Effect of catechol on the discharge of muscle spindle afferents from the hind limb of the rat

Abstract

Previous work had shown that some of the effects of catechol could be via the fusimotor system. In order to determine the extent of fusimotor involvement, recordings have been made from muscle spindle afferents in split dorsal root filaments of anaesthetised rats. Catechol failed to excite de-efferented muscle spindles therefore eliminating many possible non-fusimotor effects. Over 80% of spindle afferents with intact efferents showed increased discharge frequency 1 min after injection, this increase often following a biphasic pattern with a pronounced pre-myoclonic burst and decline followed by a more sustained period of activity during the myoclonic phase. Analysis of spontaneous twitches or twitches evoked by ipsilateral auditory stimulation showed in addition a phasic increase in discharge suggesting α-γ co-activation. Both primary and secondary afferents from many muscle groups around ankle and toes had their discharge frequencies elevated. Elevation of discharge frequencies of secondaries implies increased γ-s activity confirmed by a decrease in dynamic index. Both γ-d and γ-s involvement in catechol action on primaries is suggested by dynamic index measurements. Perhaps a more continuous form of testing such as sinusoidal stretches would reveal any rapid switching between activities in the two systems.