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BIOB32 Lecture notes Winter 2021
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Lecture notes of 130 pages for the course BIOB32 at U of T (N/A)
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BIOB32 – Lecture 1: Taste Perception in MammalsOne of the emerging symptoms of COVID - 19 infection is the loss of taste and smell.
Why does this happen?
o Viruses need to get into our cells to access our cellular
machinery. So, the COVID-19 virus uses the ACE-2 receptor to
get into our cells.
o The ACE-2 receptors are broadly distributed within mammal
bodies.
o There are ACE-2 receptors expressed in our taste receptor cells,
neuronal cells that are responsible for processing taste
information, and nasal epithelial cells which accounts for the
loss of smell.
o COVID-19 viruses uses the ACE-2 receptor to gain access to
different cell types. At the same time, macrophages (part of the
body's innate immune system) will detect the presence of the
virus and the cells that have been infected/killed by the virus.
o Then, they release a variety of
signalling molecules known as
cytokines (IL6).
There's a strong correlation
between the plasma levels of IL6 in
an individual's body and the
likelihood of losing taste or smell
during a COVID-19 infection.
o Cytokines activate cytotoxic T-
cells (immune cells that search and
kill the virus infected cells to contain
the virus).
o Any virus infected taste receptor cells ultimately get killed off by our immune system. We can regenerate
these cells which is why we gain our ability to taste/smell.
o Similar phenomenon is witnessed in the gustatory cortex of the brain. It is responsible for translating taste
sensation into taste perception (awareness of taste). If those cells are also being killed in order to contain
the virus, then, it could explain why we lose our sense of taste and smell.
Why can mammals taste?
Taste likely did not evolve to make eating a pleasurable experience.
Mesolimbic pathway: a neural circuit within our brain that creates a sense of pleasure for us in
response to certain behaviors, as a way of reinforcing those behaviors and encouraging us to conduct
those behaviors repeatedly.
, There are dopamine neurons within the VTA which
produce dopamine as their major neurotransmitter. The
axons of these neurons extend into another brain region
known as the nucleus accumbens.
When we carry out a behavior that the brain thinks is
beneficial to our survival, the VTA will release dopamine
into the nucleus accumbens. As a result, there is a rise in
dopamine levels within the nucleus accumbens when we
engage in some kind of rewarding behavior.
So, we can monitor how rewarding a particular behavior
is by assessing the levels of dopamine that are being
released into the nucleus accumbens.
Mice have been engineered so that they do not contain a
very key component of the sugar detecting pathway.
o WT - Wild type (can detect sugar)
o KO - knock out (cannot detect sugar)
There are two types of water bottles: a sucrose solution and
water.
They monitored the frequency of the mice licking the bottle
with the sucrose solution vs. the bottle with water.
RESULT: the WT mice have shown to prefer the sugar
containing water bottle regardless of the sucrose
concentration three times more than the KO mice. Thus,
suggesting that the KO mice have indeed lost their capacity
to taste sugar, whereas the WT mice have a capacity to take
sugar to taste sugar. Hence, they prefer to lick the sugar
containing bottle
Regardless of whether these mice could taste sugar or not, when
they fed upon sucrose, there was a significant elevation in dopamine
levels within their nucleus accumbens which would then dip back to
baseline within 40 - 60 minutes.
Thus, suggesting that the pleasure experience the mice are
feeling in response to feeding on sucrose is largely
INDEPENDENT of their capacity to taste the sweetness of the
sucrose.
At the 20-minute mark, the dopamine levels in WT are higher
than in the KO (did not rise to the statistical significance). So, one
could argue that there's some minor contribution of taste to the
rewarding sensation that occurs during feeding.
They fed the mice sucralose (an artificial sweetener; does not contain any
calories)
The dopamine levels within the nucleus accumbens in response to
feeding on sucralose did not rise as much when compared to sucrose
which is consistent with the fact that taste is not really a big factor here
because sucralose tastes sweet.
The caloric content of our food is responsible for creating the rewarding
sensation of feeding and taste may only be a very minor contributor to the
rewarding sensation that we get from feeding.
,Why did taste evolve?
To facilitate the acceptance versus rejection of items that are placed into an animal's mouth.
