1 Glucose Sensing

Question 1

Why is hyperglycaemia dangerous?

Answer 1

Releases free radicals primarily through a process called “glucose autoxidation,” where excess glucose in the bloodstream spontaneously reacts with oxygen, resulting in ROS

Diabetic ketoacidosis (DKA) occurs when the body doesn’t have enough insulin to use blood sugar for energy. This causes the body to break down fat instead, which produces ketones.

Question 2

Where is glucose sensed?

Answer 2

Pancreas
Liver
Muscle
Adipose
BRAIN

Question 3

Why does the brain need so much glucose?

Answer 3

Because it cannot store fuel

Could use lactate and fat = not as effficient as glucose

Question 4

What happens when glucose is low in the periphery?

Answer 4

More blood flow is needed to go to the brain

Question 5

What are key glucose sensing reigons of the brain?

Answer 5

Ventral medial hypothalamus (VMH)

Medial amygdala nucleus (MAN)

Question 6

Where do GABAergic neruones project from and to?

Answer 6

GABAergic neurones in medial amygdala nucleus directly project to ventromedial nucleus (hypothalamus)

Question 7

What is 2-DG?

Answer 7

2-deoxyglucose

Glycolysis inhibitor, increases the food intake in rats

Acts as a glucose analog, meaning it mimics glucose but cannot be fully metabolized by the body, effectively inhibiting the glycolysis pathway and restricting energy production within cells, making it a potential anti-cancer agent by starving rapidly dividing cancer cells of energy needed for growth

Question 8

What are counterregulatory hormones?

Answer 8

A group of hormones that work to prevent hypoglycemia and maintain fuel supplies in the body

Question 9

What happend when 2-DG was injected into the brain locally?

Answer 9

This made the brain think it was hypoglycaemic because glycolysis could not continue

The rest of the body was euglycemic but the brain cause an increase of counterreggulatory hormones

These hormones work to prevent hypoglycemia

Question 10

What happened when the brain was euglycemic but the body was hypoglycemic?

Answer 10

Counterregulatory hormones were decreased

This shows that the brain controls glucose sensing and homeostasis

Question 11

What is glucose clamping?

Answer 11

Holding glucose at a set level

Hypo-, eu-, hyperglycamic level

Question 12

What is VGLUT2?

Answer 12

Synaptic vesicular transporters = required for vesicular uptake and, consequently, synaptic release of neurotransmitters

(VGLUTs for glutamate and VGAT for GABA)

Question 13

What expresses VGLUT2?

Answer 13

Ventromedial hypothalamic (VMH) neurons are predominantly glutamatergic and express VGLUT2

Question 14

Where are SF1 neurones found?

Answer 14

Major subset of VMH neurons

Question 15

Why did they generate mice lacking VGLUT2 selectively in SF1 neurons?

Answer 15

To evaluate the role of glutamate release from VMH neurons

Question 16

What happened to mice lacking VGLUT2?

Answer 16

Hypoglycemia during fasting

Impaired fasting-induced increases in glucagon (glucose-raising pancreatic hormone)

Defective counterregulatory responses to insulin-induced hypoglycemia and 2-deoxyglucose (an antimetabolite)

Question 17

Why is glutamate release from VMH neurones important?***

Answer 17

Functions to prevent hypoglycemia

Question 18

Why may hypoglycaemia come about?

Answer 18

When insulin acts to decrease blood glucose, if glucagon is not responding = cannot create more glucose in the body

Question 19

Why do mice with VGLUT2 KO need higher rate of glucose infusion during hypoglycaemia clamp?

Answer 19

Because decreased rates of endogenous glucose production

Question 20

What happens when VGLUT2 is KOd?

Answer 20

Hypoglycemia induced a large increase in plasma glucagon levels in control mice
This response was undetectable in Sf1-Cre;Vglut2flox/flox mice (Figure 5E).

Similarly, hypoglycemia induced a large increase in blood levels of epinephrine in control mice, and this response was blunted in Sf1-Cre;Vglut2flox/flox mice

These results demonstrate that Sf1-Cre;Vglut2flox/flox mice have an impaired counterregulatory response to insulin-induced hypoglycemia.

Question 21

Why is using 2-DG useful?

Answer 21

Not confounded by the presence of hyperinsulinemia, scince it cannot be metabolized

(inhibits glucose metabolism in cells)

Question 22

What is MAN?

Answer 22

Medial Amygdala Nucleus

Question 23

How do we know MAN and VMN communicate?

Answer 23

Because they share several roles

We can also use retrograde labelling

Question 24

What roles do MAN and VMN share?

Answer 24

Body weight control

Ovulation

Sexual Behaviour

Fear / Anxiety behaviours

Question 25

What is retrograde labelling used for?

Answer 25

Technique used in neuroscience to identify the cell bodies of neurons that project to a specific target area by injecting a tracer substance into that target area To determine the location of the cells of origin of a nervous system pathway

Question 26

How does retrograde labelling work?

Answer 26

Injecting a tracer substance into a target region of the nervous system It is then taken up by the synaptic terminals of neurons and transported backwards along the axon to the cell body (forward = cell body to axon)

Question 27

What does glutamic acid decarboxylate do?

Answer 27

Converts glutamate into GABA

Question 28

Where are GABA and its receptors made and expressed?

Answer 28

VMH

Question 29

Where is GABA A a3 expressed?

Answer 29

GABAA receptor subunit α3 is expressed throughout most of the dorsomedial region

Question 30

Where is GABA A a5 expressed?

