Pharmacological Modulation Of 5-HT3 Receptor Flashcards
What family of receptions do 5-HT3 receptors belong to?
Cys loop facility
Same as GABA, nAch and glycine
Subunit structure?
Extracellular N TM1 TM2 TM3 HA region TM4 Extracellular C terminus
Why are they called cys loop receptors?
Cysteine form disulphide bridge in extracellular domain in N terminus
Between the two Cysteines is a conserved sequence of 13 amino acids which all members of this family
have
Why is the cys loop important?
Communication between neurotransmitter binding site and ion channel
How many subunits come together to from the 5ht3 R?
5
What mains 5HT3 receptors unique from other 5HT receptors?
This is a ionotropic receptors
All others are GPCRS
What is the equilibrium potential of 5HT3 receptors?
0mV
Selective for monovalent actions
Sodium in
Potassium out
In gross terms, what are the different areas of the nAchR (shares homology with 5HT3)
Extracellular parts which are predominantly from from beta pleated sheets
TMDs are also a helixs
Intracellular part interacts with scaffold proteins
What governs nAchR selectivity?
TM2 regions forms selectivity pore - ligand will widen pore
Selectivity governed by vertically aligned TM2 regions
Also, 3 rings of negativity charged residues flank the internal and external boundaries of TM2 which make it selective for cations
Where is the ligand binding site on 5HT3 receptors?
Located in electronegative cleft between two subunits
What is the essential amino acids for binding and where is it located?
Tyrosine 141
Loop E of 3A subunits
Also site of competitive antagonism
How can we crystallise 5HT3 receptors?
Too large to crystallise alone Make antibody to 5-HT3 R Express receptor in HEK cell Purify receptor Split with trypsin proteolysis Crystallise fragments with nanobody/VHH
What is a VHH?
Antigen binding variable domain of H chain of Heavy chain
What is the graph for current amplitude in relation of concentration of 5-HT?
Classic dose responce
3A homomers have a lower EC50 than 3A 3B hetromers
Therefore 5-ht more potent on 3A homomers
What are the subunits of 5-HT3 receptors?
A-E
Where are A and B subunits expressed?
Highly in brain and GI tract
Heteropentamers can form from 3A and 3B subunits.
What is the only pattern configuration?
BBABA
What is the only homomer which forms a functional channel?
3A (homomers of other subunits do not form channels)
3a contains tyrosine 14- essential for ligand binding
Can heteromers of 5HT3A with 3B, 3c, 3D, 3e form functions channels?
Yes, but they all have varying channel conductance properties
How do 3A homomers and 3A+B heteromers show different electrophysiology properties?
3A is an inward rectifier
3A+B has a linear conductance to voltage relationship
What is single channel conductance for 3A homomers?
0.5ps
What si single channel conductance for 3A+B heteromers
About 14ps
Why do homomers and heteromers show different single channel conductances?
Chimeric studies where parts of 3B subunit are fused into 3A subunit will show increased channel conductance if the 3B sequence replaces the respective 3A subunit position which governs its ions conductance. Through doing this the reasearchers discovered area HA in the ICL between M3 and M4
What is important about area HA?
3A subunit contains conserved string of arginines (which are positivity charge thus decrease conductance)
3B subunit does not contain arginine string thus has a larger conductance
What will the effect be of mutations the arginines in the HA region of 3A?
Point mutantions which swap out arginine = increase channel conductance to a similar level as 3B
What is the physiological role of 5-HT3 receptors?
Gut motility
Secretion
Sensation
Where are 5-HT3 receptors found?
The brain
Peripheral sensory neurons
The GI tract in the mucosa, submucosa and myenteric plexus (sensory part of vagus nerve)
Where do the different subunits exist?
Not much is know about subunit location thus may provide difficult for drug selectivity
However, 3E is found only in intestine and stomach
What cells release 5-HT within the GI tract? What is their role?
Enterochomaffin cells (enteroendocrine and neuroendocrine cell) Role in motility and secretion in GI tract These cells respond to chemical, neurological and mechanical stimuli which cause them to release 5-HT which modules peristalsis and secretion (depends on conc of 5-HT)
What stimuli cause the EC cells to release 5-HT? What do these cells act on?
Noxious stimuli, chemical (cisplatin), mechanical, toxins
Act of vagal afferents which project to solitary nucleus, the chemoreceptor trigger zone and vomiting centre
Release more 5-Ht here to cause vomiting
Name some 5-HT3 receptor antagonists? What is their effect?
Alosetron
Ondansetron
Act competitively with 5-HT due to structural homology and reduce the amount of current through channel
What makes us vomit?
Pregnancy RICP toxins Smells Rotation Stretching of stomach
Where are 5-HT3 receptors found in the brain?
CTZ
vomiting centre
What factors input to the CTZ?
Toxins from the blood (no BBB)
Vestibular apparatus
Higher centres (smell, sights and emotion)
What things feed into the vomiting centre?
The CTZ sends projections releasing 5-HT to vomiting centre
Afferents from stomach which detect drugs, toxins, irritation etc
Higher centres
What is the output of the vomiting centre?
Efferents which cause vomiting
- contraction of pyloric sphincter
- relaxation of cardiac and oesophagus
- abdominal wall contraction
- epiglottis closes the trachea
What can cause nausea and vomiting (medical interventions) ?
Cytotoxic cancer chemo drugs
Radiotherapy
Post operative N+V
Opioids
How can alosetron be used for IBS?
Treat diarrhoea variant As decreases gut transit time Increases water absorption Decreases pain May causes abdominal obstruction secondary to faecal impaction
What may 5-HT3 receptor antagonists be used to treat in future?
Anxiety
Improvise congnitive function in neurodegenative diseases
Pain (esp migraine)
What are the current uses of 5-HT3 antagonists?
Antiemetic
IBS
antidepressants (mitazipine)
Antipsychotics such as clozapine
What does 5HT stand for?
5-hydroxytrypamine
