The nervous system

In this chapter are described plants having a direct action on the central nervous system (CNS) and those plants that are used in the treatment of mental and nervous diseases, including those acting via the cholinergic and adrenergic systems.

The CNS, which is best considered as a whole, controls all sensory and integrated motor activity. Plants which interefere with its function may be classified as follows: I CNS stimulants Analeptics, stimulants Antidepressants, hallucinogens II CNS depressants

General anaesthetics, narcotic analgesics Analgesic-antipyretics Hypnotics, sedatives and tranquillizers Anticonvulsants and antiepileptics

III Peripherally acting depressants of the CNS Local anaesthetics (on sensory nerves)

Neuromuscular blockers (curare action) (on motor nerves) and anticonvulsants

IV Those with cholinergic and adrenergic actions

Depressants acting on both autonomic nervous system (ANS) and CNS terminals

Antispasmodics acting mainly on sympathetic terminals Stimulants of the cholinergic and adrenergic systems However, in practice no such clear classification can be made. Even a single plant constituent can act on both the CNS and the ANS. The plants are therefore grouped by the main resulting effect when both the CNS and ANS are involved. Also, most plants have many different constituents and these often have divergent actions (Table 3.1). In addition only a restricted number of the constituents may be known and pharmacological screening has in many cases been carried out on extracts containing several constituents in varying proportions or doses. An arbitrary decision may therefore have to be taken, in many cases based on our very partial knowledge about the plants, in considering the quantitatively and qualitatively most important constituent(s) of the plant and the most frequently reported effects.

I still consider it important to classify the plants by their clinical indications, in spite of the fact that the greatest amount of research is still to come, to keep in mind the aim, and to see rapidly and more clearly which of the far too numerous utilizations of each plant by local healers appear the most important and are confirmed by scientific observations. The enormous treasure of natural remedies should not be used in a haphazard way. A simple extraction might in some cases enable a healer to increase or isolate a fraction with a certain action or to eliminate a toxic constituent. The experience with the same plants in similar climates even in different continents should not be ignored (although the amount or quality of the constituents can vary and should be checked).

We saw in mentioning their mode of action in the Introduction (Chapter 1) that the activity of drugs acting via the nervous system can be based on their interference with the chemical mediators of nervous transmission (acetylcholine and catecholamines) at their receptor level. Anticholinergic action can be localized:

(1) at the level of the parasympathetic terminals, thus producing antispasmodic and antisecretory activities (used to prepare for anaesthesia) and mydriasis in ophthalmology (atropine action);

(2) at the level of the sympathetic and parasympathetic ganglia;

(3) at the level of the neuromuscular junctions (curare action).

On the other hand, the drugs can interfere with the catecholamines (noradrenaline and adrenaline) or the chemical intermediates in their synthesis (e.g. dopamine) in the post-ganglionic sympathetic nerves and their terminals. This action is called 'adrenergic' action. The catecholamines are also involved in central activity. In this field, the so-called 'antidepressant' drugs, for example, are supposed to act in correcting the basic biochemical trouble of the depression which could be the insufficiency of noradrenaline at the level of the encephalic synapses through degradation of cerebral catecholamines. The enzyme responsible for intracellular degradation of catecholamines is monoamine oxidase (MAO). The activity of this enzyme is blocked by a group of drugs called monoamine oxidase inhibitors (MAOIs), often found in the Rubiaceae (Table 3.2), which thereby inhibit the inactivation of noradrenaline. Unfortunately, these prevent not only degradation of cerebral catecholamines but also that of the catecholamines of the peripheral sympathetic system and of certain substrates. Thus, the biochemical properties of the MAOIs explain their long-lasting action, the numerous secondary effects observed and the diversity of the accidents produced. Their effects are very difficult to control and they are now seldom employed for psychiatric treatments. They cannot be taken together with alcohol, or with food including biogenic amines (especially tryptamine) or amino glucosidic antibiotics, in order to avoid attacks of hypertension, nausea and cephalgia (Lechat et al., 1978; Goodman and Gilman, 1980).

Table 3.1. More important constituents and divergency ofpharmacology of West African Menispermaceae speciesa

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