Bradykinin Research
BK Receptors
It has been discussed how bradykinin, a key neuropeptide in the development of inflammation and pain in the periphery, may also play a role in the CNS. Migraine development is thought to be dependent on the sensitisation, inflammation and stimulation of trigeminal neurones in the CNS. 18 TG Neurones were examined, a number of control measurement were taken, and these included tests to aid the classification of the neurones. The results indicate that it is most likely that the neurones examined were Polymodal C-Fibres. The neurones were then exposed to 100nM of bradykinin and the effects were measured.
The results indicate that BK has no effect on the majority of measurements within the Action Potential. However, it has a marked and consistent effect on the threshold and membrane potential of each neurone. However, the question remains as to how it produces its effects.
BK is known to act on two receptor subtypes: BK1 and BK2, the latter being the more predominately found receptor while BK1 is thought to be developed in chronic inflammation. To answer the question of how BK elicits its effects the neurones were tested with a number of compounds known to act at the BK receptors. The neurones were thoroughly washed after applications to ensure that the effects observed were only due to the drug administered immediately before testing.
Results
- des-Arg BK, a potent BK1 agonist was administered. This agonist did not appear to have any affect on the neurones firing.
- The potent BK2 agonist Hyp3BK was administered. Hyp3BK produced significant changes in the firing properties of the neurone: the threshold appeared to have been lowered and the firing rate greatly increased.
- Hoe140, a BK2 antagonist was used to examine its effects on a neurones response to BK. Hoe was administered initially alone and no effect was seen. Whilst the neurone was subject to Hoe140 100nM of BK was administered, the result was that the neurone no longer responded to BK. To ensure that BK did produce a response without Hoe140 being present the neurone was washed and the same amount of BK was applied, the result was membrane depolarisation.
- The neurones’ responses to 100nM of BK, des-Arg BK and Hyp3BK were consecutively examined and compared. BK produces a significant effect, depolarising the membrane. Des-Arg BK produces no effect and Hyp3BK produces a significant effect, greater than that produced with BK.
Discussion
The results show clearly that the receptor involved in the trigeminal nerves’ response to BK is the BK2 receptor. This is unsurprising due to the limited distribution of BK1 receptors. The replication of results with BK2 agonists and the lack of effect of BK1 agonists, it is clear that this is the case. However, as BK1 is not constitutionally expressed in normal tissue this may be different under extreme / chronic cases of inflammation. It may be that the effects are similarly produced with BK1 receptor activation but the tissue tested did not have BK1 present. This is difficult to test in these circumstances as the tissue must be exposed to prolonged inflammation circumstances to produce BK1 and these may not necessarily be produced in such a culture as used here. Consequently, the possible involvement of BK1 in addition to BK2 remains an open question and an area where further research could be carried out.
Given the evidence, it is certain that BK acts on BK2 receptors in trigeminal neurones. However, the mechanism by which it affects the cells electrophysiological properties following receptor activation is uncertain. BK is known to cause the production of other messengers, which may act in this case to produce the observed effects or it may be a direct effect. These possibilities are investigated in the section BK Mechanisms.
Graph showing Effects of DesArg BK. Click the thumbnail above for a larger
version.
Graph showing comparison of DesArg BK, Hyp3BK and BK effects. Click the thumbnail
above for a larger version.
Graph showing the effects of Hoe BK on neuronal response to BK. Click the
thumbnail above for a larger version.
Graph showing effects of Hyp3 BK, a BK2 agonist. Click the thumbnail above
for a larger version.
