Lecture 5 - Pain

Question 1

What are the cells that respond to pain known as?

Answer 1

nociceptors

Question 2

Where doe nociceptors terminate?

Answer 2

free nerve endings in the skin

Question 3

Are pain signals carried by the same neurons or cells that respond to touch and how was the answer found?

Answer 3

no they are carried by separate cells and pathways than mechanosensory signals; experiment recording from cells that respond to light stimulus like a Bruch do not respond to pressure pinch and crush but another cell only responded to pressure and punch and crush

Question 4

What types of fibers carry pain information?

Answer 4

Adelta and C fibers

Question 5

What type of pain information do Adelta fibers carry and what is a physical characteristic of them in comparison to C fibers?

Answer 5

they can carry sharp pain or first pain; have greater axonal diameter than C fibers

Question 6

What type of pain information do C fibers carry and what is a physical characteristic of them in comparison to Adelta fibers?

Answer 6

slower, dull pain (second pain); smaller axonal diameter

Question 7

Why was it difficult to try to find the molecules that are sensors or receptors for temperature and how was this solved?

Answer 7

Problem - temperature is very nonspecific since all molecules change conformation and enzyme reactions speed up

Solution - use capsaicin from chili peppers to find the capsizing receptor and then see if this is also a temperature receptor

Question 8

What is the capsaicin receptor?

Answer 8

TRPV1

Question 9

How was the capsaicin receptor or TRPV1 found and cloned?

Answer 9

1. isolated mRNA from DRG and made cDNA and put it into a plasmid to propagate
2. create a cDNA library
3. transfect HEK-293 cells with cDNA
4. add capsaicin and measure the intracellular calcium levels using calcium imaging
5. if cells is expressing the calcium receptor they will show an elevated calcium level in response to calcium
6. have a large pool of HEK-293 cells with 1000 different cDNAs which confer sensitivity to capsaicin
7. subdivide the library to dins the single cDNA that confers response to capsaicin
8. gene encodes a membrane protein that looks like an ion channel

Question 10

What did the capsaicin ion channel seem to have transmembrane domain wise and what was it in regards to monomer, dimer, trimer, etc.?

Answer 10

six transmembrane domains, tetramer, has a p loop or selectivity filter

Question 11

What three things does TRPV1 respond to and how was this found?

Answer 11

heat, H+, and chili peppers
-introduces the TRPV1 mRNA into frog oocytes and measured current at -80mV in response to extracts of chili pepper and found a greater response to chilies with greater capsaicin

Question 12

What did the TRPV1 knockout illustrate?

Answer 12

• the knockout mouse cannot taste chili peppers; knockout mice are less sensitive to hot temperatures but to higher hot temperatures it did meaning other heat channels are also involved

Question 13

What animals cannot taste capsaicin and why?

Answer 13

birds and it is because they have a mutation in their capsaicin receptor but the receptor is still sensitive to heat

Question 14

Why can capsaicin be used to treat pain?

Answer 14

prolonged activation of nociceptors and high calcium levels induced by binding of capsaicin kills them (high calcium levels form precipitates and they are cytotoxic and the cell dies)

Question 15

What large family of ion channels if TRPV1 a part of?

Answer 15

the family of TRP or transient receptor potential channels

Question 16

What is the cold receptor and how was it found and what does it respond to?

Answer 16

TRPM8
-as cloned and a similar strategy was used to identify it as TRPV1 but menthol instead of capsaicin was used
-TRPM8 responds to cold and menthol and mint

Question 17

At what temperatures does TRPV1 respond to and TRPM8 respond to?

Answer 17

TRPV1 - 40 degrees C
TRPM8 - 15 degrees C

Question 18

What does TRPA1 respond to?

Answer 18

wasabi receptor, brown mustard, irritating chemicals, acrolein (car exhaust), cinnamon, garlic, onion, formalin; the response helps prevent real tissue damage because the pain will make you move

Question 19

What are the eyes innervated by a great deal of?

Answer 19

nociceptors

Question 20

What does a TRPA1 knockout cause and what was the experiment that was performed in regard to this?

Answer 20

-eliminates the pain response
-inject formalin into the paw of a mouse and measure the time spend licking the paw in the WT and KO mice
-result - all phases of the pain response are decreased in the KO (less licking)

Question 21

What is similar for the molecules that sense temperature and pain?

Answer 21

all are ion channels that are directly gated by the stimulus and that carry an influx of calcium and sodium and they are non-selective cation channels like glutamate receptors

Question 22

What are released in response to tissue damage like a sunburn?

Answer 22

inflammatory mediators known as the inflammatory soup
-this causes a release of things by mass cells and pain fibers

Question 23

What is NGF, what is it released by, and in response to what?

Answer 23

never growth factor, released by sensory neurons in response to inflammation

Question 24

What is the effect of NGF on TRPV1?

Answer 24

activity of the TRPV1 receptor is unregulated by BGF released in response to inflammation so it is now activated by mild temperatures

(response of a never fiber to heat is enhanced after NGF; response of a DRG neuron to capsaicin after NGF is also increased)

Question 25

How is pain and temperatures differentiated from each other?

Answer 25

cells that activate upon leading to pain have both TRPA1 and TRPV1 while heat only cells respond to only TRPV1; how we tell the difference between wasabi (pain) and chilli peppers (heat)

Question 26

Where do nociceptors synapse in the spinal cord and what happens to their afferent fibers?

Answer 26

synapse at the same level they enter in the spinal cord and their afferent fibers cross the midline and then ascend through the anterolateral tract

Question 27

What is the difference between the anterolateral nociceptor tract and the dorsal column-mediated leminiscal touch tract.

Answer 27

Touch travels on the ipsilateral side of the spinal cord while pain is contralateral which is why if you have a lesion on one side of the spinal cord you will lose touch on the ipsilateral side as the lesion and lose pain on the contralateral side of the lesion

Question 28

What is the gate theory of pain?

Answer 28

-during WWII soldiers wounded in battle reported little pain meaning pain depends on context (they were received to be removed from the battlefield which is why they reported less pain) (descending control of the perception of pain)

Question 29

What is the role of the placebo effect in the gate theory of pain?

Answer 29

patients given saline reported less pain even when not given opiates like other patients since they produced endogenous opiates

Question 30

What gates pain and how?

Answer 30

descending inputs gate pain
-they inhibit transmission of information from C fibers to the dorsal horn
-use enkephalins (natural opiates)

Question 31

What are enkephalins?

Answer 31

natural or endogenous opiates used by descending input to inhibit the transmission of info about pain from C fibers to the dorsal horn

Question 32

How can pain be gated by touch?

Answer 32

Abeta (AB) fibers inhibit second order neurons or C fibers that carry pain information

Question 33

What does pain relief take advantage of and how?

Answer 33

the gate theory of pain
-electrical stimulation of the periaqueductal gray area of the brain provides relief from pain due to declining control of the dorsal horn cells
-skin mechanoreceptors can reduce pain (rubbing the site is a natural response to acute pain)

Question 34

What is CIP and what is it caused by (mutation in both copies of what channel)?

Answer 34

-congenital insensitive to pain and it is a recessive genetic disorder that eliminates pain
-the sodium channel SCN9a is a specific isoform only expressed in nociceptors and patients with mutations in both copies have CIP