants, which have served humankind as sources of biologically active molecules since its earliest beginnings, continue to play a key role in the world health. Compounds from higher plants are of great potential value as medicinal agents, as ”leads“ or model compounds for synthetic or semisynthetic structure modifications and optimization, as biochemical and / or pharmacological probes. As a consequence of the renewed interest in the search of new substances from natural sources as potential candidates in the drug development, since 1980 our research group has been involved in investigation of higher plants employed in Italian, Chinese, African and South-American traditional medicine. Our primary objectives are: - to isolate as many secondary metabolites as possible for the phytochemical knowledge of the plants studied - to identify active principles in plants with claimed biological activity - to evaluate pharmacological effects of plant extracts, fractions and pure compounds in relationship to the parent plant material - to subject the isolated compounds to biological screenings on the basis of their structural relationship with known drugs. One of our approach to the study of medicinal plants is the preliminary pharmacological screening of the plant extracts, followed by a bioassay-guided fractionation of the extracts leading to the isolation of the pure active constituents. Such a strategy has been used in the isolation of a number of antispasmodic alkaloids from the extracts of South-American medicinal plants which showed a pronounced inhibitory activity on the electrical induced contractions of isolated guinea-pig ileum (E.C.I.) and on morphine withdrawal. The alkaloids represent the group of natural products that has had the major impact throughout history on the economic, medical, political, and social affairs of humans. Many of these agents have potent physiological effects on mammalian systems as well as other organisms, and as a consequence, some constitute important therapeutic agents. In the plant kingdom, the alkaloids appear to have a restricted distribution in certain families and genera particularly Apocynaceae, Papaveraceae, Ranunculaceae, Rubiaceae, Solanaceae, and Berberidaceae are out-standing for alkaloid-yielding plants. Alkaloids are usually classified according to the nature of the aminoacids or their derivatives from which they are biosynthetized. Our interest has been centered on alkaloids derived from the aromatic aminoacids and in particular on isoquinoline alkaloids (biologically derived from phenylalanine) from Argemone mexicana (Papaveraceae), Aristolochia constricta (Aristolochiaceae) and on alkaloids with an indole nucleus (biologically derived from tryptophane) from Sickingia williamsii and Sickingia tinctoria (Rubiaceae).