Lecture 10: The Brain and How It Controls Body Weight

What is Energy Balance?

• Energy intake (from food) is equal to or balanced with energy expenditure (basal metabolic rate, exercise and keeping warm)

• It keeps body weight in balance

  • Most individuals maintain a stable weight over time, with body weight regulated over a set point

How can a disrupted energy balance lead to a higher body weight?

• There is a divergence from ideal body weight if we eat more than needed or don’t expend enough energy (e.g. exercise)

• Energy intake increases and energy expendature decreases → out of balance

• This can lead to the stabilisation at a higher body weight, with individuals carrying around higher amounts of fat in their organs, e.g. adipose tissue and liver

How can a disrupted energy balance lead to a lower body weight?

• There is a divergence from ideal body weight if we eat less than needed to keep up with energy requirements or expenditure, or if energy intake is normal and energy expenditure is increased

• Energy expenditure > energy intake = weight loss

Why Is Excessive Weight Loss Due to Dieting or Illness Dangerous?

• It can lead to cachexia, the breakdown of proteins once carbohydrate and fat stores are used up and depleted

• This loss of proteins often causes death in chronic diseases e.g. cancer, HIV-AIDs

• Certain mental health illnesses may lead to a reduction in body weight, e.g. anorexia nervousa or binge eating disorder

What is Body Mass Index (BMI)?

• It was used as a predictor of mortality by insurance companies in the 1930s

• A quick and simple way to estimate fatness based on height and weight

  • = Kg

m²

• BMI:17.5-25 (normal)→ Low risk of dying; BMI: 25 → overweight; BMI: 30 → Obese

  • Mortality increases exponentially on either side of this range

• It provides little information about the health and fitness characteristics of an individual. → dangerous to make assumptions based on this measure alone

• Not a useful measure for everyone, e.g. children and black people (overestimates body fat)

What is the Waist to Hip Ratio (WHR), and why is it useful?

• A simple morphometric measure used to track population-level changes in body shape over time.

• Measures abdominal fatness → useful in populations prone to abdominal obesity (e.g., Asian groups).

• Typical patterns:

  • Men: apple-shaped (more abdominal fat)

  • Women: pear-shaped (more hip fat)

• Abdominal fat is associated with increased LDL levels and so a higher risk of heart attack and stroke.

• Abdominal obesity is defined as a WHR > 0.90 in men and > 0.85 in women

• Useful for comparing populations, not individuals, over time

What can explain the rapid rise in the proportion of the population with a BMI > 30 across countries?

• Population measurements of BMI show a rapid increase in obesity (BMI ≥ 30) over only a few generations.

• While body weight is highly heritable (~0.7), genes cannot change this quickly or drastically.

• Therefore, the increase is likely driven by environmental changes, not genetics.

• Modern environments (e.g., Westernised diets, high-energy foods, reduced physical activity) interact with existing genes → greater obesity risk.

• Example: countries like Mexico show rising obesity due to the rapid Westernisation of diet and lifestyle.

What have GWAS meta-analyses revealed about genetic influences on BMI and WHR?

• GWAS links traits (e.g., BMI, WHR) with genetic variation across large populations.

• Single genes (e.g., FTO) have minimal effects on body weight.

• Obesity risk arises from many small genetic variants (changes in a group of genes), not one major gene, which can affect our response to the environment

• BMI is best described by genes expressed in the brain (regulation of appetite, energy balance).

• WHR is best described by genes expressed in adipose tissue (fat storage and distribution).

What early evidence showed that the brain regulates metabolism?

• 19th-century observations demonstrated that the brain plays a key role in metabolic control.

• Pituitary tumours compressing the hypothalamus led to obesity (Frohlich’s syndrome) → early evidence that the hypothalamus regulates body weight.

• Claude Bernard showed that stimulating areas of the brainstem of dogs (likely the dorsal vagal complex) increased blood glucose, demonstrating central control of physiology.

What is the “Dual Centre” theory of feeding and satiety in the hypothalamus?

