Topic 14: Orexigenic Peptides

Question 1

What are orexigenic peptides?

Answer 1

peptides have an orexigenic effect - stimulating the appetite or increasing food intake

most originally identified in the periphery - especially as GI hormones

many function as both peptide hormones and as neuromodulators in the CNS: neuropeptide Y, ghrelin, orexins

Question 2

What is the gut-brain axis?

Answer 2

gut-brain axis is responsible for control of energy homeostasis

sent from gut to brain: vagal afferent, spinal afferents, gut hormones, immune mediators (cytokines), microbial-derived signaling molecules

sent from brain to gut: sympathetic efferents, parasympathetic efferents, neuroendocrine (adrenal cortex, adrenal medulla)

Question 3

What is leptin?

Answer 3

not a neuropeptide

discovered as a spontaneous mutation in a mouse line at Jackson labs in 1949

obese mice (ob/ob) carried a recessive mutation leading to spontaneous hyperphagy and weight gain

later identified as a mutation in the leptin gene

Question 4

How is leptin orexilytic?

Answer 4

leptin is a 16 kDa (167 amino acids) protein hormones released from adipose tissue to attenuate appetite

release signals from site of energy storage to decrease caloric intake

leptin receptors are found in the arcuate nucleus of the hypothalamus (main site of action)

inability to signal satiety in ob/ob mice leads to constant hunger

Question 5

What is neuropeptide Y (NPY)?

Answer 5

expressed from the NPY gene as preproNPY: NPY, C-flanking peptide, and CPON

identified in 1982 in the porcine hypothalamus: NPY expression is highest in the arcuate nucleus of the hypothalamus

Question 6

What are the orexigenic effects of NPY?

Answer 6

IV administration of NPY increases food intake in rodents: hypothalamus has increased permeability of the BBB for monitoring of blood-borne hormones and release of hypothalamus hormones

agonists of NPY increase food intake

antagonists of NPY receptors decrease food intake

5 types of NPY GPCRs: NPYR 1,5 are orexigenic, NPYR 2,4 are autoreceptors (selective agonists decrease food intake)

Question 7

How is NPY related to obesity?

Answer 7

genetically obese mice tend to have high levels of NPY in the hypothalamus

NPY interacts with environmental stress: monkeys subject to chronic stress show increased NPY levels, chronic stress and a high calorie diet lead to increased abdominal adipose deposits (trunk obesity)

Question 8

What is the therapeutic potential of NPY?

Answer 8

NPY, as with other neuropeptides, has poor pharmacokinetics: short half-life circulation, IV administration required (peptides are both highly hydrophilic and sensitive to acid hydrolysis)

NPY receptor antagonism leads to transient effects on NPY signaling: short-term efficacy only - NPY levels readily compensate for changes, pharmacodynamic tolerance develops extremely quickly

Question 9

What is ghrelin?

Answer 9

first identified in 1999

GHRL gene expresses preproghrelin and obestatin (which may not have much to do with obesity)

GHRL functions as a classic gastrointestinal hormone and is released from the GI to stimulate hunger

GHRL is released constitutively but is inhibited by a mechanosensitive process when the stomach is stretched

GHRL release stimulus hunger (orexigenic) and inhibition attenuates hunger

Question 10

What are GHRL receptors?

Answer 10

ghrelin acts at the growth hormone secretagogue type 1A receptor (GHSR)

ghrelin was identified as endogenous agonist for the GHSR (hence the disparity in naming)

GHSR is expressed widely in the periphery (esp. vagus nerve) and in the CNS on NPY secreting cells of the arcuate nucleus of the hypothalamus

GHSR activation leads to secretion of NPY

Question 11

What are the orexigenic effects of ghrelin?

Answer 11

systemic administration of ghrelin increases food intake

gastric bypass surgery decreases ghrelin levels in obese patients

ghrelin also interacts with numerous aspects of mood, stress, and diet to affect food intake

ghrelin increases hedonic aspects of food intake

Question 12

What is GHSR expression in the CNS?

Answer 12

in addition to the hypothalamus, GHSR is expressed in the ventral tegmental area, substantia nigra, raphe nuclei, hippocampus (dentate gyrus)

ghrelin has pronounced effects on the mesolimbic cholinergic-dopaminergic reward network: natural role for ghrelin in signaling reinforcing aspects of food reward

Question 13

What is the posttranslational modification of GHRL?

Answer 13

ghrelin is modified at the S3 position by a medium-chain fatty acid (octanoic acid)

enzyme responsible for acylation is Ghrelin-O-acyltransferase (GOAT)

GOAT activity increases after intake of medium-chain fatty acids in the diet-leading to increased acetylation of ghrelin and increasing hunger stimuli

Question 14

What is the relationship between ghrelin and sleep?

Answer 14

ghrelin release follows a circadian rhythm: increased near expected meal times, slow steady increase from midnight to dawn, ghrelin is co-expressed with circadian clock proteins in the gut

ghrelin expression has a negative correlation with sleep time: decreased sleep leads to elevated ghrelin, increased sleep leads to decreased ghrelin

sleep disruption can disrupt ghrelin rhythms leading to increased ghrelin levels: sensitive to light levels during sleep phase (lights, light pollution, possibly backlit screens)

Question 15

What are the long-term changes in ghrelin?

Answer 15

ghrelin release is also inversely proportional to body weight

weight loss leads to increased ghrelin release
weight gain leads to decreased ghrelin release

proposed to be an endogenous mediator of weight and energy stores

ghrelin release increases with stress - stress eating

Question 16

What is ghrelin as a potential therapeutic target?

