Photo by D. Sillman
One of my favorite events of Summer and Fall is picking and eating whatever the deer and woodchucks have left me out in my garden. By now the peas and beans have come and gone and the tomatoes are beginning to fade out, but my hot peppers are just coming into their own abundance and glory. So, to me, hot peppers are a very welcome “Sign of Fall!”
Some years I plant chili peppers (sometimes red, sometimes green), other years I plant cayenne peppers (they dry beautifully and last all winter!). This year I planted jalapenos.
All of these peppers are species and varieties of the genus Capsicum. Five of Capsicum’s species are agricultural (i.e. “domesticated”), while twenty-two others (or so) are “wild” species that grow in a great variety of climates and habitats. The domesticated Capsicum range from green peppers to jalapenos to chilies and beyond, and the feature that ties all of these species and varieties together is a group of chemicals synthesized in varying amounts in the different types of peppers: the capsaicinoids! The primary capsaicinoid in hot peppers is (E)-N[(4-hydroxy-3-methophenyl) methyl]-8-methylnon-6-enamide which is better known as “capsaicin” (molecular model just below), and it is capsaicin that gives a hot pepper its heat!
B. MIlls, Public Domain
In a pepper capsaicin is especially concentrated in the inner tissues that surround and support the seeds. It is also found, in lower concentrations, in the actual flesh of the pepper itself. So, if you are trying to concoct a really hot salsa or pot of chili, be sure to leave the seeds of your peppers in your mixture!
It would be fun to share recipes for salsa and talk about experiences eating incredibly well spiced foods, but, instead, since this is an ecology blog we are going to look at capsaicin from both an ecological and an evolutionary point of view.
D. Risacher, Wikimedia Commons
Ecology: my immediate ecological observation is that no deer, no woodchucks, and no raccoons have ever eaten the hot peppers growing in my garden (and, as I have mentioned many times in this blog, these beasts seem to eat EVERYTHING else!). Insects, though, do eat the leaves of the pepper plants and there is some evidence that birds (crows and blue jays) may peck at the ripe peppers, but the birds have never been significant pests in my garden (there is too much great bird seed out in the front yard feeders!). A logical inference from these observations is that the capsaicin’s ecological role is to prevent mammals from eating the peppers and their seeds. This inference seems even more logical when you examine how delicate the individual pepper seeds are and how massively crushing the molar teeth of mammals are. The seeds would be destroyed by mammalian mastication. But, seeds of a plant do need to be dispersed! Further research into wild Capsicum species indicates that their seed dispersals are primarily accomplished by birds whose beaks and gizzards are not capable of crushing and destroying the delicate pepper seeds.
But why aren’t birds affected by the capsaicin around the pepper seeds?
Bohog2, Wikimedia Commons
Now we need some molecular biology. Capsaicin primarily reacts with a specific receptor protein that is located on sensory nerve endings of the skin and of the mucous membranes of the body (including the nerve endings that line the mouth and cover the tongue). This receptor protein is called the “TRPV1” receptor (a molecular diagram of this protein receptor is to the left). So, the “taste” of hot peppers works on these nerve endings rather than on taste buds or on the myriad of olfactory (“smell”) nerve endings up in the roof of the nose. The nerves that have the TRVP1 receptors are for the most part pain-sensory and heat-sensory nerves, and their stimulation by capsaicin tricks the body into thinking that some part of the body (like the mouth or the tongue) is being burned! Actually, the capsaicin cannot really burn the skin or the mucous membrane of the mouth and throat although the nerve stimulation can trigger a vigorous, local inflammation response! Mammals all have TRVP1 receptors that can be stimulated by capsaicin. Birds, though, have very different TRVP1 receptors that are not stimulated by capsaicin!
The evolutionary success, then, of the Capsicum pepper species, exploited the differences between the mammalian and avian TRVP1 receptors by developing a chemical that would trigger the pain and heat nerve impulses in the mammalian receptors only! Pepper plants that made this particular pain-inducing molecule had more seeds that survived predation and more seeds that got widely dispersed by birds. They thus passed their capsaicin-synthesizing genes increasingly onto subsequent generations of pepper plants!
Capsaicin also inhibits the growth of pathogenic fungi on peppers, and the higher the levels of capsaicin, the greater the degree of this protection! This relationship could only have made the evolution of this chemical in these plants (at greatly increasing concentration levels) even more beneficial to their propagation and survival.
There are many studies that have looked into the impacts of capsaicin on human physiology and health. Many of these have explored the release of brain endorphins in response to the nerve impulses arriving in the brain from the TRVP1 receptor nerve fibers. These endorphins reduce the perception of pain and generate a marvelous sense of well being and pleasure. Learning to eat hot, spicy foods as a child (or, in my case, as a college student) definitely involves the causal recognition of and desire for this euphoric-reward state of mind!
Eric at SF, Wikimedia Commons
Other studies have explored the myriad of anecdotal allegations of capsaicin’s ability to prevent the development and or growth of cancerous cells. Broad impacts have not been observed in rigorous clinical trials, but both positive and negative impacts have been seen in narrower, more focused studies. For example, capsaicin reduces carcinogenicity of prostate cancer cells, but it also triggers the formation of skin cancers when it is applied to the skin in an ointment. Further, a recent (2014) study found that capsaicin inhibits the activity of immune system cells (the “natural killer” white blood cells) that are an important component of the body’s innate system to fight and destroy cancerous growths.
TRPV1 receptors become quickly exhausted and stop responding when continuously exposed to capsaicin. This “sensory fatigue” response is obvious to someone who is eating their tenth or twelfth jalapeno pepper (they just don’t taste as hot as the first few did!). This response is also the basis of using capsaicin ointments as pain relief treatments. Deadening the TRPV1 receptors on the skin, or on a muscle, or in an arthritic joint can substantially reduce the pain nerve impulses that had been originating from these sites.
So, happy fall everyone! Time to make some pots of spicy chili to help us get through the cold days ahead!