INCREASED INTRAMUSCULAR CONCENTRATION OF BRADYKININ INCREASES THE STATIC FUSIMOTOR DRIVE TO MUSCLE-SPINDLES IN NECK MUSCLES OF THE CAT

The aim of the present study was to investigate if increased intramusc ular concentrations of bradykinin (BK) in one muscle influence the act ivity in primary and secondary muscle spindle afferents (MSAs) origina ting from both ipsi- and contralateral muscles, via fusimotor reflexes . The ipsilateral trapezius (TR) and the splenius (SP) muscles were su bjected to sinusoidal stretches and 2-3 MSAs were simultaneously recor ded from these muscles. Responses of 29 MSAs (15 SP and 14 TR) were re gistered in five adult cats anaesthetised with alpha-chloralose. Intra muscular injections of 0.5 ml BK (6-86 mu g/ml) were administered to b oth the ipsi- and contralateral SP and TR muscles. Similar doses of BK (5-10 mu g) have been shown to induce muscle pain when injected into the temporal muscle in man. The responsiveness of the MSAs to the inje ctions of BK was 86% and 87.5% from the contralateral TR and SP muscle s, respectively. The effects were predominantly static onto the MSAs. The duration of the effects was on average 3.5-4 min, however some eff ects lasted for more than 15 min. The effects were always abolished af ter cutting the nerve to the injected muscle. The large majority of th e spindle afferents were unresponsive to i.m. Tyrode injections (23 of 29). For the afferents that were responsive to injection of Tyrode, t he effects were always considerably smaller and with shorter duration than those evoked by BK injections. Thus, increased intramuscular conc entrations of BK may excite primary and secondary MSAs from ipsi- and contralateral muscles, via fusimotor reflexes evoked most probably by activity in chemosensitive muscle afferents. The results are discussed in relation to a recent hypothesis on pathohysiological mechanisms be hind genesis, spread and perpetuation of muscle tension and pain in ch ronic musculoskeletal pain syndromes. (C) 1997 International Associati on for the Study of Pain.