1 Glucose Sensing Flashcards
Why is hyperglycaemia dangerous?
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.
Where is glucose sensed?
Pancreas
Liver
Muscle
Adipose
BRAIN
Why does the brain need so much glucose?
Because it cannot store fuel
Could use lactate and fat = not as effficient as glucose
What happens when glucose is low in the periphery?
More blood flow is needed to go to the brain
What are key glucose sensing reigons of the brain?
Ventral medial hypothalamus (VMH)
Medial amygdala nucleus (MAN)
Where do GABAergic neruones project from and to?
GABAergic neurones in medial amygdala nucleus directly project to ventromedial nucleus (hypothalamus)
What is 2-DG?
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
What are counterregulatory hormones?
A group of hormones that work to prevent hypoglycemia and maintain fuel supplies in the body
What happend when 2-DG was injected into the brain locally?
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
What happened when the brain was euglycemic but the body was hypoglycemic?
Counterregulatory hormones were decreased
This shows that the brain controls glucose sensing and homeostasis
What is glucose clamping?
Holding glucose at a set level
Hypo-, eu-, hyperglycamic level
What is VGLUT2?
Synaptic vesicular transporters = required for vesicular uptake and, consequently, synaptic release of neurotransmitters
(VGLUTs for glutamate and VGAT for GABA)
What expresses VGLUT2?
Ventromedial hypothalamic (VMH) neurons are predominantly glutamatergic and express VGLUT2
Where are SF1 neurones found?
Major subset of VMH neurons
Why did they generate mice lacking VGLUT2 selectively in SF1 neurons?
To evaluate the role of glutamate release from VMH neurons
What happened to mice lacking VGLUT2?
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)
Why is glutamate release from VMH neurones important?***
Functions to prevent hypoglycemia
Why may hypoglycaemia come about?
When insulin acts to decrease blood glucose, if glucagon is not responding = cannot create more glucose in the body
Why do mice with VGLUT2 KO need higher rate of glucose infusion during hypoglycaemia clamp?
Because decreased rates of endogenous glucose production
What happens when VGLUT2 is KOd?
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.
Why is using 2-DG useful?
Not confounded by the presence of hyperinsulinemia, scince it cannot be metabolized
(inhibits glucose metabolism in cells)
What is MAN?
Medial Amygdala Nucleus
How do we know MAN and VMN communicate?
Because they share several roles
We can also use retrograde labelling
What roles do MAN and VMN share?
Body weight control
Ovulation
Sexual Behaviour
Fear / Anxiety behaviours
What is retrograde labelling used for?
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
How does retrograde labelling work?
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)
What does glutamic acid decarboxylate do?
Converts glutamate into GABA
Where are GABA and its receptors made and expressed?
VMH
Where is GABA A a3 expressed?
GABAA receptor subunit α3 is expressed throughout most of the dorsomedial region
Where is GABA A a5 expressed?
GABAA receptor subunit α5 is localized to the central region
Where is GABA A b2, b3 + g3 expressed?
the GABAA receptor subunits β2, β3, γ3 are expressed in the most ventrolateral portions
What does muscimol do?
Muscimol increases GABA’s affinity for the receptor, which enhances neuronal inhibition and causes a subsequent reduction in pain sensation
What does bicuculline do?
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
How is glutamic acid decarboxylase releated to diabetes?
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
What are the two isoforms of GAD?
GAD 65 = rapid neurotransmission
GAD 67 = sustained GABA production
Where do GABAergic neurones in the MAN directly project?
VMN
Where are the origins of GABAergic inputs to VMN from?
Forebrain
Hindbrain inputs to VMN are non-GABAergic
What is a direct connection between the brain and pancreas?
Autonomic innervation of islet alpha-cell
Strong communication between brain and pancreas
ACh acts directly on alpha-cell to stimulate glucagon production
Under hypoglycaemic conditions, what roles does the VMH play in glucagon secretion?
Stimulating the release of glucagon and other counterregulatory hormones, via changes in the activity of specialized glucose-sensing neurons
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?
Insulin acts directly on the VMH to selectively suppress glucagon secretion independent of intraislet insulin or input from circulating catecholamines
What does an insulin affibody do?
Binds and blocks the insulin receptor (instead of activating it)
Why does a decrease in GIR occur because of anti-insulin affibody?
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.
Why does glucose infusion rate drop when the brain is under hypoglycaemic conditions and there is no insulin?
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.
Why does glucagon increase when anti-insulin affibody is present?
Because normally insulin inhibits glucagon production
What does Phloridzin do?
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
How do we know hypothalamus insulin regulates alpha-cell glucagon release under hypoglycaemia?
When insulin is blocked and glucose is infused normally glucagon will decrease
But when insulin is blocked
What happens to GIR and glucagon when VMH insulin is blocked under insulin-induced hypoglycaemic conditions and why?
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
What happens to GIR and glucagon when VMH insulin is administered under phloridzin-induced hypoglycaemic conditions and why?
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.
What is muscimol?
GABA receptor agonist
What mediates insulin’s effect on the VMH and how do we know?
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.
Why does muscimol prevent increase in glucagon release?
Muscimol prevents the increase in glucagon release by enhancing GABAergic inhibition in the VMH, thereby reducing the neural stimulation of glucagon release
