135
8
adaptation to the higher body temperatures of birds. So, why can this be beneficial to
the plant? C. chacoense is a chilli plant with fruits similar to hot chilli pepper but without capsaicin. Experiments showed that consumption of their non-pungent seeds by
small mammals abolished germination because they are destroyed when they are
chewed by their molar teeth. By contrast, seeds consumed by birds remained unharmed
providing an ideal system for seed dispersal (Tewksbury and Nabhan 2001). This way,
the presence of capsaicin in the fruits creates a positive selection of wanted feeders. This
is also taken advantage of in industrial applications, where capsaicin is used as an additive to bird feeds in order to deter rodents. Interestingly, it was also shown that capsaicin content of a fruit is not only genetically determined, resulting in different capsaicin
content of different pepper varieties, but is also dependent on plant growth conditions
(Harvell and Bosland 1997). Environmental conditions such as drought seem to boost
capsaicin biosynthesis indicating that there might be more to capsaicin then just its
effect as a feeding deterrent.
Capsaicin, similar to caffeine and nicotine, is a good example for the impact of humans
on the global spreading of plants. In contrast to the native small mammals, human covet
the pungent taste of chillies, which resulted in the widespread distribution and the breeding of new variants all over the world.
While capsaicinoids are exclusively found in chilli peppers, other plant-derived secondary metabolites also target the TRPV1 receptor (Premkumar 2014). These include
campher, thymol, vanillin, piperine, eugenol and also cannabidiol; however, most of them
have a rather low degree of “hotness” as measured by the Scoville scale (number of times
an ethanol extract of a substance has to be diluted to lose its pungency). For example,
piperine, a TRPV1 agonist from black pepper, is about 100 times less potent than capsaicin. By contrast, the most infamous TRPV1 agonists are resiniferatoxin and tinyatoxin,
diterpenes (of the daphnane family) that are found in the latex of Euphorbia resinifera and
Euphorbia poissonii. Resiniferatoxin has an estimated rating on the Scoville scale of 16
billion and thereby 1000 times the score of pure capsaicin. In contrast to capsaicin, resiniferatoxin is found in the latex of the plant indicating that it rather acts as a deterrent
against herbivory.
In contrast to TRPV1, TRPM8 channels are normally activated by cold. Similar to the
action of capsaicin on TRPV1, some plant-derived compounds can bind to the TRPM8
channel and trigger a cooling sensation even under elevated temperatures when eaten,
inhaled or applied to the skin (Kamatou et al. 2013). Such compounds include the monoterpenes menthol, the active ingredient found in plants of the Mentha genus (. Fig. 8.8)
such as wild mint (Mentha arvensis), horse mint (Mentha longifolia) or peppermint
(Mentha x piperita), as well as eucalyptol (1,8-cineol), the major constituent of eucalyptus
oil. These plants are used as ingredients in food or drinks but also as part of medicinal
remedies, e.g. to relief skin irritations or minor pains. Menthol is also used as an additive
to cigarettes; however, many countries have started to ban this practice since menthol is
believed to exacerbate smoking behaviour. Several modes of action might be coming
together for menthol to increase the addictive potential of smoking (Wickham 2015). Due
to its activation of the cold-receptor TRPM8, menthol seems to be able to suppress smoking-induced irritation of the mouth, throat and lungs as well as natural defence reactions
such as coughing that usually would become effective as involuntary resistance against the
inhalation of fumes. Furthermore, menthol is a negative allosteric modulator of nicotinic
acetylcholine receptors and also seems to affect nicotine metabolism thereby increasing
nicotine reinforcement.
8.6 · TRPV and TRPM8 Receptors: Capsaicin and Menthol
8
adaptation to the higher body temperatures of birds. So, why can this be beneficial to
the plant? C. chacoense is a chilli plant with fruits similar to hot chilli pepper but without capsaicin. Experiments showed that consumption of their non-pungent seeds by
small mammals abolished germination because they are destroyed when they are
chewed by their molar teeth. By contrast, seeds consumed by birds remained unharmed
providing an ideal system for seed dispersal (Tewksbury and Nabhan 2001). This way,
the presence of capsaicin in the fruits creates a positive selection of wanted feeders. This
is also taken advantage of in industrial applications, where capsaicin is used as an additive to bird feeds in order to deter rodents. Interestingly, it was also shown that capsaicin content of a fruit is not only genetically determined, resulting in different capsaicin
content of different pepper varieties, but is also dependent on plant growth conditions
(Harvell and Bosland 1997). Environmental conditions such as drought seem to boost
capsaicin biosynthesis indicating that there might be more to capsaicin then just its
effect as a feeding deterrent.
Capsaicin, similar to caffeine and nicotine, is a good example for the impact of humans
on the global spreading of plants. In contrast to the native small mammals, human covet
the pungent taste of chillies, which resulted in the widespread distribution and the breeding of new variants all over the world.
While capsaicinoids are exclusively found in chilli peppers, other plant-derived secondary metabolites also target the TRPV1 receptor (Premkumar 2014). These include
campher, thymol, vanillin, piperine, eugenol and also cannabidiol; however, most of them
have a rather low degree of “hotness” as measured by the Scoville scale (number of times
an ethanol extract of a substance has to be diluted to lose its pungency). For example,
piperine, a TRPV1 agonist from black pepper, is about 100 times less potent than capsaicin. By contrast, the most infamous TRPV1 agonists are resiniferatoxin and tinyatoxin,
diterpenes (of the daphnane family) that are found in the latex of Euphorbia resinifera and
Euphorbia poissonii. Resiniferatoxin has an estimated rating on the Scoville scale of 16
billion and thereby 1000 times the score of pure capsaicin. In contrast to capsaicin, resiniferatoxin is found in the latex of the plant indicating that it rather acts as a deterrent
against herbivory.
In contrast to TRPV1, TRPM8 channels are normally activated by cold. Similar to the
action of capsaicin on TRPV1, some plant-derived compounds can bind to the TRPM8
channel and trigger a cooling sensation even under elevated temperatures when eaten,
inhaled or applied to the skin (Kamatou et al. 2013). Such compounds include the monoterpenes menthol, the active ingredient found in plants of the Mentha genus (. Fig. 8.8)
such as wild mint (Mentha arvensis), horse mint (Mentha longifolia) or peppermint
(Mentha x piperita), as well as eucalyptol (1,8-cineol), the major constituent of eucalyptus
oil. These plants are used as ingredients in food or drinks but also as part of medicinal
remedies, e.g. to relief skin irritations or minor pains. Menthol is also used as an additive
to cigarettes; however, many countries have started to ban this practice since menthol is
believed to exacerbate smoking behaviour. Several modes of action might be coming
together for menthol to increase the addictive potential of smoking (Wickham 2015). Due
to its activation of the cold-receptor TRPM8, menthol seems to be able to suppress smoking-induced irritation of the mouth, throat and lungs as well as natural defence reactions
such as coughing that usually would become effective as involuntary resistance against the
inhalation of fumes. Furthermore, menthol is a negative allosteric modulator of nicotinic
acetylcholine receptors and also seems to affect nicotine metabolism thereby increasing
nicotine reinforcement.
8.6 · TRPV and TRPM8 Receptors: Capsaicin and Menthol