133
8
International Olympic Committee. It was later generally banned in the USA and Europe
due to its psychedelic properties and because it is believed to cause addiction (Brown
2013). Nevertheless, it was investigated – and is still used in alternative medicine – as a
treatment for addiction to many drugs such as heroin, cocaine and methamphetamine. It
was also used to treat depression and post-traumatic stress disorder (Zubaran 2000).
Kainic and domoic acids are non-protenogenic amino acids acting on ionotropic glutamate receptors (Olney et al. 1974; Mos 2001). They are named after the Japanese word for
the red algae from where they were isolated and have long been used in Japan as an anthelminthic to expel parasitic worm. Kainic acid, found in certain red algae, is an agonist of
the kainic type of ionotropic glutamate receptors. While stimulating in small doses, large
doses of kainic acid produce immediate neuronal death by excitotoxic lesion. Domoic acid
is found in red algae as well as diatoms of the genus Pseudonitzschia and is an antagonist
of the AMPA and kainic acid type of ionotropic glutamate receptors. It can accumulate in
marine organisms such as mollusks and other shellfish that feed on phytoplankton and is
the causal agent of amnesic shellfish poisoning and certain harmful algal blooms.
Interestingly, diatoms grown in axenic culture have significantly reduced domoic acid
production, which is enhanced when marine bacteria are reintroduced into the culture,
indicating a correlation between the presence of bacteria and domoic acid synthesis (Bates
et al. 1995).
8.6 TRPV and TRPM8 Receptors: Capsaicin and Menthol
That plant metabolites act on TRP receptors that humans and animals use to detect taste
is maybe not very surprising. They can entice animals to eat fruits and thereby spread the
seeds at a larger distance from the mother plant and/or provide a fertile environment for
their growth after they are discarded with the excrements of the animal. Many of these
compounds, e.g. sugars or amino acids, are primary metabolites and are therefore not
covered here. By contrast, secondary metabolites in fruits are often a means to deter animals either to avoid premature consumption of unripe fruits and seeds or to prevent consumption by unwanted feeders. If the aim is to avoid premature consumption, the non-ripe
fruits lack enticing substances and also often advertise their premature status by their
colour. The animals learn that these fruits are not yet mature and are discouraged from
eating them. Prevention of consumption by unwanted feeders only works if the compound
that acts as deterrent is not recognized by those animals that should eat the fruit/seed for
dispersion.
In humans and many animals, TRPV1 channels mediate signals associated with heat
and pain. They are therefore a perfect target for plants metabolites meant to discourage
feeding. The best-known plant-derived compound reacting with this receptor is capsaicin,
an alkylamide found in some plants of the Solanaceae family, such as hot chilli peppers
(C. frutescens). Because it binds to the TRPV1 receptor, capsaicin creates the sensation of
burning when it comes in contact with mucous membranes and is therefore an irritant for
animals and humans consuming chillies. In small quantities, it is used as a spice; however,
larger quantities cause severe irritation and also affect sensitive skin or eye tissue. The
genus Capsicum originated from South America, and it was suggested that chilli peppers
were first cultivated around 6100 years ago in the lowland rainforests of Western Ecuador
(Perry et al. 2007). From there, they were brought to Europe, Africa and Asia, where they
are used as spice but also quickly found entrance in ethnic medicine (Meghvansi et al.
8.6 · TRPV and TRPM8 Receptors: Capsaicin and Menthol
8
International Olympic Committee. It was later generally banned in the USA and Europe
due to its psychedelic properties and because it is believed to cause addiction (Brown
2013). Nevertheless, it was investigated – and is still used in alternative medicine – as a
treatment for addiction to many drugs such as heroin, cocaine and methamphetamine. It
was also used to treat depression and post-traumatic stress disorder (Zubaran 2000).
Kainic and domoic acids are non-protenogenic amino acids acting on ionotropic glutamate receptors (Olney et al. 1974; Mos 2001). They are named after the Japanese word for
the red algae from where they were isolated and have long been used in Japan as an anthelminthic to expel parasitic worm. Kainic acid, found in certain red algae, is an agonist of
the kainic type of ionotropic glutamate receptors. While stimulating in small doses, large
doses of kainic acid produce immediate neuronal death by excitotoxic lesion. Domoic acid
is found in red algae as well as diatoms of the genus Pseudonitzschia and is an antagonist
of the AMPA and kainic acid type of ionotropic glutamate receptors. It can accumulate in
marine organisms such as mollusks and other shellfish that feed on phytoplankton and is
the causal agent of amnesic shellfish poisoning and certain harmful algal blooms.
Interestingly, diatoms grown in axenic culture have significantly reduced domoic acid
production, which is enhanced when marine bacteria are reintroduced into the culture,
indicating a correlation between the presence of bacteria and domoic acid synthesis (Bates
et al. 1995).
8.6 TRPV and TRPM8 Receptors: Capsaicin and Menthol
That plant metabolites act on TRP receptors that humans and animals use to detect taste
is maybe not very surprising. They can entice animals to eat fruits and thereby spread the
seeds at a larger distance from the mother plant and/or provide a fertile environment for
their growth after they are discarded with the excrements of the animal. Many of these
compounds, e.g. sugars or amino acids, are primary metabolites and are therefore not
covered here. By contrast, secondary metabolites in fruits are often a means to deter animals either to avoid premature consumption of unripe fruits and seeds or to prevent consumption by unwanted feeders. If the aim is to avoid premature consumption, the non-ripe
fruits lack enticing substances and also often advertise their premature status by their
colour. The animals learn that these fruits are not yet mature and are discouraged from
eating them. Prevention of consumption by unwanted feeders only works if the compound
that acts as deterrent is not recognized by those animals that should eat the fruit/seed for
dispersion.
In humans and many animals, TRPV1 channels mediate signals associated with heat
and pain. They are therefore a perfect target for plants metabolites meant to discourage
feeding. The best-known plant-derived compound reacting with this receptor is capsaicin,
an alkylamide found in some plants of the Solanaceae family, such as hot chilli peppers
(C. frutescens). Because it binds to the TRPV1 receptor, capsaicin creates the sensation of
burning when it comes in contact with mucous membranes and is therefore an irritant for
animals and humans consuming chillies. In small quantities, it is used as a spice; however,
larger quantities cause severe irritation and also affect sensitive skin or eye tissue. The
genus Capsicum originated from South America, and it was suggested that chilli peppers
were first cultivated around 6100 years ago in the lowland rainforests of Western Ecuador
(Perry et al. 2007). From there, they were brought to Europe, Africa and Asia, where they
are used as spice but also quickly found entrance in ethnic medicine (Meghvansi et al.
8.6 · TRPV and TRPM8 Receptors: Capsaicin and Menthol