Behavioural Neuroscience 2: Neural Circuits Of Hunger

Front 1

Differentiate between Pavlovian and habitual stimulus-stimulus control of behaviour.

give ex.

How are they similar?

Back 1

- with stim-stim, both stimuli are neutral, but with Pavlovian, one is unconditioned and has instinctual responses

- both are committed without conscious consideration of outcome

- thus, Pavlovian behaviours are pre-programmed habits that manifest in response to a conditioned stimuli

- hearing crunching of chips can induce salivation (Pav.) vs going into room and turning on light (habitual)

Front 2

Does Pavlovian or habitual control drive the behaviour of feeding?

give ex.

Back 2

- feeding behaviour is driven (partially) by Pavlovian

- salivation, sweating, or the approach to sweet food is all Pavlovian

- opening the fridge even though youre not hungry would be habitual

Front 3

Goal-directed control and feeding; is it always employed? give 3 examples of BOLD activity findings

Back 3

- in addition to Pavlovian response, goal directed behaviour works to balance the value of outcomes, but it is not always employed.

- in health eaters, the dlPFC sees to calculate the health aspect of a food, but this is not seen in healthy eaters

- vmPFC of healthy eaters incorporated taste and health info but unhealthy eaters only calculated taste

- reminding people to pay attention to healthiness of food increases PFC network

Front 4

What is Cholecystokinin (CCK)? where is it secreted from? Can we use it to cause weight loss?

Back 4

- peptide that promotes feeling of satiety/fullness

- secreted by GI tract in response to food ingestion

- administering it will cause a reduction in meal size at first, but it wont cause real weightloss since its effects are short lived (brain is only sensitive during onset)

Front 5

Where is leptin secreted from? Where are Leptin receptors located (2)? what kind of signal is it?

Back 5

- Secreted from adipocytes (fat cells)

- located in hypothalamus cells that control hunger by detecting fullness

- located on dopamine neurons (in midbrain, NAc, amygdala, hypo) that control reinforcing value of stimuli like food

- leptin is a negative homeostatic feedback signal like insulin (although leptin is more potent)

Front 6

What are (4) results of increasing leptin?

Back 6

- decreases hunger/ meal size

- make people more sensitive to CCK/ other satiety peptides

- decreases likelihood that people will desire to choose the same food again

- lots of leptin means you find food less enjoyable, less reinforcing

Front 7

What does weight loss do chemically?

Back 7

lowers blood plasma levels of leptin and insulin which lowers bodys sensitivity to CCK and other signals, causing reduced feeling of fullness after meals and increased meal size

Front 8

What is Ghrelin? What does it do (2) ? what does injecting it cause?

Back 8

- peptide hormone that is secreted by the gut and fluctuates throughout day with CCK

- levels increase with hunger and fall with fullness

- seems to increase value that people place on food, and impacts dopamine system to increase reinforcing value of food

- injecting it increases food intake

Front 9

Define energy homeostasis.

What is Adiposity negative feedback?

Back 9

Energy Homeostasis= the process by which the brain matches energy intake with energy expenditure;

Adiposity negative feedback signal controls homeostasis.

Circulating hormone signals form periphery (leptin, insulin) inform the brain of available energy stores.

Adiposity neg feedback signals influence the hedonic value of food.

Front 10

Describe the hedonic aspects of hunger.

What do "emergency circuits" do?

Back 10

-both the motivational and reinforcing properties of food fluctuate in accordance with hunger and available energy stores.

-Satiety reduces both rewarding and reinforcing value of food; Hunger increases them;

emergency circuits override homeostatic feeding!

Front 11

leptin and insulin signals represent___.

If you are getting CCK and leptin signalling, can you have a desire to eat?

Back 11

how much immediate and stored energy in body;

YES

Front 12

Emergency circuit 1: Hypoglycemia (cause and 4 effects)

Back 12

-when brain senses that it does not have enough glucose to support normal brain function (via glucose sensing neurons) which launches an emergency cascade:

- slows insulin secretion to keep sugar in blood

- triggers glucose production in liver

- Slows energy expenditure (basal metabolic rate) and halts growth and reproduction systems

- promotes a potent and sustained feeling of hunger

* this stimulates feeding irrespective of the energy stores (indicated by leptin and insulin)

Front 13

Emergency circuit 2: Hypergycemia (Cause, and 4 effects)

Back 13

Disruptions in insulin cause sugar to remain in blood, causing progressive fat loss, resulting in a plunge in leptin.

