Activation of fine articular afferent units by bradykinin

Abstract The responses of single fine articular afferent units to close intra-arterial injection of KCl and bradykinin were recorded from filaments of the saphenous nerve of the cat's right hindlimb. All units included in this study were sensitive to local mechanical probing of the medial and anteromedial aspects of the knee joint. The units were identified by conduction velocity as belonging either to group III (2.5–20 m/s, 17 units) or group IV ( Bolus injections of KCl were used to test the accessibility of the units via the blood vessels. Such injections elicited a rapid burst of impulses at short latencies of less than 1 s. If this discharge did not occur, no test with bradykinin was carried out. There was no difference in latency and time course between such discharges in group III and group IV units. With only 3 exceptions the 40 units excited by KCl were also activated by bradykinin which was applied in doses from 0.026 to 26 μg. Higher doses were not used. For most group III and IV units the minimal effective dose of bradykinin for a clear excitation was usually either 0.26 or 2.6 μg. In both groups of units the bradykinin-evoked discharge generally had a uniform latency and a time course with a total duration well under 1 min. In the course of repetitive injections at intervals of 3–5 min, the latency of the evoked discharge increased gradually and its magnitude became successively smaller. This tachyphylaxis was usually very pronounced, regardless of whether low or high doses of bradykinin were administered. No differences in the bradykinin sensitivity were found between units with very low to very high thresholds to local mechanical stimulation (tested with von Frey hairs) and between units belonging to the 4 different categories of response behavior to passive innocuous and noxious joint movements. These results indicate that the sensitivity to bradykinin is shared by all fine articular afferent units, regardless of their thresholds to local mechanical stimulation and joint movement and, hence, their functional role in signaling innocuous or noxious mechanical events at the knee joint. In contrast, fast group II articular units (conduction velocity > 20 m/s) are readily excited by KCl, but not by bradykinin administration.