Answer 30

GABAA receptor subunit α5 is localized to the central region

Question 31

Where is GABA A b2, b3 + g3 expressed?

Answer 31

the GABAA receptor subunits β2, β3, γ3 are expressed in the most ventrolateral portions

Question 32

What does muscimol do?

Answer 32

Muscimol increases GABA's affinity for the receptor, which enhances neuronal inhibition and causes a subsequent reduction in pain sensation

Question 33

What does bicuculline do?

Answer 33

Bicuculline acts as a competitive antagonist at the GABA receptor, meaning it blocks the action of the neurotransmitter GABA, effectively preventing it from binding to its receptor and causing an excitatory effect, often leading to seizures when administered in research settings

Question 34

How is glutamic acid decarboxylase releated to diabetes?

Answer 34

Glutamic acid decarboxylase (GAD) is considered to be one of the strongest candidate autoantigens involved in triggering beta-cell-specific autoimmunity The body's immune system mistakenly attacks and destroys pancreatic beta cells that produce insulin, leading to diabetes, because it produces autoantibodies against GAD, which is present in these beta cells

Question 35

What are the two isoforms of GAD?

Answer 35

GAD 65 = rapid neurotransmission GAD 67 = sustained GABA production

Question 36

Where do GABAergic neurones in the MAN directly project?

Answer 36

VMN

Question 37

Where are the origins of GABAergic inputs to VMN from?

Answer 37

Forebrain Hindbrain inputs to VMN are non-GABAergic

Question 38

What is a direct connection between the brain and pancreas?

Answer 38

Autonomic innervation of islet alpha-cell Strong communication between brain and pancreas ACh acts directly on alpha-cell to stimulate glucagon production

Question 39

Under hypoglycaemic conditions, what roles does the VMH play in glucagon secretion?

Answer 39

Stimulating the release of glucagon and other counterregulatory hormones, via changes in the activity of specialized glucose-sensing neurons

Question 40

Blockade of insulin within the VMH had no effect on the suppression of either C-peptide or catecholamine levels during hypoglycemia, what does this suggest?

Answer 40

Insulin acts directly on the VMH to selectively suppress glucagon secretion independent of intraislet insulin or input from circulating catecholamines

Question 41

What does an insulin affibody do?

Answer 41

Binds and blocks the insulin receptor (instead of activating it)

Question 42

Why does a decrease in GIR occur because of anti-insulin affibody?

Answer 42

Less glucose is needed to maintain a normal blood glucose because of IMPAIRED glucose utilization/clearance Too much glucose in the blood A lower GIR means that less glucose is required to maintain the same blood glucose level, which suggests that the body is more efficient at regulating glucose.

Question 43

Why does glucose infusion rate drop when the brain is under hypoglycaemic conditions and there is no insulin?

Answer 43

The brain thinks there is not enough glucose, so it wants to increase glucagon. When insulin is inhibitted, then glucagon levels increase to try to produce more glucose. There is also a decrease in glucose infusion rate, because less glucose is needed to maintain glucose levels since glucose is being produced. The lack of insulin action in the VMH leads to greater glucagon release, which typically would raise blood glucose. This is because insulin normally suppresses glucagon in the VMH, and without insulin receptors functioning in this area, the suppression is removed.

Question 44

Why does glucagon increase when anti-insulin affibody is present?

Answer 44

Because normally insulin inhibits glucagon production

Question 45

What does Phloridzin do?

Answer 45

Phloridzin induces hypoglycemia by acting as a competitive inhibitor of the sodium-glucose cotransporter (SGLT) in the kidneys, which prevents the reabsorption of glucose from the urine, effectively causing the body to excrete more glucose and lowering blood sugar levels

Question 46

How do we know hypothalamus insulin regulates alpha-cell glucagon release under hypoglycaemia?

Answer 46

When insulin- vs phloridzin-induced hypoglycaemia were compared in the VMH Only when insulin was present did glucose decrease

Question 47

What happens to GIR and glucagon when VMH insulin is blocked under insulin-induced hypoglycaemic conditions and why?

Answer 47

Glucose infusion rate decreases and glucagon increases Blockade of insulin action within the VMH reduced by 50% The GIR required to maintain the same hypoglycemic plateau compared with microinjection of a control affibody Because insulin was blocked, it seemed there wa

Question 48

What happens to GIR and glucagon when VMH insulin is administered under phloridzin-induced hypoglycaemic conditions and why?

Answer 48

Glucose infusion rate increases and glucagon decreases. Glucagon decreases because local insulin levels are still high; this signals to the body that there are still high levels of energy and inhibits glucagon secretion. GIR increases because there is less glucose in the blood.

Question 49

What is muscimol?

Answer 49

GABA receptor agonist

Question 50

What mediates insulin's effect on the VMH and how do we know?

Answer 50

The rapid increase in glucagon release caused by local blockade of VMH insulin was completely abolished by the simultaneous injection of muscimol. In contrast, we have previously shown that in overnight-fasted rats VMH microinjection of muscimol alone has no effect on either glucagon or glucose levels. Thus, insulin's effect on the VMH may be mediated, at least in part, by an enhancement of local GABA tone.

Question 51

Why does muscimol prevent increase in glucagon release?

Answer 51

Muscimol prevents the increase in glucagon release by enhancing GABAergic inhibition in the VMH, thereby reducing the neural stimulation of glucagon release