• Rat studies (Hetherington & Ranson, 1940s) using bilateral electrolytic lesions in the hypothalamus revealed two key regions:

  • Ventromedial hypothalamus (VMH) → lesions caused hyperphagia & obesity → proposed satiety centre.

  • Lateral hypothalamus (LH) → lesions caused aphagia & weight loss → proposed feeding centre.

• Lesion studies were interpreted with caution as electrodes also damaged the fibres of passage.

• The theory regained support in the 1990s “peptide revolution,” when anatomical and peptide signalling data confirmed distinct hypothalamic roles in energy balance. → proivded a functional basis to the theory

What is the Body-Weight set-point theory and how is it regulated by the Hypothalamus?

• The brain, specifically the hypothalamus, is believed to encode a body-weight set point.

• Homeostasis within the body is maintained through a series of negative feedback systems similar to temperature or glucose control.

• When a change is sensed by the body a controller (hypothalamus) compares the difference of the current physiological value with a genetically determined set point.

• If a mismatch occurs, the brain activates effectors to reduce the difference (e.g., appetite, energy expenditure)

• Set point is not genetically determined → regualte dby the hypothalamus

How Does the Temperature Regulation System Operate?

• A controller located in the hypothalamus and pre-optic area contains thermosensitive neurons that detect changes between the set point and the hypothalamus triggers compensatory responses (shivering, sweating) to restore the temperatureWha to the set point.

What is the settling-point theory of body-weight regulation?

• Body weight may not have a fixed set point, but a settling point reached passively.

• Analogy: Body energy stores = water in a mountain lake

  • Input (rainfall) = energy intake (food)

  • Output (river) = energy expenditure (basal metabolic rate, movement, thermogenesis)

• Energy expenditure is proportional to body energy stores (river outflow is proportional to lake depth/size)

• The settling point occurs when intake = expenditure → stable body weight.

How does increased energy intake affect the settling point?

• Analogy: More rainfall → lake water rises → new settling point (expenditure = intake)

• Eating more → gain fat and lean tissue → not metabolically inert and requires energy to maintain, so energy expenditure passively increases until it matches intake

• The body stabilises at a heavier weight

• Explains why it’s possible to gain weight and maintain it without active regulation

How does decreased energy intake affect the settling point and dieting outcomes?

• Analogy: Less rainfall → lake water falls → new settling point

• Eating less → lose fat + lean tissue → weight loss

• Lower energy expenditure because less tissue to maintain (it is lost)

• Explains why dieting is difficult – need to maintain a lower intake to reach a new lower weight with a new settling point reached

• Increasing intake again → body weight returns to the previous settling point

• Obese individuals have more tissue, → higher metabolic rate → higher passive energy expenditure to maintain this tissue

What are the key principles of the settling point theory and how does environment affect it?

• Key variable: body energy stores

  • Input = food intake (independent of body size)

  • Output = energy expenditure (dependent on body size)

• The settling point varies in proportion to energy intake

• Human food intake is strongly affected by environmental & socioeconomic factors

• Changes in the environment (e.g., energy-dense foods) drive the obesity pandemic

• Genes determine how individuals respond to the environment

What is the Dual Intervention Point Model

• It is a hybrid model which suggests that body weight is regulated passively within a range, similar to the settling point theory.

• Within this range, body weight is largely determined by environmental and socioeconomic factors

• Two critical points define the range:

  • Lower Intervention Point (LIP) → prevents weight dropping too low.

  • Upper Intervention Point (UIP) → limits excessive weight gain.

What is the lower intervention point (LIP) in body weight regulation?

• Triggered when body weight is too low.

• Genetic and physiological mechanisms prevent starvation and movement beyond this range, and from going beyond the critical point

• It ensures survival, reproduction, and growth

  • Evolutionary pressure

• Overrides environmental pressures favouring weight loss (e.g., poor food quality or availability).

What is the Upper Intervention Point (UIP) in body weight regulation?

• Triggered when body weight is too high and when environmental factors, e.g. energy energy-dense foods, favor weight gain

• Prevents excessive weight gain that could impair survival (e.g., foraging, escaping predators).

