Lecture 13: Clocks and Metabolism I

Question 1

Define metabolism

Answer 1

every chemical reaction and transformation which occurs within an organism.

Question 2

define metabolic rate

Answer 2

The overall balance of anabolism/catabolism determines metabolic rate.

Question 3

what is catabolism?

Answer 3

breakdown of large molecules/proteins to smaller ones.

The creation of these wastes is usually an oxidation process involving a release of chemical free energy, some of which is lost as heat, but the rest of which is used to drive the synthesis of adenosine triphosphate (ATP)

Question 4

what is anabolism?

Answer 4

the synthesis of complex molecules in living organisms from simpler ones together with the storage of energy; constructive metabolism.

mostly driven by ATP

Question 5

how can metabolism exist on different levels?

Answer 5

Cellular level:
Anabolic/catabolic pathways
ATP/AMP (energy) levels
Redox status (NAD+/NADH, NADP/NADPH…)

Tissue/System level:
Coordination of metabolic pathways between multiple tissues
e.g. Glucose homeostasis (glucose production/storage; release/uptake)

Organism level:
metabolic rate (VO2, VCO2, RQ)
thermogenesis and body temperature
feeding behaviour
(ENERGY BALANCE!)

Question 6

how is the circadian system and metabolism linked?

Answer 6

reciprocal connection?

food is also a zeitgeber that can act on peripheral tissues and the brain/scn.

circadian clock is closely coupled to metabolic processes.

food may not align with light so the clock needs to be able to adjust accordingly.

Question 7

Early evidence for clock metabolism coupling?

Answer 7

1920s.
normally feed mice at night, change schedule to feed during the day.

Either set smaller amounts of food or a smaller window.

within days they anticipate and wake before food arrives. Show FAA (food anticipatory activity).

However their SCN has stayed entrained to the LD cycle, as when put in DD and given no food they free run from night time.

Question 8

what are types of FAA?

Answer 8

Increased activity leading up to expected mealtime

can also be seen in Body temperature, hopper visits, operant behaviour…

Question 9

What is the FEO?

Answer 9

Food-Entrainable Oscillator -
Clock structure(s) which time feeding schedules

Question 10

What are characteristics of the FEO?

3

Answer 10

Food entrainment is independent of the SCN.

1. FAA does not depend on presence of LD cycle
   Rats housed under LD or LL show robust FAA to restricted feeding.
2. FAA is not affected by the phase relationship with LD cycle. Rats housed under LD or LL show robust FAA to restricted feeding.
   measured by motion sensor/lever pressing.
3. Can show both free running SCN and FAA simultaneously.
   Rats under LL show restricted feeding driven anticipation and SCN driven free running.

Question 11

What happens to rats with SCN lesions under RF?

Answer 11

still show FAA.
after RF fasted for 3 days, still show FAA.

feed for a week, fast again, still show FAA at same time. continues for weeks.

arrythmic when normally fed.

not sure about that exactly.

but FAA after arrythmia suggests clock.

can change feeding time to not be 24 hour schedules, delay slightly each day, still show FAA.

Question 12

FEO characteristics that are comparable to circadian clock?

Answer 12

TRANSIENCE:
A clock takes time to re-entrain after a shift in the phase of the zeitgeber.

change feeding time +/-8hr, not an instant change in FAA, takes time to change.

LIMITS OF ENTRAINMENT:
the highest and lowest frequency cycle to which the clock can entrain.

FAA can be entrained only if food is presented within the circadian range (between 22.5 and 29h)
Stephan and Becker, 1989
kept in LL.

suggests FEO is a clockyboi.

Question 13

Clock gene rhythms and food entrainment?

Liver vs SCN (which genes)

Answer 13

clock gene rhythms are entrained to food in the peripheral oscillators (but not in the SCN).

Liver:
at night Per1 + Dbp peak.
during the day Per2, Cry1 peak.
these are all reversed if switch to day feeding.

peripheral tissues switch clock genes to train to food.

SCN does not shift to food ever, even in LL/DD, stays entrained to LD.

mRNA in SCN in situ hybridisation, levels the same if day/night fed.

when under RF, show FAA and also normal activity, free run from normal activity in DD/no food.

