Body weight Flashcards
What is energy balance?
When energy intake equals energy expenditure
Why was BMI created?
- In 1930s incurance companies wanted to determine how likely someone was to die
- The best determiner of this was BMI
What is the healthy BMI range?
17.5-25
Either side of this likelihood to die increases exponentially
How do you work out BMI?
Kg/m^2
What BMI is clinically obese?
Above 30
What is a different way to look at body morphology besides BMI?
Waist to hip ratio
What are the good values for waist to hip ratio for men and women?
- Men, less than 0.85
- Women, less than 0.80
When over 30 BMI what are some co-morbidity risks?
- Diabetes x7 in men and x28 in women
- Cardiovascular disease x3
- Sleep apnea x3
- Some cancers such as ovarian, uterine and bowel
How much a year does it cost the NHS to treat obesity comorbidities?
10 Bill
What has caused the increase in obesity over the years?
Genes haven’t changes much therefore the changing environemnt and where the genes are acting is causing this.
What is the hetirability of obesity?
70%
What did GWAS of BMI show?
- Certain loci have different associations with BMI
- Lots of different genes involved
- Willer et al 2009
What was an exciting gene discovered in GWAS studies and why?
- FTO
- Huge amount of variation in terms of BMI but changes are small
- In KO mice, only 1-2% change
Where are genes showing associations with BMI expressed?
In the brain
Where are genes for waist to hip ratio found?
Expressed in adipose tissue
Who discovered the importancce of the brain in metabolism?
- Clause Bernard
- Stimulated different parts of a dog brain
- If stimulate a part of the spinal cord, increased circulation of blood glucose
What happens in patients with cancer causing the expansion of the pituitary gland?
- Frolics syndrome causing obesity
- KO pituitary in dogs- no change in body weight
- The pituitary is pushing against the hypothalamus
What is the dual centre theory?
- 1940s did a set of experiments, made bilateral electrolytic lesions in the ventromedial hypothalamus
- Rat got fat
- If made in the lateral part, the rats lost weight
- During the 1990s they could identify the neurons in these areas which were a good basis for the theory
What is the homeostasis and set point theory for body weight?
- There is a sensor
- Controller (hypothalamus?)
- And effector (eating, metabolism)
- These cause a body weight set point like blood glucose levels and temperature homeostasis
Does the homeostasis and set point theory for body weight hold up?
- No
- More variation per person in weight, if there was a set point, there wouldn’t be obesity
- Cannot measure our body weight intrinsically
What is the settling point theory?
- Body weight is determined by input and output
- The level of output (energy expenditure) is determined by weight
- If eat more, weight increases and so will energy expenditure as have more tissue to maintain (direct proportion)
- If eat less, energy expenditure goes down
- We reach a settling point as body weight is proportional to unregulated input
What is the dual intervention point model?
- The settling theory holds between a certain range
- If body weight is too low, we hit the lower intervention point- cannot lose more weight as won’t grow, reproduce and will die
- If body weight is too high we hit the upper intervention limit. Would become a prime target for predators and cannot hunt etc
- Genetic factors detemine the intervention points. However, the evolutionary pressure for the upper intervention point has been removed allowing genetic drift and obesity
- Speakman et al 2011
What two theories did electrolytic lesioning lead to?
- Hypothalamus responds to short term signals such as glucose (Sean Mayer)
- Static theory suggesting that hypothalamus responds to longer term regulators such as fat tissue, measuring the energy that is being stored (Gordon Kennedy)
What did the electrolytic short term signal theory suggest?
- As glucose drops, the hypothalamus detects this and we eat (glucose static theory)
- Isn’t necessarily true
Outline support for the static theory?
- Bred a large mouse, hyperphagic and diabetic (Db/Db) who had a single gene mutation
- Bred another large mouse with different genes Ob/Ob
- Because the phenotype was so similar for the two mice, must be a similar pathway?
Outline the parabiosis experiment in support of static theory
- Douglas Coleman took Ob/Ob mouse and joined its circulation to a normal mouse
- Ob/Ob mouse lost weight, suggesting there must be a factor in the circulation which it is missing
- Same with Db/Db mouse, it did not lose weight but the joined mouse did rapidly
- Db/Db has lots of the factor, so much so that it could lend some and the mouse lost weight. Perhaps obese due to a receptor mutation
- When Ob/Ob and db/db connected, ob/ob lost weight as got the factor from db/db
What was discovered to be the factor in which ob/ob mouse was lacking?