• In humans, evolutionary pressures on UIP are reduced → genes controlling upper limit drift, leading to greater variation in body weight.

• Some individuals may no longer have effective genetic protection against obesity (genes preventing obesity may no longer be effective)

What did hypothalamic lesioning studies reveal about feeding regulation?

• Hypothalamus responds to external signals to switch feeding on or off.

• Two main theories emerged, both involving set points:

  1. Glucostatic theory – feeding is regulated by blood glucose levels.

  2. Lipostatic theory – feeding is regulated by body fat stores.

What is the Glucostatic Theory?

• Hypothalamus responds to short-term energy signals (e.g., blood glucose) to regulate feeding

• The Hypothalamus contains glucose-sensing neurons.

  • They are not critical for initiating meals but maintain glucose levels within set limits.

• Evidence for this theory is weak, as it mainly explains the need to eat when blood glucose drops, to protect circulating glucose.

What is the Lipiostatic Theory

• Hypothalamus responds to long-term signals from fat tissue to regulate energy stores.

• Proposed by Gordon Kennedy (1950s).

• Provides feedback about body fat levels, influencing energy intake and expenditure.

What experimental evidence supports the lipostatic theory?

• Jackson Labs’ discovered obese (ob/ob) and diabetic (db/db) mice caused by single-gene mutations.

  • Mutations were located on different chromosomes but had similar phenotypes and so were thought to affect the same pathway in the body

• Douglas Coleman’s parabiosis experiments into:

  • Blood circulation of an ob/ob mouse connected to wild-type mouse.

  • The ob/ob mouse lost weight, showing it lacked an important circulating factor (which it received from the WT) controlling fat.

• Suggests fat-derived signals regulate body weight via the hypothalamus

What happened when a db/db mouse was connected to a normal mouse, and what does it suggest?

• db/db mouse stayed obese.

• A normal mouse lost weight rapidly; risk of starvation if not stopped.

• This suggests that the db/db mouse likely produces the circulating factor but cannot respond due to a defective receptor.

What happens when an ob/ob mouse is joined with a db/db mouse, and what does it show?

• The ob/ob mouse loses weight → received the circulating factor and could respond.

• db/db mouse remains obese → cannot respond to the factor

What is Leptin and Its Function?

• A permissive protein hormone produced by the white adipose tissue → production proportionate to tissue

• It is encoded by Ob gene (discovered in 1914), and its discovery sparked the peptide revolution in understanding how the brain affects body weight regulation.

• It acts on receptors present in the brain to influence metabolism.

• It is a permissive hormone that allows reduced energy intake and increased energy expenditure → helps maintain body weight.

  • Described as a satiety factor and a long term regulator of adipostiy

How do different levels of leptin affect body function and energy balance?

• Low leptin: signals low energy stores → signals brain that there is not enough energy for normal functioning (growth, repair)→ brain reduces non-critical functions, increases feeding, decreases energy expenditure → Protects against starvation (near low intervention point).

  • Occurs in response to weight loss → cessation of leptin production

• Normal leptin: acts on brain circuits to reduce energy intake and increase expenditure → maintains body weight within a healthy range.

• High leptin/obesity: More adipose tissue = more leptin → individuals can become leptin-resistant or insensitive→ hormone no longer effective at reducing intake or increasing expenditure → unable to maintain a low body weight

What characterises leptin-deficient obesity, what causes it, and how does it respond to treatment?

• Not a common form of obesity – caused by rare mutations in the ob gene (leptin) or its receptor (db).

• Seen clinically in children, such as a 3-year-old with an insatiable appetite (hyperphagia) and severe early-onset obesity.

• Treatment with recombinant leptin:

  • Appetite immediately normalises,

  • Dramatic weight loss,

  • Normal puberty begins (leptin is required for reproductive axis activation).

• Over time, patients may become resistant to exogenous leptin.

• Only a few dozen families worldwide have true leptin-gene or receptor mutations.

Why does giving leptin not treat common forms of obesity, and what is leptin’s physiological role?

• High levels of leptin in obese individuals → large amounts of white adipose tissue.

• Giving more leptin does not help because they are resistant to leptin’s effects.