- brain believes there is no stored energy so it:

- decreases insulin

- triggers glucose production in liver

- slows energy expentiture

- feeling of profound hunger

(same as hypoglycemia!)

Front 14

Emergency circuit 3: Stress-Induced lack of hunger

Back 14

- stress can override the homeostatic and emergency control of feeding!

- occurs when animal is given new food - neophobia- they will eat a little bit and wait

- feeding will not resume until the stress has passed

Front 15

Leptin deficiency vs Leptin Resistance, what does this cause? (3)

Back 15

- people can be born with or become insensitive to leptin

- this causes constant hunger and obesity

- you can give them daily injections of leptin and they can live a normal life;

Overweight people often have an elevated leptin set point and blunted responses to increases in leptin

this causes:

- reduction in leptins ability to cross blood-brain

- reduction in neuronal response to leptin (POMC and AgRP)

- reduction in consequences of leptin neurons

Front 16

POMC neurons (what, activated/ inhibited by, release?)

Back 16

-neurons in the arcuate nucleus of the hypothalamus play a large role in reducing the feeling hunger

- neurons are activated by leptin and inhibited by neighbouring AgRP neurons

- they release alpha-MSH peptide which is made from pro-opiomelanocortin protein, activates melanocortin receptors and this reduces hunger

Front 17

Melanocotrin Receptor MC4R (activated by, located, mutations);

Stimulating POMC neurons causes what?

Back 17

- activated by POMC

- located in the PVN or hypothalamus

- mutations in MC4R account for 6% of obesity;

stimulate POMC= reduces hunger

inhibiting POMC= increases hunger

BUT influences long term not short term feeding

Front 18

Place POMC, AgRP, Leptin, PVN in a circuitry if you are not hungry vs hungry

Back 18

For you to not feel hungry:

leptin activates POMC and inhibits AgRP's inhibitory signal on PVN, which activates the PVN neurons (these neurons must be firing for you to not be hungry!)

For you to feel hungry:

Leptin inhibits POMC which excite AgRP which inhibit PVN neurons = hunger

Front 19

AgRP Neurons (location, release what, activated/inhibited by what)

What does activating them cause??

Back 19

- located in Arcuate Nucleus of hypothalamus

- release AgRP peptide, GABA and neuropeptide Y

- activated by ghrelin and inhibited by leptin and insulin

Activating them= release of AgRp, GABA and NPY which results in a cascade of signals, the net result of which is and increase in hunger and a decrease in energy expenditure

Front 20

Why do we think that PVN neurons play a role in low leptin-induced emergency feeding?

Back 20

because excess firing here (remember that excitability= not hungry) does not prevent feeding triggered by other parts of the circuitry.

Front 21

What is the result of AgRP release from AgRP neurons? NPY and GABA?

Back 21

-> AgRP release blocks Melanocortin receptors. it increases food intake, but only over LONGTERM= opposite effect of POMC- push pull effect!

-> NPY and GABA rapidly stimulate feeding

Front 22

what does it mean that AgRP is an "inverse agonist" of melanocortin receptors?

AgRP neurons are ______!!

Back 22

It means it blocks binding of alpha- MSH, and even reduces the baseline activity of its g-protein coupled receptor

--> AgRP neurons are INHIBITORY- they inhibit the circuit in order to increase hunger/food intake!

Front 23

What happens when we remove leptin receptors from POMC or AgRP neurons?

Back 23

--> removing leptin receptors from AgRP AND POMC only causes mild obesity, whereas removing from whole body causes severe

Front 24

Prader- Willi Syndrome

Back 24

-PVN neurons must be firing for you to not be hungry

- specifically, oxytocin PVN neurons

- People with this syndrome have 7 genes deleted from chromosome 15, one of which is critical for these PVN cells

- at first no interest in food, but by 2-8 years old develop an intense hunger that never leaves

- most die at 30 from obesity

Front 25

What happens if NTS neurons are firing too much? How can we induce this?

Back 25

- feeding circuits are shut off and animals starve to death

- simple PVN, CCK and leptin inputs to NTS arent strong enough to cause this, but if you put in serotonin input, it pushes NTS above threshold

Front 26

NTS neurons (nucleus of solitary tract of hindbrain) are sensitive to activation from what (4) things? What is the meaning of activation of NTS neurons from each?