- Ob gene encodes for leptin
- Leptin is produced in white adipose tissue
- Acts as a permissive hormone for things such as growth and the lower intervention level
- If introduce leptin into the brain can get rid of the obese phenotype
Explain where leptin deficiency has been seen in humans
- Montague et al 1997
- Early onset obesity
- Bood test showed mutation in the leptin gene
- Injected with recombinant leptin- feeding and body weight dropped immediately
Is leptin deficiency a common form of obesity?
No
Normally late onset not early onset obesity
What neurons in the brain are a target for leptin?
- Arcate nucleus
- VMN
What cells in the arcate nucleus are targetted by leptin and how do we know this?
- Leptin induced phospho STAT3
- Showed pom C cells
- Also SF1 transcriptoin factor
- Hosoi et al 2002
What is creocombinase?
Bacteriophage enzyme
Explain what the cre-lox system showed about pom C cells and leptin?
- Made pom C creocombinase (normal expression of pom-Cre)
- Inserted Lox P into exon 17 of other mouse creating flox mouse
- Cross the two mice, get both trans genes expressed in the same cell
- The creocombinase cutes the lox P sites and mutates the leptin receptor
- Study the offspring
- With both genes, the offspring were obese- leptin receptors in this neuronal site are important for regulation
- Balthasar et al 2004
Where have POMC mutations been discovered?
- Issues in the pathway of POMC in pituitary
- POMC
- Peptide convertases
- Melanocortin receptors
What does the VMN contain?
Leptin receptor and Sf1
What happened if there were no leptin receptors in the VMN?
They no longer responded to leptin
- Dhillon et al 2006
What happened if SF1 neurons were KO’d in the VMN
- Flox for SF1 and Leprin were obese
- Dhillon et al 2006
What happened to Sf1-Cre, Pom-cre, lepr mice?
- The most obese
- Dhillon et al 2006
What is appetite?
Any desires to fulfil bodily needs
What is appetite in relation to hunger?
To eat food driven by hunger and fulfil the bodies metabolic needs (homeostatic feeding)
What is Ghrelin?
- Hormone isolated in 1999
- Natural ligand for synthetic growth hormones
- Produced in the stomach due to pre hormone
- Is the acetylated version which binds to serine 3 amd produces appetite controlling effects
- Desacyl ghrelin also circulates
How is Ghrelin acetylated?
- By Ghrelin O-acetyltransferase (GOAT)
- Adds an octanoyl group
What is ghrelin’s circulation profile throughout the day?
- At each mealtime, there is a increase in the amount of circulating ghrelin
- Because it is released from the stomach, may be a hunger hormone
What happens if ghrelin is injected into the circulation or brain?
Increased food intake
What neurons does ghrelin act on?
Archaic nucleus neurons AgRP in hypothalamus
What transcription factor do the NPY, AgRP and GABA neurons release in the archaic nucleus after injected with ghrelin?
Fos C- useful marker for cellular activity
What can be given to reduce the effects of ghrelin?
NPY antagonist
What neuron has opposing effects to AgRP, explain.
- AgRP increase feeding and decrease energy expenditure
- POMC decrease feeding and increase energy expenditure
- Respond to the same input but in opposing directions
What are AgRP neuron inhibited by?
Leptin
What areas to AgRP and POMC project to in the hypothalamus?
- PVN
- DMN
- Act on cells with receptors for NPY and melanocortin
What is a breakdown product of POMC which has opposing effects to AgRP?
- MSH
- Is an agonist at MC4 melanocortin
- AgRP is an antagonist at this receptor
- Functional antagonisms between the target neurons
Explain the channel rhodopsin-assisted circuit mapping of POMC and AgRP
- Create AgRP-Cre mouse
- Normally, cre cuts the single lox p site but if use two different types of lox in an antiparallel formation, cre flips it into the other orientation and recombines in the same point
- Then inject an adeno-virus containing rhodopsin into the archaic nucleus
- Trans fene is flipped so the AgRP cells only express channel rhodopsin and red fluorescent indicator
- Also crossed cre with POMC
- Drove the expression og a green fluorescent marker, to show that channel rhodopsin is only expressed in the AgRP neuron
- Scott Stoneston
Using optogenetics, how did they do channel rhodopsin-assisted mapping for AgRP?
- Put optical fibre above the nucleus to pulse blue light into the brain and picks up channel rhodopsin
- Saw an increased food intake when pulsing for an hour
- Aponte et al 2011
- Then encorporated into the membrane of neurons and transported down axons to stimulate the terminals of neurons
- Put an optic fibre into areas where AgRP was projecting to
- Stimulated the terminals
- Found PVN increased in feeding and PBN no increase
- Saw where the AgRP neurons project to
- Atasoy et al 2012
What are 4 areas that AGRP/NPY neurons project to and 3 that they don’t based on channel-rhodopsin mapping studies?