• Leptin fits the lipostatic theory (fat stores signal the brain), but does NOT imply a fixed adipostatic set point.

• Low leptin (e.g., during weight loss) is a strong signal to the brain to increase hunger and decrease energy expenditure at the LIP

• But when fat is high, leptin is poor at triggering the opposite response (reducing eating).

• Leptin receptors are throughout the body, but the hypothalamic receptors are the key regulators of metabolism.

How does leptin act on the arcuate nucleus, and what evidence identifies these neurons as key metabolic targets?

• Leptin acts throughout multiple brain regions involved in metabolic regulation, but the arcuate nucleus of the hypothalamus is a primary target.

  • It was the first brain region shown to contain neurons with leptin receptor mRNA.

• When leptin is administered to mice, it activates arcuate neurons, shown by induction of phospho-STAT3, a marker of leptin-dependent cell activation.

• Within the arcuate nucleus, there are two leptin-sensitive cell types

  • One of these cell populations produces POMC → major role in appetite suppression and energy balance

How was the Cre-Lox system used to test the role of leptin receptors on POMC neurons?

• Brad Lowell’s group used Cre-Lox technology to selectively delete leptin receptors in POMC neurons.

• A POMC-Cre mouse was engineered where Cre recombinase is expressed only in POMC cells.

• A separate floxed leptin receptor Tg mouse was created with loxP sites flanking exon 17 of the leptin receptor gene.

• Individually, both mice are normal in phenotype and body weight.

• When crossed, Cre recombinase cuts out the floxed exon only in POMC neurons, producing offspring lacking leptin receptors specifically in POMC cells.

• This allows researchers to test how loss of leptin signalling in POMC neurons affects metabolism.

What happens when leptin receptors are selectively removed from POMC neurons?

• The floxed control mouse (with intact leptin receptors) shows a normal growth curve.

• Mice with leptin receptors knocked out only in POMC neurons become mildly obese, showing that POMC neurons contribute to leptin’s anti-obesity effects.

How is POMC processed and how does it regulate food intake?

• POMC (pro-opiomelanocortin) is a 32-kDa precursor protein expressed in multiple brain regions and processed differently across tissues by protein convertases.

• In the arcuate nucleus, POMC is cleaved into several peptides, the most important being α-MSH (alpha-melanocyte-stimulating hormone).

• α-MSH acts on melanocortin receptors (MC3R/MC4R) to reduce food intake and regulate body weight.

• Monogenic mouse models show that loss of POMC, processing enzymes, or melanocortin receptors leads to severe obesity, proving the melanocortin pathway is essential for appetite control.

What have human genetic studies revealed about monogenic obesity in the POMC/melanocortin pathway?

• Exome sequencing (focusing on the 1% of DNA that encodes proteins) has identified human mutations in POMC, peptide convertase enzymes, and melanocortin receptors, mirroring mouse models.

• These mutations are rare but account for ~5% of severe, early-onset obesity, with affected individuals obese from a very young age.

• These cases are not typical of obesity: most obesity is polygenic, arising from many small variations in the genome interacting with the environment over time.

• Monogenic mouse mutants have been critical for revealing how the brain regulates metabolism through the POMC-melanocortin pathway

Why are VMN neurons important in leptin signalling, and how is Sf1 used to study them?

• The ventromedial nucleus (VMN) contains neurons that express leptin receptor mRNA, and leptin induced phospho-STAT3 expression, showing these neurons respond to leptin.

• VMN neurons require the transcription factor Sf1 for proper development.

• Sf1 is uniquely expressed, only in the VMN in the brain, allowing researchers to use Sf1-Cre recombinase to selectively target and manipulate VMN neurons as a whole (cannot distinguish among different neuron subtypes).

What is the effect of deleting leptin receptors specifically in Sf1-positive neurons in the VMN?

• KO of leptin receptors in VMN produces a mildly obese mouse (~15–20% increase in body weight), similar to POMC-specific KO.

• Shows VMN neurons contribute to leptin-regulated body-weight control but are only part of the system.