Back 26

-sensitive to leptin, CCK signals as well as PVN inputs from hypothalamus, and serotonin input from raphe nuclei

Leptin= body has enough fat

CCK activation= enough food in GI

PVN= no low leptin emergency

Serotonin= lose appetite (through nausea?)

Front 27

How does the PBN act like an emergency off-switch? Where does it receive input from?

Back 27

- excess firing in the NTS causes excess firing in PBN which causes excess firing in the amygdala

= stress response, hunger stops!

basically it receives input from NTS, but also AgRP (which project to PVN which projects to NTS...)

Front 28

What happens when you selectively kill AgRP neurons? How did they do this?

Back 28

- the animal starves to death

- mice are immune to Dipheria Toxin because they dont have the receptor but if they did, the DT would kill the cells. so they give DNA for ArGP neurons the receptor and then give them DT

Front 29

What are two suggestions of circuits that might explain why animals starve to death when you selectively kill AgRP neurons?

Back 29

1) without AgRP the PVN (which projects to NTS) will still fire so maybe AgRP and NTS also

- perhaps there is some direct or indirect connection btwn AgRP and NTS and when it stops inhibiting, NTS becomes overexcited and rats starve = NO EVIDENCE

2) maybe AgRP acts on the PVN which acts on NTS which then excites the PBN which causes starvation (emergency OFF)

- they do find that, if you kill AgRP but then block excitation in PBN (with a GABA agonist) the animal wont starve!

Front 30

What do they conclude is the reason animals starve to death when PBN is excited (through killing AgRP)? Where in the brain is this acting?

Back 30

- Serotonin drives nausea which inhibits feeding

- they found that giving a AgRP-lacking mouse serotonin antagonist through the 4th ventricle (subcutaneous didnt result in enough getting across barrier), the mouse will not starve to death

- if you block serotonin receptors in the NTS but NOT the PBN

- this serotonin activity in NTS promotes activity in PBN which causes nausea and starvation

Front 31

What would happen in an AgRP lesioned rat if you blocked all serotonin input to the NTS? (how might you accomplish this)

Back 31

This would prevent against AgRP-lesion induced starvation!

stopping 5ht activity in NTS stops excitation of PBN and thus no nausea.

method: LoxP sites on the gene that makes the enzyme for serotonin (Tph2) . Give CAV virus containing Cre to the NTS so that any neurons that make serotonin coming in will lose ability to make serotonin.

Front 32

The Serotonergic input to the NTS is coming form where? How might you discover this?

Back 32

Turns out it is coming from the RapheOB and Raphe Magnes neurons (NOT the dorsal Raphe)

5HT neurons express 2 dif enzymes: tryptophan Hydroxylase (Tph2) and aromatic amino acid decarboxylase (AADC)

- took mice whos inputs to the NTS did not have Tph2 and see where they are coming from by staining for both proteins and seeing which only contain AADC

Front 33

Other than stopping serotonin input to NTS or blocking PBN via GABA agonist, what are other ways to stop starvation in ArGP lesioned animals?

Back 33

1) Remove glutamate fro NTS neurons- Make DNA with LoxP site on either site of a glutamate packaging protein. Give cre and the neurons won’t be able to release glutamate. Put AAV-Cre into the NTS neurons

2) Remove Glutamate from PBN- loxP sites surrounding the gene that makes NMDA receptor, inject cre into the PBN via AAV virus

Front 34

Describe the large circuit involving: AgRP, PBN, NTS, RMg/ROb, and their various inputs...

Back 34

1) If PBN is allowed to fire, animals become nauseous and starve.

2) NTS receives 5-HT input from RMg and ROb and sends excitatory (Glutamate) signals to PGN which bind to the NMDAR

3) Leptin etc. acts on AgRP, which is inhibiting the PBN always so that it doesnt fire

Front 35

If you were to leave AgRP neurons intact but stop glutamate excitatory signals from NTS to PBN what would happen?;

Which areas become more active when you have food poisoning?

Back 35

the mice gain weight, but only a little;

NTS and PBN

Front 36

How do you map a circuit?

Back 36

1) determine in each of the downstream structures if the (AgRP) axons actually make synapses there?

2) what does this do to behaviour- when I pick one neurons and stimulate it what is the effect?

3) Alter activity in (AgRp) axons in each downstream structure, but then counteract the ensuing inhibition, to be sure that they are the sole reason for behavioural outcome

Front 37

Do AgRP neurons synapse on POMC neurons? How did they determine this?

