Bradykinin and serotonin effects on various types of cutaneous nerve fibres
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Bradykinin (BKN) and Serotonin (5-HT) are endogeneous substances yielded from injured tissue and evoke pain reactions in man and animals when brought into the skin even in small quantities. In order to test which nervous elements mediate the sensations evoked by these “local hormones” we studied the spike responses of single afferent fibres in cutaneous nerves of the cat's hind leg upon intraarterial administration of small doses (5–30 μg) of 5-HT and BKN.
Thick myelinated, thin myelinated, and unmyelinated fibres could be excited by both substances. A further classification of the fibres was done according to the receptor types supplied by them. Among the unmyelinated fibres we tested, al receptor classes were activated by 5-HT and BKN, but among the myelinated fibres only definite groups of receptors proved to be responsive, i.e. the slowly adapting low and high threshold mechanoreceptors with thin myelinated afferents and the slowly adapting mechanoreceptors supplied by thick myelinated fibres.
The spike responses started 10–2- sec after injection and persisted up to 4 min in units not firing spontaneously. Discharge frequencies usually were much lower than upon physical stimulation of the receptors.
Repeated injection of the same agent at a rate as low as 1:15 min abolished the response to it. However this tachyphylaxis was found not to diminish the effect of the other substance. This might be an indication of an action of BKN and 5-HT on different pharmacological receptors on the nervous membranes.
BKN injections were found to enhance the spike discharges of unmyelinated nocipeptive fibres upon noxious radiant heat stimulation for a period of 2–5 min after a single injection of 5–30 μg. It was concluded therefrom that this agent might play a role in the hyperpathia of inflamed skin.
Key wordsBradykinin Serotonin Pain Mechanisms Cutaneous Receptors
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