Topic 13: Neuropeptides

Question 1

What are neuropeptides?

Answer 1

similar in design and function to many peptide hormones of pituitary or gastrointestinal systems

many pituitary and GI hormones are neuroactive and used at selective sites in the CNS

far more neuropeptides than classical neurotransmitters

over 100 identified neuroactive peptides currently identified

at least 10 families, over 90 genes, many responsible for expression of multiple neuropeptides

Question 2

How are neuropeptides formed?

Answer 2

neuroactive peptides derive from proteins

peptides formed from cleavage of polypeptides

inactive proteins that function exclusively as precursors to peptides

specific polypeptide precursors are termed propeptides or pre-propeptides

contain 2 or more amino acid linked by a peptide bond

smaller than proteins

Question 3

What are the structures of peptides?

Answer 3

like proteins, peptides and pre-propeptides have a specific sequence of amino acids (N- and C- terminus)

peptides with similar structure often have very different functions

phylogenetically old: jellyfish, hydras, and corals often use peptides rather than classical neurotransmitters

Question 4

How are peptides synthesized?

Answer 4

peptides are synthesized as polypeptide precursors, generally at least 90 amino acids

same general process as precursor proteins biosynthesis

occurs only in cell body

metabolism to active peptide is tissue specific: most precursor are expressed in more than one tissue and the processing is yields tissue specific peptide

Question 5

How are pre-propeptides processed and transported?

Answer 5

pre-propeptides typically contain a series of hydrophobic amino acids at the N-terminus: signal sequence targets the transcribed polypeptide to the endoplasmic reticulum

in the ER the signal sequence is cleaved by a signal peptidase

cleavage of the signal sequence produces an inactive propeptide

propeptides are packaged into large dense-core vesicles (LDCV) for transport to the nerve terminal

Question 6

How are propeptides cleaved into active peptides?

Answer 6

pro-opiomelanocrtin (POMC) gene produces a propeptide for:

alpha-, beta-, and gamma-melanocyte stimulating hormones (MSH)

adrenocorticotropic hormone (ACTH)

beta-endorphins

beta- and gamma-lipoproteins (LPH)

corticotropin-like intermediate peptide (CLIP)

Question 7

Where does propeptide cleavage occur?

Answer 7

propeptide cleavage to active peptides occurs inside trafficking vesicles by synthesizing peptidases

Question 8

What are signal peptidases?

Answer 8

endoplasmic reticulum

cleave signal sequence from pre-propeptide to generate propeptide

Question 9

What are synthesizing peptidases?

Answer 9

LDCV

cleave propeptides to generate neuroactive peptides

Question 10

What are catabolic peptidases?

Answer 10

extracellular

cleave active peptides to inactivate signalling

Question 11

What are exopeptidases?

Answer 11

cleave single amino acid residues from either end of a peptide

catabolic pepetidases

Question 12

What are endopeptidases?

Answer 12

cleave peptides within the sequence of the peptide

signaling and synthesizing peptidases are typically endopeptidases

Question 13

What are the characteristics of neurotransmitters?

Answer 13

synthesized in the nerve terminal: synthetic machinery transported to nerve terminal from soma

released from small synaptic vesicles by exocytosis: closely coupled to calcium channels, relatively low calcium-sensitivity, calcium from external sources

recycled at the nerve terminal

high concentrations at nerve terminal: receptors respond to relatively high concentrations of NT

release occurs at synapse

Question 14

What are the characteristics of neuropeptides?

Answer 14

synthesized only in the cell body: propeptides transported to nerve terminal from soma

released from LDCV by exocytosis: distant from sites of calcium-entry, highly sesnitive to calcium, calcium from internal or external sources

degraded after release, never recycled

low concentrations at nerve terminal: receptors respond to relatively low concentrations of neuropeptides

release can be extrasynaptic

Question 15

How do neuropeptides function as modulators?

Answer 15

neuropeptides are proposed to function as modulators of classic neurotransmitter systems

neuropeptide release can strengthen or prolong actions of primary neurotransmitters: correspondingly, most neuropeptide receptors are G-protein coupled receptors

there are more receptors than peptides (subtypes exist for most neuropeptides)

receptors are often found at sites distal to synapses

Question 16

What sites do neuropeptides act on?

Answer 16

direct action on postsynaptic cell

presynaptic sites on the releasing cell (autocrine function)

on adjacent cells (juxtracrine functions)

on close cells (paracrine effects)

at distant sites requiring transport through circulatory system (endocrine effects)

Question 17

What are neuroactive peptides?

Answer 17

tachykinin peptides: Substance P, neurokinins, neuromedins, neuropeptides K and gamma

cholecystokinin peptides: CCL & Gastrins

cocaine and amphetamine regulated trancript (CART)

orexigenic peptides: neuropeptide Y, ghrelin, orexin

oxytocin/vasopressin

Question 18

What are tachykinin peptides (substance P)?

Answer 18

one of the earliest neuroactive peptides identified

Tachykinin family has at least 7 peptides

Question 19

What are tachykinin genes?

Answer 19

two pre-protachykinin genes express all known tachykinin peptides

TAC1: substance P, neurokinin A, neuropeptide K, neuropeptide Y

TAC3: neuromedin K, neurokiin B

Question 20

What are the three mammalian tachykinin receptors?

