In 1975 I began to be interested in the study of the active principles of African medicinal plants and searched for relevant literature. At the time I was particularly interested in receiving more information on West African plants as we were developing experimental work in collaboration with the Chemistry Department of the Nigerian University of Nsukka and I became aware of the great difficulty in finding fairly reliable documentation even on some of the best-known traditional herbal remedies.

Factors which may account for this are that the local uses were very numerous and often differed from one tribe, village or healer to another. Also, not only did superstition play an important part (often both magical purposes and empirical beliefs were attributed to the plants) but purgatives, diuretics and emetics were often used to chase the evil influences the people did not understand.

The patient work of some distinguished scholars working in the field in Africa provided an important contribution to our acquisition of knowledge on the traditional uses but only a few publications gave a more selective view on the subject. Among these one of the most relevant to me was the book of Dr Bep Oliver (Oliver-Bever), Medicinal Plants in Nigeria. She selected uses which were confirmed by the use of the same plants as cures by primitive populations in other parts of the world with similar climate, and also those which were likely to have real therapeutic value from a consideration of the then known chemical and pharmacological information.

Introduction: the action of plants on hormone secretion in Man

A number of plant constituents can act as a substitute for natural hormones in cases of hormone deficiency. Some of these constituents have been found to be chemically identical with the natural hormone (e.g. oestrone in the kernels of date and oil palm). Other plant components have a structural similarity to the hormone (e.g. Funtumia and Holarrhena alkaloids and corticosteroids). Others again have an entirely different chemical structure (e.g. coumestans and isoflavones with oestrogenic action). All these constituents can act as a substitute for hormones and replace them in their biological functions.

Plant components often have an indirect action on the secretion of certain hormones by stimulating or inhibiting other areas like the hypothalamus and the pituitary gland which can control the function of most other glands (e.g. gonadotrophic, thyrotrophic and corticotrophic action). (For more details see Plants acting on sex hormones.) However, great precaution should be taken in using the hypothalamus–pituitary axis as a narrow interrelationship exists and often hormonal or pharmacological actions other than those desired can be obtained (Bianchi, 1962; Goodman and Gilman, 1976). Thus stimulation of lactation has been obtained with dried thyroid gland or thyroxine (Robinson, 1947; Naish, 1954), no doubt through indirect action via the pituitary gland and an impairment of the pituitary adrenal response to acute stress is observed in alloxan diabetes (Kraus, 1949).

Sex hormones

Female sex hormones

Oestrogens are the hormones concerned with the maturation of the female genital tract, the development of the secondary sexual characteristics and bone formation. The main oestrogens, oestradiol and oestrone, are secreted by the theca interna and appear in the fluid of the Graafian Follicles. Oestrogens are also secreted by the placenta and to a much lesser degree by the adrenal cortex. They are rapidly metabolized in the liver.

Progesterone is the hormone concerned with the maintenance of pregnancy. It is secreted by the corpus luteum and acts only on tissues formerly sensitized by oestrogens. It inhibits ovulation during pregnancy, depresses the action of oestrogens and produces further development of the breasts. It also plays a role in the development of the placenta and depresses uterine contractility.

The production of steroid hormones is controlled by the pituitary gonadotrophins (follicle-stimulating and luteinizing hormones). In large doses oestrogens can depress the gonadotrophic and lactogenic activities of the anterior pituitary gland. Because of their rapid metabolism by the liver, oral administration of the natural oestrogens is less effective than parenteral administration, with the possible exception of oestriol, which is claimed to be as potent when given orally as when given by injection.

The oestrogens are used to treat menopausal disturbances and cases of dysmenorrhoea and menorrhagia. Progesterone is used chiefly in the treatment of functional uterine haemorrhagia.

Insulin, a hormone secreted by the β-cells of the Islets of Langerhans in the pancreas, is of utmost importance for the correct metabolism of carbohydrates and fats. It induces the oxidative breakdown of glucose, has a stimulating effect on the synthesis of liver glycogen from glucose and inhibits the formation of liver glycogen from protein and fat.

Diabetes, a condition characterized by hyperglycaemia, is caused by insufficient secretion of insulin or to insufficiency of its peripheral efficacy. Excess of insulin leads to hypoglycaemia; this is easily counteracted if dextrose or a few lumps of sugar are taken at once.

Modern investigations into the biochemistry of diabetes show that its causes and the sites of intervention in the biochemical processes are diverse. Somatostatin, the pituitary and sex hormones, corticosteroids, prostaglandins and vascular modifications of the pancreas can all be involved, together with a straightforward inadequacy in insulin production (Randle et al., 1963; Gupta et al., 1966; Burkhard et al., 1968).

A number of plants have constituents which have antidiabetic properties when taken orally (Oliver-Bever and Zahnd, 1979). There is great diversity in the nature and action of these constituents, but a number of them do seem to belong to certain chemical groups, such as sitosterol glycosides, alkaloids, sulphur oils and flavonoids, and in this chapter I have assembled the plants described into a few groups based on their possible active chemical constituents while at the same time taking into consideration their relation to, or identity with, hypoglycaemic plant constituents found elsewhere.