DO
- BNST
- PVN
- LHA
- PVT
DON’T
- PAG
- CEA
- PBN
- Atasoy et al 2012, Betley et al 2012
How is it suggested (2 ways) that AgRP neurons cause increased feeding?
- Targets for AgrP neurons contain sim1 transcription factor and perhaps oxytocin
- AgRP neurons are inhibitory as are these neurons. Inhibition of these causes disinhibition of the PAG causing feeding
- Alternatively, another population of MC4 receptors (which AgRP target) are excitatory and project to the PBN
- Neurons here cause satiety. So when inhibit, we stop the activation of the satiety neurons and get feeding
- Stachniac et al 2012, Garfield et al 2015
Outline how fibre photometry is used in studying AgRP
- Record the activity of AgRP in normally behaving mice
- Neurons of interest express activation dependent calcium
- Most increase calcium when become activated
- Took the cre animals and crossed with a calcium indicator (GCAMP)
- Only the AgRP neurons could flip the GCAMP into the right orientation
- Optic fibre shines excitatory light wavelengths into the brain and is picked up by the indicator
- Light is emitted on a different wavelength depending on how much calcium is in the cell (more calcium means more reflected wavelength)
Outline the study that found AgRP neurons are inhibited by food detection
- First experiment was in fed animals, expect AgRP to be lower
- Measured the calcium in these neurons and injected saline (nothing happened) and ghrelin (AgRP was activated)
- If provided the animals with food at the same time as ghrelin, activity dropped
- Experiments in fasted animals. Placed food into the cage, activity of AgRP drops immediately
- Suggestion that these neurons don’t conteol eating itself but telling the animal to go and find food
- If the animal eats, the activity remains low but if they don’t it increases
- Repeated this in fed animals, Presenting with food doesn’t make a difference
Chen et al 2015
How was conditioned place preference study used to determine whether ghrelin cause negative or positive valence?
Pre-test
- Mouse roamed freely between the chambers for 20 mins and preffered side was measured
- Put animal into the preferred side and give ghrelin but it cannot move (20 mins), repeat 3 times
- Inject the mouse with saline in the non-preferred side (20 mins), repeat 3 times
- measure how much food they eat when returned to cage
Test
- Mouse freely choses which chamber it wants to be in
- Mouse spent less time in the preffered side with ghrelin- negative valence as hunger
- When put back in its cage, mouse ate more after ghrelin compared to saline injections
- Schele et al 2017
Explain the closed loop conditioned place preference for ghrelin
- Similar to place preference
- During conditioning, the animal is free to roam but when in the preffered side, stimulate AgRP neurons
- After the conditioning, in the test, lost the preference for side showing the AgRP neuron is causing negative valence
- Betley et al 2015
What are the direct regulators of feeding?
- Feedforward and feedback mechanisms
- Food is in the mouth, we chew, then swallow, then move along digestive tract (feedforward)
- As we continue to eat feedback tells us when we have reached satiation
Where are the direct regulators of feeding located and whats the proof?
- Neuronal apparatus are part of the atrial region of the brainstem
- Because it is an automated process
- Did work on a decerebrate rat, if put food into the rats mouth would continue to eat until it reached satiation
What is satiety?
Period between meals when we don’t have hunger
What are the indirect regulators of feeding and what do they rely on?
- Environmental cues such as…
- How much glucose is in the blood stream
- Stored energy (in fat tissue)
- If food is available
- Time of day
- These need higher brain centres and won’t work in decerebrate animals
What are hormones released by the stomach called?
Enteroendocrine cells releasing peptide hormones
When are enteroendocrine cells release their peptide hormones?
When we are full
Where are enteroendocrine cells located?
- Dispersed in the epithelial lining of the gut
- Are sparse with an apical surface pointing into the lumen and sense nutrients in the gut
- Can act as paracrine hormones on local cells
- Some enter general circulation and act as endocrine hormones
What is the primary substance that enteroendocrine cells detect?
Triglycerides (esters of glycerol and long chain fatty acids)
Whats an 8 carbon chained triglyceride called?
Octanoic acid
Whats an 10 carbon chained triglyceride called?
Decanoic acid
Whats an 12 carbon chained triglyceride called?
Dodecanoic
What is the fatty acid cut off point for the release of CCK?
C12, C18
McLaughlin et al 1999
What are most dietary fatty acid lipid chain lengths?
> C14
McLaughlin et al 1999
Why is C12 around the cut off for CCK?
- Human body temperature is 37 degrees
- C12 melting poing is 42 degrees whereas C11 is 34 degrees
- McLaughlin et al 1999
How does CCK act to cause breakdown of fatty acids?
