Sensitization of articular afferents to mechanical stimuli by bradykinin.

In 18 cats anaesthetized with alpha-chloralose, we recorded from thin myelinated and unmyelinated articular afferents of the medial articular nerve of the knee joint. Bradykinin was injected intra-arterially close to the knee, alone and in combination with prostaglandin E2 (PGE2), and changes of the responses of single afferents to movements of the knee were monitored. Bradykinin changed the mechanosensitivity in 20 of 28 afferents inducing movement sensitivity in initially unresponsive units, lowering the threshold for movements in high-threshold afferents and/or enhancing pre-existing responses to innocuous and/or noxious joint movements in low and high threshold units. Also the application of PGE2 and bradykinin within a short interval sensitized the majority of these afferents, and in about 50% of the afferents the effect of the combination was superior to those induced by the single substances. We conclude that the inflammatory mediator bradykinin is able to sensitize articular afferents for movement stimuli and that PGE2 may enhance this effect. It is suggested that in arthritis inflammatory mediators act synergistically in the initiation and stabilization of the increased mechanosensitivity of slowly conducting articular afferents.