Answer 16

ghrelin is interesting as a target for obesity treatment

an “obesity vaccine” has been demonstrated in pigs: immune reaction against GHSR…

however… increased ghrelin levels correlate with antidepressant and anxiolytic effects: high potential for side effects on mood

Question 17

How is GHRL involved in addictions?

Answer 17

ghrelin is involved in reinforcing aspects of food

ghrelin also involved in signaling reinforcing aspects of alcohol: expected as alcohol is also a high calorie vice

ghrelin has been shown to modulate addictive behaviors in response to non-caloric substances

ghrelin administration increases motivation to self-administer heroin

ghrelin antagonists reduce the behavioral response to cocaine and nicotine

Question 18

What is an operant conditioning apparatus?

Answer 18

modified Skinner box

intravenous or oral self-administration (infusion or liquid drop)

operant training for drug/substance reward

animals have catheter implanted into jugular and affixed to infusion pump

lever press (or nose/beak poke) results in dose of drug of choice

used to study motivation to self-administer

Question 19

How is motivation measured in a progressive ratio schedule?

Answer 19

after acquisition of self-administration rats are tested for motivation

response demand is increased progressively starting from 1 (increased effort required to receive reward)

designed to so animal will not achieve response criteria in a given time frame

Question 20

How is motivation measured in a breaking point?

Answer 20

the highest completed ratio of demand to response is termed the break point

break point reflects motivation - amount of work willing to perform to achieve reward

ghrelin administration increases the break point of heroin self-administration

Question 21

What is conditioned place preference?

Answer 21

Pavlovian conditioning paradigm

drug injection (unconditioned stimulus) paired with a neutral environmental stimulus to create a conditioned stimulus

2-3 chamber conditioning box

Ghrelin administration alone can induce a CPP in animal models

Question 22

What is the three phrase conditioned place preference?

Answer 22

pre-conditioning (habituation): open exploration, 15 minutes each for 3 days

conditioning: alternate injections of drug and vehicle in alternative chambers

post-conditioning (test): open exploration, time spent in each chamber measured

Question 23

What is orexin?

Answer 23

identified by two groups in 1998

highly conserved in all major vertebrates

two peptides (orexin A/B or hypocretin 1/2) formed from a single prepropeptide off the HCRT gene

Question 24

Where are orexin-secreting neurons located in the brain?

Answer 24

orexin-secreting neurons are found in the lateral hypothalamus but extensively innervate the brain and spinal cord

Question 25

What are the orexigenic functions of orexin?

Answer 25

orexin neurons are downstream of ghrelin and leptin sensitive neurons of the arcuate nucleus

orexin k/o mice show no orexigenic effects of ghrelin administration

Question 26

What are the sleep functions of orexin?

Answer 26

proposed to link sleep and metabolism

long-term sleep deprivation (in rodents) increases food intake and energy metabolism (with eventual lethal consequences)

orexin administration stimulates wakefulness

Question 27

What is the relationship between orexin and narcolepsy?

Answer 27

orexin receptors were identified as a common cause of canine narcolepsy: receptor loss-of-function mutation leads to decreased function of orexin

similar mutations have been found in human narcolepsy

a common cause of narcolepsy is a mutation in the HLA antigen leading to autoimmune disease against orexin neurons

knockout mice for orexin develop narcolepsy

Question 28

What is the relationship between orexins and wakefulness?

Answer 28

orexinergic neurons receive indirect input from suprachiasmic nucleus - important in circadian rhythms

orexin projects to systems involved in wakefulness: locus coeruleus norepinephrine systems, dorsal raphe serotonin systems, tuberomammillary histamine systems, basal forebrain cholinergic system (BFCS)

orexin activity increases just before waking, orexin neurons are more active wake period than sleep: in mouse optogenetic stimulation of orexin neurons cause waking within several seconds

Question 29

How is orexin a pharmacological target for sleep disorder?

Answer 29

Merck has tested an orexin receptor antagonist for sleep disorders

Suvorexant is an OX receptor antagonist (dual orexin receptor antagonists, DORA) for insomnia treatment

earlier trials were cancelled due to adverse effects

Question 30

How are orexins involved in addictions?

Answer 30

as with the other orexigenic peptides, orexin is implicated in addictions: expected as the systems interact in the ARC of hypothalamus, interactions with the dopaminergic reward pathways in VTA

OX receptor antagonists decrease self-administration of alcohol, opiates, nicotine

administration of orexin into the lateral hypothalamus results in increased alcohol, opiates, nicotine

administration of orexin into the lateral hypothalamus results i increased alcohol consumption or reinstatement of extinguished alcohol or nicotine seeking

Question 31

What is the relationship between orexins and emotional memory?

Answer 31

orexinergic neurons receive inputs from the limbic system: modulate physiological responses to emotional stimuli

orexin k/o mice show decreases responses to intruder mice, air-jet stress: OX receptor antagonists decrease cue-conditioned fear responses

human narcolepsy patients show diminished autonomic responses to emotional stimuli, especially aversive

altered orexin levels observed in patients with anxiety or PTSD

proposed to maintain wakefulness in response to emotional arousal via limbic inputs

Question 32

What is the relationship between orexins and motivational activation?

Answer 32

orexins interact with systems involved in both wakefulness/arousal and reward, motivation, and salience (dopamine)

roles of orexin in food intake and wakefulness suggest involvement in motivational activation

during food deprivation orexin stimulates food seeking behavior

orexins facilitate reward seeking only when motivated by an underlying physiological (e.g. hunger) or psychological need (e.g., cues, stressors)