PHYSL 371 Transient Receptor Potential (TRP) channels

what are TRP channels

transient receptor potential channels that act as cellular sensors for a wide variety of physical and chemical stimuli

why are TRP channels polymodal channels

they respond to many different forms of stimulation

what is the selectivity of TRP channels

they are non selective as they are permeable to K+, Na+, and Ca2+

what is the reversal potential of TRP channels

around 0mV because they are non-selective

what activates TRP channels

various different stimuli like temp, pressure, stretch, chemicals, voltage, pH

what is the general structure of TRP channels

6 transmembrane domains with an intracellular C and N terminus

where is the selectivity filter for TRP channels located

between domains S5 and S6

what organism were TRP channels first identified in

drosophilla

what is capsaicin

the main pungent ingredient in hot chilli peppers responsible for the spicy/burning sensation

what happens when capsaicin was introduced into a culture if rat DRG (Dorsal root ganglion) neurons

as concentration of capsaicin increased, the more Ca2+ that was released (depolarization)

capsazepine

a completive capsaicin antagonist

what was observed when capsazepine was added in addition to capsaicin

there was no depolarization of the membrane

what are the chemical properties of capsaicin

it is hydrophobic due to a long hydrocarbon chain

what were the 2 hypotheses for why capsaicin caused Ca2+ release

1. direct disturbance if membrane lipids by hydrophobic capsaicin

2. activation of a specific capsaicin receptor on or within sensory neurons which

what neurons responded to capsaicin

dorsal root ganglion (DRG) cells

"gain of function" screening for capsaicin channels

1. isolate mRNA from DRG neurons which are responsive to capsaicin

2. reverse translate mRNA into a complementary DNA (cDNA) library

3. search through the cDNA library for the supposed capsaicin receptor gene

4. transform HEK cells with the supposed genes

5. HEK cells now become responsive to capsaicin when they were not before

calcium imaging

microscopy technique that visually tracks changes in Ca2+ concentration within cells, tissues, or even living organisms by using fluorescent indicators (excitation is around 380nm when no Ca2+ is present and 340nm when it is therefore as the ratio of Ca2+ to no Ca2+ increases, there is more Ca2+ present in the cell)

calcium imaging for capsaicin

multiple different receptors were tested fro see their response to capsaicin being added and TRPV1 receptors were the only ones that had a shift in wavelength excitation indicating Ca2+ influx

whole cell mode measures

measures over 99% of the whole cell current

what ions does TRPV1 conduct

Na+, K+, Cs2+, Mg2+, and a much higher preference for Ca2+

NMDG

large molecules that are non-permeable;e to channels

why does the TRPV1 channel have a higher preference for Ca2+

the RP for Na+, K+, and Cs2+ are all around 0mV combined but the RP for Ca2+ is more positive therefore when the channel gets activated, it is easier for Ca2+ to enter

reversal potential

the specific membrane voltage at which the net flow of ions across a membrane reverses direction and becomes a zero net current

open probability

the proportion of time a channel is in its open state relative to the total recording time

inside-out patch whole-cell mode measures

measures effects of intracellular solutes on channels within isolated patch

outside-out patch whole-cell mode measures

measures effects of extracellular solutes on channels within isolated patch

what happens when heat is put on a TRPV1 receptor

it also responds like capsaicin by evoking a Ca2+ influx

Densensitization

repeated stimulation by a ligand causing the channel to enter a non-conductive desensitized state and close even though the wigan remains bound

how can densensitization be helpful

it sets in rapid if harmful heat persists which protects the organism from cell swelling, inflammation, and tissue damage indued by harmful heat

what has densensitization thought to arise from

conformational changes in several channel structures including the channel pore in both C and N termini

platypus densensitization

platypi have TRPV1 channels that do not become desensitized and have relatively low body temperatures that therefore cannot adapt to temperatures higher than 25 degrees celsius resulting in death

TRPV1 mutant

loose function to respond to capsaicin even with increasing concentrations (TRPV1 N331K)

how can TRPV1 be a good target for drug development

drugs that have high concentrations of capsaicin will target TRPV1 channels and cause rapid densensitization leading to reduced pain signalling

Qutenza

very high (8%) capsaicin concentration which leads to activation and densensitization of TRPV1 receptors

what is capsaicins antagonist and what can it be used for drug wise

capsazepine can be used to revise the effect of the TRPV1 target drugs

what are side effects of using qutenza

hyperthermia (increased body temperature) and tumerogenesis (tumor production)

menthol sensation

responsible for eliciting a cooling sensation

cyclohexanol

an inactive synthetic menthol analogue (does no elicit the same response as menthol)

menthone

essential oils from plants like peppermint (smaller response than menthol)

what type of current does menthol induce

both inward and outward currents

what channel is responsible for sensing cooling sensations

TRPM8

what is the structure of TRPM8 channels

similar tetrameric channel architecture to TRPV1, pore region for ion selectivity, and an intracellular domain responsible for regulating channel gating

selectivity filter

a narrow region of the pore that determine which ions can pass through

activation gate

the residue downstream of the selectivity filter that physically opens or closes the pore in response to a stimulus

what compounds activate TRPM8 channels

cooling compounds like menthol, eucalyptol (medium response), icilin

what compounds do not activate (or result in small activation of) TRPM8 channels

menthone, cyclohexanol, camphor, capsaicin

what happens when you increase temperature on a TRPM8 channel

depolarization stops

what happens when you decrease temperature on a TRPM8 channel

depolarization increases

what happens when Ca2+ is present when TRP channels get stimulated

they become densensitized

why is it sometimes difficult to the tell difference between painful hot vs cold

some neurons exhibit both TRPM8 and TRPV1 channels making it difficult to distinguish because they both induce similar effects on the neuron

acoltremon (tryptyr)

treatment for eye disease (new approval) by activating the trigeminal nerve to increase basal tear production

TRPM8 agonist

acts as an activator and opens TRPM8 channels to producing cooling/sensory effects