Question 14

Which clocks in the body respond to food?

Answer 14

All peripheral tissues.

SCN doesn’t

Question 15

Feeding rhythms and SCN rhythms?

Answer 15

Normally, feeding rhythms reinforce SCN-driven rhythms to maintain synchrony.

When feeding is opposed to the SCN, meal timing dominates with regards to peripheral clock entrainment.

Question 16

summary of support for FEO?

Answer 16

Independent of the SCN and LD cycle
Persist after removal of Zeitgeber
Can ‘free-run’ with similar period to previous RFS (in some cases)

RFS is capable of entraining the molecular clocks outside the SCN
More powerful than light for entrainment of peripheral clocks

Therefore supporting the existence of a Food entrainable oscillator
(FEO)

Question 17

Neural control of energy balance?

Answer 17

brain stem hypothalamus.

Inputs:
Neural inputs (eg. Vagal inputs)

Nutrient signals (eg. glucose)

Hormone signals (eg. Leptin)

Outputs:
Neural outputs (ANS)

Hormone signals (eg. ACTH, TSH)

Question 18

Which are semi autonomous oscillators?

Answer 18

still oscillate in isolation from SCN/brain.

OB
Hb
DMH
ARC

Question 19

What are slave oscillators?

Answer 19

in vivo have rhythm, not in isolation from SCN/brain.

Question 20

rhythmic energy signals?

Answer 20

we tend to eat at the same times every day.

Nutrients:
Glucose, FA, AA

Hormones:
Insulin, Glucagon
Leptin, Adiponectin
Ghrelin
CCK, PYY

Question 21

Brain areas controlling food and eating?

Answer 21

ARC/DMN - control food intake, energy metabolism.

NAc/pre frontal - reward areas/feeding reward.

Amyg/brainstem - recieve vagal inputs

Question 22

Circadian clocks and energy balance?

Answer 22

Homeostatic centers involved in energy balance (feeding behaviour and energy expenditure)
house circadian clocks and/or are closely tied to clock structures.

Question 23

Neurons in ARC?

Answer 23

Orexigenic:
Increase food intake
Decrease energy expenditure

Anorexigenic:
Decrease food intake
Increase energy expenditure

Balance dictates a lot of our feeding behaviour and energy metabolism.

per2::luc expression rhythmic.

Question 24

Strategies for locating the FEO?

Answer 24

Examine rhythms in clock gene expression and/or neuronal activity which match FAA meal timing

Lesion (destroy) discrete brain regions and assess food entrainment

Remove clock genes in discrete brain regions

Question 25

DMH gene expression?
DMH c-fos?

good indicator of FEO?

Answer 25

PER1 genes entrain to food.

c-fos is a marker of neuronal activation.
c-fos activated at expercted feeding time.

not really narrowed down the search since lots of areas entrain to food

Question 26

DMH lesions and related studies?

Results, problems and contradictions

Answer 26

critical for anticipation/expression of food entrainable circadian rhythms.

BUT others did same experiment and found DMH lesion did show anticipation/food entrainment.

Bmal1 KO mice, then put back Bmal1 into just SCN or DMH.

found food anticipatory activity in DMH not SCN.

BUT
Another group showed Bmal KO mice show food anticipatory activity.

BUT
fraudulent data, same actogram.

so people ignored DMH

Question 27

Lesions and looking for FEO

Answer 27

Lesioned shit loads, didn’t remove FAA.

Rest of areas can’t be lesioned, ie in brainstem, and be functioning normally.

Question 28

DMH effect on SCN?

Answer 28

inhibits SCN.
SCN also inhibitory.

suppresses the suppression.
allows FAA to be expressed.

Question 29

summary of evidence for FEO?

Answer 29

In summary:

Multiple sites in brain and periphery ‘time’ rhythmic food intake
(RFS entrains peripheral tissues and extra-SCN neuronal oscillators)

Simple lesion studies have failed to locate a single area (FEO)
Almost certainly an interconnected network
responding to many peripheral energy signals

Example is reciprocal inhibition between the SCN and DMH