lecture 18 Flashcards
what is substance P?
- active neuropeptide found in gut
- 11 amino acids long
how was substance P found by Von Euler?
- found intestinal extract contained material that stimulated atropine-resistant contractions of ileum
what are the 3 major mammalian tachykinins?
- substance P
- substance K (neurokinin A)
- neurokinin B
which two tachykinins are most similar?
- substance P and K are cosynthesised and coreleased
- have common feature at C terminus
what does the amino acid residue at position 4 from the C terminus mean?
- determines the affinity
what neurones are immunopositive for substance P?
- neurones in myenteric plexus (MY) that extend in orad direction to the circular muscle
- neurones found to contain neuropeptides
- project short distances
- VIP negative but some contain ACh
what does electrical stimulation of substance P neurones cause?
- smooth muscle contraction
- excitatory junction potentials which are resistant to atropine
what are excitatory junction potentials?
- rapid, temporary depolarisations that occur in smooth muscle cell arteries
- occur when sympathetic nerves are stimulated
what does depolarising stimuli in myenteric neurones cause?
- release of substance P (detected by radioimmunoassay)
what is the stimulus for substance P (and K) release?
- distension
what does substance P cause direct excitation (contraction) of?
- gastrointestinal smooth muscle
what experiment did Grider and Makhlouf test the effects of in 1989?
- measuring the effects of ascending contraction
- using the glass rod experiment, stretching at the caudad end
what did figure 2 show?
- the greater the caudad stretch, the larger the increase in ascending contraction
- reaches a plateau at caudad stretch of 10g
what is the response in figure 2 blocked by?
- tetrodotoxin -> shows its a neuro-reflex
- hexamethonium -> blocks nicotinic cholinergic receptor so blocks the interneurones
what does caudad stretch activate?
- neuro-reflex
- activates enteric nervous system in wall of colon
- causes release of neurotransmitters to cause contraction of tissue above the point of stimulus
what occurred when atropine was added?
- effective at low levels of stretch to block (almost completely inhibits)
- less effective when greater stretch occurs
what is the neurotransmitter used at low stretch levels?
- acetylcholine
what happens when substance P antagonist is added?
- at low stretch, antagonist doesnt do much
- the greater the stretch, the bigger the effect of the antagonist
what does this tell us?
- substance P neurotransmitter is important at large stretch
what happens when both neurotransmitters are combined?
- almost completely blocks the response of ascending contraction
- effects are specific
- neither effect the descending relaxation
what did Grider find in 1989?
- both P and K released during ascending contraction
- substance P and K are neither released during orad stretch in descending relaxation
- levels decrease from basal
- shows baseline tachykinins are being inhibited by orad stretch
what does figure 5 in Griders 1989 experiment show?
- no significant difference between substance P and K release
what does treating with antibody for substance P cause?
- decrease in response for both substance P and K
- contractile response
what does caudad stretch cause?
- ascending contraction
what is ascending contraction inhibited by?
- antibodies for substance P and K
what is contraction a combination of?
- substance P
- substance K
- acetylcholine
what blocks the release of substance P and K?
- tetrodotoxin
- hexamethonium
- shows its released from the neural pathway
what is CGRP?
- calcitonin gene related peptide
what has CGRP been immunolocalised to?
- primary sensory/afferent neurones in GI tract
where do the neurones releasing CGRP extend into?
- myenteric plexus
when is CGRP released?
- when afferent neurones are stimulated
how is CGRP release detected?
- by radioimmunoassay
where is serotonin (5-HT) found in?
- paracrine cells -> enterochromaffin cells in the mucosa
when is serotonin released?
- through chemical stimulation
- or increased pressure locally due to distension
what does serotonin cause?
- release of CGRP from the sensory neurones
what did Grider test in 1998?
- neurotransmitter release during contraction and relaxation
what is the method Grider used to test this?
- cut intestine tissue open and pinned to bottom of tissue bath
- separate into separate compartments (central, orad, caudad)
- measure what’s been released from tissue therefore what’s been stimulated
what is the central compartment the point of?
- distension
what is the caudad section the point of?
- descending relaxation
what is the orad component the point of?
- ascending contraction
what happened when a 5-HT selective agonist was added to central compartment?
- dose deponent increase in CGRP in central compartment
- substance P was released from orad compartment
- VIP released from caudad compartment
what was added next alongside the 5-HT agonist?
- hexamethonium
what occurred with the addition of hexamethonium?
- no effect on the CGRP release
what does this tell us about CGRP?
- that its released before the neurones are activated
- release is early in the pathway
what does the addition of CGRP antagonist cause?
- blocked effects of CGRP (blocking the ascending contraction and descending relaxation)
what does this tell us about CGRP?
- has to be released to allow the activation of interneurone
