Some days ago, an attorney general in the USA claimed that pepper spray is not a chemical irritant. No idea if he was grasping at straws in a hopeless attempt to justify the use of the spray against protesters, or if this claim was a result of his ignorance. In fact, pepper spray is highly irritant, and this feature is mainly due to capsaicinoids, compounds that are extracted from chili peppers. Do not get fooled: the fact that they have this natural origin, does not make them any less harmful.
The main compound of this kind is capsaicin (8-methyl-N-vanillyl-6-nonenamide). This specialized metabolite (along with other structural analogues) is produced by Capsicum species, like chili peppers and many other species and varieties (1). Capsaicin is responsible for the hot and pungent “taste” of Capsicum fruits, but it is also irritant to human skin and eyes. These properties are also shared by the other capsaicinoids, but being capsaicin the most abundant one, we will focus on it. Interestingly enough, besides causing pain, capsaicin also acts as an analgesic (2). Although this might seem counterintuitive, actually we will see now how these three actions, i.e. the reaction caused when we eat it, the pain-causing, and the analgesic effect of capsaicin are, at least partially, based on the same mechanism.
Spiciness is not a taste modality
Do you remember how many taste modalities we learned in school? Let’s see…saltiness, sweetness, sourness, bitterness, and umami. Hotness or spiciness (or, to use the technical term, pungency) are clearly not there. We perceive the different tastes based on receptors localized on the buds on our tongue and more in general in the oral cavity. However, there is no receptor for spiciness. The sensation we get when we eat spicy food is a mix of heat and pain and an example of chemesthesis, a phenomenon in which chemical compounds activate receptors mediating pain, touch, or thermal perception (3). Capsaicin activates the cation channel called vanilloid type 1 (TRPV1) receptor, localized to peripheral terminals of neurons that sense both pain and heat (1). This protein is found in many tissues, including oral and nasal mucosa, and skin (4). The interaction of capsaicin with the protein causes a cascade of responses that finally cause thermal (heat) and nociceptive (pain) sensation (5). We also have a series of phenomena, such as salivation, perspiration, and increases of systolic blood pressure, heartrate, body core and surface temperatures (6). It was also shown that this interaction stimulates adrenaline release (6). Well, it seems that there is a lot going on! The same mechanism underlies the pain and burning sensation we feel when capsaicin gets inadvertently in contact for example with the area around our eyes. For what concerns the analgesic effect, this is due to a desensitization of the receptors that respond to pain. After exposure to a high or repeated dose of capsaicin, the TRPV1 receptors begin a refractory state, during which they are unable to respond to stimuli (5).
Going back to the “taste”, hotness is measured in Scoville units which indicate the number of times the substance needs to be diluted in order to perceive no pungency (and in case of chili peppers, it is also an indication of capsaicin content).
An important point is that capsaicin in not water soluble. This means that, even if the natural reaction to the burning sensation in our mouth is to drink, this is not going to help us.
Many people over the world enjoy spiciness, while many do not tolerate it. However, the use of spices is also linked to the fact that many of them have antimicrobial properties, therefore helping preserving food. Nevertheless, we are the only species that intentionally seeks spicy food, while most animals are repelled by fruits containing capsaicin.
What’s the function of capsaicin in pepper fruits?
In peppers, capsaicin (along with other capsaicinoids) content can be highly variable. In the varieties that produce it, capsaicin is however only found in the fruits (and not in the seeds, although there might be capsaicinoids on the seed surface), being especially abundant in the part where seeds are attached (the white structure, called placenta) and its concentration is the highest in ripe fruits.
Capsaicinoids might serve to protect the fruits and, by extension, the seeds from microbial infections and from the attack of several invertebrates (7). The Capsicum plants are small shrubs, which are accessible both to mammals and birds. However, capsaicin repels or poisons mammals, and this seems to be a paradox (7, 8). Indeed, peppers need animals to disperse their seeds: animals will eat the fruits and release the seeds with their feces. Luckily, birds do not care about the presence of capsaicin and so they are the main disperses of wild chili seeds. This has been especially shown for curved-billed thrashers (7). It therefore seems that the chemicals accumulated in pepper fruits selectively discourage seed predators, while avoiding deterring beneficial dispersers, in accordance with the directed-deterrence hypothesis. Why would birds be better dispersers than mammals and what qualifies mammals as predators rather than dispersers? Well, first of all, it was shown that when mammals ingest seeds of Capsicum species (and they only eat the non-pungent fruits), these seeds do not germinate, while when birds do, the seeds show a germination rate that is similar to the one of seeds directly planted from fruits (7). This seems already a reason good enough. However, birds also fly, covering in general larger distances than the ones covered by the small mammals that would also eat peppers. It was also shown that in birds capsaicin induces longer retention times of the seeds in the gut (8). This means that Capsicum seeds coming from capsaicin containing fruits are dispersed even more widely by birds, a further benefit to the parent plants. Furthermore, it was shown that birds mostly disperse seeds in shaded places, but especially places shaded by shrubs and trees producing other bird-dispersed fruits, since birds are foraging there. In these areas, seedlings are more easily established and the fruits produced by these new plants will also have larger chances to be eaten by the dispersers, which is surely an advantage (7).
The fact that plant chemicals may shape the plant-animal interaction is interesting, isn’t it? These mechanisms have been selected during evolution. By producing capsaicin and capsaicinoids, Capsicum plants protect their fruits from microbial infection and deter animals that by eating these fruits would not provide them any advantage, therefore assuring better dispersal of their seeds and consequently better chances for their offspring. It is also quite interesting to me that while we should also be deterred by the presence of capsaicin, many of us (me included) actually love adding it to their food.