- Breaking down relies on bile salts from the gall bladder
- CCK directly acts on the gall bladder causing the release of bile to solubilise hydrophobis, long-chain FAs
- Also release of enzymes from the acini site in the pancreas
- Causes a reduction in gastric emptying, slows down the rate at which food leaves the stomach and regulates flow
- Also signals when we have eaten a meal to clock when full
- McLaughlin et al 1999
Where are CCK receptors found and how this was found?
- In the nerve terminals of the vagus nerve
- Nodose ganglion is near the brain- can stain transportation into the cell bodies
- Can co-stain cells for CCK-1 transported down to the gut enabled by vagus terminals
- They are bipolar, one axon detecting gut and a second to the brainstem in the medulla oblongata nucleus tractus solitarus
- Dockray et al 2009
How does the medulla oblongata control satiety through CCK?
- CCK signal to medulla oblongata via nodose ganglion of vagus nerve
- Medulla oblongata signals to PBN, PVN, areas of the dorsal, vagal complex
- Inhibitory synapse between DMX and NTS reduces stomach emptying
- Controls signals back to the stomach through reflex arcs in the direct regulation of feeding
Where are Prolactin-releasing peptide (PrRP) neurons found?
Expressed in the NTS
What are PrRP neurons activated by?
CCK neurons
Lawrence et al 2002
Explain the LSL-PrRP x cre transgenic mice study
- Conditional KO mice- stop codon in front of PrRP gene
- Either side of the stop codon is lox c
- Cross animal with cre, cut away the stop codon and get re-expression
- Then add nestin-cre expression (expressed in all all brain cells), re-expression of PrRP in all the brain regions should be epressed
- Then add TH-cre (catecholinergic neurons), cross with these neurons, which shows expression only in the brainstem region
- Wild type CCK had a reduction in food intake
- Knock out is no longer effective, response comes back, hypothalamus may be more important with energy expenditure
- With Th-cre. response comes back with brainstem neurons showing brainstem importance
- Dodd et al 2014
Explain the PrRP-cre DREADD experiment
- PrRP-cre crossed with DREADD
- Only get expression of designer receptor in the neurons when crossed
- In the NTS, switch on CNO neurons
- PrRP, through CNO can stop the animal and mediate natural stiety from the brain
Where is CCK produced?
Proximal gut
Where is PYY produced?
- Colon- far back in the gut
- Is a glucagon-like peptide
What is a suggestion of temporal PYY release?
- Released into the blood stream before food reaches the colon
- Could be a reflex arc of CCK
What happens if high doses of PYY are given?
Nausea
What happens to the behavioural satiety sequence when given LiCL?
Causes nausea, completely disturbing the sequence
What happened to the behavioural satiety sequence when given PYY?
- Acts on Y2 receptor located on NPY neurons (is an autoreceptor)
- Maintains the sequence but shifts it to the left
- When give Y2 antagonist with PYY, returns to the vehicle
- Scott et al 2005
Explain how triglycerides cross the epithelial membrane
- Cross the epithelial membrane as get broken by lipases into monoglycerides
- Then reformed on the other side by fatty acid binding proteins in the endoplasmic reticulum
How does orlistat work?
Stops the lipase from breaking down triglycerides
They then pass out through the body without being digested
What is redux?
- Seretonin releaser
- Popular in the 70s and early 80s
- 30 mill prescriptions in the US alone
- Was good at causing weight loss but people were dying of cardiac valve disease
What is sibutramine?
- An SSRI
- Effective at causing weight loss
- Contrary indicated against people with susceptibility of heart problems
- Removed from the market as higher incidence of heart attack and strokes
What is Lorcaserin?
- Is a selective serotonin receptor agonist for 5HT2C
- Were no issues in clinical trials
- But caused dizziness, headaches and possibly some types of cancer
What was the idea when creating drugs targeting the cannabinoid system to combat obesity?
- THC mimics the effect of the natural cannabinoid system
- Feeling reward and euphoria leading to the muchies
- If block these receptors using functional antagonist or inverse agonist, could reverse food intake?
What is rimonabant (as a drug treatment?)
- Blocks cannabinoids
- People on the drug were suffering from depression as they block the reward pathways in the brain indiscriminately
What are 3 bariatric surgeries for obesity?
- Adjustable gastric banding
- Vertical banded gastroplasty
- Roux-en-Y gastric bypass
Outline how an adjustable gastric band works?