Five “Primary” Tastes
Good Tastes:
o In terms of sweet, there are compounds that taste sweet such as glucose, sucrose, fructose. They are
energy rich nutrients that the animals need to
power the various work and biosynthesis that
they are doing. Since it is very energy rich, we
have developed a sensation to eat more of it to
get more energy to survive.
We are in an energy surplus which is
one of the contributors to obesity.
Whereas our ancestors had to invest
more energy in order to detect energy
rich food.
o In terms of umami, it is mainly amino acids
(protein) that contributes to good taste. As majority of our body is comprised of proteins, we have the
need for amino acids as building blocks for our body. Thus, we have evolved the ability to detect the
presence of amino acids within the food that we eat (umami flavour is an indication that amino acids are
present in the food).
Bad tastes:
o Bitter food is usually associated with poisonous or noxious compounds. Hence, we tend to reject/spit
bitter food to minimize the chances of the poisonous/noxious compounds ends up getting into our general
circulation, and therefore, having the potential to negatively impact the function of our body.
o When we eat sour food, we are detecting the acidic nature of the food we are eating. We do not consume
acidic foods either because they could be pathogen rich or the acidic nature of those foods could damage
our body directly.
Spoiled food contains bacterial
pathogens that perform various
fermentation reactions (e.g., lactic acid
fermentation, mixed acid fermentation).
So, when we eat that spoiled food, we're
going to detect those acids and realize
that there's potentially pathogens that
are in that food. As we don't want those
pathogens into our body, when we taste
sour food, we tend to reject it.
Plants take advantage and ensure that unripe components such as unripe fruits taste sour as a way
of deterring animals from eating them. When fruits are ripe and the seeds within it are ready to be
dispersed throughout the environment, the acidic content is decreased, and sugar is increased in
order for animals to take advantage of the fact that animals need that energy.
Neutral:
o Salty foods can taste good or bad depending on how much one consumes
(e.g., sprinkling some salt over French fries vs. eating a tablespoon full of
salt).
o We have evolved the ability to detect salt for electrolyte balance. We want
a little bit of salt to taste good so that it encourages the body to bring in a
little bit of sodium and chloride and potassium in the foods that we eat.
However, high concentrations of salt tastes bad to us because bringing in
an excess of ions in our diet means we have to excrete more ions through
urine. Being terrestrial organisms, we do not have much access to water.
Hence, we do not consume salty food.
, To understand the impact exercising had on the preferred concentration of salt in their food by having participants
to exercise one day and to not exercise on the other.
o Prior to exercising, both the non-exercise and exercising group had a similar preference for salt in their
diet.
o After exercising, there was a significant increase in how much salt was found to be pleasurable in the
food of those patients who had exercised which
continued to be observable even the next
morning.
WHY DOES THIS HAPPEN?
When we exercise, we produce
extra heat. To dissipate that
heat from the body, we sweat
which
happens when sodium and
chloride ions are pumped into
our sweat glands. Since exercise
has induced an increased loss in
NaCl from the body, the brain
increases the amount of
concentration of sodium that we find pleasurable and desirable, as a way of trying to
encourage us to consume more NaCl to replace the additional amount that has been lost
through sweat via exercise.
Obese people may not necessarily taste things quite as sweet as those who are very thin, because the body is
trying to adjust our perception of sweetness, so that we have an energy supply and demand balance.
Sour as a Good Taste?
Types of groups
o Adults
o Low sour kids - didn't like the taste of sour foods
o High sour kids – liked the taste of sour foods
They fed jello to adults and kids which contained different amounts of citric acid to make it more or less acidic.
ADULTS & LOW SOUR: as citric acid is added to the jello, they tend to find lower and lower preference to the
jello because they find it off putting.
HIGH SOUR: their preference for jello increases as citric acid increases in the jello.
The capacity to find sour tasty decreases with adulthood (there can be variability)
All the three groups were able to detect which jello was most acidic regardless of their preference to sour taste.
WHY DO SOME CHILDREN PREFER SOUR FOOD?
o There are two hypotheses:
Children seek adventure compared to adults (no support for it)
Children need more Vitamin C (ascorbic acid) for the synthesis of collagen, which is a major
structural protein in our bodies. One of the major sources of vitamin C are citric foods whichalso
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