The effect of baclofen on the transmission in spinal pathways in spastic multiple sclerosis patients

Objectives: To measure the effect of baclofen on the transmission in differ ent spinal pathways to soleus motoneurones in spastic multiple sclerosis pa tients. Methods: Baclofen was administered orally in 14 and intrathecally in 8 pati ents. H-max/M-max, presynaptic inhibition by biceps femoris tendon tap of f emoral nerve stimulation, depression of the soleus H-reflex following previ ous activation of the Ia afferents from the soleus muscle (i.e. postactivat ion depression), disynaptic reciprocal Ia inhibition of the soleus H-reflex and the number of backpropagating action potentials in primary afferents, which may be a sign of presynaptic inhibition, were examined. Results: Baclofen depressed the soleus H-max/M-max ratio significantly foll owing oral and intrathecal baclofen, None of the two tests of presynaptic i nhibition, or the postactivation depression or the disynaptic reciprocal Ia inhibition of the soleus H-reflex were affected by baclofen administration . Also the action potentials of the primary afferents were unchanged during baclofen administration. Conclusions: The antispastic effect of baclofen is not caused by an effect on the transmitter release from Ia afferents or on disynaptic reciprocal Ia inhibition. One possible explanation of the depression of the H-reflex by baclofen is suggested to be a direct depression of motoneuronal excitabilit y. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.