- A band around the stomach to produce a small pouch
- People eat more slowly and point of satiation is reached more quickly
- The band is adjustable so can change the size over time and is also reversibe
Outline vertical banded gastroplasty
- Involved a band and stapling to reduce the stomach size
- Further advancement called a sleeve gastrectomy is where they completely remove the greater curvature of the stomach
Outline Roux-en-Y gastric bypass
- Aim is to reduce the area where nutrients are absorbed
- Stapling to produce a stomach pouch and missing out the duodena
- Nobody knows why its successful, people beleive its not down to nutrient reabsoption but changing the repetative behaviour
How does a roux-en-Y gastric bypass change hunger and fullness?
- Instantly report higher feelings of fulless and lower hunger
- may be because changing the number of hormones being released by the gut
- Release of PYY goes up rapidly in the direction which would cause a reduction in hunger
Do hormones have a role in bariatric surgery?
- Huge variation in results
- Overall think that they do
How does a gastric bypass change glycaemic control?
- Long lasting improvement in glycaemic control
- More so than pharmacological treatment
- Could be due to gut hormones such as glucagon peptide 1
- Mingrone et al 2012
Why is leptin not a good hormone to target pharmacologically?
People who are overweight produce a lot of circulating leptin but they become resistant to the hormones
Why is PYY a good hormone to target phamacologically?
Clinical studies show that it is at low cocentrations in obese people. They are still responsive to it
What is the effect of PYY in lean vs obese patients?
- Both reduce food intake
- Also effect hunger
- If give patients a meal, over the following hoursm hunger scores go up but if give PYY, the scores stay low
- Batterham et al 2003
What happens when ghrelin and PYY are given in conjunction?
- The effect of ghrelin is reduced and food intake is still reduced
- Action on the AgRP NPY neuons in the archaic nucleus
What is PYY-36 a selective agonist for?
Y2 NPY inhibitory autoreceptor found on all NPY neurons in the brain including the AGRP neurons in the archaic nucleus
What is NN1273?
- Experimentally made by Novo
- Has a great affinity for Y2
- Longer half-life in the bloodstream than PYY
What is a problem with PYY and its affinity?
- PYY doesn’t just bind to Y2 but also Y1 and Y3.
- These NPY receptors have the opposite effect on food intake and increase it instead
What were the findings of NN1273 studies?
- Implanted mice with osmotic pumps which can administer drugs overtime
- Can show that there is a dose dependent reduction in body weight of mice receiving NN1273
- Some mice aren’t given any of the drug but the same amount of food as the animals receiving the drug
- They lose the same amount of weight suggesting a loss in appetite and not energy expenditure
- Jones et al 2019
What happens if CCK is infused in patients?
- Get a reduction of the size of individual meals
- Then feedback suggests that not enough energy has been taken on
- Compensates by eating more meals
- No change in body weight
What is the solution to problems with CCK infusion?
Administer intermittently
What happened when infused with natural CCK while eating?
- Hunger drops
- Fullness goes up
Where are the effects of CCK mediated?
Receptor 1
What is loxiglumide?
CCK1R antagonist
What happened when loxidlumide was paired with CCK while eating?
Combatted the effects of CCK and scores of hunger and fullness went back to normal
What is NN9056?
An analogue of CCK with a greater half-life in the bloodstream
What happened when NN9056 was injected into mini pigs?
- Dose response effect on food intake when inject the drug on a daily basis
- Different experiment using lower daily doses showerd a reduction in body weight still
- Effetcs of the drugs aren’t huge but are potential for cotherapy
- Christoffersen et al 2020
What are two incretin receptor mimetics?
- Glucagon-like peptide 1 (GLP-1)
- Gastric inhibitory peptide (glucose-dependent insulinotropic polypeptide- GIP)
What cells produce GLP-1 and GIP?
L and K cells in the gut respectively
What is the effect of GLP-1 and GIP?
To improve the release of insulin after the effect
When is GLP-1 released?
Each time we eat a meal
Where do GIP and GLP-1 travel to?
- Released from the gut
- Travel to the pancreas along the portal vein
- Enhance glucose-dependent insulin (incretin effect)
- Holst et al 2007
What does insulin do?
Takes up glucose out of the blood stream and put into other tissues such as adipose or muscles where they store it or use it as fuel
What is DPP-IV?
- An enzyme that breaks down GLP-1 and GIP
- Means that by the time the hormones reach the pancreas and liver, they have almost completely been broken down
What is sitagliptin?
- Inhibits the breaking down of GLP-1
- Changes the amino acid sequence or adds a side chain to increase the half-life
- The peptide doesn’t get broken down so fast
What are two examples of incretin mimetic drugs?
- Exendin 4 (half-life still low)
- Liraglutide (big impact)
Outline why liraglutide was important
- Is effective at treating diabetes and also reduced body weight
- Normally diabetes drugs cause weight gain as they release insulin causing the body to take up fatty acids
- Development of injectable and oral formations of this drug to treat obesity
When are the oral and injectable lirglutide prescribed?