This book is a sequel to the monograph Medicinal Plants in Nigeria, written in 1960 (Oliver, 1960), which was a critical survey of the scattered information available about drug plants found in Nigeria; it suggested a first choice of the plant material which seemed potentially most important, and made suggestions concerning points requiring further scientific investigation (constituents, pharmacology, etc.).

As medical science develops and becomes more organized in the West African countries, the time would seem to have come to reassemble and update our knowledge of the subject and extend it to the whole of tropical West Africa. Furthermore, greater importance is now being attached to the use of locally available medicines as a means of reducing reliance on expensive imported drugs.

Since the first book appeared, a number of papers dealing with the chemical analysis, pharmacology and clinical action of West African plants have been published. Supplementary information now available about individual plants will be included here, and the range of plants considered can thus be more selective. This time an attempt is made to classify the drugs according to their established or possible medical uses, this being the best way of rapidly assessing the medical interest of any particular drug.

The value of a drug will depend on several factors:

1. (1) whether it is the only drug, or one of the few drugs, used in the treatment of a disease;

2. (2) whether the disease in the treatment of which it is used is a common one;

3. […]

Plants in tropical West Africa with an action on the cardiovascular system

In the particular field of cardiovascular drugs, plants still provide the basis of treatment, even in orthodox pharmacy. However, some of the plants accepted by most Pharmacopoeias, such as Digitalis, Convallaria, Adonis, Helleborus and Crataegus, which act mainly on the heart, and Hydrastis, Veratrum, Amni visnagi, Viscum album and Aesculus hippocastanum, which act more specifically on the blood vessels, do not grow in West Africa. On the other hand, the possibilities of many plants that are locally available have not yet been fully investigated. Also, some of the currently used cardiotonics have a high toxicity; less toxic but yet active constituents might be found amongst the West African plants. As mentioned in the general introduction only a limited number of local uses have been indicated.

Most herbalists will know that many plants in this group (several formerly used as arrow poisons or even in ordeals) are very toxic and will avoid using them. A few healers, however, may, in view of the fact that they are also emetics, purgatives or diuretics, be tempted to make use of them. But these plants should be employed only after complete extraction and with very exact dosages of the active constituents, and then only by physicians in possession of a full clinical diagnosis. In this, these plants differ from many others, which may be given as a decoction, an infusion or in dried or powder form.

Hormones have important regulatory roles in pregnancy from the moment of ovulation and fertilization to the delivery of young; in this chapter we shall be concerned with how hormones regulate the maternal adjustments to pregnancy, and how their secretion is controlled to meet the needs of uterine gestation. When viviparity emerged as the preferred mode of reproduction in eutherian and marsupial mammals, hormones were exploited to control the physiological adjustments of the mother. The success of viviparity in eutherians and marsupials was due to several important features: reduction in the yolk content of the eggs, development of the placenta, retention of the young within the female genital tract, and parental care of the offspring, especially including their nutrition. Such functions presented problems for maternal homeostasis, and one of the ways by which these were solved involved the exploitation of pre-existing molecules (steroids and polypeptides) as regulators of organ function. The sex hormones and gonadotrophins of mammals are widely distributed in Nature and are also involved in reproduction in many non-mammalian species. A study of the role of hormones in pregnancy therefore provides an exciting opportunity to see the uses to which these conserved molecules have been put in the evolution of viviparity.

Endocrine regulation of pregnancy primarily involves hormones of the pituitary, the ovary, and the placenta, and a distinction can be drawn between these organs and those, such as thyroid, adrenal and parathyroid, whose role is more of a supportive or permissive one.

Unlike the testes, the ovaries of all mammals remain within the abdominal cavity where they are well protected from injury by external agents. The ovary is subdivided into a series of specialized compartments or structures, each with its own precisely regulated micro-environment. In this way the oocytes can be nursed through from the start of oogenesis until ovulation (see Book 1, Chapters 1 and 2, Second Edition). The endocrine function of the ovary ensures the regular production of healthy oocytes at a time when they will have a maximum chance of being fertilized. Hence, although it is often convenient to consider the oogenic and endocrine functions of the ovary separately, the two are intimately interconnected. An ovary devoid of oocytes cannot function normally as an endocrine gland.

In the mature animal the structure and function of the ovary is continually changing. Gonadotrophins secreted by the anterior pituitary gland stimulate the growth of Graafian follicles (folliculogenesis), ovulation, and the formation of corpora lutea. The time taken for follicles and corpora lutea to develop differs from species to species and is reflected in different patterns of ovarian cycles. Hence while ovarian function is closely regulated by a feedback system involving the hypothalamus and anterior pituitary, the ‘zeitgeber’ or biological clock which determines the length of the cycle is the ovary itself.

Morphology of the ovary

The ovaries are paired organs situated within the abdominal cavity and covered by a single layer of surface epithelium, formerly misleadingly called the germinal epithelium, which is continuous with the lining of the peritoneal cavity.