Back 37

- have AgRP cre mice and put in cre dependent CHr in ARC nucleus so that when you pulse light here you activate these neurons.

- put in DNA with POMC promotor and GFP

- Thus AgRP neurons are sensitive to light, and pOMC are allowing green

- when they pulsed the AgRP and recorded from the POMC, there was an inhibtory effect on the POMC neurons- firing drops to ZERO

Front 38

What id pharmacogenetics and how is it used in these papers?

Back 38

- Uses DREADD:designer receptors exclusively activated by designer drugs

-cause a subset of cells to express a receptor that is not found in the brain normally and is not activated by anything found in the brain

- then find an agonist of that receptor

hM4D is a designer inhibitory receptor and CNO is the designer drug used

- in a POMC-cre mouse, they injected the hM4d + GFP through a virus and the inject the CNO which shuts down firing

Front 39

In an animal that expresses the hM4D receptor in the POMC neurons, CNO injections will..... What does this suggest?

What is a way of checking this

Back 39

increase food intake slowly over 24 hrs, and only slightly...- suggests that the response of POMC neurons to AgRP neurons stimulation is not the main reason that food intake increases when AgRp neurons are stimulated ;

They checked by simultaneously activating the POMC and AgRP neurons, which should not result in increased feeding if AgRP POMC inhibition was the only reason for the increase in feeding

(they found that the animals still eat, strengthening the idea that AgRP-POMC connections arent all that is important)

Front 40

Describe the relaitonship between the AgRP neruons and POMC

Back 40

- the inhibition depends completely on GABA release so if NPY and AgRP are being released, they cause no obvious consequence to the POMC neurons

- thus the inhibition can be blocked with a GABA antagonist (PTX)

- Activation of AgRP inhibits the POMC, but this inhibition is not the only thing driving the behavioural response of increased feeding

Front 41

Do agrp neurons synapse in the PVH ? PBN? How do they determine this?

Back 41

- they put ChR into AgRP neurons and then shine light into the PBN and PVH
AgRP—> PVH does have synapses, the connection is inhibitory obvi, and the connection is completely blocked by the GABA antagonist (PTX)
Similarly, AgRp—>PBN does have synapses, the connection is inhibitory obvi, and the connection is completely blocked by the GABA antagonist (PTX)

Front 42

Do the projections from the AgRP to the PVH or PBN promote feeding? Obviously they want to double check in some complicated way... what do they do?

Back 42

- if you shine light into ChR mice in these areas to activate the AgRP axons there

- When you activate AgRP axons in PVH animals immediately activate feeding. But not in the PBN

they want to double check with DREADD....

artificially inhibiting PVH with hM4D and CNO does cause feeding, and lots of it

Front 43

How is AgRP axons stimulation in the PVH mediated? How was this found?

Back 43

- it is mediated through the inhibition of the oxytocin containing neurons in the PVH

- they target ChR to both the AgRP and the PVH and stimulate both, blocking the feeding effect, but then if you stimulate the Oxy neurons, it counteracts the effect

Front 44

Does just stimulation of oxy PVH neurons on their own (without AgRP) cause feeding? (this would suggest the two are part of separate circuits). What was their double check (contra vs ipsi)

Back 44

Nope! stimulation of these neurons doesnt affect feeding unless feeding is already being promoted by AgRP neurons stimulation.

this suggests that the two are in the same circuit.

stimulate AgRP axons on one side then either stimulate Oxy neurons on ipsilateral or contralateral. Ipsilateral stimulation blocks the feeding much more than the contralateral, which makes sense wince the AgRP-PVH projection is primarily ipsilateral (3:1 ratio)

Front 45

Are all three of AgRP, NPY and GABA important for AgRP neuron stimulation induced feeding?

Back 45

- stimulating AgRP release doesnt cause acute feeding behaviour (only long term)

- blocking NPY or GABA receptors in the PVH attenuates the feeding, so both molecules seem to be necessary, which is weird since a synaptic level effect wasnt seen

Front 46

Summarize: why does AgRP stimulation increase feeding? (3)

Back 46

-AgRP neurons in ARC inhibit the POMC neurons, but this only modulates longer feeding responses

- AgRP neurons strongly inhibit the PBN neurons but this does not alter feeding behaviour (from the last paper we know this is needed to prevent starvation)

-AgRP neurons inhibit the PVH which is responsible for a large portion of the acute increases in feeding behaviour that results from stimulating the AgRP neurons