• Electrophysiological recordings in normal Sf1-Cre mice, leptin increases firing of VMN neurons; in Sf1-Cre × floxed-LepR mice, leptin has no effect, confirming the KO worked.

What happens when leptin receptors are knocked out in Sf1-positive neurons of the VMN, and how does this affect body weight?

• Knockout of leptin receptors in VMN neurons results in a mildly obese mouse, with a 15-20% increase in body weight, similar to the POMC-specific knockout mouse.

• Crossing an Sf1-Cre mouse with a POMC knockout mouse also results in a fat mouse.

• Complete knockout of leptin receptors in the brain (e.g., in both VMN and POMC neurons) leads to a 60% increase in body weight, demonstrating that leptin receptors in the VMN contribute to weight regulation

How does leptin affect the firing rate of VMN neurons in Sf1-Cre mice, and how does this change when leptin receptors are knocked out?

• In wild-type Sf1-Cre mice (with normal leptin receptors), leptin increases the firing rate of VMN neurons, confiring its role in regulating neuronal activity

• In Sf1-Cre × floxed-Lepr mice, where leptin receptors are knocked out in the VMN, leptin has no effect on the firing rate, confirming that the knockout disrupted leptin signalling

  • Results in mild obesity (15-20% weight increase), similar to mice with POMC-specific leptin receptor knockout.

• This electrophysiological result shows that leptin signalling in Sf1-positive neurons is crucial for regulating the activity of VMN neurons and body weight control.

How do genes and the environment interact to influence human body weight

• Genes play an important role in how individuals respond to environmental and socioeconomic factors, which are the primary influencers of human body weight.

• Leptin resistance and obesity can result from genetic predispositions interacting with lifestyle and environmental factors, leading to differences in how individuals respond to diet, exercise, and other external conditions

What happens when leptin receptors are removed from both POMC and Sf1 neurons, and what does this reveal?

• Removing leptin receptors from either POMC or Sf1 neurons alone causes mild obesity (~15–20% ↑ body weight).

• Knocking out leptin receptors in both POMC + Sf1 neurons produces an additive increase in body weight, showing both populations contribute to leptin’s anti-obesity effects.

• However, these mice are still far less obese than db/db mice, which lack all leptin receptors and have ~60% higher body weight.

• This indicates additional leptin-responsive neuronal populations remain undiscovered and also play key roles in body-weight regulation.

What is Ghrelin

• A prohormone produced in the stomach;

• Mature hormone is 28 amino acids long → requires post-translational acylation (octanoic acid added to serine-3 by GOAT) to become active and bind its receptor.

• Functions: Stimulates growth hormone secretion and acts as a hunger signal, with circulating levels rising before meals and dropping after eating.

• 2 forms in circulation:

  • Acylated ghrelin – active, binds receptor.

  • Desacyl ghrelin – inactive, currently no known role.

How does ghrelin stimulate feeding through hypothalamic neurons?

• Ghrelin (from the stomach) acts as a hunger hormone, increasing feeding in a dose-dependent manner when injected systemically or into the brain of rats.

• This effect is mediated by direct activation of the NPY/AgRP neurons in the arcuate nucleus, which express the ghrelin receptor.

• c-Fos protein acts as an activation marker and concentrates in the nucleus of activated neurons (similar to phospho-STAT3 for leptin).

• In transgenic mouse studies, NPY neurons labelled with GFP show increased activity when ghrelin is applied.

• Feeding effects mediated by ghrelin can be partially blocked by NPY receptor antagonists.

What are the key features of AgRP neurons and how does ghrelin influence them?

• Expresses neuropeptides:

  • Agouti-related peptide (AgRP) – unique to these neurons.

  • Neuropeptide Y (NPY) – widely expressed elsewhere in the brain.

• GABA is also colocalised with AgRP/NPY.

• Ghrelin increases the activity of AgRP neurons in the Arcuate nucleus, leading to increased feeding.

  • Feeding can be blocked using NPY antagonists

• Significance: AgRP neurons are a critical node linking stomach-derived ghrelin to hypothalamic feeding circuits.

