In their primary environment, the main function of most (if not all), naturally-occurring psychoactive chemicals is to act as defensive molecules for the organisms that made them.
Some of these compounds include four molecules that are quite addictive drugs to humans: caffeine, nicotine, cocaine and morphine.
Caffeine, Nicotine, Cocaine and Morphine: Addictive Alkaloids
All four compounds are alkaloids, which simply mean that there is a nitrogen atom as part of a ring structure. You may have noticed that all their names end in “-ine”; that’s another clue to their nitrogen content.
Alkaloids are full of surprises. For example, did you know that there are measurable amounts of nicotine not only in tobacco plants, but in a rather wide variety of other plants as well? Some of these unexpected sources of nicotine include potatoes, eggplant, cauliflower, and tomato; in fact, most plants belonging to the Solanaceae family will produce some nicotine.
However, do not fear. A 1993 paper published in the New England Journal of Medicine estimated that you’ll have to eat an average of 140, 10, 160, and 94 pounds of potatoes, eggplant, cauliflower, and tomato respectively, to get the same amount of nicotine that you would get via secondhand smoke in about three hours (if you are curious, the amount is one microgram). Tobacco plants are still the main source of nicotine, of course.
As for cocaine, their distribution in nature is much more limited. Cocaine is only found in some 200 species of plants that belong to the genus Erythroxylum. You will only normally find these plants in South America and oddly enough, in the island of Madagascar!
Caffeine is the most widely used psychoactive substance, bar none, and if I may, it is my favorite. In addition to proper coffee plants, other natural sources of caffeine include cacao, tea leaves and guarana berries.
Finally, many people consider morphine, primarily extracted from opium poppy seeds, as the first active principle (a specific compound responsible for a physiological effect) ever chemically isolated; this happened in 1804. Many say that drug addiction began with morphine and related substances.
Alkaloids: First Natural Insecticides
In its original context, most scholars consider alkaloids like the ones above, as some of the first examples of natural insecticides. Plants seem to use these compounds to defend themselves against pests. Evidence for this is that neither cocaine nor nicotine does much else for the plant, as they do not seem to participate in any endogenous metabolic reactions or in their germination and development processes, for example.
Fifty years ago, the biologists Paul R. Ehrlich and Peter H. Raven published a paper in the journal Evolution titled Butterflies and Plants: A study in coevolution (Scientific American later published a modified version of the paper, just titled Butterflies and Plants) that explained how plants use compounds as defensive molecules.
The authors proposed that certain plants and butterflies exist in a coevolutionary relationship. In a nutshell, they proposed that the chemistry of the plant influenced the evolution of resistance to such chemicals by butterflies and vice versa. For example, if a species of butterfly developed an enhanced ability to cope with a certain plant chemical, this selected for either higher amounts of the plant chemical, the development of alternate chemicals by the plants, or both. This type of coevolution is sometimes called an “evolutionary arms race”, and there are many examples of these in nature. Please note that this is not limited to butterflies, other types of insects can go through this process too. In modern times, humans were the agent of evolutionary change in some insects, which rapidly developed into the well-known phenomenon of pesticide resistance, an important and quite troubling problem.
What Does All This Have to do With Psychoactive Substances?
Defensive compounds from plants, like nicotine and cocaine, usually target nervous system components in insects. These components include proteins that have important roles on the insect’s physiology, which may include specific receptors, ion channels, enzymes, etc. In most cases, the defensive chemical kills the insect by interfering with one or more of these proteins; in other cases, the chemicals just make the plant distasteful for the insect, and therefore, the bug will leave the plant alone.
Well, we humans share our biology with most other organisms on this Earth of ours; this is another consequence of evolution. This means that our nervous systems will have molecular components rather similar (and sometimes identical) to the insect proteins that are affected by the plant’s defensive chemicals. Therefore, a human exposed to such chemicals will be affected as well. Some of these effects will be toxic, some psychoactive, and some even addictive.
Exploring Nature Through Chemistry
So you see, we humans have developed the ability to explore nature in various ways; one of them is the chemical sciences. The more we learn about it, the more we realize that in many ways, nature made it first; a case in point is the wide variety of natural psychoactive drugs that we just talked about. After all, mother nature has had literally millions of years to tinker with chemical structures. Despite all our progress, we find new bioactive compounds virtually every day. The era of drug discovery, particularly psychoactive substances, is far from over.
~Editorial Note: *Some material based on Pagán OR (2014a,b), see resources.