- To treat obesity of BMI over 30
- Or over 27 if had other comorbidities such as diabetes and high blood pressure
What percentage of body weight was lost with semaglutide?
- 15% overtime
- But if came off, put weight back on
- Wilding et al 2021
What is semaglutide?
Higher formation of GLP1 mimetic to be given to obese people with additional comorbidities
What is tirzepatide?
Single molecule with affinity for GLP1 and GIP receptors
What drug was more effective semaglutide or tirzepatide?
Tirzepatide
Frias et al 2021
What are some side effects with the incretin mimetics?
- Nausea and sickness
- However incidence of this is lower in tirzepatide
What is the proposed mechanism of how the incretin mimetics function?
- Still relatively unknown
- Perhaps, natural hormones (GLP-1 and GIP) are broken down in the bloodstream and don’t reach high enough concentrations to reach the brain
- But these mimetics have longer half-lifes and may be able to get access to certain areas of the brain
- May mimic natural brain GLP-1
What two neurons release GLP-1 and where?
Preproglucagon (PPG) neurnons in the body
Prolactin-releasing peptide (PrRP) neurons in the brain
What happens to mice when chemogenetic activation of PPG neurons?
GLP-1 cre mice
Reduction in fast-induced refeeding
Where does tirzepatide enter the brain and where is activated?
- Access of flurescent tirzepatide systemically in a mouse
- Gains access through circumventricular organs (no BBB)
- See mismatched activation of GLP-1 receptors in the brain
- May be that it accesses is happening at a certain circumventricular organ and this activates the receptors in this area
- These may then transynaptically activate other areas of the brain causing reductions in food intake
- find reference in prep
What is a GLP-1 X cre study showing the requirement of the brainstem?
- Use GLP1 receptor x cre
- Ant cells containing GLP1 express cre recombinase
- Inject into different areas of the mouse with a destructive virus called caspase which kills the cells
- If the mice have the caspase ablate the GLP-1 in the nodose ganglion- no effect
- If ablate in the archaic effect- there is a greater effect of the hormone
- If ablate in the brainstem- get rid of the hormone effects
- Huang et al 2024
How has a GLP1-Flox mouse demonstrated the importance of brainstem receptors?
- GLP1 flox mouse which has a lox p either side of the GLP1 gene
- Inject cre recombinase virus into the brainstem
- Neurons in this area will have the GLP1 neurons destroyed
- Lose the effect of body weight reduction
- find reference in prep
What neurons does Exendin-4 activate?
- GLP1
- Gfral
- CCK
What happens when Exendin 4 is injected into the brain?
- Activates CCK, one of the most widely distributed neurotransmitters in the brain
- CCK can express GLP1 receptors in certain areas
- The CCK neurons with the GLP1 receptor also have a Gfral receptor (for cytokine GDF15)
- The cytokine is released by damaged or diseased tissues, accts on the brainstem to cause nausea and reduce appetite
- What we see in patients that take these drugs
- Costa et al 2022
What happens if we disable CCK neurons in the tractus solitarius?
- Get reduction in food intake normally with GDF15 and EX4
- If disable these by using tetanus toxin, we get rid of the effect
- Tetanus toxin stops the neurotransmitters from releasing neurotransmitters
- Worth et al 2020
How was nausea due to GLP1 mimetics studied?
- Injected a retrograde tracer into parabrachial nucleus
- Stain cell bodies in the NTS and LP
- Also stained for Gfral and CCK
- Gave an injection of GDF15 and EXN4
- Shows they project to the parabrachial nucleus
- Worth et al 2020
How was the negative valence in the parabrachial nucleus studied?
- Optogenetic stimulation of CCK terminals that end in the parabrachial
- Reduction in food intake but also negative valence
- Control virus didn’t change much but the channel rhodopsin spent less time in the preferred side
- Pathway cases nausea
- D’Agostino et al 2016
What is the main difference in action between GIP and GLP-1?
- Calcium imaging in a slice with GCAMP shows exitation of glutamatergic GLP-1 neurons
- GIP doesn’t affect the glutamatergic neurons but local inhibitory GABBA neurons (may reduce nausea)
What is a study into exendin-induced anorexia and aversion?
- Condition the mice to different flavours
- Give WT mice banana milkshake (they like it and increase consumption)
- On conditioning day they give chocolate milkshake and pair this with either a saline or exendin4 injection
- Test day, they give them the chocolate milkshake
- If had saline injection, would drink
- If had EXN4, would not drink as causes negative valence
- Both animals respond to banana normally
- If have CCK knocked out and repeat this, the mice do drink the chocolate milshake even with EXN4 injection
- CCK neurons are important in this response
- Costa et al 2022
What happens to negative valence if EXN4 is paired with GIP?