How do POMC and AgRP neurons respond to leptin and ghrelin in terms of feeding and energy balance?

• They respond to the same stimuli in opposing ways.

• POMC neurons:

  • Respond to leptin → decrease food intake and increase energy expenditure.

  • α-MSH (produced by POMC neurons) acts as an agonist at MC4R, reducing food intake.

• AgRP neurons:

  • Stimulated by ghrelin → increases food intake and decreases energy expenditure.

  • Inhibited by leptin, opposing the actions of POMC neurons.

How do POMC and AgRP neurons interact with each other, and what is their role in energy balance?

• AgRP neurons are inhibitory and release GABA, which directly inhibits POMC neurons, reducing their activity.

• Both POMC and AgRP neurons project to similar areas in the hypothalamus, including the DMN and PVM.

• Receptors on target neurons:

  • POMC-derived α-MSH is an agonist at MC4R → reduces feeding.

  • AgRP acts as a functional antagonist at MC4R, increasing feeding.

• This creates an opposing regulation of feeding behaviour and energy expenditure in these brain regions.

How does channel rhodopsin-assisted circuit mapping work to study AgRP neurons?

• Use of a transgenic mouse model with the AgRP gene drives expression of Cre recombinase.

• Cre recombinase can cut DNA sequences flanked by loxP sites.

• Using two different lox sites in an anti-parallel orientation allows Cre to invert the DNA sequence and recombine it rather than just excising it.

• Adeno-associated virus (AAV) containing a channel rhodopsin-tdTomato transgene construct is injected into the arcuate nucleus.

  • The virus infects all cells, but only cells expressing Cr (AgRP neurons) flip the transgene into the correct orientation, resulting in channel rhodopsin and fluorescent marker (tdTomato) expression in AgRP neurons

How do the fluorescent markers and channel rhodopsin work to study AgRP neurons?

• Channel rhodopsin is a light-gated ion channel that, when activated by light, opens channels and activates the neuron.

• TdTomato: A fluorescent reporter that labels the AgRP neurons expressing channel rhodopsin.

• A second transgenic line with Pomc promoter driving green topaz fluorescent protein (GFP) allows identification of POMC neurons.

• Result: AgRP neurons show red fluorescence (from tdTomato), while POMC neurons show green fluorescence, allowing selective identification and manipulation of AgRP and POMC neurons in the arcuate nucleus.

How does channel rhodopsin-assisted circuit mapping reveal the role of AgRP neurons in feeding behaviour?

• An implanted optic fibre directs pulses of blue light to selectively activate channel rhodopsin in AgRP neurons in a freely-behaving mouse.

• Experimental procedure: Light pulses are applied for one hour, and the number of food pellets eaten is recorded.

• Results: During light stimulation, the mouse eats significantly more food compared to periods when no light is applied, demonstrating the role of AgRP neuron activation in stimulating feeding behaviour.

What do we learn about AgRP neuron projections and their effects on feeding behaviour from channel rhodopsin-assisted circuit mapping?

• AgRP neurons project to multiple brain regions, and fibre optic stimulation can target nerve terminals rather than cell bodies.

• Channel rhodopsin is transported to these terminals, allowing fluorescent visualisation of AgRP neuron projections, for example, in the hypothalamic paraventricular nucleus (PVN) and hindbrain parabrachial nucleus (PBN).

• When light is applied to PVN, it increases food intake due to transmitter release from the terminals.

• When light is applied to PBN of the brain stem, there is no feeding, suggesting inhibition of feeding behaviour in this region.

What Are The Downstream Targets of AgRP and NPY Neurons

• Sternson’s group studied the targets of AgRP neuron projections.

  • AgRP terminals in the Paraventricular nucleus of the hypothalams (PVN) : anterolateral bed nucleus of the stria terminalis (aBNST), Lateral hypothalamic area (LHA): and Paraventricular thalamus (PVT) are capable of inducing a feeding response when activated.

  • AgRP terminals in, Periaqueductal gray (PAG) Central nucleus of the amygdala (CEA) Parabrachial nucleus (PBN) does not induce feeding when stimulated, indicating a lack of feeding respons