Answer 20

NK1, NK2, NK3

all are GPCR that signal through Gq

PLC –> IP3 and DAG –> Ca2+ release and PKC activation

Question 21

How is substance P involved in nociception?

Answer 21

substance P is involved in pain transmission at the level of the spinal cord - involved in pain sensitization

substance P is co-released from glutamatergic sensory afferents

transmission of information from damaged tissues to peripheral nerves

regulates sensitization of pain fibers (C fibers)

proposed to be involved in fibromyalgia and neuropathic pain

NK2 and NK3 agonists reduce the response threshold for noxious stimuli

antagonists for NK1 and NK2 are being explored as possible targets for analgesic drugs

Question 22

What is capsaicin?

Answer 22

active component of chili peppers

produces intense burning sensation on contact with tissues

analgesic effect in topical application, depletes substance P

Question 23

How is substance P involved in the vomit centre?

Answer 23

the chemoreceptor trigger zone (CTZ) of the area postrema (medulla) senses toxins in the bloodstream: BB permeable area, also detects excess 5HT from the gut via 5HT3 channels

the tachykinin receptor NK1 is expressed in late, convergent steps of the vomit pathway

substance P release in the CTZ is a final triggering step of the vomit reflex

apripitant is an NK1 substance P antagonist used as an anti-emetic for chemotherapy and post-operative nausea

Question 24

How are tachykinins involved in psychiatric disease?

Answer 24

pharmaceuticals affecting the tachykinin receptors are being explored in psychiatric diseases and suggest a role for tachykinins in depression, schizophrenia, anxiety, and addictions

NK1 antagonists have antidepressant effects in animal models

NK1 knockout mice show decreased voluntary alcohol consumption and NK1 antagonists decreased alcohol cravings in preclinical trials of detoxified alcoholic inpatients

NK2 antagonists have anxiolytic and antidepressant effects in animal models

NK3 antagonists have antipsychotic effects in clinical trials (with very limited side effects)

Question 25

What are CCK family peptides?

Answer 25

Cholecystokinin (CCK) family includes CCK and gastrins

derived from two pre-proCCK, pre-proGastrin genes

gastrointestinal peptide hormones that normally triggers digestion of fat and protein: triggers release of digestive enzymes and bile from the pancreas and gallbladder, respectively

acts as a hunger suppressant in response to presence of fat/protein rich foods

Question 26

How is the CCK family peptides involved in the CNS?

Answer 26

CCK peptides are a family designated based on the length of amino acids: CCK4, CCK8, CCK22, CCK33, CCK58 are commonly found

CCK receptors are widely expressed in the CNS

administration of CCK into systemic circulation triggers nausea and emesis, as well as satiety: thought to act through the vagus nerve as circulating peptides are generally unable to cross the BBB

CCKR polymorphisms are associated with panic disorder and schizophrenia

Question 27

How is CCK4 involved in anxiety?

Answer 27

CCK4 administration is used as a model of anxiety

IV administration induces anxiety and panic attacks

can be used to test anxiolytic drugs in healthy volunteers

administration of peptides into systemic circulation elicits very transient effects as the peptides are rapidly metabolized

CCK plays some role in nociception

CCK may play an important role in anxiety disorders

Question 28

What is the relationship between CCK and benzodiazepines?

Answer 28

CCK receptor antagonists share structure and affinity with benzodiazepines

chronic benzodiazepine treatment decreases neural responsiveness to CCK

CCK receptor density is upregulated during benzodiazepine withdrawal: especially hippocampus and frontal cortex

Question 29

How do CCK receptor antagonists produce anxiolytic effects in animal models?

Answer 29

proglumide is a CCKA and CCKB antagonist used to treat stomach ulcers

anxiolytic in animal models

increases the analgesic effect of opioids and decreases the development of opioid analgesic tolerance in humans

prevents the development of analgesic tolerance to other pain treatments, e.g., transcutaneous electrical nerve stimulation (animal models)

Question 30

What is the nocebo effect?

Answer 30

nocebo is the opposite phenomenon to the placebo effect: expectation of symptom worsening leads to negative outcome

Question 31

How does the CCK family peptides relate to the nocebo effect?

Answer 31

research models of the nocebo in healthy volunteers uses verbally induced nocebo hyperalgesia (increased sensitivity to pain on expectancy)

decreased pain threshold is associated with increased activation of the HPA axis (stress response leading to cortisol release)

diazepam treatment reduces both hyperalgesia and HPA axis activity suggesting anxiety contributes to the nocebo effect

CCK receptor antagonists (proglumide) blocked the hyperalgesia of verbally induced nocebo but not the HPA axis activity: suggests CCK affect nocebo independent or downstream of anxiety

Question 32

What is the cocaine- and amphetamine-regulated transcript (CART)?

Answer 32

endogenous psychostimulant and anorexic peptide

upregulated by cocaine or amphetamine

may be responsible for some central effects of psychostimulants

CART alone induces locomotor hyperactivity BUT co-administration with cocaine inhibits motor hyperactivity

CART expression is modified by alcohol, nicotine, opioids

CART administration prevents reinstatement of abuse (animal models)

Question 33

What are the effects of CART being highly expressed in the hypothalamus?

Answer 33

inhibits known orexigenic pathways

CART signals downstream of 5HT in suppressing appetite in the hypothalamus

deficits in CART expression have been associated with binge eating (animal models)

treatments for binge-eating (rimonabant) increase CART expression