Welcome to an introductory video tutorial on the mammalian food entrainable oscillator.
In this video, we will discuss what a food entrainable oscillator is, how it was discovered,
and what we know about where it's located and how it works.
Light is an important synchronizer of circadian rhythms through the suprachiasmatic nucleus,
or SCN.
However, in conditions where food availability is limited, the feeding schedule
can also regulate circadian rhythms, leading to the idea of Food Entrainable Oscillators(FEO).
We will now review the most basic evidence that led to the discovery of the existence
of an FEO that controls behavioral rhythms, and demonstration that it is separate from
the SCN.
Here is model actogram from a rat in a normal light dark cycle.
In the first part of the experiment, food is available ad libitum, which means it's
freely available.
Since rats are nocturnal, they are active during the night time.
After a couple of days, the food is removed.
Rats can tolerate periods of food restriction without ill effect.
Food restriction alone doesn´t substantially affect the locomotor
rhythm.
After that, the food is available for only two hours during the day, when rats are usually
inactive.
The time of food availability is represented by the blue rectangle.
We can observe that, after the first few days of food restriction, the rat becomes very
active and shows high levels of locomotor activity for approximately one to four hours
before the time of food availability.
This is called Food Anticipatory Activity, or FAA.
To show FAA, the animals must be exposed to a set food availability schedule to be able
to predict when the food is going to be available, and anticipate it in the absence of any other
cue.
For example, the rat does not show FAA on the first day of food restriction, but the
FAA arises only after the restriction pattern is established.
Sometimes, the animals show less intense activity at the end of the
night after FAA, indicated by the fading activity bars.
After FAA is established during several days of restricted feeding, the food is removed
entirely for three days.
As you can see, the FAA still occurs, anticipating when food used to be available.
If the FAA were a homeostatic hunger response, we might expect the rat to have increasing
FAA behavior with every hour since it last ate, persisting until the rat was fed.
Instead, the FAA has discrete occurrence around the time of previous food availability.
This demonstrates that FAA is not due to the presence of the stimulus - in this case, food
- but rather is controlled by an oscillator that is entrained to food availability and
continues to keep time after the food is removed.
After that, the food is again given ad libitum, and the rat resumes its usual LD cycle and
FAA disappears quickly.
Three days later, food is removed again, and the FAA comes back at the same time of the
day as it occurred before, which allows us to verify that the phase is still controlled
after the release in the food-restricted condition.
Even after three days of unlimited food availability, the FAA appears again as soon as food is removed.
These findings suggested that an entrained oscillator underlies FAA, hence the use of
the terminology, Food Entrainable Oscillator.
However, because these experiments were done in light/dark conditions, it is impossible
to know if the FAA is part of the light entrainable oscillator or is controlled by some other
independent oscillator.
These next experiments address the contributions of light and the SCN.
They are performed in a different group of rats, in constant darkness and with a lesioned
SCN.
In the first days, the food is available ad libitum.
The rat is arrhythmic under these conditions due to the SCN lesion.
When the food availability is restricted, the rat shows FAA prior to food availability,
just like the intact rat before.
The fact that the SCN-lesioned animal still exhibits FAA in constant darkness means that
light entrainment and the SCN are not required for
the FEO to drive FAA.
Next, the experimenters delay food access by 2 hours.
The rat quickly entrains to the imposed cycle again and does so in the absence of any other
cue.
The period of the rhythms clearly matches the period of food availability, when food
is available approximately every 24 hours.We don't show it here, but if food is available
in a non-circadian pattern, such as every 18 hours, the FAA will not entrain.
We can also tell that the phase relationship is repeatable, because the onset
of FAA always occurs a set period of time before food is available, regardless of when
the food is presented during the day.
Next, the experimenters delay food access by 8 hours.
Here, activity is first seen when the food used to be available, but only persists for
a few hours and stops prior to food availability.
Again, if FAA were simply a function of time since last feeding, we'd expect the activity
to persist until food was available.
However, it does not, and instead slowly drifts to the time of the new food availability until
the rhythms become entrained again.
Altogether, the results of these two experiments demonstrate that FAA is not the result of
masking or hunger, but of an oscillatory system that can be entrained by a fixed time of food
availability, and that is independent of the SCN and of any photic input.
We know that SCN-dependent and many other circadian oscillations are mediated by a transcription-translation
feedback loop of the canonical clock genes.
Mutations of the Period and Cryptochrome genes can alter the period of FAA expression, suggesting
that the molecular circadian clock contributes to the oscillator that drives FAA.
However, multiple studies show that FAA persists in arrhythmic rodent models including the
Clock delta 19 and Bmal1 knockout mutants.
In these mutant animals, the FAA persists in light-dark
conditions, and also during fasting and constant darkness, demonstrating that neither the SCN
nor canonical clock genes are necessary for FAA.
Because we know the SCN is not required for this FEO to function, a lot of research has
looked for the anatomical basis of the FEO , but there is no consensus.
Some believe that it is not located in any specific tissue, but it is rather a widely
distributed network where no single part is essential, which would explain why it is so
hard to find.
In this case, lesion of a single area would not be sufficient to block FAA.
Let's review how the locomotor FEO meets the properties of a circadian oscillator.
To test constant conditions, food would need to be restricted long enough to see a free-running
rhythm of the FAA, and it is not humane or practical to starve rodents for a prolonged
period.
Therefore, we cannot easily test this property of the FEO.
The period of the FEO matches the zeitgeiber.
FAA follows 24 hour cycles of food availability, except when it is re-entraining after a phase
shift in the feeding schedule.
The FEO has a stable phase relationship with the zeitgeber, because stable FAA starts a
set number of hours before food availability each day.
The phase of the FEO is controlled after release into constant conditions when FAA reappears
during complete food restriction and maintains the same phase as with the original feeding
cue.
We don't know if the FEO is temperature-compensated.
If we knew where the FEO is located, we could evaluate temperature compensation with tissue
culture experiments, but we'd need to identify the tissue first.
We may never know if the FEO meets all of the requirements of a circadian oscillator.
However, scientists have shown that it has oscillator-like properties that make it interesting
to study.