THE MODE OF ACTION OF THE GABA DERIVATIVE BACLOFEN IN HUMAN SPASTICITY

Abstract

Baclofen is chemically related to GABA and has proved to have a clinical effect on spasticity and on flexor spasms. The mode of action of baclofen was studied by quantitative measurements of some reflexes and of voluntary power in relation to intravenous injections of the drug (average dose 0.38 mg per kg body weight) in 44 patients with spasticity due to spinal or cerebral lesion or multiple sclerosis; a total of 53 injections were given. In most of the patients studied, depression occurred in the phasic stretch reflex (Achilles tendon), tonic stretch reflex, flexor reflex, reflexes from the pelvic floor and in the tonic vibration reflex; whereas the fl reflex changed in only a few cases, and voluntary power did not change at all. In some patients a pronounced increase was observed in the vibration-induced depression of the H reflex, which after baclofen approached normal. Baclofen can reduce the fusimotor drive, whereas a general primary effect on the α motoneurones is unlikely. It acts on the interneurones and can increase ‘presynaptic’ inhibition of the la nerve fibres. It is capable of exerting its action exclusively at the spinal level and can depress hyperactive stretch reflexes, whether they are caused by a spinal or cerebral lesion.