Reverses the negative valence of EXN4
Costa et al 2022
What is LY343843 (retatrutide)?
Triple acting agonist on GLP1, GIP and glucagon receptors
Is more effective than tirzepatide
Costa et al 2022
What are the three components of energy expenditure?
- Obligatory EE
- Physical exercise
- Adaptive thermogenesis
What is obligatory EE?
- Basal metabolic rate
- Digestion, growth, reproduction
- Cannot control between individuals
- Is up to 60% of EE
What is physical exercise as a component of energy expenditure?
- Form of energy expenditure to do with lifestyle
- Also non-exercise activity thermogeneis which is the energy spent moving around
What are the two types of adaptive thermogenesis?
- Shivering thermogenesis
- Non-shivering thermogenesis
What is non-shivering thermogenesis?
Eventually we stop shivering and produce heat through sympathetic nervous system and the stimulation of brown adipose tissue
This can also be acitvated from diet
What is white adipose tissue?
- Single large lipid droplet where triglycerides are stored
- The rest of the cytoplasm forms a thin rind around the periphery
- Requires energy for its maintenance, explaining why overweight people have a high basal metabolic rate
- Also have adipokines like leptin
What is brown adipose tissue?
- Cells have multiple lipid droplets and lots of mitochondria
- Due to the stimulation from the sympathetic nervous system, these mitochondria can become uncoupled from using ATP to produce heat
- The process of BAT mediated thermogenesis uses up a lot of fuel from the doplets stored or active take up of glucose
What is the enzyme called that causes the uncoupling of ATP in brown adipose tissue?
Uncoupling protein 1
What species is BAT most present in?
Small mammals and babies
On the brain and between the shoulders (intrascapular fatpad)
What does PET scan show about glucose uptake after cold stimulation?
- Spent 2 hours prior to the examination in a warm room (25 degrees)
- Some uptake and accumulation of radioactive fluorodeoxyglucose into active tissues such as the heart
- Days later was exposed to cold (18 degrees)
- Cold induced uptake of radioactive glucose is more extensive
- Biopsies confirmed BAT
- Virtanen et al 2009
What is the corelation between location living and BAT?
Further north the subject lvies, the colder the environment and the more BAT
What is the correlation with BAT and BMI?
- Negative correlation with BAT and BMI
- Lean people have more BAT
What is the origin of BAT?
- As seen in the interscapula fat pads
- Comes from the same lineage as skeletal muscle
- Kajimura et al 2010
What is white adipose tissues origin?
Mesodermal
Kajimura et al 2010
What are brite cells?
- Prolonged cold exposure can seem to the apparent browning of white adipose
- However, there is a seperate mesodermal lineage
- Dormant cells which are recruitable proginators that ca n lie dormant in white adipose depos
- Brite cells (brown in white) can be stimulated to develop fully if exposed to cold or sympathetic nervous system is activated
How does adipose browning occur?
- In basal conditions, the inguinal fat pat is subcutaneous and deep (typical white)
- When thermogenic stimulus, same fat depo becomes brown
- Adaptive thermogenesis uses stored fat in the tissue which in the longer term needs to be released from white lipid stores
- Chao et al 2011
Outline the central pathway involved in adaptive thermogenesis
- Cold centres in the skin project information through the DRG, and up to the parabrachial nucleus
- BAT tissue is controlled by the brain and the sympathetic nervous system
- Information is relayed to the preoptic area, some neurons here are intrinsically sensitive to temp
- Signals from the periphery and central heat centres are integrated and passed onto the DMH
- Regulatory signal from DMH to serotonergic neurons in the rRPa is then sent to preganglionic sympathetic neurons in the intermedial lateral nucleus of the spinal cord
- DMH can also be stimulated by diet, if fed a high energy diet, increases BAT-mediated non-shivering thermogenesis
- Causes sympathetic activation and the release of noradrenaline to act on adrenergic receptors
- Heat is generated by the uncoupling of oxidative phosphorylation
- Nakamura and Madden et al 2008
How does leptin act on the central pathways involved in adaptive thermogenesis?
Has leptin sensitive cells (unidentified) that maybe integrate different stimuli and induce thermogenesis
Think a population of leptin sensitive neurons contain PrRP
Are there DMN neurons that are responsive to leptin?
- DMN PrRP is an important target for leptin
- Have leptin receptors but only a small percentage
- Dodd et al 2014
What happens in kockouts of leptin receptors in PrRP neurons?
- Selectively in the hypothalamus
- Offspring become obese
- Similar to when leptin is knocked out of OMC or Sf1 neurons
- Pop of neurons in DMH nucleus poses leptin receptors and are important for leptin signalling
- When receve an injection after awaking, PrRP receptors do not cause thermogenesis because they do not switch on brain adipose tissue
- Normally leptin causes sustained rise in temp
- Dodd et al 2014
What is the receptor for PrRP?
A GPCR- GPR10
What happens in knockout GPR10 mice?
- Males and females become obese on a low calorie diet when compared with WT mice
- Daily food intake is normal, and also normal locomotor activity
- They have low overall energy expenditure (measured using O2 consumptionm indirect colourimetry)
- These mice also gave a very low mean arterial BP
- Luckman
How have human GRP10 mutations been found?
- Using exome sequencing of a large obese cohort of humans
- Found 15 mutations in the human GRP10 gene
- Not a lot of phenotypic data but interestingly they have low BP
- Most common variant of the receptor found is a single nucleotide substitution of proline for serine at amino acid residue 193 (same as in mice)
- Talbot et al 2021
How could BAT thermogenesis be switched on?
- Administering beta 3 adrenerfic agonists
- Such drugs would be unsafe as it is hard to target BAT selectively without activating other sympathetic nervous system (increasing BP and HR0
- Central targets such as PrRP might have adverse effects on BP
What happens if inject 2-DG into a rat?
Causes them to eat as perceives it is low in glucose (negative valence)- mimicking homeostasis
Dodd et al 2010
What is 2-DG?
Form of glucose taken up by cells but cannot be phosphorylated by hexokinase to make ATP
When is THC effective in causing increased food intake?
- Only effective when the rats have been fed previously
- Homeostatic viewpoint no feeding is required- hedonistic
- If not pre-fed, will not cause an increase in eating
- Dodd et al 2010
What is Hemopressin?
Is a novel peptide that activates homeostatic systems
What happens when inducing homeostatic feeding using hemopressin (fMRI)?
Increased activity in the VMN of the hypothalamus after injection
What is AM251?
Drug containing cannbinoid receptors
What happens when inducing hedonistic feeding using AM251 (fMRI)?
- Hypothalamic areas are activated
- Also VTA, NA, PFC
- These are part of the reward circuitary
- Cannabinoid act to effect motivation to eat
How were liking and wanting differentiated?
- Liking is when we tase something like suger, responses are innate and help us survive
- For example, sucrose on the tongue of a baby, can measure positive orofacial expressions but if bitter can see negative ones
- Different tastes are either postitive, neutral or aversive and can be modified by experience
- For example if pair a sweet with lithium chloride it will dislike or can stimulated parts of the brain with opioid or cannabinoids to make like (these are hedonic hotspots)
- The innate orofacial responses to taske only require the brainstem but higher areas modify these responses positively or negatively
- The hotspots lie in regions of the brain comprising of the reward structures but doesn’t involve the dopiminergic system
- However wanting does involve this
Outline mesolimbic dopiminergic pathways in wanting food
- Linking the forebrain and the limbic system signalling the valence of the reward
- The flooding of dopamine in the forebrain was evolved not as a drug pathway but as a system to help a species survive
- Useful behaviours such as eating energy dense foods have positive valence
- Stimulus is reinforced by releasing dopamine from the VTA
How do dopamine receptors in the NA function in obese individuals?
- PET scans
- The first used uptake of radioactive glucose molecule to see in normal and obese humans (was similar)
- Second used radiolabelled dopamine agonist, can measure binding in vivo
- Obese patients have decreased dopamine receptors in the forebrain, NA in particular
- There are negative correlations between dopamine binding and BMI
- Have downregulated dopamine receptors, need more to satisfy the drive
Are homeostatic and hedonistic functions completely seperate?
- No, they interact
- We are aware salience is dependent on nutritional status
- Food looks and tastes better when we are hungry
- For example, food pictures are rated higher and produce a larger activation of the reward systems in the brain when hungry (but not when fed in comparison to non-food pics)
What happens to hunger when leptin deficient participants are treated with leptin?
- Before treatment, subjective scores on rating food and non-food pics
- Food pics are rated higher always no matter if thet have eaten
- Treated for 7 days
- Respond differently to pictures of food once they were fed and corresponding with the reward systems
- They lost the motivation to eat
- Farooqi et al 2007
Why was overeating a benefit?
- Allows animals to stock up with energy dense food when it is available in the environemnt
- Would have been an evolutionary pressure not to get fat however, due to predation and hunting but this has been removed from the environment
